INTERMOLECULAR INTERACTIONS IN TERT-BUTYL ALCOHOL DIMETHYL-SULFOXIDE H2O - CHEMICAL-POTENTIALS, PARTIAL MOLAR ENTROPIES AND VOLUMES

The excess chemical potentials, the excess partial molar entropies, an d the partial molar volumes in tert-butyl alcohol (TBA)-dimethyl sulfo xide (DMSO)-H2O mixtures were determined. These data, together with pr eviously published excess partial molar enthalpies (Fluid Phase Equili b. 1997, 136, 207) were used to evaluate intermolecular interactions. The TBA-TBA and TBA-DMSO, and DMSO-DMSO interactions were found to be crucially dependent on the composition. The net interaction in terms o f chemical potential is very intricate. For example, net interactions of DMSO with a hydrophobic moiety (represented here by TEA) change fro m attractive to repulsive as the composition changes. This suggests th at general discussions of the affinity of DMSO for nonpolar groups (or surfaces) are meaningful only by specifying the composition region. T he interactions in terms of enthalpy and entropy are an order of magni tude larger and strongly compensating. Anomalous changes in the enthal pic/entropic interactions and hence qualitative changes in the mixing scheme of the solution, previously described in respective binary TBA- H2O and DMSO-H2O systems, are also apparent in this ternary system. II was found that as the mole fraction, x(D), of DMSO (third component) increases, the transition in mixing scheme occurred at a progressively lower value of x(B). The behavior of partial molar volume indicated t hat as x(B) increases, the initial increase in the partial molar volum e of H2O on increasing x(D), reminiscent to ''iceberg formation'', dim inished. This suggests that existing TEA molecules already made their contribution to the ''iceberg formation''. The DMSO-DMSO interaction i n terms of volume also showed that the transition occurred at a smalle r value of x(D) than that for x(B) = 0. The boundary between the two m ixing schemes in the present ternary mixture was a straight line in th e x(D)-x(B) field, suggesting that the effect of TEA and DMSO on H2O, causing the transition in the mixing scheme, is additive.