Surfactant Effects on Hydrate Crystallization at the Water–Oil Interface

Communication pubs.acs.org/crystal

Surfactant Effects on Hydrate Crystallization at the Water−Oil Interface: Hollow-Conical Crystals Prasad U. Karanjkar,†,‡ Jae W. Lee,† and Jeffrey F. Morris*,†,‡ †

Levich Institute and ‡Department of Chemical Engineering, City College of City University of New York, New York, New York 10031, United States S Supporting Information *

ABSTRACT: Clathrate hydrates are icelike crystalline compounds with small guest molecules trapped inside the cages of hydrogen bonded water molecules. Clathrate hydrate crystal growth is studied for the specific case of the guest molecule cyclopentane. Cyclopentane hydrate formation is visualized at a millimeter-scale water drop. Crystal formation takes place at the water−organic interface and has been shown in prior work to occur in a three-step sequence of nucleation, lateral surface growth, and radial growth. This study describes cyclopentane hydrate crystal characteristics during the lateral surface growth and demonstrates the effect of the oil-soluble surfactant sorbitan monooleate (Span 80) on the hydrate crystal growth. A faceted polycrystalline hydrate shell is formed around the water drop in the absence of surfactant. A unique hollow-conical crystal is observed at Span 80 concentrations greater than 0.01% by volume in cyclopentane; the critical micelle concentration is 0.03% Span 80. The conical crystals have a polygonal base, usually hexagonal, which is pinned at the water−cyclopentane interface; growth occurs at this base and drives the previously formed crystal into the water phase. Morphology is dependent upon the growth taking place at the interface where both components of hydrate (i.e., water and cyclopentane) are present at large concentration. The hollow-conical crystals grow to over 100 μm in linear dimensions in all directions. The morphology described is seen at a range of Span 80 and cyclopentane concentrations. A hypothesis is proposed to relate crowding of surfactant molecules at the interface to the observed hollow-conical crystal shape.

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experimental setup, which has qualitative similarities to the ice growth cell by Thomson et al.6 used for grain boundary visualization. The test section, an aluminum microscope slide, has a circular well of 12.5 mm diameter and 2 mm in depth. The well is filled with cyclopentane (density approximately 0.75 g cm−3), and a 4 μL drop of the denser (approximately 1.0 g cm−3) water is placed in the center, so that it sits on the bottom. The aluminum microscope slide is placed inside a Linkam Peltier stage (LTS 120); this stage provides temperature control and is equipped with a quartz visualization window for a top view of the water drop, as seen in Figure 1c. A glass coverslip placed over the well, combined with an airtight lid and the low temperatures (usually less than 10 °C), minimize cyclopentane evaporation. A typical temperature protocol is as follows: (1) decrease the temperature from 10 °C to −25 at 5 °C/min to convert the water drop to ice; (2) increase the temperature to Thold = 0.2 °C, with a temperature ramp of 5 °C/min; (3) hold the temperature fixed between 0 °C and the minimum reported cyclopentane hydrate equilibrium dissociation temperature, Teqm, to melt the ice and form cyclopentane hydrates; reported values vary from

lathrate hydrates are solid crystalline compounds with cagelike structures of water molecules. The cavities in these structures formed by the host water are usually occupied by small guest molecules which serve to stabilize the lattice.1,2 Guest molecules include light hydrocarbons, with examples being methane, ethane, and propane, as well as hydrogen and carbon dioxide. In the case of such molecules which are gases at standard pressure, the “gas hydrates” are stable only at conditions of low temperature and high pressure. However, certain guest molecules are liquid at standard conditions. We work with such a liquid-state hydrate former, cyclopentane, which forms structure II hydrates at atmospheric pressure. This removes the need to work in a pressurized environment and allows us more experimental freedom in our examination of hydrate crystals. Our purpose is to report previously unreported hollow-conical crystals associated with the interfacial crystallization of a clathrate hydrate. The only other hollow conical crystals of which we are aware, of cadmium sulfide3,4 for example, are much smaller and are formed from a gas phase. We study cyclopentane hydrate formation at a millimeterscale water drop suspended in cyclopentane. A 99% reagent grade cyclopentane and sorbitan monooleate (Span 80) are obtained from Sigma Aldrich. Span 80 is a nonionic, oil-soluble surfactant with a hydrophilic−lipophilic balance (HLB)5 of 4 ± 1 and, thus, tends to have very little solubility in water. Deionized water is used. Figure 1 shows the schematic of an © 2012 American Chemical Society

