Journal of Colloid and Interface Science 256, 190–193 (2002) doi:10.1006/jcis.2002.8223 Phase Behavior of N-Alkyl-2-pyrrolidones in Aqueous and Nonaqueous Systems and the Effect of Additives Anjing Lou, 1 B. A. Pethica, 2 P. Somasundaran, and Xiang Yu 3 Langmuir Center for Colloids and Interfaces, Columbia University, 500 W, 120th Street, Room 911, Mudd Building, New York, New York 10027 Received June 13, 2001; accepted January 12, 2002; published online August 26, 2002 The phase behavior of N-cyclohexyl-2-pyrrolidone and a homol- ogous series of N-alkyl-2-pyrrolidones mixed with water or with paraffins has been studied, and the effect of additives investigated. Depending upon the alkyl chain length, concentration, and temper- ature, the compounds can be either partially or completely miscible with water or paraffins. Several mixtures of the pyrrolidones and water show a lower consolute temperature (LCT). The LCTs can be changed to higher or lower temperatures by addition of salts, acids, and other compounds. Mixtures of the pyrrolidone compounds with paraffins, on the other hand, show an upper consolute temper- ature (UCT) behavior. The UCT decreases with the alkyl chain length of the pyrrolidones, but increases with the paraffin chain length. The miscibility of a given paraffin/pyrrolidone mixture can be improved by adding a longer chain pyrrolidone as third compo- nent. C  2002 Elsevier Science (USA) Key Words: N-alkyl-2-pyrrolidone; phase separation; lower/ upper consolute temperature. INTRODUCTION Phase separation occurring in micellar solutions, especially for nonionic surfactant solutions, has been the subject of inten- sive experimental and theoretical study due to the importance of the subject in advancing micellar theory and industrial appli- cations (1, 2). Many nonionic surfactant/water systems exhibit phase diagrams with lower consolute temperatures (LCT) (2). Some, such as polyethylene glycol/water systems (3), may show both upper and lower consolute temperatures so that the two- phase region takes the form of a closed loop. The effect of ad- ditives, especially inorganic salts, on the colloidal properties and solubility of nonionic surfactants in aqueous solution has been well studied (1, 2, 4). It is found that the effect of the anions of the neutral salts usually follows the Hofmeister series. The increase in cloud point on addition of thiocyanates and urea is considered to be related to their action as breakers of water 1 Current address: Sun Chemical Corporation, 631 Central Avenue, Carlstadt, NJ 07072. 2 To whom correspondence should be addressed at current address: Depart- ment of Chemical Engineering, A220C, Engineering Quadrangle, Princeton University, Princeton NJ 08544-5263. 3 Current address: International Specialty Products, Wayne, NJ. structure in the bulk and also around the hydrophobic parts of nonionic surfactants (5, 6). A recent report by Balzer (7) on the phase behavior of alkyl polyglucosides in water indicated that the entire coacervation region and the LCT can be affected by very small amounts of inorganic salts. The effect of additives on the LCT is potentially useful since solid/liquid separation and liquid /liquid extraction may be realized by modest shifts in temperature around the LCT. As reported previously (8–11), N -alkyl-2-pyrrolidones are surface-active compounds with low vapor pressure and low tox- icity, and are nonionic at neutral and alkaline pH. Many are liq- uids at ambient temperatures and over a large temperature range. Their advantageous characteristics have attracted increasing in- dustrial interest for a variety of applications, such as solvents in cleaning printing presses and coating strippers in the elec- tronic industry. The surface activity and adsorption of a series of N -alkyl-2-pyrrolidones at the air/aqueous solution interface at low surface pressures have been reported and interpreted in terms of two-dimensional virial coefficients and lateral intermolecular forces in the monolayers (9). Micellization and adsorption prop- erties of hexyl, butyl, and cyclohexyl pyrrolidones in aqueous system were also studied (11–13). In addition, the interfacial properties of the partially miscible short-chain alkyl pyrroli- dones with paraffins and the adsorption of the longer chain pyrrolidones at these nonaqueous liquid/liquid interfaces have been reported (14). In this work, phase separation and the effect of additives on the LCTs of the mixtures of N -hexyl, cyclohexyl, and octyl-2- pyrrolidones and water were investigated. The phase behavior of N -alkyl-2-pyrrolidones with paraffins was also studied in terms of the effect of the alkyl chain length of the pyrrolidones and the paraffins on the upper consolute temperatures (UCT) of the binary liquid systems. EXPERIMENTAL The N -alkyl-2-pyrrolidones were obtained from International Specialty Products Inc. and are better than 99.5% pure by capil- lary gas chromatography analysis. The impurities were mostly lower homologs or ring-substituted isomers. These low-level im- purities are not expected to interfere with the phase separation experiments. Water contents of the compounds, measured using 190 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.