The aggregation of nonionic surfactants in the presence of poly(methacrylic acid)

Abstract

The solution properties of homogeneous hexaethylene and octaethylene glycol mono(n-dodecyl) ethers, C₁₂E₆ and C₁₂E₈, respectively, and octaethylene glycol mono(n-decyl) ether, C₁₀E₈, with poly(methacrylic acid) (PMA) were investigated by dye solubilization, surface tension, fluorescence, viscosity, and pH measurements. The data were discussed regarding non-cooperative and cooperative binding of surfactant to polymer. Whereas in the interaction with poly(acrylic acid) (PAA), the critical aggregation concentrations (cac or T ₁) of these surfactants were lower than the respective critical micelle concentration (cmc), in that with the more hydrophobic PMA, T ₁’s of C₁₂E₆ and C₁₂E₈ were higher than the respective cmc, but that of C₁₀E₈ was lower than its cmc. These may be ascribed to the hydrophobic microdomains (HMD) of the PMA coil in water, probably in its inside. It is considered that some surfactants are bound first to the HMD non-cooperatively and then they are abruptly bound cooperatively at T ₁. This raises T ₁ higher than cmc when the cmc is low, and the amount bound by the HMD is relatively large and vice versa. T ₁ of C₁₂E₆ or C₁₂E₈ is the former case, and that of C₁₀E₈ is the latter. Thus, different from PAA, T ₁ for PMA + nonionic surfactant system consists of the amount of non-cooperative binding and the cac of the cooperative binding in equilibrium. Therefore, this T ₁ has a different meaning from that for PAA and should be called apparent T ₁. As the binding to the HMD is dependent on PMA concentration and cac is not, which is like in the PAA system, separation of apparent T ₁ from the HMD binding was achieved by extrapolating T ₁’s to zero PMA concentration (denoted intrinsic T ₁). This value for C₁₂E₈ was found to be lower than the respective cmc and also lower than the respective T ₁ for PAA. With increase in surfactant concentration, the pH of PMA solution rose and demonstrated a peak. This pH rise and fall may be induced by loosening of the HMD coil due to binding increase and by rearrangement of PMA + surfactant complex in high surfactant concentrations region. By raising the initial pH, the HMD were loosened; consequently, T ₁ rose a little, and at higher pH, no surfactant binding took place.