Simultaneous control of electrostatic micellar partition and electroosmotic flow-rate by anion-dominated partition into zwitterionic micelles

The zeta potentials of zwitterionic micelles and capillary walls have been evaluated with capillary electrophoresis. The zeta potential of the micelles is predominantly determined by the nature of anions, while cations of identical valence have marginal effects; the linear relation has been found between the induced zeta potential and the hydration energy of an anion. The zeta potential of the capillary wall is also varied with anionic natures, and there is a good correlation between micellar and capillary wall zeta potential. This strongly suggests that the zeta potential of capillary walls is determined by the partition of anions into the surfactant layer formed on the capillary wall. Thus, we can simultaneously control both the electroosmotic flow-rate and micellar surface potential (in turn electrostatic interaction between micelles and ionic solutes) by varying the type and concentration of electrolytes. This idea has been applied to the separation of aromatic cationic solutes.     

ENERGY TRANSFER FROM MICELLAR SODIUM PHENYLUNDECANOATE TO SOLUBILIZED NAPHTHALENE

Abstract— Excitation energy transfer from the phenyl groups of surface active phenylundecanoate ions to naphthalene molecules has been studied under conditions such that the naphthalene molecules have been solubilized by micelles of phenylundecanoate. From measurements of the naphthalene fluorescence intensity in solutions of varying surfactant concentration the critical micelle concentration has been determined as 0·0091 M. The product of the micellar aggregation number and the efficiency of energy transfer has been obtained as 75 from measurements of both the sensitized naphthalene fluorescence and the quenching of the phenylgroup fluorescence. In the evaluation of the experimental data it has been assumed that the partition of naphthalene between the micelles and the aqueous phase may be treated as a distribution equilibrium, and that the solubilized naphthalene molecules are partitioned among the micelles according to a Poisson distribution. With this model, the naphthalene fluorescence intensities may be accounted for over the whole range of surfactant concentrations.
At high naphthalene concentrations, emission from naphthalene excimers has been observed. The possibility of self-quenching via excimer formation is considered.
The experimental results point to a quantum efficiency near unity for the transfer of excitation energy from the phenyl groups of the surfactant ions that form a micelle to a single solubilized naphthalene molecule. The high efficiency suggests that the naphthalene molecule and the phenyl groups are present inside the micelles.     

Zeta potential of dodecyltrimethylammonium hydroxide micelles in water

 The electrophoretical mobility of dodecyltrimethyl-ammonium hydroxide micelles has been measured at two different concentrations giving values similar to that determined in other surfac-tants. There is a good agreement between micelle ionization degrees computed from zeta potential measurements and those from ion-selective electrodes experiments. This demonstrates that electrophoresis experiments may be replaced by the simpler ion-selective electrode measurements to determine micelle surface potential. It has also been concluded that ion-selective electrodes detect only the non-micellised ions, that only free ions contribute to the intermicellar solution ionic strength, and micelles do not affect the result, and that the dependence of the electrophoretic mobility on the soap concentration is due to the reduction of the micelle net charge when the ionic strength of the intermicellar solution arises. Received: 2 December 1996 Accepted: 24 February 1997     

Dielectric relaxation spectroscopy of aqueous micellar electrolyte solutions: A novel application to infer Dukhin number and zeta potential of a micelle

The complex permittivities of aqueous SDS solutions, with and without the addition of sodium chloride (NaCl), are measured in the frequency range from 200 MHz to 14 GHz. The SDS concentrations are chosen such that the SDS molecules aggregate to micelles. In this frequency range, the measured spectra allow for the identification of two different relaxation processes. That is, the relaxation of the water molecules at frequencies above 1 GHz and the micellar relaxation at frequencies lower than 1 GHz. It is found that the addition of NaCl to the system mostly affects the micellar relaxation process. In detail, the time constant as well as the amplitude of the relaxation decrease by adding NaCl. These effects are attributed to the change in the solution conductivity that changes the properties of the micelle's electrical double layer. We also extract the Dukhin number of the micelles as a function of surfactant and electrolyte content from the measurements. The Dukhin number is a dimensionless group that describes the influence of the surface conductivity on a phenomena. A regression between Dukhin numbers and free sodium ions is found so that all data collapses on a single curve independent of the surfactant concentration. The surface conductivity is a manifestation of the electrical double layer and we use the Bikerman equation to infer the zeta potential of the micelles. Comparison to literature data shows very good agreement and proves that dielectric relaxation spectroscopy can be engaged to infer the zeta potential of micelles. Abbreviations: CMC critical micelle concentration, DRS dielectric relaxation spectroscopy, EDL electrical double layer     

