Cationic and Anionic Poly(N-isopropylacrylamide) Based Submicron Gel Particles: Electrokinetic Properties and Colloidal Stability Teresa Lo ´ pez-Leo ´ n, †,‡ Juan L. Ortega-Vinuesa,* ,† Delfi Bastos-Gonza ´ lez, † and Abdelhamid Elaı 1 ssari ‡ Biocolloid and Fluid Physics Group, Department of Applied Physics, UniVersity of Granada, AV. FuentenueVa S/N, 18071 Granada, Spain, and CNRS-bioMe ´ rieux, UMR-2714, ENS de Lyon, 46 alle ´ e d’Italie, 69364 Lyon Cedex 07, France ReceiVed: July 22, 2005; In Final Form: December 15, 2005 A cationic and an anionic poly(N-isopropylacrylamide) (poly(NIPAM)) microgel latex were synthesized via batch radical polymerization under emulsifier-free conditions. The hydrodynamic properties, colloidal stability, and electrokinetic characteristics of these two samples were studied. The hydrodynamic particle size variation was discussed by considering the effect of salinity and temperature on the shrinkage of the thermally sensitive polymer domains. The colloidal stability also depended on temperature and electrolyte concentration. A stability diagram with two well-defined domains (stable and unstable) was obtained. The flow from one domain to the other was fully reversible due to the peculiar (de)hydration properties of the polymer. The electrokinetic behavior, which depends on electrical and frictional properties of the particles, was analyzed via electrophoretic mobility measurements. Results were discussed by considering both the particle structure dependence on temperature and salinity, and the electric double layer compression. In addition, the electrophoretic mobility data were analyzed using Ohshima’s equations for particles covered by an ion-penetrable surface charged layer, as well as using another simpler equation for charges located on a hydrodynamic equivalent hard sphere. Differences between the properties of both latexes were justified by the presence of a hydrophilic comonomer, aminoethyl methacrylate hydrochloride (AEMH), in the cationic microgel. Introduction Microgels are cross-linked polymer particles in the colloidal size range that swell in good solvents and that have a large number of applications in fields as diverse as medicine biology, 1,2 industry, 3,4 or environmental cleanup. 5 One of the most striking properties of microgels is their ability to undergo large volume transitions. This phase change is controlled by competing elastic, solvency, and ionic contributions. 6 Special attention has been paid to temperature-sensitive aqueous mi- crogels since cross-linked poly(N-isopropylacrylamide) (poly- (NIPAM)) particles were prepared by Chibante in 1978 and subsequently reported in 1986. 7 Poly(NIPAM) particles exhibit a temperature-induced volume transition. It is generally believed that hydration of the poly(NIPAM) chains originates local ordering in the water molecules around the amide group by means of hydrogen bonding. However, an increase in temper- ature increases molecular agitation, which in turn causes a disruption of the H-bonding between water and the amide groups. This leads to a breakdown of local water structure around the poly(NIPAM) chains that triggers hydrophobic attraction among isopropyl groups. This feature causes hydration of polymer chains below the lower critical solution temperature (LCST) and, consequently, microgel particles are swollen, while above the LCST the particles collapse. The LCST for poly- (NIPAM) in water is around 32 °C. 8,9 There are numerous publications which refer to the charac- terization of anionic poly(NIPAM) particles, many of them reviewed by Saunders and Vincent 10 and recently by Pelton. 11 Characterization can involve gel structure, swelling, surface activity, rheology, electrical properties, colloidal stability, and interactions with other molecules (surfactants, drugs, proteins, etc). However, characterization of cationic poly(NIPAM) mi- crogels is not so usual in the literature, as these kind of particles have appeared in the past decade. Cationic poly(NIPAM) latexes are usually obtained by adding a positive comonomer together with the NIPAM in the polymerization reaction 12 as well as a cationic initiator, commonly the 2-2′-azobis-(amidinopropane) di-hydrochloride (V50). 13-15 Some cationic comonomers that can be found in the literature are (in acronyms) MADAP, 16,17 DMAEMA, 18 and DMAPAA, 19 but few authors 14,15 use the aminoethyl methacrylate hydrochloride (AEMH) employed in the present work. Some properties of cationic and anionic poly(NIPAM) microgels have been analyzed as a function of salinity, tem- perature, and ionic specificity. Specifically, this paper aims to establish the hydrodynamic properties, the electrokinetic char- acteristics and colloidal stability of a cationic poly(NIPAM) latex, comparing the results with those of a classic anionic one. This analysis is only performed as a function of salinity (using an inert electrolyte, NaCl) and temperature. A second part 20 will be focused on evidencing ionic specificities 21-23 in the two poly- (NIPAM) samples working with different electrolytes. It should be noted that Daly and Saunders 24 published an excellent paper in which they studied the electrophoretic mobility and hydro- dynamic diameter of anionic polyNIPAM particles, presenting a novel model for polyNIPAM volume phase transitions. Some mobility measurements were analyzed by applying Ohshima’s theory. In fact, Ohshima et al. 25,26 previously used such a theory * To whom correspondence should be addressed. E-mail: jlortega@ugr.es. Phone: +34 958 240018. Fax: +34 958 243214. † University of Granada. ‡ ENS de Lyon. 4629 J. Phys. Chem. B 2006, 110, 4629-4636 10.1021/jp0540508 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/03/2006