ABSTRACTS

The following is a list of the abstracts for papers which will be presented in THE THIRD INTERNATIONAL SYMPOSIUM ON ACID-BASE INTERACTIONS: RELEVANCE TO ADHESION. The listing is alphabetical by presenting author. This list is updated continually to add abstracts as they become available and to make appropriate corrections. This list may be conveniently searched by using the editor provided with most popular browsers (e.g. Microsoft Explorer, Netscape, ... etc.)

O. Erda; Department of Wood Science & Technology, faculty of Agriculture, Kyoto University, Kyoto, JAPAN

Sulfuric Acid-Catalyzed Liquefied Wood Wastes and Production of Based Resol-Type Adhesives from Them

It is well known that the majority of the raw materials used in the production of synthetic adhesives are petrochemicals or their derivatives. The synthetic adhesives are considered the most expensive components in the production of wood-based composite materials due to increased oil prices.

Recently, several attempts have intensively been made to prepare adhesives from a variety of lignocellulosic materials such as lignin, spent sulphite liquor, untreated wood, wood wastes chemically modifed by esterification, etherification etc., tannin, tree barks so as to find out a new alternative to phenol largely consumed in the production of phenol-formaldehyde resins. It is known the fact that many studies have been done on preparing lignin and tannin-based resol resin adhesives and their mechanisms. However, very restricted researchers have studied on making resol-type aqueous phenol resin adhesives from the whole untreated wood or the modified wood wastes.

In this study, wood (Betula maximowicziana Regel) wastes were liquefied with phenol in the presence of NaOH at an elevated temperature. The liquefied wood was resinified with formaldehyde and the resulting resol-type resin adhesives were applied to the production of plywood. The results indicated that resol-type resin adhesives prepared met the Japanese Industrial standard as far as dry shear adhesive strengths of plywoods were concerned. However, it could be produced boiling water-resistant adhesives prepared through the resinification of NaOH-catalyzed liquefied wood-phenol solution with formaldehyde at phenol:formaldehyde ratios of over 1:2.0 and by the addition of the appropriate crosslinking agent such as MDI.

1) ICSI-CNRS ,15, rue Starcky, BP 2488, 68057-Mulhouse (France)

2) IGC Lab Carreau Rodolphe, route de Guebwiller, 68840-Pulversheim (France)

3) Wacker Chemie GmbH, Werk Burghausen, D-84480 Burghausen (Germany)

Study of Acid-base Surface Properties of Fumed Silicas Samples With Their Specific Surface Area Using IGC Techniques.

The surface properties of fumed silica samples from Wacker Chemie society, having specific surface area starting from 45 m²/g to 380 m²/g were examined using IGC in infinite dilution conditions. By injection of polar probes having known acid-base parameters based on Gutmann's scale, one has access to an assessment of the acid-base properties of the silica samples. It was observed that they are dependent from the specific area of the studied samples, that says the acid character decreases when the specific surface area increases whereas the base one decreases correlatively. Taking into account that the specific area of a fumed silica is strongly dependent of the flame temperature, higher the temperature is, lower the specific area is, the number and the more or less randomly repartition of the silanol groups on the surface will vary. Moreover, fumed silica having specific surface areas higher 200 m2/g, exhibit rough surface at the molecular level. A simple model of the formation of the fumed primary particles in the flame, supported by some solid state NMR and IR spectroscopy measurements was proposed that permits to explain the observed evolution of the acid base properties with the specific surface area

1) Department of Physics, University of Jaén, 23071 Jaén, Spain

2) Department of Applied Physics, Faculty of Science, University of Granada,

18071 Granada, Spain

Determination of Total Surface Free Energy of a Solid From Contact Angle Hysteresis: a Comparison to The Components Approach

