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Inhibition of Tetrahydrofuran Hydrate Formation in the Presence of Polyol-modified Glass Surfaces Jeffrey R. Hall, and Paul W Baures Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00666 • Publication Date (Web): 07 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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Inhibition of Tetrahydrofuran Hydrate Formation in the Presence of Polyol-modified Glass Surfaces Jeffrey R. Hall† and Paul W. Baures#,* †Department of Chemistry & Biochemistry, University of California Santa Barbara, Santa Barbara, CA 93106-9510 #

Department of Chemistry, Keene State College, 229 Main Street, Keene, NH 03435-2001

*

To whom correspondence should be addressed.

Telephone +1 603-358-2769.

E-mail:

[email protected]

KEYWORDS Gas hydrates, tetrahydrofuran hydrate, polyols, antifreeze protein

Abstract

Glycerol was conjugated to glass test tube surfaces in four configurations by employing two different silane spacers, covalent attachment to glycerol at either the 1- or 2-position, and with a succinic acid spacer. The resulting surfaces were tested for their ability to inhibit the nucleation of tetrahydrofuran hydrate (THF hydrate) in comparison with polyvinylpyrrolidone (PVP), a known polymeric inhibitor of THF hydrate formation. Contact angle measurements were used as an indication of surface modification throughout the glass derivatization steps. Of the four final

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surfaces modified with glycerol, only the coating with (3-aminopropyl)triethoxysilane (APTES) and glycerol coupled at the 1-position (leaving a free 1,2-diol) showed significant inhibition of the formation of THF hydrate. The corresponding N-[3-(trimethoxysilyl)propyl]ethylenediamine (AEAPTMS) coating with glycerol coupled at the 1-position did not show a significant difference over the untreated test tubes. Attachment of glycerol at the 2-position yielded a coating with no benefit over the untreated test tubes regardless of the silane used, and a surface modified with APTES and succinic acid alone enhanced the formation of THF hydrate. The ability to inhibit THF hydrate formation using a polyol-modified surface is a first step in the development of a coating that, alone or in combination with known gas hydrate inhibitors, could be used to prevent gas hydrates from plugging pipelines in field applications.

Introduction There is a need for improved methods to prevent the formation of gas hydrates in pipelines in order to stave off production losses and to improve worker safety.1-3 The industry historically used alcohols like methanol or glycols as thermodynamic inhibitors of gas hydrate formation, though kinetic inhibitors are replacing those approaches and their continued development is under investigation.2-3 Kinetic inhibitors are used at low dosage as compared to thermodynamic inhibitors and operate by delaying nucleation and growth of gas hydrates.4 These kinetic inhibitors are also differentiated from the low dosage anti-agglomerates that operate mechanistically by preventing crystal agglomeration during the hydrate growth.2 Certain antifreeze proteins from fish, insects, or plants operate mechanistically as antiagglomerates by binding to ice nuclei and preventing the formation of larger crystals.5-6 Antifreeze proteins demonstrate a thermal hysteresis whereby the freezing point of water is


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reduced in a non-colligative manner.7 It has been further shown that antifreeze proteins can inhibit gas hydrate formation,8-9 but that they are not an economically viable option for the industry. On the other hand, researchers identified water-soluble polymers like poly(vinylpyrrolidone) (PVP) or poly(vinylcaprolactam) (PVCap) as gas hydrate inhibitors, and these as well as a variety of related polymers are now used in industrial applications.2 There are also efforts to identify small molecules inhibitors of gas hydrate formation.10 The conjugation of antifreeze proteins to polymers and then to glass surfaces has been reported to yield a coating that delays frosting, supporting the functional behavior of the antifreeze protein when covalently attached to a surface.11 By analogy one might reason that the inhibition of gas hydrate formation would also be possible with a surface coated with antifreeze protein, though the stability of the protein would still limit the potential applications. The antifreeze properties of poly(vinylalcohol)12 and the role that surface hydroxyl groups from serine and threonine side chains in antifreeze proteins have toward their functional behavior13 led us to hypothesize that a glass surface appropriately modified with designed polyols could demonstrate the inhibition of gas hydrate formation. This work communicates the results of measuring the formation of THF gas hydrate in the presence of glass surfaces modified with propylene glycol covalently attached in two different positions and with two different silane linkers. Observed changes in THF hydrate formation are determined by following the hydrate nucleation events of tetrahydrofuran-water solutions over a 12 h period, and is compared with the observed hydrate nucleation events of untreated tubes as well as tubes containing a PVP standard. Materials and Methods


