Surface characteristics of comblike copolymers from hexadecylacrylamide and acrylic acid at the air/water interface

Abstract

The interfacial organization characteristics of comblike copolymers from hexadecylacrylamide and acrylic acid at the air/water interface have been investigated using a Langmuir film balance at pH 6.5 and 10.0. The homopolymer and the copolymers (CHXP1, CHXP2) exhibit A₀ values of about 0.26 nm²/molecule with a π_max value of about 40 mN/m. The small differences in the surface characteristics of the copolymers suggest that the hexadecylacrylamide polymers undergo little changes in packing characteristics with the introduction of acrylic acid even up to 15 mol %. However, the copolymer consisting of 22 mol % acrylic acid tends to form a more expanded film, as is suggested from the comparatively higher A₀ (0.28 nm²/molecule) and compressibility coefficient, κ of 1.07×10^-2 m/mN at π=25 mN/m. The high surface concentration (Γ), 2.0-2.2 mg/m², and π_max of 40 mN/m of the investigated polymers are in support of the packing of the hexadecyl side chain at the interface. The results of the π -A isotherm and hysteresis experiments at pH 10.0 suggest that an introduction of an optimum level of 15 mol % acrylic acid in the copolymer imparts stabilization of the hexadecylacrylamide polymer film at the interface. The presence of 22 mol % acrylic acid shifts the amphiphilic balance of the copolymer to a more hydrophilic structure and promotes the desorption of the film into an aqueous subphase, especially under alkaline conditions leading to instability.