Structure of Model Waxes:  Conformational Disorder and Chain Packing in Crystalline Multicomponent n-Alkane Solid Solutions

The interlamellar disorder in waxes is a major factor in determining their unique physical properties. We have measured by Raman and infrared spectroscopies the conformational disorder and chain packing in the interlamellar region of a number of model n -alkane waxes consisting of 2, 3, and 4 components with chain lengths in the range from 20 to 40 carbons. Most of the mixtures studied have a constant chain length difference of four carbons between their components, in which case they are crystalline solid solutions with lamellar structures similar to pure n -alkanes with orthorhombic subcells. Chain packing density is lower in the interfacial region due to the chain length mismatch. Void volume and lamellar surface roughness are reduced by the formation of gauche bonds near the chain ends and through longitudinal displacement of the chains, so that the interfacial packing resembles that found in the high-temperature hexagonal phase of pure n -alkanes. The relative degree of disordering is highly sensitive to the chain length difference but is rather insensitive to the number of components or their average chain length. We also report on (i) the distribution of disorder among the individual components, (ii) the concentration of disorder for the orthorhombic (room temperature) and hexagonal (high temperature) phases, as measured by specific conformational sequences, and (iii) the temperature dependence of disorder for the individual components of a three-component mixture as it is warmed.