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1. General Methods. Premium microscope slides (3” ´ 1”) and heavy wall borosilicate glass culture tubes (13 ´ 100 mm) were supplied by Fisher Scientific. Aminosilanes and additional reagents were purchased from Acros Organics and used without further purification: acetone (99.5%), 3-aminopropyltriethoxysilane (APTES); N-[3(trimethoxysilyl)propyl]ethylendiamine (AEAPTMS); polyvinylpyrrolidone (average M.W. 58,000); 4-dimethylaminopyridine. Chemical reagents purchased from Aldrich Chem. Co. were used without further purification: cis-1,3-O-benzylideneglycerol; DL-1,2isopropylideneglycerol; commercial grade N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride; sodium hydroxide pellets, succinic anhydride. Tetrahydrofuran was dispensed from an MBRAUN MB-SPS solvent purification system prior to individual experiments. All references to H2O in text, relevant to hydrate experimentation, refer to LC-MS grade water purchased from Fisher Scientific. LC-MS grade methanol and dichloromethane solvents were purchased from Fisher Scientific and used without further purification. Concentrated technical grade HCl solution provided by Fisher Scientific was diluted to the desired concentrations. Thin-layer chromatography was done on Analtech 250 µm silica gel HLF Uniplates. 1H and 13C NMR spectra were measured at 400 and 100.5 MHz, respectively, in CDCl3 with CHCl3 as the internal reference for 1H (d 7.26) and CDCl3 as the internal reference for 13C (d 77.06). 2. Synthesis. The protected glycols were reacted with succinic anhydride as shown in Schemes 1 and 2 by using a modification of published procedures.14-16 4-((2,2-Dimethyl-1,3-dioxolan-4-yl)methoxy)-4-oxobutanoic acid (1.3). To an oven-dried 50 mL round-bottom flask was added 1.006 g (10.05 mmol) of succinic anhydride (1.2), which was dissolved into 30 mL of dichloromethane (DCM) by gentle heating and stirring. To the


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solution was added 1.19 mL (9.57 mmol) of D,L-1,2-isopropylideneglycerol (1.1). A catalytic amount of 4-(dimethylamino)pyridine (DMAP, 5 mol%) was then delivered to the solution. The colorless, clear solution was heated to reflux and allowed to react for 72 hours. The reaction was cooled to room temperature and concentrated to approximately 10 mL. The solution was washed with 0.1 M HCl (10 mL) and extracted two times with DCM (10 mL). The combined organic layer was washed with brine (10 mL) and the aqueous layer extracted with DCM (10 mL). The combined organic layer was dried over MgSO4, filtered, and concentrated under vacuum. The resulting white solid had mass 1.737 g (78.1%). 1H NMR (400 MHz, CDCl3): δ 1.352 (s, 3H, CH3); 1.417 (s, 3H, -CH3); 2.672 (m, 4H, -CH2-CH2-); 3.725-3.745 (m, 2H, -CH2); 4.044-4.191 (m, 2H, -CH2-CH-CH2-); 4.289-4.383 (m, 1H, -CH2-CH-CH2-); 9.112 (broad s, 1H, -OH).

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C

NMR (CDCl3): δ 177.47 (COOH); 172.10 (COOR); 109.95 (C); 73.50 (CH); 66.20 (CH2); 64.97 (CH2); 28.80 (CH2); 28.72 (CH2); 26.65 (CH3); 25.34 (CH3). 4-Oxo-4-(((2S,5S)-2-phenyl-1,3-dioxan-5-yl)oxy)butanoic acid (2.2). To an oven-dried 50 mL round-bottom flask was delivered 0.907 g (9.06 mmol) of succinic anhydride (1.2) and 0.055 g of 4-(dimethylamino)pyridine (DMAP, 5.0 mol%), which were then dissolved into 20 mL of dichloromethane (DCM) by gentle heating and stirring. To the solution was then added 1.636 g (9.079 mmol) of cis-1,3-0-benzylideneglycerol (2.1). The colorless, clear solution was then heated to reflux and allowed to react for 96 hours. The reaction was cooled to room temperature and a white precipitate was collected by filtration and dried under vacuum, resulting in a white solid with mass 2.496 g (98.3%).

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H NMR (400 MHz, CDCl3): δ 2.715 (m, 4H, -CH2-CH2-);

4.137 (m, 2H, -CH2-CH-CH2-); 4.260 (m, 2H, -CH2-CH-CH2-); 4.741 (m, 1H, -CH2-CH-CH2-); 5.548 (s, 1H, -CH); 7.354 (m, 3H, -CH); 7.487 (m, 2H, -CH).

