Raman Spectroscopic Observations of the Ion Association between

Article pubs.acs.org/JPCA

Raman Spectroscopic Observations of the Ion Association between Mg2+ and SO42− in MgSO4‑Saturated Droplets at Temperatures of ≤380 °C Ye Wan,† Xiaolin Wang,*,†,‡ Wenxuan Hu,† and I-Ming Chou§

Downloaded by UNIV OF NEBRASKA-LINCOLN on August 23, 2015 | http://pubs.acs.org Publication Date (Web): August 17, 2015 | doi: 10.1021/acs.jpca.5b02938

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State Key Laboratory for Mineral Deposit Research & Institute of Energy Sciences, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, P. R. China ‡ State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, P. R. China § Laboratory of Experimental Study Under Deep-sea Extreme Conditions, Sanya Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya, Hainan 572000, P. R. China ABSTRACT: Liquid−liquid 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 SO42−/Mg2+ ratio at a constant MgSO 4 concentration. At a MgSO4/H2SO4 ratio of 5, the liquid− liquid phase separation temperature decreases with an increase in MgSO4 concentration up to ∼1.0 mol/kg and then increases at higher concentrations, showing a typical macroscale property of polymer solutions with a lower critical solution temperature (LCST) of ∼271.4 °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−, which does not readily form complexes with Mg2+, tends to accumulate in fluid F2. Analyses of the v1(SO42−) bands confirmed the presence of four-sulfate species of unassociated SO42− (∼980 cm−1), contact ion pairs (CIPs; ∼995 cm−1), and triple ion pairs (TIPs; ∼1005 cm−1) in aqueous solution, and more complex ion pair chain structure (∼1020 cm−1) in fluid F1. Comparison of the sulfate species in fluids F1 and F2 at 280 °C suggests that SO42− 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+−SO42− ion association might be responsible for the liquid−liquid 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− in fluid F1. water budget on Mars.10−13 Epsomite (MgSO4·7H2O) is regarded as the main form of magnesium sulfate salt and consitutes a major source of water on the surface of Europa.13−15 Moreover, as MgSO4 is a typical high-valent electrolyte, the model of ion association in aqueous MgSO4 solutions was used to describe the ion complexation in many other strong electrolyte solutions.16−18 Previous studies have employed various techniques to characterize the ion association in aqueous MgSO4 solutions or even supersaturated droplets, such as dielectric relaxation spectroscopy,19,20 ultrasonic absorption measurements,2,3,17 Raman spectroscopy,4,6,21−26 infrared (IR) spectroscopy,27 and computer simulations,25,26,28 at broad P−T conditions. These studies show that Mg2+ and SO42− can form various ion

1. INTRODUCTION The interaction between Mg2+ and SO42− has attracted a great deal of attention because MgSO4 is very common and plays important roles in many biological, geological, and chemical processes. For example, MgSO4 has significant biological functions as magnesium deficiency can lead to abnormal mineral deposits inducing arteriosclerosis.1 MgSO4 is the second most abundant component of seawater and contributes a great deal to sound absorption,2−5 and its occurrence as an atmospheric aerosol has significant influence on the hygroscopic properties of the atmosphere affecting air quality, visibility, and global climate.6 Sulfate-bearing fluids are also common in aqueous solutions and melts in the lithosphere and play an important role in some geological processes such as mineralization and oil formation.7−9 In addition, much evidence suggests that H2O in hydrated MgSO4 in extraterrestrial environments is the main form of water. For example, the transformation between different MgSO4 hydrates affects the © XXXX American Chemical Society

