The surface tension of aqueous solutions of cationic, nonionic, and anionic surfactants was measured as a function of the size of drop formed under the tip of a glass capillary and the time for which the drop is allowed to stand by a previously developed apparatus based on the drop volume method. The surface tension of cationic and nonionic surfactant solutions was slightly dependent on the drop size and the adsorption time at low concentrations, and we could easily obtain the adsorption-equilibrium surface tension values by use of an intact capillary. However, the surface tension of anionic surfactant solutions showed a strong dependence on these factors, and the equilibrium surface tension values could not be obtained by use of the intact capillary at all. This situation was drastically improved by use of the water-repellent capillary coated with a silane-coupling reagent. We proposed that the equilibrium surface tension value of surfactant solutions can be determined by adopting the adsorption time at which the surface tension value becomes constant on the surface tension vs the drop size curves. Further, the conditions under which the equilibrium surface tension value of surfactant solutions could be acquired by means of the drop volume method were quite different than those of the Wilhelmy plate method.
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