An infrared‐spectroscopic study on the state of water in alcohols and in inverted micelles

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Abstract
The shifts of the antisymmetric stretching vibration bands of water (σ3) and of the ionized carboxylate group (σcoo) in the fundamental infrared region has been used to investigate the properties of water when mixed with pentanol or decanol and when engaged in the association to inverted micelles in the same solvents at 298 K. The systems studied were water + sodium n‐octanoate in pentanol and decanol, respectively. In order to identify the states occupied by water a method was developed to resolve the observed average wave number into the contribution of the different states. The Boltzmann‐distribution is utilized to give a rough estimate of the amount of water present in each state. When water is dissolved in pentanol it is initially bound to strong complexes with the alcohol molecules. In the high concentration range a part of the water seems to be dispersed free in the solvent. Diluted in decanol all the water seems to be engaged in intermolecular complexes characterized by strong hydrogen bonds. Present as hydration water of the ionic groups of the surfactant monomers the water is bound with an average strength resembling that of liquid water. The micellization seems to be initiated in all solutions investigated when the amount of hydrated surfactant is sufficiently high. Independent of the relative amount of water present the sodium ions seem then to be partly released from the strong ionic interaction found in the premicellar solutions. On the other hand, the fraction of water available is of prime importance for the state of the carboxylate group.

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