Letter Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9645-9650
pubs.acs.org/journal/ascecg
Enhanced Stabilization of Water/scCO2 Interface by Block-Like Spontaneous Gradient Copolymers Xuan Liu,† Mingxi Wang,† Simon Harrisson,† Antoine Debuigne,‡ Jean-Daniel Marty,*,† and Mathias Destarac*,† †
IMRCP, UMR CNRS 5623, Université de Toulouse, 118, route de Narbonne, F-31062 Toulouse, Cedex 9, France CERM, CESAM, University of Liège, 11, Allée du Six Août, B-4000 Liège, Belgium
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ABSTRACT: There is an increasing interest in the specific physicochemical properties of gradient copolymers at interfaces. In this work, the phase behavior and interfacial properties of amphiphilic gradient copolymers at the water/CO2 interface are explored and compared to that of diblock copolymer counterparts. It is observed that spontaneous amphiphilic block-like gradient copolymers made of N,Ndimethylacrylamide, vinyl pivalate, and vinyl acetate exhibit slightly lower cloud point pressures in supercritical carbon dioxide (scCO2) than the corresponding diblock copolymers. Much more pronounced differences are established at the water/scCO2 interface, with larger critical aggregation concentration (CAC), much faster adsorption kinetics and equilibration, and lower surface tension for gradient copolymers. RAFT/MADIX polymerization allows the control of molar mass, composition, and microstructure of the copolymers of the study. These findings shed light on how microstructural control in amphiphilic copolymers can give access to a new range of macromolecular emulsifiers for CO2 media with improved properties. KEYWORDS: Phase behavior, Surface tension, Gradient copolymer, Supercritical carbon dioxide, RAFT/MADIX
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sions.3−5 This contrasts with their polymeric counterparts: while the stabilization of W/C microemulsions with Krytoxtype perfluoropolyethers has been widely reported,6 examples using amphiphilic copolymers are scarce in the literature7−10 and exclusively based on fluorinated poly((meth)acrylates) as a consequence of their lower surface energy as compared to nonfluorinated polymers and poly(siloxanes). Yet, their toxicity and high price severely limit their industrial potential. Hence, the identification of poly(oxyhydrocarbons) for the minimization of interfacial tension between water and scCO2 remains a very important challenge. In this class of polymers, poly(alkyleneoxides),11 poly(ethercarbonates),12 and poly(vinylesters)13 are those whose phase behavior in scCO2 has been the most reported in the literature. Poly(vinyl esters) are a promising family of CO2philic polymers that present the practical advantages of favorable price and low toxicity. Among them, poly(vinyl acetate) (PVAc) is known to be relatively well soluble in scCO2, with a strong dependence of molar mass on solubility.14 Nevertheless, stronger interactions between PVAc chains as well as a lower entropy of mixing may explain their lower
INTRODUCTION CO2 and water are the two most abundant and inexpensive molecules on Earth. They are environmentally benign, nontoxic, and nonflammable fluids. Their combination in chemical processes would offer greener alternatives to traditional organic solvents. In comparison with other fluids, the supercritical state for CO2 (scCO2) is easily accessible since its critical point is located at 31.1 °C and 73.8 bar. Using scCO2 as a reaction medium for chemical reactions offers many advantages such as complete elimination of the solvent via simple depressurization. Since water is a highly cohesive and polar solvent, its solubility in scCO2 is extremely low (
