Langmuir 2000, 16, 7939-7945
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Solubilization of Trilaurin in Surfactant Solutions I. D. Robb* and P. S. Stevenson Centre for Water Soluble Polymers, North East Wales Institute, Wrexham, Wales LL11 2AW, U.K. Received January 27, 2000. In Final Form: July 3, 2000 We have studied the process of solubilization of dispersions of trilaurin by ethoxylated nonionic surfactants using optical microscopy and differential scanning calorimetry (DSC). The equilibrium solubility of the oil in the nonionic surfactant solution increases with increasing temperature and decreasing ethoxylate length, as would be expected for a system below the phase inversion temperature. The DSC data show that the nonionic surfactant partitions into the oil, producing a number of new structures that are not polymorphs of trilaurin. One of these new structures may be a mesophase that melts about 1-1.5 °C below that of pure trilaurin. Upon cooling this structure, which increases the affinity of the ethoxylate headgroup for water, further crystallizing and melting of the structure is prevented, possibly because it transforms into micellized trilaurin or into small droplets that are unable to nucleate. Creaming of the dispersion of trilaurin in the calorimeter cell complicates the results, though the heat changes measured are reproducible and not affected by creaming. Addition of poly(acrylic acid) to the trilaurin/surfactant dispersion slightly reduces the amount of oil solubilized and retards the distribution of the surfactant into the oil.
Introduction The solubilization of oils by aqueous surfactant solutions is of interest in detergency, the production of mineral oil, and the recovery of natural oils from agricultural waste. As expected, solubilization depends on the nature of both the oil and the surfactant, with maxima in solubilization often occurring near phase inversion temperatures (PITs). For the simplest systems of linear hydrocarbons in contact with ethoxylated nonionic surfactants, the oil/water interfacial tension1-3 passes through a minimum at a temperature T* which corresponds to the PIT. T* increases with the chain length of the oil being solubilized. After initial contact of oils such as eicosane4 or glycerol tripalmitate5 with the surfactant solution, water and/or surfactant penetrate the oil, after which solubilization into the surfactant micelles takes place. The maximum in solubilization or removal6 of the oil from a solid surface occurs under conditions that usually correspond to the minimum in surface tension or the PIT, though the removal process may be complicated7,8 by contact angle effects. The rate of solubilization9,10 of single component oils into nonionic surfactant solutions increases with decreasing molecular mass of oil and increasing temperature, approaching a maximum at the cloud point. The solubilization of mixed oils is similar, except that the presence of a more polar moiety (e.g., an alcohol in the hydrocarbon) can have significant effects on the solubilization. For mixed component11 oils, such as hexadecane/ (1) Aveyard, R.; Lawless, T. A. J. Chem. Soc., Faraday Trans 1 1986, 82, 2951. (2) Aveyard, R.; Binks, B. P.; Lawless, T. A.; Mead, J. J. Chem. Soc., Faraday Trans. 1 1985, 85, 2155. (3) Aveyard, R.; Binks, B. P.; Mead, J. J. Chem. Soc., Faraday Trans. 1 1988, 84, 675. (4) Scheuing, D. R.; Hsieh, J. C. L. Langmuir 1988, 4, 1277. (5) Weerawardena, A.; Drummond, C. J.; Caruso, F.; McCormick, M. Langmuir 1998, 14, 575. (6) Malmsten, M.; Lindman, B. Langmuir 1989, 5, 1105. (7) Thompson, L. J. Colloid Interface Sci. 1994, 163, 61. (8) Thompson, L.; Walsh, J. M.; Tiddy, J. G. T. Colloids Surf., A 1996, 106, 223. (9) Carroll, B.; Doyle, P. J. J. Pharm. Pharmacol. 1988, 40, 229. (10) Shaeiwitz, J. A.; Chan, A. F-C.; Cussler, E. L.; Evans, D. F. J. Colloid Interface Sci. 1981, 84, 47.