In-situ polymerization at the interfaces of micelles: a "grafting from" method to prepare micelles with mixed coronal chains

Herein we describe a new strategy for producing micelles with mixed coronal chains. This method involves attachment of an atom transfer radical polymerization (ATRP) initiator at the interface of a micelle and preparation of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes at the interface by a "grafting from" method. Poly(ethylene glycol)- block-polystyrene (PEG- b-PS) diblock copolymer was achieved by ATRP. After the sulfonation reaction PS blocks were partly sulfonated. In aqueous solution at low pH the sulfonated block copolymer self-assembled into micelles with PS cores and PEG coronae and sodium 4-styrenesulfonate groups were distributed at the interfaces of the micelles. An ATRP initiator consisting of a quaternary ammonium salt moiety and a 2-bromo-2-methyl propionate moiety was ion exchanged onto the interface of the micelle. ATRP of DMAEMA was initiated at the interface, and micelles with PEG/PDMAEMA mixed coronal chains were prepared by ATRP. The structures of the micelles were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and zeta potential measurements. The size and morphology of the micelles were controlled by pH in aqueous solution. At high pH, PDMAEMA brushes collapse, forming nanodomains on the surface of the micelles. PDMAEMA brushes in the coronae of the micelles could be used as a template for preparation of gold nanoparticles.     

Gas–liquid mass transfer in the presence of ionic surfactant: effect of counter‐ions and interfacial turbulence

This work investigated the effect of counter‐ions and interfacial turbulence on oxygen transfer from gas to liquid phase containing ionic surfactant, and experiments were performed in a mechanically stirred reactor with flat gas–liquid interface. Counter‐ions in terms of hydration ability and polarizability influence the interfacial coverage of ionic surfactants (i.e. cetytrimethylammonium bromide (CTAB) and cetytrimethylammonium chloride) with the same hydrocarbon chain length, producing hindrance but in different extent on oxygen transfer. The addition of electrolyte (NH₄Br) substantially reduced the interfacial tension and surface charge of micelles (zeta potential) in CTAB system, and this salt effect greatly compressed interfacial double layer leading to gas transfer inhibition. The surface charge, aggregation number as well as stability of micelles formed above the critical micelle concentration could also alter interfacial configuration of surfactant layer reflected by gas absorption rate. Liquid turbulence was analyzed to decide the role of surfactant present in water on gas–liquid mass transfer, since Marangoni instability effect playing positive role should be taken into consideration under moderate liquid flow, while in turbulent system, contribution of Marangoni effect became overshadowed and consequently surfactant pose ‘barrier’ effect on gas transfer due to its surface active nature. Copyright © 2013 John Wiley & Sons, Ltd.     

The gibbs energy,chemical potential,and state parameters of the surface of an ionic micelle in the form of an ellipsoid of revolution

A molecular-thermodynamic model of an ionic micelle in the form of an ellipsoid of revolution was suggested. Equations for the chemical potential of an ellipsoidal micelle, the work of transfer of ions from solution volume into the micelle, and the state parameters of the surface of the micelle (fraction of free hydrophobic surface, surface tension coefficient, and surface charge density) were obtained. The Gibbs energy, chemical potential, work of transfer, and state parameters of an ellipsoidal micelle were determined as depending on the form factor of the corresponding ellipsoid. The model was verified for the example of sodium decyl sulfate micelles. The calculated parameters were close to the thermochemical data on the asymmetrization of spherical micelles in a solution of sodium decyl sulfate. The equilibrium ion-micellar solution composition was estimated.     

Thermosensitive pluronic micelles stabilized by shell cross-linking with gold nanoparticles

Gold nanoparticles were employed to prepare shell cross-linked Pluronic micelles that exhibit a reversibly thermosensitive swelling/shrinking behavior. Two terminal hydroxyl groups of Pluronic F127 were thiol-functionalized to form self-assembling Pluronic micelles in aqueous solution with exposed -SH groups in an outer shell layer. The thiol groups present in the outer shell were cross-linked by gold nanoparticles synthesized through NaBH4 reduction of gold precursor anions. The resultant shell cross-linked gold-Pluronic micelles exhibited a temperature-dependent volume transition: their hydrodynamic diameter was changed from 157.1 +/- 15.6 nm at 15 degrees C to 53.4 +/- 5.5 nm at 37 degrees C as determined by dynamic light scattering. The critical micelle temperature measured by a pyrene solubilization technique suggested that the reversible swelling/shrinking behavior of the micelles was caused by hydrophobic interactions of cross-linked or grafted Pluronic copolymer chains in the micelle structure with increasing temperature. Transmission electron microscopy directly revealed that the shell cross-linked micelles were indeed produced by gold nanoparticles covalently clustered on the surface. These novel self-assembled organic/inorganic hybrid micelles would hold great potential for diagnostic and therapeutic applications.     

