Biomedical subjects

M E Leser

Publications and source records attributed to M E Leser.

12 recordsLinked to original sources

PubMed2006

Composite interfacial layers containing micro-size and nano-size particles.

Surface layers of micro- and nanoparticles at fluid/liquid interfaces in absence and presence of surfactants are of large importance in the process of re-discovering Pickering systems, i.e. emulsions and foams stabilized by particles. The surface pressure/area isotherms of such layers can provide information about the properties of the used particles (dimensions, interfacial contact angles), the structure of interfacial layers, the interactions between the particles as well as about relaxation processes within the layers. For a correct description of Pi-A isotherms of composite surface layers containing particles the significant difference in size of these particles to that of solvent and surfactant molecules should be taken into account. Corresponding equations can be derived on the basis of the two-dimensional solution theory. The gained equations provide satisfactory agreement with experimental data and predict realistic values for the area of particles at the interface. Also equations of state and of the dilational elasticity for composite surface layers containing particles can be obtained in the framework of the presented methodology.

Adsorption↗

PubMed2006

Reversibility and irreversibility of adsorption of surfactants and proteins at liquid interfaces.

It is shown experimentally that the desorption of sodium decyl sulphate from the liquid/air interface is purely diffusion controlled, while the desorption of higher surface active surfactants such as the non-ionic surfactants Triton X-100 and tridecyl dimethyl phosphine oxide obeys a mixed mechanism. The desorption kinetics of beta-lactoglobulin (BLG) and beta-casein is, however, determined by a barrier mechanism. From the analysis of the BLG and beta-casein desorption kinetics at different temperatures the activation energy of desorption is calculated. The values obtained are rather close to the free energy of adsorption. The theoretical model of desorption kinetics predicts that these two energetic parameters are similar if the adsorption activation energy is low. This explains why substances with a higher adsorption activity have a lower desorption rate. Adsorption kinetics studies for beta-casein with and without forced convection show the same equilibrium surface tension values. This leads to the conclusion that the protein adsorption at liquid interfaces is thermodynamically reversible, although the slow desorption kinetics would allow to assume it to be an irreversible process.

Adsorption↗

PubMed2006

Surface-pressure isotherms of monolayers formed by microsize and nanosize particles.

The thermodynamic model of a 2D solution developed earlier for protein monolayers at liquid interfaces is generalized for monolayers composed of micro- and nanoparticles. Surface pressure isotherms of particle monolayers published in the literature for a wide range of particles sizes (between 75 microm and 7.5 nm) are described by the theoretical model with one modification. The calculations of surface pressure pi on area A provide satisfactory agreement with the experimental data. The theory also yields reasonable cross-sectional area values of the solvent molecule water in the range between 0.12 and 0.18 nm2, which is almost independent of particle size. Also, the area per particle in a closely packed monolayer obtained from the theory is quite realistic.

Membranes, Artificial↗

PubMed2006

Crystallography of dispersed liquid crystalline phases studied by cryo-transmission electron microscopy.

Low molecular weight surfactants, for example monoglycerides and phospholipids, form a multitude of self-assembled structures, such as inverted cubic or hexagonal mesophases, if brought into contact with water/oil. These mesophases can be dispersed in water using adequate surface-active materials such as low molecular weight surfactants or surface active polymers. In order to use such mesophase particles for incorporating drugs and aromas, it is essential to determine their internal crystallographic structure and to understand their mechanism of stabilization. Cryo-transmission electron microscopy was used to investigate the internal structure of different dispersed particles at various temperatures and oil contents. It is shown here that cryo-transmission electron microscopy, in combination with fast Fourier transform and tilting experiments, is effective in obtaining information on crystallographic structure, space group and morphology of particles with reversed bicontinuous cubic and hexagonal structures. In particular, using the presence or the absence of the {111} reflections and viewing the same particle under different axes of observation allows one to discriminate between the Im3m and Pn3m space groups. A major advantage of cryo-transmission electron microscopy is the ability to analyse single particles. This allows the identification of particles present at very low concentrations and the coexistence of particles with different internal self-assembly structures. With this technique we have obtained strong evidence for the presence of two cubic internal self-assembly structures with different space groups within the same dispersion. In addition, we found that cryo-transmission electron microscopy combined with tilting experiments enables the analysis of internal particle morphology, allowing the discussion of mechanisms for hexosome stabilization.

