Dielectric Study of the Slow Motional Processes in the Polymorphic

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J. Phys. Chem. B 2006, 110, 8268-8273

Dielectric Study of the Slow Motional Processes in the Polymorphic States of Anhydrous Caffeine Joaquim J. Moura Ramos* and Nata´ lia T. Correia† Centro de Quı´mica-Fı´sica Molecular, Complexo I, IST, AV. RoVisco Pais, 1049-001 Lisboa, Portugal

Hermı´nio P. Diogo Centro de Quı´mica Estrutural, Complexo I, IST, AV. RoVisco Pais, 1049-001 Lisboa, Portugal

Marc Descamps Laboratoire de Dynamique et Structure des Mate´ riaux Mole´ culaires (ESA CNRS 8024), Baˆ timent P5, 59655 VilleneuVe d’Ascq Cedex, France ReceiVed: December 6, 2005; In Final Form: February 14, 2006

Molecular mobility in crystalline anhydrous caffeine was studied by the dielectric technique of thermally stimulated depolarization currents (TSDC). Two relaxational processes were found, one appearing at ∼ -10 °C that is ascribed to a reorientational glass transition, and a higher temperature one that probably arises from local molecular motions that are precursors of diffusion and sublimation. The experimental results suggest that both crystalline phases II and I of caffeine, that have distinct crystal structures, are solid rotator phases. Furthermore, this dynamic reorientational disorder shows a reorientational glass transition at the same temperature in phase II and in metastable phase I.

Introduction Caffeine is a well-known and important substance, with agrochemical and pharmaceutical applications, that was isolated for the first time in 1820, and has the appearance of an odorless white crystalline powder. From a chemical point of view, it belongs to the group of alkaloids (with purine as the basic unit), and occurs principally in tea, coffee, gurana paste, cola nuts, and in small amounts in cacao. It is widely used in medicine as a psychoactive of the central nervous system by blocking some inhibitory adenosine receptors,1 in veterinary science as a cardiac and respiratory stimulant, and in the beverage industry essentially due to its stimulating effects and as an enhancer of soft drinks due to its bitter taste.2 It is reported that a hydrated form of caffeine exists, but that it easily dehydrates at room temperature into an anhydrous phase (called phase II or β).3 The experimental results obtained and discussed here concern the anhydrous form. Phase II converts on heating into phase I (or R). The II f I phase transformation is first order, and the corresponding endothermic DSC peak shows a maximum at 155 °C (431 K),3 with an onset at 143 °C.4 Phase I subsequently sublimes on further heating before melting (Tm = 240 °C): sublimation is easy in all the temperature domains of phase I.5 Furthermore, phase I can be easily undercooled below the transition temperature, Tt ) 155 °C, becoming metastable. However, the I f II transformation slowly occurs below Tt.4 It was suggested6 that it is not possible to produce crystallographically pure samples of phase II, despite the fact that this is the stable form at room * Corresponding author phone: 351-218419253; fax: 351-218464457; e-mail: [email protected]. † On postdoctoral leave at the Laboratoire de Dynamique et Structure des Mate´riaux Mole´culaires, UFR Physique, Universite´ de Lille I, France.

temperature. A given and unpredictable amount of phase I seems to be always present as an “impurity”. The structural and dynamic properties of phase I of caffeine have been recently studied by X-ray and dielectric relaxation spectroscopy.7 It was found that this is a solid rotator phase that can be undercooled to give rise to an orientational glass, with an apparent orientational glass transition at Tg = -13 °C. Note that different reviews of apparent thermal transitions of this type in amorphous, liquid crystalline and rotator phase crystalline materials have been published by H. Suga.8,9 In the present work the dynamic disorder in the crystalline phases of anhydrous caffeine is approached using the dielectric technique of thermally stimulated depolarization currents (TSDC). The main objective is to complement the information provided by dielectric relaxation, looking for a deeper insight into the understanding of the molecular motions in anhydrous caffeine. Experimental Section Caffeine (1,3,7-trimethylxanthine, C8H10N4O2, CAS no. 5808-2), was purchased from Aldrich (catalog no. C5-3, lot no 2206HA-313, 99% purity) and was used without further purification. Some of the TSDC and DSC results were confirmed by using a sample purchased from Sigma (catalog no. C8960, lot no. 023K0054). The chemical structure of this substance is shown in Figure 1. The low symmetry of the molecular shape, together with the presence of electronegative heteroatoms in the molecular structure, give rise to a strong dipole moment (µ ) 3.7 D measured in benzene solution, and µ ) 4.6 D in dioxane solution10). As a consequence of this strong molecular polarity, dielectric techniques appear as very adequate to study molecular mobility in such a system.

