Micellar Radius Research Articles

Gel Filtration Chromatographic Study on the Micelle Formation of Triton X-100

Abstract The aggregation properties of Triton X-100 are investigated by frontal gel (Sephadex G-200) filtration chromatography (GFC) at 25 °C and considered in the framework of the monomer–m-mer equilibrium on the basis of both asymptotic and plate theories. The analysis of GFC data by asymptotic theory yields an equilibrium constant of K=1.3×1068 (mmol dm−3)−138 and a micellar elution volume of Vm=15.30 cm3. By using this Vm value, we calculate a micellar hydrodynamic radius of 4.6 nm and the monomer concentration C1 for Triton X-100. Values of cmc=0.30 mmol dm−3 (UV at 245 nm) and 0.31 mmol dm−3 (UV at 265 nm and GFC) are determined and are very close to the cmc value theoretically calculated by using Phillips’ definition of the cmc. The C1 value above the cmc is larger than these cmc values by about 0.06 mmol dm−3. The derivative elution pattern at the trailing boundary exhibits two maxima and a minimum (Vmin), and indicates the formation of small micelles near the cmc. The concentration at Vmin on the elution curve, Cmin, is close to the cmc, as we have already predicted by both asymptotic and plate theories. The present work demonstrates the utility of these theories for analyzing the GFC pattern of self-associating systems and will serve to understand the actions of Triton X-100 on membrane bound proteins and on lipid bilayers.

Counterion association to aqueous sodium dodecyl sulfate micelles in the presence of organic additives

The effect of polar, nonpolar, and mixed polar-nonpolar additives on the counterion association, the structure, and the size of aqueous sodium dodecyl sulfate micelles has been examined by measuring the conductivity, the viscosity, and the electromotive force at 298.15 K. Octane does not affect the degree of counterion association, but the mobility of the micelles decreases. This is mainly due to an increased micellar volume. Small quantities of 1-butanol and 1-hexanol decrease the degree of counterion association. At higher hexanol molalities larger micellar aggregates, rod- or disk-like micelles are formed. Long-chained α,ω-alkanediols, 1,9-nonanediol and 1,10-decanediol, also reduce the degree of counterion association. The shorter diols apparently affect the micellar system mainly through their effect on the solvent. The effect of the mixed additives is intermediate to that of pure components. Changes in the aggregation number, the micellar radius, and the number of associated counterions have been estimated from the conductivity and electromotive force measurements. Addition of octane does not affect the degree of associated counterions, but the micellar aggregation number increases. Addition of hexanol lowers both the micellar aggregation number and the fraction of associated counterions.

Interpretation of positron lifetime spectra arising from micellar solutions: Determination of the ortho-positronium diffusion coefficient in the solvent

Positron lifetime spectra arising from micellar solutions of sodium dodecylsulphate (SDS) are interpreted in terms of a classical positronium diffusion model published earlier. Unlike the generally accepted assumptions, this model results in a non-exponential ortho-positronium ( o-Ps) lifetime density function. A new method is presented for the simultaneous fitting of this lifetime density function to independent lifetime spectra recorded under the same experimental conditions. Among the fit parameters D p, the diffusion coefficient of o-Ps in the solvent (i.e. heavy water) phase is studied in detail; a detailed error analysis for D p is also given. Provided that the mean aggregation number of SDS micelles is about 60, the published D p values show that the diffusion coefficient of o-Ps in (heavy) water at room temperature is lower than that of small ions and molecules and the Arrhenius plot indicates a strong o-Ps localization in the solvent. The hydrodynamic radius of o-Ps is calculated from the o-Ps and micellar diffusion coefficients and from the micellar radius; it is greater than that of small ions and molecules and this can be considered as an independent indirect proof for the existence of o-Ps bubble in the (heavy) water.

Total intensity and quasielastic light scattering studies on the association of amphiphilic drugs in aqueous electrolyte solutions

Total intensity and quasielastic light scattering (QELS) techniques have been used to examine the effect of electrolyte on the micellar properties of the antidepressant drug imipramine hydrochloride in aqueous solution. A progressive increase of aggregation number and micellar radius was observed with increase of sodium chloride concentration over the range 0.05 to 0.6 M. A marked decrease of the diffusion coefficient with increase of drug concentration at concentrations just in excess of the critical micelle concentration has been noted and discussed. QELS measurements of the drug trimetaphan camphorsulfonate, which exhibits continuous association, showed a complex variation of effective diffusion coefficient with drug and electrolyte concentration.

