Phase Transitions in Mixed Monolayers of Sodium Dodecyl Sulfate and Dodecanol at the Air/Water Interface

A two-dimensional phase transition has been studied in a mixed monolayer of sodium dodecyl sulfate (SDS) and dodecanol at the air/water interface by sum-frequency spectroscopy and ellipsometry. At low temperatures, a monolayer at the surface of a 6 mM solution containing 99.9% SDS and 0.1% dodecanol (w/w) is conformationally ordered and has a surface coverage comparable to that of a monolayer of pure dodecanol at the same temperature. At 16 °C there is a first-order transition to a phase that is less dense and more conformationally disordered. The high-temperature monolayer phase is more disordered than the corresponding liquid phase in pure dodecanol. The solid phase contains equimolar amounts of SDS and dodecanol while the liquid phase contains SDS and dodecanol in a ratio of approximately 3:2. The change in composition of the mixed monolayer can be understood qualitatively by a reduction in the interaction parameter in the monolayer at the phase transition.