Abstract
The enthalpies of solution (text-decoration:overlineH2–H02) of some homogeneous decyl poly(oxyethylene glycol) ether (C10En) 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 (ΔmicH) and the heat capacities of monomeric (ΔCp, mono) and micellar (ΔCp, mic) forms as well as the heat capacities of micellization (ΔCp, m) have been evaluated from the enthalpy data. ΔmicH is less positive in aqueous urea solutions than in water and decreases with increasing urea concentration and temperature. ΔCp, 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.
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