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C NMR (CDCl3): δ 177.11


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(COOH); 172.22 (COOR); 137.80 (CH); 129.22 (CH); 128.42 (CH); 126.12 (CH); 101.34 (CH); 69.08 (CH2), 66.43 (CH); 29.139 (CH2); 28.79 (CH2). 3. Glass Modification. Glass substrates were prepared for modification by a general cleaning methodology previously reported.17 The glass surface (slide or test tube) was exposed to a 2.5 M NaOH solution for 24 h, sonication in H2O for 10 min, treated with a 0.1 M HCl solution for 10 min, sonication in H2O for 10 min a second time, followed by exposure to methanol for 5 min prior to silanization steps. Silanization of glass substrates was performed by dip coating or soaking in a 1% silane solution in acetone for 15 min, shaking in methanol for 5 min, a rinse in H2O for 10 min, spin drying (2,000 rpm) for 5 min, and lastly the glass substrates were baked (383 K) for 15 min. The coated glass substrates were stored in a refrigerator (~283 K) after preparation and prior to analysis or use for coating reactions. The glass surfaces were modified with the protected polyols by following published chemistry by using N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride and catalytic 4-(dimethylamino)pyridine in dichloromethane for 24 h.18-19 The polyol and carbodiimide solutions were prepared at 0.1 M in order to provide an excess reagent as compared with the surface amines. Surfaces modified with succinic acid were prepared by treating the surface amines with 0.1 M succinic anhydride and catalytic (dimethylamino)pyridine in dichloromethane for 24 h. Three slides were reacted in triplicate in glass slide holders containing the reaction solution, while 5-6 mL of the reaction solution was added to the each of the test tubes. Enough test tubes were prepared with each of the protected polyols as to afford saving a set of 16 tubes with the protected polyols for testing as a control in the induction measurements.


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The glass surfaces were rinsed with dichloromethane following the reaction. Three slides and a set of 16 test tubes for each polyol were subjected to acidic conditions to deprotect the polyol, starting from published conditions.20 We used a model compound to test the necessary conditions to deprotect each of these compounds using a minimal acidity and temperature, ultimately settling on 20% aq. CH3CO2H (v/v) solution at 50 °C for 2 h. The deprotected slides and test tubes were rinsed with water and dried before analysis. 4. Contact Angle Measurements. The setup for contact angle measurements is illustrated in the supporting information and was based on a reported method.21 All slides were treated identically during collection of the contact angle measurements. Individual slides at room temperature were aligned on the stage. Three LC-MS grade water droplets (5 µL each) were placed on the slide, the camera was brought into focus, and an image captured. The total process from the placement of the first drop on the slide to the captured image was ~30 seconds. Contact angle were measured by averaging left and right angles of the three droplets per glass substrate in triplicate replication to total 18 measurements per treatment. Images were collected with a Bodelin Technologies (USA) ProScope 5 MP digital microscope (PS-EDU-100) and analyzed using the ImageJ image processing software22 with the Contact Angle plugin.23 5. Induction Time Measurements. The methods for determining induction times of tetrahydrofuran hydrate were based on a published procedure.24 The induction time of a sample was defined as the difference between the onset of hydrate formation and the time at which the bath temperature reached 273 K. A solution for the formation of THF hydrate was prepared by combining THF and water in a 1:15 molar ratio; while a stoichiometric ratio of 1:17 is sufficient for hydrate formation a slight excess of THF ensured hydrate rather than ice formation would occur during experimentation. Poly(vinylpyrrolidinone) solutions were prepared at 0.25 mM, to


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facilitate a comparison with earlier work,25 by dissolving solid polymer into a 1:15 THF and water solution. Induction times were monitored in an apparatus (supporting information) capable of following 16 culture tubes per experiment, with each individual tube containing a stir bar and ~4 mL of freshly prepared test solution. In 12 h experiments, a NESLAB RTE-211 circulating bath chiller was utilized to cool an external bath from 278 K to 273 K at a rate equivalent to the chillers cooling capability (~45 min) and the bath was held at 273 K for the remainder of the 12 h. This is a subcooling of +4.4 K based on the stability equilibrium temperature for a stoichiometric (1:17 molar ratio) THF hydrate of 277.4 K,26 and is also a close approximation for the 1:15 molar ratio of THF to water based on the phase diagram.27 Culture tube solutions were individually monitored by Vernier (USA) thermocouples with Logger Pro software.28 Hydrate formation was indicated by a sudden temperature spike (~+2 K) within the sample solution. Samples that observed freezing prior to the bath temperature decreasing to 273 K were considered ‘flash freeze samples’ and not considered within induction measurements. If a sample did not yield an observed hydrate formation during the 12 h experiment, the induction time was assigned as >12 h. A graph of a typical experimental is found in the supporting information. A minimum of 50 induction time data points were collected per each glass substrate modification, and used to assess inhibition activities of the coatings. Results and Discussion Silylation of glass surfaces is a well-developed research technique29 and standard protocols are available that can be followed to yield surfaces supporting functionalization with other reagents like DNA30 or proteins.31 Our surface modification experiments followed these proven protocols with APTES and AEAPTMS to create glass surfaces adorned with amino groups for further derivatization.17, 32 It was not possible for us to definitely support the presence, much less


