Published: June 03, 2011 r2011 American Chemical Society 3593 dx.doi.org/10.1021/cg200553t | Cryst. Growth Des. 2011, 11, 3593–3599 ARTICLE pubs.acs.org/crystal Activation Energy of Crystallization for Trihydroxystearin, Stearic Acid, and 12-Hydroxystearic Acid under Nonisothermal Cooling Conditions Ricky Sze Ho Lam † and Michael A. Rogers* ,‡ † Department of Food and Bioproduct Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N5A8 ‡ Department of Food Science, Rutgers University, The State University of New Jersey, New Brunswick, New Jersey 08901-8554, United States ’ INTRODUCTION Often, crystallization is thought to occur in three-dimensional (3-D) space, whereby spherulitic aggregates result and minimize the surface area to volume ratio and interfacial tension. 1 However, recent advances in crystal physics have thrust low-dimensionality crystals (i.e., two-dimensional (2-D) platelets and one-dimensional (1-D) fibers) into the forefront of numerous fields, including pharmacology, 2À4 medicine, 5À7 and nanomaterials. 8À10 Currently, there is a limited understanding of how these molecules nucleate and form supramolecular aggregates which no longer minimize the surface area of the crystal. 11 It has been well established that changes to the crystallization conditions greatly affect the mechanical properties, the flow behavior, and the supramolecular structure of materials regardless of the dimensionality of crystal growth. 12,13 For example, increasing the cooling rate increases the number of nuclei, decreases the crystal size, and modifies the polymorphism and crystal imperfection. 14,15 The thermodynamic conditions during nucleation and crystal growth dictate the structures formed by 12HSA, stearic acid, and trihydroxystearin crystals. 15 At high cooling rates, highly branched networks are formed due to crystallographic mismatches, while, at low cooling rates, large aggregates with few branching points are formed. 15 In industrial applications, crystallization often occurs under nonisothermal crystallization conditions where the tempera- ture changes as crystallization progresses. 16,17 Understanding how nucleation and crystallization occurs under nonisothermal cooling conditions is of paramount importance. Three systems, 12HSA, stearic acid, and trihydroxystearin, were chosen to be modeled to determine the activation energy of nucleation due to their molecular similarities and different dimen- sionalities of crystal growth. Upon crystallization, 12HSA forms 1-D nanofibers, 11,18À23 stearic acid forms 2-D platelets, 24À26 and trihydroxystearin forms 3-D spherulitic crystals. 15,27 Traditionally, nucleation is studied under isothermal cooling conditions due to the ease of experimental design and application of theoretical models. The activation energy for nucleation may be determined under nonisothermal cooling conditions using a super- cooling-time trajectory parameter (β). 16,17 β describes the amount of supercooling experienced by a system during crystallization. 16,17 Polarized light microscopy (PLM) is utilized to visualize the number of nucleation sites formed as a function of time. 16,17 The number of nucleation sites is used to determine the dependence of Received: May 1, 2011 Revised: June 2, 2011 ABSTRACT: The nucleation activation energy under non- isothermal cooling conditions was determined for 12-hydro- xystearic acid (12HSA) (1-D crystals), stearic acid (2-D crystals), and trihydroxystearin (3-D crystals). The relative nucleation rates of trihydroxystearin and stearic acid were inversely proportional to the supercooling-time trajectory para- meter (β), while 12HSA was linearly proportional to β. The differences in the proportionality to β are attributed to micro- scopic versus macroscopic phase separation. This suggests that both stearic acid and trihydroxystearin follow a probability density function for the number of molecules which crystallize as a function of supercooling (i.e., the greater the cooling rate, the greater the number of molecules which are incorporated into the crystal lattice). On the other hand, 12HSA molecules all crystallize when supercooled. The activation energies for stearic acid, 12HSA, trihydroxystearin, and triglycerides were 1.52, 5.40, 7.87, and 24.80 kJ/mol, respectively. The activation energy is partly affected by the polarity of the crystallizing molecules relative to the solvent. As the polarity of the crystallizing molecules increases, the activation energy decreases. However, this was not always observed because the activation energy for stearic acid was less than that of 12HSA. Therefore, the activation energy is not only a function of the molecular polarity but also due to a specific interaction between the nucleating molecules. The specific interaction affects the ability of the polar regions of the molecule to phase separate from the apolar solvent. As 12HSA and stearic acid dimerize, the carboxylic acid regions of the molecule are shielded from the solvent, but 12HSA cannot effectively shield the hydroxyl groups from the crystalline surface, resulting in a higher interfacial tension and, thus, higher activation energy.