Article pubs.acs.org/IECR
The Role of n‑Alkane Solvent Carbon Number on the Gelation of Long-Chained n‑Alkanes in Solution Michael Senra,* M. Gregory Grewal, and John H. Jarboe Department of Chemical and Biomolecular Engineering, Lafayette College, 740 High St., Easton, Pennsylvania 18042, United States ABSTRACT: Crystallization and subsequent gelation of n-alkanes in subsea oil pipelines is an issue of great economic importance to the petroleum industry. To develop better wax remediation strategies, it is of interest to gain a better understanding of how n-alkanes crystallize in gel in solution. In this work, the role of the solvent on gelation properties was explored. Model oil solutions containing one or two long-chain n-alkanes (carbon numbers of 28, 32, and 36) solutes were dissolved at varying wax fractions in short-chain n-alkanes (carbon numbers of 7, 10, 12, and 16). The cloud points, gel points, and pour points were measured for these solutions. Results showed that all of the cloud points could be condensed into one relationship using only the mole fraction of the least-soluble solute, a result of the solutions all following ideal solution theory. Rheometric results were consistent for samples where the solvent was decane or dodecane. However, hexadecane samples provided results that were much different, which is an indication that the chain length of the solvent can play a role in the formation of a gel. It is hypothesized that the difference between the trends seen is caused by differences in how the crystals nucleate, grow, and aggregate.
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of the complex fluid is greater than the liquidlike behavior.9 In addition to the properties noted for cloud point, these parameters are also functions of such variables as shear history (in the case of the gel point), nucleation kinetics, and precipitation kinetics.10 Pour points are found by cooling the sample at set increments and observing movement of the sample (or the lack thereof).9 Several methods have been utilized for gel point determination, but a commonly used technique involves the application of a small oscillatory stress to the sample, using a rheometer.10 Work has shown that the gelation temperature can be defined as the point where the storage modulus (G′, the solidlike behavior) is equal to the loss modulus (G″, the liquidlike behavior) when a sample is being cooled at a set cooling rate.11,12 As noted earlier, the composition of crude oil dictates the thermodynamic and gelation parameters. Therefore, in order to optimize these remediation strategies from a practical and economic standpoint, greater understanding of these deposits is necessary. Although crude oil is a multicomponent mixture often containing hundreds of different compounds, it is generally accepted that the major component of the waxes are n-alkanes (n-paraffins) with carbon numbers greater than 20.8 Waxes are predominantly n-alkanes, because of their lower solubility in most oil-based solvents, in comparison to other molecules commonly present in crudes such as aromatics, asphaltenes, resins, and branched alkanes.13 Their ability to crystallize and form gels is greatly influenced by the other components present in the oil, even if these other molecules remain in the liquid phase, and, in some cases, are present in
INTRODUCTION The study of cold flow properties has been one of great interest in the energy industry for many years. The ability of fuels to flow, crystallize, and gel at lower temperatures impact many aspects of the oil industry, including transportation, processing, formulation, and salability.1,2 Of particular interest to the petroleum industry is the formation of gels that can result in production losses and/or blockages.3 These gels can form because of the crystallization of particular components of crude oil in subsea oil pipelines when the oil reaches a temperature below the cloud point (the temperature where the solution begins to become cloudy, indicative of precipitation) of the crude oil. Although crudes exit reservoirs at temperatures ranging from 70 °C to 150 °C, they are cooled by heat transfer with the cold water (as low as 4 °C) on the ocean floor.4 Subsequent gelation can occur if sufficient crystals have precipitated out of solution to form a volume spanning threedimensional network where the solid crystals trap the liquid crude as a wax-oil gel.5,6 These gels form as deposits on the walls of the subsea pipeline and are capable of strengthening via molecular diffusion and counterdiffusion in a process known as aging.7 They can cause partial or complete blockages of the pipelines, a flow assurance problem in the petroleum industry that can cost on the order of tens of millions of dollars.4 To prevent significant deposit growth, petroleum companies several oil characteristics to develop appropriate remediation strategies. Some of these parameters focus on the transitions that the oil experiences as it cools. The cloud point is dependent on many properties, most notably, the pressure, the thermal history, and the composition of the crude oil. Another transition point of interest is the point at which the fluid no longer behaves as a Newtonian fluid and, instead, acts as a gelled solid. Ways this transition can be defined include the pour point, which measures when a fluid ceases to flow under static conditions, and the gelation temperature (gel point), which represents the temperature where the solidlike behavior © 2015 American Chemical Society
Special Issue: Scott Fogler Festschrift Received: Revised: Accepted: Published: 4505
October 15, 2014 March 19, 2015 March 24, 2015 March 24, 2015 DOI: 10.1021/ie504083y Ind. Eng. Chem. Res. 2015, 54, 4505−4511
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whether short-chain n-alkane solvents influence the thermodynamic and gelation properties of polydisperse model oils, some of which exhibit co-crystallization and some of which that are incapable of co-crystallizing. The variables explored in this work were the cloud point, pour point, and gel point. Results from this work can help optimize the development of remediation strategies and thermodynamic models for waxes and gels formed in subsea oil pipelines.
