It can be shown that poly-(ethylene oxide) in its monomeric form, at molecular weights greater than about 1,000, cannot be soluble in water. Nonetheless, in actual practice poly-(ethylene oxide) is widely used for its hydrophilicity and for its unlimited aqueous solubility. The explanation for this apparent contradiction lies in the fact that poly-(ethylene oxide) molecules form a nonionic surfactant of a novel category, with its hydrophilic and hydrophobic chains attached to each other over their entire length so that its hydrophilic ethylene oxide side is exposed to the water interface while the hydrophobic poly-(ethylene) side remains hidden thanks to the formation of micelles.

There is a current surge of interest in the hazards associated with human exposure to nanoparticles.Nevertheless, the dangers of some naturally nanoparticulate minerals are well known.This paper reviews existing data in order to elucidate the common features conferring toxicity, which are also possessed, it is concluded, by novel manufactured nanofibres such as carbon nanotubes and gold nanorods.

The surface tension components and parameters for nine naturally occurring colloidal-sized volcanic ash samples were determined by thin-layer wicking. These materials include rhyolites, trachytes, phonolites, and an andesite, and were collected from deposits in California, Arizona, Italy and Martinique. The samples are principally volcanic glass (with minor amounts of mixed materials of various compositions) with silica contents ranging (average values) from 74% (rhyolite) to 57% (andesite). The surface tension components were determined by means of thin-layer wicking to provide contact angles which were used to solve the Young-Dupre equation. The Lifshitz-van der Waals component of the surface tension, gamma(LW), for these ash samples varies between 26.6 mJ/m(2) and 38.7 mJ/m(2). The Lewis acid parameter, gamma(circle plus), values are small with values ranging from 0 mJ/m(2) to 1.8 mJ/m(2), while the Lewis base parameter, gamma(circle plus), values varied between 15.6 mJ/m(2) and 45.6 mJ/m(2). Of the nine samples, five were hydrophobic and the other four were hydrophilic. The hydrophobic ashes were produced by eruptions with a dominantly magmatic rupture mechanism: pyroclastic flow, nuee ardente, ash cloud surge. The hydrophilic ashes were generated by a strong hydromagmatic mechanism and most formed accretionary lapilli. The zeta-potential values were determined from the electrophoretic mobility, and varied between -32.1 mV and -55.2 mV. Utilizing the surface tension values and the zeta-potential, an extended DLVO-type (XDLVO) calculation of the free energy of interaction versus distance, including the Lewis acid-base contribution, between 1 mu m spherical particles, showed that for the five hydrophobic ash samples there is a net attraction between particles in the presence of water, and for the four hydrophilic samples there is a strong net repulsion between particles. Assuming that the stability of an ash deposit (in the presence of water) on an inclined slope is related to the forces acting between neighboring ash particles, those deposits with repulsive interparticle forces would be more mechanically unstable and pose a risk of forming mud flows on remobilization.

By means of contact angle measurements on dry layers of electrostatically neutral dextran with pure water ( pH 6.1), water acidified with HCl ( to pH 1.94) and water made alkaline with NaOH ( to pH 12.8), it could be shown that there was essentially no change as a function of pH in the ratio of gamma(+)/gamma(-) of water as compared with the aqueous acid and alkaline solutions. ( Here gamma(+) is the Lewis acid parameter of the polar surface tension component of water and gamma(-) is its Lewis base parameter). In contrast, with contact angles measured with the same liquids on negatively charged clean glass, a significant decrease in contact angle was observed with water at pH 12.8, which was caused by the fact that at this alkaline pH an increase in surface hydrophilicity took place. This is because surfaces that have a given surface electrical potential at neutral pH generally acquire an even higher surface potential under more alkaline conditions which, concomitantly, also gives rise to an increase in surface hydrophilicity, and thus to lower contact angles with water. Finally, contact angles with acid water, pure water, and alkaline water, deposited on hydrophobic Parafilm surfaces, were exactly the same.

AbstractFor Abstract see ChemInform Abstract in Full Text.