10.1021/jp0571022 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/04/2006

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Figure 1. Chemical structure of the caffeine molecule.

Thermally stimulated depolarization current (TSDC) experiments were carried out with a TSC/RMA spectrometer (TherMold, Stamford, CT) covering the range from -170 to +400 °C. For TSDC measurements, the sample was placed between the electrodes of a parallel plane capacitor with effective area of ∼38 mm2. It was prepared in the form of a compressed disk of ∼0.5 mm thickness, which was shaped under the pressure of ∼735 MPa. The sample is immersed in an atmosphere of high purity helium (1.1 bar). The TSDC technique is adequate to probe slow molecular motions (20-3000 seconds). The fact that the relaxation time of the motional processes is temperature dependent and becomes longer as temperature decreases, enables to immobilize them by cooling. This is the basis of the technique. To analyze specific regions of the TSDC spectrum, different methods of polarizing the sample can be used, namely the so-called TSDC global experiment and the thermal sampling (TS) experiment (often called thermal windowing or cleaning, or partial polarization). The TS method, where the polarizing field is applied in a narrow temperature interval, enables the global peak to resolve into its individual relaxation modes. The thermal sampling procedure allows to retain (or to freeze) a polarization that arises from a narrow variety of dipolar motions. In the limit of a very narrow polarization window, the retained polarization (and of course the current peak that is the result of a thermal sampling (TS) experiment) would correspond to a single, individual dipolar motion.11,12 The physical background of the TSDC technique is presented elsewhere.13-16 The basic description of the TSDC experiment, and the discussion of the nature of the information it provides, is presented in detail in recent publications.11,12

Figure 2. Results of TSDC global experiments carried out on caffeine. The experimental conditions of all the experiments were the same: polarizing electric field, E ) 400 V.mm-1; polarization temperature, TP ) 55 °C; freezing temperature, T0 ) -125 °C; final temperature of the heating ramp, Tf ) 110 °C; heating rate, r ) 10 °C.min-1. The dashed line (lower intensity peak) is the result of an experiment carried out on the as-received sample (mainly in phase II). The continuous lines are a selection of the results of a series of 27 identical and successive experiments where the first (line 1) corresponds to pure phase I. The order numbers of all the selected experiments are indicated. Before the first experiment (higher intensity peak), the sample was cooled fast from 180 °C down to the polarization temperature. This thermal treatment warrants that the sample is in phase I at the beginning of the first experiment. In the subsequent experiments, the sample was cooled to the polarization temperature directly from the final temperature of the linear heating ramp of the previous TSDC experiment, which was Tf ) 110 °C for all experiments.

Results and Discussion As stated before, phase II is the stable phase of caffeine at room temperature. We thus expect the as-received caffeine to be predominantly in phase II, remembering, however, that the difficulty of getting phase II in the pure state was reported.6 Figure 2 shows the result of different TSDC global experiments (wide polarization window) carried out on a caffeine sample. All the experiments have the same experimental protocol, as described in the caption of Figure 2. The dashed line is the result of an experiment carried out on a as-received caffeine sample (predominantly in phase II). The continuous line with higher intensity (numbered 1), on the other hand, shows the result of a similar experiment now carried on a caffeine sample freshly prepared in phase I (obtained by heating above the II f I transition temperature at Tt ) 155 °C) and quenching to room temperature. The other lines in Figure 2 will be discussed later. From the observation of the previously referred lines we conclude that there are two relaxation peaks, one with maximum intensity at ∼ -10 °C, stronger in phase I (line 1) in comparison with the as-received sample (dashed line), and the other at higher temperature (at ∼60 °C in Figure 2) that seems to display identical features in both phases.

