Ultrasonic Relaxations in Aqueous Solutions of Alcohols and the Balance between Hydrophobicity and Hydrophilicity in the Solutes

Article abstract of DOI:10.1021/j100116a017

3-Propoxy-1-propanol was synthesized with 1,3-propanediol, anhydrous potassium carbonate, and propyl bromide, and the ultrasonic absorption coefficents in the frequency range from 6.5 to 220 MHz, sound velocity, and density were measured in the aqueous solutions at various concentrations.It was speculated that a double relaxational equation of the Debye type is appropriate to represent the absorption data in the concentration range from 1.0 up to 5.0 mol dm^-3 while a single one is fitted in those less than 1.0 down to 0.79 mol dm^-3.For explaining the ultrasonic relaxation phenomena, the analysis was carried out with a reaction model associated with solute-solvent interaction and with an aggregation reaction of the solute expressed as A + B <*> AB <*> AB^*.From the concentration dependence of the relaxation frequencies, the rate and the thermodynamic parameters were evaluated for the two reactions.The propoxy group in alcohol molecules was evaluated to have a relatively high hydrophobic character, and it plays an important role for stabilizing the water structure.From the comparison of the present results with those for other alcohol solutions reported previously, the effects of methoxyl, ethoxyl, and propoxyl groups on the water structure were systematically considered and a linear relationship was found between the standard free energy change of two-state, i.e. hydrogen-bonded and non-hydrogen-bonded, water in the solutions and the carbon number of the alkoxyl groups.It was shown quantitatively that the methoxyl group acts as a structure breaker for water, the ethoxyl group influences the water structure slightly, and the propoxyl group promotes the structure.