The air side of the water-air interface is the most hydrophobic surface known. In quantitative terms the water-air interface is about 30% more hydrophobic than the surfaces of nonpolar condensed-phase compounds or materials such as octane or Teflon. The hyperhydrophobicity of the air side of the water-air interface is the main cause of the large increase in contact angle of drops of water deposited upon rough surfaces of apolar materials, as compared with the water contact angle on smooth surfaces of the same materials. A, water drop supported on a very porous fractal surface, encountering only about 1% solid support and 99% air, can reach a contact angle of 174 degrees, which is exceedingly close to the (albeit unattainable) maximum of 180 degrees. The water-air interface hydrophobically attracts completely apolar molecules, as well as the apolar side of amphiphilic molecules (such as surfactants). Thus, for instance, dissolved surfactant molecules aggregate at a high concentration at the water-air interface when dissolved in water. On the other hand, the water-air interface repels dissolved hydrophilic (or near-hydrophilic) solutes, such as sugars and polysaccharides, mainly via net repulsive van der Waals forces. Thus, the water-air interface is depleted of such hydrophilic (or near-hydrophilic) solutes, leaving a significantly higher concentration of these solutes in the bulk of the aqueous medium than at its air interface. As both of these contrasting phenomena result in strongly anisotropic concentration distributions in liquid drops and as contact angle determinations depend on a known and homogeneous free energy of cohesion of the liquid throughout the drop, one should never measure contact angles on solid surfaces for the purpose of measuring their surface thermodynamic properties by using aqueous solutions, mixtures, or solutions in or mixtures of other polar or partly polar liquids.Finally, the peculiar properties of the water-air interface give! rise to what at first sight appears to be paradoxical behavior of air bubbles in water: in pure deionized water, air bubbles attract one another and coalesce. On the other hand, upon the addition of salt (e.g., NaCl), air bubbles repel each other and thus do not coalesce, all in apparent contradiction of the classical rules governing the stability or instability of colloidal suspensions in water.

The interaction between water and solid surfaces is of great interest to geological materials, polymers, biomaterials, cells, microbes, other particles, etc. Yet little is still known about the structure, bonding, and function of the first few water layers in contact with solids or solutes, when these are immersed in water. Mineralogical and colloidal surface studies suggest how water may interact with both hydrophobic and hydrophilic surfaces. We know from surface-thermodynamic analyses of the attractive interactions between apolar (hydrophobic) molecules or particles, and of the repulsive interactions between polar (hydrophilic) molecules or particles, all immersed in water, that the first ones are driven by the strong hydrogen-bonding free energy of cohesion between the surrounding water molecules, whilst the second ones are mainly caused by the molecules of water of hydration when these are very strongly hydrogen-bonded to polar molecules or particles. However, this knowledge does not necessarily give us a direct insight into the molecular-scale changes occurring in the conformation of the water molecules in the first few layers of water of hydration closest to a solid/water or to a non-aqueous liquid/water interface. At the interface with apolar (hydrophobic) surfaces the first one or two layers of water molecules of hydration are exclusively attracted via (apolar) Lifshitz-van der Waals (LW) forces, as a thin net of water molecule clusters of diminished polarity in a manner that resembles similar to the structure of the water molecules participating in clathrate hydrates. On the other hand, at the interface with polar (hydrophilic) surfaces the first one or two layers of water molecules of hydration are bound, in addition to LW forces, for up to slightly more than 50% by Lewis acid-base (AB) forces, which cause a significant part of the most proximal water molecules of hydration to be oriented with their H-atoms hydrogen-bonded to the polar surfaces, and with their O-atoms protruding into the bulk liquid. Neither mode of conformation of water molecules of hydration bears any obvious resemblance to any of the forms of crystal formation occurring in ice. The different conformation gradients of the water molecules of hydration at the interface with apolar (hydrophobic) and with polar (hydrophilic) entities permit action at a distance well into the bulk water, in the case of hydrophobic attraction as well as of hydrophilic repulsion. In both cases, the interaction energies decay exponentially as a function of distance with a decay length of water of approximately 1.0nm. This (experimentally found) value for the decay length of water agrees more with a radius of gyration for small clusters of 4 to 5 water molecules than for single molecules of water. Further consequences of these considerations are discussed in detail, with particular stress on the influence of temperature on the interaction of partly polar entities immersed in water, on the necessity of using the extended DLVO (XDLVO) theory [extended by the inclusion of Lewis AB interactions in aqueous media] and on the unusual and sometimes extreme properties of the water-air interface and its manifestations.

