Thermodynamic studies were carried out on aqueous solutions of some nonelectrolytes. The quantities proportional to the second derivatives of Gibbs energy were measured directly and in small increments in mole fraction or temperature. Therefore, we were able to differentiate once more with respect to mole fraction or temperature. Generally, the higher the order of the derivative, the more detailed the information it contains. Using these second and third derivatives, an attempt was made at elucidating the mixing schemes, the way in which solute and solvent H2O molecules mix with each other. For the following nonelectrolytes studied so far, 2-butoxyethanol, tert-butyl alcohol, 2-butanone, isobutyric acid, and dimethyl sulfoxide, there are separate regions in the single-phase domain of the temperature−mole fraction field, in each of which the mixing scheme is qualitatively different from those of the other regions. The details of each mixing scheme were elucidated from the behavior of the second and third derivatives of Gibbs energy. In the water-rich region (I), the effect of a solute was suggested to enhance the hydrogen bonds of H2O in the vicinity of the solute but to diminish the hydrogen bond probability of the bulk H2O away from the solute. When the hydrogen bond probability at a certain region of bulk water decreases to that of the percolation threshold, this mixing scheme is no longer operative, and mixing scheme II sets in. The transition from the mixing scheme of region I to that in region (II) was found to accompany anomalies in the third derivatives of Gibbs energy, in contrast to the normal phase transitions which are associated with anomalies in the second derivatives of Gibbs energy. In the intermediate region (II), it was suggested that the solution consists of two kinds of clusters rich in each component. In the solute-rich region (III), at least for the 2-butoxyethanol and dimethyl sulfoxide cases, the clusters of purely solute molecules exist and H2O molecules are “adsorbed” on the surfaces of such clusters.
We studied the hydration characteristics of room-temperature ionic liquids (IL). We experimentally determined the excess chemical potentials, , the excess partial molar enthalpies, , and the excess partial molar entropies in IL−H2O systems at 25 °C. The ionic liquids studied were 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4) and the iodide ([bmim]I). From these data, the excess (integral) molar enthalpy and entropy, and , and the IL−IL enthalpic interaction, , were calculated. Using these thermodynamic data, we deduced the mixing schemes, or the “solution structures”, of IL−H2O systems. At infinite dilution IL dissociates in H2O, but the subsequent hydration is much weaker than for NaCl. As the concentration of IL increases, [bmim]+ ions and the counteranions begin to attract each other up to a threshold mole fraction, x IL = 0.015 for [bmim]BF4 and 0.013 for [bmim]I. At still higher mole fractions, IL ions start to organize themselves, directly or in an H2O-mediated manner. Eventually for x IL > 0.5−0.6, IL molecules form clusters of their own kind, as in their pure states. We show that , a third derivative of G, provided finer details than and , second derivatives, which in turn gave more detailed information than and , first derivative quantities.
The excess partial molar enthalpy of 1-propanol (1P), , was measured at 28 degrees C in the ternary mixture of 1P-1-butyl-3-methylimidazolium chloride ([bmim]Cl)-H(2)O in the H(2)O-rich composition range. From these data we evaluated what we call the 1P-1P enthalpic interaction function, . Its changes induced by addition of [bmim]Cl of the pattern of were used as a probe to elucidate the effect of [bmim]Cl on the molecular organization of H(2)O. It was found that the effect of Cl(-) was not conspicuous within this methodology, and the observed dependence is predominantly due to the hydration of [bmim](+). The changes in the x(1P)-dependence of were compared with those brought about by temperature increase, or by the addition of fructose or glycerol. It was found that the effect of [bmim](+) is similar to that of fructose or increased temperature. We speculate that in the H(2)O-rich composition region a number of H(2)O molecules are attracted to the delocalized positive charge of the imidazolium ring and the bulk of H(2)O is influenced in such a manner that the global hydrogen bond probability is reduced.
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