Influence of end-group modification on interaction of amphiphilic poly(oxyethylene)-b-poly(oxybutylene) block copolymers with ionic surfactants

Abstract

Amphiphilic poly(oxyethylene)-b-poly(oxybutylene) block copolymers having hydrophilic block end-capped with neutral dimethylamino (DE79B34) and cationic trimethyl ammonium (TE79B34) groups, respectively were investigated for their interactions with ionic surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) using tensiometry, conductometry and dynamic light scattering. The self-assembly of DE79B34 and TE79B34 occur at 0.7 g/dm3 and 0.8 g/dm3, respectively. TE79B34 binds favorably with oppositely charged SDS, causing a substantial change in surface tension compared to other surfactant-polymer systems. In mixed polymer-surfactant systems, the micellization was promoted in the presence of SDS, but it was suppressed with CTAB. Such behavior is expected for cationic block copolymers, since they can engage into ion-pair formation with anionic SDS. For neutral polymers, the behavior is reflective of surfactants’ head group hydrophobicity. The head groups of CTAB are more hydrophobic and their existence in the proximity of hydrophobic segments of polymers induces shape transition to non-spherical morphologies. Gibb’s free energy of adsorption at air – water interface is negative for SDS, CTAB and surfactant-block copolymer systems, indicating that the process is highly spontaneous. The increase in entropy of TE79B34 during micelle formation with temperature is due to disturbance of hydrophobic structure of water molecules, thus hydrophobic parts are removed from bulk solution to the interface and also in the interior of micelle the freedom of hydrophobic part is increased. The dynamic laser light scattering results revealed that due to presence of block copolymers pre-micellar aggregates were favored.