oleyl alcohol in contact with a C12E6 ethoxylated nonionic surfactant, the polar component is preferentially adsorbed at the oil/water interface, contributing to the lowering of the interfacial tension. Spontaneous emulsification can take place when mixed oils are brought into contact with water, aided by the formation of a lamellar or microemulsion phase. Mixtures of hexadecane, oleyl alcohol, and C12E6 nonionic surfactant were found12 to initially absorb water, producing a lamellar phase into which the C16H34 was solubilized in small droplets. Spontaneous emulsification of hexadecane was retarded13 when there was little or no alcohol in the oil, and the formation of a lamellar phase occurred generally when the surfactant system was above its PIT. The solubilization of triglycerides is generally slower and less14,15 than that for linear hydrocarbons, partly because their larger size restricts mixing with the alkyl chains of the surfactant. The PITs of nonionic surfactants14,16 with triolein are higher than those of surfactants with linear hydrocarbons, whereas they are reduced17 with polar molecules such as alcohols. Solubilization17 and detergency14 of pure triolein by the nonionic surfactants C12E4 and C12E5 approached a maximum at the PIT. Solubilization of triolein is enhanced by the addition of fatty acid,18,19 linear hydrocarbon,16 or alcohol19,20 to the triglyceride, and a Marangoni flow at the micelle/triglyceride interface has been proposed16,20 to assist in the solubilization. The enhanced removal of the triglyceride in the presence of various polar additives has been (11) Graciaa, A.; Lachaise, J.; Cucuphat, C.; Bourrel, M.; Salager, J-L. Langmuir 1993, 9, 1473. (12) Rang, M-J.; Miller, C. J. Colloid Interface Sci. 1999, 209, 179. (13) Lim, J-C.; Miller, C. A. Langmuir 1991, 7, 2021. (14) Mori, F.; Lim, J. C.; Raney, O. G.; Elsik, C. M.; Miller, C. A. Colloids Surf., A 1989, 40, 323. (15) Minana-Perez, M.; Graciaa, A.; Lachaise, J.; Salager, J-L. Colloids Surf., A 1995, 100, 217. (16) Raney, K. H.; Benton, W. J.; Miller, C. A. J. Colloid Interface Sci. 1987, 117, 282. (17) Raney, K. H.; Miller, C. A. J. Colloid Interface Sci. 1987, 119, 539. (18) Chen, B-H.; Miller, C. A.; Garrett, P. R. Langmuir 1998, 14, 31. (19) Lim, J-C,; Miller, C. A.; Yang, C. Colloids Surf., A 1992, 66, 45. (20) Chen, B-H.; Miller, C. A.; Garrett, P. R. Colloids Surf., A 1997, 128, 129.
10.1021/la0001070 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/15/2000
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Robb and Stevenson Table 1. Properties of Surfactants
name
supplier
composition
cloud point, °C at 10% w/w
Synperonic NP 13 Neodol 1-9 Neodol 1-7 sodium octyl sulfate
ICI Surfactants Shell Chemicals Shell Chemicals Lancaster Chemicals
C9C6H4EO13 C10EO8-9 C10EO7 NaC8SO4; >99% pure
88 84 68
attributed21 to the formation of a lamellar phase, which would be expected near the PIT for a surfactant/oil system. The rate of solubilization of linear hydrocarbons into nonionic surfactants has been shown to be proportional22 to their equilibrium solubilities, although this was not the case20 for triolein mixed with hexadecane, possibly because there was preferential solubilization of the hexadecane. In this paper we have studied the solubilization of trilaurin (1,2,3-tridodecanoylglycerol) by ethoxylated nonionic surfactants chosen to have a range of PITs and radii of curvature of the micelle. Differential scanning calorimetry (DSC) was used to study the effect of surfactants that partition into the triglyceride. In addition we have studied the effect of adding polymers to the surfactant/fat system to establish whether the polymer/surfactant interaction has any influence on the solubilization. It is known23,24 that poly(acrylic acid) forms complexes with ethoxylated nonionic surfactants, reducing their cloud points. The solubility of hydrocarbons usually increases with decreasing cloud point (below the phase inversion temperature) as the spontaneous curvature of the micelle approaches a flat layer, where maximum solubilization occurs in microemulsions. Experimental Section The solubility of trilaurin in surfactant solutions was measured in two ways. Supersaturated solutions of trilaurin in nonionic surfactants were produced by sonicating various amounts of triglyceride (1-2% w/w) in surfactant (10% w/w) for 1-2 min, by which time the mixture had become warm (∼60 °C) and clarified. These one-phase systems were placed in a rotating holder in a water thermostat at various temperatures and allowed to equilibrate, with the existence of two phases being judged visually. An alternative method was to allow various amounts of trilaurin (1-2%) in surfactant solution to equilibrate at different temperatures while rotating to establish the triglyceride concentration at which the system became just one phase. The solubility was taken as the highest concentration at which the system remained in one phase. Examination of the dissolution process was also made using a Nikon Optiphot microscope with a heated sample stage. The pH of the solutions is that of the final mixture.
