Orientational fluctuations in long chain molecules at the air/water interface were detected by secondharmonic spectroscopy. The fluctuations depend on the polarization of the incident (co) and detected (2co) light showing that the fluctuations are orientational rather than density in origin. The phenomenon is attributed to an orientational phase transition that is dependent on chain length, head group, and charge. Scaling experiments based on increasing the laser beam area indicated a correlation length (domain size) greater than 6 fim near the transition point.PACS numbers: 68.10.-m We wish to report the coverage-dependent orientational fluctuations of lipid molecules at the air/water interface, which we attribute to a heretofore unknown orientational phase transition. These lipid fluctuations were detected by the time-dependent variations (fluctuations) in the second-harmonic signal. They were shown to be orientational by the observations that the fluctuations depended on the polarization of the incident light field and that of the second-harmonic light field that was measured. The experimental results of this study bear on the organization of lipid molecules at the air/water interface, a subject that has been and remains one of intense experimental and theoretical activity [l]. A range of techniques has been used to study the various equilibrium phases and phase transitions of insoluble lipid monolayers. They include surface pressure, interface electric potential [2], light scattering [3], ellipsometry [4], fluorescence microscopy [5], x-ray and neutron diffraction, and reflection [6]. There have been a number of experimental and theoretical studies concerning orientational ordering [7] and orientational phase transitions [8]. However, it remains a difficult task to experimentally explore the relative importance of orientational and density ordering parameters, Xzll -J Ns W"jgr <*£La$L 0^) + a^icosO)] ,where 0 is the angle between the molecular symmetry axis^> and the surface normal z, and if/ is the rotation angle of the molecule about its symmetry axis^>. From Eqs.(1) and(2) we see that Xxll and Xzll differ only in the nonlinear polarizability coefficients of (cos#>, namely, a~^ andThe SH signal corresponding to these two elements, xiVx and xzll lEqs. (1) and (2)1, were measured as a function of time using a synchronously pumped picosecond dye laser and a single-photon-counting detection system [12]. At sufficiently low lipid densities, e.g., greater than 38 A 2 , the system is in the liquid-gas coex-especially since they can be coupled, in lipid phase transitions. The technique we have used in this research, second-harmonic generation (SHG) [9], is a relatively new technique sensitive to both the interface molecular density and molecular orientation. The surface sensitivity of SHG is a result of the fact that it is electric dipole forbidden in a centrosymmetric medium, such as a bulk liquid or gas, but allowed in the interfacial region where the symmetry is broken [10].In this work we report on our studies ...
