X-ray Investigations of Mixed Charge Amphiphilic Systems

The liquid vapor interface is of a fundamental importance for a wide range of applications from physics, chemistry and biology to material science and nanotechnology. Soft matter thin films composed of mixture of cationic and anionic surfactants are of great interest because of their unique interfacial properties that arise from the strong electrostatic interactions between the oppositely charged headgroups. In order to exploit these properties a better understanding of the molecular scale structure of these catanionic systems is needed. Surface sensitive synchrotron X-ray scattering techniques at room temperature were used to study structural properties of a catanionic system composed of the phospholipid PIP2 and Trimethyloctadecylammonium bromide. Through a series of surface pressure versus time and surface pressure - area isotherm measurements it was established that the catanionic mixture is more surface active compared to its individual components. Furthermore, the surface pressure - area isotherm of the mixed charge system was monotonous without any phase transition indicating that the film is in a liquid expanded phase. The monlayer stability compression-decompression cycles showed that the formed Langmuir film was stable over the course of many days. Specular X-ray reflectivity measurements determined the electron density profile in the direction normal to the interface averaged over the inplane region of the interface. X-ray reflectivity, normalized to the Fresnel reflectivity of an ideal flat interface, was fit to a model system consisting of 2 layers. The occupancy of the components obtained from the electron density profile analysis gave an estimate which was in excellent agreement with the macroscopic calculation from the molecular weight, amount deposited and available area of the film. Another independent technique, Grazing Incidence Diffraction, probed for an inplane ordering of this two dimensional structure. It showed highly ordered domains composed of untitled acyl chains with hexagonal unit cell. Bragg rod analysis of the variation of the scattered intensity along the specular direction was in agreement with the findings from the X-ray reflectivity analysis. X-ray data showed a stable ordering of this soft matter system with the same regularity as that of a solid crystal. What makes this type of ordering so special is that none of the individual components of the system on their own can form such a state but, because of their cooperative and collective behavior, it becomes possible to achieve a very stable configuration in this catanionic system.