Contact angle of surfactant solutions on precipitated surfactant surfaces. II. Effects of surfactant structure, presence of a subsaturated surfactant, pH, and counterion/surfactant ratio

Abstract

The contact angle of a saturated aqueous surfactant solution on the precipitate of that surfactant was measured by using the sessile drop method. The sodium and calcium salts of alkyl sulfates (C 12 , C 14 , and C 18 ) had advancing contact angles higher than those of alkyl trimethylammonium bromides (C 14 , C 16 , and C 18 ). The measured advancing contact angles for several surfactant solutions did not substantially change with varying surfactant/counterion ratios; therefore, the precipitating counterion concentration (e.g., water hardness) had little effect on the wettability. The contact angles of fatty acid (C 12 and C 16 ) solutions did not show any dependence on pH between a pH of 4 and 10. The contact angles of saturated calcium dodecanoate (CaC 12 ) solutions containing a second subsaturated surfactant (sodium dodecyl sulfate: NaDS) decreased with increasing NaDS concentrations until reaching the critical micelle concentration of the surfactant mixture. These results show that the second suractant can act as a wetting agent in this saturated surfactant system. Application of Young's equation to contact angles showed that the solid/liquid surface tension can change substantially with surfactant concentration and be important in addition to the liquid/vapor surface tension in reducing contact angles. Application of the Zisman equation results in a "critical" surface tension for the CaC 12 or soap scum of 25.5 mN/m, which is comparable to difluoroethene.

Contact angle of surfactant solutions on precipitated surfactant surfaces. III. Effects of subsaturated anionic and nonionic surfactants and NaCl

“…This work further supports the position that the Zisman equation should not generally be used for wetting of surfactant solutions, as concluded in our previous work on these systems (13). Here, only one of the three surfactants studied obeyed the Zisman equation even over a limited range of concentrations.…”

“…As described in Part II of this series (13), the reason the empirical Zisman equation works in this range is that the reduction in γ LV as surfactant concentration increases is approximately proportional to the reduction in γ SL [or increase in (γ SL 0 -γ SL )]. Clearly, this is also the case for NaDS with added NaCl from Figures 3 and 11 in the applicable concentration range.…”

“…There can be a substantial difference between advancing and receding contact angles, or hysteresis, for these systems (10). Various techniques are used for measuring the contact angle (11); the most commonly used method is the sessile drop measurement (12), which involves direct measurement of the contact angle with a goniometer scale from a drop of liquid resting on a flat surface of the solid, or from a photograph of the drop profile as done in our previous work (10,13). This paper is Part III of a series of papers concerning the wetting of surfactant solutions onto precipitated surfactant solid.…”

“…In Part I (10), the contact angle of aqueous solutions saturated in the precipitated surfactant onto that precipitate was measured for a wide variety of surfactants. In Part II (13), the effect of an unsaturated surfactant in the solution on contact angle on the surface of the precipitated surfactant was studied. Specifically, a subsaturated sodium dodecylsulfate (NaDS) solution onto precipitated calcium dedecanoate or CaC 12 (representing soap scum) was shown to enhance wettability (decrease contact angle) due to reduction of vapor-liquid and solid-liquid IFT due to the NaDS.…”

Numerical investigation of heterogeneous nucleation of water vapour on PM10 for particulate abatement

“…As shown in Table , when the wetting agent concentration increases, the surface tension σ lv and the contact angle θ exhibit a general tendency of decreasing at first and then increasing. This phenomenon has been distinctly observed in previous studies and can be explained by the effect of impurities with a surface activity higher than the wetting agent . According to the classical nucleation theory, smaller surface tension σ lv and contact angle θ indicate better wettability and sequential better nucleation capability, leading to lower nucleation barrier and critical saturation ratio.…”