The effect of environmental conditions on the steric stabilization of casein micelles

In an attempt to characterize the steric stabilizing sheath around the casein micelles of bovine milk, photon correlation spectroscopy techniques have been used to measure the micellar radius on exposure to ethanolic buffers of varying pH, ionic strength and calcium concentration. It is shown that on exposure to alcohol, the stabilizing protein sheath undergoes dimensional collapse and that immediately prior to aggregation, a minimum or core radius is reached, characteristic of the diluting buffer conditions. Defining barrier thickness as the difference between the micellar radius in alcohol-free buffer and this minimum radius, the same linear relationship is observed between barrier thickness and the critical ethanol concentration required to reach the core radius and induce subsequent aggregation, whether those variations in barrier thickness were achieved by altering the pH, ionic strength or calcium level of the buffer. Considering the initial rate of response to added ethanol as a measure of barrier strength, it is observed that thicker barriers are weaker whereas thinner barriers are more resistant to collapse and hence intrinsically stronger. This paradox is qualitatively resolved by considering the stabilizing sheath to possess some of the characteristics of a weak or soft gel, whose rigidity or extent of cross-linking is influenced by the variations in buffer conditions.

Steric stabilization and casein micelle stability

Using photon correlation spectroscopy techniques to measure hydrodynamic size, it is found that the radius of casein micelles decreases on exposure to ethanol solutions at levels less than those required to induce aggregation and precipitation. Measurements of turbidity and serum protein levels show that such decreases in average radius are not the result of micellar dissociation or splitting. The decrease is also found to be reversible in that the micellar radius recovers its former value on reducing the ethanol level. It is suggested that the decrease is due to the conformational collapse of a sterically-stabilizing “hairy” layer on exposure to poor solvent conditions. The thickness of this “hairy” layer increases with micelle size and correlates positively with the concentration of ethanol required to induce aggregation of the casein micelles. The significance of the collapse of the “hairy” layer is discussed in the context of the theories of steric stabilization.

Microscopic diffusion model for calculating orthopositronium lifetimes in micellar solutions

A microscopic diffusion model was formulated to describe the orthopositronium (o-Ps) lifetime density function in micellar solutions with the following main assumptions: the o-Ps lifetime in the solvent is different from that in the micelles; o-Ps and micelles diffuse in the solvent; the low atomic density region of the micellar boundary serves as an effective o-Ps trap. A nonexponential o-Ps lifetime density function has been derived by approximately solving an o-Ps diffusion problem with randomly distributed spherical absorbers. The parameters of the function are the formation and annihilation probabilities of o-Ps in the solvent and in the micellar pseudo-phase, the diffusion coefficients of o-Ps and the micelles in the solvent, as well as the structural data of the micellar solution, viz. the surfactant concentration and the micellar aggregation number and radius. 44 references, 6 figures.

The micellar properties of the poly(oxyethylene) - poly(oxypropylene) copolymer Pluronic F127 in water and electrolyte solution

The micellar properties of the ABA poly(oxyethylene)-poly(oxypropylene) block copolymer Pluronic F127 have been examined in water and electrolyte solution by photon correlation spectroscopy, total intensity light scattering and viscometric techniques. The hydrodynamic micellar radius in water, as calculated from diffusion data at the critical micelle concentration, remained constant at 10.2 ± 0.1 nm, over the temperature range 35–45 °C. Viscometric studies, however, showed a progressive dehydration of the micelles with temperature increase and it is clear from total intensity light scattering that this dehydration is accompanied by a concomitant increase in anhydrous volume such that the overall hydrated volume is unaffected by temperature change. Increase of solution concentration above about 10 g · dl −1 over this temperature range caused a marked increase of mean micellar size and of the polydispersity of micellar sizes, indicative of the onset of gelation. The influence of temperature on micellar size and hydration was less pronounced in the presence of 0.5 mol-dm −3 sodium chloride and gelation commenced at higher solution concentrations. The relevance of these studies to the thermal gelation of this poloxamer is discussed.

