Advances in Colloid and Interface Science (v.91, #3)
Hydrophobic interaction between macroscopic and microscopic surfaces. Unification using surface thermodynamics by Jiřı́ Škvarla (335-390).
Comparing the values of the contact interaction force between macroscopic surfaces in gases, as determined by the separation force, ESA-MBI, crack-growth, JKR-type, and contact angle experiments, the adhesion mechanics and the effect of the surface roughness on the interaction is evaluated. For mica surfaces, the JKR adhesion mechanics with the negligible surface roughness effect is predicted. This predestines the mica surface to be a reference substrate. Low-energy polymer surfaces also obey the JKR adhesion mechanics. Despite the relatively high surface roughness (up to 2 nm), its effect may be marginal. The combining rule is verified for systems with different surfaces. Metals undergo a plastic deformation but the effect of the surface roughness is comparable to that determined for polymers. Adhesion ceases when the height of the surface asperities attains a value close to 10 nm. For rigid, high-energy surfaces (especially oxides) in the AFM, a dramatic decrease of adhesion is noticed even though the roughness is relatively low, making the distinction between the actual adhesion mechanics impossible. The contact hydrophobic force is evaluated for the macroscopic surfaces in aqueous solutions by the separation force measurements, contact extrapolations of the hydrophobic force profiles and the contact angle analysis using the Neumann equation for interfacial tensions as a good approximation. For the molecularly smooth mica surfaces covered by adsorption or LB monolayers, the values of the contact hydrophobic forces, as evaluated by the three independent methods, coincide. Therefore, no effect of the surface roughness can be expected in the SFA. The separation is governed by the JKR adhesion mechanics. The hydrophobic force is of a short-range character; it extends only to the intersurface separation of approximately 10 or 15 nm and can be fitted by the single-term exponential function with the constant decay length of approximately 1 nm. The preexponential factor reflects the surface hydrophobicity. However, for LB monolayers the short part of the hydrophobic force merges into a longer-range part, apparently insensitive to the surface hydrophobicity. By all appearances, this extra branch is a manifestation of another mechanism of the hydrophobic force. For silica surfaces in the AFM, the contact hydrophobic force is lowered in proportion to the surface roughness. The inverse effect of the surface heterogeneity on the extension of the hydrophobic force can be observed. The geometric mean combining rule is verified for the contact hydrophobic force between different surfaces. For emulsions, the short-range, exponentially decaying hydrophobic force is confirmed with the preexponential factor being proportional to the tension of smooth and homogeneous hydrocarbon–water interface of the emulsion droplets. In suspensions, however, a long-range hydrophobic force is expected.
Keywords: Hydrophobic force; Hydrophobic coagulation; Surface forces;
Direct measurements of the force between hydrophobic surfaces in water by Hugo K. Christenson; Per M. Claesson (391-436).
Direct measurements of the force between hydrophobic surfaces across aqueous solutions are reviewed. The results are presented according to the method of preparation of the hydrophobic surfaces. No single model appears to fit all published results, and an attempt is made to classify the measured interactions in three different categories. The large variation of the measured interaction, often within each class, depending on the type of hydrophobic surface is emphasized. (I) Stable hydrophobic surfaces show only a comparatively short-range interaction, although little quantitative data on this attraction have been published. (II) Many results showing very long-range attractive forces are most likely due to the presence of sub-microscopic bubbles on the hydrophobic surfaces. Such an interaction is typically measured between silica surfaces made hydrophobic by silylation. Between self-assembled thiol layers on gold surfaces very short-range attractive forces are possibly due to the presence or nucleation of bubbles. The reason for the apparent stability of these bubbles is not clear and warrants further investigation. (III) Results obtained with LB films of surfactants or lipids on mica appear to give rise to a different type of force that fits neither of these two categories. This force is an exponentially decaying attraction, often of considerable range. The force turns more attractive at smaller separations, and may at short range be similar to the interaction measured between stable hydrophobic surfaces. An apparently similar, exponential attraction is also found between mica surfaces bearing surfactants adsorbed from cyclohexane, between silylated, plasma-treated mica surfaces and between both mica and silica surfaces with surfactants adsorbed in situ. This type of force also occurs between some surfaces of relatively low hydrophobicity as well as between one such hydrophobic surface and a hydrophilic surface. No convincing model can explain this third type of interaction for all systems in which it has been observed. This review of work to date points to the importance of the morphology and structure of the hydrophobic surface, and how it may change during the interaction of two surfaces.
Keywords: Hydrophobic attraction; Hydrophobic force; Hydrophobic surface; Force measurements;
Interfacial rheological properties of adsorbed protein layers and surfactants: a review by Martin A. Bos; Ton van Vliet (437-471).
Proteins and low molecular weight (LMW) surfactants are widely used for the physical stabilisation of many emulsions and foam based food products. The formation and stabilisation of these emulsions and foams depend strongly on the interfacial properties of the proteins and the LMW surfactants. Therefore these properties have been studied extensively. In this review an overview is given of interfacial properties of proteins at a mesoscopic scale and the effect of LMW surfactants (emulsifiers) on them. Properties addressed are the adsorbed amount, surface tension (reduction), network formation at interfaces and possible conformational changes during and after adsorption. Special attention is given to interfacial rheological behaviour of proteins at expanding and compressing interfaces, which simulate the behaviour in real emulsions and foams. It will be illustrated that information on interfacial rheological properties, protein conformation and interactions between adsorbed molecules can be obtained by changing experimental conditions. The relation between interfacial rheology and emulsion and foam stabilisation is subsequently addressed. It is concluded that there is a need for new measuring devices that monitor several interfacial properties on a mesoscopic and microscopic scale at the same time, and for physical models describing the various processes of importance for proteins. Real progress will only be possible if both are combined in an innovative way.
Keywords: Interfacial rheology; Protein; Emulsion and foam stability; Surfactants;
Nano-sized cluster nucleation by Terry A. Ring (473-499).
Classical nucleation theory good for slowly nucleating systems is reviewed. A new theory for fast nucleating systems, based on a population balance, is presented that uses the energetics of clusters of various geometries determined from ab initio quantum mechanical calculations. This theory predicts cluster population dynamics during nucleation, as well as the spectrum of light emitted during nucleation-crystalloluminescence.
Keywords: Nucleation; Cluster dynamics; Crystalloluminescence; Nano-synthesis;