Thermodynamic and kinetic aspects of fat crystallization

Is universal, simple melting point prediction possible?

An investigation of the melting points of 520 organic 1:1 salts is presented with the aim of developing a universal, simple, physically well-founded prediction scheme. The general reliability and reproducibility of the recorded experimental data are discussed with respect to purity, phase behavior, disorder and thermal history of a given substance. Additionally, mistakes, systematic errors, or lack of conventions can lead to considerable differences in the experimental measurements. A rough error bar for the reproducibility of the melting points of organic salts of ±5 to ±15 °C can be assigned. With this restraint, we developed two simple, semiempirical, five- and nine-parameter schemes with easy-to-calculate quantities. With these, we could predict the melting temperature of a given organic salt in the temperature range of -25 to +300 °C with an average error of 33.5 °C and a relative error of 9.3%. All calculated quantities are assessed with the help of conventional DFT, COSMO and COSMO-RS calculations, and are currently implemented into the IL-Prop module of the upcoming version of COSMOtherm. These prediction schemes are suitable for high-throughput computational screening of substances in the context of "computer-aided synthesis". Therefore, they are valuable tools to find a compound with a suitable melting point before its first synthesis.

Development of a coarse-grained model for simulations of tridecanoin liquid-solid phase transitions

Novel coarse-grained models for molecular dynamics of tridecanoin melts are here proposed as results of a coarse-graining step procedure. The procedure is implemented to develop three coarse-grained models of increasing number of particle types from two to four. Force fields are computed by minimization of the deviations of appropriate distribution functions of the coarse-grained models from those of a reference atomistic one. Density, diffusivity and shear viscosity are computed by numerical simulation and compared with experimental values. The ability of each model to describe liquid-solid transitions is also analyzed. In particular, the model with four types of coarse-grained beads shows a transition from a liquid to a crystal phase.

Pressure-induced phase transitions in triacylglycerides

The melting point of triacylglycerides (TAGs) under atmospheric pressure depends on both the fatty acid composition and crystalline structure of the polymorphic state, which are influenced by the temperature treatment history of the TAG. In this contribution, the additional effect of high hydrostatic pressure is described. Samples were placed in a temperature-controlled cell and pressurized up to 450 MPa. The phase transition was investigated either by perpendicular light scattering and transmission or with a polarized-light microscope. The high-pressure polarized-light microscope allows a precise determination of the melting point. The investigated TAGs showed a significant nonlinear increase of the melting point with pressure. Light scattering and transmission were used to observe the phase change in the high-pressure cell. Similar to supercooling in temperature-induced phase transition, we found a dramatic increase of the delay time in our pressure-induced solidification. Even the dependency of this induction time on the control parameter pressure was similar to that in temperature-driven crystallization. We propose that different crystalline structures may be obtained by superpressuring instead of supercooling.

Evidence of critical cooling rates in the nonisothermal crystallization of triacylglycerols: a case for the existence and selection of growth modes of a lipid crystal network

The isoconversional method, a model-free analysis of the kinetics of liquid-solid transformations, was used to determine the effective activation energy of the nonisothermal crystallization of melts of pure and complex systems of triacylglycerols (TAGs). The method was applied to data from differential scanning calorimetry (DSC) measurements of the heat of crystallization of purified 1,3-dilauroyl-2-stearoyl-sn-glycerol (LSL) and commercially available cocoa butter melts. The method conclusively demonstrated the existence of specific growth modes and critical rates of cooling at specific degrees of conversion. The existence of critical rates suggests that the crystallization mechanism is composed of growth modes that can be effectively treated as mutually exclusive, each being predominant for one range of cooling rates and extent of conversion. Importantly, the data suggests that knowledge of the critical cooling rates at specific rates of conversion can be exploited to select preferred growth modes for lipid networks, with concomitant benefits of structural organization and resultant physical functionality. Differences in transport phenomena induced by different cooling rates suggest the existence of thresholds for particular growth mechanisms and help to explain the overall complexity of lipid crystallization. The results of this model-free analysis may be attributed to the relative importance of nucleation and growth at different stages of crystallization. A mechanistic explanation based on the competing effects of the thermodynamic driving force and limiting heat and transport phenomena is provided to explain the observed behavior. This work, furthermore, offers satisfactory explanations for the noted effect of cooling-rate-induced changes in the physical functionality of lipid networks.

