Adsorption of sodium dodecyl sulphate at the silicone fluid/water interface

Abstract

The adsorption of sodium dodecyl sulphate (SDS) was studied at silicone fluid/water interfaces at 297 K, as a function of siloxane chain length, surfactant concentration, in the presence of electrolytes and in mixtures with fatty alcohols or silanols. Although the interfacial tensions against water of dimethylpolysiloxanes with 2 to ∼13 siloxane units was constant at around 42.6 mNm^–1, the adsorption of SDS at the dimethylpolysiloxane/water interface depended slightly on the number of siloxane units of the fluid, i.e., the limiting surface area per SDS molecule increased from 0.551 nm² for 3 siloxane units to 0.623 nm² for ∼13 siloxane units. Similarly, changes were observed for the surface excess (decrease with number of siloxane units) and interfacial tension above the critical micelle concentration (c.m.c)(increase with number of siloxane units) but the c.m.c. did not show any systematic change with the number of siloxane units. In the presence of electrolytes, NaCl and CuCl₂, the plot of log (c.m.c.) against log [counter ions] was a straight line with a slope of 0.68. When SDS and fatty alcohols or silanols were simultaneously adsorbed at the silicone fluid/water interface there was no indication from the interfacial tension lowering of the system of molecular interaction between them.