Evaluation of surfactant ion activity in micellar solutions of dodecylpyridinium chloride from partition data and counter ion activity measurements

The partition isotherm of dodecylpyridinium chloride in the water-1,2-dichloroethane system, measured over a broad concentration range, served for the evaluation of the critical micelle concentration and of the effective concentration and mean activity of the surfactant ions in the micellar concentration range. On the basis of the charged phase separation model for the micelles and EMF measurements of counter-ion activity, the surfactant cation activity and the apparent degree of counter ion association to the micelle () of 0.69 were estimated. The results indicate a gradual decrease of surfactant cation activity with increasing total concentration in the micellar range and support literature data derived from EMF measurements that apply surfactant selective plastic exchange membrane electrodes.     

Incorporation of block copolymer micelles into multilayer films for use as nanodelivery systems

This work demonstrates the potential application of stimulus responsive block copolymer micelles as triggerable delivery systems for use within multilayer films. Cationic, pH-responsive micelles of poly[2-(dimethylamino)ethyl methacrylate-block-poly(2-(diethylamino)ethyl methacrylate)] (PDMA-PDEA) were deposited on anionic polystyrene latex particles. The charge reversal of the surface and the amount of adsorbed polymer were monitored by zeta potential measurements and colloidal titrations, respectively. Prior to adsorption, the PDMA-PDEA micelles were loaded with a hydrophobic dye, and UV-vis spectroscopy was used to determine the amount of dye encapsulated within a monolayer of micelles. It was found that subtle chemical modification of the PDMA-PDEA diblock copolymer via permanent quaternization of the PDEA block results in micelles with tunable loading capacities. Multilayers of cationic micelles of partially quaternized PDMA-PDEA and anionic polyelectrolyte (poly(sodium 4-styrene sulfonate)) were deposited on the surface of polystyrene latex particles by sequential adsorption. UV-vis analysis of the dye present within the multilayer after the addition of each layer demonstrates that the micelles are sufficiently robust to retain encapsulated dye after multiple adsorption/washing cycles and can thus create a film that can be increasingly loaded with dye as more micelle layers are adsorbed. Multiple washing cycles were performed on micellar monolayers of PDMA-PDEA to demonstrate how such systems can be used to bring about triggerable release of actives. When performing several consecutive washing steps at pH 9.3, the micelle structure of the PDMA-PDEA micelles in the monolayer is retained, resulting in only a small reduction in the amount of encapsulated dye. In contrast, washing at pH 4, the structure of the micelle layers is severely disrupted, resulting in a fast release of the encapsulated dye into the bulk. Finally, if a sufficient number of micelle/homopolyelectrolyte layers are adsorbed, it is possible to selectively dissolve the latex template, resulting in hollow capsules.     

Molecular-dynamics simulation of the surface layer of a nonionic micelle

A spherical micelle structure has been studied for cationic (n-dodecyltrimethylammonium chloride) and nonionic (hexaethylene glycol mono-n-hexyl ether) surfactants in pure water and a sodium chloride solution. The molecular-dynamics has been used to simulate the self-assembly of aggregates from an initially homogeneous mixture of water and surfactant molecules and to gain insight into the structure of micelles and their surface layers. The radial distribution functions obtained for charged components have been employed to calculate the local electric potentials of the micelles and the contributions from the charges of water atoms, ions, and a surfactant to it. It has been shown that, similarly to previously studied ionic micelles, in nonionic surfactant micelles, the contributions from water molecules and polar groups (and ions in the case of the salt solution) to the electric potential are mutually compensated in the region of the electrical double layer. Therefore, the resultant electric potential of the surface layer rapidly tends to zero.     

Structure and interactions in micellar solutions: molecular simulations of pluronic L64 aqueous solutions

Coarse-grained, implicit solvent molecular simulations have been conducted to investigate the structure and interactions of L64 Pluronic micelles in aqueous solutions. Simulations of an L64 solution beginning with monodisperse micelles (aggregation number Nagg = 40 chains) resulted in a narrow Gaussian distribution of Nagg centered around 40. While not fully equilibrated, this distribution supports the supposition that L64 micelles with Nagg = 40 are representative of the conditions considered and model employed. Detailed analysis of intramicellar monomers distribution and micelle shapes revealed that L64 micelles have a scalene ellipsoidal shape. Additional simulations of solutions containing 125 micelles constrained to have Nagg = 40 at polymer volume fractions of 0.024 and 0.110 were performed to study micelle-micelle structure factor, single micelle form factor, and total scattering intensity. The ability of various models utilized in analysis of scattering profiles in micellar solutions to describe the structure of the model L64 solutions was investigated. Investigation of the potential of mean force between two micelles reveals that the interactions between micelles are repulsive but on a length scale smaller than the mean micelle diameter, indicating that the micellar shape fluctuations are important in determining intermicellar interactions.     