This review analyzes various ways by which the surfaces of clay particles and related minerals can be non-covalently modified, as well as some of the analytical approaches with which these surface modifications can be measured and quantified. Among the principal methods of modification of the surface properties of clays and other metal oxide surfaces is treatment with cationic surfactants comprising long-chain alkyl groups. Using the Cassie equation, the percentage coverage by alkyl groups can be ascertained, from advancing contact angle data. Also, by measuring both advancing and retreating contact angles (e.g., with drops of water) on various surfaces, the percentage liquid coverage of such surface (or, if one wishes, the percentage contamination by the contact angle liquid) can be ascertained, using the Cassie equation. It was found that talc, treated with octadecyl amine (OA) became 100% covered with octadecyl groups when mildly heated with 1 wt.% OA. Treatment with higher concentrations did not increase the coverage, but gave the appearance as if the particles were covered with pentyl, not octadecyl groups, judging by the lowering of the apolar surface tension component of the treated talc surface. Coating of glass or mica surfaces with hexadecyl groups, via hexadecyl-quaternary ammonium groups, yielded about 42% coverage. As an improvement, coating with hexadecyl cationic surfactants yielded 82% coverage. However, when coating was done by application of pre-compressed monolayers formed on a Langmuir trough, 100% coverage (of eicosyl groups) could be achieved. Finally, if solidity of attachment, in addition to 100% coverage, is needed (as is very desirable in coatings applied to mica-covered half cylinders in Israelachvili's force balance), the compressed Langmuir trough layer approach, coupled to a further 2 hours worth of annealing at 100degreesC yields the most robust attachment, as shown by Wood and Sharma (Wood, J.; Sharma, R. How long is the long-range hydrophobic attraction. Langmuir 1995a, 11, 4797-4802). There are two other, totally different ways of making hydrophilic mineral particles more hydrophobic. The first one is by addition of plurivalent cations (e.g., Ca2+, La3+) to negatively charged inorganic particles, such as ground glass, ground calcite, or montmorillonite. When such initially hydrophilic, negatively charged particles, in stable aqueous suspension, have their negative surface potential diminished by the addition of small amounts of plurivalent cations, the particles become hydrophobic, and flocculate (Wu, W; Giese, R.F.; van Oss, C.J. Linkage between zeta-potential and electron donicity of charged polar surfaces 1. Implications for the mechanism of flocculation of particle suspensions with plurivalent counterions. Colloids. Surfaces. A 1994, 89, 241-252) even though the electrostatic repulsion energy still exceeds the mutual van der Waals attraction, thus furnishing a new and drastically modified explanation of the Schultze-Hardy effect. Finally, grinding hydrophilic solids or particles makes them hydrophobic by causing a strong decrease in their surface electron-donicity, most likely as a consequence of an increased liberation of electron-accepting sites through diminution, causing the neutralization of a substantial part of the electron-donating sites which, prior to grinding, were responsible for the material's hydrophilicity.