The problem of solid surface free energy determination is still open. The method proposed by van Oss et al. for determination of apolar Lifshitz-van der Waals and polar acid-base electron donor and electron acceptor components gives scattered values which depend on the kind of three probe liquids used for the determination. In this paper a new approach is offered. It is based on three measurable parameters; advancing and receding contact angles (hysteresis) of one liquid and its surface tension. The derived equation allows calculation of total surface free energy of the solid. However, no the free energy components can be determined by this method. The approach was tested on microscope glass slides and poly(methylmetha)crylate, PMMA, surfaces using six probe liquids. It was found that thus determined the surface free energy also depended on the liquid used. The surface free energy components of these solids calculated from van Oss et al´s method obviously also varied, depending which set of the contact angles was used for the calculations. Therefore, it is concluded that no specific surface free energy or its components can be determined for a solid with the help of other condensed phase (liquid) because an "induction effect" is present. The magnitude of this effect depends on the kind and strength of the interactions taking place (kind of the liquid used). However, it appeared that the average value of the total surface free energy (both for the glass and PMMA) from both methods is practically the same. It means that if sufficient number of the probe liquids is used, thus determined average value of the energy and its components give information about real surface and interfacial interactions.

Comparison of the Acid-base Components and Contact Angle Hysteresis Approaches to the Solid Surface Free Energy Determination

Total surface free energy g_s^tot for several solids (glass, PMMA, duraluminium, cadmium, and steel) was calculated from the energy components; apolar Lifshitz van der Waals g_s^LW , and acid-base electron donor g_s^- and electron acceptor g_s⁺. Using van Oss et al´s approach, the components were determined from advancing contact angles of the probe liquids: diidodomethane, water, formamide, ethylene glycol, 1-bromonaphthalene, dimethylsulfoxide, ethylene glycol, and glycerol. Simultaneously, the receding contact angles were also measured for the probe liquids and then applying the approach very recently proposed by Chibowski, total surface free energy for these solids was calculated from the contact angle hysteresis.

Although it could be expected that thus determined the total surface free energy for particular solids would depend on the kind of three probe liquids used (the component approach), as well as on the kind of the liquid (contact angle hysteresis approach), so surprisingly the average values of the energy from the two approaches agreed very well.These results point that both approaches can deliver useful information about free energy of a solid surface.

WITHDRAWN

Study of Acid-base Interactions of Glass Textile Materials by Inverse Gas Chromatography

In many previous studies, we used inverse gas chromatography(IGC) technique at infinite dilution to calculate the acidic and basic surface characteristics of some solid substrates like oxides : MgO, ZnO, SiO₂ and Al₂O3, four different carbon fibres and polymers as PMMA at various tacticities adsorbed on silica and on alumina, systems, that are known to interact strongly through acid-base interactions and ionic bonds. We determined the specific interactions between them and model organic molecules and showed the amphoteric feature of such solids.

In this paper, we used IGC technique to characterize three textile materials (in glass fibres), noted respectively A, B and C. We used the usual relation giving the specific enthalpy of adsorption (Hsp) of a polar molecule adsorbed on a solid:

(-Hsp) = (KA DN + KD AN)

Results obtained by this model showed that the regression coefficient r obtained of different fibres is very poor (r» 0.650).We proved that the previous relationship is not correct and we used a new relationship that we had proposed in other studies:

(-Hsp) = KA.DN + KD.AN - K. AN.DN

The parameter K reflects the amphoteric character of the solid. By using this new relationship and Mathematica program, we obtain the following results:

Table I : Acid-base constants of the various glass textile fibres by using our model

Values of K_A and KD obtained by this method allow to give the following conclusions :

• Material A has an acidic character and it is more acidic than the two other materials (that have been chemically treated). Acid constant of A is twice greater than its basic constant.

• Material B has an amphoteric character with a basic tendency. This comes from the methylamine silane chemical treatment of B that contains NH groups rich in basic character because of the presence of electron-pair on N.

• Fibre C is the more basic material relatively to the two others, with a basic constant three times more important than its acid constant. This is due to the treatment of C by 3-chloropropyl trimethyloxysilane which is an electron donor and then a base in Lewis terms.