Figure 3. Thermal sampling (TS) components of the relaxation at ∼ -10 °C of crystalline caffeine in phase I. The polarization temperatures, TP, varied between -70 and 0 °C (203.15 and 273.15 K), with intervals of 5 °C. The other experimental conditions were as follows: strength of the polarizing electric field, E ) 450 V.mm-1; polarization time, tP ) 5 min.; width of the polarization window, ∆T ) 2 °C; heating rate, r ) 4 °C.min-1. The final temperature of the heating ramp never exceeded Tf ) 50 °C in order to prevent the transformation of phase I into phase II.

The Relaxation at ∼ -10 °C. As shown before, the TSDC peak at -10 °C shows higher intensity in phase I, compared with the as-received sample (predominantly in phase II) and has identical temperature location in both phases. Some thermal sampling (TS) components (narrow polarization window) of this relaxation at -10 °C in phase I are shown in Figure 3. The analysis of these TS peaks, each of which roughly corresponds to a single dipolar motion, provide the kinetic parameters associated with such a motion. The activation enthalpies of the different fractional peaks of caffeine are shown in Figure 6 (Appendix A) as a function of the peak’s location, Tm. The set of points corresponding to the relaxation at -10

8270 J. Phys. Chem. B, Vol. 110, No. 16, 2006 °C is in the left-hand side of the figure (Tm < 20 °C), and show a peculiar curvature that is often ascribed to a glass transition (see Appendix A). It is the apparent orientational glass transition that arises from the dynamic disorder that exists in the higher temperature phase I of caffeine, and shows a narrow activation enthalpy distribution between 45 and 90 kJ.mol-1. In this context, the peak with higher intensity in Figure 3 is the manifestation of a mobility component that is characteristic of the glass transition,10 and the respective analysis allows the determination of the activation energy for the structural relaxation, and of the fragility index of the glass forming system. This peak has a temperature of maximum intensity at TM ) -10 °C, that is the reorientational glass transition temperature of caffeine provided by the TSDC technique (at 4 °C.min-1), and corresponds to the temperature at which the reorientational freedom is arrested on cooling. This temperature is in good agreement with the value (Tg = - 13 °C) obtained by extrapolation to τ ) 100 s of the dielectric relaxation data and with the value obtained by DSC (Tg = - 17 °C),7 which strengthens our previous attribution of the referred peak to the glass transition. From the analysis of the higher intensity peak it comes out that the activation energy of the corresponding motional mode is Ea ) 76 kJ.mol-1, and the relaxation time at the temperature of maximum intensity is τ(TM) ) 107 s. The fragility index calculated from those values17 is m ) 16, in the strong limit, in good agreement with the value (m ) 19) obtained from the dielectric relaxation results.7 As stated before, and shown in Figure 2, the relaxation peak at ∼ -10 °C is also present in phase II (as-received caffeine), but with lower intensity. Two hypotheses can be formulated in order to interpret these peaks. One is to consider that the commercial state of the sample is a mixture of two phases, a residue of the disordered phase I in the ordered phase II, in which case the peak at -10 °C in the as-received sample (with lower intensity) corresponds to the dynamic disorder of the residual amount of I. The other hypothesis is that this peak is the manifestation of a dynamic disorder that is intrinsic of the low-temperature phase II. The discussion of this problem was conducted before based on dielectric relaxation results,7 and the conclusion was that there is a mobility proper to phase II, very similar to that observed in the metastable phase I at the same temperature, but with a time scale slightly larger. The detailed analysis of this problem on the basis of our TSDC results is presented in Appendix B. From this analysis, the following can be concluded: (1) no modification of the peaks position (global peaks as in Figure 2, partial polarization peaks in Figure 7) is observed when we compare the results of identical experiments carried out in phase I and in the as-received sample (predominantly II); (2) no apparent slowing down of the reorientational motion is observed in phase II (compared with phase I). The critical analysis of this problem will be pursued later. The Transformation From Phase I to Phase II. The endothermic DSC peak associated with the phase transformation II f I in our studied samples show an onset at 141 °C and a maximum intensity at 154 °C (at 10 °C.min-1). Oppositely, the transformation I f II is rather difficult to observe on cooling given the slowness of the process. Since the peak at ∼ -10 °C shows markedly different intensities in the two phases I and II, it can be used as a probe to monitor the transformation I f II from the metastable phase I into the stable phase II. Figure 2 shows the result of a long series of 27 identical and successive TSDC global (wide polarization window) experiments performed after heating the sample up to 180 °C, to prepare the phase I. The continuous decrease of the intensity (and of the

