Structure and Interdigitation of Chain-Asymmetric Phosphatidylcholines and Milk Sphingomyelin in the Fluid Phase

We addressed the frequent occurrence of mixed-chain lipids in biological membranes and their impact on membrane structure by studying several chain-asymmetric phosphatidylcholines and the highly asymmetric milk sphingomyelin. Specifically, we report trans-membrane structures of the corresponding fluid lamellar phases using small-angle X-ray and neutron scattering, which were jointly analyzed in terms of a membrane composition-specific model, including a headgroup hydration shell. Focusing on terminal methyl groups at the bilayer center, we found a linear relation between hydrocarbon chain length mismatch and the methyl-overlap for phosphatidylcholines, and a non-negligible impact of the glycerol backbone-tilting, letting the sn1-chain penetrate deeper into the opposing leaflet by half a CH2 group. That is, penetration-depth differences due to the ester-linked hydrocarbons at the glycerol backbone, previously reported for gel phase structures, also extend to the more relevant physiological fluid phase, but are significantly reduced. Moreover, milk sphingomyelin was found to follow the same linear relationship suggesting a similar tilt of the sphingosine backbone. Complementarily performed molecular dynamics simulations revealed that there is always a part of the lipid tails bending back, even if there is a high interdigitation with the opposing chains. The extent of this back-bending was similar to that in chain symmetric bilayers. For both cases of adaptation to chain length mismatch, chain-asymmetry has a large impact on hydrocarbon chain ordering, inducing disorder in the longer of the two hydrocarbons.

Photoresponsive, switchable, pressure-sensitive adhesives: influence of UV intensity and hydrocarbon chain length of low molecular weight azobenzene compounds

A photoresponsive switchable pressure-sensitive adhesive (PSA) was fabricated with an azobenzene-containing polymer and low molecular weight compounds. Its adhesion force was activated/deactivated rapidly by UV/visible light irradiation.

Crystal Memory near Discontinuous Triacylglycerol Phase Transitions: Models, Metastable Regimes, and Critical Points

It is proposed that “crystal memory”, observed in a discontinuous solid-liquid phase transition of saturated triacylglycerol (TAG) molecules, is due to the coexistence of solid TAG crystalline phases and a liquid TAG phase, in a superheated metastable regime. Such a coexistence has been detected. Solid crystals can act as heterogeneous nuclei onto which molecules can condense as the temperature is lowered. We outlined a mathematical model, with a single phase transition, that shows how the time-temperature observations can be explained, makes predictions, and relates them to recent experimental data. A modified Vogel-Fulcher-Tammann (VFT) equation is used to predict time-temperature relations for the observation of “crystal memory” and to show boundaries beyond which “crystal memory” is not observed. A plot of the lifetime of a metastable state versus temperature, using the modified VFT equation, agrees with recent time-temperature data. The model can be falsified through its predictions: the model possesses a critical point and we outline a procedure describing how it could be observed by changing the hydrocarbon chain length. We make predictions about how thermodynamic functions will change as the critical point is reached and as the system enters a crossover regime. The model predicts that the phenomenon of “crystal memory” will not be observed unless the system is cooled from a superheated metastable regime associated with a discontinuous phase transition.

The Role of Surfactant Structure on the Development of a Sustainable and Effective Cutting Fluid for Machining Titanium Alloys

In cutting operations of titanium alloys, most of the problems are related to the high consumption of cutting tools due to excessive wear. An improvement of metalworking fluid (MWF) technology would increase the productivity, sustainability, and quality of machining processes by lubricating and cooling. In this research article, the authors varied the surfactant’s charge, the hydrocarbon chain length, and the ethoxylation degree. Surfactants were dispersed at 1.2 mM in water and trimethylolpropane oleate to produce water-based MWF. Infrared reflection absorption spectroscopy and total organic carbon analysis were used to study the influence of surfactant structure on the film forming ability of the emulsion and performance was studied on Ti6Al4V using tapping torque test. The results showed that by changing the molecular structure of the surfactant, it is possible to vary the affinity between the ester and the substrate and reach an optimal combination, which improves the formation of a tribofilm. The mixture with anionic surfactants has good tribology performance, while non-ionic surfactants shorten the tool’s life. Moreover, the increase in the hydrocarbon chain length and the number of ethoxylations of surfactants promotes the adhesion of ester onto the metal surface, improving the lubricity properties of environmentally friendly MWF.