Modification of polymers by adsorption phenomena

The adsorption of ions and amphiphilic molecules on solid polymers is investigated by direct force measurements using an atomic force microscope (AFM). It is shown that electrolyte ions are changing the surface potentials of the solid polymers as well as their adhesive properties. The experiments show that the interaction with a negatively charged probe is dramatically decreased by the adsorption of anions. The adsorption isotherms are determined by zeta potential measurements (streaming potential of flat plates). In presence of adsorbing anions, the attractive interaction and the adhesion are reduced and can be eliminated completely. So, even solutions of simple electrolyte ions can be applied intentionally in order to modify the interaction of polymer surfaces. A wide variety of technological applications becomes accessible.     

Electrophoretic properties of dodecyltrimethylammonium bromide micelles in KBr solution

Charge in ionic micelles determines the trends of their stability and their practical applications. Charge can be calculated from zeta potential (zeta) measurements, which, in turn, can be obtained by Doppler microelectrophoresis. In this study, the electrophoretic properties of dodecyltrimethylammonium bromide (DTAB) in KBr aqueous solution (0-6 mM) were determined by Doppler microelectrophoresis. At very low surfactant concentrations (up to 6 mM), zeta potential was quite constant and due to the ionized monomers (DTA+). Above 6 mM, zeta potential increased to a maximum at surfactant concentrations still below the critical micellar concentration (CMC). This increase could be explained by a formation of nonmicellar aggregates of DTAB. Then, above the CMC, zeta potential underwent an abrupt reduction, which was dependent qualitatively and quantitatively on KBr concentration, and which could be due to an increase of the number of counterions adsorbed on the micelle surface. Calculation of effective micellar charge from zeta potential gave the surface charge density. Comparing this value with the theoretical, obtained from geometrical considerations, a fraction of 0.29 of charged micellar headgroups was obtained when DTAB was in aqueous solution, which is consistent with the value obtained by conductivity measurements.     

USING ELECTROPHORESIS TO DETERMINE ZETA POTENTIAL OF MICELLES AND CRITICAL MICELLE CONCENTRATION

A heterodyne laser Doppler electrophoresis method for measuring the zeta potential of a surfactant micelle has been developed. Details of the method and results obtained will be presented in this paper. In comparison with the tracer electrophoresis method used in determining the zeta potential of a surfactant micelle, this method is much time saving and less laborious. The zeta potential values of one anionic and three cationic surfactant micelles have been determined by using this method. From the zeta potential versus concentration curve, one can determine the micellar dissociation concentration and the critical mi cellar concentration of a surfactant. The values obtained agree quite well with the surface tension measurement. The surfactant systems were studied in the presence and absence of NaCI. The zeta potential appears to decrease with the increase of NaCI concentration.     

Shell cross-linked stearic acid grafted chitosan oligosaccharide self-aggregated micelles for controlled release of paclitaxel

Stearic acid grafted chitosan oligosaccharide (CSO-SA) with different degree of amino substitution (SD) was synthesized by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC)-mediated coupling reaction. The critical micelle concentration (CMC) of CSO-SA with different SD was about 0.06, 0.04, 0.01 mg/ml, respectively. With the increase of micelle concentration, the micelle size decreased, and the zeta potential increased. On the other hand, with the increase of SD of CSO-SA, the micelle size and zeta potential decreased due to the increased hydrophobic interaction of SA and the reduced free amino groups. To increase the stability of the micelle in vivo and controll drug release, the shells of micelles were cross-linked by glutaraldehyde. By controlling the molar ratio of CSO-SA to glutaraldehyde, the cross-linking of intra-micelle could be reached, and the nanoparticle with smaller size than that of its initial micelle was obtained. Paclitaxel was then used as model drug to incorporate into the micelles, and the surfaces of the micelles were further cross-linked by glutaraldehyde to form drug loaded and shell cross-linked nanoparticles. The effects of drug loading, SD of CSO-SA and cross-link degree on the size, zeta potential, drug entrapment efficiency and in vitro drug release behavior of micelles and its cross-linked nanoparticles were investigated. The higher drug entrapment efficiencies (above 94%) were observed in all case. The charged amounts of drug did not affect the drug release behavior. The drug release rate decreased with the increase of SD of CSO-SA and cross-link degree.     

