Surfactant Effects on Hydrate Crystallization at the Water−Oil Interface: Hollow-Conical Crystals

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. C 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. 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 sulfide 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) of 4 ± 1 and, thus, tends to have very little solubility in water. Deionized water is used. Figure 1 shows the schematic of an experimental setup, which has qualitative similarities to the ice growth cell by Thomson et al. 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 Received: February 21, 2012 Revised: May 3, 2012 Published: June 25, 2012 Communication pubs.acs.org/crystal © 2012 American Chemical Society 3817 dx.doi.org/10.1021/cg300255g | Cryst. Growth Des. 2012, 12, 3817−3824 Teqm ≈ 7 to 7.7 °C. Zhang et al. 7 and Sakemoto et al. reported 7 °C while Sloan and Koh and Aman et al. 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 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