Abstract
Structures of Langmuir mono layers of cholesterol, ganglioside GM1, and their 1: 1 (in mole) mixture at the air/water interface have been investigated at 20°C by grazing incidence X-ray diffraction (GIXD). Film balance experiments show that cholesterol forms a condensed monolayer whereas the GMl monolayer undergoes a first-order phase transition from an expanded fluid to a condensed phase. The mixture takes a more condensed state at 30 mN/m than that ·expected for the ideally mixed one, indicating the mutualphilicity between two components. The GIXD patterns exhibit for all the monolayers that the film molecules were in the hexagonal rotator phase. Molecular cell dimensions, molecular areas, and lateral and vertical coherence lengths are presented. The molecular area of cholesterol estimated from GIXD almost coincides with that obtained from the isotherm. For GM1, it is suggested that a portion of long hydrophobic chain takes part in crystallization in the monolayer because of the large and electrostatically repulsive head group, and this packing structure would make a difference between the molecular area obtained from the isotherm and that from the GIXD. The mutualphilicity is also indicated by the diffraction experiment for the mixed monolayer. However, the mixing remarkably reduces the crystallinity of the monolayer.
