PRELIMINARY PROGRAM:SECOND INTERNATIONAL SYMPOSIUM ON ACID-BASE INTERACTIONS: RELEVANCE TO ADHESION

Dr. Tanweer Ahsan ;English China Clays International, Sandersville, GA

Acid-base properties of untreated and surface-modified minerals

Dr. Ronald E. Allred¹ , and Dr. Sheldon P. Wesson ²; 1. Adherent Technologies, Inc., Albuquerque, NM; 2. Ynir Instrumentation Software, Princeton, NJ,

Effect of Acid/base Interactions on Adhesion to Carbon Fiber at Polycarbonate Composite Interfaces

Interfacial bond strength is often a performance limiting factor of carbon fiber reinforced thermoplastic composites. This limitation is due, in part, to the unreactive nature of engineering thermoplastics and limitations inherent in commercial carbon fiber surface treatments. Radio frequency glow discharge plasmas are an effective means of modifying carbon fiber surface acid/base characteristics. Acidic plasmas are especially effective compared to electro- oxidative treatments for tailoring carbon fiber surfaces as shown by titrations, ESCA, wetting, and inverse gas chromatography measurements. Plasma modified fiber, both basic and acidic, showed increased adhesion with polycarbonate. We hypothesize that increased adhesion is due to ester interchange reactions between plasma formed surface groups and polycarbonate chains. Use of an esterification catalyst with acid plasma treated fibers resulted in further increases in interfacial adhesion. Increased interfacial adhesion also resulted in improved moisture resistance of IM7/polycarbonate composites.

Dr.Herve Arribart , Marie-Jose Azzopardi, Jean-Marc Berquier, Valerie Coustet, Francois Creuzet, Jacques Jupille and Xue-Yun Lin; Laboratorie CNRS/Saint-Gobain <>, BP 135, F-93303 Aubervilliers,FRANCE

Hydroxyl Groups and Acid-Base Properties of Oxide Surfaces

Although the acid-base properties of high area materials is widely documented, very few have been done in the past about flat surfaces, the major reason for that being that relevant techniques were lacking. This has prompted us to develop methods able to characterize the hydroxyl groups and their behavior on such surfaces. The formation of hydroxyl groups have been directly evidenced on magnesium oxide, alumina and silica surfaces by high resolution electron loss spectroscopy and photoemission spectroscopy under high-vacuum conditions. The values obtained in that way compare well with measurements performed with conventional methods, which validate our approaches. Finally, infrared spectroscopy has been used to investigate the role of surface hydroxyl groups in the grafting of organosilanes and in the adhesion of polymers at the surface of silica.

Dr.Henri BALARD and E.PAPIRER; ICSI-CNRS, 15 rue Starcky, BP 2488, F-68057 Mulhouse, FRANCE

Determination of the acid/base properties of solid surfaces using Inverse Gas Chromatography: Advantages and limitations

Inverse Gas Chromatography at infinite dilution conditions (IGC-DI) is a convenient method for the examination of the surface properties of a solid. The comparison of apolar (n-alkanes) and polar probe adsorption behaviors leads to an estimation of the specific (polar, acid-base) interaction capacity of the solid surface. Knowing the acid/base characteristics of the probes, one may assess the acid/base properties of the solid surface itself. The different approaches proposed in the literature will be reviewed and exhaustively discussed. The choice of an (empirical) acid-base scale is of importance and will also be examined. Furthermore, the probe-surface interaction depends on steric factors. Recently, we proposed a procedure to take this parameter into account. However, the limitations of the IGC-DI method for the determination of the acid/base properties of a solid surface are essentially related to the unavoidable surface heterogeneity of any solid surface. Indeed, the IGC-DI fundamental equation assumes that the solid’s surface is quite homogeneous. In fact, this condition is scarcely fulfilled because of the presence of surface structural defects, variation of nature and topology of surface groups… As a consequence, the sites having the highest interaction potential will principally contribute to the retention time of the probe on the chromatographic column leading to acid/base parameters that are not representative of the whole surface of the solid.

Dr. M. G. Barthes¹ and J-C. JOUD²; 1. CNRS - Centre d'Etudes de Chimie Métallurgique, 15 rue Georges Urbain, 94407 Vitry Cedex, France; 2. Institut National Polytechnique de Grenoble - Laboratoire de Thermodynamique et Physico-Chimie Métallurgique, 38402 Saint Martin d'Hères, France

Acid-Base Characterization of Metallic Materials and the Use of Model Molecules in the Study of Adhesion Mechanisms

As initially proposed by Fowkes [1], the adhesion of polymers to inorganic solids depends on acid-base interactions between the acidic (basic) surface sites and the basic (acidic) functional sites of the polymer. A precise acid-base characterisation of the surface of the solid substrate is therefore an essential preliminary step to understand adhesion mechanisms. The PZC (point of zero charge) value and the maximal surface charge can be deduced either from contact angle experiments where the equilibrium contact angle is measured as a function of the pH of the sessile drop in a two liquids configuration or from AFM measurements [2]. Both methods allow characterisation of the Brønsted acido-basicity of the surface, whereas the Lewis acidic and basic sites can be characterised by following with XPS the adsorption of molecular probes on the surface. Finally, a detailed understanding of adhesion mechanisms can be achieved using XPS by following the adsorption of small model molecules which can mimic the basic functionalities of the polymers.

Examples of acid-base characterisation of the surfaces will be given for passive films formed on metallic surfaces like Cr, Fe, Ti and stainless steel. The influence of the nature of the acidic (basic) sites on the adhesion mechanism will be illustrated in the case of an epoxy resin/titanium interface.

References

1. F.M. Fowkes and M.A. Mosfata, Ind. Eng. Chem. Proc. Res. Dev. 17 (1978) 3
2. P.E. Dubois, C. Vittoz, M. Mantel and JC Joud, EURADH'98, 6-11 September 1998, Garmish, Germany

Dr. R. Bos , G.I. Geertsema-Doornbusch, H.C. van der Mei and H.J. Busscher; Laboratory for Materia Technica, University of Groningen, Bloemsingel 10, 9712 KZ Groningen, The Netherlands.

