A thermodynamic and solvatochromic investigation of the effect of water on the phase-transfer properties of octan-1-ol

The octan-1-ol-water partition coefficient has played a central role in quantitative structure-activity relationships and has been applied to drug design, to model biological membranes, and to predict toxicity and transport of pollutants. The room temperature solubility of water in octan-1-ol is determined to be 2.5 mol dm^-3, which is equivalent to a mole fraction of 0.29. Despite this very high solubility very little is known about the effect of water on the chemical properties of the octan-1-ol phase. We have used head-space gas chromatography to measure the free energy of transfer of 14 chemically very different solutes, from neat octan-1-ol to octan-1-ol saturated with water. In addition, the infinite dilution activity coefficients of a series of 11 alcohols were measured in both phases. In general, the presence of water in octan-1-ol increases the concentration of polar and hydrogen-bonding solutes and decreases the concentration of non-polar solutes in this phase. For all solutes, the effect of water on the change in the standard free energy of transfer was less than 200 cal mol^-1.† These small changes were validated by investigating the effect of added water on the solvatochromic properties of the indicators N,N-diethyl-p-nitroaniline, p-nitroanisole, p-nitroaniline, p-nitrophenol, ET-30, bis[α-(2-pyridyl)benzylidene-3,4-dimethylaniline] bis(cyano)iron(II), Phenol Blue, and Nile Red in octan-1-ol. Regardless of the indicator used the addition of water had almost no effect on the wavelength of maximum adsorption. These results show that the water is almost completely associated with octan-1-ol and scarcely effects the properties of the media.