Apparent molal volumes of n-alkylamine hydrobromides in water at 25°C: hydrophobic hydration and volume changes

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Abstract
The densities of aqueous solutions of the homologous salts RNH3Br, where R varies from H to n-octyl, have been measured at 25 °C to the sixth decimal place by a float method. From this series, it is possible to study the transition in behavior of typical simple salts to long-chain micellar salts. The apparent molal volumes extrapolated to infinite dilution can be used to evaluate the additivity of the volume of the methylene groups. The concentration dependence of the apparent molal volumes and the change in volume during micelle formation can be interpreted in terms of a competition between the increase in ice-likeness of water and loss of free space near the solute during hydrophobic hydration. The present results show no indications of dimerization of the cations below the critical micelle concentration.