Theoretical description of aggregation of cationic gemini surfactants in the bulk solution and on the silica surface
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Theoretical description of aggregation of cationic gemini surfactants in the bulk solution and on the silica surface Mateusz Drach · Anna Andrzejewska · Jolanta Ciesla · Jolanta Narkiewicz-Michałek
Received: 30 April 2007 / Revised: 26 September 2007 / Accepted: 18 April 2008 / Published online: 1 May 2008 © Springer Science+Business Media, LLC 2008
Abstract A theory of cationic dimeric (gemini) surfactant adsorption onto negatively charged surface is presented. In the proposed model it is assumed that the adsorbed phase is a mixture of singly dispersed molecules of surfactant and spherical, globular and cylindrical aggregates of different dimensions. Only the “excluded area” interactions between the adsorbed species are considered and the effects of surface heterogeneity on monomer adsorption are taken into account. The aggregation behavior of gemini surfactants is based on the additive free energy model proposed by Camesano and Nagarajan (2000). The calculated surfactant adsorption isotherms and the differential molar enthalpies of micellisation and adsorption are compared with the experimental results obtained for a series of gemini surfactants depending on the length of a spacer, temperature or the presence of electrolyte. On the basis of theoretical results the evolution of adsorbed phase of gemini surfactants with the increasing adsorption is discussed. It is shown that the evaluated cmc values and the dimensions of surfactant aggregates are in a good agreement with experiment. Unfortunately, the theoretical model does not describe properly the temperature dependence of micellisation process. Keywords Gemini surfactants · Surfactants adsorption · Surface aggregation
M. Drach · A. Andrzejewska · J. Ciesla · J. Narkiewicz-Michałek () Department of Theoretical Chemistry, Faculty of Chemistry, Maria Curie-Skłodowska University, 20-031 Lublin, Poland e-mail: [email protected]
1 Introduction Gemini surfactants, called also dimeric surfactants, represent a new class of surface active compounds. Most often they are built of two, at times three or more, monomers of conventional surfactants. Their molecules usually consist of two hydrophobic tails and two hydrophilic groups which are joined by a spacer—usually a hydrocarbon. The spacer can be rigid (e.g. aromatic ring) or flexible (e.g. alkyl chain). Despite the fact that gemini surfactants are relatively new compounds they found a huge application in numerous branches of industry and environmental protection. The success of gemini surfactants application has had a good reason. Namely, dimeric surface active agents are characterized by better interfacial properties than the corresponding monomeric conventional surfactants (i.e. surfactant molecules with the same single hydrophilic and hydrophobic groups). It should be mentioned that the critical micelle concentration (cmc) of geminis can be two orders of magnitude lower than the cmc of corresponding conventional surfactants (Atkin et al. 2003b; Menger and Keiper 2000; Hait and Moulik 2002). For instance, t
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