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the density, of the amine groups on the glass surface with the available spectroscopic tools, even after reacting with ninhydrin.33 Thus, we used contact angle measurements as evidence of molecular changes occurring on the glass surface. The expected coatings for two of the four final surfaces are depicted in Figure 1. The progression of glass surface modifications for the APTES treated surfaces as determined by changes in contact angles is shown in Figure 2, and the values for the contact angles of all slides is given in Table 1. A cleaned slide has a very small contact angle (Figure 2A, 12 ± 2°), whereas the contact angle for a slide treated with APTES is significantly greater (Figure 2B, 38 ± 2°). Modification with succinic acid lessens the contact angle (Figure 2C, 28 ± 3°), the addition of the isopropylidene protected 1,2-glycol slightly increases the contact angle (Figure 2D, 34 ± 2°), and removal of the ketal to yield the deprotected 1,2-glycol again reduces the contact angle (Figure 2E, 27 ± 2°). We also note that the contact angles for the APTES surfaces modified with the 1,3-disubstituted glycol (both protected, 40 ± 3° and deprotected, 42 ± 4°) are different from comparable surfaces for the 1,2-disubstituted glycol surfaces (34 ± 2° and 27 ± 2° respectively). On the other hand, the AEAPTMS versus APTES treated slides have similar contact angles (36 ± 7° and 38 ± 2°, respectively) and remain comparable when substituted with succinic acid (30 ± 5° and 28 ± 3°, respectively). The glycol treated surfaces have larger contact angles prior to deprotection, with the 1,3-protected diol (PD) yielding a more hydrophobic surface than the 1,2protected diol (IG) regardless of the silane. Yet, the AEAPTMS surfaces resulted in significantly smaller contact angles in comparison to the APTES surfaces while the glycols were still protected, presumably due to the interaction of the alkylamine backbone with the water. Following deprotection of the glycols, the contact angles for the APTES and AEAPTMS


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surfaces are much more comparable regardless of whether the glycol is 1,2-disubstituted or 1,3disubstituted. The value of the contact angle for APTES treated glass at 38 ± 2° is lower than comparable surfaces reported in the literature at 59.0°,34 and could be an indication our surfaces are not modified to the same level. The contact angle for the untreated glass, 12 ± 2°, is also lower than the 25.7° that was observed in that same report.34 It is unclear whether this difference is an influence of the type of glass, as a contact angle for untreated Fisherbrand glass microscope slides has been reported as 29.4°.35 Yet, a different group measured a contact angle of only 8° on untreated glass slide, though their contact angles for APTES treated slides ranged from 63.869.9°.36 All of these studies used a 5 µL water drop to measure the contact angle, but different methods for cleaning and coating the slides with silane. One plausible explanation for these differences in contact angles between this work and previous reports is that our surfaces are not as densely coated as those in prior studies. However, it has also been reported that contact angles of APTES treated glass changes as a function of residence time of the drop,37 and since previous reports did not mention the time between the placement of the drop and the determination of the contact angle, it is possible that the protocol is responsible for the difference. Nonetheless, a comparison of contact angles for untreated and treated glass surfaces within this study is still valuable for indirectly determining the presence of surface modifications. The induction time measurements were performed as previously described,24 with the exception that our experiments were monitored for 12 h instead of 24 h. We made this modification because the control THF-water solutions were reported to have all frozen by the 12 h timeframe, and our interest in using the induction time measurements was to demonstrate a