1) Polymer Science & Engineering, Conte Polymer Center, University of Massachusetts, Amherst, MA 01003

2) Toyohashi University of Technology, Tempaku-cho, Toyohashi 441-8580, JAPAN

Studies on Surface Characteristics of Modified Silica Fillers and

Poly(ethyleneterephthalate) Fibers Evaluated by Means of Inverse Gas Chromatography

We analyzed the surface free energy of silica fillers modified by alcohols or silanes and poly(ethylene-terephthalate)(PET) fibers by means of inverse gas chromatography in order to investigate the relationship between their surface characteristics and the performance of composite formed from these materials. It was found that the dispersive component of the surface free energy for modified silica fillers was mostly lower than that for original silica filler. The polar component of the surface free energy for ethylene glycol modified silica filler became large, while that for n-butanol modified silica filler decreased remarkably. It was also found that original silica filler exhibited high acidity, while modified silica fillers exhibited low acidity. Although the surface free energy of PET fibers could not be determined quantitatively because of the surface change during their pretreatment, the polar component of the surface free energy seemed to decrease when pretreatment was made at a temperature higher than the glass-transition temperature of PET. It was indicated that the interaction between modified silica fillers and PET fibers correlated well with the basicity of the fillers, but not with their acidity.

Acid-Base Effects in Polymer Adhesion at Metal Surfaces

The surface isoelectric point for air-formed oxide films on various metals has been determined by measurement of contact angles at the hexadecane/aqueous solution interface as a function of pH of the aqueous phase. Application of Young's equation, the Gibbs equation, and surface equilibria conditions for hydroxylated oxide films leads to a mathematical expression which shows that the contact angle goes though a maximum at the isoelectric point of the oxide. Using this approach, surface isolectric points have been determined for the oxide films on aluminum, chromium, and tantalum. The acid-base properties of various polymers, including a commercially available pressure sensitive adhesive, were determined by measuring the contributions y_s⁺ and y_s^- to the solid surface free energy using the contact angle approach of van Oss and Good. Adhesion measurements for an acidic pressure sensitive adhesive show that the peel strength is greatest when the metal substrate has a surface oxide film of a basic character. The pull-off strength of poly(methyl methacrylate), a basic polymer, is greatest on the most acidic oxide surfaces.

Interfcial Bonding Heterogenity and Synergism in Polymer- Polymer Adhesion Strength

Polymer adhesion and its evaluation is very important from academic and industrial points of view. On one hand this phenomenon is mainly dependent on interfacial banding and its strength. 0n the other hand, the deformability of adherends is important as another adhesion controlling factor. Using a combination of acid-base interaction and coupling agents at the interface of a SBR-Polyurethane system, a synergistic adhesion promotion was observed, The surface of SBR was treated by acidified aqueous solution of sodium hypochlorite to establish polar groups which can interact with isocyanate groups of the polyurethane system. The appropriate coupling agent also is transferred to the SBR surface via aforementioned aqueous solution. The polar surface of the SBR apparently keeps the polar site of the coupling agent at the surface and pushes the non- polar site to the depth of the SBR bulk. A synergetic adhesion promotion was observed in the strength of the surface treated along with coupling agent SBR-Polyurethane system. This interesting finding is attributed to the tortuous path for the crack growth at the interface which was implemented by interfacial heterogeneity of bond strengths. For getting the highest synergism, a certain ratio of bond heterogeneity should be implemented which by itself depends on the deformability ratio of the adherends.

Sugimoto 3, Sumiyoshi-ku, Osaka 558-8585 JAPAN

Drop Size Dependence of Contact Angle Due to Acid-base Reactions and its Application to Measurement of Acid-base Interactions

The drop size dependence of contact angle of water caused by the acid-base interactions is studied theoretically. Suppose that a drop of aqueous solution comes into contact with a solid having ionizable functional groups on it. Then, according to the solution pH, functional groups on the solid surface will ionize via acid-base (proton transfer) reactions. At that time, hydrogen ions are released from the solid surface into the solution. For a small (e.g., a few micro-liter) drop of unbuffered solution, they cause pH variation because of the change in the hydrogen ion concentration. The magnitude of the pH variation depends on the drop size: The smaller the drop volume is, the larger the magnitude of the pH variation becomes. Consequently, small drops show contact angle different from that of large drops owing to the dependence of contact angleon solution pH. This is a drop size effect of contact angle resulting from acid-base reactions. In this paper, we estimate how large the magnitude of this drop size dependence of contact angle is. Moreover, its application to measurement of acid-base interactions is discussed.

Determination of Thermodynamic Parameters of Solids by Frontal Inverse Gas Chromatography

One of the most powerful methods for the determination of acid- base interactions is inverse gas chromatography (IGC). IGC involves the sorption of a known adsorptive onto an unknown adsorbent (solid sample). This approach inverts the conventional relationship between mobile and stationary phases found in analytical chromatography.

