Mixed micelles of tetradecyltrimethylammonium bromide and chlorhexidine digluconate in aqueous solutions of isopropyl alcohol

The influence of isopropyl alcohol (IPA) on the size and composition of the mixed micelles in mixtures of tetradecyltrimethylammonium bromide (TTAB) and chlorhexidine digluconate (CG) has been determined as a function of the composition of the systems. The addition of 0.5 M and 1.0 M IPA had little significant effect on the composition of the mixed micelles as determined both by analysis of critical micelle concentration (CMC) data using a theoretical treatment based on excess thermodynamic quantities and by an empirical treatment of conductivity data. Static and quasielastic light scattering measurements showed a progressive decrease of the aggregation number and hydrodynamic radius of TTAB micelles on addition of IPA, but minimal changes in the properties of the small CG aggregates. The results show that the micellar weight in the TTAB/CG/IPA solutions is determined by the ratio of the surfactants in the system and for each TTAB/CG ratio decreases on addition of IPA.     

Detailed structure of hairy mixed micelles formed by phosphatidylcholine and PEGylated phospholipids in aqueous media

Aqueous dispersions of mixed egg yolk phosphatidylcholine (PC) and poly(ethylene glycol) (PEG) modified distearoyl phosphatidylethanolamine (DSPE) were investigated with the purpose of determining shape, size, and conformation of the formed mixed micelles. The samples were prepared at a range of DSPEPEG to PC molar ratios ([DSPEPEG/PC] from 100:0 to 30:70) and with, respectively, DSPEPEG2000 and DSPEPEG5000, where 2000 and 5000 refer to the molar masses of the PEG chains. Particle shape and internal structure were studied using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). The contrast of the micelles is different for X-rays and neutrons, and by combining SANS and SAXS, complementary information about the micelle structure was obtained. The detailed structure of the micelles was determined in a self-consistent way by fitting a model for the micelles to SANS and SAXS data simultaneously. In general, a model for the micelles with a hydrophobic core, surrounded by a dense hydrophilic layer that is again surrounded by a corona of PEG chains in the form of Gaussian random coils attached to the outer surface, is in good agreement with the scattering data. At high DSPEPEG contents, nearly spherical micelles are formed. As the PC content increases the micelles elongate, and at a DSPEPEG/PC ratio of 30:70, rodlike micelles longer than 1000 angstroms are formed. We demonstrate that by mixing DSPEPEG and PC a considerable latitude in controlling the particle shape is obtained. Our results indicate that the PEG chains in the corona are in a relatively unperturbed Gaussian random coil conformation even though the chains are far above the coil-coil overlap concentration and, therefore, interpenetrating. This observation in combination with the observed growth behavior questions that the "mushroom-brush"transition is the single dominating factor for determining the particle shape as assumed in previous theoretical work (Hristova, K.; Needham, D. Macromolecules 1995, 28, 991-1002).     

Self-assembly of tetradecyltrimethylammonium bromide in glycerol aqueous mixtures: A thermodynamic and structural study

In this paper, we have studied the effect of glycerol on the micelle formation of tetradecyltrimethylammonium bromide. Changes in both the critical micelle concentration and the degree of counterion binding of the surfactant upon the addition of glycerol across a temperature range (20-40 degrees C) were examined by using the conductance method. The equilibrium model of micelle formation was applied to obtain the thermodynamic parameters of micellization. An enthalpy-entropy compensation effect was observed in all the solvent systems, but whereas the micellization of the surfactant in the medium with 20% glycerol occurs under the same structural conditions as in pure water, in glycerol rich mixtures the results suggest that the lower aggregation in these media is due to the minor cohesive energy of the solvent system in relation to water. It was also observed that the micellar aggregation number, as obtained by the static quenching method, decreases with the glycerol content. This fact was attributed to an increase in the surface area per headgroup of the surfactant as a consequence of an enhanced solvation, probably induced by the incorporation of some glycerol molecules in the micellar solvation layer. Although the pyrene 1:3 ratio index does not indicate significant changes in the micropolarity at the micelle-bulk interface, the data of fluorescence anisotropy of coumarin 6 and fluorescein are compatible with the formation of a more compact solvation layer.     

Block copolymer micelles in aqueous media

Polystyrene-block-poly(methacrylic acid) and poly(methacrylic acid)-block-polystyrene-block-poly(methacrylic acid) di- and triblock copolymers form micelles with polystyrene cores and poly(methacrylic acid) shells when dissolved in water/1, 4-dioxane mixtures, rich in dioxane. These micelles can be transferred into water rich mixtures, into water, and into aqueous buffers by stepwise dialysis. Quasielastic light scattering and sedimentation velocity experiments show that in dioxane rich mixtures exists a dynamic micellization equilibrium, while in dioxane poor solutions and in aqueous buffers the equilibrium is frozen. The process of mixed micelles formation was observed in dioxane rich solutions.     

Mixed micelles formed by SDS and a bolaamphiphile with carbohydrate headgroups

The formation of micelles in aqueous mixtures of a carbohydrate-based bolaamphiphile and sodium dodecyl sulfate (SDS) is investigated by surface tension and small-angle neutron scattering. The obtained values of critical micelle concentration (CMC) are analyzed within the framework of regular solution theory. Synergetic interactions between the bolaamphiphile and SDS are observed (parameter beta is negative; a minimum in the plot CMC vs composition). SANS data are collected for mixtures containing protonated and deuterated SDS. This gives us the possibility to conclude that mixed micelles with a homogeneous distribution of surfactant molecules within the micelle are formed. The shape of the micelles is found to be slightly oblate.     

Enthalpies of dilution of aqueous tetradecyltrimethylammonium bromide from 50 to 175°C

Enthalpies of dilution of aqueous tetradecyltrimethylammonium bromide have been measured from 0.3 to about 0.002 mol-kg^–1 and from 323 to 448 K at 1.03 MPa. Different methods of obtaining the enthalpy of micellization from experimental data are examined. Enthalpies of micellization calculated from these methods disagree by large amounts. From consideration of the temperature dependence of the average aggregation number and multiple equilibria equations for micellization, it is shown that there is an enormous dependence of micellizaton enthalpy on aggregation number of a micelle.     

Mixed micelles of benzyldimethyltetradecylammonium chloride with tetradecyltrimethylammonium and tetradecyltriphenylphosphonium bromides: a head group contribution

Mixed micelle formation by tetradecyltrimethylammonium (TTAB) and tetradecyltriphenylphosphonium bromides (TTPB) with benzyldimethyltetradecylammonium chloride (BTDACl) was studied with the help of conductivity and Kraft point measurements. The BTDACl + TTAB mixtures showed synergistic interactions whereas those of BTDACl + TTPB indicated weak antagonistic behavior. From Kraft temperature measurements, the enthalpy of fusion (H(1)(0)) from solid hydrated BTDACl to the liquid state in the presence of TTAB or TTPB was computed. It was found that DeltaH(1)(0) was much more positive for BTDACl + TTPB than for BTDACl + TTAB mixtures.     

Wormlike micelles formed by mixed cationic and anionic gemini surfactants in aqueous solution

The formation and the properties of wormlike micelles in aqueous solutions of mixed cationic and anionic gemini surfactants, 2-hydroxyl-propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3(OH)-12) and O,O'-bis(sodium 2-dodecylcarboxylate)-p-benzenediol (C(12)?C(12)), have been studied by steady-state and dynamic rheological measurements at 25°C. With the addition of a small amount of C(12)?C(12) into the solution of 12-3(OH)-12, the total surfactant concentration of which was always kept at 80 mmol L(-1), the solution viscosity was strongly enhanced and its maximum was much larger than that of the mixed system of propanediyl-α,ω-bis(dimethyldodecylammonium bromide) (12-3-12) and C(12)?C(12). The results of dynamic rheology measurements showed that 12-3(OH)-12/C(12)?C(12) formed longer wormlike micelles in comparison with 12-3-12/C(12)?C(12). This was attributed to the effect of hydrogen bonding occurring between 12-3(OH)-12 molecules, which was an effective driving force promoting micellar growth. As few C(12)?C(12) participated in the micelles, the electrostatic attraction between the oppositely charged head groups of 12-3(OH)-12 and C(12)?C(12) made the molecules in the aggregates pack more tightly. This reinforced the hydrogen-bonding interactions and greatly promoted the micellar growth.     