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difference between the treated glass surfaces with the PVP standard and control (untreated) test tubes. Figure 3 illustrates the induction time measurements for the progression of functionality in APTES treated test tubes as compared with a THF-water solution in untreated test tubes and THF-water solutions containing 0.25 mM PVP as a positive control. Our experiments started with stirred solutions at 278 K that are subsequently cooled to 273 K over approximately 45 min, subject to the rate of cooling of the recirculating chiller. A total of 34% of the THF-water solutions taken into the experiments froze before the temperature of the bath equilibrated at 273 K. These samples were not included in the analysis of the induction time measurements. The remaining samples are used to create the induction time curve for the THF-water samples in untreated tubes in Figure 3 by plotting the number of remaining samples unfrozen over the total number of samples passing through the aforementioned equilibration period as a function of time. A total of 20% of the THF-water samples used in the induction time measurements remained unfrozen at the end of the 12 h period. By comparison, only 14% of the 0.25 mM PVP solutions froze during the equilibration period, and 50% of the PVP solutions used in the induction time measurements remained unfrozen at the end of 12 h. This difference in induction time measurements between THF-water solutions alone and those containing 0.25 mM PVP is consistent with the previously reported results using this methodology.24 The APTES treated test tubes did not result in a significantly different induction time measurement curve as compared with the untreated tubes alone (Figure 3). Interestingly, the succinic acid treated tubes (APTES SA) resulted in a curve that indicated an increased nucleation rate with a decreased percentage of tubes (7%) remaining unfrozen at the end of the 12 h period. This was also evident in the percentage (49%) of samples in the APTES SA treated tubes that


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frozen prior to equilibration. Of the two polyol-modified tubes, only the surface bearing the 1,2substituted glycol (APTES SA IG, Figure 1B) showed a greater percentage (29%) of samples remaining unfrozen at the end of 12 h. However, these samples also had a greater percentage (45%) that froze prior to equilibrium as compared with the untreated tubes (34%). A total of 18% of 1,3-substituted glycol (APTES SA PD, Figure 1A) remained unfrozen at 12 h and this set of treated tubes also had 37% of the samples freeze prior to equilibration, both percentages that are consistent with the untreated tubes. The induction time measurements with the AEAPTMS treated tubes are shown in Figure 4, alongside the untreated tubes and PVP control. In contrast with the APTES SA treated tubes which accelerated freezing, the AEAPTMS SA tubes did not alter freezing in the first five hours of the time course and resulted in an increased percentage (28%) of samples that remained unfrozen at 12 h as compared with untreated tubes (20%). There was no significant difference in the percentage (37%) of these samples that froze prior to equilibrium as compared with untreated tubes (34%). The AEAPTMS SA IG (1,2-glycol) showed an increase in the percentage (30%) of samples that remain unfrozen at the end of 12 h as compared with untreated tubes, and is comparable to the APTES SA IG percentage (29%) that remained unfrozen after 12 h. On the other hand, the first two hours of the inductive time curve for AEAPTMS SA IG did not appear very different from the untreated tubes. The APTES SA PD behaved nearly identical to the untreated tubes in both the percentage of samples that froze before equilibration and those samples remaining unfrozen at the end of 12 h. The AEAPTMS SA PD treated tubes, however, showed an increase in percentage of samples that froze early in the time course, with 58% freezing prior to equilibration. Yet, these samples returned to track alongside the untreated tubes


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in the second half of the time course, ending with the same percentage (20%) of tubes left unfrozen after 12 h. To more fully determine the significance associated with the results from these samples, we plotted the induction time data with the Kaplan Meier38 estimator in GraphPad Prism 7TM. The curves along with the computed standard errors are shown in Figure 5, and illustrate the significance between some of these samples. We compared the curves with a log-rank (MantelCox) test, which confirms the PVP results are very different and statistically significant (P < 0.0001) as compared with the untreated tubes. Also by this analysis, the APTES SA treated tubes resulted in a strongly significant (P < 0.0006) increase in the nucleation of the THF hydrate. Both of the two 1,2-polyol surfaces resulted in a higher percentage of tubes remaining unfrozen after 12 h as compared with the untreated tubes. However, only the results with the APTES SA IG treated tubes demonstrated reasonable significance (P = 0.0381), while the AEAPTMS SA IG tubes tracked more closely with the untreated tubes in the first part of the experiment and resulted in a curve considered insignificant (P = 0.1601) by this analysis. The increased percentage of total samples that froze within the equilibration period for the treated glass tubes is not apparent from the induction time curves in Figures 3 and 4. To obtain each induction time curve, we continued to run experiments until a minimum of 50 samples were included in the induction time data. The required number of tubes ranged from a minimum of 111 runs with tubes treated with APTES to a maximum of 157 runs with tubes treated with AEAPTMS SA PD. The bar chart in Figure 6 shows the percentages of samples used for the induction time curve, those that froze prior to equilibration (flash freeze), and those that remained unfrozen after 12 h. The untreated tubes containing PVP solutions were strongly


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inhibited from freezing, as expected from the literature and use of this polymer as a kinetic inhibitor of gas hydrate formation.2 Stoichiometric THF to water (1:17) ratios nucleate THF hydrate mostly in the bulk phase, consistent with a homogeneous mechanism for hydrate formation.39 The two proposed mechanisms for gas hydrate formation (labile cluster hypothesis and local structure hypothesis) cannot be experimentally distinguished from one another at present time.40 Importantly, in a homogeneous nucleation mechanism there would be no explanation for the role of a glass surface, and it becomes challenging to explain why APTES SA IG treated tubes inhibit THF hydrate formation while the APTES SA treated tubes favor THF hydrate formation and AEAPTMS SA IG treated tubes do not appear to significantly alter THF hydrate formation under these conditions. We note a similar anomaly in the reports of hydrophobized glass surfaces that accelerate the nucleation of THF hydrate from a 1:17 THF to water solution by helping template water at the surface.41-42 It is reasoned that inhibiting gas hydrate formation by frustrating the crystallization process or crystal growth is conceptually analogous to accelerating nucleation by serving as a template for hydrate formation. However, the requirements for a nucleating surface may be more exacting for gas hydrate formation as opposed to inhibition as you have a smaller window of distances that would be acceptable to nucleate a gas hydrate as opposed to the window of distances that would hold interfacial water far enough apart so as to preclude direct nucleation. The fine line between promotion and inhibition of gas hydrates has been observed recently for ethylene glycol, which was found to promote methane hydrate formation at low (1%) concentrations and inhibit methane hydrate formation at higher (5%) concentrations.43


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Less is known about the nucleation of gas hydrates as compared with the nucleation mechanism and rates for water to ice formation; however, the fundamentals that govern the formation of ice at surfaces are also ambiguous.40 A recent screen of 27 water-soluble dyes found one compound, safranine O chloride, that aggregated to form a supramolecular surface capable of inhibiting ice formation. In this work, the researchers postulated that in the aggregate the dye mimicked the presentation of surface functional groups found in antifreeze proteins and thereby preorganized a water layer that led to inhibition of ice nucleation.44 There are additional factors that can play a role in the organization and templating of ice formation on a surface. For instance, ice nucleation on monolayers of long chain alcohols correlates to the organization of interfacial water at these surfaces, though as the alcohol chain length is increased, nucleation is inhibited as a result of increased flexibility of the surface creating more of a structural mismatch to ice.45-46 In this way, the dynamics of the surface assumes a role in the nucleation process. This can also be true for gas hydrates, as the nucleation of CO2 hydrate on a hydrated silica surface due to the organization of water and dissolved gas at the surface was attributed to the bound water layers immediately away from the solid surface, where the water is not yet in the bulk phase, but has more mobility than at the solid-liquid interface.47-48 Recent results on ice nucleation with antifreeze proteins non-covalently bound to derivitized silicon substrates demonstrated that the ice-binding face favored nucleation, while the non-icebinding face depressed nucleation.49 This is consistent with the mechanism of some antifreeze proteins that inhibit ice nucleation by binding to ice surfaces and preventing further grown, and is an important demonstration of protein surface dependent behavior of antifreeze proteins.50 A computational study on the nucleation of gas hydrates resulted in a thermodynamic landscape that the authors compared to a protein-folding funnel, complete with varying


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trajectories that represent different microstates and nuclei.51 This supports the idea that a single surface could both promote the formation of some microstates while inhibiting others relevant to the potential energy landscape for crystallization. In this way, the change from a hydrate forming surface to a hydrate inhibiting surface due to small variations in structure could be anticipated and consistent with both theory and experimental results, even if it is still unpredictable. Conclusions This effort has demonstrated the ability of a polyol coating (APTES SA IG) in test tubes to inhibit the formation of tetrahydrofuran hydrate, and is a promising first step in identifying improved coatings that could be used in field applications. This result represents a useful direction whereby coatings could reduce the need for large amounts of kinetic hydrate inhibitors currently being used in gas pipelines. Indeed, the synergistic benefits of a thermodynamic inhibitor like mono-ethyleneglycol (MEG) with kinetic inhibitors like PVP,52 PVCap,53 Luvicap,54 or PEO-coVCap-155 have already been shown. The variables demonstrated as significant in controlling gas hydrate nucleation in recent research results, such as the flexibility of the coating, addition of a non-ice binding component or face, as well as the synergistic interaction of a coating with a known kinetic inhibitor will be useful explorations in future efforts. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energy-fuels.####### 1

H and 13C NMR spectra for 1.3 and 2.2, 1H NMR spectra for the deprotection tests of 2.2,

experimental setup for time induction time experiments, model induction time results with


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defined time points, experimental setup for determining contact angles, and contact angle progression for AETPS with 2.2, AEAPTMS with 1.3, and AEAPTMS with 2.2 (PDF) Corresponding Author *Telephone: +1-603-358-2769. E-mail: [email protected]. Notes The authors declare no competing financial interest. Present Addresses †Department of Chemistry & Biochemistry, University of California Santa Barbara, Santa Barbara, CA 93106-9510 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgements Acknowledgement is made to the Donors of the American Chemical Society Petroleum Research Fund for support of this research. In addition, the authors thank the National Science Foundation for support of the KSC NMR facility (1337206). Partial research support was provided by an Institutional Development Award (IDeA) from the National Institutes of General Medical Sciences of the National Institutes of Health under grant number P20GM103506 and is appreciated.


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14. Carnahan, M. A.; Grinstaff, M. W. Macromolecules 2001, 34, 7648-7655. 15. Luman, N. R.; Kim, T.; Grinstaff, M. W. Pure Appl. Chem. 2004, 76, 1375-1385. 16. Maconi, E.; Potenza, D.; Valenti, M.; Aragozzini, F., Annali di Microbiologia ed Enzimologia 1990, 40, 177-186. 17. Metwalli, E.; Haines, D.; Becker, O.; Conzone, S.; Pantano, C. J. Colloid Interface Sci. 2006, 298, 825-831. 18. Shelkov, R.; Nahmany, M.; Melman, A. Org. Biomol. Chem. 2004, 2, 397-401. 19. Hassner, A.; Alexanian, V. Tet. Letters 1978, 19, 4475-4478. 20. Babler, J. H.; Malek, N. C.; Coghlan, M. J. J. Org. Chem. 1978, 43, 1821-1823. 21. Lamour, G.; Hamraoui, A.; Buvailo, A.; Xing, Y.; Keuleyan, S.; Prakash, V.; EftekhariBafrooei, A.; Borguet, E. J. Chem. Ed. 2010, 87, 1403-1407. 22. Rasband,

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ImageJ,

Version

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software),

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M.,

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ImageJ

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26. Larsen, R.; Knight, C. A.; Sloan, E. D. Fluid Phase Equilibria 1998, 150, 353-360. 27. Strobel, T. A.; Koh, C. A.; Sloan, E. D. Fluid Phase Equilibria 2009, 280, 61-67. 28. Vernier, Logger Pro 3; https://www.vernier.com/products/software/lp/ (accessed Mar 2017). 29. Asenath Smith, E.; Chen, W. Langmuir 2008, 24, 12405-12409. 30. Chrisey, L. A.; Lee, G. U.; O'Ferrall, C. E. Nucleic Acids Res. 1996, 24, 3031-3039. 31. Carvalho, F.; Paradiso, P.; Saramago, B.; Ferraria, A. M.; do Rego, A. M. B.; Fernandes, P. Journal of Molecular Catalysis B: Enzymatic 2016, 125, 64-74. 32. Zhu, M.; Lerum, M. Z.; Chen, W. Langmuir 2011, 28, 416-423. 33. Soto-Cantu, E.; Cueto, R.; Koch, J.; Russo, P. S. Langmuir 2012, 28, 5562-5569. 34. Qin, M.; Hou, S.; Wang, L.; Feng, X.; Wang, R.; Yang, Y.; Wang, C.; Yu, L.; Shao, B.; Qiao, M. Colloids Surf. B: Biointerfaces 2007, 60, 243-249. 35. Kanan, S. M.; Tze, W. T.; Tripp, C. P. Langmuir 2002, 18, 6623-6627. 36. Jang, L.-S.; Liu, H.-J. Biomed. Microdevices 2009, 11, 331-338. 37. Zeng, X.; Xu, G.; Gao, Y.; An, Y. J. Phys. Chem. B 2010, 115, 450-454. 38. Goel, M.; Khanna, P.; Kishore, J. Int. J. Ayurveda Res. 2010, 1, 274. 39. Zhang, W.; Creek, J. L.; Koh, C. A. Meas. Sci. Technol. 2001, 12, 1620-1630.