very small amounts. Petroleum companies have taken advantage of the role oil composition plays by developing additives that can influence many of the parameters that are considered to be crucial in flow assurance. These additives, generally polymeric in nature, can improve the flowability of the crude by acting as pour point depressants, wax crystal modifiers, and paraffin inhibitors.14 Although significant research has been focused on wax inhibitors, less work has been dedicated to analyzing how the components of the oil itself influence the thermodynamic and rheological properties of a crude oil.14−19 Studies have been conducted to observe how n-alkane polydispersity influences relevant properties. Previous work has shown that n-alkanes with carbon numbers that are sufficiently close together are capable of crystallizing out of solution at the solubility limit of the longer carbon number n-alkane as co-crystals.18,19 Cocrystallization has been shown to influence the heat of crystallization, the magnitude and wax fraction of a deposit, the gelation temperature, and the yield stress of a gel, generally by impacting crystal morphology.18−22 Crystal morphology will be influenced because a co-crystal will form by the longer molecule bending to associate with the shorter molecule.23 This molecular bending leads to a weakness in the crystal structure, making it more difficult to form the larger crystals that are conducive in forming volume-spanning networks that are necessary to form a gel. In addition to co-crystallization, work showed that the presence of n-alkanes of moderate carbon numbers that were incapable of co-crystallizing with the longer n-alkanes still influenced the gelation, deposition, and, to a lesser degree, thermodynamic properties of the solution.21 Of most interest were the results that these shorter n-alkanes would have no influence on the gelation of a longer one, if present in small amounts. However, at sufficiently high amounts (i.e., 4 mass % C28 in a dodecane solution with 4 mass % C36), the presence of the shorter molecule would cause a significant decrease in the gel point and pour point (as much as 20 °C) and the gelation characteristics became dependent on the shorter molecule, even though an overwhelming majority of the longer molecule had already crystallized.21 It was hypothesized that the molecular size of these shorter molecules hampered the ability of the longer crystals to form the necessary volume-spanning network, even though co-crystallization was not occurring. In addition to the crystallizable materials, some work has also been completed looking at the role solvent plays in the crystallization and gelation of multicomponent waxy solutions. Differential scanning calorimetry (DSC) analysis showed that the chemical structure of the solvent can greatly influence the crystallization rate, with the rate being slower for systems with aromatic solvents that slows crystal growth, because of its impact on how n-alkane molecules align.24 In addition, the solvent type also influenced the enthalpies of crystallization and melting. Other work, which focused on a variety of organic solvents, including n-alkane solvents, showed that the solubility of a wax increases as the n-alkane solvent size decreases, because the smaller solvent can more easily contact the solute, allowing for solvation.2 Additional analysis in that work showed that solvent strength was inversely related to the ability of the solvent to self-associate. Taking advantage of the insights provided in previous research, the major objectives of this work are to analyze how the effects of co-crystallization and polydispersity are influenced by the type of solvent used. This work will particularly focus on