Thermodynamic studies are usually carried out at infinite dilution since these parameters reflect exclusively the interaction between the solid surface and the probe molecule under these conditions. Therefore pulse-IGC has been successfully applied to study various materials. The pulse technique implies the injection of a certain amount of probe molecule into a carrier gas stream and the resulting chromatogram shows a peak. The retention volume, which can be calculated from the peak maximum, is directly related to the interesting thermodynamic properties.

In the case of higher surface area materials, pulse-IGC is only applied at high temperatures since peaks become too small and broad at low temperatures. In this case it is difficult to distinguish the peak from the baseline or to recognize a peak maximum. This limitation restricts the applicability of IGC since many materials show a strong dependence of their properties on the temperature range.

Frontal-IGC is a continuous technique, which overcomes this problem. Using this approach, the retention volumes can be calculated from the turning point of the break-through curve. This method has been applied only at finite concentration in the past since it is difficult to maintain an accurate flow of small concentrations at infinite dilution.

Nowadays, however, with flow control technology the frontal method can be applied to the infinite dilution region for the determination of thermodynamic parameters at low temperatures.

New experimental results will be shown to illustrate the potential of frontal-IGC at infinite dilution for studying adsorption site energetics on different organic and inorganic materials at low temperatures.

M. Morra, C. Cassinelli; NobilBio, VillaFranca d'Asti

M. Anderle, G. Speranza, R. Canteri, C. Pederzolli, G. Gottardi, S. Janikowska; ITC, Trento

Recent Theoretical and Experimental Advancements in the Application of Good-van Oss Acid-base Theory to the Analysis of Polymer Surfaces.(I). General.

The acid-base theory as developed by Good, Chaudury and van Oss is a powerful instrument to analyze the free energy of polymer surfaces; however some problems can be encountered in its applications and some of the authors have shown as these problems can be theoretically fronted with an original approach which considers this theory as an example of the so-called LFER theories. From this point of view the definition of a well-defined scale of acid-basestrength and the use of a wide and well-equilibrated, opportune set ofsolvents is very important. In the present paper some recent results are presented which are based on the mathematical approach discussed by Della Volpe and Siboni in previous papers.

A wide set of polymer surfaces, whose purity has been controlled by high vacuum spectroscopies (ESCA and TOF-SIMS) have been analysed by a wide set of liquids, chosen on the base of the optimization of their condition numbers and the results have been analysed using an acid- base scale in which the acid-base ratio for the reference material was different from that proposed by the original theory, and based on recent theoretical results. The general problems of the preparation and of the analysis of the samples are also discussed.

M. Morra, C. Cassinelli; NobilBio, VillaFranca d'Asti

M. Anderle, G. Speranza, R. Canteri, C. Pederzolli, G. Gottardi, S. Janikowska

ITC, Trento

Recent Theoretical and Experimental Advancements in the Application of Good-van Oss Acid-base Theory to the Analysis of Polymer Surfaces.(Ii) Some Peculiar Cases.

Based on the general points of view presented in the first portion of this paper some examples of the results obtained with common polymers, are discussed; particularly, the case of PVC, which has been often considered as peculiar in the literature is fully analysed on the base of a new set of well prepared samples, whose composition was controlled by high vacuum spectroscopies, with "equilibrium" contact angles, obtained by an original experimental technique, in a well-balanced set of liquids, chosen on the base of their low condition number, are presented. The physico-chemical results obtained on these samples have been also compared with the adhesion properties of some bacterial cells, commonly found as infective agents in biomaterials surfaces to rationalize these results in the theoretical framework of acid-base theory. Prevalently acidic and prevalently basic inorganic materials are also compared with the organic polymers.

Contact Angle and Film Pressure : the Influence of Adsorption on Characterisation of Solid Surfaces

(Abstract not yet available)

Yuelin Wang, Wanhua Lee and Hechang Lee; GuangDong Baiyun Sealant R&D center, Guangzhou,China,510510

Acid-base Theory of Rtv-1 Silicone Sealants and Similar Model Compounds

RTV-1 silicone sealants are combination of base polymer, filler, crosslinker, catalyst, additive etc. components. Many of them have some extent of acidic or basic in nature. Based on acid-base characters of crosslinkers and fillers, similar RTV-1 model compounds are made and an acid-base theory was gave. It was found consistency in acidic or basic will benefit for RTV-1 compound, but inconsistency in acidic and basic will cause unstable and lead to gelation. This was summarized as ABN rule.