Among the three different non-covalent forces acting in aqueous media, i.e. Lifshitz-van der Waals (LW), Lewis acid-base (AB) and electrical double layer (EL) forces, the AB forces or electron-acceptor/electron-donor interactions are quantitatively by far the predominant ones. A subset of the AB forces acting in water causes the hydrophobic effect, which is the attraction caused by the hydrogen-bonding (AB) free energy of cohesion between the water molecules which surround all apolar as well as polar molecules and particles when they are immersed in water. As the polar energy of cohesion among water molecules is an innate property of water, the hydrophobic attraction (due to the hydrophobic effect) is unavoidably always present in aqueous media and has a value of DeltaG(hydrophobic) = -102 mJ/m(2), at 20degreesC, being equal to the AB free energy of cohesion between the water molecules at that temperature. The strong underlying hydrophobic attraction due to this effect can, however, be surmounted by very hydrophilic molecules and particles that attract water molecules more strongly than the free energy of attraction of these molecules or particles for one another, plus the hydrogen-bonding free energy of cohesion between the water molecules, thus resulting in a net non-electrical double layer repulsion. Each of the three non-covalent forces, LW, AB or EL, any of which can be independently attractive or repulsive, decays, dependent on the circumstances, as a function of distance according to different rules. These rules, following an extended DLVO (XDLVO) approach, are given, as well as the measurement methods for the LW, AB and EL surface thermodynamic properties, determined at 'contact'. The implications of the resulting hydrophobic attractive and hydrophilic repulsive free energies, as a function of distance, are discussed with respect to specific and aspecific interactions in biological systems. The discussion furnishes a description of the manner by which shorter-range specific attractions can surmount the usually much stronger long-range aspecific repulsion, and ends with examples of in vitro and in vivo effects of hydrophilization of biopolymers, particles or surfaces by linkage with polyethylene oxide (PEO; also called polyethylene glycol, PEG). Copyright (C) 2003 John Wiley Sons, Ltd.

To avoid aspecific attractions between carrier surfaces for either ligand or receptor molecules in, e.g., immunoassays, or kinetic rate constant measurements, it has long been established that a background consisting of an aspecific, very hydrophilic carrier surface is generally quite effective. However, it is not often realized that one achieves such a non-reactive background, even with electrostatically neutral materials, at the price of creating a strong (polar) hydrophilic repulsion between dissolved biopolymer (e.g., protein) molecules and the non-adsorbing carrier surface. To investigate the quantitative effects of this type of repulsion in systems involving streptavidin, the surface properties of a streptavidin-coated glass plate were determined by contact angle measurements, from which the aspecific, macroscopic-scale free energies of repulsion between a streptavidin-coated surface and dissolved proteins such as immunoglobulin-G (IgG), and human serum albumin (HSA), could be derived. Streptavidin, even at neutral pH (at which it has virtually no electric surface charge as determined by electrophoresis) is very hydrophilic and strongly repels both IgG and HSA molecules. At neutral pH, molecules such as IgG and HSA, in aqueous solution, cannot approach a streptavidin layer more closely than to approximately 3.0 nm, which suffices to prevent IgG or HSA from any aspecific adherence to the streptavidin layer (as determined by extended DLVO analysis). This aspecific repulsion however also has the (usually unsuspected) effect of causing a decreased specific attachment between ligand and receptor molecules. In addition, it decreases the measured kinetic on-rate constants, often by about two decimal orders of magnitude. However, once the surface-thermodynamic properties of all the aspecific (macroscopic-scale) and specific (microscopic-scale) entities, as well as the specific equilibrium binding constant are known, the real kinetic on-rate constant between just the ligand and the receptor determinants can be determined, yielding the value it would have if the measurement of that constant were unhindered by the repulsive interactions exerted by the background of hydrophilic carrier molecules.

