J. Phys. Chem. C 2008, 112, 12381–12385
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Adsorption of Sodium Dodecyl Sulfate at THF Hydrate/Liquid Interface J. S. Zhang,† C. Lo,† P. Somasundaran,‡ S. Lu,‡ A. Couzis,† and J. W. Lee*,† Department of Chemical Engineering, The City College of New York, 140th Street and ConVent AVenue, New York, New York 10031, and Department of Earth and EnVironmental Engineering, Columbia UniVersity, New York, New York 10027 ReceiVed: March 5, 2008; ReVised Manuscript ReceiVed: June 10, 2008
Understanding the interaction between sodium dodecyl sulfate (SDS) and gas hydrates provides insight into the role of SDS in promoting gas hydrate formation. The aim of this study was to investigate the relationship between tetrahydrofuran (THF) hydrate induction and SDS adsorption at the hydrate/liquid interface. The adsorption behavior was studied by ζ-potential and pyrene fluorescence measurements. The negative charge of the hydrate particles remains constant at SDS concentrations of 0 to 0.17 mM. The ζ-potential becomes more negative as the SDS concentration increases from 0.17 to 3.4 mM. The micropolarity of the THF hydrate/ liquid interface decreases with increasing SDS concentrations, and then it remains almost unchanged at SDS concentrations above 0.17 mM. A monolayer of DS- is completed at a SDS concentration of 0.17 mM. The reduction of induction time in the presence of SDS levels off at a SDS concentration of 0.17 mM. This provides strong evidence that the short induction is due to the adsorption of DS- at the hydrate/liquid interface. The adsorption study of SDS on THF hydrates can be extended to other systems and we may screen suitable surfactants for accelerating or retarding gas hydrate formation. 1. Introduction Gas hydrates are nonstoichiometric crystalline compounds in which guest molecules such as methane, propane, carbon dioxide, and tetrahydrofuran (THF) stabilize the hydrogenbonded water cavities.1 The size of the guest molecules determines the crystal structures of gas hydrates; e.g., methane forms sI hydrates, and propane forms sII hydrates. If all of the cavities of sI and sII hydrates are singly occupied by gas molecules, one volume of hydrates can store up to ∼170 volumes of gas at STP conditions. Gas hydrates usually form at ambient temperatures (typically 0.6 MPa).2 Thus, gas hydrates are a promising medium for gas storage. One obstacle to industrial application of hydrate storage is the long induction time and low growth rate. Many studies have found that sodium dodecyl sulfate (SDS) not only reduces the induction time but also accelerates the growth rate under static conditions.3–6 It was proposed that SDS forms micelles in the aqueous phase that act as nucleate sites for hydrate nucleation.3 However, several recent studies have suggested that SDS micelles are not present in the aqueous phase under hydrate formation conditions if the formation temperature is below the normal Krafft point (281-289 K) for SDS.5–11 One possible mechanism is that SDS molecules adsorb on hydrate nuclei, which reduces the energy barrier of hydrate nucleation.6 Thus, detailed knowledge of this interaction between SDS and hydrates can clarify the role of SDS in hydrate formation. However, the subject of this topic is not well understood at the molecular level. The main goal of this paper is to understand the relationship between SDS adsorption at the hydrate/liquid interface and gas hydrate induction times. * Author to whom correspondence should be addressed. Telephone: 212650-6688. Fax: 212-650-6660. E-mail:
[email protected]. † Department of Chemical Engineering, The City College of New York. ‡ Department of Earth and Environmental Engineering, Columbia University.