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MATERIALS AND METHODS In this work, model oils were developed that contained solely long-chain n-alkanes as the solute and short-chain n-alkanes as the solvent. The long-chain n-alkanes used include hexatriacontane (C36H74, 98% purity), dotriacontane (C32H66, 97% purity), and octacosane (C28H58, 99% purity) all of which were purchased from Sigma−Aldrich. Solvents used include heptane, decane, dodecane, and hexadecane, all with a purity of at least 99% and were purchased from Sigma−Aldrich. This work focused on determining the cloud point, pour point, and gel point of varying compositions of either one or two solutes in the pure solvent. The cloud point and gel point were determined by using an Endocal Model RTE-9DD refrigeration circulated bath. Since the cloud points and gel points for the aforementioned samples were sufficiently higher than the freezing point of water, water was used as the circulating fluid. To determine the cloud points, ∼10 g of a sample in a vial were placed in the bath at a temperature at least 10−15 °C above the possible cloud point. The bath was held at this temperature for ∼30−45 min, to eliminate thermal history. The bath temperature was then lowered and the solution was held at the temperature for another 30−45 min. The vials were then removed from the bath. If no cloudiness in the vial was observed, the temperature was further reduced and the process was repeated. If cloudiness in the vial was observed, this temperature represented a lower bound for the cloud point. Additional experiments are then performed to narrow the range of the potential cloud point, until it is ∼0.5 °C wide. The methodology used for the pour point was similar, except instead of looking for clouding of the solution, the vial was inverted to determine if flow occurred. If the solution did not flow, the temperature of the bath represented a lower bound for the pour point. If the solution did flow, the procedure was restarted because the inversion of the fluid impacted the ability of the fluid to form the volume-spanning network necessary for a gel. All pour points were found within ∼0.5 °C. The gel point was found by using a TA Instruments Model ARES-DHR1 controlled stress rheometer. A cone (2°)-andplate geometry was utilized for the experimentation, along with a solvent trap to minimize solvent evaporation. To find the gel point, the fluid was heated to a temperature ∼10−15 °C higher than the known cloud point found using the methodology just described. The solution is held at this temperature for 1 h, to eliminate any thermal history. The solution is cooled at a constant rate of 1 °C/min while an oscillatory stress of 0.1 Pa with a frequency of 0.1 Hz is applied. Each sample was done in triplicate to ensure accuracy.
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RESULTS AND DISCUSSION All of the polydisperse systems for this study were designed such that C36 would be the least soluble material in the model oil. Therefore, initial analysis was conducted on monodisperse 4506
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Industrial & Engineering Chemistry Research C36 systems in a range of n-alkane solvents. Previous work has shown that these solute−solvent systems tend to obey van’t Hoff solubility theory.1,2,19,25 Some of the key premises of van’t Hoff solubility theory are that the solution behaves ideally and that the heat capacity difference between the solute and solvent are negligible.1,19 If these assumptions hold, then the van’t Hoff equation, linearized as eq 1, will be valid. ⎛ 1 ⎞ ΔHdiss ΔSdiss ln⎜ − ⎟= RT R ⎝ xsolute ⎠
Table 1. Values of the Enthalpy (ΔHdiss) and Entropy (ΔSdiss) of Dissolution of C36 in n-Alkane Solvents
(1)
where xsolute is the mole fraction of solute, ΔHdiss the dissolution enthalpy (kJ/mol), ΔSdiss the dissolution entropy (J/(mol K)), R the universal gas constant (J/(mol K)), and T the cloud point temperature (K). Therefore, a graph of ln(1/xsolute) vs 1/T will generate a straight line if the solution obeys van’t Hoff solubility theory. Figure 1 shows this graph. The cloud point temperatures found
solvent
ΔHdiss (kJ/mol)
ΔSdiss (J/(mol K))
heptane decane dodecane hexadecane
113 109 109 113
325 312 311 322
overall
105
297
Figure 1. Determination of accordance with van’t Hoff solubility theory: Various amounts of C36 were mixed with four different solvents: (◆) heptane, (▲) decane, (■) dodecane, and (×) hexadecane. The dashed line represents a best-fit line drawn through all trials.
