Pressure-induced changes in molecular conformation in liquid alkanes

Abstract

LIQUID linear alkanes at room temperature and pressure exist in a mixture of molecular shapes. The n-heptane molecule, for example, has 13 possible distinct conformations with up to four gauche bonds distributed along its length. In the crystal, however, all the molecules assume the all-trans, straight chain shape. We have studied the conformations of chain molecules in the liquid state in various conditions using Raman scattering as a probe1. We have looked particularly at what happens to the shapes of linear alkane molecules subjected to high pressures in the liquid state. We speculated that as the pressure on a liquid of chain molecules was increased toward the freezing point that the chains might begin to straighten out. In fact, we found that an increase in pressure caused an increase in the number of gauche bonds, that is, pressure caused the molecules to become more globular.