J. Phys. Chem. B 2008, 112, 5403-5411
5403
Crystallization and Melting Transitions of Hexadecane Droplets in Polystyrene Nanocapsules Emily V. Fette, Anthony Pham, and Thorsteinn Adalsteinsson* Department of Chemistry, Santa Clara UniVersity, Santa Clara, California 95053 ReceiVed: October 24, 2007; In Final Form: December 14, 2007
Shifts to lower transition temperatures are observed for the freezing and melting of submicron-sized hexadecane droplets encapsulated within thin polystyrene shells. Supercooling of ∼14 K is observed in the first-time cooling scans for the oil. We attribute this lowering predominantly to nucleation of the phase change originating from the oil/polymer/water interface. We obtain a rough estimate of the interfacial tension between the hexadecane oil droplet and the polystyrene of 14 mN/m assuming the Gibbs-Thomson relationship. Melting points for the hexadecane are 1-2 K below the bulk transition temperatures. This effect is connected with the surface/volume ratio of the capsules. Both the supercooling and the melting point depression approach the bulk-phase transition temperature when the sample is taken through multiple cool/heat cycles. The heating and cooling rates affects the number of cycles required before bulk-like behavior is observed. The thickness of the capsule wall is also observed to be critical to how many cooling cycles are required. Two hypotheses to explain this behavior are presented.
Introduction Liquids confined in submicron pores and microscopic dispersed droplets are known to have freezing and melting temperatures that differ from the bulk transition temperatures. There are two main theories for explaining this phenomenon. One is that the difference arises from the overwhelming effect that the thermodynamics of the interface has on the behavior of systems where the ratio of surface area to bulk volume is large. This argument can be developed from equilibrium thermodynamics to yield the classical Gibbs-Thomson equation:
Tb - Tdr )
γijυiTb K ) ∆htr‚r r
(1)
Here, Tb is the bulk transition temperature, γij is the interfacial tension between the liquid and the enclosing surface, νi is the molar volume of the liquid, Tdr is the observed transition temperature in the droplet, and ∆htr is the molar enthalpy for the phase transition.1,2 The inverse size relationship is observed in the case of water confined in mesoporous silica systems.3,4 In these systems, the solubility of water in the silica is negligible, and the pore size is small (between 3 and 30 nm). The observed Gibbs-Thomson relation for these systems is actually observed to follow a modified version of the theory, where Tb - Tdr ) K/(r - t). Here, t is the thickness of a surface-layer of water adsorbed to the silica surface. Similar 1/r behavior for hydrocarbon systems has not been verified in hydrocarbon systems.5 The hydrocarbon systems most commonly studied, viz., surfactant-mediated oil/water emulsions, differ significantly from the water/silica systems. First of all, the oil droplets are much larger in size than the porous silica system and are also generally more dispersed in size. Finally, the surface tension between a surfactant and oil may not be * Corresponding author.
easily determined via measurements, although it is possible to measure the interfacial tension between air and surfactant easily. It is therefore likely that the surface effect is rather small, and the Gibbs-Thomson effect is negligible in these systems. The second approach to understanding supercooling of confined liquids is based on kinetic reasoning, with supercooling being regarded as a nonequilibrium phenomenon, or kinetically arrested state.6,7 For a phase transition in a supersaturated system, a nucleation event must occur.7Two distinct processes for nucleation are identified: heterogeneous nucleation and homogeneous nucleation. In the former, an unspecified impurity, e.g., a dust particle, in the supersaturated phase acts as a catalytic center to overcome a free energy barrier. This mechanism is considered to govern transitions in the bulk phase, as it can be nearly impossible to obtain a dust-free bulk phase. By dividing a phase into smaller, isolated partitions, every partition requires a separate nucleation event. When the partition becomes microscopic, the probability of each micro-droplet containing sufficient impurities arguably is small, and nucleation must be initiated from the molecules in the phase itself.8-10 The rate of nucleation is strongly temperature dependent, but the growth rate is related to the extent of supercooling in the system. Turnbull reported that, in n-alkanes, the nucleation rate decreased by a factor of 5000-8000 per degree.10 Thus, a large magnitude of supercooling in freezing transitions is connected with homogeneous nucleation. Freezing transitions of n-alkanes in the bulk phase are commonly observed to have small magnitudes of supercooling, viz., ∼1-2 K. For droplets of n-alkanes confined in small droplets such as surfactant emulsions, supercooling is found to be much larger, with Tb - Tdr ∼ 10-16 K.5,10-12 This supercooling is much less than that observed in emulsified mercury droplets9 and for water in mesoporous silica, where the supercooling is up to 40 K.3,13,14 Nonetheless, supercooling of around 15 K in the liquid-solid transition for n-alkanes is taken in the literature to indicate homogeneous nucleation in the oil.10,15
10.1021/jp7102879 CCC: $40.75 © 2008 American Chemical Society Published on Web 04/05/2008
5404 J. Phys. Chem. B, Vol. 112, No. 17, 2008 An additional reason for considerable supercooling in the freezing transition may be from soluble impurities in the oil phase. Herhold et al. argue that impurities in the oil become excluded from the solid alkane crystals during freezing.12 Although this does not affect the bulk freezing point significantly, given the volume of the bulk, this exclusion causes a significant change in concentration during freezing of a microdroplet. This could therefore lower the freezing point for thermodynamic reasons. This point is very important for emulsified oil droplets, where there may be partitioning of all compounds in the system between the three or more phases in the system: bulk, surfactant layer, and oil microdroplet. Emulsion droplets may also change in shape and morphology during cooling and freezing.16-18 Herhold et al. noted that the polydispersity in the size of oil droplets had little effect of the type of function suitable for fitting the droplet nucleation dynamics.12 It may therefore be unlikely that small variations in droplet size during phase change has a significant effect. Interestingly, the literature is in some disagreement of how significant the effect of impurities is on nucleation for initiating phase change in alkane microdroplets.12,15,19-26 In this work, we study an oil microdroplet system related to emulsion droplets. Our oil droplets are encased in a thin, rigid polystyrene shell, however. This system was chosen since it should allow multiple cooling cycles without a significant change in the enclosure containing the droplet or in the composition of the droplet during freezing. Furthermore, the system is relatively free of surfactant that may affect the phase transition behavior of the n-hexadecane (HD) via colloid-tocolloid exchange. As it turns out, we suspect that the capsules undergo a subtle change during the heat cycling without actually disintegrating or falling apart. This apparent change in the capsule is one of the main conclusions based on the observations reported in this paper. The method for preparing the capsules was initially reported by Tiarks and co-workers,27,28 but several publications from the group of Landfester have enhanced the synthetic capabilities and expanded the range of available capsule architectures.29-31 Many other methods for producing core-shell oil-droplets exist in the literature,32,33 but a review of these is outside the scope of this paper. We elected the encapsulation protocol as it enables us to prepare fairly monodispersed polymer core-shell particles reproducibly. The particles studied here all have similar overall size, but varying fractions of HD, and thereby varying oil-droplet sizes at the cost of thinner capsule walls. Experimental Section Materials. Styrene (Fluka) was purified by double distillation under vacuum to remove free radical inhibitor before use. Purity was verified by 1H NMR spectroscopy. Sodium dodecyl sulfate (SDS, Sigma-Aldrich) was purified by repeated crystallization from 2-propanol. 