Small-angle X-ray scattering studies on inhomogeneity in aqueous mixtures Small-Angle X-Ray Scattering Studies onInhomogeneity In Aqueous Mixtures

博士論文

Aqueous solutions of small organic molecules often show anoma- lous behavior in various transport and thermal properties. The formation of some kind of molecular aggregates or microinhomogeneity has generally been invoked for explaining these peculiarities. However, details of the microinhomogeneity are not clear as yet. 　　　The small angle X-ray scattering (SAXS) method is one of the most direct ones to study microinhomogeneity. In the analysis, SAXS intensity curve (I(s)) is expanded as follows: 　　　　I(s)=r<small>0</small>-r<small>2</small>s²+r<small>4</small>s⁴-r<small>6</small>s⁶+ ... ...(1) where 　　　　r<small>2i</small>=(1/(2i+1)i)<Δρ<small>e</small>(O)Δρ<small>e</small>(r)>r²ⁱ 4πr²dr. . . . (2) Here s is the scattering parameter(s=4πsinθ/λ,where 2θ is the scattering angle and λ is the wavelength of X-rays), Δρ<small>e</small>(r) is the difference in the electron density from the average at position r, and < > denotes the ensemble average. The 'moment' determined through a SAXS measure- ment (r<small>2i</small>) reflects various properties of the solution. The zero-angle scat- tering intensity (I(O)), which relates to the square of the number of mole cules concerning the microinhomogeneity, is equal to r<small>0</small> . The mean square fluctuation is concentration (<N>(ΔC<small>1</small>)²>) is derived from I(O). The correla- tion length (ξ), which indicates the average size of the microinhomogenei- ty, is calculated as (r<small>2</small>/r<small>0</small>)^0.5. These two (r<small>0</small> and r<small>2</small> ) have been chief concern in the previous SAXS studies. 　　　Higher order coefficients in eq. (1), which have not been employed in previous SAXS studies, have a possibility to give additional information about microinhomogeneity as long as accurate enough SAXS data can be accumulated. To understand the complicated mixing state of the molecular aqueous solutions, it is required to use as many kinds of indicators of microinhomogeneity as possible. Thus, in this thesis, the author has attempted to apply the higher order coefficients to studying the mixing state. 　　　A point-focusing diffractometer with a double-bent LiF crystal monochromator as well as a position-sensitive proportional counter has been constructed to obtain accurate SAXS data. With this diffractometer, small-angle resolution of better than 0.42 is achieved. To evaluate the performance of the diffractometer, SAXS data on several pure liquids and aqueous solutions were measured. The results showed that this diffrac- tometer produced incident beams intense enough to measure SAXS on solutions, making a collimation effect negligible simultaneously. 　　　With the diffractometer mentioned above, it is possible to accurately determine the higher order coefficients. Accordingly, the following new parameter (χ) is proposed: 　　　　χ=r<small>0</small> r<small>4</small>/r<small>2</small>² . . (3) Being related with r<small>4</small> , this dimensionless parameter reflects the shape of I(s). Several theoretical SAXS functions were examined in order to under- stand the physical meaning of χ further, and χ has been interpreted to represent the size dispersion of fluctuating clusters formed in solution: a large/small χ corresponds to a large/small size dispersion. 　　　To examine whether or not an analysis in terms of χ is meaningful in understanding the mixing state, the SAXS curve shape of 2-butoxyeth- anol (BE) and 1-propanol (abbreviated to NPA) aqueous solution has been extensively studies. The behavior of χ for the two aqueous solutions shows marked contrast. 　　　BE aqueous solution has a lower critical solution temperature near room temperature, and its correlation length (ξ) and concentration fluctua- tion (<N><(ΔC<small>1</small>)²>) showed very large variations with temperature and concentration. In accordance with <N><(ΔC<small>1</small>)²> and ξ,χ markedly changed with both concentration and temperature. 　　　On the other hand, NPA mixes with water at any concentration and at any temperature. ξ changed gradually with concentration but showed almost no change with temperature. The χ of NPA aqueous solution changed smoothly with concentration but varied only slightly with tempera- ture; modest/immodest local structure change with temperature and concentration of NPA/BE aqueous solution is reflected on χ. Thus, it has been experimentally proved that χ is very sensitive to the mixing state and is accordingly available as a useful parameter. 　　　In terms of the obtained χ's, a new piece of information about clus- ters formed in the two aqueous solutions can be discussed. In BE aque- ous solution, χ is the smallest and is about 0.8 near the critical composi- tion (around BE 5 mole%) at all the temperature studied and near the< miscibility curve in phase diagram. In NPA aqueous solution, χ takes a minimum value and is about 1.1 around NPA 15 mole% at all the tempera- ture studied. Because χ is small, it is predicted that clusters having a fairly well-defined size are dominant in these regions. This interesting sugges- tion should be ascertained in future work. 　　　A use of the diffractometer constructed in this work has also made it possible to obtain SAXS data within a short time. Hence, the growth of nickel and iron silicates in the mixed solution of metal ethylene glycolates, water, and tetraethoxysilane has been monitored with SAXS. At an early stage of polymerization, the fractal dimension (D) increases, reaching a constant value while the reaction is still in progress. The D value suggests that the structure of the gels containing metals is different from that made without metals. Useful catalysts are prepared by the calcination of the gels. A relationship between the structures of the gel and of the catalyst is discussed.

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