X-ray scattering study of water intercalation into thin lyotropic multilayers. I. Molecular ordering and distinct states of hydration

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Abstract
We present an x-ray scattering study of the intralayer molecular, and the lamellar ordering, in thin substrate-deposited multilayers of the phospholipid dipalmitoylphosphatidylcholine, maintained in a dry or water-saturated helium atmosphere. Unexpectedly, we find that, in the regime of low hydration, water adsorption and desorption do not conform to the widely held notion of a continuous swelling process, but instead occur in discrete steps. As a result, a series of distinct states of hydration appears. These are homogeneously hydrated states of the multilayer in which an integral number of water monolayers is associated with each bilayer. Disordered intermediate states arise in the multilayer in the course of water intercalation as a consequence of the coexistence of two such hydration states. In the present and the accompanying article we show how a quantitative theory of x-ray scattering, developed on the basis of this simple and physically appealing picture, provides a unifying account of a wide variety of observations when applied to the analysis of the lamellar, or (00l) scattering line shapes. This analysis permits conclusions pertaining to the type of layer sequences adopted in the disordered intermediate states. Inspection of the intralayer ordering reveals that throughout the regime of low hydration (≲15% water by weight) examined here the multilayers are in a state consistent with that of tilted hexatic phases.