1-Dodecanol, HD, 2,2′-azobisisobutyronitrile (AIBN), and potassium persulfate (KPS), also from SigmaAldrich, were used as received. Preparation of Polymer-Encapsulated Oil Droplets. A stock surfactant solution was prepared by dissolving 4.00 g of surfactant in 96.0 g of deionized water. The surfactant consisted of 3.8 g of SDS with 0.2 g of 1-dodecanol added as cosurfactant. The solution was gently stirred in a warm water bath for 20-30 min until clear. The monomer/oil mixture used in the polymerization reaction was preformulated by combining styrene and HD at the desired ratio. A small amount of AIBN initiator was added to give 0.020 g of initiator per gram of monomer. Immediately after adding the initiator, 0.40 mL of
Fette et al. the monomer/oil mixture was added to 3.00 mL of the surfactant solution. The solution was placed in an ice-water bath and sonicated under a flow of argon for 2 min using a Misonix Microson Ultrasonic Cell Distrupton XL on power setting 6. The samples were capped and transferred to a water bath at 78 ( 2 °C for 75 min, a period of time known to be sufficient for full polymerization to occur. Polymerizations using AIBN were more successful than trials using the water-soluble initiator (KPS). The sample was allowed to cool to room temperature and then purified by centrifuging at 16 400 rpm for 20 min. In the centrifugation, the oil-filled capsules float on top, but unreacted emulsion droplets remain in suspension. The isolated capsules were redispersed in deionized water, and centrifugation was repeated at least twice more. Excessive purification eventually leads to aggregation of the capsules, as this removes surfactant from the colloids. The sample was finally dispersed in 1 mL of water. No turbidity gradient was visible in the samples over several days. Analytical Methods. A 90-Plus Particle Size Analyzer (Brookhaven Instruments) outfitted with an avalanche photodiode was used for particle sizing. The dynamic light scattering (DLS) samples were prepared by prefiltering 4.5 mL of 10 µM potassium chloride (KCl) through a 0.20 µm Whatman organic membrane filter into a clean polystyrene cuvette. The solution was tested to ensure the absence of particles over 5 nm in size. Then approximately 0.05 mL (one drop) of the purified particle suspension was added, and the sample was gently mixed. In some cases, further dilution was required for stable readout from the DLS instrument. The particle size was calculated assuming spherical particles and using the refractive index of polystyrene and the viscosity of water at 25.0 °C. Differential scanning calorimetry (DSC) measurements were obtained using a Mettler Toledo DSC823e instrument and sealed 40 µL aluminum pans with punctured lids. This ensures that the pressure during the experiment remains constant and the measured heat flow is the enthalpy change in the system. In the sample pans, 20.0 µL of purified sample was added along with 10.0 µL of filtered 1.0 M aqueous sodium chloride (NaCl). Addition of the NaCl lowers the available temperature range in the DSC to -20 °C to avoid any overlap of the liquid-solid transition temperature of the HD with that of the water. The addition of salt also causes aggregation of the colloids in the sample, but this affects neither the transition temperature of the encapsulated oil drops nor the enthalpy of fusion. The DSC protocol used started at 25.00 °C, followed by cooling to -10.00 °C at a constant rate. The heating and cooling rates were controlled at 2.00 K/min here, except where other rates are specified. Then the sample was warmed to 25.00 °C at the same rate to complete a cycle. This cycle was repeated five times per measurement. Results Capsule Synthesis. The polymer capsules studied here are defined by the volume ratio of HD/styrene, (xHD), that was used in the synthesis. Apart from this ratio, the synthesis was controlled so as to maintain constant values of the ratios of surfactant/water, surfactant/oil, surfactant/co-surfactant, as well as the sonication time. All these variables, especially the sonication time, affect the final particle size.5,27,28 The use of a co-surfactant, here 1-dodecanol, along with the surfactant, followed by ultrasonication, generates metastable emulsions, classified as mini-emulsions. The proposed mechanism for this metastability is that the co-surfactant inserts into the surfactant layer. generating a sharp osmotic gradient across the surfactant
Melting Transitions of HD in Polystyrene Shells
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5405
TABLE 1: Capsule Samples Synthesized volume fraction of HD to styrene
RH/nm (DLS)
roil/nm
shell thickness/nm
0.30 0.40 0.50 0.60 0.70 0.80 0.85 0.90 0.95
67.9 74.9 68.2 85.1 90.2 182.8 104.6 104.8 115.1
45.5 55.2 54.1 71.8 80.1 170 99.1 101 113
22.4 19.7 14.1 13.3 10.1 13.1 5.5 3.6 2.0
layer. These mini-emulsions have been demonstrated to be highly resilient toward Ostwald ripening,34,35 thus creating a confinement for the emulsified oils that shows little or no micelle-to-micelle exchange of surfactant and encapsulated phase.35 Minimizing exchange by this means is a key aspect in the synthesis of capsules, since polymerization within each micelle will lead to the formation of a polymer that is insoluble in HD.27 Given the correct order of the spreading coefficients between the water/polymer, polymer/oil and water/oil interfaces,30 the polystyrene will precipitate at the oil/surfactant interface and form a thin polymer film surrounding the remaining HD. Formulations in which the styrene fraction was high (low xHD) gave colloids that had bimodal size distributions and were hard or impossible to purify by our method. The lowest xHD that gave “well-behaved” colloids was xHD ) 0.3. The yield of polymer from monomer was found to be low, or roughly 50% by the weight of styrene. This low yield is an indication that the micelle-micelle exchange is negligible. Statistically, only half of the micelles may be initiated with an odd number of initiator molecules.34,36 The molecular weight of one colloid sample was determined by viscometry and was estimated to exceed 6 × 105 g/mol, which was the limit of our current analytical method. It should be noted that the volume of styrene prior to synthesis is different from that of the polystyrene making up the capsule, since the densities of the two are different, viz., Fstyrene ) 0.909 g/mL and Fps ) 1.050 g/mL. Additionally, other factors, such as film thickness and molecular weight, may influence the actual density of the polystyrene. Here, we elect to maintain notation from the synthesis for consistency. Assuming that the synthesis of mini-emulsions led to a formation of a true polystyrene/HD core-shell particle, we can estimate the internal radius of the average enclosed HD droplet, using eq 2, where xv is the volume fraction of the oil to styrene and RH is the radius of the colloid, measured as the hydrodynamic radius.
