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 \(\theta_{t}=53\)