Hydrophobic interaction between macroscopic and microscopic surfaces. Unification using surface thermodynamics

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.