Acid-base interactions in microbial adhesion to hexadecane and chloroform

Acid-base interactions play an important role in adhesion, including microbial adhesion to interfaces. An interesting approach to qualitatively demonstrate acid-base interactions in microbial adhesion is to compare adhesion to hexadecane (a negatively charged interface in aqueous solutions, unable to exert acid-base interactions) and chloroform (able to exert acid-base interactions) from aqueous suspensions of microorganisms. For 32 different microbial strains (4 E. coli, 4 E. faecalis, 4 Lactobacilli, 10 Staphylococci, 10 Streptococci) in low and high ionic strength potassium phosphate suspensions, adhesion to chloroform was initially faster and in a stationary end-point more extensive than to hexadecane, due to acid-base interactions, additionally operative from the chloroform. However, a (quantitative) surface thermodynamical analysis of acid-base interactions based on water, formamide, methyleneiodine and -bromonaphthalene contact angles completely failed for the data obtained at low ionic strength, likely because electrostatic interactions intervened. A weak relationship between initial removal rates and the acid-base interfacial free energy of adhesion was observed for the high ionic strength data, provided the analysis was confined to strains with an absolute zeta potential less than 10 mV. It is concluded that:

1. A surface thermodynamical analysis of microbial adhesion must include a consideration of electrostatic interactions.

2. Acid-base interactions may not become fully operative in adhesion of various oral microorganisms because structural surface features as fibrils and fimbriae impede a close enough approach.

3. Gel-like polysaccharide matrices, in which many staphylococci are embedded, impede a definition of the actually interacting surface.

4. Qualititatively, the role of acid-base interactions in microbial adhesion cannot be denied, but quantitative analysis is still largely inadequate.

Leanne Britcher, Susanne Grasser and Janis G. Matisons; Polymer Science Group, Ian Wark Research Institute, University of South Australia, Mawson Lakes 5095, South Australia

Acid-Base Glass Surface Chemistry

Glass fibres are often used to reinforce polymer resins. A key factor in the successful employment of such matrials is the interaction of various coupling agents at the glass-fiber interface. Recent work by the Polymer Science Group has shown that siloxane oligomers bearing the required functional groups may, in fact, produce better sizes for composite manufacture, than conventional silane coupling agents. Glass fibers can and do adsorb a host of agents and reagents, having both basic and acidic properties, due to the existence of both acidic and basic sites on the glass surface.

The present study examines the attachment of not only g-APS, a common silane coupling agent, but also amines and amino acids to the treated E-glass fibers and attempts to identify and evaluate the acid-base surface parameters influencing their attachment. A close study using XPS, SEM, contact angles and DRIFT has revealed that significant details of the treated glass surface can be evaluated. Two glass pre-treatment processes were examined:

• Drying the fibers at 550-650 C followed by a solution graqfting treatment
• Acidic washing of the fibers to remove sodium, calcium and aluminum ions prior to a solution grafting treatment.

The influence of both pretreatments on the subsequent interactions with amino acids will be assessed.

A. R. Burns , J. E. Houston, R. W. Carpick and T. A. Michalske; Sandia National Laboratories, Albuquerque, NM

Acid-Base Interactions at the Molecular Level: Adhesion Studies with Interfacial Force Microscopy

To understand adhesive interactions at the molecular level, we are engaged in a program which utilizes scanning probe microscopy to explore molecular forces at well defined interfaces. In particular, we measure displacement-controlled adhesive forces between self assembled monolayers of alkanethiols strongly bound to a gold substrate and a single asperity contact tip. Inlike cantilever-based atomic force microscopes, the Interfacial Force Microscope (IFM) developed at Sandia¹ is a noncompliant, mechanically stable probe that provides a complete adhesive profile without jump-to-contact. By systematically varying the alkanethiol end group chemistry, we are able to relate the measured work of adhesion to bond energies for specific molecular level acid-base, hydrogen bonding and van der Waals interactions. More recently, we have constructed a new instrument with decoupled lateral and normal force sensors to simultaneously observe the onset of both friction and chemical bond formation. Measurements made on alkanethiols with chemically different head groups show that friction can be directly attributed to bond formation and rupture well before repulsive contact. Thus we are able to separate chemical friction from more traditional mechanical sources of energy dissipation. Finally, we show how these single asperity contact measurements can be used to predict the mechanical performance of micromachined devices with realistic rough surfaces.

1. S. A. Joyce and J. E. Houston, Rev. Sci Instrum., 62, 710 (1991).

Dr. Christophe BUREAU¹ , Delano P. CHONG², Spyridon KRANIAS ¹ and Gerard LECAYON ¹;1. CEA-Saclay, DSM-DRECAM-SRSIM, bat.466, F-91191 Gif-sur-Yvette Cedex (France); 2. Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada).

Accurate Calculation of Core-Electron Binding Energies via Density Functional Theory :Towards an Evidencing of Intermolecular Effects at Real Interfaces via XPS

A recent procedure for computing core-electron binding energies (CEBEs) will be presented, which makes use of Density Functional Theory (DFT). It is based on a generalization of the Transition-State concept of Slater, and affords calculated CEBEs with a systematic average absolute deviation from experiment of the order of 0.2 eV on the 200 benchmark systems on which it was tested. The favorable CPU-time scaling of DFT allows this procedure to be applied to fairly large systems such as polymers and interfaces. By requiring a systematic agreement of the order of 0.2 to 0.3 eV between theory and experiment, one may use this procedure as a powerful tool to identify unexpectedly weak effects on XPS spectra. It will be shown that measurable shifts may even be obtained in the case of several intermolecular effects such as ion-pairing, dipolar interactions and hydrogen bonding within polymer phases. Possible extensions of this method to the calculation of CEBEs of molecules chemisorbed on metallic surfaces will also be considered.

Dr. Michael Butkus¹ and Dr. Domenico Grasso²;1. Department of Geography and Environmental Engineering United States Military Academy West Point, NY; 2. Environmental Engineering Program,The University of Connecticut, Storrs, CT 06269-2037

Employing the VOCG Technique to Measure Changes in Particle Hydrophilicity as a Function of Anion Surface Complexation

We examined the utility of an extended Derjaguin-Landau-Verwey-Overbeek (DLVO_EX) model for predicting changes in colloidal ferric hydroxide hydrophilicity, as a function of phosphate adsorption. Increase in phosphate adsorption, below monolayer coverage, resulted in increased hydrophilicity as indicated by decreased mean ferric hydroxide aggregate diameter. Results of the DLVOEX model suggested that adsorption of phosphate resulted in an increase in the colloid’s Lewis Base component (g-) of surface tension. The increase in g- resulted in a concomitant increase (and change of sign) in the predicted free energy of interaction (DG1w1) between colloidal aggregates. The change in hydrophilicity suggested by DG1w1 is conceptually congruent with the Gibbs Isotherm Equation, which predicts a decrease in solid-liquid interfacial energy with increased adsorption. The increase in predicted aggregate hydrophilicity is also consistent with the observed decrease in aggregate size discussed above. Phosphate adsorption beyond monolayer coverage resulted in an increase in mean ferric hydroxide aggregate diameter suggesting a decrease in particle hydrophilicity. This decrease in ferric hydroxide hydrophilicity may be a result of increased ionic strength and/or phosphate bridging.