Acid-base theory and model compounds

Crosslinkers and fillers are classified by acidic (A) or basic(B) and neural (N)characters. Crosslinkers/ acid cure (A); hydroxylamine and amine cure (B); methanol or alcohol cure, acetone cure, amide cure and oxime cure (N) Fillers/ fumed silica and ground quartz (N); untreated calcium carbonate (B); and stearic acid treated calcium carbonate is regarded as (A) for Basic crosslinker and (B) for acidic crosslinker. Compatible model RTV-1 systems follow Crosslinker/filler parter match as [A,N],[B,(B or N)],[N, (A or B or N)]. We summarized this Acid-Base match as ABN rule for RTV-1 silicone.

When an Acid filler (stearic acid treated calcium carbonate) was added to a compatible B/B match crosslinker/filler compound, it was found a RTV-2 silicone was made. This is regarded as anti-ABN rule. And it could be another way for RTV-2 making.

Beyond the model, catalyst, additive etc. components also have acid or base tendency. we believe they fit for ABN rule too.

1) Department of Applied Physics, Faculty of Sciences. University of Granada, 18071 Granada. Spain.

2) Department of Physics, Faculty of Sciences, Abdelmalek Essaadi University, 95000 Tetouan, Morocco.

Modifications of the Hydrophilicity of Clay Minerals by Adsorption of Metals Ions and Humic Substances

The aim of this work is to investigate the effect of metals ions and humic acid adsorption on the energetic (surface free energy) properties of clay minerals. From these data, the Lifshitz-van der Waals (LW) and acid-base (AB) free energy of interaction between clay particles in water solution (DG₁₃₁) have been estimated. This allows to quantify the changes produced in the hydrophilic repulsion between the dispersed particles.

Two different clay minerals have been used: sodium montmorillonite (NaMt), obtained from natural bentonite, and a fluorinated sodium montmorillonite, from natural Rassoul. The surface free energy components(Lifshitz-van der Waals g^LW, electron-acceptor g⁺, and electron-donor g^-) of both clays were obtained from contact angle and thin-layer wicking measurements.The results indicate that NaMt particles posses a pronounced monopolar (g⁺ = 0; g^- >>0) and strong hydrophilic character, while Rassoul has a certain dipolar character (g⁺ and g^- ¹ 0). The adsorption of alkaline and alkaline-earth cations (up to 10^-1 M solutions) does not implies a significant modification of the surface free energy. However, the adsorption of heavy metals (Cd²⁺, Hg²⁺, Zn²⁺, Pb²⁺) significantly reduces the values of the electron-donor parameter and, as a consequence, the hydrophilic nature of the surface.

The adsorption of humic acid (HA) has been investigated in the range 50-300 mg/l, and in 10^-2 M NaCl solution (pH 3-9). To elucidate the preferential HA adsorption sites (faces or edges) on the laminar clay particles, the adsorption of HA was also studied on silica and alumina particles, because the chemical composition of the edges of clay particles is similar to that two metal oxides. SEM pictures showed an evident change in the texture and shape of clay particles by the action of the HA molecules. The effect of HA adsorption was: i) on clay, a significant reduction in the value of g^- at acid pH; ii) on silica, no significant modification is observed; iii) on alumina, g^- is significantly reduced whatever the pH of the solution. Thus, it is concluded that HA adsorption is preferentially produced on aluminol (Al-OH) sites of the edges of clay particles, mainly because of the electrostatic attraction between the negative HA molecules and the positive or neutral aluminol sites.

Acknowledgements: To projects MAT98-0940 (Ministerio Educación y Cultura, Spain) and INTAS99-0510.

