Monolayer orientational fluctuations and a new phase transition at the air water interface detected by second harmonic generation

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Abstract
Fluctuations in the second harmonic intensity were observed in a number of insoluble amphilphiles spread at an air/aqueous interface. The fluctuations, which depend on the polarization of both the incident light and the second harmonic light generated, indicate a new phase transition for which the orientation of the head group is the order parameter. It is found that the phase transition occurs in certain long chain aromatic species but not in others of the same chain length with different head groups. The phase transition is identified as a weak first order transition. Agreement is obtained with a modified Landau–Ginzburg equation in which the chain–chain interactions provide a friction for the motions of the head groups.