Received: February 21, 2012 Revised: May 3, 2012 Published: June 25, 2012 3817

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Figure 1. Crystal visualization chamber: aluminum slide with circular well inside a stage with Peltier temperature control (Linkam LTS 120) and a quartz viewing window.

Figure 2. Faceted cyclopentane hydrate shell morphology formed from a water drop immersed in cyclopentane, with no surfactant, at T = 0.2 °C: (a) initial water drop; (b) hydrate ball (white dotted line distinguishes between hydrate and aluminum surface); and (c) enlarged view of the hydrate ball.

Figure 3. Effect of Span 80 on cyclopentane hydrate morphology: water drop in cyclopentane, CS80 = 0.1% (v/v), T = 0.2 °C. (a) Initial water drop (pointed by dotted circle), and (b) mushy/hairy ball after essentially complete water conversion to cyclopentane hydrate; (c) enlarged view of the hydrate ball.

Teqm ≈ 7 to 7.7 °C. Zhang et al.7 and Sakemoto et al.8 reported 7 °C while Sloan and Koh9 and Aman et al.10 have reported 7.7 °C. It is found that water from freshly melted ice leads to immediate hydrate formation, overcoming the extended and stochastic induction time associated with cyclopentane hydrate nucleation. Figure 2 shows the morphology of cyclopentane hydrate crystals formed with a water drop immersed in cyclopentane

alone, with no surfactant. Hydrate nucleation occurs, apparently randomly, at the water−cyclopentane interface as soon as free liquid water becomes available through ice melting. The presence of ice in close proximity to the liquid water− cyclopentane interface may have caused the observed rapid hydrate nucleation. Note that hydrate nuclei are too small to be observed. The first hydrate crystals we are able to observe, of O(10) μm, are at the water−cyclopentane interface, suggesting that nucleation occurred at the interface as well. These small 3818

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Figure 4. Cyclopentane hydrate crystal morphologies during the lateral surface growth: (a−c) faceted shell, no surfactant; (d−g) hollow-conical crystals, CS80 = 0.1% (v/v), T = 0.2 °C.

Information) show that the interior of the cone is filled with cyclopentane during this pinned phase. The pinning becomes unstable as the crystal grows larger, typically with the crystal >100 μm across the open base of the cone, and water is observed to flow in to replace cyclopentane; the hydrate surface is preferentially wetted by water relative to cyclopentane. As a consequence of water filling the cone, the crystal becomes immersed in the water phase, regenerating the water− cyclopentane interface for further hydrate formation. The sequence of lateral crystal growth in the conical form and crystal entry to the water phase occurs repeatedly until the water drop volume is packed with hydrate crystals but retains some liquid water. This residual water then undergoes outward radial flow and converts to hydrate where it contacts the cyclopentane. Cyclopentane flows into the void space within the crystal packing, and thus, the observed hairy/porous morphology shown in Figure 3 is developed. While all of the noted processes of nucleation, lateral surface growth, and radial growth are important in deciding the final hydrate morphology, the focus here is on the hollow-conical crystals observed during the lateral surface growth. Before discussing the conditions leading to the hollow conical crystals, we summarize the visually observed role of surfactant. For the surfactant-free case, lateral surface growth exhibits facet-like hydrate crystals which lead to formation of a hydrate shell around the water drop with unreacted water trapped inside. The faceted crystal structure apparently has junctions sufficiently robust that mass transfer (guest cyclopentane diffusing through the solid, based on work of Jakobsen et al.16 studying CCl3F hydrate) limits the water conversion to a very low rate. This faceted structure is replaced by a 3D hollowconical shape at CS80 = 0.1% (v/v). Figure 4 shows additional images of hollow-conical and faceted crystals observed during the lateral surface growth with (Figure 4d−g) and without (Figure 4a−c) the surfactant, respectively. In the growth of hydrate crystals, a factor of critical importance is that hydrate formation is an interfacial process. The “reaction” of water and cyclopentane to form a clathrate structure can only take place at the rates observed at the water− cyclopentane interface, as the two components are simulta-

