Properties of monolayers at the air/water interface. Part 2. - Effect of temperature on the double layer and water dipole contributions to the surface potential of sodium octadecyl sulphate

Accurate surface potentials measured by the radioactive electrode method are given for monolayers of sodium octadecyl sulphate spread on several aqueous sodium chloride solutions. In the temperature range 5-18°C, the magnitude of the surface potential increases with temperature for all salt solutions. This is interpreted as showing that increasing temperature disorients water dipoles more at the clean air/water interface than at the film-covered interface for long-chain sulphate over the temperature range studied. It is not possible from the data to determine uniquely the orientation of water molecules at the clean air/water interface, but it is almost certain that they are arranged with the oxygen atoms pointing towards the air phase. The general breakdown of the Gouy-Chapman model of the ionic double layer is confirmed at all temperatures. The Stern model as modified by van Voorst Vader and van den Tempel cannot explain the data. The classical Stern adsorption isotherm covers the results, giving Stern adsorption energies for the sodium ion which are negative (except possibly at very high charge densities) and vary significantly with salt concentration but do not vary with temperature and monolayer density in the area range 300 to 1200 Ų molecule^-1.