Michael A. Rogers

Solvent-Modulated Nucleation and Crystallization Kinetics of 12-Hydroxystearic Acid: A Nonisothermal Approach†

The solvent type strongly affects the nucleation behavior of 12HSA and therefore strongly influences the peak nucleation rate, fiber length, spatial distribution of mass, and degree of branching. Using nonisothermal kinetic models, a correlation was established among the activation energy of nucleation, fiber length, and induction time of 12HSA nucleation in different solvents. However, there was no correlation between any measurable parameter and solvent polarity. Activation energies varied from 2.2 kJ/mol in methyl oleate to 15.8 kJ/mol in glycerol. Nucleation behavior and structure were strong functions of the cooling rate, with distinct regions observed above and below 5-7 degrees C/min for fiber length, induction time, rate constant, and peak nucleation rate. The abrupt changes in the rate of nucleation, crystal growth rate constant, and degree of branching around this cooling rate are related to whether the nucleation and crystal growth processes are governed by mass transfer or thermodynamics. Furthermore, the Avrami equation accurately predicted several structural features of the fibrillar network such as fiber length and, to a lesser extent, induction time.

Engineering the oil binding capacity and crystallinity of self-assembled fibrillar networks of 12-hydroxystearic acid in edible oils

The crystallinity and oil binding capacity of 12-hydroxystearic acid (12HSA)-vegetable oil organogels was modified by changing the post-crystallization annealing temperature from 5 °C to 30 °C for 24 h. The gels stored at 5 °C had a highly branched crystalline structure with small uniform pores, as determined by cryo-scanning electron microscopy. Large T2proton relaxation peaks at 50 to 70 ms determined by pulse nuclear magnetic resonance (pNMR) suggested the presence of highly immobilized oil at 5 °C. When the gels were stored at 30 °C, longer fibers and a less branched network were observed. At 30 °C, the 12HSA networks crystallinity was enhanced with fewer inclusions of liquid oil as determined by pNMR. When the gels were stored at 30 °C, a significantly shorter T2 relaxation peak was observed. The increased crystallinity, at 30 °C, was attributed to a reduction in bulk supersaturation, resulting in a very high crystallographic mismatch nucleation barrier (ΔG*) which favored one-dimensional fiber growth. However, at a lower crystallization temperature of 5 °C, there is an increase in the supersaturation and hence the crystallographic mismatch barrier is significantly lower, increasing fiber tip branching. The nucleation-growth-branching-growth model for self-assembled fibrillar networks explains the differences in crystallinity, pore size and oil syneresis observed for the 12HSA-vegetable oil organogels. It was found that the gels stored at 30 °C syneresised 1.35 times faster than the gels stored at 5 °C. Furthermore, the change in the T2 relaxations and the ratio of the complex viscosity/pore radius were 1.35 and 1.30 respectively.

Harnessing Hansen solubility parameters to predict organogel formation

Hansen solubility parameters predict the capacity of molecular gels to form in a vast array of organic solvents. The prediction ability for 12-hydroxystearic acid is closely associated with the hydrogen-bonding Hansen solubility parameter (δh). Solvents with a hydrogen-bonding Hansen solubility parameter less than 4.7 MPa1/2 produce clear organogels, opaque organogel formed between 4.7 < δh < 5.1 MPa1/2 and solutions remained when the hydrogen-bonding Hansen solubility parameter is greater than 5.1 MPa1/2. Furthermore, the critical gelator concentration is linearly correlated with the hydrogen-bonding Hansen solubility parameter. Solvents with the same functional group, which varied only by chain length, have correlations between the static relative permittivity, Hansen solubility parameter, dispersive HSP, polar HSP and hydrogen-bonding HSP and the critical gelator concentration.

Oil organogels: the fat of the future?

Organogelation of edible oils using food-derived ceramides may represent a viable non-conventional strategy for structuring liquid oils into semisolid fats without the use of saturated or trans fats.

Structural basis for the yield stress in plastic disperse systems

The apparent yield stress is a fundamental rheological characteristic of a plastic material. In this work, we propose a model that relates the structure of a fractal particle network to its yield stress. The model predicts the yield stress to be a function of particle size, solid-liquid surface free energy, the amount of network material, and the fractal dimension of the network. The predicted yield stress corresponded to the stress at the limit of linearity in fat crystal networks. Moreover, a structural definition for the yield stress is proposed.

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

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−1 and 3200 cm−1 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 (ϕ). 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−1. At cooling rates greater than 5 to 7 °C min−1 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−1), 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−1) short branched fibers are produced with a fractal dimension between 1.15 and 1.32.

Comparing and correlating solubility parameters governing the self-assembly of molecular gels using 1,3:2,4-dibenzylidene sorbitol as the gelator.