hydrate crystals grow laterally along the water drop surface, leading to a faceted, polycrystalline hydrate shell as shown in Figure 2c. The shell formed by the joining of numerous hydrate facets slows further hydrate formation due to mass transfer limitations. A similar faceted hydrate shell formation at the water−cyclopentane interface has been reported before.8,11 The effect of Span 80 on the ultimate cyclopentane hydrate morphology, after what is believed to be full water conversion, is shown in Figure 3. The Span 80 concentration, CS80, is fixed at 0.1% (v/v) based on the cyclopentane volume. A hairy or mushy hydrate morphology is observed as compared to a faceted shell formation observed in the absence of Span 80. This structure has significant porosity, as evidenced by the final hydrate ball size being ∼1.3 times the initial water drop diameter, or above two times the initial water volume; the difference in density of hydrate and water can account for only a few percent increase. A similar modification in the morphology of cyclopentane hydrates due to the presence of Span 80 is also observed by Nakajima et al.12 and Aman et al.13 On the basis of a similar hairy morphology observed for tetrahydrofuran hydrates in the presence of Span 20,14 Aman et al. hypothesized that the observed hydrate surface morphology changes are independent of the hydrate former and may be related to the addition of Span-class surfactants in the bulk fluid phase. The process leading to the formation of a hairy and porous morphology in cyclopentane hydrate with sufficient surfactant present involves a three-step process of nucleation, lateral growth, and radial growth as described in prior work.15 The process begins similarly to the surfactant-free case. Nucleation is followed by small hydrate crystals growing laterally along the water drop surface. However, with surfactant at sufficient levels, the hydrate crystals display a remarkable structure which is of primary interest here. Specifically, the hydrate crystals exhibit a three-dimensional (3D) hollow-conical morphology. The hollow-conical crystal has its base, predominantly hexagonal, pinned at the water−cyclopentane interface with the vertex pointing into the water phase. The half-apex angle of this conical crystal is found to be approximately between 18° and 20°. Still images and video sequences (see Supporting 3819

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neously present at high concentrations in the interfacial region. Any modification in the interfacial properties must be expected to affect the hydrate formation, both in rate and in the mechanistic details of the process. Figure 5 shows the

Table 1. Langmuir Adsorption Isotherm Parameters for Span 80 at the Water−Cyclopentane Interface cmc in cyclopentane (% v/v) γcmc (mN m−1) Γ∞ (×10−6 mol m−2) α/β (×10−3 mol m−2)

