Introduction
Oleogelation is a structuring technique that has gained much interest in the past two decades as a promising alternative to produce healthy oleogels that resemble semi-solid fats. Oleogelation is the process of using particular molecules, the oleogelators (10% or less w/w), to create a three-dimensional (3-D) scaffolding that immobilizes edible liquid oil(s). The strategy used to disperse the oleogelator in the oil dictates if a direct or indirect method is needed . In adirect method, the network structure is formed by using an olegelator that is brought, together with the oil, to a sufficiently high temperature to be in a melted phase. As the mixture cools, the oleogelator crystallizes into a network that entraps the vegetable oil. Oleogelators that fall into this category are triacylglycerols, fatty acids, fatty alcohols, waxy esters, waxes, and monoglycerides. Theindirect method uses a pre-step in which the olegelator is dissolved in a solvent where the 3-D structure is created. The solvent is then removed, and the 3-D porous structure is used to absorb liquid oils. Several oleogelators have been reported in the literature with natural vegetable waxes, like sunflower, ricebran, candelilla, and beesewax being the most popular in the food industry . However, these studies have shown that the properties of the gels are very variable depending on the source of the wax and the type of oil used. Some authors attribute these differences to the presence of minor components in the oil and/or in the wax . The reality is that the waxes used as oleogelators have a heterogeneous chemical composition consisting of a mixture of esters, hydrocarbons, free fatty acids, and free alcohols . The roles that each of these components play in wax crystallization and therefore in the formation of oleogels is unknown.
Two molecules, triacontane (TC), a hydrocarbon and, behenyl lignocerate (BL) a wax ester, were studied in this work. The molecules were chosen for their simplicity of being mathematical modelled and for the purpose of starting with a simple simulation before moving into more complex molecules. These linear molecules were amongst those studied using elastic X-ray scattering as described in another paper Peyronel et al. (in review) In that paper, a theory was successfully used to predict some average molecular tilt angles in a monolayer when the molecules are in the solid state. The paper also explains why no crystals involving two or three types of molecules were observed, which is not covered here. The theory depends upon the assumption that, at the temperatures utilized, the molecules were constrained to be linear with zero trans-gauche twisting.
The energies used in elastic X-ray scattering enable a user to correctly state that X-ray scattering is a non-invasive technique that can help elucidate the atomic and molecular structure of crystalline materials . Hydrocarbons, MAGS, DAGS, TAGS have been extensively studied using the powder x-ray diffraction technique . In particular, the SAXS region is used to reveal information about the lamella thickness or monolayer (thickness of the layer made by the molecules) and the thickness of the crystal . By measuring the q-value position of a Bagg Peak and using\(L=\ \frac{2\ \pi}{q}\) the monolayer thickness (L) can be computed. Previous work done by our group on TC and BL produced q-values of 0.18 Å-1, and 0.12 Å-1 for TC, and BL respectively. These values resulted in monolayer thicknesses of 34.9 Å, and 52.4 Å for TC, and BL, respectively. Because the length of a TC hydrocarbon chain in its (fully-extended) all-trans state is 38.6 Å, TC monolayers comprising molecules perpendicular to the monolayer surface, are expected to exhibit a repeat distance of ~39.9 Å, corresponding to a q-value of 0.16 Å-1. The difference between 0.16 Å-1 and the experimental observation of 0.18 Å-1 cannot be accounted for by appealing to experimental confidence limits shown in another paper from Peyronel et al. (in review)
Therefore, the objective of this study was to use computer simulations using the Metropolis Monte Carlo (MMC) algorithm to calculate the average distance between the end groups in the a crystalline monolayer made by either TC or BL molecules. The atomic distribution in each molecule obtained throughout the MMC algorithm was used to understand if the molecules formed trans-gauche bond excitations (twists) when the molecule was in thermal equilibrium.
Competing interactions such as electrostatic interactions between glycerol moieties in triacylglycerol molecules, or between polar or charged head groups of phospholipid molecules in aqueous solutions, and the dispersion interactions between hydrocarbon chains, can contribute to the existence of tilted phases in which the chains exhibit a tilt angle, \(\theta_{t}\), with respect to the orientation of the layer interface. In n- alkanes there are no such competing interactions, only the dispersion interaction; hence it would appear that one can predict that such molecules in a solid state will exhibit\(\theta_{t}=0\). This is not in agreement with the prediction obtained from the theory presented by Peyronel et al. (in review) that, with the assumption of extended hydrocarbon chain rigidity such chains in their solid state exhibit free energetically-favorable local minimums for \(\theta_{t}=33\) and \(\theta_{t}=53\)