Effects of Halide Anions on the Solution Behavior of Double Hydrophilic Carboxy-Sulfobetaine Block Copolymers

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Abstract
The solution behavior of the double polybetaine block copolymer poly(2-((2-(methacryloyloxy)ethyl)dimethylammonio)acetate)-block-poly(3-((2-(methacryloyloxy)¬ethyl)dimethylammonio)propane-1-sulfonate (PGLBT-b-PSPE) in sodium halide aqueous solutions was investigated. In the presence of salt ions, the unimer-to-micelle transition of PGLBT-b-PSPE that originated by Coulombic attraction between PSPE motifs was suppressed and shifted to much lower temperatures. The transition was more hindered by increases in the salt concentration due to additional counterion binding on the ionized site of PGLBT-b-PSPE chains which screens the dipole-dipole attractions. The specific ion effect was investigated on four different halides, Cl-, Br-, I- and F-. Cl- and two chaotropes (Br- and I-) apparently prevented micelle formation, and the hindering effectiveness on the PSPE pairing followed the general Hofmeister series of anions: I- > Br- > Cl-. More chaotropic anions strongly maintained the polymer chains in a fully hydrated state when the same amount of salts was incorporated. However, F-, which is classified as kosmotrope, only did small contribution on lowering the transition point and led to abrupt transition without showing a gradual phase change prior to the transition. The variations of hydrodynamic radius and zeta potentials of unimers and micelles gave hints of the solvation state of salt-incorporated PGLBT-b-PSPEs in each state. These results suggest that chaotropes tend to exist in the vicinity of the diblock polybetaine chain surface thus prominently influenced the thermoresponsive solution behavior, whereas kosmotropes prefer water molecules and cause minor changes in the PGLBT-b-PSPE aqueous solution.

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