Moura Ramos et al. area) of the peaks at ∼ -10 °C, from the peak numbered 1 until the peak 27, is the manifestation of the progressive phase transformation I f II. As shown in Appendix C, this transformation is very slow at temperatures lower than ∼50 °C, is effective at temperatures higher than that value, and the maximum transformation rate occurs at ∼90-95 °C. The continuous decrease of the intensity shown in Figure 2 thus arises from the fact that, in each experiment, the sample is allowed to rest, for a given period of time, in a temperature region where the transformation occurs efficiently (above 50 °C and up to 110 °C). Note that a relaxation at a higher temperature is observed which does not show any apparent evolution. The slow and progressive transformation shown in Figure 2 suggests that the evolution of the relaxation peak at ∼ -10 °C tends to a stationary limit. This stationary limit seems to be the dashed line in Figure 2 that corresponds to the result of an experiment, with exactly the same experimental protocol, carried out on the as-received sample. Two interpretative scenarios are compatible with the results previously reported. The first is to assume that this residual peak at -10 °C in the as-received sample corresponds to the presence of a given amount of phase I in phase II. If this is the case, the peak at -10 °C is a feature of phase I and not of phase II, and the area under this peak (the dielectric strength of the relaxation) will be proportional to the amount of phase I in the sample. From our results, we are thus forced to conclude that the commercial form of caffeine (the as-received sample) is a mixture rich in phase II, containing ∼25% of phase I. According to the second possible interpretative scenario, the as-received sample is in phase II, and the residual peak at -10 °C observed in this sample is nothing but the manifestation of a dynamic orientational disorder that is proper to phase II. If this is the case, our TSDC results show that this molecular mobility is identical to that observed in phase I, with the same temperature location, similar activation parameters, and identical distribution of relaxation times (see Appendix B). Furthermore, the dynamic reorientational disorder observed in the two phases can be frozen by cooling at the same orientational glass transition temperature. The difference is that the relaxation at -10 °C observed in phase I is much more prominent than that in phase II, meaning that the dielectric strength of the relaxation is higher in phase I than in phase II. The difference of the crystal structures of the two phases would be, in the context of this scenario, at the origin of the decrease of the dielectric strength of the relaxation in phase II (reduction of the number of involved dipole entities and/or restriction on the amplitude of the dipolar reorientations). The problem now is the choice between the two scenarios. In our opinion, it is not easy to understand why the I f II transformation, that is kinetically efficient at 90-100 °C, does not go through completion, but instead tends to a particular mixture of phases with 25% of I. Furthermore, we observed that this ratio of 0.25 was not changed by using different lots of commercial caffeine, or by recrystallization. We believe that it is not reasonable to accept that phase II of caffeine avoids states of purity higher than 75%, i.e., we do not find thermodynamic or kinetic arguments that justify 75% as the higher limit of the purity of phase II in caffeine. As a consequence, we cannot attribute the peak observed in phase II to an impurity of phase I, and we tend to suggest the second scenario as the most probable. Furthermore, the fact that the enthalpy of phase transition II f I obtained with our samples is 3.7-3.9 kJ.mol-1

Motional Processes in Anhydrous Caffeine

Figure 4. Results of successive global experiments on the as-received caffeine illustrating the different degrees of activation of the hightemperature relaxation as a function of the polarization temperature. The electric field strength was E ) 350 V.mm-1, the polarization time was tP ) 5 min, freezing temperature T0 ) - 125 °C, and the heating rate r ) 8 °C.min-1. The polarization temperature, TP, and the final temperature of the heating ramp, Tf, from right to left, were as follows: TP ) 60 °C and Tf ) 110 °C; TP ) 55 °C and Tf ) 105 °C; TP ) 50 °C and Tf ) 100 °C; TP ) 40 °C and Tf ) 90 °C; TP ) 20 °C and Tf ) 70 °C. Note that Tf is chosen in order to warrant full depolarization of the sample.

