Micellar Diffusion Coefficients Research Articles

Dynamic Interfacial Behavior of Poly(oxyethylene) Lauryl Ether Based Surfactant Mixtures

The adsorption behavior of binary mixtures comprising nonionic surfactants at the air–water interface has been studied by bubble pressure tensiometry at concentrations above and below their critical micelle concentrations. Surfactants with the same hydrocarbon chains but different degree of ethoxylations were chosen as the components to understand their mixing behavior at equilibrium and dynamic conditions. At short times, the adsorption is found to be diffusion limited for individual components as well as for the mixtures, as predicted by the Ward and Tordai model. The effective diffusion coefficient of the monomers in the mixed state displays a dynamic synergism, consistent with the molecular thermodynamic model for dynamic surface tension. However, the equilibrium surface tension and micellar diffusion coefficient of the mixtures exhibit ideal behavior.

A Simple Approach to Calculate the Micelle Aggregation Numbers of Ionic Liquid-Based Surfactants: Electrochemical Behavior of Aggregate-Solubilized Ferrocene Studied by Microelectrode Voltammetry

Micellar diffusion coefficient (DMDC), micellar hydrodynamic radius (RH) and average aggregation numbers (Nagg) of 1-alquil-3- methylimidazolium chloride (CxMeImCl x = 12, 14, 16) surfactants in aqueous solution have been determined by using voltammetric information obtained with disk microelectrodes. Ferrocene was used as redox-active electrochemical probe. DMDC values determined by extrapolation of the plot of diffusion coefficient versus surfactant concentration to the critical micelle concentration were found to be in good agreement with the literature values, based on static light scattering and theoretical calculations.

Research on the Micellar Dispersal Behaviors in Mixed Surfactant System by Cyclic Voltammetry

Cyclic voltammetry method was used to investigate the dispersal behaviors of micelles in mixed solutions of cationic surfactant CTAB (cetyltrimethyl ammonium bromide) and anionic surfactant SDS (sodium dodecyl sulfate) at 313 K using ferrocene as the probe. Voltammetric behaviors of ferrocene on the electrode were reversible, indicating the diffusion of ferrocene in the mixtures of cationic and anionic surfactants was controlled by the microstructures of the mixtures. The results showed that the anionic micellar diffusion coefficient initially decreased and then increased with CTAB content due to the structural transformation of micelles. However, the cationic micellar diffusion coefficient curve appeared a monotonous decreasing trend with the increase of the CTAB content in the experimental range, and the diffusion coefficient of cationic micelle was smaller than that of anionic micelle.

Location of ethanol in sodium dodecyl sulfate aggregates

The hexagonal liquid crystalline phase of SDS (Sodium dodecyl sulfate)/H2O system changes into lamellar liquid crystal and the effective length of surfactant molecule d0/2 in the lamellar liquid crystal decreases with the addition of ethanol. The micellar aggregation number N of SDS decreases and the micellar diffusion coefficient increases with the added ethanol. Under a constant concentration of SDS, the molecule number ratio of ethanol to SDS in the micelle increases with the concentration of ethanol and even exceeds 10 when ethanol concentration is 1. 085 mol/L. All these results show that ethanol, even though a short chain alcohol and soluble in water, can partly exist in the interphase of the amphiphilic aggregates showing some properties of co-surfactant.

A general diffusion model for mass transfer in colloidal liquid aphron systems

A general diffusion model for solute transport in colloidal liquid aphron (CLA) systems is proposed in terms of diffusive mass transfer. The model incorporates five specific mass transfer resistances occurred at interfaces and in liquid phases. The experimental cases conducted in CLA systems are simulated successfully to verify the validity of numerical procedure and underlying assumptions. Both numerical results and experimental data show that apparent diffusion coefficient of CLA systems is far smaller than that in pure solvent systems. This can be well explained considering the role of surfactants, which cause the interfacial resistance and micelles (or reverse micelles) hindrance. The interfacial resistances of adsorbed surfactants and formed micelles (or reverse micelles) hindrance are estimated by the simplex method from the experimental data. It is shown that both interfacial transfer coefficient and micellar diffusion coefficient decrease with the increase of surfactants concentration.

