HYDROPHOBIC INTERACTION - EXCESS THERMODYNAMIC PROPERTIES OF NORMAL AND HEAVY-WATER SOLUTIONS OF TETRABUTYLAMMONIUM BROMIDE

Based on differential vapour pressure measurements, excess free energi es, excess enthalpies and excess entropies of the solvent in normal an d heavy water solutions of n-Bu(4)NBr have been determined over wide t emperature and concentration ranges. Solvent isotope effects on the ap parent and partial molar volumes were also measured. The extremes - ob served with the partial molar volumes at 0.8 mol and with the excess e ntropies and enthalpies at 1.7 mol - suggest that in moderately dilute solutions a ''cage''-structure is formed with a composition of n-Bu(4 )NBr .(63+/-6)H2O, which is transferred with increasing concentration to a ''clathrate-like'' structure corresponding to the solid clathrate composition, i.e., n-Bu(4)NBr . 33H(2)O. Separation of the solvent-so lvent contribution in the excess entropy shows that while it dominates the behaviour of the dilute solution, its contribution rapidly decrea ses with increasing concentration.