Journal of Colloid and Interface Science 247, 404–411 (2002) doi:10.1006/jcis.2001.8042, available online at http://www.idealibrary.com on Adsorption of Nonionic Mixtures at the Air–Water Interface: Effects of Temperature and Electrolyte J. Penfold, ∗, 1 E. Staples,† I. Tucker,† L. Thompson,† and R. K. Thomas‡ ∗ ISIS Facility, CLRC, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, U.K.; †Port Sunlight Laboratory, Unilever Research, Quarry Road East, Bebington, Wirral, U.K.; and ‡Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, U.K. Received January 9, 2001; accepted October 15, 2001 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 sul- phate, Na 2 SO 4 ) electrolyte results in reduced and enhanced ad- sorption, respectively, for C 12 EO 8 , whereas both types of electrolyte result in enhanced adsorption for C 12 EO 12 . The addition of elec- trolyte does not substantially alter the temperature dependence of the adsorption of the pure monolayers. For the nonionic mix- tures of C 12 EO 3 /C 12 EO 8 increasing temperature results in a surface richer in the least surface-active component, C 12 EO 8 . For the same nonionic mixture, the addition of “salting in” and “salting out” electrolyte results in an reduced and increased adsorption, respec- tively. The addition of “salting in” electrolyte results in a surface more rich in C 12 EO 3 , whereas for the addition of both “salting in” and “salting out” electrolyte the surface composition is essentially unaltered. C  2002 Elsevier Science (USA) Key Words: mixed surfactants; adsorption; interfaces; reflectivity. INTRODUCTION The nature of the adsorption of surfactant mixtures at inter- faces is important, in that most of the domestic, technological, and industrial applications of surfactants involve mixtures. In the context of such applications the nonionic alkyl poly-oxyethylene oxide surfactants are an important class of surfactants. In the diverse applications such surfactant mixtures are required to op- erate in an environment of varying solvent quality, due to tem- perature or different electrolytes and electrolyte concentrations. There is relatively little information in the literature on the effect of varying solvent quality on the adsorption of surfactants (1) or mixed surfactants (2) at interfaces. 1 To whom correspondence should be addressed. We have previously shown that specular neutron reflection, in combination with H/D isotopic substitution, is a suitably sensitive technique for the study of the adsorption of mixed surfactants at interfaces (3) over a wide range of surfactant con- centrations. We have exploited that sensitivity here to study the effect of temperature and added electrolyte on nonionic surfac- tant adsorption at the air–water interface. We report here an investigation of the effects of temperature and the addition of “salting in” and “salting out” electrolyte on the adsorption of pure nonionic surfactants at the air–water inter- face, from C 12 EO 3 to C 12 EO 12 . Extending some earlier reported results on the adsorption of the C 12 EO 3 /C 12 EO 8 mixture at the air–water interface (3, 4), we have also investigated the effect of temperature and electrolyte on the surface composition of the same nonionic surfactant mixtures. We have previously reported some preliminary results on the effects on temperature on the adsorption of nonionic surfactants and nonionic surfactant mix- tures (5). The purpose of this paper is to compare the effects of temperature and electrolyte, as different means of changing solvent quality, on nonionic surfactant adsorption. EXPERIMENTAL DETAILS The specular reflection of neutrons provides information about inhomogeneities normal to the surfaces, and has been de- scribed in detail elsewhere (6). The essence of a neutron reflec- tion measurement is that the variation on specular reflection with Q, the wave-vector transfer normal to the surface, and defined as Q = 4π sin θ/λ, where θ is the glancing angle of incidence and λ the neutron wavelength, is simply related to the compo- sition and concentration profile normal to the surface. In the kinematic approximation (7), the specular reflectivity, R( Q), is given by R( Q) = 16π 2 Q 2 |ρ ( Q)| 2 , [1] where ρ ( Q) is the one-dimensional Fourier transform of ρ (z ), the average scattering length density profile normal to the 404 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.