ChemInform Abstract: NONPLANAR CONFORMERS AND THE PHASE BEHAVIOR OF SOLID N‐ALKANES

Abstract

Solidsolid phase transitions of the odd n-alkanes n-C17H36 through n-C29H60 were studied by using differential scanning calorimetry (DSC) and infrared (IR) spectroscopy. Two phase transitions were found in C25, C2,, and C29 in addition to the previously reported highest-temperature solid-solid transition (the so-called rotator transition). IR spectra revealed that, as the temperature is raised, the concentration of nonplanar conformers successively increases through each phase transition. Three types of nonplanar have been identified in the highest-temperature phase of all the n-alkanes: (gt ...), kink (...gtg'...), and (...gg...). The end-gauche defect was observed in the lower-temperature phases as well. Kink conformers, however, were observed only in the highest-temperature phase while double-gauche were found in measurable concentrations only within a few degrees of the melting point. The concentrations of nonplanar conformers in the highest-temperature phases increase with increasing chain length. For example, roughly 70% of C29 molcules are nonplanar prior to melting, in contrast to 5-10% for C17. The relationship between the existence of nonplanar conformers and the various distinct solid phases presents a major puzzle. Normal alkanes undergo a remarkable series of solidsolid phase transitions prior to melting. Analogous premelting transitions are also found in a number of more complex systems which contain hydrocarbon chains as major constituents. These systems include polyethylene,' variously substituted alkanes,2 and biological ~nembranes.~ In the latter cases, the phase changes are believed to be intimately related to the state of the hydrocarbon component. For example, current models of the lipid bilayer phase transitions require detailed consideration of the changes in conformation of the alkane chains4 Since the n-alkanes themselves are the sim- plest of these chain-molecule solids, they are a natural starting point for understanding the premelting transitions exhibited by all of these systems. The phase behavior of even the n-alkanes is complex. All odd n-alkanes between C9 and C45 undergo at least one premelting phase transition, and the longer members show no less than four high-temperature solid phases in addition to the low-temperature crystalline phases5 The nature of these phases is not well un- derstood although it is clear that the higher-temperature phases involve increasing degrees of disorder. By here we mean any deviation from the regular arrangement of all-trans chains found in the low-temperature solid. In this paper we focus mainly on one aspect of the disordering processes occurring in these solids-that involving conformational changes. We show that infrared spectroscopy is sensitive to the presence of various nonplanar (not all-trans) conformers and to some types of intermolecular order. We will find that disorder occurs in a series of apparently solid phases. The question then arises how can the apparent short-range implied by these nonplanar conformations be reconciled with the existence of a series of phases showing long-range order? This question which is fundamental to the understanding of the more complex systems such as biolipid bilayers will be taken up again in the Conclusion. The body of this paper presents the infrared spectra of the alkanes as a function of temperature with an analysis of the conformational defects seen in the various phases.