Effects of KBr and n-decanol on the properties of cetyltrimethylammonium bromide micelles in aqueous solutions: A microelectrode voltammetric study

A platinum disc microelectrode was employed to measure the diffusion coefficient of cetyltrimethylammonium bromide, CTABr, aggregates in aqueous solution and in the presence of additives: KBr, n-decanol, and KBr + n-decanol. Other micellar parameters such as the hydrodynamic radii and aggregation numbers were calculated from the micellar diffusion coefficients. The data obtained were rationalized in terms of electrolyte-induced aggregate size increase and sphere-to-rod morphology change. In the presence of solubilized n-decanol, this transition occurred at a lower KBr concentration, confirming that the micelle surface charge density, hence its dimension, is strongly influenced by adsorption of the fatty alcohol.

Poly(ethylene glycol)-b-Poly(ε-caprolactone) and PEG-Phospholipid Form Stable Mixed Micelles in Aqueous Media

Novel mixed polymeric micelles formed from biocompatible polymers, poly(ethylene glycol)-b-poly(epsilon-caprolactone) (PEG(5000)-b-PCL(x)) and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-methoxy poly(ethylene glycol) (PEG-DSPE), possess small size and high thermodynamic stability, raising their potential as long circulating carriers in the context of delivery of antineoplastic and antibiotic drugs. Formation of mixed polymeric micelles was confirmed using size exclusion chromatography and 1H NMR NOESY. Steady-state fluorescence measurements revealed depressed critical micellar concentrations indicative of a cooperative interaction between component hydrophobic blocks, which was quantified using the pseudophase model for micellization. Steady-state fluorescence measurements indicated that the mixed polymeric micelle cores possess intermediate micropolarity and high microviscosity. Pulsed field gradient spin-echo measurements were used to characterize micellar diffusion coefficients, which agree well with those obtained using dynamic light scattering. NOE spectra suggested that the hydrophobic polymer segments from individual components are in close proximity, giving evidence for the formation of a relatively homogeneous core. Contrary to one-component PEG(5000)-b-PCL(x) micelles, the mixed polymeric micelles could incorporate clinically relevant levels of the poorly water soluble antibiotic, amphotericin B (AmB). AmB encapsulation and release studies revealed an interesting composition-dependent interaction of the drug with the mixed polymeric micelle core.

Micelle structures in aqueous solutions of glucose-based surfactants having an isoprenoid-type hydrophobic chain

Surfactant self-diffusion coefficients have been measured on a binary system of 1- O- β-3,7-dimethyloctyl- d-maltopyranoside ( β-Mal 2(Ger))/water and a mixed surfactant system of β-Mal 2(Ger)/1- O- β-3,7-dimethyloctyl- d-glucopyranoside ( β-Glc(Ger))/water at 25 °C. For comparison, measurements have also been made on 1- O- β-decyl- d-maltopyranoside ( β-Mal 2C 10)/water and β-Mal 2C 10/1- O- β-decyl- d-glucopyranoside ( β-GlcC 10)/water. The hydrodynamic radius of β-Mal 2(Ger) micelles obtained from the micellar diffusion coefficient is around 3 nm and nearly equal to that of β-GlcC 10 micelles within experimental error. In the mixed surfactant systems, the hydrodynamic radii for both systems increase with increasing X G (the mole fraction of β-Glc(Ger) or β-GlcC 10 in the total mixed solute) above X G ≅ 0.4 when the total surfactant concentration is kept constant at 2 wt%. The R H of β-Glc(Ger)/Mal 2(Ger) micelles increases more rapidly than β-GlcC 10/ β-Mal 2C 10 micelles, and then phase separation occurs at X G ≅ 0.65 . On the other hand, the R H of β-GlcC 10/ β-Mal 2C 10 micelles continues to increase until the phase separation occurs at X G ≅ 0.92 . Measurements have also been performed as a function of total surfactant concentration at constant X G (=0.6). The CMC of the β-Glc(Ger)/Mal 2(Ger) system is larger than that of the β-GlcC 10/ β-Mal 2C 10 system as expected from the results of the pure surfactant systems published previously. The R H increases with increasing total surfactant concentration for both systems. At higher concentrations, the R H of β-Glc(Ger)/Mal 2(Ger) micelles increases more rapidly than β-GlcC 10/ β-Mal 2C 10 micelles. These results can be explained by the fact that the geranyl and decyl chains have the same volume but different chain lengths.