and C28 are added to the system, the mole fraction of the solute will change. However, this difference is relatively minimal, because a majority of the mass is the lower-molecular-weight shorter n-alkane. As an example, a 4 mass % C36 solution in dodecane would have a mole fraction of 0.0138, which corresponds to a cloud point of 314.3 K, using the aforementioned universal equation. By adding 8 mass % C28 (a material whose cloud point in dodecane is less than that of 4 mass % C36) to the system, the mole fraction changes to 0.0145, which corresponds to a cloud point of 314.7 K. Therefore, it would be expected that the cloud point of the system would not be impacted by the replacement of solvent with additional solutes, as long as the solubility limit of the shorter-chain solute does not pass the solubility limit of the longer n-alkane. The work done in this study corroborates this observation, because minimal changes in the cloud point were observed as either C28 or C32 was added to a system where C36 was the chief crystallizing component and systems and solutions where C28 was added to C32 systems. The gelation properties of these monodisperse C36 systems were also studied. Heptane samples were removed from this analysis, because of their rapid evaporation when conducting the rheometric studies. These results are presented in Figure 2. Although the gel point and pour point are used as methods to determine when the solutions gel, their methodologies are different. Therefore, it would not be expected for these temperatures to be identical, but it would be expected that similar trends would be observed. Figure 2 shows that, as the mass % of C36 is increased, both the gel point and the pour
are consistent with cloud points found in the literature using a variety of techniques, including DSC.2,19,21,24,26 The graph shows that not only does each individual solvent obey van’t Hoff solubility theory, but all of the solvents could also be combined and modeled via one relationship. The ability to develop one unifying relationship is reasonable, because van’t Hoff solubility theory assumes that the solution is ideal. nAlkane solvents would make good candidates for ideal solvents, because they lack any functional groups, meaning that they are unlikely candidates for solvent self-association. To further confirm the ideality of these solutions, the values of ΔHdiss and ΔSdiss were calculated for each individual solvent and for the compilation of all of the trials. These values are provided in Table 1, results consistent with previous work.19,25 With an overall ΔHdiss and ΔSdiss that are both within 10% of the respective values found for the individual solvents, the universal equation (ln(1/xsol) = 12563/T − 35.684) can be used to predict the cloud point of monodisperse C36 in short-chain nalkane solvents. This universal equation can also be extended to systems where C36 will crystallize out first. As shorter solutes such as C32
Figure 2. Determination of gel point and pour point for monodisperse C36. Various mass percentages of C36 were mixed with three different solvents: (◇,◆) decane, (□,■) dodecane, and (△,▲) hexadecane. The filled data points represent the gel points, while the unfilled data points represent the pour points. The solid error bars represent the error bars for the gel points and the dashed error bars represent the error bars for the pour points. 4507
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Industrial & Engineering Chemistry Research point increased. Moreover, for a given mass percentage, the gel point and pour point increased as the chain length of the solvent increased. These results are expected because, in order to form a gel, a sufficient amount of solute must precipitate out of solution in order to form the volume-spanning network that is capable of entrapping the liquid. Because solutions with higher mass percentages have higher cloud points, it would follow that sufficient crystallization needed to form a gel would occur at higher temperature. The trend in the carbon number of the solvent is seen because the mass fraction of C36 at 4 mass % is higher for the longer n-alkane solvents, which as Figure 2 shows, corresponds to a higher cloud point. However, an interesting trend can be seen at the lowest mass percentage observed in Figure 2, which shows all of the solvents converging to one common temperature. This result can be explained by the presence of a lower bound of wax necessary for the formation of a gel to occur. As discussed in previous work, crystallization in these model fluids occurs via homogeneous nucleation and can be expressed in three regimes: (1) A nucleation lag phase, where the solution has reached its solubility limit but precipitation has yet to commence, because the critical nuclei number density has yet to be reached; (2) A supersaturation growth regime, where a majority of the precipitation occurs, which is driven both by the nucleation lag and the decreasing solubility of the solute; and (3) Equilibrium growth, where the crystallization is driven solely by changes in solubility with temperature.25 At higher mass percentages, only a small portion of the C36 is needed to crystallize to form a gel. Therefore, this