The surface properties of tetradecyltrimethylammonium bromide observed by capillary electrophoresis

The electroosmotic flow (EOF) is measured as a function of tetradecyltrimethylammonium bromide (TTAB) concentration and is shown to have distinct zones that are pH dependent. The data is correlated with previously proposed surface structures ranging from unimolecular adsorption to hemimicelles and micelles of TTAB adsorbed on the hydrated fused silica. A plot of the TTAB concentration at zero EOF versus pH shows that the zero point of charge (zpc) is pH dependent and that a linear extrapolation of the data intercepts close to the pH value for the zpc of a fused-silica surface. This shows that different surface properties at different pH values at any given TTAB concentration are generally dealt with. Therefore, these pH-dependent structures of the fused-silica surface have to be taken into account while studying these phenomena.     

The thermodynamics of dilution ofn-dodecyltrimethylammonium bromide micelles in aqueous solutions

Summary The relative partial molal thermodynamic functions ₁ and have been derived from reported experimental light scattering and calorimetric measurements on micellar solutions ofn-dodecyltrimethylammonium bromide. It is shown that the magnitudes of the thermodynamic functions are considerably larger than can be accounted for by the interaction of the electrical double layers surrounding the micelles. Possible enthalpy changes arising from a change in micelle shape, size and degree of dissociation on dilution are discussed.

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Zusammenfassung Die relativen molaren thermodynamischen Partialfunktionen , andS ₁wurden abgeleitet aus Literaturwerten von Lichtstreuungsmessungen und kalorimetrischen Untersuchungen an mizellaren Lösungen vonn-Dodecyltrimethylammoniumbromid. Es wird gezeigt, daß die Zahlenwerte der thermodynamischen Funktionen erheblich größer sind als von der Wechselwirkung der elektrischen Doppelschichten zu erwarten ist, die die Mizellen umgeben. Mögliche Enthalpieänderungen, die sich aus Änderungen von Größe, Gestalt und Dissoziationsgrad bei Verdünnung herleiten, werden diskutiert.

     

Stabilized micelles of amphoteric polyurethane formed by thermoresponsive micellization in HCl aqueous solution

The thermoresponsive micellization behavior of amphoteric polyurethane (APU) was studied in HCl aqueous solution (pH 2.0) through light scattering, transmission electron microscopy, and fluorescent measurement. When APU concentration is high enough, nonreversible assembly of macromolecules can be observed with temperature decreasing from 25 to 4 degrees C. However, micelles reaching equilibrium at 4 degrees C can self-assemble reversibly in the temperature range of 4-55 degrees C. According to our research, we found it is the temperature sensitivity of the poly(propylene oxide) (PPO) segments that leads to the reassembly of APU at lower temperature. We proposed that core-shell-corona micelles ultimately form with hydrophobic core, PPO shell, and hydrophilic corona when temperature increases from 4 to 25 degrees C. This structure is very stable and does not change at higher temperatures (25-55 degrees C). That provides a new way to obtain stable micelles with small size and narrow size distribution at higher concentration of APU.     