Adsorption of Nonionic Surfactants on Cellulose Surfaces: Adsorbed Amounts and Kinetics

Kinetic and equilibrium aspects of three different poly(ethylene oxide) alkylethers (C12E5, C12E7, C14E7) near a flat cellulose surface are studied. The equilibrium adsorption isotherms look very similar for these surfactants, each showing three different regions with increasing surfactant concentration. At low surfactant content both the headgroup and the tail contribute to the adsorption. At higher surface concentrations, lateral attraction becomes prominent and leads to the formation of aggregates on the surface. The general shape of the isotherms and the magnitude of the adsorption resemble mostly those for hydrophilic surfaces, but both the ethylene oxide and the aliphatic segments determine affinity for the surface. The adsorption and desorption kinetics are strongly dependent on surfactant composition. At bulk concentrations below the CMC, the initial adsorption rate is attachment-controlled. Above the CMC, the micellar diffusion coefficient and the micellar dissociation rate play a crucial role. For the most hydrophilic surfactant, C12E7, both parameters are relatively large. In this case, the initial adsorption rate increases with increasing surfactant concentration, also above the CMC. For C12E5 and C14E7 there is no micellar contribution to the initial adsorption rate. The initial desorption kinetics are governed by monomer detachment from the surface aggregates. The desorption rate constants scale with the CMC, indicating an analogy between the surface aggregates and those formed in solution.

Cyclic voltammetry investigation of the mixed micelles of cationic surfactants with pluronic F68 and TritonX-100

The cyclic voltammetry (CV) measurements for the combinations of hexadecyltrimethylammonium bromide (HTAB), tetradecyltrimethylammonium bromide (TTAB), dimethylene bis(tetradecyldimethylammonium bromide) (14-2-14), and dimethylene bis(hexadecyldimethylammonium bromide) (16-2-16) with TritonX-100 (TX-100) and triblock polymer, Pluronic F68, have been carried out in the presence of 0.1 M KCl. The critical micelle concentrations of these binary mixtures over the whole mixing range have been determined by using electroactive probe 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). A variation in the various micellar parameters and diffusion coefficient of the electroactive probe indicates that the mixed micelle formation between the components of monomeric/dimeric and TX-100/F68 binary mixtures was synergistic in nature. The stronger synergistic interactions were observed for the cationic combinations with F68 rather than TX-100. The results have been discussed on the basis of reduction in the cationic polar head group repulsions by the intercalation of non-ionic surfactants in the mixed micelle.

Marangoni Effects in Liquid Jets of Non-Ionic Surfactants

The adsorption of nonionic surfactants in the CnE8 family at the air–water interface has been studied on the millisecond timescale in a free liquid jet. The amount of adsorbed surfactant was measured by ellipsometry. The rates of adsorption are compared with a diffusion-controlled adsorption model. In the case of C10E8, which is below its cmc, the monomer diffusion coefficient provides a good fit to the experimental data. For n = 12, 14, and 16, micelles control the mass transport. The best fit diffusion coefficients are close to, but not identical with, the literature values for the micellar diffusion coefficients. Laser Doppler velocimetry was used to measure the change in surface velocity arising from adsorption of the surfactant, for n = 12, 14, and 16. There was a qualitative correlation between the retardation of the surface velocity and the surface tension gradients.

Ultramicroelectrode Voltammetric Investigation of the Micellar Phase of Sodium Dodecyl Sulfate in Aqueous Solution

Abstract Ultramicroelectrode voltammetric measurements have been made on Sodium Dodecyl Sulfate (SDS) in aqueous solutions using K4Fe(CN)6 as the electroactive probe and H2SO4 as the supporting electrolyte. By extrapolation, the SDS micellar diffusion coefficient (D m), hydrodynamic radius (R m), aggregation number (n), and the molecular weight of SDS micellar in aqueous solutions at various surfactant concentrations are obtained. The partition coefficients, KD , of the electroactive probe between micelle and water continuous phase are also estimated. The results suggest that the probe is sensitive to the surfactant concentration and reflect the changes of the structure and the hydrophobic nature of micelles.