(compared with 4.0 kJ.mol-1 reported for the USP reference standard6) is compatible with the previous suggestion. The Higher Temperature Relaxation. A relaxational peak appears at higher temperature, above the peak at ∼ -10 °C, in phase II as well as in phase I (see Figure 2). Figure 4 clearly shows the two relaxation peaks obtained in global experiments carried out on the sample in phase II. In the experiment with polarization temperature TP ) 20 °C, the field fully activates the relaxation at ∼ -10 °C, but it is not able to activate the higher temperature relaxation. Higher polarization temperatures allow, as shown in Figure 4, different degrees of activation of the higher temperature relaxation. The fact that the lower temperature peak does not change from experiment to experiment in Figure 4 demonstrates the independence of the two relaxation mechanisms. The fact that sublimation of caffeine becomes effective above ∼ 140 °C5 precludes a complete characterization and analysis of this relaxation process. Despite this restriction, we were able to obtain some TS peaks of this relaxation, some of them shown in Figure 5. The activation enthalpy of many fractional peaks obtained on the higher temperature relaxation of caffeine (in phase I and in the as-received sample) are also shown in Figure 6 (Appendix A) as a function of the peak’s location, Tm. The set of points associated to the higher temperature relaxation is in the righthand side of the Figure 6 (Tm > 20 °C), the triangles corresponding to phase I and the circles to the as-received sample. We can conclude the following: (1) The set of points show a higher dispersion (when compared with those of the apparent orientational glass transition relaxation), which indicates a lower experimental reproducibility and (2) the activation enthalpies of the corresponding motional processes are small, with the consequence that the activation entropies are negligibly small (localized mobility). The fact that this higher temperature peak corresponds to a mobility that occurs in both phases, with similar characteristics (temperature position and kinetic parameters), probably arises from the structural similarity of the two phases.18 The attribution at the molecular level of this high temperature peak of caffeine, is not straightforward. One possibility is to associate it to a local

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Figure 5. Thermal sampling (TS) components of the higher temperature relaxation of crystalline caffeine in phase II. The polarization temperatures, TP, varied between 40 and 90 °C (313.15 and 363.15 K), with intervals of 5 °C. The other experimental conditions were as follows: strength of the polarizing electric field, E ) 450 V.mm-1; polarization time, tP ) 5 min.; width of the polarization window, ∆T ) 2 °C; heating rate, r ) 4 °C.min-1.

and noncooperative mobility, probably ascribable to out-of-plane motions that are precursors of molecular diffusion and sublimation. Another possibility is that it is a consequence of the presence in the sample of extrinsic ionic species (the so-called F-peak). The discussion of this problem needs more experimental information eventually arising from other techniques. Conclusions The TSDC results show that phase I of caffeine is a solid rotator phase with a prominent relaxation at ∼ -10 °C. This relaxation corresponds to the apparent orientational glass transition associated to the freezing, or cooling, of the dynamic reorientational disorder into an orientational glassy state. The fragility index of this glassy crystal was found to be m ) 16, in the strong limit of the Angell scale. The relaxation peak at ∼ -10 °C was also found in the commercial state of caffeine (essentially phase II, the most stable at room temperature), but with much lower intensity. The careful comparison of this relaxation, in phase I and in the as-received sample, indicates that it has exactly the same features in the two states, with the same location in the temperature axis and the same activation parameters. Furthermore, the ratio between the areas of the TSDC peaks at ∼ -10 °C, in the as-received sample and in phase I, is ∼0.25 which rules out the possibility of attributing the peak observed in phase II to an impurity of phase I. On the contrary, it seems to correspond to a dynamic orientational disorder intrinsic of phase II. A higher temperature dipolar relaxation peak was also found in both phases of caffeine. The characterization of this mobility is difficult due to the interference of sublimation, but it appears as a localized motion with low activation energy attributable to out-of-plane molecular motions. Acknowledgment. N.T.C. acknowledges a postdoctoral grant from the Fundac¸ a˜o para a Cieˆncia e Tecnologia. Support by a franco-portuguese cooperation protocol (GRICES/CNRS) is gratefully acknowledged. Appendix A General Features of the Partial Polarization Components (Narrow Polarization Window) of the Relaxational Processes in the Crystalline Phases of Caffeine. The motional processes present in phases I and II of caffeine have been studied by the

