MICROPHASE SEPARATION IN BULK AND SURFACE-ADSORBED BINARY ALKANE SYSTEMS. E.P. Gilbert, P.A. Reynolds and J.W. White, Research School of Chemistry The Australian National University Canberra. ACT 0200. AUSTRALIA.

Certain binary mixtures of n-alkanes, when quenched from the melt to room temperature, undergo spontaneous demixing from the solid solution to form a lamellar microphase¹. In this 'bulk' system, demixing is essentially longitudinal, the molecules slipping along their long axes to separate into lamellar components which neutron scattering can easily detect when isotopic substitution of one of the components is employed².

STM studies have shown that when these mixed hydrocarbons are spread onto a clean graphite surface from a non-polar solvent, only the longer chain is imaged. This preference indicates a strong demixing enthalpy in the two-dimensional layer.

We have investigated microphase formation in binary C_n:C₃₆ systems (20<n<34) not only in the bulk but also in which the two-component protonated/deuterated melt is quenched inside the gallery structures present in exfoliated graphite. In the latter system, it is possible for phase separation not only to occur in the longitudinal direction, as in the 'bulk', but also in a transverse direction due to the graphite surface forces.

We report here how the microphase structure and its rate of formation are determined by the presence of the graphite substrate, alkane composition, isotopic labelling, quench temperature and chain-length mismatch³.

¹ Mazee, W.M. Prepr. - Am. Chem. Soc. Div. Pet. Chem. 3 (1958) 35.

² White, J.W.; Dorset, D.L.; Epperson, J.E.; Snyder, R.G. Chem. Phys. Lett. 166 (1990) 560.

³ Gilbert, E.P.; Reynolds, P.A.; White, J.W. to be submitted to Mol. Phys. (1996).