Thomas Engels

Molecular dynamics simulations of stratum corneum lipid models: fatty acids and cholesterol

We report the results of an investigation on stratum corneum lipids, which present the main barrier of the skin. Molecular dynamics simulations, thermal analysis and FTIR measurements were applied. The primary objective of this work was to study the effect of cholesterol on skin structure and dynamics. Two molecular models were constructed, a free fatty acid bilayer (stearic acid, palmitic acid) and a fatty acid/cholesterol mixture at a 1:1 molar ratio. Our simulations were performed at constant pressure and temperature on a nanosecond time scale. The resulting model structures were characterized by calculating surface areas per headgroup, conformational properties, atom densities and order parameters of the fatty acids. Analysis of the simulations indicates that the free fatty acid fraction of stratum corneum lipids stays in a highly ordered crystalline state at skin temperatures. The phase behavior is strongly influenced when cholesterol is added. Cholesterol smoothes the rigid phases of the fatty acids: the order of the hydrocarbon tails (mainly of the last eight bonds) is reduced, the area per molecule becomes larger, the fraction of trans dihedrals is lower and the hydrophobic thickness is reduced. The simulation results are in good agreement with our experimental data from FTIR analysis and NIR-FT Raman spectroscopy.

The influence of coemulsifier type on the stability of oil-in-water emulsions

Oil-in-water emulsions containing ethoxylated non-ionic surfactants may undergo a phase inversion to water-in-oil emulsions on heating. This temperature-induced phase inversion is utilized in the so-called phase inversion temperature (PIT) method for the preparation of finely dispersed bluish oil-in-water emulsions. PIT emulsions with polar oil components were prepared by adding mixtures of fatty alcohol ethoxylates with or without the coemulsifier glyceryl monostearate. In the phase inversion temperature range, microemulsion or liquid crystalline phases are formed depending on the type of emulsifier mixture used. The formation of a bicontinuous or lamellar structure is essential in order to obtain finely dispersed oil-in-water emulsions. In both phases the hydrophilic-lipophilic properties are balanced, which results in minimum interfacial tensions between oil and water phases. Fine dispersity, however, is not the only criterion for stable oil-in-water emulsions. Studies of different parameters of the emulsions give hints to the stabilization mechanism of the emulsifier/coemulsifier system. According to these results, low viscous PIT emulsions exhibit long-term storage stability, if the oil droplets are shielded against coalescence by a lamellar gel phase, which is imparted by the coemulsifier glyceryl monostearate.

Liquid crystalline surfactant phases in chemical applications

The importance of lyotropic liquid crystalline structures is shown for two selected surfactant applications,i.e. cosmetics and detergency. The properties of lyotropic liquid crystals are demonstrated for binary surfactant–water systems, ternary surfactant–oil–water systems and multicomponent systems. Detailed knowledge of the phase behavior is crucial for tailor-made product development.

MD-SIMULATION STUDY OF THE HYDROPHOBIC HYDRATION OF NONIONIC SURFACTANTS

Abstract The hydrophobic hydration of nonionic surfactants of general structure H(CH2)m(OCH2CH2)nOH (abbreviated as CmEn) as well as their constituents, namely small alkane and ether molecules, in dilute aqueous solutions have been investigated. By an extensive series of classical molecular dynamics simulations the temperature dependent association of surfactant molecules and hydrophobic test-particles are studied. The simulations were performed at constant ambient pressure conditions and temperatures between 275 and 450 K. The hydrophobic interaction has been probed by Widom’s particle insertion method. The temperature dependence of the association process can be described well by temperature independent enthalpies and entropies of transfer of the test particle from bulk to shell. These transfer properties can be reduced to group contributions. The resulting Gibbs free energy of transfer was used as a measure of the hydrophobicity and could be correlated with the experimental cloud point temperatures of binary aqueous mixtures of these surfactants. An empirical correction leads to a quantitative description of the experimental data.

Hydrophobic Aggregation of Nonionic Surfactants in Aqueous Solution: An MD Simulation Study

We have studied dilute aqueous solutions of nonionic surfactants of general structure H(CH2) m (OCH2CH2) n OH (abbreviated as C m E n ) by an extensive series of classical molecular dynamics simulations. The temperature dependent association of surfactant molecules and hydrophobic test—particles has been determined by Widom’s particle insertion method. The simulations were performed at constant ambient pressure conditions and temperatures between 275 K and 450 K. Our simulations suggest an entropy driven association process, which can be described well by temperature independent enthalpy and entropy contributions. These properties can be further reduced to group contributions. The resulting Gibbs free energy of transfer was used as a measure of the hydrophobicity and could be correlated with the experimental cloud point temperatures of binary aqueous mixtures of these surfactants. An empirical entropy correction leads to a quantitative description of the experimental data.

Lyotrope flüssigkristalline Phasen Anwendungen in Wasch-, Spül- und Reinigungsmitteln, in der Kosmetik und in technischen Prozessen

Flussigkristalline Phasen haben in den vergangenen Jahren ihren Weg in viele Anwendungen gefunden. Hierzu gehoren elektrooptische Anzeigen, Sensoren, optische Schalter und die Thermographie. Flussigkristalline Strukturen, die von amphiphilen Molekulen gebildet werden, sind die Grundlage fur Emulsionen und sind systematisch fur die Pharmazie, Kosmetik, Nahrungsmittelindustrie sowie chemische und olverarbeitende Industrie untersucht worden.