The molecular basis for sound velocity in n-alkanes, 1-alcohols and dimethylsiloxanes

Abstract

The velocity of sound has been determined in pure liquid n-alkanes (pentane to hexadecane), 1-alcohols (methanol to 1-dodecanol) and dimethylsiloxanes (200 fluid (L2, 0.65 cSt) to 200 fluid (5000 cSt)) at 20 °C. Corresponding density data have been taken from the literature and the adiabatic compressibility determined. The measured adiabatic compressibility has been compared with two molecular models of sound velocity, the Schaaffs model and a development of the Urick equation. The Urick equation approach is based on a determination of the compressibility of the methylene or siloxane repeat units which make up the chains in these linear molecules. We show that the Urick equation approach accurately predicts sound velocity and compressibility for the higher members of each series, whilst the Schaaffs approach fails for the 1-alcohols. We suggest this is because of the influence of the hydroxyl end group on nearby molecules through hydrogen bonding. The Schaaffs approach does not take into account interactions between the end units and the chain repeat units. This interaction modifies the derived compressibility of the methylene groups reducing their compressibility as compared to their compressibillity in the n-alkanes. The technique described provides valuable new insights into end-group, inter-molecular and intra-molecular interactions in liquid linear-chain molecules. We suggest that it provides a powerful new method for characterising polymeric fluid materials.