Dr. Mohamed M. Chehimi ; Institut de Topologie et de Dynamique des Systèmes (ITODYS)Université Paris 7 - Denis Diderot, associé au CNRS (UPRESA 7086)1 rue Guy de la Brosse, 75005 Paris (France)

Characterization of Acid-Base Properties of Polymer Surfaces by XPS

The background to the use of XPS to assess acid-base properties of polymer surfaces is reviewed. This method is based on :

(i) the extent of uptake of reference molecular probes by polymer surfaces
(ii) the determination of XPS chemical shifts undergone by these acidic and basic probes as a result of probe-polymer acid-base interactions.

These chemical shifts permit to evaluate heats of formation of molecule-polymer acid-base adducts, as well as physicochemical constants to rank acid-base properties of solids.

Some examples of the application of XPS to assess acid-base properties of untreated homopolymers and plasma-treated polypropylene surfaces (in relation with their adhesion aspects) will be given.

Wenji Victor Chang and Xuzhi Qin ;Department of Chemical Engineering, University of Southern California, Los Angeles, CA 90089-1211

Repulsive Acid-Base Interactions: Fantasy or Reality

About nine years ago, we proposed a phenomenological model for characterizing local thermodynamic interactions at interface. The model has been successfully applied by us and others to study wetting, adhesion, solubility and crazing in polymer-solvent systems. However, many researchers still have great reluctance to accept our model. Some even strongly object to our allowing repulsive acid-base interactions in our model. Therefore, in this paper we will first review the assumptions of the model and the need to use a single class of materials, n-alkanes, as the only pure dispersive materials. We will then show phenomenologically rigorously the existence of repulsive interactions based on both the wettability and inverse GC data. The key to address this issue is to determine the dispersive component of the model accurately. Four independent methods are used in our analysis. The dispersive component of a low surface energy material can be determined by the wettability of n-alkanes on the material. For high surface energy materials a modified two-liquid contact angle method could be used to determine it. A numeric nonlinear fitting program was developed to estimate the parameter based on wettability data. We can also apply the Lifshitz theory and use the spectroscopic data to estimate the dispersive component. All the results support the existence of the repulsive acid-base interactions as defined by our model.

Prof. Emil Chibowski ; Marie Curie-Sklodowska University, Lublin, Poland

Thin -Layer Wicking - A Method for the Acid-Base Free Energy Interaction Determination

Free energy components of a solid surface can be determined via measurements of contact angles of probe liquids on a flat surface of the solid. However, for many solids, which are interesting from both theoretical and practical viewpoints (e.g. soil, clays and pigments) the flat piece of the surface cannot be obtained. In such cases the thin-layer wicking method, although time consuming, seems to be at present, the only method which can be applied successfully. Themethod was originally proposed by van Oss et al., and then further developed by Chibowski et al. It is based on measurements of the time needed for a probe liquid to wick subsequent distances of a thin porous layer of the solid deposited usually on a glass slide or packed in a thin capillary. The relationship time-distance is described by Washburns equation, which more general form has been proposed.

x²=((Rt)/(2n))G

Where x is the penetrated distance in time t by the tested liquid, R is the effective radius of the interparticle capillaries, n is the liquid viscosity and g is the specific change in free energy accompanying the wicking process. Four cases have been distinguished in which G assumes different expressions. This i) for low energy liauid (n-alkane) wicking the thin layer layer which has been equilibrated with the liquid vapor (precontacted surface, duplex film present on the solid surface) G_p=g₁ the surface tension of the liquid. It allows determination of R in the equation. ii) The same liquid penetrates the thin layer, which surface is bare, then G_b=W_b-W, is the difference between the work of adhesion and the work of cohesion of the liquid. iii) High energy liwiud (formamide, water) penetrates into precontacted with the liquid vapor the thin layer with the free energy change G, iv) The same liquid penetrates the bare surface with G_b. It has been shown that for such liquids:

G_b-G_p=W_s=W_s-W_c=2(g_s^LWg_l^LW)^1/2+2(g_s^-g_l⁺)^1/2+2(g_s⁺g_l^-)^1/2-2g_l

thus having done at least six wicking experiments for three probe liquids (for example, n-nonane, water and formamide, for which their surface tension components are known) it is possible to solve simultaneously three equations with three unknowns and calculate the urface free energy components, apolar Lifshitz-van der Waals, g_s^LW, and polar acid-base parameters, electron donor, g_s^- and electron acceptor, g_s⁺

Prof. Emil Chibowski and Lucyna Holysz; Marie Curie-Sklodowska University, Lublin, Poland

Effect of PMMA Precoverage on Acid-Base Interactions of Carbonate Rock

Carbonate rocks can be used as materials for elevation make-up of buildings. But, because they are porous and hydrophylic, first hydrophobization of the surface, including pores, has to be done. Among others, treatment with PMMA solution may lead to the needed effect. To evaluate hydrophobicity the rate of water migration into a porous layer layer can be measured. However, determination of the surface free energy components, apolar Lifshitz-van der Waals g_s^LW, and polar acid-base; electron donor,g_s^-, and electron acceptor, g_s⁺, (van Oss et al approach) allows quantitative evaluation of water adhesion to the surface. Such experiments were conducted for the carbonate rock tested. Similar experiments were also carried out with silica (Merk,for thin layer chromatography) to compare results for better defined systems. It was found that PMMA practically did not affect the g_s^LW component which was close for both materials as well as PMMA, 38-45 mJ/m². However, the g_s^- component was reduced practically to the value of PMMA (13.7 mJ/m²) for the rock (57 mJ/m², bare surface) and for silica (53 mJ/m², bare surface) it was reduced to 38.6 mJ/m². On the other hand, the g_s⁺ component for these two solids is a bit increased after PMMA treatment, but still it is low, 3.7 and 1.64 mJ/m² for silica and rock, respectively, while that for PMMA is 0.02mJ/m². For silica these changes in the components were obtained for only ca 3% of the surface coverage with PMMA. The carbonate rock was also pretreated with anionic (sodium dodecylsulfate) and cationic (cethyltrimethylammonium bromide) surfactant solutions, but hydrophobization was less than with PMMA. It can be concluded that PMMA may be applied as an efficient hydrophobizing agent for the carbonate rock surface.

