A molecular insight into the nature of crystallographic mismatches in self-assembled fibrillar networks under non-isothermal crystallization conditions

Abstract

The lengths of the 12-hydroxystearic acid (12HSA) fibers are influenced by crystallographic mismatches resulting from the incorporation of 12HSA monomers into the crystal lattice in an imperfect manner. On a molecular level, this can be differentiated using synchrotron Fourier transform infrared (FTIR) spectroscopy by monitoring the change in area of the 1700 cm À1 and 3200 cm À1 peaks, corresponding, respectively, to the dimerization of the carboxylic acid groups and hydroxyl non-covalent interactions, during crystallization. The crystallographic mismatch is attributed to a plateau in the apparent rate constant for the dimerization of the carboxylic acid head groups while the hydroxyl interactions linearly increase as a function of cooling rate (f). The rate constant for hydroxyl interactions linearly increases as a function of cooling rate while a plateau is observed for the rate of dimerization at cooling rates between 5 and 7 C min À1 . At cooling rates greater than 5 to 7 C min À1 12HSA monomers do not effectively dimerize before being incorporated into the crystal lattice causing crystal imperfections impeding linear epitaxial crystal growth and produces branched fibers. At slow cooling rates (i.e., less than 5 to 7 C min À1 ), long fibers are produced with a fractal dimension between 0.95 and 1.05 and for rapid cooling rates (i.e., greater than 5 to 7 C min À1 ) short branched fibers are produced with a fractal dimension between 1.15 and 1.32.

Gelled Bicontinuous Microemulsions

“…This happens in the so-called 'non-permanent' or 'transient junction zones' of 12-HOA networks while the 'permanent junction zones' are branch points of the fibers which arise because of crystallographic mismatches during fiber growth [40]. Notably the shape and size of the gelator fibers and junction zones is very much dependent on the molecular structure of the gelator [29,41], on the type of the solvent in the gel [29,30,38,42], on the presence of additives [43] and on the cooling rate upon gel formation [42,44,45]. In 12-HOA gels one finds gelator fibers whose cross-section is, e.g., square when the solvent is benzene and rectangular in nitrobenzene where the 12-HOA strands are ribbon-shaped [38].…”

“…The latter is an important parameter defining the amplitude of the 44 2 Theoretical Background scattered neutron wave and thus the detected intensity. The scattering length density of, e.g., a molecule k equals the sum of the scattering lengths b i of all comprised atoms i divided by the volume of the molecule [78,79].…”

Gels with sense: supramolecular materials that respond to heat, light and sound

“…61 This theory was explored by Lam et al in a study of the gelation kinetics of 12-hydroxystearic acid in mineral oil under non-isothermal conditions. 74 Intriguingly, at cooling rates below 5 K/min, fibres in a 2.5 wt.% solution exhibit nearly constant persistent lengths of 90-100 μm and fractal dimensions of approximately 1.0, indicative of extremely limited branching. At higher cooling rates, however, the persistence length is reduced to 20-40 μm and branching becomes more prevalent, producing fractal dimensions in the range 1.1-1.…”

Rheological assessment of the sol–gel transition for self-assembling low molecular weight gelators

“…2A). Depending on the degree of undercooling, the number of nuclei will differ (Chambon & Winter, 1987;Lam et al, 2010). Once nucleation occurs, gelator molecules continue to diffuse to the crystal surface and accrete onto the growing crystal surface (Fig.…”

“…Low molecular weight gelators are unlike polymer gelators because they must first self-assemble prior to viscosity enhancement of the solution. The process of one-dimensional growth requires a meticulous balance between the contrasting parameters of solubility and those which control epitaxial growth, leading to an elongated axis (Lam, Pederson, Quaroni, & Rogers, 2010;Rogers & Marangoni, 2009;Suzuki et al, 2003). In specific cases, the supramolecular assemblies may form organogels which aggregate to form threedimensional networks entrapping the solvent phase.…”