Solvent-Induced Polymorphic Nanoscale Transitions for 12-Hydroxyoctadecanoic Acid Molecular Gels Songwei Wu, †,⊥ Jie Gao, †,⊥ Thomas J. Emge, ‡,# and Michael A. Rogers* ,†,⊥ † School of Environmental and Biological Sciences, Department of Food Science, Rutgers University, The State University of New Jersey, New Brunswick, New Jersey 08901, United States ‡ School of Arts and Science, Department of Chemistry and Chemical Biology, Rutgers University, The State University of New Jersey, Piscataway, New Jersey 08854, United States * S Supporting Information ABSTRACT: 12-Hydroxyoctadecanoic acid (12HSA) molec- ular gels have been reported to form self-assembled fibrillar network (SAFiNs) in organic solvents. For the first time, different polymorphic forms for 12HSA molecular gels have been reported. 12HSA, in alkanes and thiols, have a hexagonal subcell spacing (∼4.1 Å) and are arranged in a multilamellar fashion with a distance greater than the bimolecular length of 12HSA (∼54 Å). This polymorphic form corresponded to SAFiN with CGC less than 1 wt %. 12HSA, in nitriles, aldehydes, and ketones, have a triclinic parallel subcell (∼4.6, 3.9, and 3.8 Å) and interdigitation of the lamellar structure (38-44 Å). This polymorphic form corresponds to a less effective sphereultic supramolecular crystalline network, which immobilizes solvents at CGC greater than 1.5 wt %. ■ INTRODUCTION Molecular organogels are thermally reversible, quasi-solid materials comprised of an organic liquid (usually ≥95%) and a gelator molecule that self-assemble via physical interactions, including hydrogen-bonding, 1-4 π-π stacking, 5 dipole-dipole, 6,7 and London dispersion forces, 8 into a three-dimensional network. 9-11 Although the physical interactions between gelator molecules are central in understanding gelation, the solvent-gelator speci fic (i.e., H-bonding) and nonspecific (dipole-dipole, dipole-induced, and instantaneous dipole induced forces) intermolecular interactions are equally important. 12,13 The process of self-assembly, in molecular gels, is an intricate process that must balance the solubility and those intermolecular forces that control epitaxial growth into axially symmetric elongated aggregates. 10,13-16 During assembly, individual molecules are driven to assemble by molecular self-recognition and intermolecular noncovalent interactions into oligomers, and subsequently these oligomers assemble into fibrillar aggregates immobilizing the solvent via capillary forces. 17,18 Herein, we present an investigation of the first polymorphic transformation, for a molecular gel, induced by modifying the solvent with 12HSA as the gelator. In molecular gels, polymorphic transitions have only been noted in (R)-18-(n- alkylamino)octadecan-7-ols in CCL4 which undergoes a gel- gel polymorphic transition during heating. 9 Several other transitions have been reported in molecular gels; however differences lie at the supramolecular level of structure induced by crystallographic mismatches and not different polymorphic forms. 2,3,12,15,19-22 12HSA, a structurally simple, highly effective low molecular weight gelator (LMOG), has been studied extensively for gelation kinetics 2,23-25 and supramolecular structure forma- tion, 3,19,20,26-28 as well as to monitor surface properties, 29 Received: January 22, 2013 Published: February 5, 2013 Table 1. Critical Gelator Concentrations from ref 13 and Peak Melting Temperatures Determined in Triplicate Using Differential Scanning Calorimetry solvent CCG (wt %) melting temperature (°C) hexane 0.4 61.9 ± 0.01 heptane 0.3 62.5 ± 0.1 octane 0.3 61.6 ± 0.5 nonane 0.25 60.5 ± 0.9 decane 0.2 63.9 ± 0.1 tetradecane 0.2 64.8 ± 0.35 1-pentanethiol 0.5 NA 1-hexanethiol 0.45 45.7 ± 0.01 1-heptanethiol 0.45 49.1 ± 0.03 1-octanethiol 0.4 50.8 ± 0.04 1-decanethiol 0.3 51.9 ± 0.06 butylnitrile 2.1 64.2 ± 0.3 hexanenitrile 1.9 65.5 ± 1.2 heptylnitrile 1.5 58.4 ± 1.0 nonanenitrile 0.9 NA butylaldehyde 2.8 NA dodecylaldehyde 1.4 41.2 ± 0.57 4-heptone 2 NA 5-nonanone 2.1 32.8 ± 0.8 6-undecanone 1.6 45.6 ± 1.5 Article pubs.acs.org/crystal © 2013 American Chemical Society 1360 dx.doi.org/10.1021/cg400124e | Cryst. Growth Des. 2013, 13, 1360-1366