Received: March 27, 2015 Revised: August 2, 2015

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DOI: 10.1021/acs.jpca.5b02938 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

phase behavior can be reproduced and to obtain reliable temperatures for the liquid−liquid phase separation. The compositions of the immiscible fluids were analyzed in situ with a JY/Horiba LabRAM HR800 Raman spectrometer. Raman spectra were acquired with 532.11 nm (air-cooled frequency-doubled Nd:YAG laser) laser excitation, a 50× Olympus objective, and a 1800 groove/mm grating with a spectral resolution of ∼1 cm−1. Approximately 9.5 mW laser light was focused on a central level of the horizontal tube during the spectral measurements. Spectra were collected from 800 to 1400 cm−1 and from 2750 to 3800 cm−1 for the v1(SO42−) band and the OH stretching band [vs(H2O)], respectively. To minimize the uncertainty in the spectral fitting processes, spectra were acquired for 120 s with three accumulations per spectrum. 2.3. Calibration and Treatment of the Raman Spectra. The Raman spectrometer was calibrated with the v1 band of silicon at 520.2 cm−1.33 The peak areas of the v1(SO42−) and vs(H2O) bands were calculated with Labspec version 5.58.25. The peak area integration ranges are from 920 to 1100 cm−1 for the v1(SO42−) bands and from 2800 to 3800 cm−1 for the vs(H2O) bands. Then, the peak area ratios between the v1(SO42−) and vs(H2O) bands were used to evaluate the MgSO4 concentrations in the immiscible liquid phases. To investigate the species of immiscible fluids F1 and F2, the v1(SO42−) bands were fitted with PeakFit version 4.0 (AISN Software Inc.), using the Lorentz function. The baseline was corrected with the NParm equation, and 1% smoothing was applied to all the spectra. The Raman shift and the full width at half-maximum of vs(H2O) were analyzed with Labspec version 5.58.25 to investigate the hydrogen bonding in the two immiscible liquid phases.

pairs, including double solvent-separated ion pairs (2SIPs), solvent-separated ion pairs (SIPs), and contact ion pairs (CIPs). Investigations of the ion association between Mg2+ and SO42− in MgSO4-saturated droplets is essential for evaluating the hygroscopic properties of sulfate-containing particles, which is important to understanding the behavior of atmospheric aerosols.29 However, previous experimental investigations of the ion association in MgSO4-saturated droplets were mainly conducted at room temperature.6,20,22,23,26,27 In fact, studies of the ion association in MgSO4-saturated solutions at high temperatures may provide clues for modeling the ion complexation between Mg2+ and SO42− and in other strong electrolyte solutions. Unfortunately, such experimental studies are quite limited because of the very low solubility of kieserite (MgSO4·H2O) in aqueous solutions at high temperatures.30 Recently, Wang et al.24 reported the liquid−liquid phase separation in vapor-saturated aqueous MgSO4 solutions; at temperatures above 260 °C, the homogeneous aqueous MgSO4 solution separated into a heavy MgSO4-rich liquid phase (MgSO4-saturated fluid F1) and a light MgSO4-depleted liquid phase (fluid F2). On the basis of the experimental work of Wang et al.,24 we conducted in situ optical and Raman spectroscopic observations of the phase behavior and ion interactions in MgSO4 solutions with excess H2SO4 over a broader temperature range (25−380 °C). The major contributions of this study are as follows. (1) We confirmed the lower critical solution temperature (LCST) phenomenon in the MgSO4/H2SO4/H2O system. The LCST phenomenon was inferred from the critical homogenization between the two immiscible liquids in ref 24. (2) The sulfate concentrations and species of the two immiscible fluids are investigated via in situ Raman spectroscopy. (3) The hydrogen bonds in the two immiscible liquid fluids are also examined by evaluation of the Raman shift and the full width at half-maximum of the OH stretching band.

