Aggregation and adsorption of sodium dioctylsulfosuccinate in aqueous ammonium chloride solution: Role of mixed counterions J. Dey, U. Thapa, K. Ismail ⇑ Department of Chemistry, North-Eastern Hill University, Shillong 793022, India article info Article history: Received 20 June 2011 Accepted 22 October 2011 Available online 6 November 2011 Keywords: AOT Critical micelle concentration Counterion binding constant Mixed counterions Synergism Surface excess abstract The critical micelle concentration (cmc) of sodium dioctylsulfosuccinate (AOT) was determined at 25 °C from surface tension and fluorescence methods in aqueous NH 4 Cl solution for assessing the influence of mixed counterions on the special counterion binding behavior (SCB) of AOT. The SCB of AOT refers to a sudden twofold increase in the value of the counterion binding constant (b) in aqueous medium when the concentration (c  ) of the added 1:1 sodium salt is about 0.015 mol kg 1 , and it has been tested so far for sodium ion only. In the presence of sodium and ammonium mixed counterions also the SCB of AOT exist, but with lower c  (0.009 mol kg 1 NH 4 Cl). Synergism in the cmc occurs due to mixed counte- rions. In the case of inorganic counterions, unlike the case with organic counterions, the cmc is dependent on the total counterion concentration in solution and negligibly on the specific type of counterion. Na + and NH þ 4 bind almost equally to the micelle in the region of low b (below c  ), but in the region of high b (above c  ) NH þ 4 binds predominantly. It has been shown that the theoretical expression for the surface excess of ionic surfactant + electrolyte system containing a single counterion can also be used to evaluate the surface excess in the presence of mixed counterions if the two counterions are considered to undergo Henry-type adsorption at the air–solution interface. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Sodium dioctylsulfosuccinate (AOT) has a special counterion binding behavior (SCB) [1–4], viz. the value of its counterion bind- ing constant (b) undergoes a sudden twofold increase in aqueous NaCl solution. It has been established that shape change of AOT micelle is responsible for the abrupt change in b [4]. AOT, which is considered to be a special surfactant [5–8], is perhaps the only an- ionic surfactant whose micelle undergoes a shape change in aque- ous medium at a very low concentration (around 0.015 mol kg 1 ) of added sodium ion, since in other anionic surfactants, micellar shape change occurs at low concentration of electrolyte only if the added counterion is multivalent [9–13]. It is also reported [4] that organic coion like salicylate has suppressing effect on the sud- den increase in the value of b of AOT. Although AOT-type surfactants with different counterions were synthesized and studied [14–17], the SCB of AOT is yet to be tested for counterions other than sodium and for mixed counterions. Therefore, our main interest is to examine how the SCB of AOT is influenced by the nature of counterions. With this objective in mind, we measured in the present work surface tension of AOT in aqueous NH 4 Cl solution as this system contains a mixture of Na + and NH þ 4 counterions. The fluorescence emission method was also used to check the critical micelle concentration (cmc) values of AOT in the presence of NH 4 Cl. 2. Materials and methods AOT (Sigma, 99% assay), NH 4 Cl (sd fine chemicals, 99.8% assay), NaCl (Merck, 99.5% assay), and pyrene (Fluka) were used without further purification. Stock solutions of AOT and the salts were prepared in Milli-Q water, and the required concentrations were obtained by dilution. Surface tension measurements were taken by the Wilhelmy plate method using a Krüss K11 tensiometer attached with a thermostat (Haake DC 10). The fluorescence emission spectra of pyrene were recorded using Hitachi F4500 FL spectrophotometer. The details of solution preparation, surface tension measurement, and fluorescence emission intensity measurement are given in our earlier papers [1,18]. All measurements were taken at 25 °C. 3. Results and discussion 3.1. Surface tension, fluorescence emission, and critical micelle concentration The plots of surface tension (c) versus log[AOT] corresponding to aqueous NH 4 Cl solutions of different concentrations are shown 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.10.055 ⇑ Corresponding author. Address: Department of Chemistry, North-Eastern Hill University, NEHU Campus, Shillong 793 022, India. Fax: +91 364 2550486. E-mail addresses: kismail@nehu.ac.in, kinehu@hotmail.com (K. Ismail). Journal of Colloid and Interface Science 367 (2012) 305–310 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis