Cooling Rate Effects on the Microstructure, Solid Content, and Rheological Properties of Organogels of Amides Derived from Stearic and (R)-12-Hydroxystearic Acid in Vegetable Oil

Abstract

Using safflower oil as the liquid phase, we investigated the organogelation properties of stearic acid (SA), (R)-12-hydroxystearic acid (HSA), and different primary and secondary amides synthesized from SA and HSA. The objective was to establish the relationship between the gelator's molecular structure, solid content, and gels' microstructure that determines the rheological properties of organogels developed at two cooling rates, 1 and 20 °C/min. The results showed that the presence of a 12-OH group in the gelator molecule makes its crystallization kinetics cooling rate dependent and modifies its crystallization behavior. Thus, SA crystallizes as large platelets, while HSA crystallizes as fibers forming gels with higher solid content, particularly at 20 °C/min. The addition to HSA of a primary or a secondary amide bonded with an alkyl group resulted in gelator molecules that crystallized as fibrillar spherulites at both cooling rates. Independent of the cooling rate, gels of HSA and its amide derivatives showed thixotropic behavior. The rheological properties of the amide's organogels depend on a balance between hydrogen-bonding sites and the alkyl chain length bonded to the amide group. However, it might also be associated with the effect that the gelators' molecular weight has on crystal growth and its consequence on fiber interpenetration among vicinal spherulites. These results were compared with those obtained with candelilla wax (CW), a well-known edible gelling additive used by the food industry. CW organogels had higher elasticity than HSA gels but lower than the gels formed by amides. Additionally, CW gels showed similar or even higher thixotropic behavior than HSA and the amide's gels. These remarkable rheological properties resulted from the microstructural organization of CW organogels. We concluded that microstructure has a more important role determining the organogels' rheology than the solid content. The fitting models developed to describe the organogels rheological behavior support this argument.

Natural Waxes as Oil Structurants

“…The formation of such crystalline particles by CLW is attributed to a high proportion of linear hydrocarbons (n-alkanes) as reported previously [15]. In addition, the formation of gels with higher elasticity at lower crystalline mass fraction is in agreement with previously reported studies on CLW-oil oleogels [6,16].…”

Relationship between helical aggregates and polymorphs in a 12-hydroxystearic acid gel: their thermal stability and formation kinetics

“…Recently, a different long spacing of 12-HSA was reported by some researchers. A different long spacing at different cooling rates was reported by Weiss et al by using safflower oil as a solvent [9]. However, their result is slightly different from ours.…”

“…Therefore, the sol-gel transition of the gel is thermally reversible, i.e., the crystalline aggregates melt at high temperatures, while they are formed by crystallization again upon cooling temperature. Structural morphology of the aggregates is significantly affected by thermal history in gelation [9] or solvents used [10]. Terech et al showed that the crosssectional morphology of the fibrillar aggregates had a variety of shapes such as rod-like aggregates with circular or rectangular cross section, lamella-like aggregates, or helical aggregates by different solvents [5,10].…”

Rheological Profiling of Organogels Prepared at Critical Gelling Concentrations of Natural Waxes in a Triacylglycerol Solvent

“…For example, systems where triacylglycerol solvents (vegetable oils) are gelled using natural components, have found potential applications in biorelated fields such as drug delivery, 5−7 cosmetics, 8−10 and foods. 11−13 Among various gelators explored for gelling vegetable oils, natural waxes are considered to be the most promising ones because of their excellent oil binding properties, 14 economical value (capable of gelling oils at a significantly lower mass fractions, w c ≪ 0.1), 15,16 and availability of a number of waxes approved for use in humans. 17,18 Moreover, the gels formed using waxes have interesting properties such as themoreversibility 19,20 and emulsion stabilization, 21−23 which further justifies their popularity as organogelators for vegetable oils.…”