3. RESULTS AND DISCUSSION 3.1. Liquid−Liquid Phase Separation. The occurrence of liquid−liquid phase separation in solely inorganic aqueous systems at temperatures below 300 °C is intriguing because only a few examples exist.34 To the best of our knowledge, the liquid−liquid phase separation has been mainly reported in phosphate, sulfate, and some uranyl aqueous solutions.34−36 Recently, Wang et al.24 observed the liquid−liquid phase separation in aqueous MgSO4 solutions at temperatures from 260 to 350 °C. The liquid−liquid phase separation in vaporsaturated aqueous MgSO4 solutions was considered to be metastable because the solubility of kieserite decreases with an increase in temperature.24 The hydrolysis of MgSO4 might also contribute to the meta-stability of the liquid−liquid phase separation because the Raman band of HSO4− has been observed at elevated temperatures.24,28 The addition of H2SO4 in an aqueous MgSO4 solution can prevent the hydrolysis of MgSO4. In addition, the presence of H2SO4 in aqueous UO2SO4 solutions might increase the phase separation temperature of a constant UO2SO4 solution,37 whereas the effect of H2SO4 in an aqueous MgSO4 solution has not been documented. To better investigate the liquid−liquid phase separation in aqueous MgSO4 solutions, excess H2SO4 was added to the solution to prevent the hydrolysis of MgSO4. Figure 1 shows the phase behaviors of a 1.5 M MgSO4/0.3 M H2SO4/H2O system in a fused silica capillary capsule during heating and cooling processes. When the sample was heated to 273.3 °C along the liquid−vapor curve, the originally homogeneous aqueous phase separated into two immiscible liquid phases (Figure 1a,b), MgSO4-rich F1 droplets and the

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. The MgSO4 solutions were prepared from commercially available analytical grade anhydrous MgSO4 (Sigma, 99.5%) and distilled deionized water with the resistivity of 18.2 kΩ. Excess concentrated H2SO4 (0.5 mol/L) was added into two series of MgSO4 solutions to adjust the MgSO4/H2SO4 molar ratio to 10 and 5. The MgSO4 concentrations were prepared with the following MgSO4 molalities: 0.1, 0.5, 1.0, 1.5, and 2.0 mol/kg. The sample solutions were loaded in fused silica capillary capsules (FSCC) with a 300 μm outer diameter and a 100 μm inner diameter. The length of the prepared FSCC is ∼2 cm. The detailed FSCC preparation procedures were described in refs 31 and 24. 2.2. Microscopic Investigation and Collection of Spectra. Microscopic observation and collection of Raman spectra were conducted in the laboratory of the Institute of Energy Sciences of Nanjing University. Fused silica capillary capsules containing sample solutions were placed in a Linkam CAP500 heating−cooling stage.32 The reported temperatures are accurate to approximately ±0.1 °C. To ensure the system reaches an equilibrium state, the heating/cooling rates were set at 2 °C/min below 250 °C and 1 °C/min above 250 °C. The phase behaviors were observed through a Leica microscope with a 10× objective. A 5 megapixel CCD (QImaging Corp.) and Linkam Linksys 32 software were used to continuously record the phase behavior in the FSCC. The heating and cooling processes were repeated three times to make sure the B


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Figure 2. Relationship between the phase separation temperature and the composition of the MgSO4/H2SO4/H2O system. Filled symbols represent data for the system showing normal homogenization; the volume of fluid F1 decreased with a decrease in temperature, and immiscible fluids F1 and F2 eventually homogenized into the aqueous phase. The empty symbols represent data for systems showing critical homogenization; the boundary between fluids F1 and F2 faded away with a decrease in temperature. Data plotted are those listed in Table 1 and those reported by Wang et al.24

Figure 1. Photomicrographs of the phase behavior of a 1.5 M MgSO4/ 0.3 M H2SO4/H2O system in an FSCC along the liquid−vapor curve at elevated temperatures: (a) homogeneous MgSO4−H2SO4 solution, (b) original homogeneous solution separated into two coexisting liquid phases, (c and d) small fluid F1 droplets that coalesced into larger ones as the temperature increased, and (e−h) boundary between immiscible fluids F1 and F2 that faded away in the homogenization process during cooling.