nucleation and lateral crystal growth cause reduction in the available interfacial area for the same number of Span 80 molecules, unless desorption of the surfactant is very rapid. Our conjecture is that hydrate crystal growth laterally across the interface leads to crowding of Span 80 molecules in the remaining area, thus developing an excess surface pressure (i.e., Γ exceeds the maximum equilibrium surface concentration Γ∞). Figure 6 shows a schematic for purposes of explanation of the argument. In this scenario, the excess surface pressure acts as a mechanical barrier to lateral crystal growth and forces the crystal to grow conically into the water phase. It is relevant to note first the manner of approach to the equilibrium interfacial tension data shown in Figure 5: from an initial value of γc = 48.7 mN m−1 for the water−cyclopentane interface with no surfactantin relation to the problem at hand, this is assumed to be the case instantaneously after melting of iceγ drops immediately (before we are able to access data and thus under a second) to a value depending upon the CS80 of interest. For example, the drop is immediately to γ ≈ 36, 27, and 1.7 mN m−1 for CS80 = 0.001, 0.01, and 0.1% (v/v), respectively; the further evolution toward the equilibrium involves comparatively slower dynamics with characteristic times on scales of O(10) seconds or greater (see Supporting Information for the experimental data). Thus, there is a rapid adsorption to the interface by surfactant to values approaching the equilibrium level, with slower processes, presumed to involve mass transfer from the bulk and finally rearrangement of surfactant at the interface, bringing the interface to equilibrium in relation to the bulk surfactant concentration. Interfacial tension is immediately strongly affected by the surfactant, and thus the water−cyclopentance interface is immediately brought to conditions where competition for the interface between hydrate crystals and surfactant is expected. It is conjectured that the slowest process, observed in all cases on time scales of greater than 10 s, is surface rearrangement of surfactant; over this time scale, hydrate crystal growth following ice melting is significant, and thus, crystal growth will interact with the rearrangement. Separate desorption experiments (not reported in detail here) show that at the low value of CS80 = 0.001% (v/v), when a drop is rapidly reduced in size to instantaneously increase the surfactant interfacial concentration and hence decrease γ, the primary relaxation back toward equilibrium by surfactant desorption from the interface takes place over roughly 100 s. The conclusion from these different studies is that surfactant dynamics of rearrangement and desorption have time scales which are sufficiently slow that competition with growing hydrate crystals for the interface is to be expected. Figure 7 shows the evolution of a conical crystal at the water drop−cyclopentane interface. The growth of the conical crystal can only take place at the cone base, i.e. at the three phase contact of water−hydrate−cyclopentane. After the crystal grows sufficiently large, typically to O(100) μm in size, the pinning loses stability and water fills the cone. This causes the hydrate crystal to become immersed in the water, thus

Figure 5. Equilibrium interfacial tension of water−cyclopentane with variable surfactant (Span 80) concentration in cyclopentane.

equilibrium water−cyclopentane interfacial tension as a function of Span 80 concentration, CS80; all concentrations are on a volume percentage based on the cyclopentane volume. The interfacial tension (measured using a pendant drop method17) for a clean water−cyclopentane interface, i.e. for CS80 = 0, is γc = 48.7 mN m−1. The data in Figure 5 indicate saturation of the water−cyclopentane interface, i.e. a constant interfacial tension above a critical micelle concentration (cmc) CS80,CMC ≈ 0.03% (v/v). The data below the cmc can be fitted with a Langmuir adsorption isotherm and equation of state,18 Γe 1 = Γ∞ 1 + (α /βCS80)

(1)

⎡ ⎛ Γ ⎞⎤ γ(Γ) = γc + RT Γ∞⎢ln⎜1 − ⎟⎥ ⎢⎣ ⎝ Γ∞ ⎠⎥⎦

(2)

0.03 0.80 3.69 3.45

where γ is the interfacial tension (as noted γc is for the clean water−cyclopentane interface), CS80 is the bulk Span 80 concentration, Γ is the surface concentration (subscripts e and ∞ for the equilibrium and maximum packing concentrations, respectively), R is the gas constant, T is absolute temperature, and finally α and β are the kinetic rate constants. The fitting parameters are Γ∞ and (α/β), with the best fit values for the Span 80/water/cyclopentane system reported in Table 1. The value of Γ∞ corresponds to 45 Å2 of area per adsorbed Span 80 molecule. This value is in close agreement with reported values for the same surfactant at the water− paraffin interface.19,20 The concentration of CS80 = 0.1% (v/v) is used for the cases shown in Figure 3. This is above the cmc value of CS80,CMC ≈ 0.03% (v/v), and thus, the surface concentration at the water− cyclopentane interface will be at its maximum, Γ∞. Hydrate 3820

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Figure 6. Schematic of the proposed cyclopentane hydrate lateral surface growth hypothesis: water−cyclopentane interfacial area is reduced by the crystal growth and the surfactant crowding in the remaining area (excessive surface pressure) leads to conical crystal shape. Interface is regenerated by pushing the crystal into the bulk water phase.

