Enthalpies and heat capacities of transfer of sodium decanoate, sodium dodecanoate and sodium dodecyl sulphate from water to aqueous urea solutions

Abstract

Integral enthalpies of solution at very low concentrations of sodium decanoate (NaC₁₀) and sodium dodecyl sulphate (NaDDS) in 2 and 6 mol kg^–1 aqueous urea solutions at 298.15, 308.15 and 318.15 K and of sodium dodecanoate (NaC₁₂) in 2 mol kg^–1 aqueous urea solution at 298.15 and 308.15 K were determined calorimetrically. Enthalpies and heat capacities of transfer of these compounds from water to aqueous urea solutions were derived from integral enthalpies of solution. The results show that the enthalpies of transfer of NaC₁₀ and NaC₁₂ from water to 2 mol kg^–1 aqueous urea solution at 298.15 K are positive while that of NaDDS is negative. These values decrease at higher temperatures. The heat capacities of transfer of all these solutes from water to aqueous urea solutions at 303.15 K are negative and increase with increasing temperature. The results are discussed in terms of the structural effects of urea and of the constituent hydrophobic and hydrophilic groups of the solutes in aqueous solutions.