concentration up to 2 M, the increase in the liquid−liquid phase separation temperature at higher concentrations (>2 M) was not demonstrated in their experiments because these solutions could not be prepared at room temperature because of the limited solubility of epsomite in water. However, in the presence of H2SO4, this increase in phase separation temperature with an increase in MgSO4 concentration can be observed (Figure 2 and Table 1). At a constant MgSO4 concentration,

MgSO4-poor F2 phase. In the following heating process, the dispersed F1 droplets coalesced to form larger droplets (Figure 1c,d). During the subsequent cooling process, the phase boundary between the immiscible liquid phases faded away with a decrease in temperature and disappeared at ∼273 °C (Figure 1e−h), which was described as critical homogenization by Wang et al.24 In this study, the critical homogenization between immiscible fluids F1 and F2 was also observed in more concentrated aqueous MgSO4 solutions [2 M MgSO4/0.2 M H2SO4/H2O system (Figure 2)]. For the solutions containing 1.5 M), the concentration difference between fluids F1 and F2 is minor near the homogenization temperature, resulting in critical homogenization during the cooling process. While in dilute solutions, the MgSO4 concentrations in fluids F1 and F2 are still quite different near the liquid−liquid phase separation temperature because of the very low concentration in fluid F2, and normal homogenization between fluids F1 and F2 will occur. The v1(SO42−) bands for both immiscible fluids F1 and F2 are shown in Figure 5a−d. It should be noted that the HSO4− band instead of the v1(SO42−) band was observed in the spectra for fluid F2 at temperatures above 320 °C (Figure 5c), indicating that SO42− tends to accumulate in fluid F1, whereas HSO4− is the dominant sulfate species in fluid F2 at high temperatures (Figure 5b−d). 3.2.2. Sulfate Speciation. A free SO42− ion has Td symmetry and nine modes of internal vibration spanning the representation Γvib = A1 + E + 2F2.23,44 All of these modes are Raman active, among which v1(SO42−) (A1) is the strongest and very sensitive to the molecular environment. It can provide clues for studying ion association between metal cations and sulfates28 and therefore is the only band being discussed in this work. Even though the broader v3(SO42−) (F2) band is also observed (Figure 3), its intensity is much lower than that of the v1(SO42−) mode and can be detected only in the immiscible liquid F1 phase. The v1(SO42−) bands of the 1 M MgSO4/0.1 M H2SO4/H2O system show an overall blue-shift from 980 to 995 cm−1 in an aqueous solution, and from 995 to 1020 cm−1 in fluid F1. Meanwhile, the band became much broader with an increase in

Figure 3. vs(H2O) intensity-normalized Raman spectra of fluids F1 and F2 at 300 and 350 °C.

However, it is difficult to associate the Raman intensity of the v1(SO42−) bands directly with the actual sulfate concentrations of F1 and F2 phases. For quantitative analysis of fluid inclusions in minerals by a Raman spectrometer, Wopenka and Pasteris45 suggested a practical approach based on Placzek’s ratio method. For Raman active species a and b in the same phase, the relative concentration, C (e.g., moles or mole percent), is related to their Raman peak area, A, by the formula A a /Ab = (Ca /C b)(σa /σb)(ηa /ηb) = (Ca /C b)(Fa /Fb)

(1)

where σ, η, and F are the Raman scattering cross section, instrumental efficiency, and Raman quantification factor, respectively. This approach has been applied to measure the sulfate concentration in fluid inclusions by Dubessy et al.;46 a linear relation was reported between the concentration of sulfate and the ratio of the peak area of v1(SO42−) and vs(H2O) bands (Asulfate/Awater). Theoretically, the ratio Asulfate/Awater can be used to measure the sulfate concentration in immiscible fluids F1 and F2 because the sulfate concentration is proportional to the Asulfate/Awater ratio (eq 1). Unfortunately, there is no reported calibration curve for calculating the sulfate concentration in liquid phases at elevated temperatures, and it is impossible for us to obtain the exact sulfate concentrations in the immiscible fluids. However, the changes in the Asulfate/Awater D