Materials The surfactants are listed in Table 1. Trilaurin (puriss) was supplied by Fluka; purity g 99%. Poly(acrylic acid), PAA, was Versicol E11, supplied by Ciba Chemicals, with a molecular mass of 250 000, as determined by GPC. The cloud point of 1% Neodol 1-7 + 0.05% PAA at pH 2.5 was 57 °C Differential scanning calorimetry (DSC) was carried out on a Setaram Micro DSC II calorimeter. DSC experiments were performed in duplicate and were reproducible within the limits in Table 2.
Results The solubility of trilaurin in various nonionic surfactants was measured by the two different methods, either cooling (21) Zourab, S. M.; Miller, C. A. Colloids Surf., A 1995, 95, 173. (22) Donegan, A. C.; Ward, A. J. I. J. Pharm. Pharmocol. 1987, 39, 45. (23) Yoshida, K.; Dubin, P. L. Colloids Surf., A 1999, 147, 161. (24) Anghel, D. F.; Winnik, F. M.; Galatanu, N. Colloids and Surfaces A 1999, 149, 339.
from supersaturation or allowing the solid trilaurin to dissolve at constant temperature. The data, Figure 1, show that each method gives approximately the same result after about 2 h, indicating that the solubilities are close to equilibrium after that time. The rate of solubilization will depend on the particle size of the trilaurin crystals, which, in these experiments, were used as received. The solubility of trilaurin in different surfactant solutions as a function of temperature was measured by rotating the oil/surfactant dispersion at various constant temperatures, and the results are shown in Figure 2. The solubility increases with temperature for all the nonionic surfactants, and for a constant alkyl chain length, more triglyceride is solubilized by surfactants with shorter ethoxylate chains. The effect of adding poly(acrylic acid) (PAA) at pH 2.5 was small but reduced the amount of triglyceride solubilized. Solubilities with added PAA could not be measured at higher temperatures, since polymer/ surfactant complexes precipitated in these systems at about 49 °C. Examination under the microscope of the dissolution of trilaurin (0.5%) in Neodol 1-9 (10%) sonicated in an ice bath and heated to 48 °C on the microscope stage showed that dissolution of this amount was complete in about 7-10 min. Using crossed polarizing lenses, no mesophases were detected at the oil/surfactant solution interface during dissolution. In these sonicated systems, dissolution was faster than that with unsonicated samples, as sonication in the ice bath dispersed the trilaurin crystals to smaller particle sizes. With 2% trilaurin and Neodol 1-9 (10%), where only partial solubilization was achieved, heating the dispersion on the microscope stage caused all the fat to melt, and on cooling to 25 °C, some of the droplets either split into smaller drops or adopted a “blotchy” appearance, as shown in Figure 3. This behavior is analogous to that found by Rang and Miller12 and was observed with several systems upon cooling. The melting and solubilization behavior of trilaurin on its own and in the presence of various surfactants was investigated in the calorimeter. Crystals of trilaurin were heated and cooled in the calorimeter, giving a melting point of 45.5 °C and a crystallizing temperature of 28-31 °C. Crystallization of pure trilaurin would follow heterogeneous nucleation, and the lower temperature is a measure of the supersaturation (of the vapor pressure of solid compared to liquid) needed to induce crystallization. Of the many cooling cycles carried out in the study, several showed crystallization in the 28-34 °C range, but none occurred above this range. The measured heats of melting and crystallization are shown in Table 2. A dispersion of trilaurin (1%) in sodium octyl sulfate (0.1%) was prepared by rotating it at 48 °C for 2 h, transferring to the calorimeter cells, and cooling and heating. As this surfactant concentration was about a factor of 20 below the cmc, it was expected to have little effect on the melting and recrystallization behavior, the temperatures of which (Table 2) were essentially the same as those for pure trilaurin. The surfactant helped stabilize the dispersed particles against flocculation, though creaming of the dispersion was usually observed after temperature cycling in the calorimeter. There was no influence of the uneven distribution of trilaurin in the cells, caused