Surface Energy Approach in Analyzing Resin Bleed Behavior

Resin bleed is a phenomenon that occurs when adhesive (or component thereof) migrates over the surface of the substrate beyond its intended boundary. A problem will occur if the material spreads over an area where wirebonding is to be performed. In this paper a relatively new surface energy approach (Three-Liquid Probe Method) was used to determine, through contact angle measurements, the mechanisms of resin bleed. In addition, a surface profilometer was used to measure the roughness of the epoxy (fiber filled) substrate, and the effect of roughness on resin bleed is also discussed. Preliminary results suggest that acid-base interactions play a significant role in resin bleed while no correlation was seen with roughness. The three-liquid probe method appears to hold promise for analysis of resin bleed behavior.

Drop Size Dependence of Contact Angle Due to Acid-base Reactions and its Application to Measurement of Acid-base Interactions

(Abstract not yet available)

One of the most powerful methods for the determination of acid- base interactions is inverse gas chromatography (IGC). IGC involves the sorption of a known adsorptive onto an unknown adsorbent (solid sample). This approach inverts the conventional relationship between mobile and stationary phases found in analytical chromatography.

Thermodynamic studies are usually carried out at infinite dilution since these parameters reflect exclusively the interaction between the solid surface and the probe molecule under these conditions. Therefore pulse-IGC has been successfully applied to study various materials. The pulse technique implies the injection of a certain amount of probe molecule into a carrier gas stream and the resulting chromatogram shows a peak. The retention volume, which can be calculated from the peak maximum, is directly related to the interesting thermodynamic properties.

In the case of higher surface area materials, pulse-IGC is only applied at high temperatures since peaks become too small and broad at low temperatures. In this case it is difficult to distinguish the peak from the baseline or to recognize a peak maximum. This limitation restricts the applicability of IGC since many materials show a strong dependence of their properties on the temperature range.

Frontal-IGC is a continuous technique, which overcomes this problem. Using this approach, the retention volumes can be calculated from the turning point of the break-through curve. This method has been applied only at finite concentration in the past since it is difficult to maintain an accurate flow of small concentrations at infinite dilution. Nowadays, however, with flow control technology the frontal method can be applied to the infinite dilution region for the determination of thermodynamic parameters at low temperatures. New experimental results will be shown to illustrate the potential of frontal-IGC at infinite dilution for studying adsorption site energetics on different organic and inorganic materials at low temperatures.

World of Gamo Equation: Contact Angle (theta), Changeable from 0 to 90 at Optimum Adhesion

Young equation of nearly 200 years ago is well known today for the fundamental theory of adhesion and the contact angle theta (to 0 deg.) is optimum wetting- and adhesion-condition. But contact angle 0 deg. to 90 deg. is optimum adhesion-condition in "World of Gamo Equation". Dupre-Gamo equation W_G= S_L(1+1/cos (theta)) results in infinity when contact angle theta to 90 deg. (W_G : work of adhesion, S_L: surface tension of liquid). Optimum adhesion-condition is at this case. The Dupre-Gamo equation was verified experimentally using specific work of adhesion WG/S_L and theoretical curve 1+1/cos(theta) Namely the experimental results on the specific work of adhesion were well coincided with the theoretical curve. And Gamo equation(the same as Young equation, but the cos(theta) to reciprocal) is obtained from construction by standard axis-setting at surface of liquid droplet on solid flat-surface(i.e. the axis with contact angle (theta) arbitrary axis), obtained also from calculus of surface free energy of the same droplet system provided that the droplet volume is changeable.

1) Department of Microbiology,

2) Department of Chemical Engineering and

3) Department of Geology,

State University of New York at Buffalo, Buffalo, NY 14214-300, USA

Role of Acid-base Interactions in the Kinetics and Energetics of Protein Adsorption onto Silica and Other Metal Oxide Surfaces

If one wishes to ascertain whether a hydrophilic protein (e.g., human serum albumin, HSA, at neutral pH) in aqueous solution, would adsorb onto hydrophilic glass or silica, by taking into account all macroscopic-scale interfacial interaction energies, one would arrive at the conclusion that the strong net hydrophilic (hydration pressure-type) repulsion between HSA and glass or silica, of Lewis acid-base (AB) origin would make such an adsorption rather unlikely. One would however be entirely wrong: HSA adsorbs quite readily onto glass or silica surfaces from aqueous solutions at neutral pH. This is because, in addition to the above-mentioned aspecific AB repulsion, there is a microscopic, partly electrostatic (EL), but for the major part AB attraction between a moiety on the HSA molecule and discrete cationic sites on the otherwise negatively charged metal oxide surfaces. There is thus an interplay between a strong macroscopic repulsion of HSA by the overall glass or silica surface and a more modest microscopic attraction of HSA by cationic sites on the glass or silica surfaces. In spite of the fact that the macroscopic repulsion is much stronger than the microscopic attraction, a significant degree of HSA adsorption occurs nonetheless, following rules that were already outlined in 1917, by M. von Smoluchowski.

