C. J. van Oss

The role of van der Waals forces and hydrogen bonds in “hydrophobic interactions” between biopolymers and low energy surfaces

Abstract The thermodynamic nature of interfaces and of adhesion is reexamined in the light of the Lifshitz theory of the forces acting across condensed phases. A new term is proposed, γLW, which consists of the sum of the terms heretofore ascribed to London, Debye, and Keesom forces, LW referring to Lifshitz-van der Waals. This term and a second term γSR account for the entirety of two-phase interactions in nonionic systems; SR refers to short range forces. This new analysis of forces is of value in explaining some important biological and other phenomena. The rather strong attachment of hydrophilic proteins, e.g., human serum albumin (HSA) and human immunoglobulin G (IgG), to low energy surfaces, e.g., polytetrafluoroethylene (PTFE) and polystyrene (PST), while immersed in H2O, cannot be ascribed solely to Lifshitz-van der Waals forces (LW). For instance, it can be shown that the LW interaction would give rise to a repulsion between HSA and PTFE. The short range (SR) interactions, e.g., between H2O and HSA, are due to H-bonds, which cannot directly account for interactions with PTFE. However, the combined SR interfacial tensions between the H-bonding liquid, the biopolymer, and the low energy surface still result in a strong attraction between PTFE and HSA, immersed in H2O. This is analogous to the behavior of a liquid-air interface (where the fact that the direct interaction between a given solute and air is zero does not preclude the solute from being preferentially attracted to the interface). This SR attraction (minus the LW repulsion) between HSA and PTFE, in H2O, is of the same order of magnitude as the adsorption energy derived from the Langmuir isotherm obtained for this system. Analogous results are found with IgG and PTFE, and also with HSA and IgG, with PST. Desorption patterns (obtained by changing the γLW and γSR of the liquid medium) allow an insight into the degree of local dehydration (or “denaturation”) of adsorbed proteins under various conditions. It is suggested that the term interfacial forces more aptly describes the underlying mechanism than “hydrophobic interactions.”

Acid—base interfacial interactions in aqueous media

Abstract The role of electron-acceptor—electron-donor, i.e. Lewis acid—base (AB) interfacial interactions in polar media, especially in aqueous media, is reviewed, together with the role of interfacial Lifshitz—van der Waals (LW) interactions, which, although often relatively weak, are always present. The methodology of measurement of AB (concomitantly with LW) surface tension components and parameters is discussed, together with the development of earlier AB concepts. Also discussed are the following: AB interactions as driving force for “hydrophobic” attractions, and for hydrophilic (“hydration force”) repulsions; use of AB free energies in energy balance studies of the stability (or flocculation) of particle suspensions; role of AB forces in the aqueous solubility of polymers, surfactants and other solutes; AB repulsions in aqueous polymer phase separation, microemulsion formation, advancing freezing front interactions, and coacervation. Finally, the role of AB free energies of interaction in aqueous adsorption and adhesion phenomena is treated, including applications to protein adsorption, various modes of liquid chromatography, cell adhesion and cell—cell interactions, and in specific interactions such as antigen—antibody, lectin—carbohydrate, enzyme—substrate and ligand—receptor combinations. The review ends with a summary of the salient and novel aspects of AB interfacial interactions.

Hydrophobicity of biosurfaces — Origin, quantitative determination and interaction energies

It is shown that the “hydrophobic” attraction energy between two apolar moieties (as well as between one polar and one apolar moiety) immersed in water is the sole consequence of the hydrogen-bonding energy of cohesion of the water molecules surrounding these moieties. It is also shown that “hydrophobic” surfaces do not repel, but on the contrary attract water. The theory is given of hydrophobic interactions at a macroscopic level, as well as various methods for their quantitative measurement. The properties of hydrophobic, partly hydrophobic and hydrophilic compounds and surfaces are described, including those of amino acids, proteins (incorporating protein solubility), proteins at the air-water interface, carbohydrates, phospholipids, phospholipid layers, and nucleic acids. Finally, some effects and applications of hydrophobic interactions are discussed, including protein adsorption, protein precipitation, cell adhesion, cell fusion, and liquid chromatography approaches such as reversed-phase and hydrophobic interaction chromatography. Finally, the influence of hydrophobic forces is treated in antigen-antibody and other ligand-receptor interactions.

Microbial adhesion to solvents: a novel method to determine the electron-donor/electron-acceptor or Lewis acid-base properties of microbial cells

Abstract Lewis acid-base, i.e. electron-donor/electron-acceptor, interactions are implicated in various interffacial phenomena such as phagocytosis, biofouling and microbial adhesion. Therefore, the determination of electron-donor/electron-acceptor properties of microbial cells can be of importance in many research areas. However, until now, there has only been one method to determine these properties which is based on contact angle measurements combined with the equations of van Oss. Consequently, this method requires specific and elaborate equipment. Thus to facilitate the characterization of microbial cell surfaces, we have developed a simple, rapid and quantitative technique, the MATS (microbial adhesion to solvents) method, which is based on the comparison between microbial cell affinity to a monopolar solvent and a polar solvent. The monopolar solvent can be acidic (electron acceptor) or basic (electron donor) but both solvents must have similar surface tension Lifshitz-van der Waals components. Using this new method we have shown that Streptococcus thermophilus B (STB) and Leuconostoc mesenteroides NCDO 523 (LM 523) display maximal affinity for an acidic solvent and a low affinity for basic solvents. There was not a great difference between microbial cell adherence to basic solvents and apolar solvents, except for STB suspended in a 0.1 mol l −1 potassium phosphate buffer. These results, which demonstrate that both bacteria are strong electron donors and very weak electron acceptors, are in accordance with the energetic characteristics derived from van Osss approach.

