Fluid Phase Equilibria 327 (2012) 58–64 Contents lists available at SciVerse ScienceDirect Fluid Phase Equilibria j o ur nal homep age: www.elsevier.com/locate/fluid Micellar size and phase behavior in n-octyl--d-thioglucoside/Triton X-100 mixtures: The effect of NaCl addition J.A. Molina-Bolívar, C. Carnero Ruiz ∗ Grupo de Fluidos Estructurados y Sistemas Anfifílicos, Departamento de Física Aplicada II, Escuela de Ingenierías, Universidad de Málaga, Campus de Teatinos, 29071 Málaga, Spain a r t i c l e i n f o Article history: Received 23 February 2012 Received in revised form 4 May 2012 Accepted 11 May 2012 Available online 18 May 2012 Keywords: Mixed micelles n-Octyl--d-thioglucoside Triton X-100 Micellar size Cloud point a b s t r a c t The temperature and NaCl concentration dependence of the size of mixed micelles of n-octyl--d- thioglucoside (OTG) and Triton X-100 (TX100), two non-ionic surfactants belonging to the alkyl glucosides and polyoxyethylene alkyl ether families, respectively, has been investigated using the dynamic light scat- tering technique. Micellar size increases with temperature, with this micellar growth occurring faster when the temperature approaches the cloud point temperature. The extent of this growth is less pro- nounced at higher relative proportions of OTG in solution. In addition, the cloud point temperature was investigated for aqueous and NaCl solutions of OTG and Triton X-100 mixtures. The experimental results can be explained assuming the clouding process to be a liquid–liquid phase separation which takes place as a result of reduced hydration of the head group in the surfactant molecules caused by the tempera- ture rise. The thermodynamic parameters of the clouding phenomenon have been calculated using the phase-separation model and considering the cloud point to be the threshold temperature for solubility. This study found that the enthalpy–entropy compensation relationship holds for this process. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Amphiphilic surfactants can form a variety of nanometer-sized assemblies such as micelles and vesicles in solution whose shape and size depend on the structure and nature of the surfactant as well as on certain experimental conditions including temperature or the presence of additives. In the case of additives, the self-assembly process, and hence the structure of the aggregates, can be modified in two different ways: (i) through specific interactions with the surfactant molecules, even those participating in the supramolec- ular structure, or (ii) by altering the nature of the medium, in other words acting as a non-penetrating agent [1]. An interesting exam- ple of the first mechanism occurs when a second surfactant (or co-surfactant) is added to the system. In this case, the interac- tions between the two surfactants can result in the occurrence of synergic effects and, consequently, improved performance when compared with the individual components alone in certain appli- cations [2]. As a result of their multiple applications, mixed surfactant sys- tems containing nonionic/ionic pairs are by far the most widely studied mixed systems. For example, nonionic/anionic surfactant mixtures are extensively employed in surfactant-based applica- tions such as detergents or surfactant fluids for enhanced oil ∗ Corresponding author. E-mail address: ccarnero@uma.es (C.C. Ruiz). recovery [3]. In contrast, mixed nonionic surfactant systems have been much less studied [4]. However, due to their higher tolerance to pH changes or the presence of electrolytes, the use of nonionic species is often preferred in many applications. Polyoxyethylene alkyl ethers occupy a prominent place among nonionic surfactants and, as such, a great deal of experimental effort has been expended in exploring their physicochemical properties. In recent years, a relatively new class of carbohydrate-derived nonionic surfactants (sugar-based surfactants) has been receiving considerable atten- tion due to their numerous advantages as regards performance, health of consumers, and environmental compatibility compared to other standard products [5–8]. Alkylpolyglucosides are a very important member of the aforementioned family and have found extensive technical applications in areas such as cosmetics, con- sumer products, and industrial cleaners [5,8]. Until recently, however, the study of mixed systems containing an ethoxylated and an alkylpolyglucoside surfactant was rather limited and was almost always carried out as part of a more general investigation [9]. More recent studies by Stubenrauch et al. [10,11] and Bäver- back et al. [12] have, however, served to focus attention of this problem and, as a consequence, to highlight the interest in study- ing these kinds of mixed surfactant systems. As a result, we have recently focused on characterizing mixtures of the nonionic sur- factants n-octyl--d-thioglucoside (OTG) and octaethylene-glycol monododecyl ether (C 12 E 8 ). In this study, we have observed that although both surfactants mix ideally, a number of structural prop- erties are substantially affected by the composition of the system [4]. 0378-3812/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fluid.2012.05.009