Fractional ionization of cetyltrimethylammonium hydroxide micelles determined by dynamic light scattering

Using the method of dynamic light scattering we have measured the diffusion coefficients D of cetyltrimethylammonium hydroxide (CTAOH) micelles as a function of CTAOH and NaOH concentrations at 25°C for 0.01 ≤ [CTAOH] ≤ 0.05 M and 0.02 ≤ [NaOH] ≤ 0.5 M. Substantial enhancements in the diffusivity were observed at low [NaOH] due to the presence of strong intermicellar electrostatic repulsions. We obtained good theoretical fits to the D versus [CTAOH], [NaOH] data over the range 0.01 ≤ [NaOH] ≤ 0.5 M using linear interaction theory (including both thermodynamic and hydrodynamic corrections to D) with a DLVO-type pair interaction potential. From these fits we obtained estimates of the mean micellar hydrodynamic radius, R h ∼ 23.4 Å, and the micellar fractional ionization, α ∼ 0.5.

Temperature dependences of micellar molecular weight and radius of nonionic surface active agent in the aqueous solution

The molecular weight and diffusion constant of poly(oxyethylene) dodecyl ether (C 12OE 9·3) micelles with an average of 9.3 oxyethylene group were determined from light-scattering measurements and free boundary experiments over a wide temperature range. It was found that micelles formed by C 12OE 9·3 were monodisperse in the temperature range studied. Plots of the logarithms of micellar molecular weight and micellar radii estimated from the diffusion constants against the temperature have a break point around 45°C. This temperature agrees with a theta point where the second virial coefficient obtained from light-scattering measurements is zero. Although the number of the molecules participating in micelle formation increases with temperature, the size of the micelle remains constant up to the temperature corresponding to the theta point, because the spherical micelle is packed with increasing tightness. The increase in the size and weight beyond that point may be attributed to a departure from spherical shape.

Study of a Phase Transition in a Lyotropic Liquid Crystal of Potassium Laurate/KCI/ Water by X-ray Diffraction and Optical Microscopy

Abstract Type I lyomesophase of potassium laurate was studied by X-ray diffraction and optical microscopy techniques with temperature variation. For increasing temperatures from 22°C to about 54°C only a nematic phase (N) was observed. For temperatures between 60°C and 74°C it was observed coexistence of an isotropic (I) and hexagonal (H α) phase with lattice parameter (47 ± 1) A. For temperatures between 54°C and 60°C, in some experiments, it was observed the coexistence of phases N and H α. A binary system of potassium laurate/water (H α phase) was also studied by both techniques at room temperature; the lattice parameter is (44.9 ± 0.5) A. Possible models for the N phase are discussed and compared with the obtained lattice parameters and associated micellar radius.

Effects of taurocholate on the size of mixed lipid micelles and their associations with pigment and proteins in rat bile

Mixed lipid micelles were isolated from rat bile on taurocholate-equilibrated Sephadex G100 and G200 columns (5-60 mM) to study relationships between lipids and other constituents of bile. Phospholipid, cholesterol, and a bile salt peak co-eluted as mixed micelles at all taurocholate concentrations. The micellar radius, derived from the elution profile, increased progressively from approximately 1.6 nm to approximately 3.5 nm when the column taurocholate concentration was reduced from 40-60 mM to 5 mM (the physiological range for rat bile). Biliary bile pigment and bromsulphthalein, added in vivo, eluted as self-aggregates that were smaller than the lipid micelles. In contrast, on bromsulphthalein-equilibrated columns, unconjugated bromsulphthalein associated weakly with lipid micelles but this association accounted for less than 10% of the unconjugated dye in bile. No associations were found between lipid and proteins when SDS-polyacrylamide gel electrophoretic polypeptide patterns of column fractions were compared with the lipid elution profiles at different taurocholate concentrations. Two high molecular weight protein aggregates were demonstrated in bile (greater than 222,000 Mr) by Sephadex G200 chromatography. These studies provide a reliable estimate of rat bile lipid micelle size and suggest that bile pigment, bromsulphthalein and proteins do not form strong associations with biliary mixed micelles but exist in bile predominantly as self-aggregates.

Dynamic light scattering from cetyltrimethylammonium bromide micelles. Intermicellar interactions at low ionic strengths

Quasi-elastic light scattering measurements have been performed on aqueous solutions of CTAB (cetyltrimethylammonium bromide). Theoretic fits to the measured diffusion coefficients at low NaBr were obtained by using the generalized Stokes-Einstein equation expanded to first order in the surfactant concentration (including both thermodynamic and hydrodynamic terms), with a pair-interaction potential taken from Dlvo theory. Good fits are obtained at low NaBr concentrations by assuming a constant micellar radius for each temperature and assuming that the remaining parameters (aggregation number, fractional ionization, and Hamaker coefficient) are independent of temperature. This procedure yields an unambiguous value for the fractional ionization alpha for CTAB micelles: alpha = 0.22. The limits of validity of the theoretic fitting procedure (previously applied by Corti and Degiorgio to the system sodium n-dodecyl sulfate (SDS) + NaCl) are discussed. 65 references.