Kinetics of silica nucleation on carboxyl- and amine-terminated surfaces: insights for biomineralization

An in situ, atomic force microscopy- (AFM-)-based experimental approach is developed to directly measure the kinetics of silica nucleation on model biosubstrates under chemical conditions that mimic natural biosilica deposition environments. Relative contributions of thermodynamic and kinetic drivers to surface nucleation are quantified by use of amine-, carboxyl-, and hybrid NH(3)(+)/COO(-)-terminated surfaces as surrogates for charged and ionizable groups on silica-mineralizing organic matrices. The data show that amine-terminated surfaces do not promote silica nucleation, whereas carboxyl and hybrid NH(3)(+)/COO(-) substrates are active for silica deposition. The rate of silica nucleation is approximately 18x faster on the hybrid substrates than on carboxylated surfaces, but the free energy barriers to cluster formation are similar on both surface types. These findings suggest that surface nucleation rates are more sensitive to kinetic drivers than previously believed and that cooperative interactions between oppositely charged surface species play important roles in directing the onset of silica nucleation. Further experiments to test the importance of these cooperative interactions with patterned NH(3)(+)/COO(-) substrates, and aminated surfaces with solution-borne anionic species, confirm that silica nucleation is most rapid when oppositely charged species are proximal. By documenting the synergy that occurs between surface groups during silica formation, these findings demonstrate a new type of emergent behavior underlying the ability of self-assembled molecular templates to direct mineral formation.

Effect of surfactant surface coverage on formation of solid lipid nanoparticles (SLN)

The effect of surfactant surface coverage on formation and stability of Tween 20 stabilized tripalmitin solid lipid nanoparticles (SLN) was investigated. A lipid phase (10% w/w tripalmitin) and an aqueous phase (2% w/w Tween 20, 10 mM phosphate buffer, pH 7) were heated to 75 degrees C and then homogenized using a microfluidizer. The resulting oil-in-water emulsion was kept at a temperature (37 degrees C) above the crystallization temperature of the tripalmitin to prevent solidification of emulsion droplets, and additional surfactant at various concentrations (0-5% w/w Tween 20) was added. Droplets were then cooled to 5 degrees C to initiate crystallization and stored at 20 degrees C for 24 h. Particle size and/or aggregation were examined visually and by light scattering, and crystallization behavior was examined by differential scanning calorimetry (DSC). Excess Tween 20 concentration remaining in the aqueous phase was measured by surface tensiometry. Emulsion droplets after homogenization had a mean particle diameter of 134.1+/-2.0 nm and a polydispersity index of 0.08+/-0.01. After cooling to 5 degrees C at low Tween 20 concentrations, SLN dispersions rapidly gelled due to aggregation of particles driven by hydrophobic attraction between insufficiently covered lipid crystal surfaces. Upon addition of 1-5% w/w Tween 20, SLN dispersions became increasingly stable. At low added Tween 20 concentration (<1% w/w) the SLN formed gels but only increased slightly at higher surfactant concentrations (>1% w/w). The Tween 20 concentration in the aqueous phase decreased after tripalmitin crystallization suggesting additional surfactant adsorption onto solid surfaces. At higher Tween 20 concentrations, SLN had increasingly complex crystal structures as evidenced by the appearance of additional thermal transition peaks in the DSC. The results suggest that surfactant coverage at the interface may influence crystal structure and stability of solid lipid nanoparticles via surface-mediated crystal growth.