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Figure 6. Representation of the activation enthalpy, ∆Hq, against the temperature location of the corresponding elementary peak, Tm, for all the fractional polarization peaks of caffeine. The triangles correspond to experiments carried out on the sample in phase I, while the circles correspond to experiments carried out on the as-received sample (mainly in phase II).

technique of fractional or partial polarization, in the temperature range between -125 °C and +125 °C. Each fractional polarization peak, obtained by polarizing the sample at a given polarization temperature, Tp, is characterized, at a given heating rate, by a temperature location (temperature of maximum intensity, Tm) and by an activation energy. The graphical representation of these parameters is often used in order to differentiate the motional modes and characterize their main features. One of such representations is shown in Figure 1S where the activation enthalpy, ∆Hq, is plotted against the temperature of maximum intensity, Tm, for all the obtained fractional polarization peaks. Each point in the plot corresponds to a fractional polarization peak. The triangles in the figure correspond to experiments carried out on the sample in phase I, while the circles refer to experiments with the as-received sample (predominantly in phase II). From the observation of Figure 6 we can draw the following general conclusions: (1) Two main relaxation regions can be distinguished on the basis of two different regimens of variation of the two variables. One for Tm < ∼20 °C concerns the relaxation at ∼ -10 °C, the other for Tm > ∼20 °C concerns the higher temperature relaxation (see Figure 2 of the main text). (2) The points corresponding to experiments in phase I (triangles), and those corresponding to experiments in the asreceived sample (predominantly phase II, circles), seem to display an identical variation trend, in the lower temperature relaxation as well as in the higher temperature relaxation. This can be an indication that the relaxation peaks in both samples correspond to similar motional processes. (3) The points corresponding to the relaxation at -10 °C, in the left-hand side of Figure 6 (Tm < 20 °C), show a peculiar curvature that arise from a concomitant increase of the activation enthalpy and entropy (or of the activation energy and preexponential factor), and is often ascribed to a glass transition. This concomitant increase, often referred as a deviation from the zero entropy behavior, is indeed small given that we are in the presence of an orientational glass transition. (4) The points corresponding to the fractional peaks of the higher temperature relaxation, in the right-hand side of the figure (Tm > 20 °C), are more scattered. The analysis of these fractional peaks indicates that they concern a molecular mobility with low activation enthalpies and negligible activation entropies, suggesting a localized and noncooperative nature of those motions (see discussion in the text).

Moura Ramos et al.

Figure 7. Partial polarization peaks of the relaxation at ∼ -10 °C in phase I (higher intensity peaks) and in phase II (lower intensity peaks). The general experimental conditions of all the six experiments were: strength of the polarizing electric field E ) 400 V/mm, polarization temperature Tp ) -10 °C, polarization time tP ) 5 min, polarization window ∆T ) 2 °C, heating rate r ) 4 °C.min-1. The insert shows the τ(T) lines of the peaks.