Solvent properties play a central role in mediating the aggregation and self-assembly of molecular gelators and their growth into fibers. Numerous attempts have been made to correlate the solubility parameters of solvents and gelation abilities of molecular gelators, but a comprehensive comparison of the most important parameters has yet to appear. Here, the degree to which partition coefficients (log P), Henry’s law constants (HLC), dipole moments, static relative permittivities (εr), solvatochromic ET(30) parameters, Kamlet–Taft parameters (β, α, and π), Catalan’s solvatochromic parameters (SPP, SB, and SA), Hildebrand solubility parameters (δi), and Hansen solubility parameters (δp, δd, δh) and the associated Hansen distance (Rij) of 62 solvents (covering a wide range of properties) can be correlated with the self-assembly and gelation of 1,3:2,4-dibenzylidene sorbitol (DBS) gelation, a classic molecular gelator, is assessed systematically. The approach presented describes the basis for each of the parameters and how it can be applied. As such, it is an instructional blueprint for how to assess the appropriate type of solvent parameter for use with other molecular gelators as well as with molecules forming other types of self-assembled materials. The results also reveal several important insights into the factors favoring the gelation of solvents by DBS. The ability of a solvent to accept or donate a hydrogen bond is much more important than solvent polarity in determining whether mixtures with DBS become solutions, clear gels, or opaque gels. Thermodynamically derived parameters could not be correlated to the physical properties of the molecular gels unless they were dissected into their individual HSPs. The DBS solvent phases tend to cluster in regions of Hansen space and are highly influenced by the hydrogen-bonding HSP, δh. It is also found that the fate of this molecular gelator, unlike that of polymers, is influenced not only by the magnitude of the distance between the HSPs for DBS and the HSPs of the solvent, Rij, but also by the directionality of Rij: if the solvent has a larger hydrogen-bonding HSP (indicating stronger H-bonding) than that of the DBS, then clear gels are formed; opaque gels form when the solvent has a lower δh than does DBS.

Influence of solvent on the supramolecular architectures in molecular gels

Elucidating the molecular structures, responsible for promoting self-assembly of low-molecular weight organogelators (LMOG) into supramolecular fibers, has been an extensive area of study. Although this has been a fruitful endeavor, this study illustrates that the chemical nature of the solvent and solvent–gelator interactions are equally important. The nanostructure, microstructure and supramolecular structures, of 12HSA molecules gels, are all influenced by the chemical nature of the solvent, which correlate to the hydrogen-bonding Hansen solubility parameter (∂h). Depending on the solvent employed, the polymorphic form, arrangement of the carboxylic acid dimers, domain size, fiber morphology, microstructure, thermal properties and visual appearance of the gel all differ. Solvents that have δh 4.4 MPa1/2.

Influence of Positional Isomers on the Macroscale and Nanoscale Architectures of Aggregates of Racemic Hydroxyoctadecanoic Acids in Their Molecular Gel, Dispersion, and Solid States

Inter/intramolecular hydrogen bonding of a series of hydroxystearic acids (HSAs) are investigated. Self-assembly of molecular gels obtained from these fatty acids with isomeric hydroxyl groups is influenced by the position of the secondary hydroxyl group. 2-Hydroxystearic acid (2HSA) does not form a molecular dimer, as indicated by FT-IR, and growth along the secondary axis is inhibited because the secondary hydroxyl group is unable to form intermolecular H-bonds. As well, the XRD long spacing is shorter than the dimer length of hydroxystearic acid. 3-Hydroxystearic acid (3HSA) forms an acyclic dimer, and the hydroxyl groups are unable to hydrogen bond, preventing the crystal structure from growing along the secondary axis. Finally, isomers 6HSA, 8HSA, 10HSA, 12HSA, and 14HSA have similar XRD and FT-IR patterns, suggesting that these molecules all self-assemble in a similar fashion. The monomers form a carboxylic cyclic dimer, and the secondary hydroxyl group promotes growth along the secondary axis.

Multicomponent Hollow Tubules Formed Using Phytosterol and γ-Oryzanol-Based Compounds: An Understanding of Their Molecular Embrace

The formation kinetics of self-assembling tubules composed of phytosterol:gamma-oryzanol mixtures were investigated at the Canadian Light Source on the mid-IR beamline using synchrotron radiation and Fourier transform infrared spectroscopy (FT-IR). The Avrami model was fitted to the changing hydrogen bonding density occurring at 3450 cm(-1). The nucleation process was found to be highly dependent on the molecular structure of the phytosterol. The nucleation event for cholesterol:gamma-oryzanol was determined to be sporadic whereas 5alpha-cholestan-3beta-ol:gamma-oryzanol and beta-sitosterol:gamma-oryzanol underwent instantaneous nucleation. One-dimensional growth occurred for each phytosterol:gamma-oryzanol mixture and involved the evolution of highly specific intermolecular hydrogen bonds. More detailed studies on the cholesterol:gamma-oryzanol system indicated that the nucleation activation energy, determined from multiple rate constants, obtained using the Avrami model, was at a minimum when the two compounds were at a 1:1 weight ratio. This resulted in drastic differences to the microscopic structures and affected the macroscopic properties such as turbidity. The formation of the phytosterol:gamma-oryzanol complex was due to intermolecular hydrogen bonding, which was in agreement with the infrared spectroscopic evidence.