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Table 2. Results of DSC Scans with Trilaurin Dispersionsa temp cycle
peak maxima, °C ( 0.5
∆H, J/g trilaurin ( 10
1. trilaurin only
heat 1 cool 1
2. trilaurin (1%) + octyl sulfate (0.1%)
heat 1 cool 1 heat 1 cool 1 heat 2 cool 2 heat 3 cool 3 heat 1 cool 1 heat 2 cool 2 heat 1 cool 1 heat 2 cool 2 heat 3 cool 3 cool 1 heat 1 cool 2 heat 2 cool 3 heat 1
45.5 28-32 30-34b 45.5 30-34 45.8 31.0, 6.6 46.5, 43.7 31.3 46.5 31.1 45.9 31.6, 6.6 46.2, 43.7 31 45.6 30.9, 6.4 43.5, 45.7 31 45.9 31 5-15 44.0, 45.5 15, 25 44.5, 45.9 26 45.9
183 167 172 187 169 194 97, 26 166 (tot.) 95 98 93 190 88, 56 76, 40 88 193 72, 8 7, 95 63 68 61 77 110 (tot.) 23 44 (tot.)
cool 1 heat 2 cool 2 cool 1
25.7 46.1 26.9 27, 15, 6
181 188 177 77, 15, 18
heat 1 cool 2 heat 2
45.3 28 45.3
166 81 126
sample
3. trilaurin (1%) + Neodol 1-9 (10%) dispersed at 3 °C (Figure 4)
4. trilaurin (0.5%) + Neodol 1-9 (10%) dispersed at 3 °C
5. trilaurin (1%) + Neodol 1-7 (10%) dispersed at 3 °C
6. trilaurin (1%) + Neodol 1-9 (10%) dispersed at 48 °C (Figure 5)
7. trilaurin (1%) + Neodol 1-9 (10%) + PAA (0.5%) at pH 2.5 dispersed at 3 °C (Figure 6)
8. trilaurin (1%) + Neodol 1-9 (10%) + PAA (0.5%) at pH 2.5 dispersed at 48 °C
aData
204
collected at a heating rate of 0.3 °C/min unless otherwise indicated. bData collected at 0.05 °C/min.
Figure 1. Solubility of trilaurin in Neodol 1-7 (10%) surfactant at 48 °C: O, cooled from heated, sonicated sample; 0, dissolved at constant temperature.
Figure 2. Solubililty of trilaurin in surfactant solutions: 9, 10% Neodol 1-7; O, 10% Neodol 1-9; 2, 10% Synperonic NP13; /, 10% Synperonic NP13 + 0.5% PAA at pH 2.5.
by the creaming, on the heats of melting, although the amount of trilaurin solubilized in subsequent experiments was influenced. The melting of trilaurin (1%) dispersed in Neodol 1-9 (10% in water) was studied by pulse sonicating the mixture in an ice bath for 1 min (over a 6 min period to avoid heating above the melting point of trilaurin), transferring to the calorimeter cells, and cycling the temperature. In this system at equilibrium, about 85% of the triglyceride would be solubilized at 48 °C. The heating and cooling curves are shown in Figure 4, and the amount of heat exchanged is shown in Table 2. On heating the dispersion, a single melting peak at 45.8 °C was obtained, after which the temperature was held at 48 °C, in separate experiments, for 5 min, 1 h, and 8 h to allow varying times for solubilization. These different holding times made little
difference to the subsequent temperatures of melting or the heats measured. On cooling to 3 °C, peaks were recorded at 30 °C and between 5 and 10 °C; the temperature was held at 3 °C for 10 min, and the sample was reheated, producing two melting maxima at 43.7 and 46.5 °C. The temperature was again held at 48 °C for 10 min, with the next cooling showing only one peak at 31 °C, with no peak in the 5-10 °C range. On the third heating cycle, only the one peak at 46 °C was observed. Further heating and cooling cycles produced only single peaks at the same temperatures (46 and 31 °C, respectively). The occurrence of the additional peaks on heating and cooling is discussed below; they are assigned to the melting and crystallizing of trilaurin containing sufficient surfactant to form an unidentified mesophase. As temperature cycling caused these peaks to disappear, they may have arisen
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Figure 3. Photo from optical microscope of a system of 2% trilaurin + 10% Neodol 1-7 after heating to 50 °C and cooling to 30 °C. Some of the trilaurin particles have a “blotchy” appearance, suggesting the presence of some mesophase.
