Model Orientation Dependent Pair Distribution Functions Describing the Association of Simple Carboxylic Acids and of Ethanol in Aqueous Solution

Abstract

Nuclear magnetic relaxation rates of the CH proton of formic acid and of the methyl and methylen protons of acetic, propionic, and butyric acid and of ethanol are reported. All these substances are dissolved in water (D2O) and the organic molecules are such that the hydrogen nuclei whose relaxation is not studied are deuterons. The corresponding deuteron relaxation rates (in H2O) have also been measured and the results are converted to yield the proton intramolecular relaxation rates and the intermolecular relaxation rates. Lastly, intermolecular proton relaxation rates which are caused by the interaction with protons in other groups have been measured. All these data are given as a function of the acid mole fraction x2. From these experimental results first coordination numbers of the acids with respect to the acid and closest distances of approach between the various hydrocarbon groups have been derived. Model molecular pair distribution functions have been constructed which show that propionic acid and butyric acid molecules undergo a change in the relative pair configuration as more and more water is added such that a side by side configuration of the hydrocarbon rests is produced. In the ethanol water mixture the structural change is the reverse one and here no hydrophobic association is observable.