Counterions in Polymeric Amino Acid Based Surfactants: Effect on Physical Properties and Enantioselectivity

The influence of alkali metal counterions on micellar properties and chiral recognition of poly-N-undecenoyl-l-leucinate (poly-L-UL) surfactant was investigated by use of surface tensiometry, fluorescence spectroscopy, pulsed field gradient NMR (PFG-NMR), and micellar electrokinetic chromatography (MEKC). The larger counterions (Rb+ and Cs+) showed a dramatic increase in the critical micelle concentration. In addition, fluorescence and PFG-NMR experiments showed a general increase in micellar polarity and diffusion coefficients, respectively, with increasing counterion size. Fluorescence and PFG-NMR data were interpreted in terms of different micellar aggregates that form in the presence of the different counterions: Li+, Na+, K+, Rb+, and Cs+. Significant effects were observed in the presence of Rb+ and Cs+. A group of six chiral analytes were used to test the chiral selectivity (α), chiral resolutions (Rs), and retention factors (k‘) using the alkali metal counterions of poly-L-UL micelles in MEKC. In ...

Electrochemistry in true reverse micelles

In this communication we demonstrate for the first time the electrochemistry of a simple redox couple Fe(CN) 4/3− 6 in a true reverse micellar system (i.e. not a microemulsion). The reverse micelles were formed from aerosol-OT (AOT=sodium bis-2-ethylhexylsulfosuccinate) in isooctane with a [H 2O]/[AOT] ratio ( W) of 10 using the phase transfer technique in the absence of supporting electrolyte. Ferricyanide was introduced by dissolving in the aqueous phase prior to mixing with the AOT-containing isooctane phase. The reduction of Fe(CN) 3− 6 within the reverse micellar environment was carried out at a carbon fibre microelectrode using a two-electrode assembly with a porous-polymer-coated silver wire acting as the pseudo-reference electrode. Nernstian electrochemical responses were observed and the limiting currents were well defined from which micellar diffusion coefficients could be obtained.

Transport behavior of four bile salt micelles and cholesterol solubilized by their micelles across porous membrane

The transport behavior of bile salts (BSs) solubilizing cholesterol (Ch) or none across an artificial membrane was investigated for sodium salts of deoxycholic acid (NaDC), chenodeoxycholic acid (NaCDC), ursodeoxycholic acid (NaUDC) and cholic acid (NaC) in tetraborate–carbonate buffer solution at pH 10.0 and 37 °C. The study demonstrated that the surfactant properties such as critical micellization concentration (CMC) and micellar size or diffusion coefficient were determinable from the flux or permeability measurements. The comparison among the respective pure systems of BSs led to a conclusion that the micellar size was in the order of NaDC>NaCDC>NaUDC>NaC and determined CMC values were in agreement with those in literature. The magnitude of solubilizing power (capacity) of BS for Ch was found to decrease in the order of NaDC>NaCDC>NaC>NaUDC; this order is in accordance with that of the empirical hydrophobicity index. The hydrodynamic radii for the singly dispersed species and the micellar species of the respective BSs and of Ch-solubilizing micelles were estimated from the permeability data; the radii of the Ch-solubilizing micelles are approximately 12–15 A and interestingly, smaller than those of the respective BS alone micelles ranging from 14 to 22 A.

Determination of micellar self-diffusion coefficients by micellar electrokinetic chromatography.

A method is described by which the diffusion coefficients of electrically charged micelles can be determined using micellar electrokinetic chromatography (MEKC). The determination is based on a theory for the dispersion, at low electric field strengths, of analytes that are solubilized by only the micellar phase. The dispersion is represented by contributions from instrumental sources and from longitudinal diffusion. The latter depends on the micellar diffusion coefficient. The theory is used to determine the micellar diffusion coefficients of the surfactant sodium dodecyl sulfate in three solutions having different surfactant concentrations. These diffusion coefficients compare very favorably with those determined by diffusion-ordered two-dimensional NMR spectroscopy. An argument is presented justifying that the diffusion coefficients determined by MEKC are self-diffusion coefficients.

New assessments of dispersion in micellar electrokinetic chromatography

A recent theory for plate number N in micellar electrokinetic chromatography (MEKC) is reevaluated using more accurate measurements of micellar diffusion coefficients and instrumental contributions to dispersion. These measurements are determined using theory outlined here from the low-field dispersion of peaks of micellar markers. The theory for N accurately predicts the experimental N's of weakly and intermediately retained analytes over wide ranges of field strength and concentration of the surfactant, sodium dodecylsulfate. The departure from theory of experimental N's of highly retained species developed at high field strengths in buffers having high surfactant concentrations is shown by additional theory to be consistent with the radial variation of micellar electrophoretic mobility resulting from Joule heat. These calculations are the most accurate reported to date that reconcile with theory the observed high-field dispersion of highly retained neutral compounds in MEKC. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 479–489, 1998

