Disjoining pressures of nonionic surfactant stabilized thin liquid films and surface tensions of electolyte solutions.

摘要

Thin-liquid films stabilized by polyethylene oxide-alkyl ether nonionic surfactants are investigated experimentally using the porous-plate technique. Single, isolated films are formed in a porous-glass film holder that is sealed in an experimental cell, and the liquid in the film is in equilibrium with excess bulk solution at a reference pressure. A capillary pressure is applied to the film by precise control of the pressure in the gas phase surrounding the film, and the film thickness is measured by thin-film interferometry. By repeating the process over a range of applied capillary pressures, we construct a complete disjoining pressure isotherm.;We experimentally studied the effects of surfactant structure and concentration, ionic strength, ion valence and size, and pH on disjoining pressure isotherms of thin-liquid foam films. Films are stabilized by long-range electrostatic forces due to the adsorption of hydroxide ions at the film interfaces that results in a plane of charge. The surface charge decreases with surfactant concentration, increases slightly with ionic strength and is almost totally unaffected by ion valence or surfactant structure.;The results of the experimental investigation of thin-liquid foam films are used to formulate a self-consistent model of air/water and oil/water interfaces. We describe the adsorption of hydroxide and surfactant with a competitive adsorption mechanism. Poisson's equation is modified to include image charge forces and dispersion forces on ions due to the presence of the interface. Image charge forces repel ions from either air/water or oil/water interfaces, whereas dispersion forces attract ions to an oil/water interface and repel ions from an air/water interface. The result is a highly nonlinear second-order ordinary differential equation that is solved by finite differences and Newton-Raphson iteration.;The proposed model accurately predicts disjoining pressures in thin-liquid foam films as a function of ionic strength, surfactant concentration, and pH. The model also calculates zeta-potentials of air bubbles or oil droplets in aqueous solutions and surface tensions of electrolyte solutions with and without added nonionic surfactant. At low ionic strengths, the specific adsorption of hydroxide leads to a increase in ion concentration near the interface compared with that in the bulk. At high ionic strengths, shielding of the surface charge and image and dispersion forces result in a net desorption of ions. As a result, the proposed model predicts a surface tension minimum for extremely dilute electrolyte solutions that was first measured by Jones and Ray in 1937, but that was not explained by existing models of aqueous solution interfaces.