Journal of Colloid and Interface Science 237, 6–10 (2001) doi:10.1006/jcis.2001.7429, available online at http://www.idealibrary.com on An Enthalpic Analysis on the Aggregation of Colloidal Particles Studied by Microcalorimetry A. B. J ´ odar-Reyes, A. Mart´ ın-Rodr´ ıguez, and J. L. Ortega-Vinuesa 1 Biocolloids and Fluid Physics Group, Department of Applied Physics, University of Granada, 18071 Granada, Spain Received February 4, 2000; accepted January 22, 2001 There are different theories concerning the stability of colloidal suspensions. Most of them arise from the well-known DLVO theory which relates colloidal stability to intermolecular forces between particles. Experimental corroboration of these theories has been obtained mainly by using different optical techniques that ana- lyze changes in the optical properties of the solution while parti- cles aggregate. However, no attention has been paid to studying the aggregation process thermodynamically. This is why we have fo- cussed on studying the heat released during the agglutination of polystyrene particles. The enthalpy change in this aggregation pro- cess was detected by using a highly sensitive and modern technique called isothermal titration calorimetry. In addition, some results about repeptization, that is, reversibility in the aggregation process, are also shown. C  2001 Academic Press Key Words: colloidal stability; isothermal titration calorimetry; aggregation enthalpy; repeptization. INTRODUCTION The problem of the colloidal stability of mesoscopic systems has usually been studied considering the interaction energies or forces between particles dispersed in a medium (1, 2). Most of the theories relating to the stability and the kinetics of coagula- tion have arisen from the DLVO theory (3, 4), which considers the sum of van der Waals attractive forces and the electrical double-layer repulsive forces to calculate the total interaction energy between two colloidal particles. Most of the studies have mainly been focussed on the kinetics, and the characteristic mag- nitude that has usually been studied is the Fuchs stability factor W . However, little attention has been paid to thermodynamic as- pects related to the stability of colloidal systems. As a matter of fact, after perusing specific literature, we were only able to find a couple of papers published in the last decade. One of them (5) is actually related to the energy and entropy associated with the electrical double layer, although it does not intrinsically analyze the aggregation process. The other one (6) presents a real ther- modynamic formulation of coagulation. However, the study is based on statistical thermodynamics and coagulation is viewed as a “phase transition.” To our knowledge, no information about 1 To whom correspondence should be addressed. the enthalpy or entropy changes (H aggr , S aggr ) involved in colloidal aggregation processes has been reported. There exists a lack of both theoretical treatment and experimental data con- cerning H aggr and S aggr . This may be due to the difficulty in obtaining experimental data of the heat released during coagu- lation, which must coincide with H aggr . To detect this heat it is necessary to work with a highly sensitive microcalorimeter, and the sensitivity of most of the microcalorimeters currently available is not sufficient. However, thanks to recent advances in instrumentation, it is now possible to experimentally obtain H aggr by using isothermal titration calorimetry. The main goal of this work has been to present experimental data of the en- thalpy changes associated with the aggregation process of a la- tex system (that is, polystyrene particles dispersed in an aqueous medium). On the other hand, to support the microcalorimetry re- sults, the aggregation of the latex was also analyzed by an optical technique: photon correlation spectroscopy (PCS). The optical results gave some information on the reversibility of the coagula- tion process. According to the DLVO theory, the total interaction energy versus distance should in principle present two minima and one maximum. When aggregation occurs, the colliding par- ticles overcome the maximum and come into direct contact in the infinitely deep primary minimum. But this situation is un- likely to occur since short-range effects, such as those caused by adsorbed layers of counterions (in the Stern layer), adsorbed sol- vent molecules, and/or some roughness in the surface, may keep particles from coming into physical contact, making the primary minimum of finite depth (7, 8). If the primary minimum is deep enough, coagulation is usually assumed irreversible. However, if the minimum is of restricted depth, or if particles aggregate in the secondary minimum (flocculation), the aggregation pro- cess can become reversible by changing the ionic strength of the medium. This phenomenon is called repeptization (9–12). The PCS experiments were useful for checking that repeptization oc- curred in our latex system, provided that the salt concentration was lower than the critical coagulation concentration. MATERIALS AND METHODS All chemicals used were of analytical grade quality. Water was purified by reverse osmosis, followed by percolation through charcoal and a mixed bed of ion-exchange resins. 6 0021-9797/01 $35.00 Copyright C  2001 by Academic Press All rights of reproduction in any form reserved.