Modifying Properties of Aqueous Cetyltrimethylammonium Bromide with External Additives: Ionic Liquid 1-Hexyl-3-methylimidazolium Bromide versus Cosurfactant n-Hexyltrimethylammonium Bromide Kamalakanta Behera, Hari Om, and Siddharth Pandey* Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi - 110016, India ReceiVed: October 10, 2008; ReVised Manuscript ReceiVed: NoVember 13, 2008 Understanding the effect of external additives on the properties of aqueous surfactant solutions is of utmost importance due to widespread applications of surfactant-based systems. Role of ionic liquids (ILs) in this regard may turn out to be crucial as these substances are known to possess unusual properties. To unambiguously understand and establish the role of ILs in modifying properties of aqueous surfactant systems, changes in the physicochemical properties of aqueous cetyltrimethylammonium bromide (CTAB) upon addition of an IL 1-hexyl-3-methylimidazolium bromide ([hmim][Br]) are compared with those when a cosurfactant n-hexyltrimethylammonium bromide (HeTAB) is added to aqueous CTAB. Important physicochemical properties, such as critical micelle concentration (cmc), aggregation number (N agg ), solution conductance and microfluidity, and average aggregate size and polydispersity, are observed to change as either [hmim][Br] or HeTAB is added to aqueous CTAB; the experimental outcomes clearly imply the changes in most of the physicochemical properties to be significantly more dramatic in case of IL [hmim][Br] addition. The fact that, between the two, only IL [hmim][Br] may show cosolvent-type behavior at high concentrations is evoked to explain the differences in the behavior of the two additives. It is demonstrated that both [hmim][Br] and HeTAB show electrolytic as well as cosurfactant-type behavior within aqueous CTAB when present at low concentrations, with the changes in physicochemical properties being very similar. At high concentrations, although HeTAB still acts as a cosurfactant forming mixed micelles with CTAB, IL [hmim][Br] behaves partly as a cosolvent toward altering the physicochemical properties of aqueous CTAB. The unique role of IL in changing properties of aqueous surfactant systems is demonstrated. Introduction Aqueous surfactant solutions have immense importance due to their unparalleled application potential. 1 This is amply manifested by the extensive research efforts invested by many research groups in both academia and industry toward under- standing aqueous surfactant-based systems. 2 Within this context, assessment and subsequent modification of the properties of an aqueous surfactant system acquire utmost importance. At ambient conditions, physicochemical properties of an aqueous surfactant solution depend, among others, on the identity of the surfactant. 1 Subsequently, if the conditions remain the same, the aqueous solution of a surfactant at a given concentration possess more-or-less fixed physicochemical properties that are difficult to modulate. Other than changing temperature and pressure, the usual way to modify the physicochemical properties of a given aqueous surfactant solution is to use external additives, such as cosolvents, cosurfactants, electrolytes, polar organics, nonpolar organics, etc. 1,2 Because of their unusual and interesting properties ionic liquids (ILs) 3 as additives in modify- ing properties of aqueous surfactant systems, they may turn out to be of immense importance. 4-7 In our recent investigations we have demonstrated the uniqueness of ILs in altering the key physicochemical properties of aqueous surfactant systems composed of nonionic, 5 anionic, 6 and zwitterionic 7 surfactants. Most importantly, we have noted the dissimilarities in the nature of an IL to that of a salt or a cosolvent or a cosurfactant in altering the properties of an aqueous surfactant solution. The effectiveness and uniqueness of ILs in this respect were found to be unparalleled. Subtle, albeit distinct, differences between the chemical structure of the IL and a common salt or a cosolvent or a cosurfactant are obviously of importance in our aforementioned investigations. 4-7 For example, the cation (i.e., 1-butyl-3- methylimidazolium) and the anion (PF 6 - or BF 4 - ) of the IL in these investigations are different from the usual cations and anions of common salts generally used to alter properties of aqueous surfactant systems. Although the changes due to the addition of an IL as electrolyte (i.e., at low concentrations) are observed to be more pronounced as compared to those due to the addition of common salts, the similarities in changes are obvious. The changes in the physicochemical properties due to the addition of high concentration of ILs in these investigations are compared with the changes due to the addition of molecular cosolvents. Obviously, the molecular cosolvents are considerably different from ILs. The ionic nature inherent to the ILs results in highly unusual behavior of these neoteric substances as cosolvents in modifying properties of aqueous surfactant systems. Further, as far as cosurfactant-type behavior of ILs in these investigations is concerned, only the butyl functionality of the IL cation may induce any such behavior to some extent. IL appended with longer alkyl chains may be effectively utilized to investigate cosurfactant-type behavior, if any, of these substances. The outcomes can be conveniently compared with those from the addition of a traditional cosurfactant containing an alkyl chain of the same length. This, in our opinion, will * Corresponding Author. Phone: +91-11-26596503, Fax: +91-11- 26581102, E-mail: sipandey@chemistry.iitd.ac.in. J. Phys. Chem. B 2009, 113, 786–793 786 10.1021/jp8089787 CCC: $40.75  2009 American Chemical Society Published on Web 12/30/2008