Most gas hydrates, such as methane hydrates, are not suitable for studying the adsorption of SDS on hydrates because they are usually unstable at room temperature and under atmospheric pressure. THF forms sII hydrates with a maximum melting point of 277.9 K at atmospheric pressure12,13 and thus the THF hydrate is a good model system for investigating the interaction between SDS and hydrates. This work will present the first study of SDS adsorption at the hydrate/liquid interface by using ζ-potential and fluorescence measurements. The ζ-potential measurement provides qualitative information on the SDS adsorption density, whereas the fluorescence spectroscopy is used to obtain information on the micropolarity of the hydrate/liquid interface. We will discuss a possible configuration of adsorbed SDS at the hydrate/liquid interface based on the experimental data. 2. Experimental Section Materials. Tetrahydrofuran and sodium dodecyl sulfate with a purity of + 99% were purchased from Sigma-Aldrich (ACS reagent grade). The pyrene used for fluorescence had a purity of >99.0% and was supplied by Fluka. All chemicals were used as received without further purification. Deionized water was produced in our laboratory with a resistivity of 17 mΩ cm-1. ζ-Potentials. THF solutions (10 wt %) in different SDS concentrations were prepared. THF hydrates were formed by placing the solution in a freezer at a temperature around 269 K for 1 day. The THF hydrate and ice mixtures then were melted in an ultrasonic cleaner at room temperature to remove any bubbles from the solution by ultrasonication. A 1-mL aliquot of THF solutions was transferred to a Folded Capillary zeta cell (DTS1060, Malvern Instruments) once most, but not all, of the hydrates were dissociated. The cell was then inserted to a Zetasizer Nano ZS (Malvern Instruments) and the temperature of the cell was set to 276.4 K. The ζ-potential of THF hydrates were consecutively measured after the cell was maintained at 276.4 K for 0.5 to 1 h to allow the hydrate formation to complete inside the cuvette cell.
10.1021/jp801963c CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
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Figure 1. Schematic diagram of experimental setup.
Steady-State Fluorescence. Solutions containing 20 g of THF solution (10 wt %) in different SDS concentrations were placed in a refrigerator at 269 K until hydrates were observed, and then the sample was maintained at 276.2 K overnight. Pyrene was added to the slurry and its concentration in the liquid was approximately 10-6 M. The samples were excited using a UV light source’s 335 nm line and emission spectra were recorded between 360 and 500 nm at a temperature of 276 K. Emission was also collected between 360 and 500 nm at room temperature for THF solutions in 0 and 3.47 mM SDS. Induction Time. Figure 1 shows the schematic diagram of the experimental setup to analyze formation kinetics of THF hydrates in different SDS concentrations. The volume of the stainless steel reactor is 375 mL. The temperature of the reactor was controlled by circulating the coolant from an Isotemp 3006P thermostat (Fisher Scientific) with a stability of ( 0.01 K inside the jacket around the cell. The temperature of the reactor was monitored with two type-T thermocouples (Omega Engineering), where one was immersed in the liquid and the other was placed on the wall. The uncertainty of the temperature measurement is ( 0.2 K. The temperatures of the reactor were sampled every 20 s by the Labview interface. After every run, to remove any memory effect from hydrate particles or used water, we kept the reactor at room temperature for several hours, cleaned the reactor with tap water several times, and finally rinsed it with D.I. water followed by drying it for at least 10min. Then, 40 g of fresh THF solution (20 wt %) was charged to the reactor, followed by setting the thermostat to 278.2 K. After the reactor was kept at this temperature for 0.5 h, the reactor was cooled down to 268.2 K. We held this temperature until we observed the temperature spike due to THF hydrate formation, and we define this holding period as the induction time. 10 wt % THF Solution vs 20 wt % THF Solution. The concentration of THF used in the induction time measurements was 20 wt % to avoid the formation of ice at temperatures below 273 K whereas 10 wt % THF solution was used in the ζ-potential and fluorescence measurements. The reason why we used 10 wt % instead of 20 wt % THF solution in adsorption study is that the mass fraction of solid THF hydrates is more than 90% under measurement conditions with an initial 20 wt % THF solution according to equilibrium conditions of THF hydrates,12 which makes ζ-potential and fluorescence measurements impossible. 3. Results and Discussion ζ-Potential of THF Hydrates at 276.4 K. The hydrate dissociation temperature for 10 wt% THF solutions is 276.8 K
Zhang et al.