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Figure 5. Intensity-normalized v1(SO42−) bands of (a) aqueous solutions from 25 to 250 °C, (b) fluid F1 from 280 to 380 °C, (c) fluid F2 from 280 to 380 °C, and (d) immiscible fluids F1 and F2 at 280 °C. The deconvolution of the v1(SO42−) bands of aqueous solution at 225 °C (e) and fluid F1 at 300 °C (f) in the 1 M MgSO4/0.1 M H2SO4/H2O system.

collected for the aqueous solutions and fluid F1, have been decomposed into four sub-bands peaking at approximately 980, 995, 1005, and 1020 cm−1 (Figure 5e,f). The first three v1(SO42−) modes were generally assigned to hydrated or unassociated SO42−, a contact ion pair (CIP), and a triple ion pair (TI), respectively.6,20,21,28 The v1(SO42−) mode at ∼1020 cm−1 has been reported in several publications.23,24 Wang et al.24 assigned this v1(SO42−) mode to chain structures or polymers because experimental investigations and theoretical computations suggested that the wavenumber of the v1(SO42−)

temperature (Figure 5a,b); the full width at half-maximum (fwhm) of v1(SO42−) bands increased from 7.5 cm−1 at 25 °C to 31.6 cm−1 at 250 °C in aqueous solutions and from 47.8 cm−1 at 280 °C to 61.5 cm−1 at 380 °C in fluid F1. Previous studies suggested that the v1(SO42−) band could be used to study the interactions between SO42− and Mg2+ because the frequencies of the v1(SO42−) bands depended mainly on the type and number of Mg 2+ −SO 4 2− bonds.21,22,28,44 To investigate the ion association between Mg2+ and SO42−, the v1(SO42−) bands ranging from 920 to 1100 cm−1 in the spectra, E


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fluid F2 (Figure 5b−d). Considering that the v1(SO42−) component at ∼1020 cm−1 can be detected in only fluid F1, we speculate that the liquid−liquid phase separation might be induced by the formation of ion pair chains or polymer(s). In polymer solutions, the strong directional attractive interactions of hydrogen bonding between the polymer and water give rise to favorable heats of solution and a miscible liquid state at low temperatures (below the phase separation temperature). The extent of hydrogen bonding decreases with an increase in temperature (will be discussed in the next section), and the polymer solutions become unstable at temperatures above the phase separation temperature; the contact between water and hydrophobic surfaces approaches the minimum, and the polymers transform to much denser structures to induce liquid−liquid phase separation.39,47,48 In the MgSO4/H2SO4/ H2O system, at lower temperatures (i.e., 310 °C (Figure 5b)]. This observation supports the previously proposed idea that HSO4− is a noncomplexing anion at higher temperatures.50 For example, no Al3+−HSO4− ion pairing has been observed in aqueous Al2(SO4)3 at temperatures of ≤184 °C.51 The accumulation of HSO4− in fluid F2 also indicates the stronger ion interactions in fluid F1. The relatively weaker ion interaction between Mg2+ and HSO4− might be used to explain the increase in phase separation temperature in the MgSO4/ H2SO4/H2O system with more H2SO4 at a constant MgSO4 concentration. The second dissociation constant of H2SO4 decreased sharply with an increase in temperature, from 1.028 × 10−2 at 25 °C to 4.59 × 10−6 at 275 °C;52 hence, with the addition of H2SO4, more sulfate exists as HSO4− at elevated temperatures through the reaction


band shifted to a higher value with an increasing length of contact ion pairs.28,29 The fractions of v1(SO42−) components were calculated using the peak area ratios of corresponding v1(SO42−) modes.24 As shown in Figure 6, the fractions of unassociated SO42−

