Journal of Colloid and Interface Science 287 (2005) 312–317 www.elsevier.com/locate/jcis Stabilizing effect of low concentrations of urea on reverse micelles Asima Chakraborty, Munna Sarkar, Soumen Basak ∗ Chemical Sciences Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Received 15 September 2004; accepted 24 January 2005 Available online 10 March 2005 Abstract Urea is a well-known destabilizing agent for biopolymers like proteins and molecular aggregates like micelles and reverse micelles. Several theories have been proposed to explain the destabilizing/denaturing effect of urea. In this work, we present evidence for a stabilizing effect of a low concentration (<1 M) of urea incorporated in the central pool of AOT/n-heptane/water reverse micelles. Static light-scattering experiments were performed to measure (w 0 ) cr —the molar ratio of water to AOT beyond which the micelles become unstable—as a function of the concentration of urea in the central water pool. The stabilizing effect of urea is reflected in an increase in the value of (w 0 ) cr at low urea concentrations over that in the absence of urea. Dynamic light-scattering experiments show that the hydrodynamic radius of the micelles is smaller at low urea concentrations (<1 M) than in the absence of urea. Size-distribution analysis shows that for w 0 = 20 the microemulsion containing 0.5 M urea in its pool is significantly more monodisperse than that containing no urea. Temperature-dependent studies in the range 15–65 ◦ C indicate that the magnitude of this stabilizing effect decreases with increasing temperature, vanishing at temperatures higher than 65 ◦ C. A model is proposed to explain the above results.  2005 Elsevier Inc. All rights reserved. Keywords: AOT; Urea; Static light scattering; Dynamic light scattering; Stabilization; Renaturing effect 1. Introduction The mechanism by which urea exerts its effect on mole- cular aggregates and bio-molecules like proteins has been a subject of long-standing debate. Most commonly it is ac- cepted that urea exerts its effect through a direct or indirect mechanism [1–3]. According to the first, urea is capable of forming favorable H-bonding interactions with water, which permits it to replace water in the solvent shell around hy- drophobic components resulting in their enhanced solubil- ity. The indirect mechanism postulates that urea disrupts the three-dimensional network of water molecules, thereby de- creasing the energetic cost required to dissolve hydrophobic components in bulk solution [4–6]. While urea is generally believed to act as a water-structure breaker, existing evidence points to only an insignificant influence of urea on the three- dimensional structure of water [7,8]. * Corresponding author. Fax: +91-33-23374637. E-mail address: sbasak@hp1.saha.ernet.in (S. Basak). Wallqvist et al. have proposed an “outside-in” model for urea denaturation of proteins [9]. According to this, urea molecules preferentially adsorb onto the charged hy- drophilic residues on the surface causing repulsion between the surface residues. This allows penetration of water in- side the protein core and exposes the hydrophobic residues, thereby leading to destabilization of the native state. How- ever, several studies aimed at determining the urea concen- tration at the micellar interface have found similar concen- trations of urea at the interface as well as in the bulk aqueous phase [10]. This result contradicts the notion of preferential adsorption of urea on hydrophilic surfaces as the primary cause of its denaturing effect. More recently, another ap- proach has been adopted where the destabilizing effect of urea on micelles and reverse micelles has been attributed to the enhanced stability of hydrophilic components in solu- tion [10]. Reverse micelles are closed, spherical aggregates of sur- factants in hydrophobic solvents (typically alkanes, or oils) stabilized mainly by forces that tend to keep the polar or 0021-9797/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.01.071