Mixing of silicone oils with monolayers of nonionic surfactant at the air/water surface

Abstract

We have investigated the effects of adding small quantities of liquid silicone oils of the polydimethylsiloxane (PDMS) type to the surfaces of aqueous solutions of nonionic surfactants of general structure H(CH₂)_n(OCH₂CH₂)_mOH (abbreviated to C_nE_m). A range of surfactant head and chain lengths has been studied (n = 8, 10, 12, 14 and 16 and m = 2, 3, 4, 5 and 6). For solutions above the c.m.c. and with the 50 cS oil, placing surfactant in oil causes the oil to spread rapidly on all surfaces; otherwise spreading is slower and lenses form initially. In every case the surface tension is lowered by an amount, Δγ, due to adsorption, and with measured values of the oil/water tension, the spreading coefficient has been calculated. Initial spreading coefficients are all positive whilst equilibrium coefficients lie around zero. Δγ is weakly dependent on surfactant alkyl chain length but increases with surfactant head group size in line with the increased area per molecule spacing the chains. The complete adsorption isotherms of the volatile PDMS oils on monolayers of C₁₂E₅ above the c.m.c. have been measured by adsorption from the vapour. At unit activity of oil, the mixed layer contains ca. equal numbers of oil and surfactant molecules. For C₁₂E₅, low viscosity oils spread whilst high viscosity oils spread partially lowering the tension but an oil lens remains at long time. Opening up the monolayer of C₁₂E₅ by dilution below the c.m.c. results in displacement of surfactant from the surface by PDMS. By comparison, no mixing occurs on layers of a silicone nonionic surfactant. The extent of solubilisation of PDMS into aqueous aggregates of these two surfactants follows the same trend as that found at the planar surface.