title: Combined effect of shearing and cooling rate on the rheology of organogels developed by selected gelators.
abstract: In this study we investigated the combined effect of shearing and cooling rate in the rheology of organogels developed in high oleic safflower oil by (R)-12-hydroxystearic acid (HSA), its primary amide derivative [(R)-12-hydroxyoctadecanamide, HOA], and the N-octadecyl derivative of HOA [(R)-N-octadecyl-12-hydroxyoctadecanamide, OHOA]. The experimental set up to develop the organogels involved: 1). The use of quiescent (0s-1) or shearing (300, 600, and 1200s-1) conditions during cooling the gelator solutions (2%) just until achieving the gelator's melting temperature (TM) in the vegetable oil, to then continuing the cooling under static conditions until achieving 15°C) The use of cooling rate protocols involving a constant cooling rate of 1°C/min (CR1) or 10°C/min (CR10) in the shearing and static stages, or variable cooling rates in each stage (i.e., VR1-10or VR10-1). The elasticity of the organogels (G') was measured while cooling under static conditions, once the systems achieved 15°C, and after 60min at this temperature. The rheological results obtained at 15°C showed a cooling rate and molecular weight-dependent effect of shearing on G'. We propose that the molecular relaxation time of gelator molecules, and its increase as molecular weight increases and as temperature decreases, plays an important role on the gelator's susceptibility to go through a shear induced crystallization process. Therefore, high molecular weight molecules like OHOA (551.97Da) would remain stretched by shearing longer times than HSA (300.49Da) and HOA (299.49Da). Thus, when shearing was applied while cooling at the higher cooling rate (i.e., CR10and VR10-1), the stretched OHOA molecules would lead to the development of mesophase precursors that upon further cooling under quiescent conditions, crystallize developing a well-structured organogel. In contrast, stretched low molecular weight molecules (i.e., HSA and HOA) with shorter relaxation time would dissolve back to the isotropic state during cooling. Additionally, the rheological results of HSA and HOA organogels suggested that the shear induced crystallization process might be dependent on the gelator polarity also. These results show that the application of shear and the extent of its application as temperature decreases until achieving TM, have important implication on the self-assembly of gelator molecules, and therefore in the organization and rheology of the three-dimensional crystal network of the organogel.
authors.affiliation: Facultad de Ciencias Químicas-CIEP, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava 6, Zona Universitaria, 78210 San Luis Potosí, Mexico.
authors.forename: A
authors.initials: A
authors.lastname: De la Peña-Gil
authors.affiliation: Facultad de Ciencias Químicas-CIEP, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava 6, Zona Universitaria, 78210 San Luis Potosí, Mexico.
authors.forename: F M
authors.initials: FM
authors.lastname: Álvarez-Mitre
authors.affiliation: Facultad de Ciencias Químicas-CIEP, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava 6, Zona Universitaria, 78210 San Luis Potosí, Mexico.
authors.forename: M M
authors.initials: MM
authors.lastname: González-Chávez
authors.affiliation: Facultad de Ciencias Químicas-CIEP, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava 6, Zona Universitaria, 78210 San Luis Potosí, Mexico.
authors.forename: M A
authors.initials: MA
authors.lastname: Charó-Alonso
authors.affiliation: Facultad de Ciencias Químicas-CIEP, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava 6, Zona Universitaria, 78210 San Luis Potosí, Mexico. Electronic address: toro@uaslp.mx.
authors.forename: J F
authors.initials: JF
authors.lastname: Toro-Vazquez
mesh_terms: D003080:Cold Temperature; D003460:Crystallization; D004548:Elasticity; D005524:Food Technology; D005782:Gels / Q000737:chemistry*; D008970:Molecular Weight; D044367:Phase Transition; D012212:Rheology; D012450:Safflower Oil / Q000737:chemistry; D013228:Stearates / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D005782:Gels; D013228:Stearates; D012450:Safflower Oil
keywords: Cooling rate; Hydroxystearic acid; Oleogels; Organogels; Rheology; Shear rate; Spherulites