Figure 2. ζ-Potential of THF hydrate slurries as a function of SDS concentrations.
at atmospheric pressure.12,13 Delahaye et al.12 reported that the hydrate equilibrium temperature for 9.6 wt% THF solutions is 276.4 K. Therefore, the mass fraction of THF hydrates at 276.4 K is estimated as about 4.0% with an initial THF concentration of 10 wt % based on the phase diagram of THF-water system.12 This low conversion of THF to hydrates is utilized to ensure that the size of some THF hydrate particles falls in the range of ζ-potential measurements (up to 10 µm) because it was reported that the mean size of THF hydrate particles increases as more hydrates form.14 The particle size measured from the Zetasizer is between 3 and 10 µm. Above 6 µm, the size measurement in this machine may not be accurate. Thus, we will use another laser scattering machine to accurately measure the particle size, which is not in the scope of this work but will be reported on in another work. Figure 2 shows that a negative charge exists at the slipping plane of the THF hydrate/liquid interface in the absence of SDS, which is due to the adsorption of anions at the hydrate/liquid interface. The samples are exposed to the atmosphere during preparation and measurements, thus they are saturated with carbon dioxide. The anions that can exist in open THF + water systems are hydroxide (OH-), bicarbonate (HCO3-), and carbonate (CO32-) in fresh deionized water. The partial pressure of carbon dioxide in the air is about 3.5 × 10-5 MPa and the pH of the THF solution sample is close to 5.8, thus the concentration of bicarbonate is around 3 µM according to the Henry’s constant at 276.4 K given by Carroll and Mather,16 which is 2 orders of magnitude higher than the concentration of OH- and 5 orders of magnitude higher than the concentration of carbonate. Drzymala et al.17 reported that there is no preferential adsorption of OH- over H+ at the ice/water interface and the surface charge is dependent on pH. This will be the same for the hydrate/water interface because the molecular structure of hydrate surface is similar to that of ice.18 At pH 5.8, [H+] is greater than [OH-] in the THF hydrate suspension and then the surface charge should be positive. However, the surface charge is negative with no SDS as shown in Figure 2. Thus, we can suggest that bicarbonate (HCO3-) adsorbs at the hydrate/ water interface. A possible mechanism for the bicarbonate adsorption is hydrogen bond formation at the hydrate/liquid interface. Compoint et al.19 suggested that formic and acetic acids adsorb on the ice surface via two hydrogen bonds between oxygen and hydrogen atoms of carboxyl group (COOH) and water molecules. This could be the same for the bicarbonate case because the geometrical arrangement of hydrogen bonds
Adsorption of Sodium Dodecyl Sulfate
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12383
Figure 3. Molecular structures of formic acid, acetic acid and bicarbonate.
Figure 5. Variations of I3/I1 in THF hydrate slurries as a function of SDS concentrations.
Figure 4. Normalized fluorescence spectrum of pyrene in THF slurries. λex ) 335 nm; I1 ) 374 nm; I3 ) 384 nm; Intensity is normalized for the band at 374 nm.
in hydrate crystals is not much different from that in ice crystals and bicarbonate also has the same carboxyl group as formic and acetic acids as shown in Figure 3. The effect of SDS on the ζ-potential of THF hydrates is given in Figure 2. The ζ-potential remains constant at SDS concentrations below 0.17 mM. With a further increase in the SDS concentration, the ζ-potential first decreases linearly with a slope of -336 mV mM-1 and then with a slope of -20 mV mM-1 at SDS concentrations above 0.35 mM. The decrease in the ζ-potential is due to DS- adsorption at the hydrate/liquid interface, which brings more negative charge to the hydrate particles. The change in the slope possibly comes with different adsorption mechanisms. Fluorescence Spectra. Pyrene is a strong hydrophobic probe with a very low solubility in water, and it partitions from the aqueous phase into the hydrophobic domains if they occur in the system. The fine structure (vibronic brands) intensities in pyrene fluorescence spectra are strongly affected by the microenvironment into which it locates.20 Pyrene fluorescence has been successfully employed to detect the occurrence of hydrophobic domains in solutions and on the solid surface. Steadystate fluorescence spectra of pyrene using an excitation wavelength of λex ) 335 nm in THF hydrate slurries are shown in Figure 4. The spectrum is composed of four vibronic bands associated with the 1Lb f 1A transition.21 The intensity ratio of the third vibronic band (I3, λ ) 384 nm) to the first one (I1, λ ) 374 nm) is a qualitative measure of the microenvironment’s polarity around the probe, which varies from 0.50 to 0.80 for simple polar solvents and from 1.65 to 1.75 for hydrocarbon solvents.20 The value of I3/I1 (also called micropolarity parameter) in THF hydrate slurries is 0.54 without SDS as shown in Figure 5, close to that in water (I3/I1 is 0.49 in water) but much lower than that in THF solutions (I3/I1 is 0.71 in 10 wt % THF solutions). If the value of I3/I1 in THF hydrate slurries depends on the micropolarity of the solutions, then it should be close to 0.71 because the THF concentration in aqueous phase is close to 10 wt % due to the small fraction of the THF hydrate particles. Since this was not observed, the ratio of I3/I1 in THF slurries probes the micropolarity of the hydrate/liquid interface
Figure 6. Schematic representation of SDS adsorption at the hydrate/ liquid interface.