Dr. Cristian Contescu and James A. Schwarz; Dept. of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY

Wet and Dry Chemistry of Acid-Base Interactions on Surfaces of Inorganic Solids

We review and systematize the main elements of acid-base chemistry on surfaces of inorganc solids, both in wet and (semi) dry state. We also introduce a method for characterization of acid-base properties (the pK spectrum) of solid imorganic solids in terms of their Proton Affinity Distribution (PAD) function. The experimental procedure, based on potentiometric titration data, and sample-specific restraints are exemplefied for various inorganic materials for which the PAD of their solid/aqueous solution interface are shown. The quantitative information obtained therefrom on the number and relative acid strength of surface hydroxyls is compared with quantitative DRIFT spectra of structurally non-equivalent surface hydroxyls of the same materials in their semi-dry state.

For inorganic solids the relationship between acid-base properties of wet and (semi) dry surfaces is determined by the common structure of the outmost surface layers. In particular, for oxides, it is assumed that at both interfaces, the proton-binding/proton-donor sites are surface hydroxyl groups which terminate the oxide layer and bind the adjacent water molecules from either a liquid or (semi-dry) gaseous environment,

Most of the examples selected refer to application of the PAD method in characterization of inorganic oxides. However, the field of potential applications of this technique in materials science is much broader: from heterogeneous catalysis, microporous adsorbents and activated carbin to fillers and binders for fibers, polymers, and composite materials.

Dr. Claudio Della Volpe and S. Siboni; Dept. of Materials Engineering, Univ. of Trento, ITALY

Acid-base solid surface freeenergies and definition of scales in Good-van Oss-Chaudury theory

A new approach to Good-van Oss-Chaudury (GvOC)theory for the calculation of acid-base components of solid surface freeenergies is proposed, similar to that introduced for other scales ofacid-base strength (Drago, Abrahams). GvOC equations are treated as an overdetermined set to be solved by somebest-fit algorithm, without any assumption about the chemical nature ofsome particular liquids or solids.

The introduction of an appropriate matrix formalism reveals the invarianceof GvOC equations through a three-parameter group of lineartransformations, for which an explicit characterization can be given. Astraightforward implication is non-uniqueness of best-fit solutions, whenever defined. Indeterminacy due tothe invariance group provides the key to define scales, by suitablyassigning acid-base components of some reference liquids. More specifically, particular scales can be introduced suitable to a direct comparison with other acid-base scales. Algorithms for the computation of best fit-solutions are developed, along with areasonable mathematical procedure for the scale definition.

Some of the difficulties commonly encountered in theapplication of vOGT are also discussed (the low values of acid componentsalso measured on acid surfaces, for instance) and it is proved as they canbe overcomeby including a larger number of prevalently acid liquids in the solvent setemployed and by appropriately fixing the acid-base components of thereference solvents. By the light of the latter considerations, someother literature treatments (Lee, Li and Chang, etc.) are analysed, byshowing that they fit in the general scheme proposed and that they reduce to the choice of a particular scale of acid-base strength. Because of the non-linear character of the GvOCequation set, the choice of the acid-base components for reference liquids appears particularly critical; taking water as a reference substance, aspecific set of the above parametersis proposed which allows one to calculate chemically reasonable acid-basecomponents for common liquids and polymers. Some new data collected with the aim to develop a wider and proper set ofadvancing and receding contact angles of test liquids on polymers arecommented.

Dr. H. T. Deo ; University Dept. of Chemical Technology (UDTC)Matunga, Bombay, INDIA

Superabsorbent Polymers as Environment Friendly Coating Materials on Fibers, Papers, Films and Membranes and their Acid-Base Properties

Dr. Frank Etzler , John Simmons, Veena Thomas and Nadia Ladyzhynsky; Boehringer-Ingelheim Pharmaceuticals, Inc. Ridgefield, CT

Assessment of Acid-Base Contributions to Adhesion: A Comparison of Techniques

Dr. Douglas J. Gardner , Qiang Shi, and William Tze; University of Maine Advanced Engineered Wood Composites Center And Department of Forest Management Orono, Maine

Comparison of Acid-Base Characterization Techniques on Lignocellulosic Surfaces. Which one is right?

Lignocellulosic surfaces including wood and fiber products such as paper are adhesive bonded, coated ,or printed upon in a majority of applications, and therefore, understanding their surface properties is important. Contact angle analysis (CAA) and inverse gas chromatography (IGC) are commonly used techniques to provide data for determining the acid-base character of lignocellulosic surfaces. In this presentation, we will discuss the results of research determining acid-base character on lignocellulosic surfaces using the van Oss and Chang methods for CAA data, and the Papirer and Schultz methods for IGC data. For CAA data, both the van Oss and Chang methods inidicate that lignocellulosic surfaces are amphoteric. However, the results for the van Oss method indicates a more basic surface while the Chang method indicates a more acidic surface. For the IGC data, both the Papirer and Shultz methods indicate an amphoteric, but slightly more acidic surface for cellulose fibers and extracted wood surfaces. Wood extractives made acid-base determination problematic for IGC data on unextracted wood surfaces.

Dr. Robert J. Good; Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, NY 14260

Contact Angles: from Macroscopic Observations to Microscopic Interpretations

there are important arifacts that interfere with the use of contact angles in fundamental research. On real rough surfaces, roughness and heterogeneity, which are almost always present, lead to hysterisis. Astrong recommendation is made, to measure both advancing (theta)_a and retreating (theta)_r contact angles. If this is not done, an experimenter is very likely to report (for example) artifacts caused by roughness as being evidence for changes in surface energy. It will be shown that, in some cases, it is possible to use the advancing contact angle in a qunatitative measure of roughness. If roughness (as measured by the Wenzel ratio R)is less than about 1.05, it may be possible to use (theta)_a data directly in determining energy parameters (gamma)^LW, (gamma)⁺ and (gamma)^-. Retreating angles on a smooth surface correspond to the presence of a fraction of the surface with a high surface energy.

Other problems in interpreting contact angles in terms of microscopic energy parameters will be discussed. Can useful results be obtained when the liquid is appreciably soluble (e.g. swelling) in the solid? Answer: sometimes. Can a single parameter such as (gamma)^p, be used to interoret contact angles in terms of the polarity of a solid? Answer: No; hydrophilic vs. hydrophobic behavior requires at least 3 surface parameters to describe it.

