Colloids and Surfaces A: Physicochem. Eng. Aspects 412 (2012) 135–142 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal homep a ge: www.elsevier.com/locate/colsurfa Reactivity of a lipophilic ingredient solubilized in anionic or cationic surfactant micelles Claire C. Berton-Carabin a , John N. Coupland a , Cheng Qian b , D. Julian McClements b , Ryan J. Elias a,∗ a Department of Food Science, The Pennsylvania State University, University Park, PA 16802, USA b Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA h i g h l i g h t s ◮ The location of a lipophilic spin probe was measured in surfactant solu- tions. ◮ The probe partitioned between micelle and aqueous environments. ◮ The probe in anionic micelles decreased its reactivity with the anionic reactant. ◮ The probe in cationic micelles increased its reactivity with the anionic reactant. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 13 June 2012 Received in revised form 16 July 2012 Accepted 18 July 2012 Available online 4 August 2012 Keywords: Anionic surfactant Cationic surfactant Micelle Electron paramagnetic resonance Spin probe a b s t r a c t The aim of this work was to investigate the location and reactivity of a lipophilic spin probe, 4-phenyl- 2,2,5,5-tetramethyl-3-imidazoline-1-oxyl nitroxide (PTMIO) in anionic (sodium dodecyl sulfate, SDS) or cationic (dodecyl trimethylammonium bromide, DTAB) surfactant micelles. The analysis of electron para- magnetic resonance (EPR) spectra of PTMIO in micellar systems showed that probe molecules partitioned between two populations: a more mobile fraction in the aqueous phase and a less mobile fraction in the micelle. The fraction of PTMIO incorporated in surfactant micelles increased with surfactant concentra- tion. The rate of the reduction of the nitroxide group of PTMIO by the negatively charged, water-soluble ascorbate decreased when the probe was solubilized in anionic SDS micelles and increased when it was solubilized in cationic DTAB micelles. Thus, both the surface charge as well as the solubilization capacity of the micelles controlled the reactivity of the lipophilic molecule. © 2012 Elsevier B.V. All rights reserved. 1. Introduction A wide range of lipophilic ingredients (e.g., flavors, pigments, vitamins, drugs, antimicrobials or phytochemicals) are added to food, beverage, pharmaceutical and cosmetic products to produce a desired functionality. Various formulation strategies are used to disperse lipophilic ingredients into aqueous media including emulsions, nanoemulsions and solid lipid nanopar- ticles [1–4]. Emulsion-based delivery systems often contain ∗ Corresponding author at: Department of Food Science, The Pennsylvania State University, University Park, PA 16802, USA. Fax: +1 814 863 6132. E-mail address: elias@psu.edu (R.J. Elias). appreciable amounts of non-adsorbed emulsifiers in the aqueous phase surrounding the dispersed lipid particles. Non-adsorbed sur- factant molecules form micelles when their concentration exceeds a particular level know as the critical micelle concentration (CMC). Surfactant micelles consist of a hydrophilic shell and a hydropho- bic core that are capable of incorporating lipophilic molecules [5–7]. Consequently, it is possible for any encapsulated lipophilic component within an emulsion-based delivery system to partition between the non-polar regions of the lipid droplets and the sur- factant micelles [8–10]. Surfactant micelles may, therefore, play an important role in the solubilization and localization of lipophilic ingredients in multiphase systems. Accordingly, surfactant micelles have been described as acting as a separate phase from water, which constitutes the basis of the pseudophase model [11]. 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.07.029