ORIGINAL CONTRIBUTION Colloid Polym Sci 274:1013 1019 (1996) 9 SteinkopffVerlag 1996 J.S. Hong T. Nakahara H. Maeda Y. Kikunaga A. Kishida M. Akashi Cloud points and phase separation of aqueous poly(N- vinylacetamide) solutions in the presence of salts Received: 15 February 1996 Accepted: 21 March 1996 J.S. Hong- T. Nakahara Dr. H. Maeda ([E~) Department of Chemistry Faculty of Science Kyushu University Fukuoka 812, Japan Y. Kikunaga - A. Kishida 9 M. Akashi Department of Applied Chemistry and Chemical Engineering Faculty of Engineering Kagoshima University Kagoshima 890, Japan Abstract Aqueous poly(N-vinylace- tamide) (PNVA) solution was found to exhibit the cloud point in the presence of salt. This cloud point was shown to correspond to a liquid- liquid phase separation, as confirmed when the PNVA-salt solutions were maintained at a temperature above the cloud point. The upper layer had a higher polymer concentration and a lower salt concentration than those in the lower layer. Thus interaction between PNVA and salts are repul- sive. The lower critical solution temperatures were estimated to be 18 _+ 1 ~ for 1.25 molal (NH4)2SO4 and 25 _+ 1 ~ for 0.76 molal Na2SO4. Divalent anions such as SO42 -, SO32-, HPO 2- and CO 2- were effective in causing turbidity when examined at 25 ~ Dependence of the effect on the cationic species was similar to but significantly different from that for acetyltetraglycine ethylester. The cloud points of PNVA decreased linearly with the increase of the polymer concentration at a fixed salt concentration or with the increase of the salt concentration at a fixed polymer concentration. A parameter analogous to the salting-out constant was empirically derived from the dependencies of the cloud points on the concentrations of polymer and salt. Key words Poly(N-vinylacetamide) - cloud point - phase separation - salting out - LCST(lower critical solution temperature) Introduction Although many biopolymers including proteins, nucleic acids and polysaccharides are polyelectrolyte, short-range interactions other than the long-range electrostatic inter- action are important to understand the function and stability of the structure of these biopolymers. Thermo- dynamic study on solutions of nonionic water-soluble polymers is hoped to contribute to our understanding on various interactions in aqueous polymer solutions. Phase behavior of polyethylene glycol in water has been known to complex [1-3]. Several models have been proposed for describing the phenomena on the basis of either water structure [4, 5], hydrogen bond [6], or con- formational effects [7]. On addition of salts, cloud points were observed on polyvinylpyrrolidone [8], poly(vinyl alcohol-acetate) [9, 10], dextran [11], polymers of N- alkylacrylamides [12] and of N-alkoxylacrylamide [13], and nonionic surfactants [14-16]. Phase separation of the solutions containing two polymer components has re- ceived much attention in relation to the separation of biomaterial [11, 17-19]. Solution properties of nonionic polymers in aqueous media have been studied [20, 21]. Salt effects on the aqueous solution of nonionic solutes have been discussed on the basis of the Hofmeister series or lyotropic series [22, 23]. The studies included non- polar molecules [24-26], amides and peptides [27-30].