| Research Abstract |
1. We determined the complex dielectric spectra of alcohol/water and ethylene glycol/water mixtures in the frequency range of 0.1【less than or equal】v/GHz【less than or equal】89 using TDR and wave guide interferometry. At 0.3【less than or equal】X (molar fraction of alcohol)【less than or equal】1.0, besides the dominating low-frequency process (j=1), assigned to the cooperative dynamics of the H-bond system, two additional Debye terms (j=2 and j=3) with the relaxation times of τ_2〜10-20ps and τ_3〜1-2ps are required to reproduce the high-frequency part of the spectrum. These can respectively be assigned to the motion of singly H-bonded alcohol monomers at the ends of the chain structure and the flipping motion of free OH. At X<0.3, the j=2 process becomes no longer separable.
2. The excess activation free energy, ΔG^E, enthalpy, ΔH^E, and entropy, ΔS^E, and their partial molar quantities, ΔG^E_i, ΔH^E_i, and ΔS^E_i (i=alcohol and water) were derived from the relaxation time of the cooperati
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ve process, τ_1. In X【greater than or equal】X_b (ex. X_b〜0.15, for 2-propanol/water), ΔH^E_A and ΔS^E_A become nearly zero, suggesting the generation of zigzag H-bonded alcohol chains similar to the pure alcohol. The two pertinent maxima in ΔH^E_A and ΔS^E_A (ex. at X〜0.03 and 〜0.07 for 2-propanol/water) are interpreted as a simultaneous increase of the number and strength of H-bond and support the view that in the intermediate region, water molecules are pushed out of the first coordination shell of the non-polar group into the bulk due to solute-solute association, called hydrophobic interaction. The comparison of the result of 2-propanol/water to that of 1-propanol/water revealed that not only the size of non-polar head group but the steric hindrance to alcohol OH are the key factor that controls the cooperative dynamics of H-bond system.
3. We thoroughly investigated aqueous solutions of amino acids; glycine, L-alanine, L-valine, L-leucine, L-isoleucine, L-serine, L-thoreonine, L-proline, L-cysteine, L-methionine, and L-Phenylalanine using DRS at 25℃ in the frequency range 0.1【less than or equal】v/GHz【less than or equal】89. The three relaxation processes; the rotational diffusion of zwitterionic amino acid molecules (τ_1〜40-110ps), the cooperative dynamics of bulk water (τ_2〜8-11ps), and a fast rotation of singly H-bonded water molecules (τ_3〜1-2ps) were detected. Applying the Cavell equation respectively to the solute and solvent processes, the dipole moment of each amino acid and the effective hydration number, Z, corresponding to the number of water molecules per solute that cannot contribute to the solvent relaxation processes, were deduced. A comparison of the effective molar volume of amino acid molecules as a rotational species estimated from τ_1 using the Stokes-Einstein-Debye (SED) equation to their partial molar volume, revealed that the shape factor of the amino acids under the hydrodynamic slip boundary condition, f_⊥, representing deviation from a spherical shape, increases linearly with the size of the side chain group except the case of L-proline that possesses a rigid ring-like structure. Less
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