Figure 7. Relationship between the induction time for THF hydrates at 268.2 K and SDS concentrations.
but not the aqueous phase, indicating there is some interaction between pyrene and THF hydrates. The results presented here indicate THF hydrates are hydrophilic, similar to ice.17 The variation of I3/I1 in THF hydrate slurries as a function of SDS concentrations is presented in Figure 5. The value of
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TABLE 1: I3/I1 in Different SDS Concentrations CSDS (mM) 0.00 0.17 0.35 0.87 1.73 2.60 3.47 a
I3/I1 0.56 0.63 0.64 0.72 0.71 0.73 0.80
0.53 0.70 0.64 0.73 0.70 0.67 0.77
0.53 0.68 0.72 0.75 0.76 0.74 0.77
0.73 0.76 0.77 0.80
0.68
0.73
0.83
av
sd
se
lowera
uppera
0.54 0.68 0.69 0.73 0.72 0.73 0.78
0.01 0.04 0.06 0.02 0.03 0.04 0.03
0.01 0.02 0.03 0.01 0.02 0.02 0.02
0.52 0.64 0.63 0.71 0.68 0.69 0.74
0.56 0.72 0.75 0.75 0.76 0.77 0.82
95% confidence, av )average, sd ) standard deviation, and se ) standard error.
TABLE 2: Induction time for THF Hydrates at 268.2 K induction time (h)
a
CSDS (mM)
1st
2nd
3rd
av
sd
se
uppera
lowera
0.00 0.17 0.35 0.87 1.73 2.60 3.47
8.04 0.00 1.54 0.18 0.31 0.00 1.91
2.28 0.00 0.51 0.00 0.18 0.00 0.30
3.92 1.86 0.78 1.96 0.36 1.33 1.58
4.75 0.62 0.94 0.71 0.28 0.44 1.26
2.42 0.88 0.44 0.88 0.08 0.63 0.69
1.62 0.58 0.29 0.59 0.05 0.42 0.46
7.91 1.77 1.51 1.87 0.38 1.26 2.17
1.58 0.00 0.37 0.00 0.18 0.00 0.36
95% confidence, av ) average, sd ) standard deviation, and se ) standard error.