An investigation of the solubilization mechanism of sodium octanoate micelles by small-angle X-ray scattering

The micellar radii and the electron density of the polar and non polar regions in micelles have been determined by a small-angle X-ray scattering (SAXS) technique originally designed by Luzzati et al. and subsequently modified by Larsson. The model employed shows the micellar structure as consisting of a liquid hydrocarbon core and a polar shell comprising the ionic polar head groups plus the diffuse double layer of bound counterions. The shortcomings of the model are discussed in the light of a recent paper by Hayter & Penfold providing general support for the parameters obtained and the conclusions drawn.

Diffusion of interacting micelles: A general treatment at low salt concentrations

The micellar diffusivity, measured by quasielastic light scattering, is described by a simple phenomenological expression for a variety of surfactants over a range of surfactant concentrations and temperatures at low salt concentration where electrostatic interactions dominate. This universal expression contains the low order perturbations to the micellar diffusivity in the absence of interactions. The dominant correction scales with the ratio of surfactant to salt concentrations and results from repulsive electrostatic interactions, screened by the added salt ions. By fitting experimental data to this expression, we obtain the Stokes–Einstein micellar radius and a relative measure of the micellar ionization fraction.

Pressure effects on micellar size

Abstract The micellar radius of four cationic surfactant solutions, at high electrolyte concentration, has been determined as a function of increasing pressure and at several temperatures by quasi-elastic light-scattering techniques. The effective size of micelles may increase, decrease, or remain constant until the solubility limit is exceeded and phase separation occurs at high pressures. If the hydrodynamic radius is modified by pressure, the micelles will shrink at the lower temperatures, grow at the higher temperatures and shrink, then grow at intermediate temperatures. The pressure effect is most pronounced for cetyltrimethylammonium bromide and becomes progressively weaker as the alkyl chain is shortened from n = 16 to n = 14 and n = 12, as the electrolyte concentration is reduced and as the Br− counterion is replaced with Cl−.

Gel Filtration of Aqueous Sodium Dodecyl Sulfate

Abstract The tail analysis of gel filtration was applied to a sodium dodecyl sulfate (SDS) solution using Sephadex G-50. This method is more simple and useful than the frontal analysis method formerly reported. The relative elution rate of SDS micelle, Rm, increased with an increase in the SDS concentration above CMC, reached a maximum near 16 mmol/l and then decreased up to 100 mmol/l. The application of the Laurent-Killander equation gave a micellar radius of 17.6 Å at CMC and 11.7 Å at the maximum Rm, corresponding to aggregation numbers of 35 and 80, respectively. The apparent decrease of Rm observed above 20 mmol/l was due to an structural change of the Sephadex gel caused by the addition of SDS. The micellar weight was estimated referring to a substance of known molecular weight; it was found to be constant in the region from 15 to 100 mmol/l SDS. The results of gel filtration of the SDS solution using CPG-10 as a gel column were similar to the case of Sephadex G-50; a micellar weight and an aggregation number of 25000 and 88, respectively, in the constant Rm range were obtained.

An investigation of the size and structure of the micelles in sodium octanoate solutions by small-angle X-ray scattering

Micellar radii and the electron densities of the polar and nonpolar regions in micelles have been determined by small-angle X-ray scattering. This was done in aqueous solutions of sodium octanoate (NaC 8), in 1.2 M (=mole/liter) NaC 8 with added NaCl and in 1.2 M solutions of NaC 8 to which were added the solubilizates p-xylene, N,N-dimethyl aniline (DMA) and decanol. The micellar model developed by Luzzati, which describes the micelle in terms of different electron densities at different radii, was employed. Interpretation of the results according to this model results in a thick polar layer, indicating considerable hydration of the micelles and penetration of water between the hydrocarbon chains. Support for this interpretation is also given by the relatively low electron density in the polar layer. Solubilized decanol and DMA appears to be located in the polar region of the micelle, while p-xylene is mainly solubilized in the inner, nonpolar part. It was also shown that there is a slightly increased adsorption of counterions as their concentration in the solution increases.