Dr. Keiko Gotoh¹ , Junko Tao¹ and, Mieko Tagawa²;1. Kyoto University of Education 1 Fukakusa-Fujinomori-cho Fushimi-ku Kyoto 612-0863 JAPAN; 2. Nara Woments University, Kita-Uoya-Higashi-machi, Nara 630-8506, Japan

Adhesive Interaction in Water/ n-Alcohol Mixtures Between Silanized Silica and Polymer Particles

The deposition of polymer particles onto silanized silica was investigated in water/n-alcohol mixtures and was discussed on the basis of the adhesive interaction. Polymer particles used were spherical polyethylene and nylon having mean diameters of 4 and 5 microns respectively. Silica plates were modified with gamma-aminopropyltirethoxysilane, methyltriethoxysilane ane perfluoroethyltrimethoxysilane. Doubly distilled water mixed with methanol,ethanol,propanol or butanol was used as a medium. Polymer particles were dispersed in water/alcohol mixtures and the silica plate was perpindicularly immersed in the dispersion. After immersion of 60 minutes, the number of particles adhering to the plate was counted using a microscope. The number of particles adhered (n_a) increased by the silanization of silica and decreased with increasing volume ratio of alcohol. The Lifshitz-van der Waals and acid-base components of the surface free energies of the particles, substrates and liquids were determined by contact angle measurements. The acid-base components reduced by the silanization of silica and by the addition of ethanor to water. The absolute values of the electrokinetic potential of the particles and substrates decreased with increasing volume ratio of ethanol. The particle adhesion was discussed in terms of the Lifshitz-van der Waals, electrostatic and acid-base interactions. The particle adhesion in the present systems was found to be dominated by acid-base interactions.

Dr. V.M.Gun'ko;Institute of Surface Chemistry, 31 Prospect Nauki, Kiev, 252022, Ukraine

Theoretical Simulations of Interaction between Silanes and Oxide Surfaces and Characterization of Modified Surfaces of Oxide Fillers

Interaction of 3-aminopropyltriethoxysilane, hexamethyldisilazane, methacryloyloxymethylenemethyl diethoxysilane, methacryloyloxypropyltrimethoxysilane and other compounds with silica, alumina/silica and titania/silica was modeled in a cluster approach using ab initio and semiempirical quantum chemical methods. Dynamics of reactions between adsorbed compounds and surface groups was investigated by the dynamic reaction coordinate method, also, dynamic simulations on the base of potentials obtained by semiempirical methods wereutilized. The mechanisms of reactions

SiOH + ROSiR3' ---> SiOSi + ROH
and
SiOH + HN(SiR3)2 ---> SiOSiR3 + H2NSiR3

and influence of different factors on them were studied; the corresponding activation energies were estimated by ab initio and semiempirical methods. The mechanism of hydrolysis of SiOCR, SiNR, SiO(R)M, MOSiR, M-Hal (M = Al and Ti, Hal = F, Cl, Br, and I) and other bonds was investigated at a various amount of water molecules participating in the reaction. Solvation effects on the reactions and the properties of modified oxides were studied using AM1-SM1. Stability of MOSiR bonds was analyzed for M = Si, Al, and Ti and different R involving N and O atoms modeling thermal decomposition, electron, excitation, interaction with water and oxygen (molecules and radicals).

Dr. V.M.GUN'KO, V.I.ZARKO, E.F.VORONIN, AND E.M.PAKHLOV; Institute of Surface Chemistry, 31 Prospect Nauki, 252022 Kiev, Ukraine

SURFACE MODIFICATION OF DISPERSE SILICA, TITANIA AND MIXED OXIDES BY ORGANIC AND ORGANOSILICON COMPOUNDS FOR IMPROVEMENT OF THEIR PROPERTIES AS FILLERS

Fumed silica, different titanias (rutile, anatase, rutile covered by alumina and silica) and titania/silicas modified by di(ethylene glycol), glycerol, butanol, methanol, methacryloyloxymethylenemethyl diethoxysilane (MADES), 3-aminopropyltriethoxysilane (APTES), hexamethyldisilazane, 3- methacryloyloxypropyl trimethoxysilane, mixture of hexamethyltricyclosiloxane, octamethyltetracyclosiloxane, penta- and hexacyclosiloxanes were studied in comparison with unmodified oxides by IR and photon correlation spectroscopies, electrophysical, electrophoretic and quantum chemical methods and tested as additives to polymers for improvement of the properties of the coatings. The addition of small amount (0.5 - 2 wt %) of modified titania, silica, and TiO2/SiO2 allows to improve the adhesion properties, light and corrosion resistance of the filled polymer coatings. For example, titania/silica modified by APTES and MADES or silica modified by butanol or di(ethylene glycol) with certain content of different functional groups at the surfaces is an effective filler for improvement of the protection properties of the filled coatings. Changes in acid-base properties of mixed oxide surfaces can be governed using a mixture of different modifiers possessing the corresponding active groups, which can substitute or block the surface active sites.

Dr. S. Helt , Y. Baziard, V. Nassiet, and J. A. Petit; Ecole Nationale d'ingenieurs de Tarbes, FRANCE

Characterization of Acid-Base Properties of SiC Surfaces Using Huttingers method. Application to the Adhesive Bonding of a Sintered Silicon Carbide

Sintered Silicon Carbide (SSiC) substrates were subjected to two Surface Treatments (ST) with the view to improve their adhesion properties. The first was a chemical treatment with the etching solution of Murakami (K₃FeCN₆+NaOH) and the second was a thermal treatment in air atmosphere. Both treatments modified the physical chemistry of SSiC surfaces,as was observed using several techniques of characterization (SEM, AFM, profilometry, XPS and wettability (Fowkes procedure)). Thus, after both ST, the total work of adhesion W_SL (W_SL = W_SL^D + _SL^AB) increased sharply because of the W_SL^AB term of Acid-Base interactions. Moreover, the titration of SSiC surfaces, with the method of Huttinger, showed Bronsted Acid-Base interactions below a critical value of the carbon contaminant concentration, the SSiC surface being oxidized and then coated by a silica layer (thermal treatment) or not oxidized (chemical treatment). This critical value corresponded also to the transition between adhesive-substrate interfacial ruptures (low W_SL^AB) and cohesive ruptures in the adhesive (high W_SL^AB), for adhesively-bonded SSiC/structural epoxy assemblies.

Dr. Yash Kamath;C. J. Dansizer, and K. R. Ramaprasad TRI/Princeton, Princeton, NJ

Acid-Base Interactions in Finishing of Textile Fibers

Acid-base components of the surface energies of textile fibers such as nylon, polyethylene terephthalate (PET) and poly(propylene) (PP) have been evaluated. Surface energies were obtained by determining the wetting forces of fibers in three different liquids, using the Wilhelmy method. It was necessary to make certain assumptions to obtain meaningful information from these data. The validity of these assumptions will be discussed.

We have also made an attempt to determine the acid-base components of the surface energies of polymeric liquids used as lubricants in spin finishes. These are random copolymers of ethylene oxide and propylene oxide (EO/PO). These liquids are basic in nature because of the ether group in their structure.