I3/I1 increases from 0.54 to 0.68 as the SDS concentration increases from 0 to 0.17 mM. Then it remains almost constant at SDS concentrations up to 3.47 mM because the change in the value of I3/I1 is within the measurement error (see the lower and upper bounds in the last two columns of Table 1). The value of I3/I1 is 0.68 in 10 wt % THF solutions (no hydrates) with 3.47 mM SDS, close to that in 10 wt % THF solutions (0.71) without SDS, which means the micropolarity of the solutions is independent of SDS concentrations. Therefore, the increase in the value of I3/I1 suggests that hydrophobic domains form at the hydrate/liquid interface as SDS concentration increases, into which the pyrene is preferentially solubilized. In other words, the hydrophobic domains come from the adsorbed DS- at the hydrate/liquid interface. As the SDS concentration increases from 0.17 to 3.47 mM, more DS- adsorbs at the interface. However, the value of I3/I1 remains almost unchanged, suggesting that the micropolarity of the hydrate/liquid interface is independent of DS adsorption density at a SDS concentration range of 0.17 to 3.47 mM. Adsorption Mechanism. The ζ-potential and fluorescence measurements presented here can be used to elucidate the adsorption mechanism of DS- at the hydrate/liquid interface. In the absence of SDS, we assume that all adsorption sites on the hydrate surface are occupied by bicarbonate in an open system. When SDS is present in the system, it competes with bicarbonate for the adsorption sites. DS- interacts with hydrates by the same adsorption mechanism as that for bicarbonate. As we mentioned before, bicarbonate adsorbs at the hydrate/liquid interface via hydrogen bonds. Thus, it is also plausible that hydrogen bonds are formed between the pendant hydrogens on the hydrate surface and oxygens of DS- with the hydrocarbon chains protruding into the aqueous phase. An increase in the micropolarity parameter as the SDS concentration increases from 0 to 0.17 mM indicates that the DS- density at the hydrate/ liquid interface increases (Figure 5). However, the charge of hydrate particles at SDS concentrations below 0.17 mM is the same as hydrate particles without SDS (Figure 2), suggesting that some adsorption sites are occupied by bicarbonate whereas the others are occupied by DS-. DS- occupies all adsorption sites and thus a monolayer may complete above 0.17 mM SDS.
With a further increase in the SDS concentration, more SDS anions would associate with the adsorbed DS- through lateral interaction of the hydrocarbon chains with headgroups orienting toward the aqueous phase to offset the electrical repulsion between DS-. Further adsorption of DS- makes the hydrate surface more negative but does not change the micropolarity of the hydrate/liquid interface significantly. Therefore, the ζ-potential decreases but the value of I3/I1 remains constant as more DS- adsorbs at the hydrate/liquid interface as shown in Figures 2 and 5. At SDS concentrations above 0.35 mM, the change of ζ-potential versus SDS concentration is much less than that at a SDS concentration range of 0.17 to 0.35 mM (Figure 2). One possible explanation is that the adsorption is completed at a SDS concentration of 0.35 mM and the decrease in the ζ-potential at SDS concentrations above 0.35 mM is due to the increase in the SDS concentration in the aqueous phase. The observation that there is no significant change in the value of I3/I1 after the hydrophobic interaction becomes the predominant force for DS- adsorption is in agreement with an earlier report that the polarity of the microenvironment formed by associated DS- on the alumina surface is independent of SDS adsorption via hydrophobic interaction.22 A schematic representation of SDS adsorption at the hydrate/liquid interface is shown in Figure 6. Induction Time. The effect of SDS on the induction time of THF hydrates is shown in Figure 7. The induction time is 4.75 h in the absence of SDS, 0.62 h in 0.17 mM SDS, and 1.26 h in 3.47 mM SDS. The difference in the induction time at SDS concentrations above 0.17 mM is within the measurement error (Table 2). This result indicates that the induction time of THF hydrates decreases in the presence of SDS but it is constant at SDS concentrations ranging from 0.17 to 3.47 mM. Here, the induction time is determined by monitoring the temperature of THF solutions. Some hydrate nuclei exist in the THF solution before a temperature spike is detected in the metastable region according to the nucleation theory.1,23 Although the amount of hydrate nuclei, with a radius of less than 100 nm,24,25 is difficult to determine, the amount of hydrate nuclei is small. A dramatic change in temperature can be