roil ) RH‚xv1/3
(2)
Table 1 summarizes the particles sizes that are further discussed below. DSC Experiments. To highlight data for the following discussion, we focus on the results for a single capsule sample with a formulation of xHD ) 0.5, as this sample shows behavior in the DSC scans similar to all other samples in terms of baselines and peak structures. Figure 1 shows five sequential cool/heat cycles for this sample. The numbers below the curves indicate the order in which the curves were collected; the scans have been stacked vertically for purposes of clarity. The inset portrays the raw measurement data versus time as the sample is cooled from 25.00 °C to -10.00 °C (odd-numbered curves) and then warmed to 25.00 °C (even numbered curves). The baseline offset
Figure 1. Five-cycle DSC curve for the cooling and heating of xHD ) 0.50 by volume polystyrene/HD core-shell particle dispersion. The figure shows heat flow difference between the reference and the sample versus temperature in the reference cell. A positive peak indicates an exothermic process. The inset shows the full measurement curve versus time. The peaks are numbered in their appearance: odd-numbered peaks represent scans from 25 °C to -10 °C, even-numbered peaks are scans from -10 °C to 25 °C.
between odd- and even-numbered scans is indicative of the heat capacity of the sample and the scanning rate, which was 2.00 K/min. Three features from the figure will be discussed in the following text: (1) The deviation of the onset temperatures for the phase transition in the first cool/heat cycle from the values found for the bulk oil (here we will include experiments done for capsule samples from Table 1); (2) the shift in the extent of supercooling with repeated cool/heat cycling (this will include experiments where the dynamic rate was varied); and (3) the appearance of peak-shoulder in the exotherms that occur in repeated experiments. 1. Freezing and Melting Transition in the First Cycle. Curve 1 in Figure 1 shows the freezing exotherm for the encapsulated HD. The onset temperature of the transition is at 2.8 °C, which is 13.5° lower than the onset of the same transition for a bulk sample of the oil.39 The supercooling that we observe in Curve 1 of Figure 1 is similar to what is seen in other systems, where the oil is significantly confined. The mini-emulsion systems studied by Montenegro and co-workers showed a slightly larger supercooling of 16 degrees from the bulk-phase freezing temperature.5,37 This difference in the extent of supercooling is not unexpected. Whereas the polymerization reaction that forms the capsule is unlikely to leave much unreacted styrene, given the high molecular weight of the polystyrene we measured, it is likely that precipitation of the polymer will not produce a perfectly formed capsule. This gives a polymer/alkane interface with defects that may allow the oil to be in contact with the water phase. Judging from the extent of supercooling, our system is more analogous to mini-emulsions than to systems where side chains in polymers are micro-phase-separated.38 Curve 2 in Figure 1 is the melting endotherm of the encapsulated HD. The onset for this transition occurs at 16.9 °C, which is 0.95° below the measured melting onset in the bulk sample. A downward shift of the melting transition in microscopically confined systems has been reported for both HD in emulsions and water in micropores.3,5 Note that this shift in transition temperature is in the opposite direction to the behavior predicted by the Gibbs-Thomson equation, which predicts superheating of the solid phase. Traditionally, a downward shift in freezing point is called supercooling, whereas lowering of melting point is termed
5406 J. Phys. Chem. B, Vol. 112, No. 17, 2008
Figure 2. Collection of onset temperatures for first-freezing (upper curve, closed circles) and first-melting transitions (lower curve, open circles) for HD encapsulated within a thin polystyrene shell. Squaredivided points are omitted from the linear fit. The colloidal particles are similar in size. With increasing HD fraction, we expect a thinner polystyrene shell, and hence a larger oil droplet.