Figure 7. Lateral surface hydrate growth: hollow-conical crystals at curved water−cyclopentane interface (enlarged section of a 4 μL water drop), CS80 = 0.1% (v/v), T = 0.2 °C, scale bar = 0.1 mm (solid lines show the vertex of the conical crystal and dotted white line shows the water front filling the cone).

Figure 8. Lateral surface hydrate growth: hollow-conical crystals at a planar water−cyclopentane interface, CS80 = 0.1% (v/v), T = 0.2 °C, scale bar = 0.1 mm (solid lines show the vertex of the conical crystal, and the dotted white line shows the water front filling the cone).

regenerating the interface for further hydrate formation. The water front during filling of the cone is marked with a white dotted line, and solid lines indicate the position of the cone in Figure 7. A similar morphological characterization is carried out at a planar water−cyclopentane interface with 0.1% (v/v) Span 80. In this case, the interface is seeded with an externally grown cyclopentane hydrate seed crystal. Hydrate crystals grown at the planar interface also showed hollow-conical crystal morphology; see Figure 8. The argument of excess surface pressure (Γ > Γ∞) driving the conical crystal growth requires for its validity that the desorption rate of Span 80 be comparable to or less than the

hydrate crystal growth rate. In other words, if the Span 80 molecules desorb very rapidly from the interface, allowing equilibration of the surface concentration Γ for the new reduced area, no excess surface pressure will be generated due to crystal growth at the interface. To validate our argument, a single-drop morphology experiment is performed at a higher hold temperature (and thus reduced subcooling), Thold = 4 °C, again with CS80 = 0.1% (v/v) in cyclopentane. Reduction of the subcooling by 3.8 °C (over half the subcooling relative to the equilibrium dissociation temperature of Teqm ≈ 7 °C) should slow the hydrate crystal growth rate;21 we assume that the Span 80 desorption rate is not significantly affected by this 3821

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Figure 9. Cyclopentane hydrate morphologies in cyclopentane, CS80 = 0.1% (v/v), 4 μL water drop: (a) 0.2 °C; (b) 4 °C. Time is measured from onset of ice melting.

Figure 10. Cyclopentane hydrate crystal morphologies at various CS80 (v/v): (a) 0.0001%; (b) 0.001%; (c) 0.01%; (d) 0.1%. Water drop volume = 4 μL, T = 0.2 °C, numbers indicate the elapsed time in seconds from the start of ice melting, scale bar = 1 mm unless otherwise noted.

temperature change. At lower subcooling (higher temperature), crystal growth is much slower (note the times in Figure 9) and

most of the crystals are platelike (see Figure 11 for detailed view), in contrast to the case of conical crystals obtained at 3822

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Figure 11. Cyclopentane hydrate crystal morphologies: (a) hollow-conical at CS80 = 0.01%, 0.2 °C; (b) hollow-conical at CS80 = 0.1%, 0.2 °C; (c) platelike at CS80 = 0.1%, 4 °C. All percentages are v/v based on cyclopentane.

Figure 12. Cyclopentane hydrate morphologies at different cyclopentane concentrations in the oil phase, CS80 = 0.1% (v/v), T = 0.2 °C, (a) 65%, (b) 50% cyclopentane by volume (light mineral oil is used as a second oil component), numbers indicate the elapsed time in seconds from the onset of ice melting. Scale bar = 1 mm unless mentioned otherwise.