Adsorption of Sodium Dodecyl Sulfate observed once crystal growth occurs. Therefore, the concentration of SDS in aqueous phase keeps almost constant before the onset of crystal growth that is associated with a temperature spike. Furthermore, the adsorption of SDS on hydrate nuclei can immediately reach equilibrium under static conditions because the amount and size of hydrate nuclei are very small during nucleation. As discussed in the previous sections, the monolayer of DS- at the hydrate/liquid interface is formed with a small amount of SDS (0.17 mM) in aqueous phase. This indicates that the short induction time for THF hydrates is due to the adsorption of DS- at the nucleus/liquid interface and the induction time decreases to a constant value after the monolayer is completed. Our previous study suggested that the short induction time for methane hydrates in the presence of SDS is attributed to the adsorption of SDS on hydrate nuclei,9 consistent with the findings presented here. One possible mechanism of SDS reducing the induction time is that the interfacial tension of hydrate-liquid decreases after DS- adsorption. Kashchieva and Firoozabadib26 reported that the induction time for gas hydrates decreases with decreasing hydrate-liquid interfacial energy. At present, no report on the effect of SDS on the interfacial tension of hydrate-liquid is available to clarify this point. Another possible mechanism is that hydrate formers such as methane and THF are solubilized in the hydrophobic domains formed by adsorbed DS-, which increases the concentration of hydrate formers at the hydrate/liquid interface. Somasundaran and Moudgil27 suggested that hydrocarbon gases such as methane could coadsorb with DS- at the solid/liquid interface. 4. Conclusion In this study, we first qualitatively analyzed the adsorption of SDS at the THF hydrate/liquid interface in terms of ζ-potential and pyrene fluorescence measurements. THF hydrate particles are negatively charged in the absence of SDS and other anions except for OH- in fresh D.I. water, and the negative charge arises from the adsorption of bicarbonate in open systems. The charge of the hydrate particles at SDS concentrations below 0.17 mM is the same as that without SDS, and hydrophobic domains form at the hydrate/liquid interface under these conditions, indicating that DS- competes with bicarbonate for the adsorption by the same adsorption mechanism. A monolayer of DS- is completed at a SDS concentration of 0.17 mM, in which headgroups orient toward the surface and tails toward the aqueous phase. With a further increase in the SDS concentration, DS- associates with the adsorbed DS- by hydrophobic force with headgroups orienting toward the aqueous phase to offset the electrical repulsion between DS-. The micropolarity parameter of the hydrate/liquid interface remains unchanged after the monolayer forms. The induction time of THF hydrates decreases in the presence of SDS and then it levels off at a SDS concentration corresponding to the completion of the monolayer. Therefore, the short induction time of hydrates in the presence of SDS is due to its adsorption at the hydrate/liquid interface, which possibly results in a decrease in the interfacial energy of hydrate-liquid and/or coadsorption of hydrate formers by solubilizing them in the hydrophobic domains formed by adsorbed DS-. References and Notes (1) Sloan, E. D. Clathrate Hydrates of Natural Gas, 2nd ed.; Dekker: New York, 1998.