melting point depression and is commonly assumed to be an indication of an impurity in the phase. We argue that the observation of a lowered melting point in confined systems is not from an added impurity in the system, but is a consequence of the particle size. The preparation of samples varies dramatically between our case and in the case of the authors in the cited references above.3,5 In our case, the impurity would have to be added during the sample preparation, and since the melting point varies with the mole fraction of HD (xHD), the sample preparation would have to involve a consistently increasing addition of impurity to the HD phase with every sample. We consider this highly improbable and conclude that the effect is connected to the size of the encapsulated oil droplet. To distinguish this trend from the case of the added impurity, we will refer to this phenomenon as a downward shift, rather than melting point depression. The lowering of the observed phase transition temperatures from the bulk transition temperature for the various samples in Table 1 is shown in Figure 2. The figure shows that the downward shift of the onset of melting and the extent of supercooling vary slightly with xHD in the system. With few exceptions, downward shift of melting temperatures increases systematically with xHD. A linear fit through the melting temperatures and the freezing temperatures are shown in the figure to quantify this trend. The change in supercooling is very similar; omission of points provides linear fits where the slopes are nearly identical. The general trend within the samples is that, as xHD increases, the thickness of the polymer shell decreases. Furthermore, as xHD increases, the average oil droplet size increases. The data in Table 1 show that, at high HD fractions, the polymer shell is only several nanometers thick. With thinner shells, more imperfections and pores are likely to allow contact between the water and impurities in the water phase and the encapsulated oil. This could be connected with the decrease in the supercooling and the change in downward shift of melting temperature beyond xHD ) 0.85 (shell thickness < 10 nm) in Figure 2. Given the miniscule size of water molecules compared to what we would expect the matrix density of the polymer to be, it is likely that the water can easily penetrate the polystyrene shell, even when the thickness is greater than 10 nm. We would then assume that, as the surface area in the system increases, the contact points and the probability of nucleation from the interface should also increase. A convenient variable that takes this into account is the surface area-to-volume ratio (S/V). Turnbull discussed this type of surface nucleation in 1952.9 In his work, he showed a dramatic effect of surface contaminants
Fette et al.
Figure 3. The transition temperature versus the surface area-to-volume ratio (S/V). Open circles represent the freezing transition; the closed circles are the shift in melting point. Gray-shaded points fall outside of a statistical envelope of 2σ.
on the rate of nucleation during freezing of mercury droplets. In the system studied, highly purified Hg droplets were encapsulated within a surfactant shell made by a mixture of surfactants that formed a heterogeneous surfactant shell. The two important parameters in the system were the concentration of surfactant heterogeneities, which Turnbull could control, and the surface-to-bulk ratio, or S/V ) 3/rHg. An increase in S/V leads to more contact between the oil droplets and nucleation sites, and thus, with a decrease in the droplet radius, the rate of nucleation should increase, and the supercooling, as well as downward shift in melting, should decrease. This is in agreement with the trend apparent in Figure 3, which shows ∆T versus S/V for the encapsulated HD. It is interesting to note that the expected trend with size here is the reverse of the prediction of the Gibbs-Thomson equation. In eq 1, both ∆h and γij are positive, and the decrease in radius should lead to increased supercooling. An underlying assumption in deriving the GibbsThomson equation is that there is no partition between the adjacent phases i and j. This explains why the behavior is observed for water in porous glass, but is hard to verify for oils in emulsions. This is not to say that the system here does not behave in the manner predicted by the Gibbs-Thomson, (or Hoffman-Lauritzen) equation. The effect is simply overshadowed by the kinetic effect of nucleation and the lowering of the melting temperature of the solid HD. In order to estimate the value of the Gibbs-Thomson coefficient K in eq 1, we need to have access to the surface tension as well as the enthalpy of melting for the system. Over a narrow temperature range, the enthalpy can be estimated to be a constant value of 47.5 kJ/mol;40 we can also estimate the molar volume of the liquid from the density and molecular weight. This gives νl ) 2.92 × 10-4m3/mol. The surface tension is very unlikely to be constant over the temperature range associated with supercooling, but we would expect the surface tension to be relatively low, around 20 mN/m, given that hydrocarbon oils partially spread on polystyrene.41 Lacking a value of the bulk-transition temperature for the oil droplets, we must use the observed melting temperature for each capsule as a reference point and calculate an alternative supercooling value by taking the difference between the observed first cycle freezing point and the observed first cycle melting point. This comparison has two major drawbacks. First, the reference point for supercooling should be the freezing point of the bulk phase, and not the melting point. For bulk systems, the freezing is on the order of 1 K below the melting point. The other drawback is that the melting transitions do not occur at constant temperature within one sample, as is evident in Figure 1, but shift closer to the true bulk transition temperature with repeated cool/heat cycling. The melting point is therefore
Melting Transitions of HD in Polystyrene Shells
Figure 4. Recalibrated supercooling versus reciprocal radius of the encapsulated oil droplet. The recalibration of the supercooling is made by choosing the first observed onset of melting for each experiment is used instead of the bulk melting point for the oil. This choice is fraught with the error that the first melting onset is not constant with repeated cool/heat cycling (see Figure 1).
Figure 5. Extent of supercooling from the curves in Figure 1. Open circles represent the supercooling for the freezing transition; closed circles represent the melting transitions. The heating and cooling rate was 2 K/min.
