Abstract
The quasi-static mode of the drop volume technique has been applied for determining the equilibrium surface tension-concentration (σe-c) curves of ionic (SDS, CTAB) zwitterionic (CHAPS), and nonionic (Triton X-100) surfactants. The experimental values have been fitted in two parts by the best polynomial and straight-line equations. Determination coefficients (R2) higher than 0.98 were achieved for all σe-c curves. The critical micelle concentration (CMC) and surface tension at the CMC have been calculated at the intersection point of the two fitted curves. The results conform with those found in the literature. The polynomial equation derivative at the CMC was used in the Gibbs equation to calculate the maximum surface excess concentration and the corresponding molecular area. The space required per molecule under saturation conditions is in agreement with the surfactant structures. A comparison with the Wilhelmy-plate method was also made. It is concluded that the quasi-static drop volume method provides accurate results and presents many practical advantages.
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