J. Phys. Chem. C, Vol. 112, No. 32, 2008 12385 (2) Sloan, E. D. Fundamental principles and applications of natural gas hydrates. Nature 2003, 426, 353–359. (3) Zhong, Y.; Rogers, R. E. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000, 55, 4175–4187. (4) Gayet, P.; Dicharry, C.; Marion, G.; Graciaa, A.; Lachaise, J.; Nesteroy, A. Experimental determination of methane hydrate dissociation curve up to 55 MPa by using a small amount of surfactants as hydrate promoter. Chem. Eng. Sci. 2005, 60, 5751–5758. (5) Watanabe, K.; Imai, S.; Mori, Y. H. Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using HFC-32 and sodium dodecyl sulfate. Chem. Eng. Sci. 2005, 60, 4846–4857. (6) Zhang, J. S.; Lee, S. Y.; Lee, J. W. Kinetics of methane hydrate formation from SDS solution. Ind. Eng. Chem. Res. 2007, 46, 6353–6359. (7) Watanabe, K.; Niwa, S.; Mori, Y. H. Surface tensions of aqueous solutions of sodium alkyl sulfates in contact with methane under hydrateforming conditions. J. Chem. Eng. Data 2005, 50, 1672–1676. (8) Di Profio, P.; Arca, S.; Germani, R.; Savelli, G. Surface promoting effects on clathrate hydrate formation: Are micelles really involved. Chem. Eng. Sci. 2005, 60, 4141–4145. (9) Zhang, J. S.; Lee, S. Y.; Lee, J. W. Does SDS micellize under methane hydrate-forming conditions below the normal ambient Krafft point? J. Colloid Interface Sci. 2007, 315, 313–318. (10) Zhang, J. S.; Lee, S. Y.; Lee, J. W. Solubility of sodium dodecyl sulfate near propane and carbon dioxide hydrate-forming conditions. J. Chem. Eng. Data 2007, 52, 2480–2483. (11) Okutani, K.; Kuwabara, Y.; Mori, Y. H. Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates. Chem. Eng. Sci. 2008, 63, 183– 194. (12) Delahave, A.; Fournaison, L.; Marinhas, S.; Chatti, I.; Petitet, J. P.; Dalmzaaone, D.; Furst, W. Effect of THF on equilibrium pressure and dissociation enthalpy of CO2 hydrates applied to secondary refrigeration. Ind. Eng. Chem. Res. 2005, 45, 391–397. (13) Anderson, R.; Chapoy, A.; Tohidi, B. Phase relations and binary clathrate hydrate formation in the system H2-THF-H2O. Langmuir 2007, 23, 3440–3444. (14) Devarakonda, S.; Groysman, A.; Myerson, A. S. THF-water hydrate crystallization: an experimental investigation. J. Cryst. Growth 1999, 204, 525–538. (15) Prosser, A. J.; Franses, E. I. Adsorption and surface tension of ionic surfactants at the air-water interface: review and evaluation of equilibrium models. Colloid Surf. A. 2001, 178, 1–40. (16) Carroll, J. J.; Mather, A. E. The system carbon dioxide-water and the Krichevsky-Kasarnovsky equation. J. Solution Chem. 1992, 21, 607– 621. (17) Drzmala, J.; Sadowski, Z.; Holysz, L; Chibowski, E. Ice/water interface: Zeta potential, point of zero charge, and hydrophobicity. J. Colloid Interface Sci. 1999, 220, 229–234. (18) Suga, H.; Matsuo, T.; Yamamuro, O. Thermodynamic study of ice and clathrate hydrates. Pure Appl. Chem. 1992, 64, 17–26. (19) Compoint, M.; Toubin, C.; Picaud, S.; Hoang, P. N. M.; Girardet, C. Geometry and dynamics of formic and acetic acids adsorbed on ice. Chem. Phys. Lett. 2002, 365, 1–7. (20) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; p 318. (21) Kalyanasundaram, K.; Thomas, J. K. Environmental effects on vibronic band intensities in pyrene monomer fluorescence and their application in studies of micellar systems. J. Am. Chem. Soc. 1977, 97, 2039–2044. (22) Chandar, P.; Somasundaran, P.; Turro, N. J. Fluorescence probe studies on the structure of the adsorbed layers of dodecyl sulfate at the alumina-water interface. J. Colloid Interface Sci. 1987, 117, 31–46. (23) Mullin, J. W., Crystallization, 4th ed., Butterworth-Heinemann: Oxford, U.K., 2001, Chapter 5. (24) Englezos, P.; Kalogerakis, N.; Dholabhai, P. D.; Bishnoi, P. R. Kinetics of formation of methane and ethane gas hydrates. Chem. Eng. Sci. 1987, 42, 2647–2658. (25) Thompson, H.; Soper, A. K.; Buchanan, P.; Aldiwan, N.; Creek, J. L.; Koh, C. A. Methane hydrate formation and decomposition: Structural studies via neutron diffraction and empirical potential structure refinement. J. Chem. Phys. 2006, 124, 164508. (26) Kashchieva, D.; Firoozabadib, A. Induction time in crystallization of gas hydrates. J. Cryst. Growth 2003, 250, 499–515. (27) Somasundaran, P; Moudgil, B. M. The effect of dissolved hydrocarbon gases in surfactant solutions on froth floatation of minerals. J. Colloid Interface Sci. 1974, 47, 290–299.
JP801963C