New assessments of dispersion in micellar electrokinetic chromatography

A recent theory for plate number N in micellar electrokinetic chromatography (MEKC) is reevaluated using more accurate measurements of micellar diffusion coefficients and instrumental contributions to dispersion. These measurements are determined using theory outlined here from the low-field dispersion of peaks of micellar markers. The theory for N accurately predicts the experimental N's of weakly and intermediately retained analytes over wide ranges of field strength and concentration of the surfactant, sodium dodecylsulfate. The departure from theory of experimental N's of highly retained species developed at high field strengths in buffers having high surfactant concentrations is shown by additional theory to be consistent with the radial variation of micellar electrophoretic mobility resulting from Joule heat. These calculations are the most accurate reported to date that reconcile with theory the observed high-field dispersion of highly retained neutral compounds in MEKC. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 479–489, 1998

Drug Dissolution Into Micellar Solutions: Development of a Convective Diffusion Model and Comparison to the Film Equilibrium Model with Application to Surfactant-Facilitated Dissolution of Carbamazepine

The intrinsic dissolution rate of carbamazepine in solutions of sodium lauryl sulfate was measured to study the convective diffusion transport of drug-loaded micelles from a rotating disk. Alternative definitions for effective diffusivity and reaction factor are presented and compared with those commonly used for this type of transport problem. The conventional and alternative approaches are based on the same fundamental assumptions differing only in their interpretation of the diffusional boundary layer. For example, in this study it was observed that, above the cmc, a 2% w/v solution of sodium lauryl sulfate increased the dissolution rate approximately 6-fold and the solubility approximately 20-fold. This difference between the solubility and dissolution enhancement was attributed to the contribution to the total transport of both the enhanced solubility, a 20-fold increase, and the effective diffusivity of the drug–micelle complex, a 3-fold decrease, hence a net 6-fold increase in dissolution. The diffusivity of the drug-loaded micelle estimated from the dissolution data using the new definitions compared well with values determined by other methods (Dsm=8.4 × 10−7 cm2/s). On the basis of these results, the new definitions for the effective diffusivity and reaction factor offer a practical method for estimating micellar diffusion coefficients and predicting drug dissolution under the well-defined hydrodynamics of the rotating disk. It may also be possible to extend the application of these definitions to study the dissolution of water-insoluble drugs in other media, such as emulsions, to better understand drug dissolution under fed conditions in vivo.

Effect of Inorganic Additives on Solutions of Nonionic Surfactants: X. Micellar Properties

The study of the interaction between electrolytes and nonionic surfactants was extended to the micellar properties of octoxynol 9 at 25°C. Density, surface tension, viscosity, and static and dynamic light scattering were measured in water (where dilute octoxynol solutions had a cloud point of 64°C), in a salting-out electrolyte, 0.40 m Na 2SO 4 (cloud point lowering = 28°C), and in the salting-in electrolytes 1.80 m NaSCN (cloud point increase = 27°C) and 2.00 m Mg(NO 3) 2 (cloud point increase = 14°C). The intrinsic viscosity of octoxynol in Na 2SO 4 was twice as large as in water and NaSCN, the diffusion coefficient was 2.5 times smaller, the micellar molecular weight was 5 and 14 times larger than in water and NaSCN, respectively, and the micellar hydration was at least twice as large. The micellar properties in Mg(NO 3) 2 differed from those in the other three media: The partial specific volumes in the latter were independent of surfactant concentration and had comparable values, while decreasing significantly with increasing surfactant concentration in the former. The micellar molecular weights and aggregation numbers were nearly the same in Mg(NO 3) 2 and NaSCN and amounted to ≈ 1 3 of the corresponding values in water; the intrinsic viscosities were 98 and 83% of that in water, respectively, while the micellar diffusion coefficients at infinite dilution were 59 and 120%. The apparent diffusion coefficients in Mg(NO 3) 2 increased slightly with increasing surfactant concentration while decreasing in the other three media. Only in Mg(NO 3) 2 did the micellar hydration increase with increasing surfactant concentration. These observations point to micellar binding of Mg(NO 3) 2, presumably via complexation of Mg 2+ ions by the ether oxygens of the micellar polyoxyethylene moieties, which rendered the surfactant somewhat cationic.