Figure 6. Peak area fractions of different sulfate species in homogeneous aqueous solutions from 25 to 250 °C and in fluid F1 from 280 to 380 °C.

decrease with an increase in temperature and the typical peak of unassociated SO42− at ∼980 cm−1 cannot be detected in fluid F1 at temperatures of >310 °C. The fraction of CIP (f 995) increases with an increase in temperature at temperatures below 250 °C in aqueous solutions, whereas f 995 decreases with an increase in temperature at temperatures above 250 °C, especially in fluid F1. The fractions of TI ( f1005) and ion pair chains (f1020) increase with an increase in temperature in the investigated temperature range. The decrease in the fractions of unassociated SO42− and simple ion pair (CIP) as well as the increase in the fractions of TI and ion pair chains with an increasing temperature in fluid F1 indicates the transformation of unassociated SO42− and simple ion pair(s) to TI and Mg2+− SO42− polymer(s).24 Meanwhile, it seems that the ion interaction between SO42− and Mg2+ in fluid F2 is relatively weaker than that in fluid F1, as supported by two pieces of evidence. (1) As illustrated in Figure 5d, in the Raman spectra of fluids F1 and F2 at the same temperature, the frequency of v1(SO42−) in fluid F1 is higher than that in fluid F2, and (2) the width of the v1(SO42−) band in fluid F1 is broader than that in

H 2SO4 + SO4 2 − ⇌ 2HSO4 −

(2)

Figure 7. Raman spectra of vs(H2O) in the wavenumber range of 2800−3800 cm−1 in (a) aqueous solutions from 25 to 250 °C, (b) F1 droplets from 280 to 380 °C, and (c) fluid F2 from 280 to 380 °C for the 1 M MgSO4/0.1 M H2SO4/H2O system. F


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Figure 8. Temperature dependence of the peak position and full width at half-maximum of vs(H2O) of immiscible fluids F1and F2 in the 1 M MgSO4/0.1 M H2SO4/H2O system.

and both immiscible fluids F1 and F2 (Figures 7 and 8). As the aqueous MgSO4-bearing solution in the FSCC was heated to 380 °C along the liquid−vapor curve (near supercritical condition at temperatures above 370 °C), the variation of the pressures in the FSCC is very slight compared with that of temperature. Then, as shown in Figure 8, an increase in temperature results in an obvious decrease in the number of hydrogen bonds in aqueous solutions and fluid F2. In fluid F1, the extent of hydrogen bonding also decreases with an increase in temperature. However, at the same temperature and pressure, the hydrogen bonding in fluid F1 is stronger than that in fluid F2. Also, with an increase in temperature, the decrease in the level of hydrogen bonding in fluid F1 is less significant than in fluid F2. Mg2+ can be easily hydrated because of its large charge to radius ratio. Hydrated Mg2+ has six water molecules in the inner sphere, and the cluster is thought to be of Th symmetry.62−64 Because of the polarization effect of Mg2+ on the inner hydration layer of H2O, the hydrogen bonding between the first and second hydration layers is significantly strengthened.23,27,29 SO42− is also considered to be a “structure maker” as it can form hydrogen bonds with water molecules.65,66 The hydrogen bonding between SO42− and H2O was thought to be slightly stronger than that between water molecules.44,67 Therefore, the net effect of MgSO4 on hydrogen bonding in aqueous solutions might be positive.27 The probability of collision of H2O with Mg2+ and SO42− increases with an increase in temperature in fluid F1, whereas that between water molecules decreases, because the sulfate concentration increases with an increase in temperature (Figure 4). Although the strength of hydrogen bonding decreases with an increase in temperature, the interaction between H2O and the solute in saturated fluid F1 partly offsets the effect of temperature on hydrogen bonding. As the temperature increases, fluid F1 becomes more saturated. Thus, the ultimate situation might be that the water molecules are fitted into the framework of various Mg2+−SO42− ion pairs and ion pair chain(s). In this case, the hydrogen bonding enhanced by Mg2+ and SO42− in very concentrated fluid F1 should be stronger than that between water molecules. For fluid F2, the dominant sulfate speciation is HSO4− (Figure 5), the polarization effect of which is weaker than that of SO42−, and hydrogen bonding between HSO4− and H2O is weaker than that between SO42− and water molecules. Therefore, the

