X-ray and neutron diffraction have been used to investigate the formation of solid crystalline monolayers of all of the linear carboxylic acids from C(6) to C(14) at submonolayer coverage and from C(8) to C(14) at multilayer coverages, and to characterize their structures. X-rays and neutrons highlight different aspects of the monolayer structures, and their combination is therefore important in structural determination. For all of the acids with an odd number of carbon atoms, the unit cell is rectangular of plane group pgg containing four molecules. The members of the homologous series with an even number of carbon atoms have an oblique unit cell with two molecules per unit cell and plane group p2. This odd-even variation in crystal structure provides an explanation for the odd-even variation observed in monolayer melting points and mixing behavior. In all cases, the molecules are arranged in strongly hydrogen-bonded dimers with their extended axes parallel to the surface and the plane of the carbon skeleton essentially parallel to the graphite surface. The monolayer crystal structures have unit cell dimensions similar to certain close-packed planes of the bulk crystals, but the molecular arrangements are different. There is a 1-3% compression on increasing the coverage over a monolayer.
Solid monolayer formation by all the linear carboxylic acids from C 6 to C 20 adsorbed from their liquids to a graphite surface is demonstrated. In addition, we present the two-dimensional phase behaviour of linear monocarboxylic acid mixtures adsorbed on graphite from their liquid mixtures, determined using differential scanning calorimetry. All acid mixtures with alkyl chains that differ by two or three methylene groups (Dn ¼ 2 or 3) exhibit a significant degree of phase separation in the monolayer. Generally, the mixing tendency increases with increasing alkyl chain length for a given Dn, and with more similar chain lengths (e.g. Dn ¼ 1). We report neutron diffraction data that confirms the formation of solid acid monolayers. This structural data also allows us to compare the mixing results with a recent quantitative model for 2-D mixing. The general form of the observed behaviour agrees well with the model, although the characteristic parameters that separate complete mixing, partial mixing and phase separation are different from those found with alkane and alcohol monolayer mixtures.
There is a continuing interest in "2D solids" which appear at various interfaces for their novel structure and dynamics. Many simple molecules are adsorbed on solid surfaces, such as graphite, to form 2D layers that are solid at low temperatures. Interestingly, similar solid monolayers are formed at the interface between bulk liquid adsorbates and graphite. In this work, we report a 2D-like ice structure which appears in the initial stage of crystallisation of aqueous solutions from the undercooled state. We present results obtained by neutron scattering to demonstrate the formation of this novel structure and its temperature evolution.
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