Material and Methods
Material
Sunflower wax (6607L, Lot.nr. F1911020-001) (SFX), rice bran wax (2811,
Lot.nr. F1851015-001), carnauba wax (2442L, Lot.nr. F1806007-001),
beeswax (8108LM, Lot.nr. F1727017-001) (BWX) and candelilla wax (2039L,
Lot.nr. F1915044-001) were kindly supplied by Kahlwax GmbH & Co KG
(Trittau, Germany).
All WEs were obtained from Larodan AB (Solna, Sweden) with a purity
>99 %. WEs used were Palmityl Myristate (CN30: 16_14),
Palmityl Stearate (CN34: 16_18), Stearyl Palmitate (CN34: 18_16),
Myristyl Behenate (CN36: 14_22), Stearyl Stearate (CN36: 18_18),
Behenyl Myristate (CN36: 22_14), Arachidyl Stearate (CN38: 20_18),
Behenyl Arachidate (CN42: 22_20) and Behenyl Lignocerate (CN46:
22_24). Throughout the manuscript, the first number indicates the
carbon number of the FaOH residue, the second number the carbon number
of the FA residue.
MCT‑oil was purchased from Caesar & Lorentz GmbH (Hilden, Germany). The
FA composition of the TAGs was determined to be 56 % caprylic acid
(10:0) and 44 % capric acid (8:0) – determined by GC analysis of FA
methyl esters according to DGF method C‑VI 10a (00).
All materials were used without further purification or modification.
Methods
Oleogel preparation
The MCT‑oil was structured either by pure WEs (mono-ester gel) or binary
mixtures of WEs (mixed oleogel). The concentration of the structurant
was 10.0 % (w/w) for all oleogel systems. Besides the nine mono-ester
gels, four mixed oleogels (C30+C46; C30+C42; C36(18_18)+C38;
C36(14_22)+C36(22_14)) were each prepared at three different mixing
ratios (2:1; 1:1; 1:2).
Stock solutions, each with a total mass of 300 mg, were prepared in 1.5
mL screw neck vials. Weighing precision of components was ±0.00003 g.
Complete dissolution was ensured by heating the samples under agitation
(250 rpm) up to approximately 95 °C on a heating plate. To avoid any
changes in the gel composition, the samples for different analytical
methods were directly transferred to the respective sample configuration
while still liquid at high temperature.
Thermal properties (DSC)
All DSC measurements were performed in aluminum crucibles. An empty
aluminum pan was used as reference.
Pure wax ester sample sizes were at 5-6 mg, the oleogel samples had a
size of 8‑10 mg. Stabilized samples were produced by storing the
crucibles for 48 h at ambient temperature after having been melted at a
temperature of 105 °C and cooled to 20 °C in the DSC. The stabilized
samples were subjected to the following measurement protocol: The
melting of the stabilized samples was monitored by heating from 20 to
105 °C. After holding for five minutes, the samples were cooled down to
5 °C and kept at this temperature for 30 min. Subsequently, they were
heated to 105 °C again, kept isothermally for another five minutes and
cooled to 20 °C. All heating and cooling rates were 5 K/min. Samples
were undergoing this protocol fourfold.
The melting point temperature of the pure WE were determined from
multiple scans at different heating rates and extrapolation to a zero
scan rate . The molar heat of fusion of the pure WE was calculated
straightforwardly from the experimental heat of fusion and molecular
weight. For the gels, assigning temperatures is less well defined. In
this contribution, the onset temperature of the heating thermogram are
defined as gel-sol transition temperatures. This is done because at this
point, the solubility of WE in the oil is larger than the concentration
of the WE in the sample. The heat of fusion for the WE in the gels is
derived based on the assumption that the 10 % (w/w) dosage of WE is
completely solid in the sample. This is obviously wrong ignoring the
solubility of individual WE.
Viscoelastic behavior
(Rheology)
The viscoelastic behavior of all oleogel systems was studied on a
modular compact rheometer using a steel plate-plate geometry with
sandblasted surface (PP25‑S; d = 25 mm) to avoid slippage. A top and
base Peltier-system of the measurement geometry ensured accurate
temperature control.
Approximately 100 mg of the hot stock solution were positioned on the
center of the preheated (90 °C) plate. The measuring gap was 0.2 mm. For
standardization purposes, the samples were agitated under shear
(\(\dot{\gamma}\) = 10 s-1; t = 5 min) and then stabilized at quiescent
conditions. The samples were cooled down to 5 °C (5 K/min) and kept
isothermally for stabilization (t = 30 min), analogously to the DSC
procedure. To observe gel-sol transitions, the samples were heated to
90 °C (5 K/min). This was followed by crystallization, cooling to 5 °C
and isothermal stabilization as described above. On these samples, an
amplitude strain test ranging from 0.005 % to 100 % at an angular
viscosity of 10 rad/s was performed isothermally. Next, the sample was
heated to 90 °C (5 K/min) and the same procedure repeated. During all
measurements, an angular frequency of 10 rad/s was used and for
temperature and time tests, a strain of 0.005 % was applied. The gap
width was allowed to compensate changes in the sample density during the
test (FN = 0 N) and reset to 0.2 mm after each heating step. The
crystallization was performed in quadruple, the melting and the
amplitude sweep were performed in duplicate.
Microstructure
The samples for the bright field microscopic (BFM) images were prepared
by pouring a droplet of the hot stock solution on a pre-heated objective
slide and covering it with a cover slip. The slides were cooled down
carefully from 70 °C to 5 °C (0.8 K/min) in a constant climate chamber.
Before image analysis, the samples were stabilized (T = 5 °C; t = 12 h).
The micrographs were captured at ambient temperature.