variable. Our hope is that, since all the experiments are done at the same heating rate, fluctuations should be minimized. Figure 4 shows this re-calibrated supercooling point as a function of reciprocal radius of the oil droplet as provided in Table 1. Since the ordinate in Figure 4 is a difference between two measured quantities, the error in the value is at least double that of what we assume for the ordinate values in Figure 2. The scattering of the values in Figure 4 is also from the experimental uncertainties in the DLS experiments. The linear fit through the data points is poor as a consequence. Nevertheless, the trend shown in the figure is in agreement with the expected behavior according to the Gibbs-Thomson equation (eq 1). The value of the slope is 52 ( 10 K‚nm. Using the value for ∆h, νi, and Tb ) 289.7 K (average value for the melting of HD in the first cycle), we obtain a value for the interfacial tension between HD and polystyrene (γij) of 29 ( 7 mN/m, a reasonable value. 2. Multiple Cooling/Heating Cycles. It is immediately apparent from Figure 1 that the appearance of the initial cool/ heat cycle (curves 1 and 2) is not repeated in the subsequent cycles (curves 3, 4 and 5, 6, etc.). The difference between two subsequent cooling runs (curves 1 and 3) is more pronounced than the difference between two subsequent heating runs (curves 2 and 4). Figure 5 summarizes the observed onset temperatures for the main peaks of the curves in Figure 1. In cases where the endotherm had a shoulder on the peak, the onset temperature is for the more intense peak only. Figure 6 shows the DSC scan for an xHD ) 0.50 sample prior to polymerization. As is expected, the DSC signature for the
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5407 pre-polymerization sample is very different from the encapsulated HD sample. In short, multiple cool/heat cycles led to irreversible changes in the structure of the micelles and subsequent destabilization of the emulsion. A direct comparison between this sample and that of the sample in Figure 1 should be done with some skepticism. The unreacted sample could not be purified the same way as the reacted samples, so excess surfactant is likely present. Furthermore, here we know that the oil phase is a mixture of styrene and HD, whereas the reacted sample has little or no unreacted styrene in it. The peak position of the first cooling run of the sample appears close to -5 °C. This shift in the mini-emulsion droplet is in a good agreement with the work of Montenegro and coworkers.5 Interestingly, the pre-polymerization sample showed a much broader melting endotherm than the encapsulated analog, most likely due to the significant amount of styrene in the HD. In the repeated cooling runs, the main exotherm peak had a complex pattern, which we were unable to generate reproducibly in repeated runs. A signature from an exothermic transition is seen around -6 °C in all the cooling runs. The baseline for the scans was not level, as was the case for the capsule samples, which indicates a continuous change in the heat capacity with temperature. The sample is therefore most likely changing during the scan. The pattern of multiple peaks in both the freezing exotherms and the melting endotherms can also be related to the changes we would expect for the surfactant system upon multiple freeze-thaw cycles. In the initial cycle, the oil is encapsulated within the mini-emulsion droplets. These droplets were not polydispersed according to DLS measurements, but in some experiments, a bimodal size distribution was observed. During the first heating run, we can expect the emulsion to disintegrate and precipitate as a result of the salt in the DSC sample. In the second cooling run, the HD may therefore be in much more complex array of surfactant structures, including dispersed, swollen micelles and multi-lamellar surfactant aggregates. This would explain why the subsequent freezing exotherms show multiple sharp peaks from the HD in the lamellar structures and a broad exotherm at very low temperatures from the HD in swollen micelles. Finally, neither the melting nor the freezing transitions reach the bulk values. Since nearly half of the oil is not HD, this bulk value may not be achieved in any case. For our capsules, the DSC signature in Figure 1 was very reproducible, with the onset temperatures in near complete agreement between measurements. The peak pattern was slightly less reproducible and depended on the heating and cooling rate. Repeated cool/heat cycles of other capsule systems had similar patterns as shown in Figure 1 and the addition of excess surfactant did not lead to marked change in the DSC scans, apart from a slightly noisier baseline. The striking difference between the capsule sample in Figure 1 and the pre-polymerization sample as seen in Figure 6 indicates that our synthesis of the capsules was successful, although the evidence is not unambiguous. We use the difference to argue that the HD is encapsulated by a polymer shell, at least in the initial cooling runs. The first cooling run showed only a single peak. Two peaks could be seen in the second cooling run. For samples with low xHD, a broad, low-intensity peak appeared at high temperatures and a second sharper and more intensive peak at low temperature. For samples with high xHD, a broad peak appeared at low temperatures, but a sharper peak was observed at higher temperatures. The signature in the third cooling run was slightly more variable between samples. Most commonly, two peaks
5408 J. Phys. Chem. B, Vol. 112, No. 17, 2008
Fette et al.
Figure 6. Multiple cool/heat cycles at 2 °C/min for an unreacted xHD ) 0.50 by volume. The DSC sample was prepared and handled identically to the encapsulated samples with the exception of purification prior to the scan.
Figure 7. Collection of onset temperatures for the freezing of HD in polystyrene/HD core-shell particles. All the scans were taken at a scanning rate of 2 °C/min. HD volume fraction is indicated in the legend; square symbols represent samples with xHD > 0.50.
were visible: a broad peak at lower temperatures and a more intense, sharper peak. The broader peak was less visible for samples with high xHD. The final onset-freezing temperature after four scans was 0.8 K below the bulk freezing point. The onset of the melting transitions shows a trend similar to that of the freezing transitions, with the exception that only one peak was visible. The final onset after multiple cool/heat cycling agreed completely with the bulk onset melting temperature. Our conclusion is therefore that, with repeated cool/heat cycles, the system changes from a capsule-type DSC signature to a bulktype DSC signature. Figure 7 shows the collected onset temperatures for the freezing transitions for all the samples studied. It is notable that significant supercooling is seen for all samples in the first cooling run, but this supercooling is nearly gone in the second cooling run if xHD is greater than 0.5 (square symbols). The figure only shows the onset of the majority peaks; remnants of the capsule-type DSC signature are apparent in all the samples in the second cooling runs. The disappearance of the capsule-type DSC signature depends on the heating and cooling rate in the DSC experiment. Figure 8 shows the effect that four different rates have on the DSC signature for xHD ) 0.5 samples. The measured onset temperature for bulk HD was found to be identical for a scanning rate of 2 K/min and 10 K/min. The difference between the scans at 1 K/min and 10 K/min is particularly striking, since the 1 K/min scan reaches the bulk DSC signature in the third cooling run (curve 5), whereas the bulk DSC signature is not reached at all