Efficient spreading on yarns is an important requirement for spin finishes. We have studied this behavior by observing the spreading of a liquid drop on parallel filaments. The method can be made quantitative by measuring the length of the liquid thread formed between the filaments. The effect of the acid-base component of solid-liquid adhesion on the spreading rate has been evaluated. The spreading of aqueous liquids seems to be dominated by the acid-base interaction of water with the fiber surface.

We have also tried to study the molecular dynamics during the drawing of nylon fibers. The acid-base interaction between fluorescent dyes and the fiber surface seems to highlight these changes and can be observed by the emission of fluorescence.

Dr.D. Y. Kwok,and A. W. Neumann ; Department of Mechanical and Industrial Engineering, University of Toronto, 5 King's College Road, Toronto, Ontario, Canada M5S 3G8

Contact angles: Measurement and Interpretation

Low-rate dynamic contact angles of a large number of liquids were measured on various surfaces by Axusymmetric Drop Shape Analysis - Profile (ADSA-P). It is found that not all experimental contact angles can be used for the calculations of surface energetics: For instance, in some cases, slip and stick contact angle behavior occurs where the contact angle fluctuates by as much as 35 degrees. Thus, circumspection is necessary in the decision whether or not the experimental contactangles can be used to interpret surface energies in conjunction with Young's equation and whether the solid-liquid surface systems violate the basic assumptions made in all contact angle measurements, the liquid-vapor surface tension times cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. g^lvcos(theta) depends only on g^lv, for a given solid surface (or solid surface tension). Changing the solid surface (and hence g^sv) shifts the curves in a very regular manner. Thus, g^lvcos(theta) depends only on g^lv and g^sv. Intermolecular forces forces do not have an additional and independent effect on the contact angles. Because of Young's equation, the solid-liquid surface tension g^sl can be expressed as a function of only g^lv and g^sv. Thus acid-base surface tension component approaches do not describe these experimental contact angle patterns.

Dr. Ritalba Lamendola, M. Creatore, E. Matarrese and R. d'Agostino; Department of Chemistry, University of Bari, Via Orabona, 4 70126, Bari, Italy

NH₃, O₂ and He Plasma Treatments of PET and Kapton for Improving Al Adhesion

Polyethelene terethalate and Kapton have been plasma processed in order to increase sputtered aluminum adhesion. The effect of different plasma feedings, i.e. NH₃, O₂ and He, has been evaluated for both chemical and morphological modification of the polymers surface, and then, for the adhesion at polymer-metal interface. In the case of ammonia treatments, the measured peel strength is very well correlated to the acid-basic properties of the plasma processed polymer surface. The titration of basic functionalities on these surfaces has been achieved by exposing polymers to CHCl₃ vapour and then detecting chlorine content by X-ray Photoelectron Spectroscopy analysis. The effect of plasma processing on the polymer's morphology has been investigated by means of Atomic Force Microscopy. The competition between the chemical and the morphological modifications of the polymer surface, induced by the different plasma feeds, in effecting adhesion has been studied.

Dr. A. Leppe , J. Lara and Dr. Henry P. Schreiber; Dept. of Chemical Engineering, Ecole Polytechnique, Montreal, CANADA

Polymer Orientation at Solid Surfaces: Acid-Base Interaction Origins and Some Property Responses

The inverse gas chromatographic (IGC) route to the characterization of polymer and other solid surfaces is reviewed, and then applied to evaluate the acid-base interaction potentials of an ethylene-g-acrylic acid (EAA) copolymer and of two coated rutiles. IGC also has been used to evaluate the interaction of the polymer when adsorbed at varying mass concentrations on these pigments. When the net acidic copolymer adsorbs on a basic rutile, strong adhesion occurs, the chain orientation at the pigment and the air interface is similar, and an isotropic interphase is created. When the polymer is adsorbed on a rutile with acidic surface properties, however, the chain orientation at the pigment and air interfaces differs, resulting in an anisotropic interphase. Rheological data show that at stated mass concentration, the strongly adhering isotropic adsorbed layer is more compact than its anisotropic counterpart. Stress-strain data on EAA filled with the acidic and basic rutiles are at first indistinguishable, but on accelerated aging the system with isotropic interfaces far outperforms its counterpart, apparently because of retarded intrusion to the interface of water vapor. Implied is a general need to understand the impact of interfacial interactions on use-properties of polymer systems.

Dr. Thomas Lloyd; Lehigh University

Comparison of two procedures for assessing the acidity of polymer surfaces graded in acid content

This study was aimed primarily at testing surface acidity measuring procedures using contact angles. Plaques of polyethylene containing acrylic acid ranging from 0 to 20% were pressed and the three-liquid , contact angle approach used to grade the acidity and basicity. The data were analyzed according to two procedures : 1) van Oss, Good and Chaudhury(OGC) and 2) Chen and Chang (CC). In measuring acidity, the polar pair,water(pH6.3)-dimethylsulfoxide gave values of (V+)1/2^* (OGC) and y (CC) which tracked acrylic acid content better than the pair, water- ethylene glycol. Using water buffered at pH 13.0 was equal to pH 6.3. Since some of the (V+)1/2 values are negative, squaring to get V+ destroyed the linearity with % acrylic acid. In assessing basicity, the two polar pairs were essentially equal and gave good linearity of V- with acrylic acid (C=O) content. However, values of z (CC) at 15.5% and 20% failed to rise as expected. Presumably at these carboxyl concentrations hydrogen bonding between carboxyls is influencing the acid-base interactions with the probe liquids considerably.

* this acidic parameter (OGC) is "square root of gamma +, sub s (solid)"

Dr. A. Mavon ; Lab. de Pharmacocinétique Cutanée, Institut de Recherche Pierre Fabre, BP 74, 31322 Castanet Tolosan, France

Acid-Base Characteristics of Polymer Surfaces:Relevance to Bioadhesion

Skin surface wettability is an important factor of the skin protective function, e.g. ecosystem preservation, smoothness and resilency, barrier to chemicals but surprinsingly it had received little attention. Hence, this work aimed at calculating the acid-base characteristics of human skin, on the volar forearm, a sebum-poor area, and on the forehead, a sebum-rich area, and at assessing the influence of skin lipids, using advancing contact angle measurements of water, glycerol, formamide and diiodomethane. Contact angles on the forearm and the forehead of ten healthy volunteers were measured, using a surgical microscope fitted with a slanted mirror and equipped with a video camera linked to a computer. Both skin areas were examined before and after ether-treatment. According to the approach of van Oss et al. to solid-liquid interfacial interactions, the surface free energy components, i.e. Lifshitz-van der Waals, electron acceptor and electron donor components were calculated. From the contact angle measurements and calculations of surface free energy, the forearm skin surface was found to be an almost apolar surface and the forehead skin surface a monopolar basic surface (electron donor). Ether-treatment, turned both surfaces toward the apolar end, mostly by reducing the electron donor component. These results would suggest that skin surface lipids, mainly sebum, give the skin surface a basic character. The mechanism of the obvious wetting enhancing effect of sebum could be due to the free fatty acids, which are especially abundant in the sebum, or to the forehead skin surface hydration.

