the particles to aggregate. The dispersive action of the surfac-Nitroxide spin probes have been widely used to study the behavtants can be due to electrostatic repulsion and/or a reduction ior of surfactants in solution as well at solid-liquid interfaces. In of the attractive van der Waals forces via modification of this study the effect of temperature on the mobility of nitroxide the particle surface due to the presence of the adsorbed layer spin probes in cyclohexane and at the alumina/cyclohexane inter- (5, 6). The electrostatic repulsion depends on the nature of face is investigated in the presence of anionic aerosol-OT (AOT).
the surfactant and its tendency to dissociate and develop
The probe mobility in AOT micelles in solution is markedly depenresidual charges on the particle surface. In contrast, the alterdent on the amount of solubilized water. The variation with temation of the van der Waals energy depends on the extent of perature of the mobilities of adsorbed doxyl stearic acid with the surfactant adsorption and the microstructure of the adsorbed nitroxide probe at the 5th, 10th, and 16th positions from the {COOH group has shed some light on the mechanisms of interac-layer, i.e., the conformation and packing of the molecules at tion of the probe with the solid surface as well as the molecular the solid-liquid interface. Indirect information on molecular structure of the adsorbed layer. The results indicate that the adorientation at the interface can be obtained from a detailed sorption of the probe occurs mainly through interactions of the analysis of the adsorption isotherm as well as from measure-{COOH group with the surface while the {NO group interacts ments of thermodynamic parameters pertinent to the adsorponly weakly with the surface. The adsorption of AOT causes tion process. For example, Kipling and Wright in their work changes in probe mobility with the changes being most significant on the adsorption of fatty acids on different substrates dewhen the nitroxide group on the stearic acid chain is farthest from duced molecular orientation relative to the substrate surface the anchoring group. It is suggested that the adsorption of the from measurements of heats of immersion (7). Mills and surfactant causes an orientational rearrangement of the probe mol-Hockey discussed the possible variations in molecular conecules. The relatively low-temperature effect on the probe mobility formations with adsorption from the heats of adsorption data observed in the presence of surfactant is attributed to the compactness of the adsorbed layer which is found to be stable even at as a function of surface coverage of n-fatty acids on silica elevated temperatures.