Sugars such as sucrose and glucose, when dissolved in water, increase its measured surface tension. Polymers of the same sugars, sucrose or glucose, on the other hand, cause a decrease in the surface tension of their aqueous solutions. From the surface tension properties of solid layers of dried sucrose and glucose, and from the aqueous solubilities of these two sugars, their surface tension components and parameters in the dissolved state can be determined. The surface tension of both sugars, in the dissolved state, is about twice as high as that of water, which is mainly due to a very strong inter-molecular Lewis acid-base interaction (expressed as gamma(AB)). From the surface tension properties of both sugars it follows that their molecules are strongly repelled by the water-air interface, leaving a thin zone inside the water-air interface that is depleted of sugar molecules. This gives rise to a measured surface tension for these aqueous sugar solutions which is only a few mJ/m(2) higher than that of water, instead of more than 70 mJ/m(2) higher (for 100% sugar).In contrast with simple sugars, with sugar polymers the strong electron-donor/electron-acceptor interaction energies no longer exist, which lends ficoll and dextran a gamma(AB) value of only 42% of that of water. Both polymers are also repelled by the water-air interface, making the decrease in the measured surface tensions of their aqueous solutions also rather modest. On the other hand, partly hydrophilic/partly hydrophobic solutes such as proteins and surfactants are attracted to the water-air interface. Their bulk solution contains a (rather dilute) concentration of solute, but the solute concentration at the water-air interface is as high as, or higher than, the solubility limit, with the hydrophobic moieties of these molecules protruding into the air. This causes an exaggerated apparent decrease in the measured surface tension of the aqueous solution which is generally already quite noticeable at very low solute concentrations.

It has been shown earlier that interfacial tensions between organic liquids and a polar liquid such as water, when measured by drop shape (or drop weight) analysis only closely relate to the mutual solubility of the two liquids as well as to the free energies of interaction between and in the two liquids, when using completely apolar liquids such as alkanes. In all other cases, with partly polar liquids (even including aromatic compounds), the interfacial tension with water (as relates to the mutual solubility of the two liquids) is significantly underestimated by this method, due to unavoidable hysteresis caused by the almost instantaneous orientation of the polar moieties of the organic compound toward the water interface. It is therefore appropriate to introduce the concept of zero time dynamic (ZTD) interfacial tension, where ZTD interfacial tension is designated as gamma(jj)(0). Now, two methods for determining ZTD interfacial tensions between polar condensed-phase compounds and water correlate closely to solubilities as well as to free energies of interaction. The first one relates the compounds aqueous solubility to its contactable surface area and its ZTD interfacial tension with water This approach also allows the determination of ZTD interfacial tensions with very water-soluble solutes, such as sugars. The second one (able to furnish ZTD interfacial tensions between any two condensed-phase materials) utilizes the results of contact angle measurements, interpreted via the Young-Dupre equation, combined with the van Oss-Chaudhury-Good equation for interfacial tension. In some cases the latter approach can be circumvented by using the original Young equation. From the solubility of water in a number of organic solvents, the contactable surface area could be determined for water clusters, at 20degreesC, at about 4.5 monomeric water molecules per cluster.

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.METHODS AND RESULTS:The biosurfactant was produced according to a simple, novel technique based on cultivation on nutrient agar. Adhesion studies were performed using L. monocytogenes cells cultured at 20 or 37 degrees C.CONCLUSIONS:A substrate-dependent behaviour of the LO28 strain (larger number of cells adhering to stainless steel than to PTFE), and a significant reduction (< 90%) in microbial adhesion levels through the prior adsorption of biosurfactants on stainless steel surfaces, which can be related to a change in the electron-donor characteristics of this substratum, was demonstrated.SIGNIFICANCE AND IMPACT OF THE STUDY:The prior adsorption of biosurfactants on solid surfaces may constitute a new and effective means of combating the implantation of pathogenic micro-organisms in food processing plants.

The adsorption of human serum albumin (HSA) onto hydrophobic talc and various types of hydrophilic silica has been studied. The kinetic adsorption constants for each case were determined experimentally, using time-resolved fluorescent spectroscopy. Von-Smoluchowski's f-factor, a measure of the dominance of the macroscopic protein repulsion by the solid surface, was determined for each protein-surface system. The free energies of both macroscopic (repulsive) and microscopic (attractive) interactions between HSA and solid substrata were calculated via extended DLVO (XDLVO) analysis. The combination of such kinetic and thermodynamic data can lead to the estimation of a number of other parameters which are characteristic for each system and contain significant information, such as the mean microscopic and macroscopic-scale interaction energies between protein and solid substratum and the ratio of unfavorable to favorable protein orientations during adsorption, as well as the average net total ratio of repulsion over attraction. The latter was found to be inversely proportional to the amount of HSA adsorbed onto the various mineral particles, after 1 h exposure. Furthermore, detailed examination of all kinetic adsorption parameters leads to the conclusion that the equilibrium affinity constant, extrapolated to zero time K aff t → 0 is the parameter which rather accurately defines the specific interactions for a given protein-solid substratum system. However the most precise characterization of a given protein-substratum system is defined by both the specific (microscopic) adsorption k a mic and the specific (microscopic) desorption k d mic rate constants, which are obtained after elimination of the influence of the aspecific (macroscopic) factors which strongly influence the overall rate constants (ka and kd).