gelation will occur during the supersaturation growth regime, a kinetic regime that extends ∼5−10 °C below the cloud point. However, at 2 mass %, the solution needs a significant majority of the solute to crystallize in order to have a sufficient amount of wax crystals to form the gel. Therefore, the gelation of the system becomes dependent on equilibrium thermodynamics. In addition, at this small amount, it could be possible that the molecular size of the solvent could inhibit the ability of the wax crystals to connect together to form a volume-spanning network. Therefore, it would involve more precipitated C32 to form a gel in hexadecane than in dodecane or decane. Because of these facts, the difference in gelation between the solvents is minimized. For solutions with two solutes, three different systems were analyzed: (1) systems containing 4 mass % C36 with varying amounts of C32 added, (2) systems containing 4 mass % C36 with varying amounts of C28 added, and (3) systems containing 4 mass % C32 with varying amounts of C28 added. Previous work has shown that C36 can co-crystallize with C32 and C32 can co-crystallize with C28. However, because of the difference in molecular size, C36 is unable to co-crystallize with C28.19 Figure 3 shows how the gel point and pour point were affected by the addition of C32 in decane, dodecane, and hexadecane. As expected, the gelation temperatures increase as the solvent chain length increases; this is a function of the fact that the longer solvent chain lengths correspond to higher cloud points. Because more solid wax is present at higher temperatures, it would follow that gelation would occur at higher temperatures. However, Figure 3 also provides an interesting trend. As the carbon number of the solvent decreases, the location of the
Figure 3. Effect of varying C32 on the gelation properties of 4 mass % C36 solutions. Various mass percentages of C32 were added to 4 mass % C36 solutions in three different solvents: (◇,◆) decane, (□,■) dodecane, and (△,▲) hexadecane. The filled data points represent the gel points, whereas the unfilled data points represent the pour points. The solid error bars represent the error bars for the gel points, and the dashed error bars represent the error bars for the pour points.
minimum in the curve shifts to higher percentages in C32. The reason that a minimum can occur, despite more wax coming out of solution, is because when co-crystals are formed, there will be defects that will be caused by the longer n-alkane folding itself to incorporate the shorter n-alkane into a common crystal structure. The presence of these defects can influence the ability for the crystals to link together to form a volume-spanning network necessary for gelation to occur. This effect of inhibiting gel formation is counteracted as the mass percentage of C32 is increased by the greater amount of crystals being formed, because of the fact that C32 will crystallize out with C36, which is a phenomenon that has been observed in previous works.18,19 Therefore, as the total wax fraction increases, there will be sufficient crystals present in the solid phase to overcome the smaller crystals that can be caused by the formation of defects. The trends that have been observed for the dodecane trials are consistent with results observed in other work, where the cooling rate used in the prior work was 0.5 °C/min.21 For the solutions in decane, dodecane, and hexadecane, all of which obey ideal solution theory, it would initially appear that the trends would be the same for all three solvents. However, because there are noticeable differences in the respective trajectories in Figure 3, it indicates that the solvent has some influence in how crystallization occurs, most likely in the stages where crystals first begin to form and/or in the crystal growth stage. Previous work has shown that the amount of solid wax needed for both monodisperse and polydisperse systems in mineral oil to form a gel is not a constant value, but is rather a function of variables such as cooling rate, wax percentage, and solvent type.25 Further work showed that gels that form at higher temperatures will tend to require less solid wax.10 These variables will impact the homogeneous nucleation kinetics, which is influenced by the degree of supersaturation. If the nucleation lag regime is long, this will cause a higher degree of supersaturation. Since a large amount of crystals exit solution all at once or within a small temperature range, there is a greater chance that the crystallization will not occur in the most thermodynamically favorable state. This effect inhibits the formation and growth of the long platelike crystals conducive to 4508
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Industrial & Engineering Chemistry Research forming gels that require a smaller amount of solid wax. Previous work has shown how the growth rate of C36 crystals in petroleum ether is positively correlated with the degree of supersaturation of the system.27 To gain some further insight into this possible effect, the difference between the cloud point and pour point of the samples in Figure 3 were evaluated. These differences are presented in Figure 4.