A typical affinity binding constant of HSA onto silica is 2.8 x 108 L M^-1, corresponding to a free energy of binding of 23.5 kT. The measured kinetic rate constant of adsorption, k_a however, is only 5.5 x 104 L M^-1 sec-1, which turns out to be about a factor 4.9 x 10³ too low, compared to the microscopic k_a^mic rate constant value for the actual binding of just a the HSA

ligand to the specific cationic receptor site on the silica surface.

The specific to value can be obtained from the measured k_a value and the surface properties of the protein and the adsorbing substratum, using von Smoluchowski's approach and extended DLVO analysis. The results show that within a given type of system (e.g., HSA adsorbing onto various hycrophilic as well as hydrophobic metal oxide surfaces), the specific OR values vary remarkably little. The specific k_aff values and in particular the specific k_a^mic values, i. e., the dissociation kinetic rate constants on the other hand, vary widely. It can be concluded that whilst the specific adsorption rate constant, k_a^mic, is mainly just function of the diffusion coefficient of the protein that is being adsorbed, it is the specific desorption rate constant, k_a^mic, that is linked to the strength of the adsorption: The higher the adsorption energy, the slower the dissociation rate. In other words, for similar types of systems, the solute arrival rates are usually quite similar and uninteresting, but the departure rates vary widely: the stronger the bond, the slower the departure.

Acid-base and Lifshitz-van der Waals Interactions in the Hydration of Hydrophobic and Hydrophilic Surfaces

The forces involved in the hydration of hydrophobic (apolar) surfaces are only due to Lifshitz-van der Waals (LW) interactions which on a strongly apolar surface such as Teflon, represent an attractive energy of about 40 mJ/m2. This attractive energy is mainly enthalpic and to a lesser extent, entropic. There is no direct Lewis acid-base (AB) interaction between water molecules and completely apolar (hydrophobic) surfaces. However, the water molecules in the bulk liquid are attracted to one another via a strong energy of cohesion, which is for 70% AB and for 30% LW. The AB part of the energy of cohesion between water molecules is the sole driving force of the hydrophobic effect- A thin, flat, monomolecular layer of hydrogen-bonded water of hydration forms on apolar surfaces which, when the apolar surfaces are those of single apolar molecules that are present at a low concentration, can give rise to clathrate formation.

The forces involved in the hydration of strongly polar, hydrophilic surfaces are comprised partly of LW and partly of AB interactions. On very hydrophilic sur faces the attractive energy of hydration (which can be up to about 140 mJ/m²) is also mainly enthalpic but can be accompanied by a negative entropy of hydration.

The two different modes of hydration, on hydrophobic and on hydrophilic surfaces, each play a role in the action at a distance of hydrophobic attraction, as well as of hydrophilic repulsion, in aqueous media.

The influence of an increase in temperature (T) on the hydration of apolar (hydrophobic) surfaces is minor, as it is mainly due to the increase in T, in kT. However, an increase in T significantly decreases the degree of hydration of polar (hydrophilic) surfaces, as a consequence of an increase in the G_w⁺/G_w^- ratio with an increase in T, even though the total G_w^AB value increases very little with T. Given that most hydrophilic surfaces (s) have an elevated G_s^- parameter, an increase in G_w⁺ (with an increase in T) also causes an increase in hydrophilic repulsion, when T is increased, which explains why, e.g., dirty dishes or textiles are cleaned more effectively in hot than in cold water, even without soap.

The decrease in hydration of partly polar surfaces, which brings two hydrated surfaces closer together when T increases, also furnishes a simple explanation for the phenomenon of enthalpy-entropy compensation, without the need to evoke esoteric thermodynamic principles.