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Scheme 1 O

O

O

O

OH

O 1.1

1.2 DMAP DCM

O O

O O

OH O

1.3

Scheme 2


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O

O

O

OH

Ph O 2.1

O

1.2 DMAP DCM

O

O Ph

O O

OH O

2.2

a

b

OH OH

OH OH

HO

O O

O

O

O

NH

O Si O O

O

O O

O NH

HN

O

O NH

O

O

HN O

OH

O

O

O

HO

O

O

HO

OH

HO

OH

HO

NH

Si Si O O O O O

APTES SA PD

NH

HN

HN

O

O Si O O

Si Si O O O O O

AEAPTMS SA IG

Figure 1. a) Depiction of the coated surface of an APTES treated slide with a succinic acid (SA) linker to a glycerol from 2.2 substituted at the 2-position, leaving behind a 1,3-diol on the surface. b) Depiction of the coated surface of an AEAPTMS treated slide with a succinic acid (SA) linker to a glycerol from 1.3 substituted at the 1-position, leaving behind a 1,2-diol on the surface.


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Figure 2. Progression of slide treatments: A) untreated glass slides, B) APTES treated slides, C) APTES SA treated slides which functionalizes the amino groups with succinic acid, resulting in a terminal carboxylic acid attached to the glass surface, D) APTES SA IG p which has the diol protecting group in 1.3 still in place, E) APTES SA IG which is the final deprotected surface displaying a free 1,2-diol.

Table 1. Values for the contact angles (°) and standard deviations of the treated slides prepared in this study. silanized slide

SA

SA IG p (protected)

SA IG (deprotected)

SA PD p (protected)

SA PD (deprotected)

APTES

38 ± 2

28 ± 3

34 ± 2

27 ± 2

40 ± 3

42 ± 4

AEAPTMS

36 ± 7

30 ± 5

21 ± 3

27 ± 5

29 ± 3

36 ± 4


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Figure 3. Comparison of induction time curves for THF-water solutions in untreated tubes ( ) along with 0.25 mM PVP in untreated tubes as a positive control (

). The circles,

squares, triangles, or diamonds on each line represent the freezing of individual tubes. The additional induction time curves are for surfaces treated with APTES alone ( displaying succinic acid ( 1,2-diol (

), coatings

), as well as deprotected glycerol coatings from 1.3 that yields a

) and 2.2 that yields a 1,3-diol (

). The APTES SA treated tubes enhance

nucleation of THF hydrate, while APTES SA IG treated tubes inhibit the nucleation events. The APTES treated tubes alone and APTES SA PD treated tubes did not significantly alter induction time curves.


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Figure 4. Comparison of induction time curves for THF-water solutions in untreated tubes ( ) along with 0.25 mM PVP in untreated tubes as a positive control (

). The circles,

squares, triangles, or diamonds on each line represent the freezing of individual tubes. The additional induction time curves are for surfaces treated with AEAPTMS and displaying succinic acid (

), as well as deprotected glycerol coatings from 1.3 that yields a 1,2-diol (

2.2 that yields a 1,3-diol (

) and

). The appearance of an initial enhancement of nucleation in the

AEAPTMS SA treated tubes and appearance of a reduced nucleation in the APTES SA IG and APTES SA PD treated tubes was determined to be insignificantly under further analysis.


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Figure 5. Graph of selected induction time curves plotted as survival curves, along with the logrank(Mantel-Cox) analysis error bars for determining significance. The significance of the PVP solutions (

) in inhibiting THF hydrate formation in untreated tubes as compared with the

THF-water solutions alone (

) is evident from this analysis (P < 0.0001), as previously

demonstrated.11 The APTES SA (

) induction time curve demonstrates enhanced nucleation of

THF hydrate and is likewise statistically significant (P < 0.0006). Although the end points of the APTES SA IG (

, P = 0.0381) and AEAPTMS SA IG (

, P = 0.1601) appear the same, only

the APTES SA IG is statistically significant by this analysis.


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Figure 6. The bars for each of the experiments represent the percentage of tubes included in the induction time measurements ( ) for that set of experiments, that tubes that flash froze within the approximately 45 min before the equilibrium temperature of 0 °C was reached ( ), and those tubes that had not formed THF hydrate at the 12 h time point ( ). This comparison illustrates the general increased percentage of treated tubes that flash freeze before reaching the equilibrium temperature. On the other hand, with all of the tubes binned into one of these three categories it does not illustrate the significance of the specific inductions times observed over 12 h. This is most evident in similar the binned results are with tubes treated with APTES SA IG, which inhibits THF hydrate formation, and AEAPTMS SA IG, which did not show statistical significance in inhibiting THF hydrate formation.


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Inhibition of Tetrahydrofuran Hydrate Formation in ... - ACS Publications

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