when the scan rate is 10 K/min. However, the initial scans are all in reasonable agreement. The onset temperatures for the main peaks are provided in Figure 9. On the basis of the data presented thus far, we have two hypotheses for the observed behavior. The first hypothesis, which we call the purification hypothesis, is that HD crystals are always formed through a heterogeneous nucleation in our systems, but the observed supercooling is predominantly due to impurities dissolved in the HD. These impurities may the surfactant or the co-surfactant, as well as water dissolved in the oil, most likely in a reverse-micelle-type structure within the oil droplet. Since the capsules are ∼200 nm in diameter, the volume of oil per capsule is in the range of 10-18 L. This means that, above a 1 µM concentration of a solute (impurity) in the oil, there is, on average, one solute molecule per capsule. This level of impurity is very likely well exceeded in our systems. Here, the structure of the capsule is not affected by the repeated cool/heat cycles. In the first cycle, the impurities lower the crystallization temperature. At the early stages of crystallization, the crystals formed are pure HD, and the remaining liquid phase contains an increasing concentration of the impurities, which further lower the crystallization temperature. With complete freezing, the impurities are eventually partially forced out of the oil drop or are confined to the region of the capsule wall. Upon thawing of the HD, the impurities are not redissolved in the time frame of the DSC experiment. Re-freezing of the HD in the second cycle further excludes the impurities and yields a crystallization temperature that is higher than in the initial run. During each thawing cycle, the oil drop is gradually becoming more pure, resulting in a higher melting temperature. Lower cooling rates allow more of the impurities to be excluded from the capsule in each run. In smaller droplets xHD ) 0.30, xHD ) 0.40, and xHD ) 0.50, the impact of the impurities is more significant because of the limited volume. There are weaknesses in this hypothesis, but it agrees well with freezing point depression of impure systems and the lowering of the melting temperature that we observe. The exclusion of the impurities are not likely to be so slow that a cooling rate of 1 K/min gives a significantly different behavior from 5 K/min, as is observed. If the transition temperature is also this sensitive to the concentration of dissolved impurities
Melting Transitions of HD in Polystyrene Shells
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5409
Figure 8. DSC curves for the cooling and heating of a xHD ) 0.50 polystyrene/HD core-shell particle dispersion at various cooling/heating rates: (a) 1 K/min; (b) 2 K/min (Figure 1); (c) 5 K/min; (d) 10K/min. The insets are the raw data scans as above. A bracket around some regions in the inset of panel d indicates that the heat flow was not fully controlled by the DSC instrument during this time.
Figure 9. Collection of the onset for the freezing transitions from Figure 7. Each scan was taken at different a heating rate, shown in the inset legend to the left.
in the HD, the crystallization in droplets of larger volumes should appear at higher temperatures, which is not what we observe. The second hypothesis, which we call the capsule failure hypothesis, is based on a change in the rate of nucleation with a subsequent failure of the capsule wall. In this hypothesis, the initial freezing of the HD occurs in a complete capsule via nucleation initiated at the oil/polymer interface. Although the oil phase may contain a significant level of impurities, several authors have reported that dissolved impurities have little effect on the type of nucleation.10,12 As discussed above, we can argue the increase in supercooling follows the decrease in S/V ratio, namely, that more of the oil is exposed to this interface at higher S/V ratio. The HD has lower specific volume in the solid state than in the liquid state, so freezing causes shrinkage in the core of the capsule. If the capsule is inflexible (polystyrene is most likely in the glassy state), the capsule wall will have to bend to respond to the core, thereby causing weakening. Repeated freezing will cause more extensive weakening of the capsule, and the point of weakening becomes a solid-type nucleation site that can initiate the phase transition. Given sturdier capsule
walls where xHD ) 0.30, xHD ) 0.40, and xHD ) 0.50 by volume, more stress is required to form a point of failure in the capsule shell. The thinnest capsules may have significant weaknesses in the first scan, which could explain the deviation from the linear trend in the extent of supercooling for the xHD ) 0.85 and xHD ) 0.90 capsules in Figure 2. With faster cooling rates and more rigid shells, the liquid-to-solid transition is more likely to result in a glassy state of the encapsulated oil, with that state having higher specific volume than the crystalline oil.10 We have no indication that the capsules are fully ruptured by the thermal cycling, and the leakage of the HD out of the shell is unlikely since it would require the water to enter a hydrophobic environment. We did not detect leakage of the capsules, but, given the miniscule amount of HD in the sample and its low vapor pressure, this would have been hard to observe with the instrumentation used in this study. The weakness of this hypothesis is that it is based on changes in the capsules that are next to impossible to observe experimentally without a bias in interpreting the result. We can, however, design an experiment that measures any exchange between the interior and exterior phase, but such an experiment is not within the scope of the present study. The capsule failure hypothesis is therefore largely non-verifiable at this time. Such hand-waving explanations can and should be followed by extensive testing of different capsules where the response is expected to be different, i.e., capsules that are more flexible or more fragile. 3. The Appearance of Secondary-Peaks in the Freezing Transition Exotherms. Throughout the above discussion, we have not given an explanation to the shapes and number of the observed peaks. The DSC signature was reproducible for the capsules here, with only slight variation in the intensity and peak maxima between runs as long as the heating and cooling rates were constant. The pattern and peak numbers for the DSC signatures were discussed above and termed capsule-type DSC signature and bulk-type DSC signature, respectively.