Figure 5. Effect of varying C28 on the gelation properties of 4 mass % C32 solutions. Various mass percentages of C28 were added to 4 mass % C32 solutions in three different solvents: (◇,◆) decane, (□,■) dodecane, and (△,▲) hexadecane. The filled data points represent the gel points while the unfilled data points represent the pour points. The solid error bars represent the error bars for the gel points, and the dashed error bars represent the error bars for the pour points.
Figure 4. Difference between the gel point and the cloud point for samples containing 4 mass % C36 and various mass percentages of C32 in three different solvents: (◇) decane, (□) dodecane, and (△) hexadecane.
ratios are quite similar, it would make sense that the trends observed for both of these co-crystals would not be vastly different from each other. Figure 6 shows the results of the 4 mass % C36 trials with varying amounts of C28. C28 is a solute that is capable of
As Figure 4 shows, at 1 mass % C32, the difference between the cloud point and gel point are virtually independent of solvent. Because there is little C32 present in the system, the amount of defects in the crystals are minimized. Therefore, the effects of homogeneous nucleation kinetics and supersaturation are mitigated, because of the fact that single n-alkanes readily grow long plates in n-alkane solvents. However, as more C32 is introduced, the three solvents follow completely different trajectories. For hexadecane, which is the solvent that allows for crystallization at higher temperatures, the difference between the cloud point and gel point decreases as the amount of C32 increases. This decrease can be explained by the inability of the longer hexadecane molecule to intertwine itself into the process of the precipitation and particularly, the growth of the C36−C32 co-crystal. Therefore, gelation can occur at higher temperatures, because of the more crystallizable wax present in the solute. However, in the case of decane, its shorter molecular length helps to disrupt the growth of the co-crystal just enough to magnify the influence of the defects being caused by the imperfect co-crystal formation. Eventually, the role of solvent interference is mitigated by the shear amount of crystallizable wax present and the differences between the gel point and cloud point return to becoming virtually independent of solvent chain length. Figure 5 shows the results of the pour point and gel point analysis for the 4 mass % C32 systems with varied amounts of C28 added. These results look very similar to the results seen in Figure 3. As aforementioned, in both cases, the two solutes are capable of co-crystallizing with each other. The only difference between the two solution is the ease by which co-crystallization can occur, which is related to the molecular length because of the need of the longer molecule to bend to incorporate the shorter molecule. Because these are straight chained n-alkanes, the ratio of the molecular lengths can be closely approximated by the carbon number. Thus, C36 is ∼1.13 times the length of C32, while C32 is ∼1.14 times the length of C28. Since these
Figure 6. Effect of varying C28 on the gelation properties of 4 mass % C36 solutions: Various mass percentages of C28 were added to 4 mass % C36 solutions in three different solvents: (◇,◆) decane, (□,■) dodecane, and (△,▲) hexadecane. The filled data points represent the gel points while the unfilled data points represent the pour points. The solid error bars represent the error bars for the gel points and the dashed error bars represent the error bars for the pour points.
precipitating (at temperatures lower than C36) but is unable to co-crystallize with C36, because the difference in molecular length is too large. The results seen for the dodecane trials are consistent with results seen in other work using a cooling rate of 0.5 °C for the gel point studies.21 As seen in the trials where co-crystallization occurred in Figures 3 and 5, the trends for decane and dodecane are consistent with each other, while the trends for hexadecane are distinctly different with the changes in temperature being less drastic for the hexadecane trials. However, there are some distinct differences between the trials 4509
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hexadecane, preventing the ability of C28 to inhibit crystallization and gelation.