Figure 4. DSC traces from 1% trilaurin dispersed with sonication in 10% Neodol 1-9 in an ice bath; the first temperature cycle was heating.
from those particles that were observed with the microscope to split into smaller particles. To determine if the occurrence of the additional melting (43.7 °C) and crystallizing (5-10 °C) peaks was a result of excess trilaurin above the solubility of the surfactant
Robb and Stevenson
system, less trilaurin (0.5%) was dispersed in an ice bath by sonication in Neodol 1-9 (10%), a system where all the triglyceride can be expected to be solubilized at 48 °C at equilibrium. The temperature cycling results, which were essentially the same as those for the surfactant with 1% trilaurin, are shown in Table 2. To determine whether trilaurin, fully solubilized in micelles, showed any melting transition, systems of trilaurin (1%) in Neodol 1-7 (10%) were sonicated at 50 °C. When solubilization was complete and the system optically clear, no DSC peaks were found upon either heating or cooling, indicating that the few molecules of trilaurin per micelle present were insufficient to nucleate the solid phase. To observe some of the processes of solubilization, trilaurin (1%) was dispersed in Neodol 1-7 (10%) (where all the trilaurin would be solubilized at 48 °C) in an ice bath, and the mixture was temperature-cycled in the calorimeter, with the thermograms being similar to those in Figure 4. The initial melting occurred at 45.6 °C, and upon cooling, two crystallization processes, the larger at 30.9 °C and the smaller at 5-10 °C, occurred (data in Table 2). Upon reheating, two melting peaks were observed, the smaller one at 43.5 °C and the larger one at 45.7 °C. On recooling, only the single crystallization at 31 °C was observed, and on reheating, only the single peak at 45.9 °C was present. As the 1% trilaurin was below the solubilization limit of the Neodol 1-7 surfactant, it was expected that all the trilaurin would have been solubilized, though creaming of the emulsion probably reduced contact between the triglyceride and the surfactant in solution. After the last cooling and heating cycle produced no further change in the melting temperature, the cell was removed from the calorimeter, shaken, replaced in the calorimeter, and the temperature was recycled. On cooling, crystallization occurred at 30 and 6.6 °C followed by the double melting peaks at 43.7 and 46 °C. This change in melting behavior after removing and shaking the cell confirms that creaming retarded the dissolution process. As shown in Figure 4, the crystallization at 5-10 °C was associated with the melting at 43.7 °C, and both peaks disappeared after one complete temperature cycle. In the experiment (No. 5, Table 2), pure trilaurin formed after melting, probably because some of the solid particles creamed, preventing good contact with the surfactant in solution. To remove the effect of creaming, a mixture of trilaurin (1%) with Neodol 1-7 (10%) was rotated at 48 °C for 5 min and prior to complete solubilization was transferred to the calorimeter cells. Upon cooling, a small peak at 10 °C was observed, and on reheating, a melting peak at 43.5-44.8 °C was observed. No measurable peak was observed on the next cooling or reheating. To determine the effect of distributing surfactant more uniformly in the fat, trilaurin (1%) was rotated for 15 min at 48 °C with Neodol 1-9 (10%) in calorimeter cells and placed in the calorimeter at 48 °C. The first cooling peak, shown in Figure 5, was a “triplet” containing a crystallizing peak between 5 and 10 °C, with an additional peak at about 15 °C. The heating cycle showed melting at 44.0 and 45.5 °C, the heats from the individual peaks being difficult to resolve. Upon cooling, crystallization occurred at 25 °C and around 15 °C. On reheating, melting occurred mainly at 45.9 °C, with a small shoulder at 44.5 °C. Further temperature cycling continued to produce peaks at 25 °C on cooling and 45.5 °C on heating. In this system, all the molten trilaurin was equilibrated with surfactant prior to cooling, yet particles of pure trilaurin and trilaurincontaining surfactant were produced after cooling. It was
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Figure 6. DSC traces from 1% trilaurin dispersed with sonication in an ice bath in 10% Neodol 1-9 + 0.5% PAA at pH 2.5; the first temperature cycle was heating.
