Surface Tension Measurements with the Drop Profile Analysis Tensiometry—Consideration of the Surfactant Mass Balance in a Single Drop

Abstract

Abstract:In drop profile analysis tensiometry, the ratio of drop surfaces area S to volume V is large, i.e., S/V >> 1. In such a case, the concentration of a surfactant within the drop bulk decreases due to adsorption at the drop surface. In contrast, in bubble profile analysis tensiometry, we have S/V << 1 so that depletion due to adsorption is negligible. A protocol is presented to determine the correct adsorption parameters of surfactants from surface tension data measured by bubble and drop profile analysis tensiometry. The procedure is applied to experimental data measured for selected surfactants of different adsorption activities: C 10 OH, CTAB, Tween 20, and the equimolar mixture SDS + DoTAB. The results show that for surfactants with higher surface activity, the differences between the surface tensions measured with the drops and bubbles profile analysis tensiometry, respectively, are larger, while for less surface-active surfactants, such as SDS, the results obtained from drop and bubble profile experiments are very close. The correction procedure is based on the same set of adsorption parameters used to fit both the experimental data obtained from drop-based measurements (which involve the depletion effects) and those data measured in a way that depletion effects are negligible.

Surface Tension and Adsorption Studies by Drop Profile Analysis Tensiometry

“…The surface tension values were measured with the drop profile (filled circles) and bubble profile (open circles) analysis tensiometry in [54]; the data obtained by the bubble profile method almost coincide with the data obtained by the du Noüy ring and Wilhelmy plate methods, as reported in [72]. The bold red curve was obtained by fitting the drop-based experimental values using Eqs.…”

“…These values were used in the calculation with the Frumkin model for an electroneutral surface layer [73]; the x 0 values listed in Table 1 refer to the total area of each CTAB molecule. In this case, also the dash-dotted curve obtained from fitting the bubble-based experimental data (open diamonds, [54]) coincides with the curve obtained by the recalculation of the drop-based fitting curve on the equilibrium concentration. Figure 3 illustrates the results obtained in [76] for aqueous solutions of Inositol-C2-PAF.…”

“…Figure 3 illustrates also the experimental and calculated values obtained for Tween 20 solutions [54]; here again the dash-dotted curve referring to the data from bubble profile experiments coincides with the curve obtained by the recalculation of the drop-based fitting curve on the equilibrium concentration.…”

“…(1), (3), and (5) to the experimental data measured by the drop profile method are listed in Table 1, and ranged according to the increase of the adsorption activity coefficient b . Note, the model parameters estimated in [54] by fitting the values calculated using Eqs. (1)–(3) to the bubble-based experimental data are close to those summarized in Table 1.…”

“…We begin with the analysis of experimental data obtained using the drop profile analysis method assuming the mass balance in equilibrium conditions, referring to the results reported previously [54]. In this overview, we mainly apply the Frumkin model, which was generalised to account also for the intrinsic compressibility of the adsorbed surfactant layer.…”