Journal Article↗

PubMed2005

Surface dilational rheology of mixed beta-lactoglobulin/surfactant layers at the air/water interface.

The general theoretical model by Garrett and Joos proposed in 1976 for the estimation of the dilational elasticity of mixed surfactant solutions, and also the theoretical model proposed by Joos for the limiting elasticity of such mixtures, demonstrate quite satisfactory agreement with experimental results obtained from the oscillating bubble shape method for mixtures of a nonionic surfactant and a protein, that is, beta-lactoglobuline and decyl dimethyl phosphine oxide, C10DMPO.

Air↗

PubMed2005

Kinetics of the desorption of surfactants and proteins from adsorption layers at the solution/air interface.

It is shown by experiments that the DeSNa desorption kinetics is governed by a pure diffusion mechanism, while the desorption of more surface active surfactants such as C13DMPO and Triton X-100 obeys a mixed mechanism. The BLG desorption kinetics, as shown by experiments, is determined by a barrier mechanism. From the analysis of the temperature dependence of the BLG desorption kinetics it is possible to calculate the activation energy of this process, which is quite close to the free energy of BLG adsorption. The theoretical model of desorption kinetics predicts that these two energetic parameters are approximately equal to each other if the adsorption activation energy is low. This can explain the fact that the higher the adsorption activity of a substance is, the lower is its desorption rate.

Adsorption↗

PubMed2004

Dynamic surface tension and adsorption kinetics of beta-casein at the solution/air interface.

A diffusion model is proposed to describe the adsorption kinetics of proteins at a liquid interface. The model is based on the simultaneous solution of the Ward-Tordai equation and a set of recently developed equations describing the equilibrium state of the adsorption layer: the adsorption isotherm, the surface layer equation of state, and the function of adsorption distribution over the states with different molar areas. The new kinetics model is compared with dynamic surface tensions of beta-casein solutions measured with the drop/bubble profile and maximum bubble pressure methods. The adsorption process for low concentrations is governed by the diffusion mechanism, while at large protein concentrations this is only the case in the initial stage. The effective diffusion coefficients agree fairly well with literature data. The adsorption values calculated from the dynamic surface tension data agree very well with the used equilibrium adsorption model.

Journal Article↗

PubMed2002

Effects of non-esterified stanols in a liquid emulsion on cholesterol absorption and synthesis in hypercholesterolemic men.

UNLABELLED: BACKGROUND Numerous studies have shown that dietary plant sterols (phytosterols and phytostanols) and their esters can decrease cholesterol absorption. However, few researchers have examined the effects of plant sterols on cholesterol absorption and synthesis using stable isotope tracers, instead of relying on endogenous pathway precursors. Further, we have worked with non-esterified lecithin-solubilized stanols as opposed to the more frequently studied esterified sterols and stanols. The vehicle was an oil-in-water liquid emulsion rather than the more common spread vehicle typically employed. AIM OF THE STUDY: To determine the effects of relatively low doses of lecithin-solubilized non-esterified stanols in liquid emulsions on cholesterol absorption and synthesis in mildly hypercholesterolemic subjects. METHODS: In a randomized, double blind crossover design, 12 mildly hypercholesterolemic men received either a free phytostanol supplement (3 g/d in 3 servings) or a control treatment for 3 days. Cholesterol endogenous synthesis rate was determined using the rate of incorporation of deuterium from body water into newly formed cholesterol molecules. Cholesterol absorption at the intestinal level was determined using the dual isotope method using 13C cholesterol injected intravenously and 180 cholesterol given orally. RESULTS: Cholesterol absorption was 55.7 +/- 6.5 % for the control and 33.5 +/- 5.3% for the phytostanol treatment. This massive reduction of the cholesterol absorption did not induce, on average, a difference in cholesterol endogenous synthesis which was measured at 0.074 +/- 0.0015 pool/d for plant sterols and 0.0736 +/- 0.0015 pool/d for controls (p > 0.05). CONCLUSIONS: The results demonstrated that lecithin-solubilized stanols administrated during a short period of time (3 days) in an oil-in-water emulsion can dramatically decrease cholesterol absorption, without a consistent, concomitant increase in synthesis, which is highly suggestive of effective LDL cholesterol lowering. The effects of synthesis should be verified in a longer study with more subjects.