The binary phase behavior of 1, 3-dipalmitoyl-2-stearoyl-sn-glycerol and 1, 2-dipalmitoyl-3-stearoyl-sn-glycerol

The binary phase behavior of purified 1, 3-dipalmitoyl-2-stearoyl-sn-glycerol (PSP) and 1, 2-dipalmitoyl-3-stearoyl-sn-glycerol (PPS) was investigated at a very slow (0.1 degrees C/min) and a relatively fast (3.0 degrees C/min) cooling rate. Mixtures with molar fractions of 0.1 increments were studied in terms of melting and crystallization, polymorphism, solid fat content (SFC), hardness and microstructure. Only the alpha-form of a double chain length (DCL) structure was detected for all mixtures in both experiments. The kinetic phase diagram, constructed using heating DSC thermograms, displayed two distinct behaviors separated by a singularity at the 0.5(PSP) composition: a eutectic in the X(PSP)< or =0.5 and a monotectic in the X(PSP)< or =0.5 concentration region. The singularity was attributed to the formation of a 1:1 (mol:mol) molecular compound. Apart from the segment from 0.0(PSP) to the eutectic point, X(E), the simulation of the liquidus line using a model based on the Hildebrand equation suggested that the molecular interactions are strong and tend to favor the formation of unlike pairs in the liquid state and that the miscibility is not significantly dependent on cooling rate. The kinetic effects are manifest in all measured properties, particularly dramatically in the X(PSP)< or =X(E) concentration region. An analysis of induction time as measured by pulse nuclear magnetic resonance (pNMR) showed that PPS retards crystal growth, an effect which can explain the peculiarity of this concentration region. At both cooling rates, fit of the SFC (%) versus time curves to a modified form of the Avrami model revealed two common growth modes for all the mixtures. The polarized light microscope (PLM) of the PSP-PPS mixtures revealed networks made of spherulitic crystallites of size, growth direction and boundaries that are varied and sensitive to composition and cooling rate. The change in the microstructure and final SFC (%), particularly noticeable at compositions close to the eutectic, explain in part the differences seen in relative hardness.

Modeling of the triglyceride-rich core in lipoprotein particles

Triglycerides are a major component of many important biological entities such as lipoproteins and lipid droplets. This work focuses on two common triglycerides, tripalmitin and triolein, which have been simulated through atomistic molecular dynamics at temperatures of 310 and 350 K for 300-700 ns. In these systems, both structural and dynamical properties have been characterized, paying particular attention to understanding the packing of triglyceride molecules and their molecular conformations. Additionally, we study the liquid-to-crystalline phase transition of tripalmitin through a temperature quench from the high-temperature isotropic liquid phase to 310 K, corresponding to a polymorphic, crystalline-like phase. The transition is characterized in detail through density, average molecular shape, and, in particular, the relevant order parameter describing the transition.

Influence of lipid physical state on the in vitro digestibility of emulsified lipids

The objective of this study was to investigate the influence of the physical state of emulsified lipids on their in vitro digestibility by pancreatic lipase. A 10 wt % tripalmitin oil-in-water emulsion stabilized by sodium dodecyl sulfate (0.9 wt % SDS) was prepared at a temperature (>70 degrees C) above the melting point of the lipid phase (T(m) approximately 60 degrees C). A portion of this emulsion was cooled to a temperature (0 degrees C for 15 min) well below the crystallization temperature of the emulsified lipid (T(c) approximately 22 degrees C) and then warmed to 37 degrees C so as to have completely solid lipid particles. Another portion of the emulsion was directly cooled from 70 to 37 degrees C (which is above the T(c)) to have completely liquid (supercooled) lipid particles. Pancreatic lipase (8 mg/mL) and bile extract (5.0 mg/mL) were then added to each emulsion at 37 degrees C, and the evolution of the particle charge, particle size, appearance, and free fatty acid release were measured over a period of 2 h. It was found that the rate and extent of lipid digestion were higher in the emulsion containing liquid particles but that appreciable lipid digestion still occurred in the emulsion containing solid particles (i.e., >35% lipid digestion after 2 h). These results may have important consequences for controlling the digestion rate of lipids or for developing solid lipid particle delivery systems for lipophilic functional components.