Influence of alkoxyethanols on the mixed micelle formation by hexadecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide surfactant mixtures

 The conductances of hexadecyltrimethylammonium bromide (HTAB) and tetradecyltrimethylammonium bromide (TTAB) mixtures over the entire mole fraction range of HTAB were measured in aqueous binary mixtures of ethylene glycol monomethyl ether, monoethyl ether, and monobutyl ether, and of diethylene glycol monomethyl ether and monoethyl ether containing 10–30 wt% additive in their respective binary mixtures at 30 °C. Each conductivity curve showed a single break over the whole mole fraction range of HTAB–TTAB mixtures. From the break in the conductivity curve, various micellar parameters were calculated and the results were discussed with respect to the alkoxyethanol's additive effect on the mixed micelle formation. The micellar parameters of HTAB, TTAB, and of their mixtures showed a strong dependence both on the amount as well as on the number of repeating units in the presence of ethylene glycol derivatives, whereas a significant dependence only on the amount of additive was observed in aqueous diethylene glycol derivatives. The results in the former case were attributed to the hydrophobic hydration of the mixed micelles by the ethylene glycol derivatives, which showed a large dependence on the increase in the alkyl chain length of the additive. The hydrophobic hydration was considerably reduced in the case of diethylene glycol derivatives owing to the presence of an extra ether oxygen. An evaluation of the nonideality in the HTAB–TTAB mixtures revealed that in spite of the strong hydrophobic hydration of the HTAB–TTAB mixtures by the alkoxyethanols, the mixed micelles remain almost free from the additive molecules. Received: 11 January 2000/Accepted: 14 April 2000     

Thermodynamics and micellar properties of tetradecyltrimethylammonium bromide in formamide-water mixtures

The effect of formamide on the micellization of tetradecyltrimethylammonium bromide has been investigated by conductance and fluorescence probe experiments. The critical micelle concentration and the degree of counterion dissociation of micelles were obtained from conductance measurements in the temperature range of 20 to 40 degrees C. It was found that these two parameters increase with both temperature and formamide content in the solvent system. The thermodynamic parameters of micellization were estimated using the equilibrium model of micelle formation. The standard free energy of micellization was found to be negative in all cases and becomes less negative as the formamide content in the mixed solvent increases, but it is roughly independent of temperature. Although the entropic contribution was found to be larger than the enthalpy one, in particular at lower temperatures, an enthalpy-entropy compensation effect was observed for all systems. Micellar aggregation numbers were determined by the static quenching method, using pyrene as a probe and cetylpyridinium chloride as a quencher. The observed decrease in the micelle aggregation number, which is controlled by the increase in the surface area per headgroup, was attributed to an enhanced solvation in formamide rich solvent mixtures. Changes in the pyrene 1:3 ratio index, indicating a more polar environment, are consistent with an increased micellar solvation. Fluorescence polarization of both coumarin 6 and fluorescein are indicative of a decrease in microviscosity with cosolvent addition. The data on fluorescence anisotropy of coumarin 6 were analyzed using the wobbling in cone model. Data indicated the formation of micelles with a less ordered structure as the formamide increases in the solvent system.     

Composite films of polyaniline and molybdenum oxide formed by electrocodeposition in aqueous media

Composite films of polyaniline (PANI) and molybdenum oxide (MoO_x) were afforded through a convenient route of electrocodeposition from aniline and (NH₄)₆Mo₇O₂₄. The composite films showed characteristic redox behaviors of PANI and MoO_x, respectively, on the cyclic voltammograms. Chlorate and bromate were catalytically electroreduced with an enlarged current on the composite film at a potential ca. 0.2 V more positive than that on MoO_x. The potential window for the composite film to display pseudocapacitive properties in 1.0 mol·dm⁻³ NaNO₃ was −0.6 ∼ 0.6 V vs SCE. The cathodic potential limit shifted at least 0.4 V negatively from that of polyaniline (PANI)-based materials reported so far. The specific capacitance was 363.6 F·g⁻¹ when the composite film was charged–discharged at 1.5 mA·cm⁻², about two times of that of the similarly prepared PANI. The composite film was characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Molybdenum existed in a mixed state of +5 and +6 in the composite film based on XRD and XPS investigations. Figure PANI and (MoO_x) were electrocodeposited in aqueous solutions from aniline and (NH₄)₆Mo₇O₂₄. The composite film obtained displayed catalytic activities toward the electroreduction of oxoanions. The pseudocapacitance of the composite film is nearly two times of that of PANI with the potential window extended negatively up to −0.6 V vs SCE     

