Mixing in cationic surfactant films studied by small-angle neutron scattering

Abstract

Structure and composition of cationic surfactant films surrounding water nano-droplets in water-in-oil microemulsions has been studied by contrast variation small-angle neutron scattering (SANS). To arrive at a robust fit, and yield reliable values for structural parameters, the I(Q) curves for complementary contrasts were fitted simultaneously by a core–shell particle scattering law, as described before (J. Chem. Soc., FaradayTrans., 1996, 92, 65). The mass densities of solvents, and the surfactant layers in the microemulsions, were measured and used to calculate the scattering length densities ρ. Given known concentrations and scattering length densities, the fitted parameters were the water core radius R_c, the polydispersity p, the effective film thickness t and its scattering length density ρ_film. This latter value was used to probe the film compositions with two specific aims: (a) to ascertain the extent of alkane penetration into the curved monolayers, and (b) to measure the composition of a mixed surfactant layer. In the first case the surfactants were di-chain n-alkyl-n-dodecyldimethylammonium bromides (Cn–C12), with the first chain n C12 to C18. The effect of alkane structure, and molecular volume V_m, on the oil penetration was investigated with n-heptane (n-C7, V_m ≈ 240 ų and cyclohexane (c-C6, V_m ≈ 180 ų). Apparently, heptane is essentially absent from the layers, but cyclohexane has a greater penetration, especially as the surfactant chain lengths become more unequal. The maximum mixing was for C18–C12, where the film contained approximately 8% by volume of cyclohexane. These results suggest that oil penetration is a subtle effect, which clearly depends on the chemical structures of both surfactant and oil. The findings for microemulsions are compared with related experiments on single chain cationic surfactants and n-dodecane at air/water interfaces (J. Phys. Chem., 1992, 96, 10971). This SANS contrast method was also applied to determine compositions of mixed surfactant films, containing blends of deuteriated di-dodecyldimethylammonium bromide (d-DDAB) and protonated n-dodecyltrimethylammonium bromide (h-DTAB). The fraction of h-DTAB in the layers, obtained either by model fitting for ρ_film, or using a model independent approximation based on the absolute SANS intensities, was consistent with ideal mixing in the interface. These results demonstrate the utility of SANS for investigating complex films at oil/water interfaces.