Conductometric and microcalorimetric analysis of the alkaline-earth/alkali-metal ion exchange onto polyacrylic acid

• Published in: Colloid and polymer science Vol. 277; no. 9; pp. 818 - 826
• Main Authors: POCHARD, I, FOISSY, A, COUCHOT, P
• Format: Journal Article
• Language: English
• Published: Berlin Springer 01.09.1999; Springer Nature B.V
• Subjects: Applied sciences; Binding energy; Bond strength; Complexation; Earth; Electrostatics; Exact sciences and technology; Hydrophobicity; Lithium; Organic polymers; Physicochemistry of polymers; Polyacrylates; Precipitates; Properties and characterization; Sodium; Solution and gel properties; Cation exchange; Polyelectrolyte; Ionic selectivity; Acrylate salt polymer; Alkali metal ion; Aqueous solution; Experimental study; Alkaline earth metal ion
• ISSN: 0303-402X; 1435-1536
• DOI: 10.1007/s003960050458

The specificity of the exchange between divalent (Di^sup 2+^=Ca^sup 2+^ or Ba^sup 2+^) and monovalent (M^sup +^=Li^sup +^, Na^sup +^ or K^sup +^) ions onto a polyacrylic chain is examined using conductometric and microcalorimetric techniques. Assuming the formation of a bidentate complex between the Di^sup 2+^ and the carboxylate groups, the conductometric data give the exchange ratio (Di^sup 2+^/M^sup +^) and the speciation of the acrylic groups. No significant difference is observed between the three alkali-metal ions for a given Di^sup 2+^ ion. Comparisons between Ca^sup 2+^ and Ba^sup 2+^ show a stronger hydrophobicity of the former as it precipitates at a complexation ratio r=0.33 versus r=0.45 for the Ba^sup 2+^ salt. Microcalorimetric data show that all Di^sup 2+^/M^sup +^ exchange energies are positive and depend significantly on the type of cations. The largest displacement energy (the more positive) is found for the binding of Ca^sup 2+^ with sodium polyacrylate (8.13kJ·mol^sup -1^) and the smallest for Ba^sup 2+^ with lithium polyacrylate (1.88kJ·mol^sup -1^). The interpretation of the data leads to the conclusion that specificity of the Di^sup 2+^ binding originates in the dehydration phenomenon and specificity between the three alkali-metal ions is due to the decrease in the electrostatic bond strength with an increase in the ionic radii. The Di^sup 2+^/M^sup +^ exchange is entropically driven.[PUBLICATION ABSTRACT]