5410 J. Phys. Chem. B, Vol. 112, No. 17, 2008 The capsule-type signature was seen in the initial cooling run for all the samples. The observation is a single, slightly broad, endotherm peak for the freezing transition. The observation of a single endotherm peak indicates that the interior and surrounding environment of the capsules must be nearly identical for each of the capsules since our capsules are separated in space, and cross-seeding of the transition is unlikely. In general, we can say that a single peak is observed when a thermal absorption or thermal emission occurs simultaneously and reversibly. When systems undergo an irreversible transition (i.e., glass transition, reaction, or when one phase transition induces another), secondary peaks, shoulders, and tails are observed in the DSC signature. Irreversible transitions should also show an irreversible change in the baseline in a constant rate DSC, since the heat capacity changes. This change can be difficult to detect, but the effect can be resolved by modulated DSC methods. Secondary peaks and shoulders are commonly observed when studying phase transitions in emulsions16,18 and other microporous materials. Side peaks are observed for phase transitions of water in porous silica when the matrix is only partially filled.13,14 In this case, the argument is that the silica pores contain surface-bound water molecules, and have bulk porewater populations that change with the extent of pore filling. Schreiber et al. observed secondary peaks, rather than side-bands in the DSC signal when studying similar water/mesoporous silica systems.3 In this case, the pores were completely filled with water. In both the partially and completely filled pores, distinctive secondary peaks were observed, rather than gradually broadened peaks, which could have been correlated to the distribution of liquid states. If we consider the capsule failure mechanism as the plausible hypothesis for our observations, we would have expected to observe a distribution of nucleation rates, as the capsules of varying shell thicknesses would be expected to fail to varying degrees. We would therefore expect to see either a significant broadening of the initial peak or a gradual appearance of the final bulk-transition signature, and not an isolated intermediate state, such as the one we observed. If we take the purification hypothesis, we can envision capsules where the impurities are dispersed in inverse micelles within the oil droplets. If we assume that the number of micelles is limited because of the volume of the droplet, the disappearance of a micelle from the droplet changes the chemical environment in a stepwise fashion rather than a gradual one. This could therefore explain why we observe distinctive peaks at temperatures between the encapsulated and bulk signature peaks. With the current results and methods, we are not able to further elucidate the nature of the DSC signature. The knowledge that the transitions we observe are irreversible transitions has, however, prompted us to re-examine the system with a modulated DSC method along with attempting to vary the fragility of the capsules studied. Conclusions/Summary HD oil droplets, in the size range of 50-100 nm encapsulated within polystyrene shells a few nanometers thick were prepared by using mini-emulsion synthesis. The success of the synthesis was predominantly seen in the dramatically different DSC signatures that the encapsulated samples have from the control sample of an unpolymerized styrene analogue. The onset temperatures for the freezing of the encapsulated oil were found to be nearly 14 K below the bulk transition temperature. The
Fette et al. onset temperature for the melting of the same droplets was observed to be roughly 1 K below the bulk transition temperature. These onset temperatures varied slightly with droplet size. Trends in the onset temperatures were felt to be predominantly due to variation in the probability of nucleation from the oil droplet surface. Once a convenient bulk temperature had been selected, the variation could also be explained by the GibbsThomson-type dependence of the droplet/surface surface tension. Multiple cool/heat cycles of the capsules resulted in irreversible, but reproducible shifts in the onset temperatures from the original values to values only slightly different from those of the bulk. Two hypotheses explaining this behavior were put forth. In the first hypothesis, we argue that phase transitions in small volumes are more sensitive to the freezing out of impurities present in the oil droplet. This causes the transition temperature to shift downward. In a second hypothesis, we propose that structural failure of the capsules with repeated freezing results in a dramatic shift in the nucleation rates producing the transitions. With an intact capsule, the nucleation is slow, since the oil/polymer interface is relatively smooth. With repeated freezing, points of failure arise in the capsule, which changes the nucleation of the phase transition to a heterogeneous, rapid nucleation. Acknowledgment. The authors thank the NSF-REU program at SCU for financial and fellowship support during this work. E.V.F. was funded by NSF-REU #0453460, and A.P. was funded by an internal IBM sponsored grant at SCU. References and Notes (1) Hansen, E. W.; Gran, H. C.; Sellevold, E. J. Heat of fusion and surface tension of solids confined in porous materials derived from a combined use of NMR and calorimetry. J. Phys. Chem. B 1997, 101 (35), 7027-7032. (2) Litvan, G. G. Phase transitions of adsorbates. V. Aqueous sodium chloride solutions adsorbed of porous silica glass. J. Colloid Interface Sci. 1973, 45 (1), 154-169. (3) Schreiber, A.; Ketelsen, I.; Findenegg, G. H. Melting and freezing of water in ordered mesoporous silica materials. Phys. Chem. Chem. Phys. 2001, 3 (7), 1185-1195. (4) Schmidt, R.; Hansen, E. W.; Stocker, Akporiaye, M. D.; Ellestad, O. H. Pore-size determination of MCM-41 mesoporous materials by means of 1H NMR spectroscopy, N2 adsorption, and HREM. A preliminary study. J. Am. Chem. Soc. 1995, 117 (14), 4049-4056. (5) Montenegro, R.; Antonietti, M.; Mastai, Y.; Landfester, K. Crystallization in miniemulsion droplets. J. Phys. Chem. B 2003, 107 (21), 50885094. (6) Turnbull, D. Isothermal rate of solidification of small droplets of mercury and tin. J. Chem. Phys. 1950, 18 (2), 198-203. (7) Charoenrein, S.; Reid, D. S. The use of DSC to study the kinetics of heterogeneous and homogeneous nucleation of ice in aqueous systems. Thermochim. Acta 1989, 156, 373-381. (8) Becker, R.; Do¨ring, W. Kinetische behandlung der keimbildung in u¨bersa¨ttigten da¨mpfen. Ann. Phys. 1935, 416 (8), 719-752. (9) Turnbull, D. Kinetics of solidifaction of supercooled liquid mercury droplets. J. Chem. Phys. 1952, 20 (3), 411-424. (10) Turnbull, D.; Cormia, R. L. Kinetics of crystal nucleation in some normal alkane liquids. J. Chem. Phys. 1961, 34 (3), 820-831. (11) Schlossman, M. L. Liquid-liquid interfaces: Studied by X-ray and neutron scattering. Curr. Opin. Colloid Interface Sci. 2002, 7 (3-4), 235243. (12) Herhold, A. B.; Ertas, D.; Levine, A. J.; King, H. E. Impurity mediated nucleation in hexadecane-in-water emulsions. Phys. ReV. E 1999, 59 (6), 6946-6955. (13) Takamuku, T.; Yamagami, M.; Wakita, H.; Masuda, Y.; Yamaguchi, T. Thermal property, structure, and dynamics of supercooled water in porous silica by calorimetry, neutron scattering, and NMR relaxation. J. Phys. Chem. B 1997, 101 (29), 5730-5739. (14) Bellissentfunel, M. C.; Lal, J.; Bosio, L. Structural study of water confined in porous-glass by neutron-scattering. J. Chem. Phys. 1993, 98 (5), 4246-4252.
