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The effects of 1-propanol, 2-propanol, NaCl, urea, and [beta]-D-fructose on the molecular organization of water To, Eric Chun Hin

Abstract

Excess partial molar enthalpies of 1-propanol, HIP[sup E], were measured directly and accurately in four ternary aqueous systems by titrating small increments of 1 -propanol into mixtures comprised of 1-propanol, H₂O, and a "third component" at 25°C. Firstly, in the 1-propanol - H₂O - 2-propanol mixtures, the enthalpic interaction function between 1-propanol molecules, H[sub 1P1P][sup E] , were evaluated. Based on the knowledge accumulated in our laboratory on the mixing behaviour of each solute on the hydrogen bond network of water, the result indicates that 1 -propanol and 2-propanol modify the molecular organization of water in an identical and additive manner. Secondly, in the 1 -propanol - H₂O - NaCl mixtures, the enthalpic interaction function suggests that though NaCl acts as a structure breaker, it also modifies the hydrogen bond network of water in a positively cooperative manner with 1-propanol, a relatively hydrophobic solute. However, the results also suggest that a NaCl molecule binds to 7 to 8 water molecules, which were made unavailable for 1-propanol to interact with. The remaining bulk water away from solute NaCl would interact with 1-propanol as it is pure water. Thirdly, in the 1-propanol- H₂O -Urea mixtures, the total vapour pressures of this ternary system were measured in addition to H[sub 1P][ sup E]. The Boissonnass method based on the Gibbs-Duhem relation were employed to calculate the partial pressures, excess chemical potentials of each component. Excess partial molar entropies of 1-propanol were then evaluated. Both enthalpic and entropic interaction functions imply that urea molecules blend themselves into the hydrogen bond network of water keeping the connectivity of the hydrogen bonds intact but concurrently the water mediated solute-solute interaction is reduced. Finally, the interactions of 1P-1P in terms of enthalpy and entropy, H[sub 1P1P][sup E] and S[sub 1P1P][sup E], respectively, in 1-propanol-H₂0-β-D-Fructose mixtures suggest that fructose and 1- propanol modify the hydrogen bond network of water in a positively cooperative manner, much like the 1-propanol-H₂O-Glycerol systems studied previously. However the different appearances in the interaction functions of fructose as compared to other solutes suggests that the solvation of fructose might involve different mixing mechanisms which are yet to be determined.

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