Raman Spectroscopic Observations of the Ion Association between Mg2+ and SO42鈥?/sup> in MgSO4-Saturated Droplets at Temperatures of 鈮?80 掳C

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• 作者：Ye Wan ; Xiaolin Wang ; Wenxuan Hu ; I-Ming Chou
• 刊名：Journal of Physical Chemistry A
• 出版年：2015
• 出版时间：August 27, 2015
• 年：2015
• 卷：119
• 期：34
• 页码：9027-9036
• 全文大小：577K
• ISSN：1520-5215

文摘

Liquid鈥搇iquid phase separation was observed in aqueous MgSO₄ solutions with excess H₂SO₄ at elevated temperatures; the aqueous MgSO₄/H₂SO₄ solutions separated into MgSO₄-rich droplets (fluid F1) and a MgSO₄-poor phase (fluid F2) during heating. The phase separation temperature increases with SO₄^{2鈥?/sup>/Mg2+ ratio at a constant MgSO₄ concentration. At a MgSO₄/H₂SO₄ ratio of 5, the liquid鈥搇iquid phase separation temperature decreases with an increase in MgSO₄ concentration up to 鈭?.0 mol/kg and then increases at higher concentrations, showing a typical macroscale property of polymer solutions with a lower critical solution temperature (LCST) of 鈭?71.4 掳C. In situ Raman spectroscopic analyses show that the MgSO₄ concentration in fluid F1 increases with an increase in temperature, whereas that in fluid F2 decreases with an increase in temperature. In addition, HSO₄鈥?/sup>, which does not readily form complexes with Mg2+, tends to accumulate in fluid F2. Analyses of the v₁(SO₄2鈥?/sup>) bands confirmed the presence of four-sulfate species of unassociated SO₄2鈥?/sup> (鈭?80 cm鈥?), contact ion pairs (CIPs; 鈭?95 cm鈥?), and triple ion pairs (TIPs; 鈭?005 cm鈥?) in aqueous solution, and more complex ion pair chain structure (鈭?020 cm鈥?) in fluid F1. Comparison of the sulfate species in fluids F1 and F2 at 280 掳C suggests that SO₄2鈥?/sup> in fluid F2 is less associated with Mg2+. On the basis of in situ visual and Raman spectroscopic observations, we suggest that the formation of the complex Mg2+鈥揝O₄2鈥?/sup> ion association might be responsible for the liquid鈥搇iquid phase separation. In addition, Raman spectroscopic analyses of the OH stretching bands indicate that the hydrogen bonding in fluid F1 is stronger than that in fluid F2, which might be ascribed to the increasing probability of collision of H₂O with Mg2+ and SO₄2鈥?/sup> in fluid F1.}