MORPHOLOGY OF ORTHORHOMBIC LONG-CHAIN NORMAL ALKANES - THEORY AND OBSERVATIONS

This paper describes a theoretical and experimental investigation of the growth and especially the morphology of paraffin crystals of the odd n-alkanes (CnH2n+2, n not-equal 2n) over a range of chain lengths with 15 less-than-or-equal-to n less-than-or-equal-to 41, for growth from solution, under pure conditions and under the influence of additives. Three strong and one weak interactions between the bodies of paraffin molecules in a crystal and four weak interactions between the head and the tails of the paraffin molecules are identified and the crystal graph is derived. The elementary cell of this crystal graph is characterized by a pseudo F cell with first nearest neighbour interactions between the points. From this crystal graph ten connected nets giving rise to eight (or nine) different F faces are identified. Using different quantities like E(hkl)att, E(hkl)slice, Ising temperatures (theta(hkl)c), interplanar distance, etc. for the connected nets of paraffin crystals, growth and equilibrium forms are constructed. All crystal forms show large {001}, small thin faces {110} and often smallest faces {010}. It is shown that predicted and observed morphologies, agree quite well. In passing the sometimes dramatic change in the expected habit of crystals, caused by tailor made additives is discussed. The theory of roughening transition is applied to paraffin and it is shown how the roughening temperature depends on n, the length of the paraffin chain. An actual roughening temperature of 10.65+/-0.50-degrees-C is measured for faces of the form {110} of paraffin with 23 C atoms, growing from a slightly supersaturated solution of hexane.