Langmuir 1998, 14, 3133-3136
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Supercooling, Nucleation, Rotator Phases, and Surface Crystallization of n-Alkane Melts E. B. Sirota Corporate Research Science Laboratories, Exxon Research and Engineering Company, Route 22 East, Annandale, New Jersey 08801 Received June 5, 1997. In Final Form: March 4, 1998 We demonstrate the relation between supercooling of n-alkane melts and the appearance or nonappearance of rotator phases. Supercooling occurs for the shorter, even chain lengths. The lack of supercooling in most n-alkane melts is discussed in terms of surface crystallization. We argue that melt crystallization of the nonrotator triclinic crystal phase is likely mediated by surface-crystallization-induced nucleation of a metastable rotator phase.
Phenomena which will play a major role in n-alkane crystallization include (i) rotator phases,1-6 which are weakly ordered crystalline phases occurring in equilibrium at temperatures between the liquid phase and the melt, and (ii) surface crystallization7-10 of an equilibrium ordered monolayer at temperatures of up to a few degrees above the melting point. In the paper “An Examination of the Nucleation Kinetics of n-Alkanes in the Homologous Series C13H28 to C32H66, and Their Relationship to Structural Type, Associated with Crystallization from Stagnant Melts”, Taggart, Voogt, Clydesdale, and Roberts (TVCR)11 presented interesting measurements of the metastable zone widths (MSZWs) (i.e. supercooling) on crystallization from the melt, extrapolated linearly to zero cooling rate, and related them to the low-temperature nonrotator crystal structures. Most of those n-alkanes, however, actually crystallize into rotator phases. Here we show that the variation in MSZWs reported by TVCR are manifestations of the fact that for a few of those n-alkanes the rotator phases are either skipped on heating or are entirely absent. We reconcile the observations of TVCR with, and explain them in terms of, previous work12 performed under much slower cooling rates and more isothermal conditions, which shows MSZWs much smaller than those reported by TVCR. We also explain how the lack of supercooling in most of these n-alkanes can be related to surface crystallization. We show that melt crystallization of the nonrotator triclinic crystal phase is (1) Ewen, B.; Strobl, G. R.; Richter, D. Faraday Discuss. Chem. Soc. 1980, 69, 19. (2) Ungar, G. J. Phys. Chem. 1983, 87, 689. (3) Denicolo, I.; Craievich, A. F.; Doucet, J. J. Chem. Phys. 1984, 80, 6200. (4) Sirota, E. B.; King, H. E., Jr.; Singer, D. M.; Shao, H. H. J. Chem. Phys. 1993, 98, 5809. (5) Sirota, E. B.; King, H. E., Jr.; Shao, H. H.; Singer, D. M. J. Phys. Chem. 1995, 99, 798. (6) Sirota, E. B. Langmuir 1997, 13, 3849. (7) Earnshaw, J. C.; Hughes, C. J. Phys. Rev. A 1992, 46, 4494. (8) Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Ocko, B. M.; Deutsch, M. Phys. Rev. Lett. 1993, 70, 958. (9) Wu, X. Z.; Ocko, B. M.; Sirota, E. B.; Sinha, S. K.; Deutsch, M.; Cao, B. H.; Kim, M. W. Science 1993, 261, 1018. (10) Ocko, B. M.; Wu, X. Z.; Sirota, E. B.; Sinha, S. K.; Gang, O.; Deutsch, M. Phys. Rev. E 1997, 55, 3164. (11) Taggart, A. M.; Voogt, F.; Clydesdale, G.; Roberts, K. J. Langmuir 1996, 12, 5722. (12) Sirota, E. B.; Singer, D. M. J. Chem. Phys. 1994, 101, 10873. (13) Turnbull, D.; Cormia, R. L. J. Chem. Phys. 1961, 34, 820. (14) Uhlmann, D. R.; Kritchevsky, G.; Straff, R.; Scherer, G. J. Chem. Phys. 1975, 62, 4896. (15) Oliver, M. J.; Calvert, P. D. J. Cryst. Growth 1975, 30, 343.
