We present x-ray diffraction results of dipalmitoylphosphatidylcholine (DPPC) multilayers in three structural phases. Using pure DPPC, precision temperature control, and high angular resolution methods, we have discovered splitting of the first diffraction order due to multilayering in the P(beta) phase. This splitting permits us to calculate the amplitude of ripples in this phase. The amplitude is large enough to suggest a structural mechanism for rippling.

No abstract

We report results from x-ray experiments on mixtures of N-(p-hexyloxybenzylidene)--aminobenzonitrile (HBAB) m N-^-cyanobenzylidene-^-?2-octloxyanilitie (C BOO A) 5 very recent microscopic observations on mixtures of cyanohexyloxybiphenyl in cyanooctylojgrbiphenyl are m agreement with these results. The evidence suggests that the reentrant nematic phase is similar to the classical nematic phase but may coexist with crystalline fluctuations. It has been shown recently^ that mixtures formed with two liquid crystals could exhibit with decreasing temperature the following sequence of phases: nematic, smectic A, and then again nematic. The nematic phase which occurs at low temperature is called the reentrant nematic phase (Fig. 1). The same phenomenom has also been observed with pure compounds under pressure.^ By increasing the pressure from the nematic phase, we observe the smectic phase and then again a nematic phase at high pressure. The nematic phase which occurs at high pressure is also called the reentrant nematic phase. All these results have been obtained using the light microscope.A model has been proposed^ for the organization of the molecules in this reentrant nematic phase. In this model, the molecules are assumed to be associated in antiparallel pairs in the smectic-A phase, which then can be destabilized as shown in Fig. 2.Here we present for the first time the results of an x-ray investigation of this unusual phase, and also very recent results of a microscopic study with a new mixture of thermotropic liquid crystals (Fig.

By means of X-ray diffraction we have observed the tilting of hydrocarbon chains within a dipalmitoyl phosphatidylcholine multilayer. The tilting produces a distribution of intensity in the (4.2 A)-1 region that provides a quantitative measure of the angle of tilt. Detailed diffraction investigation of multilayers prepared by a recently developed method and mounted to form a "freestanding" sample has made possible the accurate measurement of intensity in this region. We find the intensity distribution to be different from that previously reported and proceed to develop a new model for tilted chains. The model permits calculation of the intensity distribution as a function of tilt angle which is then fit to the data.

No abstract