Thermally sensitive reversible microgels formed by poly(N-Isopropylacrylamide) charged chains: A Hofmeister effect study Teresa López-León a , Juan L. Ortega-Vinuesa b , Delfi Bastos-González b,⇑ , Abdelhamid Elaissari c,d a EC2M, UMR Gulliver CNRS-ESPCI 7083 – 10 Rue Vauquelin, F-75231 Paris Cedex 05, France b Biocolloid and Fluid Physics Group, Department of Applied Physics, University of Granada, Av. Fuentenueva S/N, 18071 Granada, Spain c University of Lyon, F-69622 Lyon, France d University of Lyon-1, Villeurbanne, CNRS, (UMR 5007), LAGEP-CPE-308G, 43 bd. du 11 Nov. 1918, F-69622 Villeurbanne, France article info Article history: Received 4 February 2014 Accepted 10 April 2014 Available online 18 April 2014 Keywords: Microgels Hofmeister effects Electrokinetic behavior abstract In this study, we present a new method to obtain anionic and cationic stable colloidal nanogels from PNIPAM charged chains. The stability of the particles formed by inter-chain aggregation stems from the charged chemical groups attached at the sides of PNIPAM polymer chains. The particle formation is fully reversible—that is, it is possible to change from stable polymer solutions to stable colloidal disper- sions and vice versa simply by varying temperature. In addition, we also demonstrate that the polymer LCST (lower critical solution temperature), the final particle size and the electrokinetic behavior of the particles formed are highly dependent on the electrolyte nature and salt concentration. These latter results are related to Hofmeister effects. The analysis of these results provides more insights about the origin of this ionic specificity, confirming that the interaction of ions with interfaces is dominated by the chaotropic/kosmotropic character of the ions and the hydrophobic/hydrophilic character of the surface in solution. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction Poly(N-Isopropylacrylamide) (hereafter called PNIPAM) has been extensively studied in the last two decades [1–18]. The great interest in this polymer is due to its extraordinary properties of solvency, which are highly dependent on the solvent characteris- tics such as salt concentration, pH, and especially, temperature [19–22]. It is well known that single polymer chains of PNIPAM dissolved in water undergo a sharp collapse transition from a highly hydrated extended coil into a compact globule when temperature is increased over a critical point, usually called lower critical solution temperature or LCST, which is around 31–34 °C for PNIPAM [23]. This peculiar behavior is due to the presence of both hydrophilic (amide groups) and hydrophobic (isopropyl groups) moieties in the NIPAM molecule. At room temperature, water behaves as a good solvent through hydrogen bonding with the amide groups. Upon heating, the water–amide hydrogen bonds get increasingly disrupted by thermal energy, causing the water to act as a poorer solvent. Above the LSCT, the monomer–monomer interactions become stronger than the monomer–solvent interactions, leading to a polymer-chain contraction as the number of monomer–monomer contacts increases [24]. For uncharged PNIPAM chains, increasing the temperature over the LSCT usually leads to a phase separation between PNIPAM and water. In this work, we show that a completely different scenario arises when attaching charged groups at the ends of the PNIPAM chains. These charged groups provide certain amphiphilic character to the PNIPAM chains, which aggregate into configurations where the charged end-groups are exposed toward the aqueous continuous phase. This leads to the formation of particles with a certain surface charge density, which is eventually capable of stabilizing the growing of the particles and preventing complete phase sepa- ration. We show that the particles formed with this method are reversible; that is, simply by varying temperature, it is possible to transform a stable polymeric solution into a monodisperse col- loidal suspension and vice versa. This novel PNIPAM-based system offers an interesting arena to study ion-specific effects or Hofmeister effects. It is widely known that different ions can specifically modify a broad range of interfa- cial phenomena from surface tensions to colloidal stability by means of ion accumulation or exclusion from the interfaces that cannot be explained simply by considering electrostatic interac- tions [25–29]. Regardless of the property studied, and with very http://dx.doi.org/10.1016/j.jcis.2014.04.020 0021-9797/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Fax: +34 958 243214. E-mail address: dbastos@ugr.es (D. Bastos-González). Journal of Colloid and Interface Science 426 (2014) 300–307 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis