Adsorption of nonionic mixtures at the air-water interface: effects of temperature and electrolyte

Specular neutron reflection has been used to investigate the effects of temperature and added electrolyte on the adsorption of nonionic surfactants and nonionic surfactant mixtures at the air-water interface. For the alkyl poly-oxyethylene oxide nonionic surfactants, C(n)EO(m), the adsorption at the air-water interface is independent of temperature for surfactants with shorter ethylene oxide groups, whereas there is an increasing tendency for increased adsorption with temperature for surfactants with longer ethylene oxide groups. The addition of "salting in" (sodium thiocyanate, NaSCN) and "salting out" (sodium chloride, NaCl, sodium sulphate, Na2SO4) electrolyte results in reduced and enhanced adsorption, respectively, for C12EO8, whereas both types of electrolyte result in enhanced adsorption for C12EO12. The addition of electrolyte does not substantially alter the temperature dependence of the adsorption of the pure monolayers. For the nonionic mixtures of C12EO3/C12EO8 increasing temperature results in a surface richer in the least surface-active component, C12EO8. For the same nonionic mixture, the addition of "salting in" and "salting out" electrolyte results in an reduced and increased adsorption, respectively. The addition of "salting in" electrolyte results in a surface more rich in C12EO3, whereas for the addition of both "salting in" and "salting out" electrolyte the surface composition is essentially unaltered.