Dr. V. Médout-Marère, S. Partyka and J.M. Douillard; L. A. M. M. I., ESA 5072 du CNRS, Case 015, Université de Montpellier 2 Sciences et techniques du Languedoc, Place Eugène Bataillon, 34095 Montpellier Cédex 5. FRANCE

Adhesion Properties of High Energy Solids vs. Water or Organic Solvents studied by Immersion Calorimetry

Understanding of wettability is essential for efficient industrial processing. Wettability is generally characterized by a macroscopic parameter: the contact angle formed on a smooth homogeneous surface, at the triple line of contact. But in actual cases, solids are in the form of powders or of porous heterogeneous rocks. Therefore the contact angle experiment appears too simple in such cases. Furthermore the important parameters in the equations describing wettability are solid/fluid surface tensions and not pure solid surface characteristics.

Therefore we develop an analysis of wettability on the basis of adsorption thermodynamics(1). It is then possible, by using adsorption isotherm and/or microcalorimetric techniques to analyze the molecular interactions between divided solids and pure fluids. We have constructed a routine(2) to deduce solid surface energy (both enthalpic and free enthalpic) components, in the Van Oss framework(3), a model developed for the analysis of contact angle. This routine can be used with or without approximations(4). At the end of the routine the solid surface tension is obtained.

The solids studied(5) are some clays: kaolinites ; illites ; talc ; and chlorite, and some silicas. REFERENCES 1. Zoungrana T., Douillard J.M. and Partyka S., J. Therm. Anal., 41, 1287, (1994)
2. Douillard J. M., Zoungrana T. and Partyka S., J. Pet. Sci. and Eng., 14, 51, (1995)
3. Van Oss, C. J., Good, R. J. and Chaudhury, M. K., Langmuir, 4, 884, (1988)
4 Douillard J.M., J. Colloid Interface Sci., 188, 511 (1997).
5. Malandrini H., Clauss F., Partyka S. and Douillard J.M., J. Colloid Interface Sci. 194, 183, (1997)

Dr. Marco MORRA; Nobil Bio Ricerche Str. S. Rocco 32, 14018 Villafranca d’Asti ITALY

Acid-Base Characteristics of Polymer Surfaces:Relevance to Bioadhesion

Bioadhesive phenomena, i.e. adhesive interactions between biological species and surfaces of synthetic materials play an important role in many technological fields. Bacterial adhesion to polymer surfaces, for instance, is still the major barrier to the extended use of many medical devices. Most bioadhesive interactions occur in water, and the central role played by water molecules in bioadhesion as been highlighted in theories that relate interfacial energetics to adhesion in aqueous media. In this communication, cells and bacterial adhesion to synthetic materials in described in the perspective of the effect of the acid-base properties of polymer surfaces on interfacial water molecules. Because of its orientation-dependent properties, water at interfaces responds to electron donor-electron acceptor properties of interfacing solid phases by changing both its partial molar volume (density) and orientation. The free energy changes associated with this water-specific effects concur in the definition of the outcome of the bioadhesive interactions. The relationship between the electron donor-electron acceptor properties of polymers, interfacial water molecules and interfacial forces is discussed on the light of some recently developed approaches. Results of cells and bacterial adhesion to polymer surfaces and to surface-modified polymers are presented and discussed in terms of the relevant acid-base properties.

Dr. Anil N. Netravali and Q. Song; Fiber Science Program, Department of Textiles and Apparel, Cornell University, Ithaca, NY

Laser Surface Modification of UHSPE Fibers: Correlation Between Acid-Base Interactions and Adhesion to Epoxies

Fibers based on ultra-high-strength polyethylene (UHSPE) such as Spectra^® have excellent mechanical properties for use in advanced composites. They have the highest specific strength among all high strength fibers. However, their poor adhesion to any common matrix material limits their use. This paper will describe the use of a pulsed eximer laser to modify the surface chemistry to increase the interfacial shear strength (IFSS) of UHSPE fibers with epoxy resins. Spectra^® 1000 fibers were treated with pulsed UV eximer laser radiation in air and other environments including ammonia, argon and helium. The changes in surface energetics and wettability , topography were characterized using several analytical techniques. Interfacial shear strength with epoxy was measured using single fiber pullout tests. The results indicate that laser treatments in air and other non innert environments incorporate oxygen and other elements to increase the acid-base component of the fiber surface and make it polar. The laser treatment also is responsible for the surface ablation and making it rougher. These changes together increase the fiber/epoxy IFSS by about 250%. Laser treatments in the inert environments do not change the acid-base characteristics but do increase the surface roughness resulting in a 100% increase in the IFSS.

Dr. Claudine Noguera , A. Pojani, J. Goniakowski¹ and F. Finocchi; Orsay, FRANCE; 1. CRMC2, Campus de Luminy, 13288 MARSEILLE (FRANCE)

Electronic properties of non-stoichiometric and polar oxide surfaces

Various oxide surfaces, such as MgO(100), MgO(111), SrTiO₃(110) and SrTiO₃(111), ZnO(0001) which are used as substrates for the growth of copper-oxide superconducting films or other types of applications, have been investigated by a combination of ab initio DFT (density functional theory) and semi-empirical Hartree-Fock methods. We will focus on the characteristics of the electronic states when the surface is either oxygen deficient or polar. We will show how localised vacancy states progressively transform into extended "metallic" states on an oxygen deficient MgO(100) plane. By performing simulations of the (1x1) planar terminations as well as of nonstoichiometric reconstructed configurations, we will compare the driving force to reconstruction and/or facetting on MgO, SrTiO₃ and ZnO polar surfaces. Preliminary results for the interaction of these surfaces with metal deposits will be presented.

Jan Rayss ; Laboratory of Optical fibers Technology, Maria Curie-Sk³odowska University, 20-031 Lublin, Poland

ACID - BASE CHARACTERISTICS OF POLYMER PROTECTIVE COATINGS OF OPTICAL FIBERS

The optical fibers may be considered as a composite material composed of two parts, greatly different in their physicochemical properties: glass inner part and the outer one, prepared from UV-cured polymers. Thus, the mechanical strength of the optical fiber depends on the mechanical properties of its two parts but also on the adhesion of polymer protective coating to the glass surface. It was demonstrated [1] that high adhesion of polymer protective coating leads to an increase of the mechanical strength of optical fibers.