Adult↗

PubMed2001

Improved oil solubilization in oil/water food grade microemulsions in the presence of polyols and ethanol.

Microemulsions based on five-component mixtures for food applications and improved oil solubilization have been studied. The compositions included water, oil phase [such as R(+)-limonene and medium-chain triglycerides (MCT)], short-chain alcohols (such as ethanol), polyols (propylene glycol and glycerol), and several surfactants and their corresponding mixtures (nonionic, such as ethoxylated sorbitan esters, polyglycerol esters, sugar ester, and anionic, such as phosphatidylcholine). The phase behavior of these systems is discussed with respect to the influence of polyols and short-chain alcohols on the degree of solubilization of oils in the aqueous phase. The alcohol and polyols modify the interfacial spontaneous curvature and the flexibility of the surfactant film, enhancing the oil solubilization capacity of the microemulsions. The solubilization of R(+)-limonene was dramatically improved in the presence of the alcohol and polyols, whereas the improvement of solubilization for triglycerides containing MCT was less pronounced. In some systems high oil solubilization was achieved, and some of them can be easily diluted to infinity both with the aqueous phase and with the oil phase. Viscosity measurements along selected dilution lines [characterized by a single continuous microemulsion region starting from a pseudo binary solution (surfactant/oil phase) to the microemulsion (water/polyol corner)] indicate that at a certain composition the system inverts from a W/O to an O/W microemulsion.

Emulsions↗

PubMed2000

Structured fluids as microreactors for flavor formation by the Maillard reaction.

Thermal reactions of cysteine/furfural and cysteine/ribose mixtures were studied in model systems to gain more insight into the influence of structured fluids such as L(2) microemulsions and cubic phases on the generation of aroma compounds. Formation of 2-furfurylthiol from cysteine/furfural was particularly efficient in L(2) microemulsions and cubic phases compared to aqueous systems. The reaction led to the formation of two new sulfur compounds, which were identified as 2-(2-furyl)thiazolidine and, tentatively, N-(2-mercaptovinyl)-2-(2-furyl)thiazolidine. Similarly, generation of 2-furfurylthiol and 2-methyl-3-furanthiol from cysteine/ribose mixtures was strongly enhanced in structured fluids. The cubic phase was shown to be even more efficient in flavor generation than the L(2) microemulsion. It was denoted "cubic catalyst" or "cubic selective microreactor". The obtained results are interpreted in terms of a surface and curvature control of the reactions defined by the structural properties of the formed surfactant associates.

Cysteine↗

PubMed2000

Some characteristics of sugar ester nonionic microemulsions in view of possible food applications.

This study explores some characteristics of microemulsions composed of sucrose monostearate (SMS), medium-chain triglycerides (MCT), or R-(+)-limonene, alcohols, and water. The systems are homogeneous, soft, and waxy solids at room temperature but liquefy and structure into homogeneous microemulsions when heated to >40 degrees C. The amount of solubilized water is enhanced as a function of the alcohol/oil ratio and is inversely proportional to the alcohol chain length. Over 60 wt % water can be solubilized in systems consisting of propanol/MCT/SMS at a weight ratio of 3:1:4 (initial weight ratio). These microemulsions are unique and differ from nonionic ethoxylated-based microemulsions in that their viscosity is very low and is reduced with increasing amounts of solubilized water. The electrical conductivity increases only slightly as a function of the water content and does not show typical bicontinuous or percolated behavior. The water in the core of the microemulsion strongly binds to the headgroups of the surfactant. Only at >15 wt % solubilization of water was free or bulk water detected in the core of the microemulsions. Such unique behavior of the core water might have a possible application in systems requiring monitoring of enzymatic (lipase) reactions carried out in the microemulsions as microreactors.

Cyclohexenes↗

PubMed1986

Application of reverse micelles for the extraction of proteins.

The solubilization of proteins in hydrocarbon solutions of reverse micelles is examined at the aim of establishing whether this process can be utilized for the separation and extraction of proteins. Two techniques of solubilization are considered, the phase transfer of proteins from an aqueous solution into a supernatant micellar solution and the direct extraction of the protein powder into the micellar solution. Basic questions concerning the influence of structural parameters of the proteins, as well as the influence of external parameters (pH, salt concentration) on the specificity of the solubilization process are discussed.

Colloids↗