Wolfgang von Rybinski

Alkyl Polyglycosides—Properties and Applications of a new Class of Surfactants

A new generation of nonionic surfactants that are widely applicable and simultaneously ecologically compatible has been developed in the alkyl polyglycosides. Their production from the renewable resources glucose and fatty alcohol and their ultimate biodegradation into carbon dioxide, water, and assimilated bacterial biomass is an example for a closed cycle. As a result of their physico-chemical properties and ecotoxicological evaluation, they have become very important as high-performance components of detergents and cosmetic preparations.

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.

Alkyl glycosides and polyglycosides

The past three years have seen significant progress in the understanding of the physical chemical properties of alkyl glycosides. Remarkable results concern the phase behavior, the oil/water interface, the microemulsions and the adsorption of this new surfactant type on solid surfaces. Interesting differences of alkyl glycosides compared with other nonionic surfactants have been shown. An increasing importance of mixtures of alkyl glycosides and other surfactants led to more systematic studies of the influence of electrolytes and surfactants on the properties of alkyl glycosides.

ALKYLPOLYGLYCOSIDE : EIGENSCHAFTEN UND ANWENDUNGEN EINER NEUEN TENSIDKLASSE

Eine neue Generation vielfaltig nutzbarer und gleichzeitig umweltvertraglicher nichtionischer Tenside sind die Alkylpolyglycoside. Ihre Herstellung aus den nachwachsenden Rohstoffen Glucose und Fettalkohol und ihr vollstandiger biologischer Abbau zu Wasser, CO2 und Biomasse sind ein Beispiel fur einen geschlossenen Stoffkreislauf. Aufgrund ihrer physikalisch-chemischen Eigenschaften sowie ihrer okotoxikologischen Bewertung haben sie mittlerweile eine grose Bedeutung als leistungsstarke Komponenten von Wasch-, Spul- und Reinigungsmitteln sowie von kosmetischen Erzeugnissen.

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.

Adsorption Isotherms of Cellulose-Based Polymers onto Cotton Fibers Determined by Means of a Direct Method of Fluorescence Spectroscopy

We present a novel method for the measurement of polymer adsorption on fibers by employing fluorescently labeled polymers. The method itself can be used for any compound that either shows fluorescence or can be labeled with a fluorescent dye, which renders it ubiquitously applicable for adsorption studies. The main advantage of the method is that the choice of adsorbent is not limited to flat surfaces, thereby allowing the investigation of fibrous and porous systems. As an example of high interest for application we determined the adsorption isotherms of various polysaccharide-based polymers with different charges and different substituents on cotton fibers. These experiments show that the extent of adsorption depends not only on the charge conditions but also very much on the specific interactions between the polymer and fiber. For instance, the cationic hydroxyethyl cellulose can become bound to an extent similar to that of the anionic alginate, while the anionic carboxymethyl cellulose of similar charge density adsorbs much less under these conditions. This shows that the adsorption of polymers depends subtly on the details of the interaction between the polymer and fiber but can be determined with good precision with our direct fluorescence method.

A – Physical Aspects of Cleaning Processes

This chapter focuses on the physical chemistry of cleaning processes in aqueous systems in a systematic way and provides more information about the physical chemical basics. The cleaning process can be divided into four different key factors: chemistry, mechanical action, thermal effects, and time. Depending on the substrate that has to be cleaned and the soil, one of these factors can be dominant or all factors can have similar influence. The physical chemistry of cleaning processes is influenced by effects at the interfaces and in the bulk phases. Interfacial phenomena are the basis for all the cleaning and washing processes. The effects range from the wetting of hard surfaces or fabrics and the dissolution of stains from hard surfaces or fabrics to the removal of ions from the washing liquor or the interaction of softeners with the fabric in the rinse cycle of a washing process. In addition to these interfacial effects, the bulk properties of the liquid system play an important role. The cleaning liquid can be either a homogeneous one-phase system or a dispersion of two or more immiscible phases, for example, a foam or an emulsion of two immiscible liquids. Also, viscosity or structures within the liquid may have a great impact.

The influence of polymers, surfactants and salt on the fine structure of cotton revealed by SANS.

Cotton is the most abundantly employed material for the production of fabrics. Therefore it is very interesting to know the influence of compounds commonly in contact with cotton during the washing process, on its mesoscopic structure. Small angle neutron scattering (SANS) is able to monitor structural changes in the range of 1 to a few 100 nm, making it a powerful tool to observe such changes. For that purpose we studied the change of fibre structure if exposed to low concentrations of polymer or surfactant, as they are relevant in the washing process. An interesting observation is that for the effectively available surface of cotton fibres their nanometric structure appears to be the central aspect. However, this local structure is only little affected by the presence of anionic surfactant. The same applies to a variety of polymers and only for the addition of a cationically modified cellulose a substantial increase of the thickness of the locally present rod-like structures by ~30% is observed.

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.