From the solubility of water in a number of apolar solvents it can be concluded that water, and in particular water of hydration on hydrophobic surfaces, forms clusters of five hydrogen-bonded water molecules each, at 20 C.

Applicability Limitations of Different Methods of Interfacial Tension Determination

There are five different approaches to determining interfacial tensions between two condensed-phase materials. For illustrative purposes one of these condensed-phase materials is here taken to be water (w), and the other a water-immiscible liquid, or a solid surface or a solute or particle (i), for which the value of Y_iw is sought. Y_iw consists of:

(1) Y_iw = Y_iw^LW + Y_iw^AB

where LW stands for Lifshitz-van der Waals and AB for Lewis acid-base interactions.

By definition:

(2) Y_iw = ½ G_iwi^IF

where G_iwi^IF is the free energy of interfacial interaction between two molecules, particles or surfaces, immersed in a liquid, which for our present purpose is water (w). As this free energy of interfacial interaction can be attractive (G_iwi^IF <0) as well as repulsive (G_iwi^IF >0 ), the value for Y_iw can be positive or negative, according to whether the interaction is attractive or repulsive (cf. eq. 2). For instance for water-soluble polymers, immersed in water,G_iwi^IF >0, so that then Y_iw < 0. It should be noted that Y_iw^LW is always positive, so that Y_iw can only be negative when Y_iw^AB is negative, see eq. 1.

The five different approaches for determining Y_iw are:1) Drop shape analysis

iw of a drop of i, in water, which is only applicable to completely apolar liquids, and partly polar, symmetrical molecules such as benzene. 2) Using the aqueous solubility of i (s_i, which is expressed in mol fractions), employing:

(3) -2 Y_iw . S_c = kT ln s_i

where S_c the contactable surface area between two molecules of i, immersed in water, k is Boltzmann's constant and T the absolute temperature in °K. This is applicable to polar liquids and other polar solutes, in solution. 3) Contact angle measurement with diiodomethane to determine Y_i^LW for use in Young's equation:

(4) Y_w cos theta = Y_i - Y_iw

applicable to apolar and monopolar materials, where Y_i = Y_i^LW . 4)Contact angle determinations with three or more different liquids, of which two liquids must be polar, using the Young-Dupre equation, including the LW and

surface tension components and parameters, in conjunction with the fullblown seven-term Y_iw equation, applicable to polar particles, surfaces and dried solutes. 5)Contact angle determinations as in 4), above, in conjunction with Young's equation (eq. 4), applicable to polar particles, solid surfaces, or

dried solutes.

Methods 4 and 5 are also both applicable to the polar moieties of amphi-

pathic solutes such as alcohols, surfactants, etc. However to obtain the

Y_iw values for the latter category, these have to be determined for the apolar and the polar moieties of these molecules separately, converted to kT units and then added together to arrive at the final Y_iw of the whole amphipathic molecule.

Y_iw is the most crucial ingredient in all surface thermodynamic approaches to quantitative determinations in adsorption, adhesion, stability, solubility, osmotic pressure and phase separation phenomena in aqueous media.

1) First Ten Angstroms, 465 Dinwiddie Street, Portsmouth, VA 23704

2) Singular Systems, 19451 Sierra Raton Road,Irvine, CA 92612-3824

3) Dept. of Materials Science and Engineering, The Pennsylvania State University, University Park PA

Young's Equation Revisited

This work uses a standard energy-minimization technique to derive drop shape (contact angle) of fluids on surfaces with varying surface energy. Results supplement the traditional vector diagram typically used to justify Young's equation. Our motivation was to develop computational methods for systems too complex to be solved analytically where explicit and implicit assumptions underlying Young's equation are either clearly not satisfied or of dubious applicability. Non-equilibrium systems of interest include those exhibiting contact angle hysteresis, especially due to surface rugosity, or those in which there is significant probe-fluid Absorption into, or extensive spreading onto, the solid surface where shape/definition of the "three phase line" loses specificity. Numerical results verify the Young equation for the case of a spherical cap for which an analytic solution is also available through tractable, but surprisingly tedious algebra. Graphical representation of general results confirm that the utility of the Young equation is highly restricted to a narrow range of possible wetting behaviors.