Appendix B Comparison Between the Relaxation Peak at ∼ -10°C as Observed in Phase I and in the As-Received Sample. In this section we compare the features of the relaxation at ∼ -10 °C in phase I on one hand, and in the as-received sample (mainly phase II) on the other hand. In Figure 7 we selected six partial polarization peaks obtained with the same experimental protocol (see Figure caption), three of them relative to the sample of caffeine in the phase I (higher intensity peaks), and the other three relative to the as-received sample (predominantly phase II, lower intensity peaks). It is to be noted that three different samples, prepared in different days, were used to obtain these results. From the observation of Figure 7, it can be concluded that all the peaks have similar locations in the temperature axis; in fact, for the peaks in phase I (higher intensity) we have Tm ) - 5, - 5, and - 4 °C, while the temperature of maximum intensity is - 6, - 6, and - 5 °C for the corresponding peaks in phase II (lower intensity). On the other hand, all six peaks have τ(T) lines nearly coincident (as shown in the insert of Figure 7) indicating that the activation parameters of the corresponding motional modes are similar. From these findings we can conclude that the relaxation peak at ∼ -10 °C, which is observed in the metastable phase I and in the as-received sample, is the manifestation of the same motional process (see discussion in the text). Appendix C Influence of the Temperature on the Rate of the Phase Transformation I f II. It was stated before that the continuous decrease of the intensity of the peak at ∼ -10 °C illustrated in Figure 2 (main text) is the manifestation of the phase transformation I f II. It occurs given that, at the end of each experiment, the sample is allowed to rest in a temperature region where the transformation occurs efficiently. To approach the problem of the temperature dependence of the transformation rate, we designed experiments where the sample in phase I was submitted to annealing periods at different annealing temperatures. Figure 8 shows the results of a series of such experiments. The experimental protocol used to probe the phase transformation is described in the caption of Figure 8. An important feature is that no significant extent of transformation is expected to occur during this experiment (with Tf ) 50 °C), since it was found that the transformation rate was very small at 50 °C and

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J. Phys. Chem. B, Vol. 110, No. 16, 2006 8273 the case of the series shown in Figure 8). The decrease of the intensity of the peak at ∼ -10 °C shown in Figure 8 is a consequence of the I f II phase transformation that occurs during the annealing step. In Figure 9 is shown the evolution of the I f II phase transformation four different annealing temperatures, more specifically for Ta ) 50, 60, 80, and 90 °C. We can conclude the following: (1) the rate of the I f II phase transformation is indeed very low at 50 °C; (2) this rate increases with increasing temperature. Furthermore, we verified that the transformation rate does not increase significantly for temperatures higher than 90 °C. References and Notes

Figure 8. Identical and successive global experiments on the relaxation at -10 °C of caffeine. The experimental conditions of all the experiments were the same: polarizing electric field, E ) 400 V.mm-1; polarization temperature, TP ) 5 °C; freezing temperature, T0 ) -125 °C; final temperature of the heating ramp, Tf ) 50 °C; heating rate, r ) 10 °C.min-1. Before the first experiment (higher intensity peak), the sample was cooled fast from 180 °C down to the polarization temperature. This thermal treatment warrants that the sample is in phase I at the beginning of the first experiment. In the subsequent experiments, it was annealed at 90 °C during 15 min.

Figure 9. Influence of the annealing temperature (annealing time ta ) 15 min) in the rate of the I f II phase transformation. The Figure shows the decrease of the maximum intensity, Im, in series of identical and successive experiments with the same annealing temperature. Im0 is the intensity of the first peak in each series, and n is the order number of the experiment in the respective series of experiments. Before the first experiment, the sample was cooled fast from 180 °C down to the polarization temperature. This thermal treatment warrants that the sample is in phase I at the beginning of the first experiment. In the subsequent experiments, was previously annealed at Ta during 15 min. The different symbols correspond to different annealing temperatures: ] Ta ) 50 °C; 0 Ta ) 60 °C; 4 Ta ) 80 °C; O Ta ) 90 °C.

lower. Prior to the first experiment in each series, the sample was heated to 180 °C, to prepare the phase I. The higher intensity peak in Figure 8 is the result of the probe experiment carried out on the sample in phase I. The other peaks in Figure 8 are the result of a series of identical and successive probe experiments, all of them preceded by an annealing of ta ) 15 min at the annealing temperature, Ta (which is Ta ) 90 °C in

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