Abstract
Drop profile analysis tensiometry is applied to determine the distribution coefficient of a nonionic surfactant for a water/hexane system. The basic idea is to measure the interfacial tension isotherm in two configurations: a hexane drop immersed in the surfactant aqueous solutions at different bulk concentrations, and a water drop immersed into a hexane solution of the same surfactant. Both types of experiments lead to an isotherm for the equilibrium interfacial tensions with the same slope but with a concentration shift between them. This shift refers exactly to the value of the distribution coefficient.
Highlights
Many studies have paid attention to the adsorption behaviour of various surface-active substances at liquid/liquid interfaces from the viewpoints of basic sciences as well as practical applications. This problem was considered in various books, such as [1,2,3,4]; a number of studies were published in which both adsorption and rheological characteristics at the surfactant aqueous solution/alkane interface are analysed [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
Besides methods based on interfacial tension methods, there are chemical procedures in which the aqueous solution is brought into contact with the oil phased, and after reaching the distribution equilibrium, the amount of surfactant is determined by a suitable analytical method
The supposition that the surface tension decrease at high adsorption times is caused by the impurities present in hexane is supported by the data shown in Figure 2, where the kinetic dependencies for the C13DMPO concentration of 30 μmol/L in hexane are shown for its solutions in hexane of 99% purity and hexane of 95% purity
Summary
Many studies have paid attention to the adsorption behaviour of various surface-active substances at liquid/liquid interfaces from the viewpoints of basic sciences as well as practical applications. Besides methods based on interfacial tension methods, there are chemical procedures in which the aqueous solution is brought into contact with the oil phased, and after reaching the distribution equilibrium, the amount of surfactant is determined by a suitable analytical method Such protocols are specific for each surfactant and not further discussed here (see for example the discussions in [7,9,11] and the references therein). The interfacial tension isotherms at the hexane/water interface were measured for hexane drops formed in the C13DMPO aqueous solution and for water drops formed in C13DMPO solutions in hexane using the drop profile analysis method From these isotherms, the interfacial distribution coefficient could be and precisely determined as the ratio of concentrations at the same interfacial tension. Any variation of concentration caused by the adsorption of C13DMPO onto the drop surface and the diffusion of C13DMPO into the drop could be neglected
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