Thermodynamics of micellization of some decyl poly(oxyethylene glycol) ethers in aqueous urea solutions

Abstract

The enthalpies of solution (H₂–H⁰₂) of some homogeneous decyl poly(oxyethylene glycol) ether (C₁₀E_n) surfactants in monomeric and micellar states in 3 and 6 mol dm^–3 aqueous urea solutions have been determined calorimetrically in the temperature range 288.15–318.15 K. The enthalpies of micellization (Δ_micH) and the heat capacities of monomeric (ΔC_{p, mono}) and micellar (ΔC_{p, mic}) forms as well as the heat capacities of micellization (ΔC_{p, m}) have been evaluated from the enthalpy data. Δ_micH is less positive in aqueous urea solutions than in water and decreases with increasing urea concentration and temperature. ΔC_{p, m} is relatively higher in aqueous urea solutions. The results indicate a decrease in hydration or structure around monomers in the pre-c.m.c. (critical micelle concentration) region and a relative increase in hydration of micelles due to enhanced hydrogen bonding between ethylene oxide groups and urea molecules in the post-c.m.c. region. Interactions between polyoxyethylene (hydrophilic) groups and urea seem to dominate over decyl (hydrophobic) group–urea interactions.