Colloids and Surfaces A: Physicochem. Eng. Aspects 261 (2005) 65–73 The surface tension of aqueous solutions of cetyltrimethylammonium cationic surfactants in presence of bromide and chloride counterions G. Para, E. Jarek, P. Warszynski ∗ Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Cracow, Poland Received 9 July 2004; accepted 2 November 2004 Available online 19 January 2005 Abstract An improved model of ionic surfactant adsorption was applied for description of CTABr (cetyltrimethylammonium bromide), CTACl and CTAHSO 4 surfactants adsorption from their aqueous solutions containing KCl and KBr at liquid/air interface. The model is based on the assumption that the surfactant ions and their counterions undergo non-equivalent adsorption within the Stern layer. The surface tension isotherms for various concentrations of surfactant ions and Br - and Cl - counterions were calculated and compared with the results of surface tension measurements performed by the pendant drop shape analysis methods. The dependences of surface excess values of all ions on CTA + /Br - /Cl - bulk concentrations were calculated. Obtained results showed much larger affinity to the interface of Br - anions than Cl - anions. This was explained in terms of a larger repulsion of chloride anions from the interface than bromide anions. It may be caused by the difference in their excess polarizability and the van der Waals interaction with the interface. © 2004 Elsevier B.V. All rights reserved. Keywords: Adsorption; Surface tension; Ionic surfactants; Counterions; Electric double layer 1. Introduction Surface tension of aqueous solutions of surfactants, either ionic or non-ionic, in the presence of added salt can provide useful information on their adsorption at the air solution inter- face. Explanation of the mechanism of adsorption is of impor- tance for a variety of practical phenomena such as thin film stability, micellisation, foamability, etc. However, theoreti- cal description of adsorption mechanism of ionic surfactants is very complicated because of multicomponent nature of such systems. They contain surfactant ions, counterions and ions of salt added to control ionic strength of the solution. The successful theoretical model should take into account electrical interactions between all ions and should describe at least semi-quantitatively the electric double layer (EDL) structure in such multicomponent systems. Due to the com- plexity of the problem there have been only a limited number ∗ Corresponding author. Tel.: +48 12 4252814; fax: +48 12 4251923. E-mail address: ncwarszy@cyf-kr.edu.pl (P. Warszynski). of attempts to formulate theoretical models of ionic surfac- tant adsorption. In general the theoretical models are based on thermodynamics concept of the Gibbs dividing surface, in which the surface excess of any ion is the sum total of its excesses in the actual monolayer and in the electrical double layer [1,2]. The model proposed by Davies and Rideal [3], then extended by Borwankar and Wasan [4] exploited the simple Gouy-Chapman model of the electrical double layer and assumed that surfactant ions adsorb in the Stern layer and the (point-like) counterions remain in the diffuse part of the electric double layer. Due to the latter assumption this model cannot describe any phenomena, which are dependent on the nature of counterion. Similar model was proposed by Fainer- man and Lucasssen-Reynders [5,6]. They included the cor- rection for activity coefficients of electrolyte solution but the role of the electric interaction of the adsorbing ionic species was not taken explicitly into account. The model proposed by Kalinin and Radke [7] considered the counterion binding to surfactant ions adsorbed in the Stern layer. They adopted Grahame’s [8] concept of the triple layer with counterions 0927-7757/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2004.11.044