Phase Characteristics of 1-Monopalmitoyl-rac-glycerol Monolayers at the Air/Water Interface.

Abstract

1-Monopalmitoyl-rac-glycerol is omnipresent in numerous biological and applied systems. Systematic GIXD measurements of 1-monopalmitoyl-rac-glycerol monolayers are carried out over a large pressure interval at 5, 10, and 15 °C to construct the phase diagram on the basis of reliable 2D lattice structures. These studies are complemented by other monolayer characteristics, such as π-A isotherms and mesoscopic domain topographies. A phase transition is found between the two orthorhombic structures with NN and NNN tilted alkyl chains at low temperatures (5 and 10 °C). It increases linearly with increasing temperature. With a further increase in temperature to 15 °C, only NN-tilted orthorhombic lattices are observed in the whole pressure region. The cross-sectional area, A0, is less affected by surface pressure and temperature and amounts to values of between 19.7 and 19.8 Å(2), as expected for a rotator phase at the lower limit. The tilt angle t with respect to the surface normal decreases with increasing pressure and is only slightly influenced by the temperature. The transition pressure to untilted alkyl chains, as determined by the extrapolation of 1/cos(t) to zero tilt angle, is >50 mN/m for all temperatures. The results of lattice distortion d versus sin 2(t) suggest for 10 and 15 °C the tilt of the aliphatic chains as the reason for the monolayer lattice distortion whereas at 5 °C the nonzero-tilt-angle intercept d0 could be an indication of the prevention of hexagonal packing. The generic π-T phase diagram of racemic monoacylglycerol monolayers is constructed on the basis of the phase diagrams of 1-monopalmitoyl-rac-glycerol and 1-monostearoyl-rac-glycerol, which shows that for 1-monopalmitoyl-rac-glycerol monolayers the oblique phase can occur only close to and below 0 °C. The possible phase behavior of other racemic monoacylglycerol monolayers with alkyl chain lengths of C14 and C20 is discussed on the basis of the generic phase diagram.