Micellization and interfacial behavior of binary and ternary mixtures of model cationic and nonionic surfactants in aqueous NaCl medium
Graphical abstract
Comparison of experimental critical micelle concentration of C16Br, C16BzCl and Brij58 in their various ternary combinations using Clint's, Blankschtein's, and Rubingh–Holland formulations.
Introduction
Mixtures of surfactants have received wide attention for several decades because of their efficient solubilization, suspension, dispersion and transportation capabilities [1]. In view of this, numerous binary and ternary combinations of ionic/ionic, ionic/nonionic and nonionic/nonionic surfactant mixtures have been studied with respect to their mixed micelle formation and adsorption at air/water interface [2], [3], [4], [5], [6], [7], [8], [9], [10]. Theories of Clint [11], Motomura [12], Rosen [13], Rubingh [14], Rubingh–Holland [6] and Blankschtein [15] have been used to analyze and compare the experimental results to understand synergism in mixed surfactant systems. The approach of regular solution [6] and thermodynamic [16], [17] theories have been applied on ternary mixtures of surfactants to predict the cmc and other micellar parameters on the basis of either solution characteristics of binary surfactant mixtures or the micellar properties of individual surfactants.
Ionic–nonionic surfactant mixtures are important from the fundamental as well as technological point of view as they exhibit strong synergism on mixing because of complimentary behavior in the mixed micelles causing decrease in cmc [18]. Cationic surfactants, being antifungal, antibacterial and antiseptic, have attracted attention with respect to their interaction with DNA and lipids [19]. On the other hand, nonionic surfactants are useful as detergents, solubilizers, emulsifiers, etc. [20]. In view of the importance of such mixtures, several ternary systems involving cationic–cationic–cationic [21], cationic–nonionic–nonionic [22], anionic–nonionic–nonionic [6], [23], and cationic–cationic–nonionic [9], [24] have been investigated in detail with respect to their mixed micelle formation and adsorption characteristics. Most of these studies have been performed in absence of salts. It has been argued that for ionic–nonionic surfactant mixtures, interaction parameters and hence micellar mole fractions obtained using Rubingh's or Rosen's models are valid when all the mixtures are kept at constant ionic strength because variation of electrical contribution of surfactants toward micellization has been neglected in these treatments [13], [14]. This therefore demands use of swamping amount of electrolyte in all the solutions. Compared to binary surfactant systems, ternaries have been scantly studied and quantification of results in terms of mutual interaction of components and ideality/nonideality has been limited. Recently, our study [24] has shown that the incorporation of nonionic surfactant in cationic–cationic surfactant mixtures increases their solubilization capabilities toward polycyclic aromatic hydrocarbons. In view of recent environmental regulatory perspective it is highly desirable to select the prospective surfactant mixtures from the existing surfactant mixtures rather than new ones.
In the present study, we have focused on the micellar (cmc, aggregation number, micropolarity and free energy of micellization) and adsorption (surface excess, minimum area per molecule and free energy of adsorption) properties of binary and ternary surfactant mixtures of hexadecyltrimethylammonium bromide (C16Br), hexadecylylbenzyldimethylammonium chloride (C16BzCl) and polyoxyethylene cetyl ether (Brij58) in 30 mM aqueous NaCl at 25 °C using tensiometry and spectrofluorimetry. The selected surfactants present similar hydrophobic chain length but dissimilar head groups. The surfactant–surfactant interaction within the micelles have been analyzed using the theories of Clint [11], Rubingh [14], Blankschtein et al. [16], [18], and Rubingh–Holland [6] with the aim to reveal the comparative performance of these models. At 30 mM NaCl the ionic strength will remain constant to ensure the validity of estimated parameters using such theories.
Section snippets
Materials
The cationic surfactants hexadecyltrimethylammonium bromide (C16Br) and hexadecylbenzyldimethylammonium chloride (C16BzCl) were obtained from Aldrich Chemical Co. (>98%) and used after recrystallization thrice from acetone–methanol mixtures. The nonionic surfactant Brij58, also an Aldrich product, was used as received. Surfactant solutions were prepared in triple-distilled water. Cetylpyridinium chloride used as quencher was also from Aldrich Chemical Co. (>98%) and was used after
Critical micelle concentration (cmc)
The cmc values of various binary (C16Br + Brij58, C16BzCl + Brij58 and C16Br + C16BzCl) and ternary (Brij58 + C16Br + C16BzCl) surfactant systems in 30 mM NaCl at 25 °C were determined from surface tension (γ) vs plots shown in Fig. 1 and the cmc values thus obtained are presented in Table 1. cmc values of C16Br, C16BzCl and Brij58 in 30 mM NaCl were found to be 0.107, 0.0537 and 0.00339 mM compared to 0.815 [21], 0.473 [25] and 0.0081 mM [26] in water at 25 °C, respectively. Reduction
Summary
The mixed micellization and adsorbed monolayer formation of C16Br, C16BzCl and Brij58 in 30 mM aqueous NaCl solution at 25 °C have been studied in their binary as well as ternary combinations using tensiometric and fluorimetric techniques. Following are the main conclusions drawn from this study:
- 1.
Although experimental cmc values of mixtures were less than predicted by Clint's equation indicating synergism in mixed micelle formation for all binary surfactant systems, aggregation numbers were
Acknowledgments
We are thankful to Prof. A.K. Nandi, Head, Polymer Science Unit, IACS, Kolkata, for providing facilities in his department for carrying out part of this work. Constant encouragement of Prof. K.U. Zaman Khan and Prof. M.A. Qurishi, Department of Chemistry, University of Kashmir, Srinagar, is gratefully acknowledged.
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