Thermodynamics of n-alkane + dimethylsiloxane mixtures. Part 4.—Surface tensions

Abstract

Surface tensions have been measured by the differential capillary rise method under orthobaric conditions for binary mixtures of four n-alkanes (n-pentane, n-heptane, n-decane and n-tetradecane, replaced in the case of dimer by n-hexadecane) with four linear dimethylsiloxanes (dimer, trimer, tetramer and pentamer) at 303.2 K. The sign and magnitude of the excess surface tension depend ultimately upon the chain lengths of the components of the mixtures irrespective of whether a volume-fraction-based or a mole-fraction-based ideality is adopted. The dependence of the volume-fraction-based excess surface tension on chain length is very similar to that previously found for the excess volume. The results are analysed in terms of Prigogine's parallel-layer theory as modified by Gaines and Prigogine's average potential theory. Only the latter gives a useful description of the complexity of the chain-length dependence of the excess surface tension, and it is qualitative at best even then.