Temperature and Pressure Effect on Cluster Formation in Diol-Water Mixtures

用差示扫描量热法(DSC)、大角度X射线散射(LAXS)、小角中子散射(SANS)和核磁共振弛豫谱研究了乙二醇(Eg)、1，3-丙二醇(PRD)、1，4-丁二醇(BUD)和1，5-戊二醇(PED)的HO-(CH_2)_n-OH结构的混合二元醇的水溶液。差示扫描量热仪的测量结果表明，二醇(n=2和4)的水溶液混合物在低温下容易结晶，而二醇(n=3和5)的水溶液在-10 0℃之前一直保持过冷状态。对具有偶数个碳原子的纯二元醇的晶体结构的X射线衍射研究表明，二元醇分子很容易通过氢键聚集。这可能是二醇(n=2和4)的水溶液混合物容易结晶的一个原因。在25℃的乙二醇水混合物的SANS实验中，没有观察到明显的小角散射强度，这表明乙二醇水分子是均一的…在解决方案中混合了更狡猾的东西。对富含H_2^&lt；17&gt；O的乙水和PRD水混合物的核磁共振弛豫测量表明，随着温度的降低，各混合物的T_1值逐渐减小。结果表明，随着温度的降低，水分子的自转运动逐渐受到限制。对于甲醇和乙醇的水溶液，已有报道在T_1值曲线上观察到拐点是摩尔分数的函数，拐点对应于结构从水结构到氢键醇链的转变。因此，我们得出结论，二元醇和水分子在二元醇-水混合物中的混合比在甲醇-水和乙醇-水混合物中更均匀，即在二元醇-水混合物中几乎没有发生剧烈的结构转变。这就是二醇-水混合物很容易保持过冷状态的一个原因。目前，利用本项目研制的高压核磁共振池对二醇-水混合物进行了高压实验。较少

Aqueous mixtures of diols, which have an HO-(CH_2)_n-OH structure, of ethylene glycol (eg), 1,3-propanediol (prd), 1,4-butandiol (bud), and 1,5-pentanediol (ped) have been investigated by using differential scanning calorimetry (DSC), large-angle X-ray scattering (LAXS), small-angle neutron scattering (SANS), and ^<17>O-NMR relaxation. The results from DSC measurements revealed that aqueous mixtures of diols (n = 2 and 4) are easily crystallized at low temperature, while those of diols (n = 3 and 5) are kept as a supercooled solution until -100℃. An X-ray diffraction study of a crystal structure of pure diol, which has even number of carbon atoms, showed that the diol molecules easily aggregate via hydrogen bonding. This may be a reason for easy crystallization of aqueous mixtures of diols (n = 2 and 4). From SANS experiments on eg-water mixtures at 25℃ small-angle scattering intensities were not significantly observed, suggesting that ethylene glycol and water molecules are homogeneou … More sly mixed in the solutions. ^<17>O-NMR relaxation measurements on eg-water and prd-water mixtures, where H_2^<17>O is enriched, indicated that T_1 value for each mixture gradually decreases with lowering temperature. It is suggested that the rotational motion of water molecules is gradually restricted with decreasing temperature. In the case of aqueous mixtures of methanol and ethanol, it has been reported that inflection points are observed in plots of T_1 value as a function of mole fraction and the inflection points correspond to structural transition from water structure to hydrogen-bonded alcohol chain. Thus, we concluded that diol and water molecules are more homogeneously mixed in the diol-water mixtures than in methanol-water and ethanol-water ones, i.e. a drastic structural transition scarcely occurs in the diol-water mixtures. This is a reason for that diol-water mixtures can be easily kept at the supercooled state. Now, experiments on diol-water mixtures at high pressures are carrying out by using high pressure NMR cell, which was prepared in this project. Less