interfacial tension is γ = 28 mN m−1 and the observed hydrate morphology is shown in Figure 10b. For this case, it is observed that a complete hydrate shell develops around the water drop immediately after ice melting and the shell eventually buckles with the bulk water exiting the shell in what appears visually almost like boiling, leading to further hydrate formation in the lumpy (but notably not hairy) morphology. The absence of conical crystals for these low Span 80 concentrations suggests that the lower Γ values at these concentrations are not enough to generate the excess surface pressure assumed to be responsible for the conical crystal growth. The system with CS80 = 0.01% (v/v) Span 80 follows a very similar pattern as the CS80 = 0.1% (v/v) case, where the nucleation is followed by lateral surface growth in the conical form, as seen in Figure 11. The conical hydrate crystals become immersed in and eventually pack the water phase, with subsequent radial hydrate growth leading to the hairy/porous morphology. At these higher Span 80 concentrationsapproximately CS80 > 0.01% (v/v)Γ values approach Γ∞ and are enough to generate the excess surface pressure during the lateral crystal growth to force the conical crystal formation. The cyclopentane hydrate morphologies of faceted shell observed without Span 80 and hollow-conical crystals observed during lateral hydrate growth and eventually leading to the hairy hydrate morphology at high

higher subcooling. The slow lateral hydrate growth, and associated slower reduction in the interfacial area, is believed to provide sufficient time for Span 80 desorption from the interface to mitigate the interfacial crowding. The observation of platelike crystals is consistent with our argument, namely that rapid hydrate growth leads to excess surface pressure due to the crowding of surfactant, and this crowding is responsible for the observed conical crystal growth. Figure 9 compares the hydrate morphologies formed at 0.2 and 4 °C. The time required for the hydrate growth increased by an order of magnitude due to the reduction in subcooling. Though the crystals at reduced subcooling are predominantly platelike rather than conical, water wetting of the hydrate causes the crystals to become immersed in the water drop, leading to the hairy/porous hydrate growth. This is qualitatively similar to the morphology at the higher subcooling level, although differing in detail, as shown in an enlarged view in Figure 9. A similar morphological characterization at 0.2 °C for different Span 80 concentrations is shown in Figure 10. The water drop volume is 4 μL in each case. A faceted hydrate shell morphology, similar to the surfactant-free case, is observed for the low concentration of CS80 = 0.0001% (v/v). Note that, for this concentration of Span 80, the interfacial tension is only reduced to γ = 46 mN m−1. For CS80 = 0.001% (v/v), the 3823

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leading to the hollow-conical growth. Although a similar hollow-conical crystal morphology has been reported previously for cadmium sulfide crystals,3,4 the conical crystals observed in the present study with dimensions ranging from 102 to 103 μm are unique in the hydrate crystallography literature to the best of our knowledge.

Span 80 concentrations (CS80 > 0.01%) are found to be highly reproducible. A similar morphological characterization is also performed with a nonhydrate forming hydrocarbon in a mixture with cyclopentane. A light mineral oil (obtained from Fisher Scientific) is used as a second oil component to adjust the concentration of cyclopentane. This is of particular importance in the case of hydrate formation in subsea crude oil transport, as hydrate-forming gases such as methane and ethane are mostly dissolved in the oil phase at elevated pressures. Figure 12 shows the cyclopentane hydrate morphologies for cyclopentane concentrations of 65% and 50% by volume in the external oil phase. The Span 80 concentration and the experimental temperature are fixed at 0.1% (v/v) and 0.2 °C, respectively. The presence of Span 80 leads to conical crystal behavior, similar to that observed for pure cyclopentane, in these oil mixtures containing 65% and 50% cyclopentane as well, as shown by Figure 13. The process of conical crystal growth and

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data, and still images and video sequences of the interior of the cone. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support from Chevron and the National Science Foundation (Grant No. CBET-085421). Discussions with Professor Charles Maldarelli (Department of Chemical Engineering, City College of New York) and the Chevron Flow Assurance Core Team are greatly appreciated.

