Sterol surfactants: from synthesis to applications.

An overview is given of sterol surfactants, including raw material aspects, classification and synthesis routes, physico-chemical behaviour and applications in pharmaceuticals and cosmetics.

Five-component food-grade microemulsions: structural characterization by SANS.

In this paper we present the structural characterization of a five-component food-grade microemulsion containing Tween 80, R(+)-limonene, ethanol, glycerol, and water. Our main approach to investigating the microstructure of dense microemulsions, and how it can be influenced by the various components, was to employ small-angle neutron scattering and the new evaluation technique for dense, interacting systems, the Generalized Indirect Fourier Transformation. We started our investigation with the impact of glycerol and ethanol on Tween 80 micelles in water. We found that glycerol increases the aggregation number and withdraws the hydrating agents from the headgroup region of the surfactant, resulting in a higher packing density of molecules in a micelle at slightly increasing size. The same trend holds when the micelles are oil swollen and/or ethanol is present. Ethanol, on the other hand, redistributes mainly between water and the interface-headgroup region of the surfactant. Part of it replaces surfactant molecules in the micelles, which increases the available interface and results in a higher number of micelles with shrinking size. The same trend holds when the micelles are oil swollen and/or glycerol is present in the aqueous phase. We also investigated samples along the dilution of a mixture of surfactant and oil phase (R(+)-limonene and ethanol), which can be diluted with aqueous phase (mixture of water and glycerol) without the occurrence of phase separation. In some samples of this dilution most probably bicontinuous structures are present. To elucidate this point, we also employed dynamic light scattering, viscosity, and conductivity measurements.

Influence of foam structure on the release kinetics of volatiles from espresso coffee prior to consumption.

The relationship between the physical structure of espresso coffee foam, called crema, and the above-the-cup aroma release was studied. Espresso coffee samples were produced using the Nespresso extraction system. The samples were extracted with water with different levels of mineral content, which resulted in liquid phases with similar volatile profiles but foams with different structure properties. The structure parameters foam volume, foam drainage, and lamella film thickness at the foam surface were quantified using computer-assisted microscopic image analysis and a digital caliper. The above-the-cup volatile concentration was measured online by using PTR-MS and headspace sampling. A correlation study was done between crema structure parameters and above-the-cup volatile concentration. In the first 2.5 min after the start of the coffee extraction, the presence of foam induced an increase of concentration of selected volatile markers, independently if the crema was of high or low stability. At times longer than 2.5 min, the aroma marker concentration depends on both the stability of the crema and the volatility of the specific aroma compounds. Mechanisms of above-the-cup volatile release involved gas bubble stability, evaporation, and diffusion. It was concluded that after the initial aroma burst (during the first 2-3 min after the beginning of extraction), for the present sample space a crema of high stability provides a stronger aroma barrier over several minutes.

Monocomponent hexa- and dodecaethylene glycol succinyl-tocopherol esters: self-assembly structures, cellular uptake and sensitivity to enzyme hydrolysis.

We have chemically synthesized two water-soluble forms of tocopherol succinate linked via an ester bond to hexaethylene glycol and dodecaethylene glycol. The self-assembly structure of the former in water is vesicular, whereas the latter forms elongated micelles. We treated Caco-2 cells with these compounds in these physical forms, in addition to a mixed micelle form. The intact compounds were taken up into the cells, influenced by both the chain length and the physical structure. In addition, the tocopherol derivatives were also metabolized into tocopherol succinate and tocopherol inside the cell. The total hydrolysis and uptake into the cells was two-fold higher from tocopherol hexaethylene glycol succinate in the form of mixed micelles than in vesicular form as assessed by analyzing intracellular tocopherol and tocopherol succinate. The longer polyethylene glycol chain gave a higher intracellular tocopherol succinate/tocopherol ratio. The major intracellular esterase in Caco-2 cells is carboxyl esterase 1 (EC 3.1.1.1), and in silico modelling studies show that the position of docking and hence the site of hydrolysis is influenced by the chain length. The in silico prediction is consistent with the in vitro data, since a longer chain length is predicted to favour hydrolysis of the ester bond between the succinate and polyethylene glycol moieties.

Structure and dynamics of micelles and cubic phase structures with ethoxylated phytosterol surfactant in water.

The self-assembly of a sterol ethoxylate surfactant with 30 oxyethylene units in water was studied by 1H NMR self-diffusion measurements in a wide concentration range in the micellar region (0-25 wt %). The data showed that the surfactant aggregates do not interact by hard sphere interactions but rather a strong concentration dependence of the diffusion coefficient was noted which was explained by polymer scaling theory. In the cubic phase (30-65 wt %), the self-diffusion data from water, from surfactant, and from free polyoxyethylene suggest spherical micelles, although water diffusion was much restricted due to binding to the surfactant headgroup. From X-ray measurements in the cubic phase, the unit cell size was calculated, and together with surfactant self-diffusion measurements the exchange dynamics between free and aggregated surfactant was obtained.

Design of double emulsions by osmotic pressure tailoring.

A method was developed allowing in situ adjustment of water-in-oil-in-water double emulsion (W/O/W) morphologies by tailoring the osmotic pressure of the water phases. The control of internal droplet size is achieved by altering the chemical potential of the external and internal water phases by dissolving neutral linear polysaccharides of suitable molecular weights. As a consequence of the different chemical potentials in the two aqueous phases, transport of water takes place modifying the initial morphology of the double emulsion. Self-diffusion 1H nuclear magnetic resonance (1H NMR) was used to assess transport mechanisms of water in oil, while a numerical model was developed to predict the swelling/shrinking behavior of W/O/W double emulsions. The model was based on a two-step procedure in which the equilibrium size of a single internal water droplet was first predicted and then the results of the single droplet were extended to the entire double emulsion. The prediction of the equilibrium size of an internal droplet was derived by the equalization of the Laplace pressure with the osmotic pressure difference of the two aqueous phases, as modeled by mean-field theory. The double emulsion equilibrium morphologies were then predicted by upscaling the results of a single drop to the droplet size distribution of the internal W/O emulsion. Good agreement was found between the theoretical predictions and the measurement of double emulsion droplet size distribution. Therefore, the present model constitutes a valuable tool for in situ control of double emulsion morphology and enables new possible applications of these colloidal systems.

Low-density lipoprotein analysis in microchip capillary electrophoresis systems.

Due to the mounting evidence for altered lipoprotein and cholesterol-lipoprotein content in several disease states, there has been an increasing interest in analytical methods for lipoprotein profiling for diagnosis. The separation of low- and high-density lipoproteins (LDL and HDL, respectively) has been recently demonstrated using a microchip capillary electrophoresis (CE) system [1]. In contrast to this previous study, the present report demonstrates that LDL analysis can be performed in an uncoated glass microchannel. Moreover, by adding sodium dodecyl sulfate (SDS) to the sample at a concentration well below the critical micellar concentration prior to injection, the LDL peak undergoes a focusing effect and exhibits an apparent efficiency of 2.2 x 10(7) plates/m. Laser light scattering experiments demonstrate that the low concentration of SDS used does not significantly alter lipoprotein particle size distribution within the time course that the analysis is performed. It is thus hypothesized that SDS nondisruptively coats LDL particles. The peak sharpening effect, observed only when SDS is added solely to the sample, is probably due to a mobility gradient created between the sample and the running buffer. The chip-based method demonstrated here has the potential for rapid analysis and sensitive detection of different LDL forms of clinical relevance.