likely mediated by surface-crystallization-induced nucleation of a metastable rotator phase. In understanding nucleation phenomena, it is crucial that the nucleating ordered phase be well characterized. In the case of the studies of homogeneous nucleation in emulsion samples13-15 where the supercooling is of the order 10-15 °C, crystallization occurs into one of the lowtemperature nonrotator crystalline modifications. In the study of TVCR, the states into which crystallization from the melt occurred, as can be determined from the phase diagrams of pure n-alkanes and mixtures,2-5 are, in fact, the rotator phases (except for the shorter pure even carbon number n < 20). Rotator phases are crystals which lack long-range order with respect to rotation about the long axis of the molecule. While the low-temperature nonrotator crystal phases are indeed triclinic, orthorhombic, and monoclinic, the rotator phases into which crystallization actually occurs are RI (distorted hexagonal) for n j 21, RII (hexagonal) for 21 j n j 26, and RIV (tilted) for n J 26. (We have used the ∼ symbol, since the carbonnumber range of the phases varies somewhat from pure materials to mixtures.5) The temperature range below the melting point over which the rotator phases are stable is shown in Figure 1a for pure n-alkanes on heating and cooling and mixtures.5,16 While the RI rotator phase (denoted β0 in many phase diagrams17) is of an orthorhombic symmetry, it is distinct from the low-temperature orthorhombic crystalline phase (denoted β) and separated by a significant first-order transition. As can be seen in the slower calorimetric measurements,12 there is negligible hysteresis at the rotator-liquid transition, while there is great hysteresis at the rotator-crystal transition, which exhibits a large well-known even-odd effect in pure materials. Because of this, pure, short, even-n alkanes will melt directly out of the crystal phase, even if freezing is into the rotator phase (as occurs for n ) 20). The spike “anomalies” in the reported MSZW as a function of n can then be attributed to whether it is rotator or crystal on the low-temperature side of the freezing or melting transition. This is made even clearer by the reported enthalpies of melting measured on heating (TVCR Figure 7), when (16) Small, D. M. The Physical Chemistry of Lipids; Plenum: New York, 1986. (17) Gerson, A. R.; Nyburg, S. C. Acta Crystallogr. 1994, B50, 252. (18) Schaerer, A. A.; Busso, C. J.; Smith, A. E.; Skinner, L. B. J. Am. Chem. Soc. 1955, 77, 2017. (19) Broadhurst, M. G. J. Res. Natl. Bur. Stand. 1962, 66A, 241.
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Figure 1. (a) Phase diagram showing the temperature range below the melting temperature over which the rotator phases are stable. For the pure n-alkanes the ranges are shown for both heating and cooling. For n (n + 1) 50:50 binary mixtures (∆n ) 0.5) where there is negligible hysteresis, only cooling is shown. The interpolated melting temperature of pure odd-n alkanes is used as the reference temperature.5 (b) Schematic showing the relative free energy of the different phases. The rotator phase line is curved because of its anomalously high specific heat.
compared to the well-known values in the literature.16,18-20 The higher values for the pure, even-n alkanes can be explained as follows: For n e 20, direct liquid-crystal transitions whose enthalpies are known to be what would be expected for the sum of liquid-rotator and rotatorcrystal enthalpies, with a heat-capacity correction described below accounting for the difference. For n ) 22 and 24, the sum of the enthalpies of the two transitions could not be separated due to the broad temperature resolution of the DSC (shown in TVCR Figure 6 for what must really be a C18/C19 mixture21). While the linear plots of cooling rate versus temperature (Figures 2 and 4 of TVCR, both linear despite the labels) suggest that ∆T extrapolates to a “zero cooling rate” value of ∼0.2 °C for samples crystallizing into the rotator phase, previous measurements12 showed a much smaller hysteresis. In those studies of alkanes from C19 to C30 with a very slow cooling rate of ∼0.005 °C/min at essentially isothermal conditions, the melting transition was seen to be sharp (0.1 °C/min, which leads to large temperature gradients at the first-order transition.22 Since the melting point on heating occurs at a well-defined temperature, the use of the DTA peak temperature to determine the melting (20) Atkinson, C. M. L.; Larkin, J. A.; Richardson, M. J. J. Chem. Thermodyn. 1969, 1, 435. (21) Robles, L.; Mondieig, D.; Haget, Y.; Cuevas-Diarte, M. A.; Alcobe, X. Mol. Cryst. Liq. Cryst. 1996, 281, 279. (22) The temperature difference shown in Figure 1 of TVCR is indicative of the gradients in the cell, since the thermistor measures an interior temperature and the temperature of the reference cell (which has no phase transition) will be closely following the temperature of the outer wall of the sample.
Sirota
temperature, instead of the temperature of its deviation from the baseline (as is used for the freezing point), would have increased the reported MSZW. It is clear that the procedure used by TVCR can intrinsically overestimate the MSZWs and thus does not contradict the previous study,12 which showed that, for the samples melting from, and cooling into, the rotator phases, they are