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REFERENCES

(1) Sloan, E. D. Energy Fuels 1998, 12, 191−196. (2) Englezos, P. Ind. Eng. Chem. Res. 1993, 32, 1251−1274. (3) Mash, D. H.; Firth, F. J. Appl. Phys. 1963, 34, 3636−3637. (4) Chandrasekharaiah, M. N.; Krishna, P. J. Cryst. Growth 1969, 5, 213−215. (5) Davis, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press Inc. (London) Ltd.: New York, N.Y., USA, 1963. (6) Thomson, E. S.; Wettlaufer, J. S.; Wilen, L. A. Rev. Sci. Instrum. 2009, 80, 103903. (7) Zhang, J. S.; Lee, J. W. J. Chem. Eng. Data 2009, 54, 659−661. (8) Sakemoto, R.; Sakamoto, H.; Shiraiwa, K.; Ohmura, R.; Uchida, T. Cryst. Growth Des. 2010, 10, 1296−1300. (9) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; CRC Press, Taylor & Francis Group: Boca Raton, FL, 2008. (10) Aman, Z. M.; Brown, E. P.; Sloan, E. D.; Sum, A. K.; Koh, C. A. Phys. Chem. Chem. Phys. 2011, 13, 19796−19806. (11) Taylor, C. J.; Miller, K. T.; Koh, C. A.; Sloan, E. D. Chem. Eng. Sci. 2007, 62, 6524−6533. (12) Nakajima, M.; Ohmura, R.; Mori, Y. H. Ind. Eng. Chem. Res. 2008, 47, 8933−8939. (13) Aman, Z. M.; Dieker, L. E.; Aspenes, G.; Sum, A. K.; Sloan, E. D.; Koh, C. A. Energy Fuels 2010, 24, 5441−5445. (14) Taylor, C. J.; Dieker, L. E.; Miller, K. T.; Koh, C. A.; Sloan, E. D. J. Colloid Interface Sci. 2007, 306, 255−261. (15) Karanjkar, P. U.; Lee, J. W.; Morris, J. F. Chem. Eng. Sci. 2012, 68, 481−491. (16) Jakobsen, T.; Sjöblom, J.; Ruoff, P. Colloids Surf., A 1996, 112, 73−84. (17) Lin, S. Y.; McKeigue, K.; Maldarelli, C. AIChE J. 1990, 36, 1785−1795. (18) Pan, R.; Green, J.; Maldarelli, C. J. Colloid Interface Sci. 1998, 205, 213−230. (19) Paltonen, L. J.; Yliruusi, J. J. Colloid Interface Sci. 2000, 227, 1−6. (20) Santini, E.; Liggieri, L.; Sacca, L.; Clausse, D.; Ravera, F. Colloids Surf., A 2007, 309, 270−279. (21) Kishimoto, M.; Iijima, S.; Ohmura, R. Ind. Eng. Chem. Res. 2012, 51, 5224−5229. (22) Karanjkar, P. U.; Lee, J. W.; Morris, J. F. Proceedings of the 7th International Conference on Gas Hydrates (ICGH); 2011.

Figure 13. Hollow-conical cyclopentane hydrate crystals; (a) and (b) correspond to the cases shown in Figure 12.

crystals merging into the water phase followed by a radial outward flow of water leads to the hairy morphology in each case. The time required for complete growth is also found to increase with the drop in cyclopentane concentration. This can be explained from the observed drop in cyclopentane hydrate equilibrium temperature with the drop in cyclopentane concentration in the oil phase, 22 Teqm ≈ 5 °C at 50% cyclopentane in the oil phase, and the corresponding drop in the extent of subcooling at Thold = 0.2 °C. We conclude with the key observations of this work. Experimental observations of cyclopentane hydrate growth show that the morphology is dramatically impacted by the surfactant loading and the temperature. A crucial feature in this system is the occurrence of a “reaction” between components predominantly found on the two sides of the interface between immiscible components, with the result being formation of crystals at that interface. As a consequence, interfacial properties have a significant impact on the hydrate crystal morphology. The faceted shell of flat cyclopentane hydrate crystals obtained in the absence of surfactant is replaced by hollow-conical crystals which grow away from the interface as well as laterally at surfactant (Span 80, sorbitan monooleate) concentrations beyond 0.01% (v/v). The observed crystal behavior is found to be dependent on Span 80 concentration and temperature, and our work supports the argument that this dependence is due to the critical role played by the relative rates of desorption of Span 80 molecules and lateral crystal growth. Growth only in the interfacial zone combined with the competition for the interface between the crystals and surfactant is believed to be the key factor in the mechanism 3824

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