Study of the 1-chloro-2,4-dinitrobenzene + oh- reaction in tetradecyltrimethylammonium bromide aqueous micellar solutions

The 1-chloro-2,4-dinitrobenzene + OH^- reaction was studied in tetradecyltrimethylammonium bromide aqueous micellar solutions. Influence of changes in [surfactant] as well as in [NaOH] on the observed rate constant were rationalized by using pseudophase kinetic models. At high hydroxide ions concentration an additional pathway across the micellar boundary had to be considered. This revised version was published online in June 2006 with corrections to the Cover Date.     

Salt-induced viscoelastic wormlike micelles formed in surface active ionic liquid aqueous solution

The growth and structure of the aqueous micellar solutions of a surface active ionic liquid, 1-hexadecyl-3-methylimidazolium bromide (C16mimBr), in the presence of an organic salt sodium tosylate (NaTos), were investigated by rheological measurements and freeze-fracture transmission electron microscopy at room temperature (298 K). As in some conventional ionic surfactant/salt aqueous systems, wormlike micelles and network structures could be formed in the C16mimBr/NaTos aqueous solutions, according to measurements of the zero-shear viscosity, the entanglement length, the average contour length, as well as application of the Cox-Merz empirical rule and Cole-Cole plots. FF-TEM images further confirmed that wormlike micelles were formed in these aqueous solutions. The wormlike micelles presented here would expand potential applications of ionic liquids in home care products, oilfield stimulation fluids, and nanobiotechnology.     

Structure and dynamics of highly PEG-ylated sterically stabilized micelles in aqueous media

Molecular assemblies of highly PEG-ylated phospholipids are important in many biomedical applications. We have studied sterically stabilized micelles (SSMs) of self-assembled DSPE–PEG2000 in pure water and isotonic HEPES-buffered saline solution. The observed SSM sizes of 2–15 nm largely depend on the solvent and the lipid concentration used. The critical micelle concentration of DSPE–PEG2000 is 10 times higher in water than in buffer, and the viscosity of the dispersion dramatically increases with the lipid concentration. To explain the experimentally observed results, we performed atomistic molecular dynamics simulations of solvated SSMs. Our modeling revealed that the observed assemblies have very different aggregation numbers (N(agg) ≈ 90 in saline solution and N(agg) < 8 in water) because of very different screening of their charged PO4(–) groups. We also demonstrate that the micelle cores can inflate and their coronas can fluctuate strongly, thus allowing storage and delivery of molecules with different chemistries.     

Local dynamics of micelles of new long-chain surfactants in aqueous media

The molecular dynamics, organization, and phase state of aqueous solutions of new long-chain cationic surfactants with saturated hydrocarbon radicals (from C₁₆ to C₂₂) containing one or two hydroxyl groups in their polar heads are studied by the spin-probe EPR spectroscopy. In the region of micellar solutions, local mobility of surfactant molecules slightly changes with an increase in the length of hydrocarbon radical, whereas the order parameter of micelles increases notably. The addition of two hydroxyl groups to the polar part of long-chain (C ₂₂) surfactant molecule considerably decreases local mobility and increases the ordering of micellar system compared to the micelles of analogous surfactant with one hydroxyl group. Phase transition from micellar to a solid state is observed in this system with a decrease in temperature. The addition of KCl to aqueous surfactant solution lowers the local mobility, increases the order parameter of micelles, and can cause changes in the phase state of a system. In the presence of salt, the correlation time of probe rotation and its order parameter depend on surfactant concentration. Apparently, this is explained by changes in the shape of micelles upon variations in surfactant concentration.