range, and consequently very little melting occurs below that of pure trilaurin, indicating that surfactant penetration of the trilaurin was reduced in the presence of PAA. Discussion
Figure 5. DSC traces from 1% trilaurin dispersed in 10% Neodol 1-9 at 48 °C; the first temperature cycle was cooling.
expected that this differentiation of molten trilaurin may have been caused by creaming, and the same system was heated to 48 °C in a calorimeter cell (in a thermostat) and cooled to 0 °C with rotation while in the thermostat. The cell was transferred to the calorimeter, and the temperature was cycled. The first cycle, heating, produced only one peak at 43.6 °C, with a small shoulder at 45.5 °C followed by a small peak at 15 °C on cooling. Subsequent heating and cooling produced small peaks at 45.5 and 15 °C, with the majority of trilaurin not crystallizing or melting. The influence of added PAA (0.5% at pH 2.5) on the system of trilaurin (1%) in Neodol 1-9 (10%) was studied by dispersing the trilaurin ultrasonically in an ice bath in the polymer/surfactant mixture. The heating and cooling runs produced peaks at 45.9, 25.7, 46.1, and 26.9 °C, as shown in Figure 6, with the heats being tabulated in Table 2. Since there was no peak recorded in the cooling cycle between 5 and 10 °C and, hence, no additional melting (at 43-44 °C) occurring below that of pure trilaurin in the presence of PAA, a sample of trilaurin (1%) in Neodol 1-9 (10%) in the presence of PAA (0.5%) at pH 2.5 was rotated at 48 °C for 15 min to allow time for the surfactant to penetrate the molten trilaurin. The sample was then placed directly into the DSC cell at 48 °C, and the temperature was cycled between 48 and 3 °C (melting transitions and heat exchanged are tabulated in Table 2). Again, very little trilaurin crystallizes in the 5-10 °C
The solubility of trilaurin increased with increasing temperature for all three nonionic surfactants, as would be expected for ethoxylated surfactants below their phase inversion temperature. With increasing temperature, the size of the surfactant headgroup shrinks, due to dehydration of the ethylene oxide (EO) chains causing the critical packing factor (CPF)25 to approach 1.0. Maximum solubilization of most oils occurs where the spontaneous curvature of the micelles is flat (i.e., where the CPF is 1.0). This corresponds to the phase inversion temperature, which depends on the nature of the oil solubilized. For polar oils, the PIT is reduced17 compared to the cloud point, whereas, for nonpolar26,27 oils, it is increased. The PIT in the presence of nonpolar oils increases1 with the chain length of the oil, and attempts to measure the PIT for trilaurin or dodecane with Neodol 1-7 were unsuccessful, indicating that it was greater than 95 °C. Thus, all trilaurin/nonionic surfactant systems studied here were in the Windsor 1 phase, with oil solubility expected to increase with temperature. In contrast, the addition of PAA at pH 2.5, while lowering the cloud point of nonionic surfactants, reduced the solubility of trilaurin in the Neodol 1-9. Lowering the cloud point and PIT by the addition of salt would be expected to raise the solubility of an oil in nonionic surfactant (if the systems were in the Windsor 1 phase). While the addition of poly(acrylic acid) lowers the cloud point by direct hydrogen bonding to some of the EO segments, it (25) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1976, 72, 1525. (26) Kunieda, H; Haishima, K. J. Colloid Interface Sci. 1990, 140, 383. (27) Kuneida, H; Asaoka, H; Shinoda, K. J. Phys. Chem. 1988, 92, 185.