which decreases the attractive forces between anions and cations. The decrease in the attractive interactions between anions and cations adversely affects the polymerization between Mg2+ and SO42−; therefore, H2SO4 functions as a repression of phase separation. 3.2.3. Hydrogen Bonding. Water is an associated liquid with hydrogen bonding among the water molecules. Previous studies have suggested that Raman spectroscopy can be used to investigate the hydrogen bonds as well as the structure of water.42,53−57 Theoretically, isolated water molecules have three Raman active vibrational modes: two for symmetry species A1 (symmetric stretching mode ν1 at 3657.05 cm−1 and bending mode ν2 near 1595 cm−1) and one for symmetry species B1 (antisymmetric stretching vibration ν3 at 3755.97 cm−1).58,59 In the liquid phase, the number of observed vibrational transitions is duplicated by intermolecular coupling through nonharmonic hydrogen bonding. In addition, overtone 2ν2 of the bending vibration falls in the region of the lowest stretching vibration, being mutually perturbed by the Fermi resonance.53 Therefore, the Raman spectrum of liquid water is a complex profile of several broad and overlapping bands in the OH stretching region (2800−3800 cm−1) with a bending mode at ∼1630 cm−1.53 Because the Raman intensity of the bending mode is weak, it is not discussed in this work. As shown in Figures 7 and 8, with an increase in temperature, the OH stretching vibration bands [vs(H2O)] of the aqueous solution and immiscible fluids F1 and F2 become sharper and shift to higher wavenumbers. With respect to immiscible liquid phases F1 and F2, the vs(H2O) band of fluid F2 is sharper and shifts to a higher wavenumber at the same temperature (Figures 7 and 8). In addition, although the vs(H2O) peak position and the fwhm show a similar variation tendency with an increase in temperature, the changes in the wavenumber and fwhm are less significant in fluid F1 than in fluid F2; from 280 to 380 °C, a 3 cm−1 blue-shift and a 38 cm−1 decrease in fwhm were observed in fluid F1, whereas a 30 cm−1 blue-shift and a 93.2 cm−1 decrease in fwhm were observed for fluid F2 (Figure 8). The variation of the frequency (or wavenumber) at maximal intensity (vs) and fwhm has been thought to be in close association with the hydrogen bonding between water molecules.40,42,60,61 The vs(H2O) bands become sharper, and the vs shifts to a higher wavenumber with an increase in temperature, indicating that the number of hydrogen bonds decreases with an increase in temperature in aqueous solution G


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hydrogen bonding in fluid F1 is stronger than that in fluid F2 at the same temperature.

However, compared with the vs(H2O) spectra in fluid F2, the vs(H2O) spectra of fluid F1 are relatively broader and shift to lower wavenumbers at the same temperature. In other words, the hydrogen bonding is stronger in fluid F1 than in fluid F2. This can be ascribed to the higher probability of collision between H2O and Mg2+ and SO42− in fluid F1 than between water molecules in fluid F2; the presence of Mg2+ and SO42− in aqueous solutions can strengthen the hydrogen bonds. With an increase in temperature, the concentration of sulfate increases in fluid F1. Finally, the water molecules might be inserted into the network formed by strong ion associations between Mg2+ and SO42−. As a result, the hydrogen bonding in fluid F1 should be much stronger than in fluid F2 at the same temperature.

