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The densities and heat capacities per unit volume were measured for the systems urea (U)-/er/-butyl alcohol (TBA)water (W) and dimethylformamide (DMF)-TBA-W at 25 °C with a flow densimeter and a flow microcalorimeter. One of the nonaqueous components was kept at a constant low concentration and whenever possible the other component was varied over the whole mole fraction range. From these data, combined with previously published data on NaCl-U-W and on Bu4NBr-U-W, volumes and heat capacities of transfer were calculated and compared with the corresponding excess functions of the binary systems to get an overall view of these ternary systems. In the water-rich region the thermodynamic properties are characterized by strong solute-W and solute-solute interactions. At the nonaqueous end the thermodynamic properties are reflecting mostly solute-solvent interactions. Through an understanding of the two ends it becomes easier to account for the various inflection points, maxima or minima in the transfer functions.Mixed aqueous solvents are used intensively in chemistry, biology, and industry to control factors such as solubility, stability of systems, and kinetics of reactions. Despite numerous studies on thermodynamic, transport, and spectral properties,211 there is still disagreement on the nature of the interactions which are responsible for the characteristic trends in the composition dependence. For example, often maxima, minima, or inflection points are observed which are then in-terpreted in terms of various effects such as structural changes in the solvent, preferential hydration, or association complexes. However, depending on the property studied, these extrema or inflection points often do not occur at the same mole fraction and depend on the nature of the solute, making many of these interpretations dubious.Much of the confusion probably comes from the limited range of most studies. A better understanding of ternary

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. Can. J. Chem. 55,856 (1977). The densities and specific neat capacities of niixtures of terr-butyl alcohol (TBA) and water (W) have been measured at 6,10, 25, 40, 55, and 65 'C. From these data the apparent lnolal volilmes (O,), heat capacities j&), and expansibilities (4,) were evaluated. Generally, the tenlperature dependence of these quantities is rather characteristic of hydrophobic solutes in water. At high co~lcentrations 6, and 4,. resemble those of surfactants, suggesting the existence of some microphase separation. At low temperatures 6, show a significant hump as a function of concentration, which is probably not due to the presence of the -OH group on the TBA molecule. A similar behaviour is observed for I+,. The 4, and $c of water in TBA have also been calculated; 4,,, is in theTBA-rich region 2 to 2.5 cm3 m o l l smaller than the molar volume of water, but & , , is of the same order of magnitude as the molar heat capacity of water. InatasductionThe numerous studies on inixtures of alcohols and water have been very well reviewed by Franks and co-workers (1-3). Most of the alcoholic systems fall in a class of solutio~ls which are generally classified as typically aqueous (3-5); the excess free energy GE is positive, I TSEI > 1 HEI (entropy-controlled mixing), the solution tends to unmix at high temperatures and the concentratioii depeildence of the thermodynamic properties shotv characteristic trends. The hydrophobic character of the cosolvent seems to be responsible for the general behaviour of these solutions.fert-Butyl alcohol (TBA) is of particular interest since, although it is quite hydrophobic, it is still miscible in all proportions with water.