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may not alter the spontaneous curvature of the micelle toward a more flattened shape. Thus, lowering of the cloud point in this case does not increase oil solubility. The melting of trilaurin28 at 45.5 °C is as expected for the pure material. The crystallization via heterogeneous nucleation occurs at ∼30 °C, and this was repeated with the emulsion using octyl sulfate, even though it had creamed. The difference in the temperatures of melting and recrystallization may have been due to the formation of polymorphs that are well-known29 to occur with triglycerides. Trilaurin has been reported30 to have at least three polymorphs having different heats and temperatures of melting, though, in the pure trilaurin sample, only one DSC peak was observed on each of the heating and cooling cycles. Thus, it is more likely that the difference in temperature is a result of undercooling rather than polymorphism. The heat of melting (190 J/g) is also in agreement with the published data.29 The heat of crystallization was usually found to be about 10% less than the heat of melting, presumably because crystal growth was not complete, even at the rate of 0.3 °C/min. Again, polymorphism29 may have contributed to this heat difference, though as no second peak was observed on the cooling cycle of the pure triglyceride, this seems unlikely. The most interesting result of the calorimetry is the occurrence of the second melting and crystallization peaks at ∼44 and 5-15 °C, respectively, which only occur in the presence of added nonionic surfactant. From the present experiments there are uncertainties in assigning the peaks, though a provisional assignment can be made on the basis of the magnitude of the heats exchanged and the species present. It is known31,32 that nonionic surfactants partition into hydrophobic solvents, with the partitioning favoring the oil phase for surfactants with longer alkyl chain lengths and shorter EO chains. In Figure 4, where the trilaurin would not be fully solubilized, even at equilibrium, the initial melting for trilaurin occurred as expected at 45.8 °C, though, on cooling, only about half the trilaurin had crystallized at the expected 30 °C. About one-third of the remainder crystallized between 5 and 10 °C. At 48 °C, where the sample had been held for 10 min, surfactant would have partitioned into some of the oil, though possibly not in a uniform way, as the emulsion creamed in the cell. This would explain why holding the sample at 48 °C for various times up to 8 h had little effect on the crystallization behavior. In addition, some trilaurin will be solubilized into micelles, though lack of agitation means that the rate would be slower than that shown in Figure 1. Thus, at the end of the first cooling cycle, the trilaurin probably existed in three main states: (a) pure crystals, (b) solubilized in micelles, and (c) as droplets containing surfactant. The heats evolved suggested the 43.6 °C melting came from the material crystallizing between 5 and 10 °C. After holding the system again at 48 °C for 10 min, the next cooling curve showed only the crystallization at 31.3 °C, with the second crystallization at 5-10 °C being absent. Further solubilization was negligible, probably due to creaming and hence poor contact between the trilaurin and surfactant. The disappearance of the second melting and crystallizing peaks probably has two causes; either the trilaurin in these second peaks becomes solubilized into micelles or the size (28) Ollivon, M.; Perron, R. Thermochim. Acta 1982, 53, 183. (29) Hagemann, J. W. Crystallisation and Polymorphism of Fats and Fatty Acids, Marcel Dekker: New York, 1988; Chapter 2. (30) Buchheim, W; Knoop, E. Naturwissenschaften 1969, 56, 560. (31) Marquez, N; Anton, R; Graciaa, A; Lachaise, J; Salager, J.-L. Colloids Surf., A 1995, 100, 225. (32) Graciaa, A; Lachaise, J; Sayous, J. G.; Grenier, P; Yiv, S; Schechter, R. S.; Wade, W. D. J. Colloid Interface Sci. 1983, 93, 474.