We have previously shown [2] that the "practical adhesion" [3] of the protective coatings is produced by the shrinkage of polymer which takes place during its UV-curing and by the dispersive and donor-acceptor interactions between the glass and polymer surfaces.

In this study the acid-base properties of UV-cured polymers (epoxy- and urethaneacrylates) commonly used as the protective coatings of optical fibers, were investigated. Two methods - calculations of dispersive and electron donor and electron acceptor components of gS using the contact angle values of the three liquids, and inverse gas chromatography were used in order to characterize the surface properties of these polymers. The results obtained show that the surfaces of both kinds of polymers are rather basic ones. Also, the influence of curing time on the polymer surface properties were investigated.

References

1. C.R. Kurkijan and D. Innis, Optical Engineering, 30, 681 (1991)
2. J. Rayss, A. Gorgol, W. Podkoœcielny, J. Widomski, M. Cho³yk, J. Adhesion Sci. Technol., 12, 293 (1998)
3. K.L. Mittal (Ed.), in: Adhesion Measurements of Films and Coatings, pp. 1-13, VSP, Utrecht, The Netherlands (1995)

Jeffrey S. Rowell , Frenze Denes and R.A. Young; University of Wisconsin-Madison

Acid/Base Interactions and Water: Influence of acid/base interactions on the wet strength of sisal/polypropylene composites.

Strong acid/base interactions are required for good wetting and chemical adhesion, but strong acid base interactions, in this case in the form of hydroxyl groups and other hydroxy functional groups, also can attract moisture. Moisture can do several things in a composites, but the two effects to be studied here are the effects of moisture on the interfacial strength of the fiber/matrix bond and the effect of fiber swelling on the fiber/matrix bond. To isolate these effects acetylation and different plasma treatments are to be applied to the fiber in such a way as to control the cell wall and surface chemistry. This will enable the effects of moisture on the interface (loss of adhesion? lubricant?) and cell wall (swelling stresses?) to be looked at independently yet within the same system. Acidic, basic and neutral surface plasma modifications will be applied to the fibers, on both acetylated fiber (where the plasma will remove all surface actetyl groups) and on non-treated fiber. In addition, plasma modification of very fine PP powder will be used so there will be all possibilities of acid/base interactions in the composites tested. To test, characterize and compound the system the following equipment will be used: Wilhelmy, ESCA, AFM (dry and wet cell), ATR, SEM, DMA, IGC, EMC, torque rheology, and tensile, flexual, and impact tests (dry and water soak). Correlation between tests within treatment groups will also be used to assess validity of methods to predict adhesive strength in wet or dry applications. Comparisons between treatments will give an idea of the relevant importance of interfacial moisture to cell wall moisture in lignocellulosic/thermoplastic composites.

Dr. Carel van Oss ; Deps. of Microbiology, Chemical Engineering and Geology, SUNY, Buffalo, NY

Acid-Base Interactions as the Driving Force for Both Hydrophobic Attraction and Hydrophilic Repulsion

Apolar molecules or particles (i), immersed in water (w) attract each other hydorphobically with a free energy G_iwi^hydrophobic = -102 mJ/m², at 20 deg. C. In contrast with this strong hydrophobic energy of attraction, the Lifshitz - van der Waals (LW) attraction between such molecules or particles, immersed in water amounts to G_iwi^LW - values between -0.5 and -0.7 mJ/m². Under the same conditions the electrostatic (EL) repulsive energy, G_iwi^EL, tends to have a positive (repulsive) value in the same range as the negative value of G_iwi^LW for most biological (macro)molecules, particles or cells. The hydrophobic attraction is driven by the Lewis acid-base (AB) free energy of cohesion between the water molecules of the liquid medium ^1,2;

G_iwi^hydrophobic=G_ww^AB (1)

Equation 1 also signifies that the hydrophobic effect, which is caused by the AB free energy of cohesion between the surrounding water molecules, and which, therefore is always present in liquid water, plays an inexorable role in all interactions taking place in aqueous media. A net hydrophilic repulsion between particles or molecules immersed in water must therefore comprise a gross repulsive free energy (i.e.,G_iwi^hydrophilic >> 0)which significantly exceeds the value of G_ww^AB = -102 mJ/m² (at 20 deg. C). In practice very hydrophilic organic as well as inorganic material (i) tend to be monopolar electron donors with a Lewis basic surface tension paramater g_i^- > 28.5 mJ/m², so that³:

G_iwi^hydrophilic = 4 (g_i^--g_i⁺)^1/2

[where g_i⁺ is the Lewis acid (electron accepting) surface tension parameter of water, equal to 25.5 mJ/m² at 20 deg. C], in order to overcome not only the omnipresent underlying hydrophobic attraction, but also to compensate for the small but non-negligible Lifshitz-van der Waals attraction G_iwi^LW.

The criterium for hydrophobicity is G_iwi^interfacial < 0, and for hydrophilicity:G_iwi^interfacial>0⁴, where:

G_iwi^interfacial = G_iwi^LW + G_iwi^AB (3)

Hydrophobic particles, for which G_iwi^interfacial < 0, clump together when immersed in water and hydrophobic molecules are insoluble in water⁵. Hydrophilic particles, for which G_iwi^interfacial >0, form stable suspensions in water (without requiring an electrostatic repulsion) and hydrophilic molecules and macromolecules are soluble in water.

1. C. J. van OSS, "Hydrophobicity of Biosurfaces - Origin, quantitative determination and Interaction Energies", Colloid Surf. B 5:91-110 (1995).
2. C. J. van Oss. "Hydrophobicity and Hydrophilicity of Biosurfaces", Opin. Colloid Interface Sci. 2:503-512 (1997).
3. C. J. van Oss, "Interfacial Forces in Aqueous Media", Marcel Dekker, New York, 1994, p. 44.
4. C. J. van Oss and R. F. Giese, "The Hydrophilicity and Hydrophobicity of Clay Minerals", Clay Clay Miner. 43:474-477 (1995).
5. C. J. van Oss and R. J. Good, "Hydrogen Bonding, Interfacial Tension and the Aqueous Solubility of Organic Compounds", J. Dispersion Scie. Tech. 17:443-449 (1996).

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   Dr. Adam M. Voelkel , POLAND

   Acid-Base Properties of Modified Fillers, Adsorbents and Polymers by means of Inverse Gas Chromatography

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