The kinetics and energetics of human serum albumin (HSA) adsorption onto silica and talc particles are compared, from the earliest moment of contact, up to 24 h exposure. The measured affinity constants are used to obtain the free energies of interaction (ΔG). The free energies of interaction are also obtained via the measured surface properties of HSA, water, silica and talc. Comparison between the ΔG-values obtained by the two different approaches makes it possible to deduct the manner and conformation of HSA adsorption at the earliest moment, as well as after onset of hysteresis. Two different modes of hysteresis are identified — one allows slow dissociation and the other approaches apparent irreversibility. The mechanisms of both are elucidated, as is the mechanism of desorption at high pH and by displacement with identical or quasi-identical molecules.

In the quantitative treatment of non-covalent inter- and intra-cellular interactions taking place in water, in vitro as well as in vivo, it is essential to treat the surrounding and pervading liquid medium as the continuous medium. In the close vicinity of inter- and intra-cellular surfaces and of biopolymers the various different non-covalent forces may locally alter the structure of water in a number of ways, but these local structural changes can be quantitatively taken into account. The operative forces are: Lifshitz-van der Waals (LW) forces. Lewis acid-base (AB) forces and electrostatic (EL) forces. Of these, the AB forces are generally the preponderant ones, in aqueous media. This is due, inter alia, to the strong cohesive and adhesive hydrogen-bonding interactions typically occurring in and by water. Among the strong AB interactions occurring in water are hydrophobic attraction (the hydrophobic effect) and hydrophilic repulsion (hydration pressure). Also treated is the function of LW, AB and EL forces in: hydration; in the stability of particle and cell suspensions, the solubility of biopolymers, small organic solutes, and electrolytes; and in specific ligand-receptor (such as antigen-antibody) interactions.

The kinetics of desorption of human serum albumin from silica and talc microparticles are presented. The evolution of hysteresis, i.e. of the progressive bond strengthening between protein molecules and solid substratum, was monitored by calculating the Langmuir adsorption isotherms, as well as the desorption rates and desorbable protein amounts after various particle–protein contact times. The effect of parameters such as temperature, ionic strength, presence of cation-complexing and protein denaturation agents, on the desorption rate and maximum desorbable amount were also examined. The results help elucidate the mechanism of protein adsorption and also the strength of the formed bonds before and after the onset of hysteresis.

The 'hydrophobic effect', i.e. the attraction between apolar molecules or particles (i), immersed in water W, is caused by the Lewis acid-base (AB) component of the free energy of cohesion of water: DeltaG(iwi)(hydrophobic) = DeltaG(ww)(AB) (coh) = - 102 mJ/m(2), at 20degreesC. Therefore, hydrophilic repulsion in water between hydrophilic molecules or particles (i) must achieve a free energy of repulsion, DeltaG(iwi)(hydrophilic) >> 102 mJ/m(2), before actual repulsion can prevail. Methods of measurement of the subcomponents and parameters of these free energies are outlined, as well as methods for calculating solubilities, or critical micelle concentrations of partly polar compounds and surfactants. Similarly methods are given for predicting stability or otherwise of aqueous particle suspensions. Finally, the linkage between the hydrophilicity vs. hydrophobicity of particles, and their surface (zeta) potential, is outlined, with indications how to modulate these parameters.