Neutral molecules and a simplified treatment for ions

Abstract

Films of neutral substances adsorbed at the oil/water interface obey the equation of state and the adsorption isotherm which are derived assuming complete mobility of the molecules in the interface. The equations are obeyed down to areas of ~ 30A 2 per molecule; deviations which occur below this value result from interactions between the adsorbed molecules. Adsorbed films of completely ionized sodium alkyl sulphates and quaternary am m onium compounds also obey these equations when allowance is m ade for the electrical contribution to the adsorption energy and for electrical interaction between the ions in the aqueous bulk phase. The activity coefficients cannot be neglected in these systems at concentrations exceeding 0.001 m. In the range 0.001 to 0.09m the ‘complete’ Debye-Huckel equation is used to calculate these coefficients; at the higher concentrations (for sodium decyl and octyl sulphates) the activity coefficients thus calculated are suspect. The electrical energy is given exactly by a modified Gouy equation, when allowance is made for the film ions being situated a finite equilibrium distance from the interface and for the fact that counter-ions will be present above the head groups. This modified equation yields linear plots for the isotherm and leads to consistent values for the free energy of adsorption of the molecules. It also ( a ) demonstrates th at the electrical potential at the phase boundary (jrd) differs from that in the plane for the film ions (i/r0) and th at the true jr0 is lower than th at calculated from the simple Gouy equation; from the m easured surface potentials (AF) interpreted by the equation of Schulm an & Hughes (1932), it is shown th at the calculated is experim entally confirmed; ( b ) predicts much more accurately than the simple equation the relation between the observed surface pressure (17) and area per molecule (equation of state). Deviations from the equations occur for short-chain compounds (C8 and C 10) in w ater at high concentrations, and for all surface-active agents in high electrolyte concentrations (> m/100), especially where A < 70A 2 . It is concluded that these deviations result m ainly from a neglect of ionic size in the theoretical treatm ent. There is no evidence in any of the systems for any specific interaction between counter ions and adsorbed film ions. The experimental results are inconsistent with the view that appreciable energy changes associated with dehydration of the surface-active ions occur as they approach the oil/water interface.