Estimation of the polar parameters of the surface tension of liquids by contact angle measurements on gels

Abstract In a previous paper it was shown that negative interfacial tensions between predominantly monopolar surfaces (i.e., surfaces with mainly H-acceptor properties) and polar liquids are real phenomena. Such negative interfacial tensions do however decay rapidly. For miscible liquids, the decay of the interface is, in general, so rapid that it practically excludes measurement of interfacial tension. However, if one liquid is present in the form of a gel, and if the other liquid is placed as a drop upon the gel, there is often enough time to measure contact angles. This may be done at various concentrations of the liquid encased in the gel, and an extrapolation made to zero concentration of the gelling agent. With this method we found the existence of negative interfacial tensions at liquid/liquid interfaces.

THE HYDROPHILICITY AND HYDROPHOBICITY OF CLAY MINERALS

AbstractThe terms “hydrophobic” and “hydrophilic” are typically used in a non-specific sense and, as such, they have a limited utility. Surface thermodynamic theory, as described here, allows a natural and potentially powerful definition of these terms. The boundary between hydrophobicity and hydrophilicity occurs when the difference between the apolar attraction and the polar repulsion between molecules or particles of material (1) immersed in water (w) is equal to the cohesive polar attraction between the water molecules. Under these conditions, the interfacial free energy of interaction between particles of 1, immersed in water (ignoring electrostatic interactions), ΔG1 w1IF exactly zero. When ΔG1 w1IF is positive, the interaction of the material with water dominates and the surface of the material is hydrophilic; when ΔG1 w1IF is negative, the polar cohesive attraction between the water molecules dominates and the material is hydrophobic. Thus, the sign of defines the nature of the surface and the magnitude of may be used as the natural quantitative measure of the surface hydrophobicity or hydrophilicity.

Determination of contact angles and pore sizes of porous media by column and thin layer wicking

A simple method is described for the determination of contact angles (0) on powdered materials such as clay particles. This is done by depositing the particles from a liquid suspension onto glass slides, by sedimentation, followed by drying. The dried thin-layer plates are then subjected to wicking in a number of liquids using the Washhurn equation to determine cos . However, one other unknown in the Washburn equation, i.e. the average interstitial pore radius R, must first be determined. This is done by wicking with low-energy spreading liquids, such as alkanes. It could be shown with spherical monosized polymer particles, as well as with clay particles, that spreading liquids pre-wet the surfaces of the particles over which they subsequently spread. Thus, it can be demonstrated that spreading coefficients, in the sense of Harkins, play no role in this type of spreading and cos 0 equals unity in the Washburn equation for all values of γ1, for all spreading liquids (L). Results were obtained by thin layer...

DLVO and non-DLVO interactions in hectorite

The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory sums the attractive van der Waals and repulsive electrostatic forces as a function of separation distance to predict the interaction between charged particles immersed in a liquid. In aqueous media, however, non-electrostatic polar (electron acceptor/electron donor or Lewis acid/base) forces between particles with high energy surfaces often are comparable to, or greater than, the components of DLVO theory. By means of contact angle measurements on smooth self-supporting clay films, the values of the polar surface forces (AB) and the van der Waals forces (LW) of hectorite were measured. Determinations of ζ were used to derive the electrostatic forces (EL). Calculations based on the values obtained for the EL, LW, and AB forces show that for smooth spheres with a radius of 1 µm in a ≥ 0.1 M NaCl solution a net attraction exists leading to flocculation. At NaCl concentrations of ≤ 0.01 M, a repulsion energy of about +500 to +1300 kT exists at separation distances ≤ 50 Å, preventing contact between particles, thus ensuring stability of the colloidal suspension. At these concentrations, theory predicts that small clay particles or edges of clay crystals having an effective radius of curvature ≤ 10 Å should be energetic enough to overcome the repulsion barrier which prevents flocculation. Experimentally, for NaCl solution concentrations of ≥ 0.1 M, suspensions of hectorite particles flocculated, whereas at concentrations of ≥ 0.01 M, the suspensions remained stable. These experimental results agree with the predictions made by summing all three forces, but contradict the calculations based on classical DLVO theory.

Adsorption of biosurfactant on solid surfaces and consequences regarding the bioadhesion of Listeria monocytogenes LO28

Aims: The influence of biosurfactant compounds produced by a strain of Pseudomonas fluorescens on the adhesion of Listeria monocytogenes LO28 to polytetrafluoroethylene (PTFE) and AISI 304 stainless steel surfaces was investigated.

Hydrophobic, hydrophilic and other interactions in epitope-paratope binding.

Macroscopic, non-covalent, aspecific interactions between hydrophilic biopolymers, particles and cells in aqueous media tend to be repulsive; they are caused by Lifshitz-van der Waals (LW), Lewis acid-base (AB) and electrostatic (EL) forces. Microscopic scale specific interactions, e.g. between epitopes and paratopes, are also non-covalent and caused by attractive LW, AB and EL forces, which locally must be able to overcome the long- to medium-range macroscopic aspecific repulsive forces. Thus epitopes and paratopes need to be able to attract each other over a distance of at least 3 nm. The medium- and long-range specific attractive forces are mainly of hydrophobic (AB) and of EL origin; in aqueous media the medium- and long-range LW attractions are usually much weaker. It has been shown that hydrophobic (AB) interactions are as often enthalpic as entropic. Upon expulsion of interstitial water of hydration between epitope and paratope, a strong interfacial bond ultimately arises which is mainly caused by LW forces.