Surface viscosity hydrophobic effect

Choi et al. [1] examined the apparent slip effects of water in hydrophobic and hydrophilic microchannels experimentally using precision measurements of flow rate versus pressure drop. They correlated their experimental results to that from analytical solution of flow through a channel with slip velocity at the wall. There was clear difference between the flows of water on a hydrophilic and hydrophobic surface indicating the effect of slip flow (Fig. 2). Neto et al. [3] have reported clear evidence of boundary slip for a sphere-flat geometry from force measurements using atomic force microscopy. The degree of slip is observed to be the function of both liquid viscosity and shear rate (Fig. 4). 

By covalently attaching reactive groups to a polyelectrolyte main chain the uncertainty as to the location of the associated reactive groups can be eliminated. The location at which the reactive groups experience the macromolecular environment critically controls the reaction rate. If a reactive group is covalently bonded to a macromolecular surface, its reactivity would be markedly influenced by interfacial effects at the boundary between the polymer skeleton and the water phase. Those effects may vary with such factors as local electrostatic potential, local polarity, local hydrophobicity, and local viscosity. The values of these local parameters should be different from those in the bulk phase. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

Heretofore, ionic liquids incorporating the 1,3-dialkylimidazolium cation have been preferred as they interact weakly with the anions and are more thermally stable than the quaternary ammonium cations. Recently, the physical properties of 1,2,3,4-tetraalkylimidazolium- and 1,3-dialkylimidazolium-containing ionic liquids in combination with various hydrophobic and hydrophilic anions have been systematically investigated (36,41). The melting point, thermal stability, density, viscosity, and other physical properties have been correlated with alkyl chain length of the imidazolium cation and the nature of the anion. The anion mainly determines water miscibility and has the most dramatic effect on the properties. An increase in the alkyl chain length of the cations from butyl to octyl, for example, increases the hydrophobicity and viscosity of the ionic liquid, whereas densities and surface tension values decrease, as expected. 

Colloids are either hydrophilic (water-loving) or hydrophobic (water-hating). Hydrophilic colloids (e.g., proteins, humic substances, bacteria, viruses, as well as iron and aluminum hydrated colloids) tend to hydrate and thereby swell. This increases the viscosity of the system and favors the stability of the colloid by reducing the interparticle interactions and its tendency to settle. These colloids are stabilized more by their affinity for the solvent than by the equalizing of surface charges. Hydrophilic colloids tend to surround the hydrophobic colloids in what is known as the protective-colloid effect, which makes them both more stable. 

In the presence of anionic surfactants, it is reasonable to expect that the hydrophobic groups of the poljrmer and of the surfactant would combine to form a mixed film at the liquid-air interface. The interactions between the cationic groups of the polymer and the anionic groups of the surfactant would further strengthen the interactions in the monolayer. These effects can be expected to increase the surface and sub-solution viscosity in lamellae and in turn enhance their stability. 

Within the layers limited by hydrophobic walls, the water molecule dipoles are oriented parallel to the surface. The effect of ordered orientation spreads to a considerable distance that is, it is of a long-range nature. Such an orientation of water molecules causes a decrease in density near the walls and an increase in the mobility of the molecules in the tangential direction. This situation is interpreted as a decrease in the viscosity of the boundary layers. From a macroscopic point of view, this effect can manifest itself as the slipping of water on the hydrophobic substrate. 

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