where co-crystallization occurs and the trials where cocrystallization does not occur, most notably that the gel and pour points exhibit a drastic dropoff at ∼3−4 mass % C28. The difference between the cloud points and gel points is provided in Figure 7, which shows that the maximum difference in the
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CONCLUSION To better develop appropriate models for wax deposition in subsea oil pipeline systems, a better understanding of how the solvent plays a role in the gelation properties of the fluid is necessary. This work specifically focused on the role n-alkane solvent chain length plays in the gelation of model fluids containing one or two longer-chained n-alkane solutes. Of specific interest were comparing systems that contained solutes that were capable of co-crystallizing to those that contained solutes that were incapable of co-crystallzing. From a solution standpoint, this work showed that these model fluid systems consisting of n-alkane solutes and solvents obey ideal solution theory and can be modeled via the van’t Hoff equation. An equation was developed to predict the cloud point of the solution that can predict the cloud point of the system with only knowledge of the mole fraction of the nalkane. Although this analysis focused on mondisperse systems, its results can be extended to many polydisperse systems as long as it is clear which n-alkane represents the component that will crystallize out first in solution. Rheometric results found in this work were consistent with previous research. The addition of even small amounts of cocrystallizable solute is capable of causing a decrease in the gel point and pour point of a fluid, because of the formation of cocrystals that have crystal defects that can inhibit the formation of a gel. However, if the additional solute is unable to cocrystallize with the primary solute, a threshold of additional solute must be passed before influencing the gelation characteristics of the system. Once this threshold is passed, a significant decrease in the gel and pour points are observed. Almost identical trends were seen when the solvent used was decane or dodecane. The results for hexadecane were quite different from the other two solvents. In all system analyzed, the addition of the shorter n-alkane solute had much less, and, in many cases, no influence on the gelation characteristics of the system. It is hypothesized that the longer chain length influenced the homogeneous nucleation step necessary to begin crystallization in these pure systems. By altering the nucleation, it enhanced the formation of long, platelike crystals often seen in the crystallization of n-alkanes in solution, which is conducive to forming a gel.
Figure 7. Difference between the gel point and the cloud point for samples containing 4 mass % C36 and various mass percentages of C28 in three different solvents: (◇) decane, (□) dodecane, and (△) hexadecane.
systems where co-crystallization does not occur is triple the amount of the maximum difference in the systems where cocrystallization does occur. Previous work hypothesized that the reason for the sharp decrease was caused by the C28 molecules still present in the liquid phase, because they have yet to reach their solubility limit, and thereby act as barriers for crystal growth and/or aggregation. In fact, the gelation temperatures of the high-mass-percentage C28 solutions approached the gelation temperatures of C28 solutions without the presence of C36.21 In that work, the results were corroborated by work using crosspolarized microscopy, which showed this impedance of crystal growth and formation in 4 mass % C36/C28 samples where the amount of C28 present was large. For samples where the mass percentage of C28 was below a threshold of ∼3−4 mass %, the samples appeared unaffected by the C28. Because C28 is acting as a barrier, a sufficient amount of C28 must be present to provide this barrier. If insufficient C28 is present, then C36 will crystallize and aggregate as if the C28 is not present, meaning that the gel point should not change in this regime, which is a trend that is observed for all solvents in Figure 6. If molecular solvent size had the same influence as the molecular solute size, then the presence of a longer solvent would cause further depressions in the gel point and, potentially, a lower threshold of C28 needed for gel point depression to occur. However, the issue of molecular size is inconsistent with the results seen in Figure 6, which is sensible because the solvent molecules are much shorter than the solute molecules and should not provide any hindrance for the C36 crystals to grow and aggregate to form a volume-spanning network. Therefore, the influence of the solvent chain length must impact the homogeneous nucleation kinetics and/or the ability of crystals to grow in multiple dimensions, which is an insight identical to the one developed for the trials where cocrystallization occurred. However, the presence of C28 does somewhat influence the gelation characteristics of these systems in hexadecane, as seen in the decrease in the gel point at 3% C28. However, the effects are minimized and, in some cases, eliminated, because C36 is better able to nucleate and grow in
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Lafayette College and, particularly, the Department of Chemical and Biomolecular Engineering for their support in this work. In addition, the authors would like to thank Tom DeFazio and Matthew Iassogna for their assistance in the development of protocols and apparatuses for the successful completion of this work. 4510
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DOI: 10.1021/ie504083y Ind. Eng. Chem. Res. 2015, 54, 4505−4511
