Effect of chain length on the interaction between modified organic salts containing hydrocarbon chains and poly(N -isopropylacrylamide- co -acrylic acid) microgel particles

A series of four hydrophobically modified, diphenylazo-based organic salts have been prepared and characterized. To achieve this a C_x (x = 4, 6, 8, or 10) hydrocarbon chain was inserted between the diphenylazo moiety and the quaternary ammonium headgroup of the salt. The absorption of each of the four modified organic salts into anionic microgel particles of poly(N- isopropylacrylamide-co-acrylic acid) has been studied at pH 8. In addition, the hydrodynamic diameters and electrophoretic mobilities of the microgel particles have been studied as a function of the organic salt concentration, also at pH 8. In addition to the electrostatic attraction between the quaternary ammonium head groups of the organic salts and the anionic groups within the microgel particles, hydrophobic association between the chains of the organic salts within the microgel particles plays a role, with this effect increasing strongly from x = 4 to 10. Desorption of the x = 4 and 6 organic salts occurs readily on changing, in situ, the pH from 8 to 2.5 (and thereby eliminating the electrostatic interaction) but is only partially achieved for the x = 8 and 10 organic salts. Indeed, for the x = 10 organic salt, only about 80% of the organic salt is desorbed upon dilution of the microgel particles into a large excess of water.