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6559spacing and orientation of the chains. It should be pointed out here that the difference between bulk hydrocarbon and apparent core densities can be explained by two reasons: (a) the two phases are different in the number of terminal hydrogen atoms per alkyl chain, and (b) the molar volume of the terminal hydrogen atom in micelles is significantly less than that in the bulk.Finally, an attempt is made for guessing the number of solvent molecules penetrated in the micellar core. The total volume occupied by solvent molecules is A@(H20) = P ( H ) -9,(H). By definition, the molar volume of solvent molecules inside the micelles is the same as in the bulk; thus, w = A@(H20)/Vw*, resulting in w = 0.45. This result is in good agreement with the upper limit obtained from SANS;2 the agreement of results originating from totally different considerations and experimental methods suggests that the present approach to w may not be entirely arbitrary.Small-angle X-ray scattering measurements have been carried out on tert-butyl alcohol (TBA)-water mixtures at 28, 40, and 55 "C. The temperature effects in the concentration fluctuation and the Kirkwood-Buff parameters have been studied.Debye's correlation length has also been determined for each temperature. With the increase of temperature, the fluctuation becomes larger and the correlation length becomes longer, which indicates that the microscopic demixing of the mixture proceeds as the temperature is raised. The temperature and concentration dependences of the mixing state of the TBA-water solution are discussed.

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Small-angle X-ray scattering measurements have been carried out on ZerZ-butyl alcohol (TBA)-water mixtures at 20 °C.The obtained zero-angle scattering intensities are used together with the isothermal compressibilities and the partial molar volumes to obtain the fluctuations in the particle number and concentration. The values of the concentration fluctuation obtained from X-ray scattering are in agreement with those from light scattering experiments. The concentration dependence of the Kirkwood-Buff parameters has been determined from the concentration fluctuations. The mixing state of the solution has been discussed.

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Conformational characteristics of poly(ethylene imine) (PEI) have been investigated by a rotational isomeric state (RIS) analysis of ab initio molecular orbital (MO) calculations and 1H and 13C NMR experiments for a monomeric model compound, N,N ‘-dimethylethylenediamine (di-MEDA). From the MO and NMR data, it was shown that the C−C and C−N bonds of di-MEDA have high gauche (71−93%) and trans (64−86%) preferences, respectively. Conformational energies of PEI were determined from the MO calculations for di-MEDA at the MP2/6-311++G(3df, 3pd)//HF/6-31G(d) level. The high gauche stability in the C−C bond was indicated to stem from a moderate and a weak intramolecular N−H···N hydrogen bonds; the interaction energies were evaluated as −1.54 and −0.58 kcal mol-1, respectively. The RIS scheme including rotational and inversional isomerizations was developed and applied to PEI to evaluate the chain dimension and diad probabilities. With the conformational energies determined as above, the characteristic ratio and meso-diad probability of PEI at 25 °C were calculated to be 2.9 and 0.63, respectively. In polar and protic solvents, the intramolecular hydrogen bonds are weakened, and consequently the PEI chain extends. Branching effects on the conformation were investigated from MO and NMR analysis for monomeric model compounds of branched PEI, N, N,N ‘-trimethylethylenediamine and N, N,N ‘ ,N ‘-tetramethylethylenediamine; the gauche preference in the C−C bonds, due to the hydrogen bonds, is reduced with increasing number of methyl groups. Ab initio MO calculations were carried out for the double-stranded helix found in anhydrous PEI crystal. The PEI chain was indicated to adopt the isotactic form exclusively. The natural bond orbital analysis showed that intermolecular N−H···N hydrogen bonds are formed between paired chains of the double helix. The enthalpy of association per repeating unit was estimated to be −3.6 kcal mol-1 at the MP2/6-311+G(2d,p)//HF/6-31G(d) level.

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