The balance of different influences on adsorption such as electrostatic attaction to an interface, repulsion between charged adsorbates and solven interactions provides the rich variety of behaviour observed with surfactants. This proposal seeks to provide an explanation of this phenomenon by studying systematically the effects of pH and added salt on aqueous solutions of hexadecyl trimethyl ammonium bromide (C16TAB) against sapphire substrates. The new data will allow sensitive tests of new models of adsorption that depend on ionic association and surface potential close to the critical micelle concentration.