Diffusiophoresis of concentrated suspensions of spherical particles with distinct ionic diffusion velocities

The diffusiophoresis of a concentrated spherical dispersion of colloidal particles subject to a small electrolyte gradient is analyzed theoretically for an arbitrary zeta potential and double layer thickness. In particular, the influence of the difference in the diffusivities of cations and anions is discussed. A unit cell model is used to simulate a spherical dispersion, and a pseudospectral method is adopted to solve the equations governing the phenomenon under consideration. We show that, as in the case of an infinitely dilute dispersion, when the diffusivities of cations and anions are different, the diffusiophoretic mobility is no longer an even function of the zeta potential or double layer thickness. In contrast to the case of identical diffusivity of cations and anions, a local electric field is induced in the present case due to an unbalanced charge distribution between higher and lower concentration regions. Depending upon the direction of this induced electric field, the diffusiophoretic mobility can be larger or smaller than that for the case of identical diffusivity. The diffusiophoretic mobility is influenced mainly by the induced electric field arising from the difference in the ionic diffusivities, the concentration gradient, and the effect of double layer polarization.     

Nonideal mixed micelles of Gemini surfactant homologues and their application as templates for mesoporous material MCM-48

The micellization properties of aqueous solutions of the mixed Gemini surfactant homologues GEM16-6-16 and GEM16-12-16 with various compositions were investigated. The measured critical micelle concentration (CMC) deviated significantly from the ideal mixing model. Good agreement was found with a nonideal mixing model, the Margules model, which has two optimal parameters, A12=-3.611 and A21=-6.318. It was shown that the properties of mixed micelles were not sensitive to the compositions, and most of the GEM16-12-16 molecules were aggregated into the micelles. Dynamic laser light-scattering measurements revealed that the mixed micelles had almost the same size and similar zeta potential. When the mixed micelles were used as templates, a series of highly ordered cubic MCM-48 mesoporous materials, characterized by XRD and TEM, were produced through self-assembly. The N2 adsorption-desorption measurements suggested that the pores of these materials had similar average diameters of 2.2-2.5 nm. This further demonstrated the nonideal behavior of the homologue mixture.     

Bipolar reverse osmosis membrane for separating mono-and divalent ions

Bipolar reverse osmosis membranes that have both negatively and positively charged layers have been prepared to enhance the selectivity towards mono- and divalent ions in respect of both cations and anions. Positively charged layers are formed on low pressure reverse osmosis membranes having negative charge (NTR-7410 and 7450) by an adsorption method using polyethyleneimine (PEI) or a quaternary ammonium polyelectrolyte (QAP). These layers attach to the membrane's dense layer, which is made of sulfonated polyether sulfone. The selectivity of mono- and divalent ions is proven by experimental results for single electrolytes (NaCl, Na₂SO₄ and MgCl₂). Although negatively charged membranes repulse divalent anions more strongly than cations and monovalent anions, bipolar reverse osmosis membranes reject both divalent cations and divalent anions better than monovalent ions. An optimal preparation method for bipolar membranes showing selectivity towards mono- and divalent ions were developed. The bipolar membranes showed good ion selectivity for artificial sea water.     

Adsorption of ions onto polymer surfaces and its influence on zeta potential and adhesion phenomena

 The adhesion behavior that governs many technologically and biologically relevant polymer properties can be investigated by zeta potential measurements with varied electrolyte concentration or pH. In a previous work [1] it was found that the difference of the adsorption free energies of Cl^- and K⁺ ions correlates with the adhesion force caused by van der Waals interactions, and that the decrease of adhesion strength by adsorption layers can be elucidated by zeta potential measurements. In order to confirm these interrelations, zeta potential measurements were combined with atomic force microscopy (AFM) measurements. Force–distance curves between poly(ether ether ketone) and fluorpolymers, respectively, and the Si₃N₄ tip of the AFM device in different electrolyte solutions were measured and analysed. The adsorption free energy of anions calculated from the Stern model correlates with their ability to prevent the adhesion between the polymer surface and the Si₃N₄ tip of the AFM device. These results demonstrate the influence of adsorption phenomena on the adhesion behavior of solids. The results obtained by AFM confirm the thesis that the electrical double layer of solid polymers in electrolyte solutions is governed by ion adsorption probably due to van der Waals interactions and that therefore van der Waals forces can be detected by zeta potential measurements. Received: 18 November 1997 Accepted: 19 January 1998