Robb and Stevenson
of the droplets becomes small enough upon temperature cycling that nucleation of crystals does not occur. The optical microscopic studies showed that this did occur, though it is not possible to associate this process with these “secondary” melting and crystallizing peaks with any certainty. The data of Rang and Miller12 show that hexadecane with about 10% oleyl alcohol in contact with water at constant temperature can form mesophases prior to splitting into smaller droplets. A similar process may occur in this system when the temperature is lowered. Lowering the temperature increases the affinity of the ethoxylate headgroups for water, which may cause more water to be incorporated into the trilaurin droplets, leading to their splitting into smaller particles or forming a “blotchy” mesophase. A photograph of what this mesophase might be is shown in Figure 3. Again, it seems unlikely that these additional DSC peaks arise from polymorphism, as they do not correspond to any of the published polymorph transitions29,30 and disappear after one complete temperature cycle. Similar results were obtained with 0.5% trilaurin and 10% Neodol 1-9, where complete solubilization would be expected at equilibrium, showing that the creamed, pure trilaurin remains unaffected in the calorimeter cell. When the molten trilaurin (1% at 48 °C) was dispersed in Neodol 1-9 (10%) for 15 min (Figure 5), there was an opportunity for the surfactant to distribute uniformly across all of the oil. On cooling, crystallization of pure trilaurin (at 30 °C) did not occur, with only about half the trilaurin crystallizing at the “triplet” between 5 and 15 °C. The remainder of the trilaurin would have been solubilized in surfactant micelles. This triplet contained a new peak at about 15 °C that we provisionally assign to trilaurin crystallizing in the presence of a concentration of surfactant too low to form a mesophase with the oil. This material may have creamed and not been in contact with micelles. Upon heating, two peaks at 43.5 and 45.7 °C appeared, corresponding to trilaurin-containing (mesophase) surfactant and pure trilaurin. Further cooling produced the main crystallizing peak at 25 °C, and reheating showed essentially only one melting peak from pure trilaurin. The occurrence of two forms of trilaurin from the molten oil was surprising, though the subsequent experiment where the oil and surfactant were cooled in the calorimeter cell while being rotated confirmed that the creaming caused the differentiation of the trilaurin crystallization. The effect of the added polymer was to prevent the formation of the intermediate phase containing the surfactant in the mesophase. This probably arose from some of the surfactant being in the polymer/ surfactant complex rather than distributing into the oil, thus reducing the concentration of surfactant in the oil. By comparison with Rang and Miller’s results,12 mesophase formation requires a minimum concentration of surfactant in the oil. Some insight into the mechanism of solubilization of trilaurin may be obtained. The occurrence of the “secondary “ peaks which are assigned to trilaurin-containing surfactant in a mesophase indicates that penetration of the oil by the surfactant precedes its solubilization and suggests a slightly different mechanism for solubilization than that put forward earlier.18 These data,18 taken at constant temperature, were consistent with micelle adsorption being the rate-determining step. However, micelle adsorption at a surface seems unlikely, given that the dynamic surface tension33 of surfactants usually increases for chain lengths above about C12. The reason for this is the slower rate of micelle breakdown with increasing alkyl
Solubilization of Trilaurin in Surfactant
chain length; if micelle adsorption occurred, dynamic surface tensions probably would not show this chain length dependence. The data here indicate that surfactant partitioning is much quicker than solubilization and may occur by monomer exchange with the surface, though there are no definitive results for either mechanism. Where a change in affinity of the surfactant for oil occurs (e.g., by changing the temperature), the flow of water across the interface can lead to smaller drop sizes, analogous to previous results.18 Conclusions The solubility of trilaurin in ethoxylated nonionic surfactant increased with increasing temperature, as expected for a system below the PIT, where the critical packing factor approaches 1.0. Solubility was greater for the surfactant with the shorter headgroup, which had the (33) Eastoe, J.; Dalton, J. S.; Rogueda, G. A.; Crooks, E. R.; Pitt, A. R.; Simister, E. A. J. Colloid Interface Sci. 1997, 188, 423.
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lower cloud point. The addition of poly(acrylic acid), which formed a complex with the surfactant, lowered the solubility of the trilaurin. Solubilization of the trilaurin was preceded by penetration of the surfactant into the oil. Trilaurin-containing surfactant probably formed some mesophase with a melting and a crystallizing temperature below that of the pure fat. Temperature cycling of this phase was followed by the oil either being micellized or forming small droplets that did not nucleate on cooling to 3 °C. Partitioning of the surfactant into the oil appeared to be much more rapid than solubilization of the oil into micelles. Added poly(acrylic acid), by forming a complex with the surfactant, probably reduced the penetration of the surfacatnt into the oil. Acknowledgment. The authors would like to thank Unilever Research, Port Sunlight, for sponsoring PSS during this work. LA0001070