4. CONCLUSION Aqueous MgSO4/H2SO4 solutions with MgSO4/H2SO4 ratios of 10 and 5 were found to separate into two immiscible liquid fluids F1 and F2 at elevated temperatures (>266.5 °C) along the liquid−vapor curve. The compositions of homogeneous aqueous solutions and immiscible fluids were analyzed via in situ Raman spectroscopy at temperatures up to 380 °C. We suggested that the liquid−liquid phase separation was triggered by the formation of Mg2+−SO42− ion pair chain(s). The main contributions of this work are as follows. (1) We observed the liquid−liquid phase separation in aqueous MgSO4 solutions with excess H2SO4. With MgSO4 solutions with a MgSO4/H2SO4 ratio of 5 as an example, the aqueous solution separated into MgSO4-rich droplets (fluid F1) and a MgSO4-depleted liquid phase (fluid F2) at temperatures above 266.5 °C. The liquid−liquid phase separation temperature decreases with an increase in MgSO4 concentration in the low MgSO4 concentration range (1.0 M). This unusual phase behavior exhibits a lower critical solution temperature (LCST) phenomenon, which is a typically macroscale property of polymer solutions. The LCST phenomenon indicates that complex ion association may occur in aqueous MgSO4 solutions at elevated temperatures. In addition, the liquid− liquid phase separation temperature increases with an increase in H2SO4 concentration in a constant MgSO4 solution. The LCST also increases with H2SO4 concentration and tends to occur in more dilute MgSO4 solutions. (2) The ratio of integrated Raman intensity between v1(SO42−) and vs(H2O) bands was used to reflect the sulfate concentration of the two immiscible liquid fluids. Our observations show that the sulfate concentration increases with an increase in temperature in fluid F1, whereas the sulfate concentration of fluid F2 decreases with an increase in temperature. The deconvolution of the v1(SO42−) bands shows the existence of unassociated SO42−, a contact ion pair (CIP), and a triple ion pair (TI) at approximately 980, 995, and 1005 cm−1, respectively. Besides, an unusual v1(SO42−) mode was detected only in fluid F1 at ∼1020 cm−1. Considering the v1(SO42−) band shifts to a higher wavenumber with an increase in the length of contact ion pairs, the v1(SO42−) mode at ∼1020 cm−1 should be assigned to ion association more complex than TI, probably ion pair chain(s) or polymer(s). The v1(SO42−) intensity-normalized Raman spectra of liquid fluids F1 and F2 show that the dominant sulfate species in fluid F1 are various ion pairs and polymer(s), whereas fluid F2 is characterized by the predominant signal of HSO4− near 1050 cm−1, which is very weak in fluid F1. In addition, only the signals of unassociated SO42−, CIP, and TI were observed in the v1(SO42−) bands of fluid F2, while the bands of ion pair chain(s) were obvious in fluid F1. These observations suggest that the ion complexation is stronger in fluid F1 than that in fluid F2, and the ion interactions between Mg2+ and SO42− are stronger than those between Mg2+ and HSO4−. (3) The vs(H2O) band, with an increase in temperature, becomes sharper and shifts to a higher wavenumber, and the fwhm decreases in aqueous solutions and both immiscible liquid fluids. These observations suggest that the level of hydrogen bonding decreases with an increase in temperature.

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AUTHOR INFORMATION

Corresponding Author

*Department of Earth Sciences, Nanjing University, 163 Xianlin Ave., Nanjing, Jiangsu 210023, P. R. China. E-mail: [email protected]. Telephone: +86 (25) 89680867. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the associate editors, Profs. James Lisy and Martin T. Zanni, and three anonymous referees for their careful reviews and constructive comments and suggestions. This work was supported by National Natural Science Foundation of China (Grants 41203045 and 41230312), the Knowledge Innovation Program (SIDSSE-201302), China’s National Science & Technology Special Project (Grant 2011ZX05005-002008HZ), the Hadal-trench Research Program (Grant XDB06060100) of the Chinese Academy of Sciences, and the foundation of the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Grant PRP/ open-1203).

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