Vibrational Spectra, Orientations, and Phase Transitions in Long-chain Amphiphiles at the Air/Water Interface: Probing the Head and Tail Groups by Sum Frequency Generation

IR + visible sum frequency generation (SFG) has been used to obtain the vibrational spectra and orientations for both the nitrile (CN) head group and the methyl (CD3) tail of the insoluble long-chain amphiphile CD3(CH2)1&N at the aidwater interface. The results show that the orientations of the head group and the terminal group vary with amphiphile surface density, but in markedly different ways. It is found at the amphiphile density, which corresponds to the phase transition from the gasfliquid coexistence region to the liquid region, that the hydrogen bonds of the CN head groups to water are broken, the water is squeezed out of the monolayer, and the orientation of the head group changes sharply. The possibility that the rupturing of the hydrogen bonds and reorientation of the CN head groups may be the trigger for the phase transition is discussed. In addition to the SFG study of the amphiphile at the aidwater interface, a method was discovered that enables SFG to remove the ambiguity in the ratio of the two Raman polarizability elements for a symmetric stretch. Depolarization measurements in linear Raman experiments cannot determine whether the ratio of elements is greater than or less than one, whereas the SFG method can.