Melting Transitions of HD in Polystyrene Shells (15) Hindle, S.; Povey, M. J. W.; Smith, K. Kinetics of crystallization in n-hexadecane and cocoa butter oil-in-water emulsions accounting for droplet collision-mediated nucleation. J. Colloid Interface Sci. 2000, 232 (2), 370-380. (16) Clausse, D.; Gomez, F.; Pezron, I.; Komunjer, L.; Dalmazzone, C. Morphology characterization of emulsions by differential scanning calorimetry. AdV. Colloid Interface Sci. 2005, 117 (1-3), 59-74. (17) Hindmarsh, J. P.; Hollingsworth, K. G.; Wilson, D. I.; Johns, M. L. An NMR study of the freezing of emulsion-containing drops. J. Colloid Interface Sci. 2004, 275 (1), 165-171. (18) Clausse, D. Thermal behaviour of emulsions studied by differential scanning calorimetry. J. Therm. Anal. Calorim. 1998, 51 (1), 191-201. (19) McClements, J. D.; Dickinson, E.; Povey, M. J. W. Crystallization in hydrocarbon in water emulsions containing a mixture of solid and liquid droplets. Chem. Phys. Lett. 1990, 172 (6), 449-452. (20) Yano, J.; Furedi-Milhofer, H.; Wachtel, E.; Garti, N. Crystallization of organic compounds in reversed micelles. II. Crystallization of glycine and l-phenylalanine in water-isooctane-AOT microemulsions. Langmuir 2000, 16 (26), 10005-10014. (21) Coupland, J. N. Crystallization in emulsions. Curr. Opin. Colloid Interface Sci. 2002, 7 (5-6), 445-450. (22) Herhold, A. B.; King, H. E.; Sirota, E. B. A vanishing nucleation barrier for the n-alkane rotator-to-crystal transformation. J. Chem. Phys. 2002, 116 (20), 9036-9050. (23) McClements, D. J.; Dungan, S. R. Effect of colloidal interactions on the rate of interdroplet heterogeneous nucleation in oil-in-water emulsions. J. Colloid Interface Sci. 1997, 186 (1), 17-28. (24) Dickinson, E.; Ma, J. G.; Povey, M. J. W. Crystallization kinetics in oil-in-water emulsions containing a mixture of solid and liquid droplets. J. Chem. Soc., Faraday Trans. 1996, 92 (7), 1213-1215. (25) Dickinson, E.; Kruizenga, F. J.; Povey, M. J. W.; Vandermolen, M. Crystallization in oil-in-water emulsions containing liquid and solid droplets. Colloids Surf. A: Physicochem. Eng. Aspects 1993, 81, 273279. (26) McClements, D. J.; Dickinson, E.; Dungan, S. R. Kinsella, J. E.; Ma, J. G.; Povey, M. J. W. Effect of emulsifier type on the crystallization kinetics of oil-in-water emulsions containing a mixture of solid and liquid droplets. J. Colloid Interface Sci. 1993, 160 (2), 293-297. (27) Tiarks, F.; Landfester, K.; Antonietti, M. Preparation of polymeric nanocapsules by miniemulsion polymerization. Langmuir 2001, 17 (3), 908918. (28) Antonietti, M.; Landfester, K. Polyreactions in miniemulsions. Prog. Polym. Sci. 2002, 27 (4), 689-757.
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5411 (29) Landfester, K. The generation of nanoparticles in miniemulsions. AdV. Mater. 2001, 13 (10), 765-768. (30) Landfester, K.; Bechthold, N.; Tiarks, F.; Antonietti, M. Formulation and stability mechanisms of polymerizable miniemulsions. Macromolecules 1999, 32 (16), 5222-5228. (31) Landfester, K.; Spiess, H. W. Characterization of interphases in core-shell latexes by solid-state NMR. Acta Polym. 1998, 49 (9), 451464. (32) Advincula, R. C. Surface initiated polymerization from nanoparticle surfaces. J. Dispersion Sci. Technol. 2003, 24 (3-4), 343-361. (33) Ballauff, M. Nanoscopic polymer particles with a well-defined surface: Synthesis, characterization, and properties. Macromol. Chem. Phys. 2003, 204 (2), 220-234. (34) El-Aasser, M. S.; Sudol, E. D. Emulsion Polymerization and Emulsion Polymers; El-Aasser, M. S., Ed.; John Wiley & Sons, Inc.: New York, 1997; p 826. (35) El-Aasser, M. S.; Sudol, E. D. Miniemulsions: Overview of research and applications. JCT Res. 2004, 1 (1), 21-31. (36) Capek, I.; Chern, C. S. Radical polymerization in direct miniemulsion systems. In New Polymerization Techniques and Synthetic Methodologies; Springer: Berlin/New York, 2001; pp 101-165. (37) Alvarado, J. L.; Marsh, C.; Sohn, C.; Vilceus, M.; Hock, V.; Phetteplace, G.; Newell, T. Characterization of supercooling suppression of microencapsulated phase change material by using DSC. J. Therm. Anal. Calorim. 2006, 86 (2), 505-509. (38) Hempel, E.; Budde, H.; Horing, S.; Beiner, M. On the crystallization behavior of frustrated alkyl groups in poly(n-octadecyl methacrylate). J. Non-Cryst. Solids 2006, 352 (42-49), 5013-5020. (39) The onset temperature is the intersection between a tangential line drawn through the maximum slope of the peak and the interpolated baseline. This point represents the true transition point of the material and is not quantity dependent. The onset may vary slightly with the cooling rate of the experiment, but the instrument manufacturer, Mettler-Toledo, has advanced their calibration routine, which largely corrects for this. When the samples are heated or cooled at the same rate, the onset temperatures between two sample sizes are comparable. (40) Values taken from condensed phase thermochemical data provided on webbook.nist.gov. (41) The nonzero surface tension and spreading coefficient between the polystyrene and HD are requirements for the successful precipitation of the polystyrene during the synthesis and the subsequent formation of the capsule. Also, the interfacial tension between water and polystyrene must be smaller than the interfacial tension between water and HD.
