Hydrophobic Forces in Free Thin Films of Water in the Presence and Absence of Surfactants
In this study, two different system were studied to investigate the forces interacting in thin liquid films. In the former, direct force measurements were conducted with bitumen-coated mica surfaces using a Mark IV surface force analyzer. Besides long-range electrostatic repulsive forces at separation distances above approximately 70 nm, unexpectedly strong repulsive forces were measured at shorter separation distances. These non-DLVO forces may be attributed to the steric repulsion between asphaltenes extracted on the bitumen surface. The steric forces increased with pH and temperature, which may be explained by the increased solubilization of asphaltenes in water. The steric force also increased with electrolyte concentration, possibly due to a decrease in the mobility of the tails of the asphaltenes on the surface.
The kinetics of coalescence of bitumen droplets was studied by measuring the induction time. There was a general agreement between the force data and the kinetic information, suggesting that bitumen suspensions are stabilized by asphaltene. The steric forces identified by the direct force measurements may have profound implications on the hot water processing of Alberta Tar Sands.
In the later, a thin film balance (TFB) of Scheludko and Exerowa-type was used to determine equilibrium film thicknesses of dodecylammonium chloride and sodium dodecyl sulfate solutions. The hydrophobic force as a third component of the extended DLVO theory was represented as a power law. The results showed that at low surfactant concentrations, the hydrophobicity parameter, K232 is positive and decreases with increasing surfactant concentration. By extrapolating this data for the zero surfactant concentration, K232 was found about 10-17 J, which is approximately 270 times larger than the Hamaker constant, A232. These results suggest that air bubbles are hydrophobic and the hydrophobicity decreases with increasing surfactant concentration..
The disjoining pressure isotherm for both surfactant systems in the presence of inorganic electrolyte (NaCl) were also obtained. The results indicated that the K232 values estimated from the equilibrium film thickness measurements can be used to fit the experimentally obtained disjoining pressure isotherm. Consideration of hydrophobic force predicted a rupture thickness larger than predicted using the DLVO theory, but is substantially smaller than the experimental result. This discrepancy may be ascribed to the hydrodynamic force operating in the film thinning process.
To compare the hydrophobicity of air bubbles and solid surfaces, an air bubble was simulated by using a hydrophobic solid surfaces. The equilibrium contact angle of aqueous surfactant solutions on hydrophobic surfaces was measured using a contact angle goniometer. The results indicated that the nature of both the air-water interface and the solid-water interface is altered gradually from hydrophobic to hydrophilic with increasing surfactant concentration. The results also suggested that the hydrophobicity of both systems are comparable, i.e., K232 is about 10-17 J, which is estimated using the extended DLVO theory, and K131 is in the order of 10-16 J, which is obtained by direct force measurements between the hydrophobic surfaces. It was showed that hydrophobic forces much higher than the London-van der Waals forces for both systems need to be included when the stability of foams and colloids is considered.