Micellar effects upon the reaction of hydroxide ion with 2-phenoxyquinoxaline

Cationic micelles of alkyltrimethylammonium chloride and bromide (alkyl = n? C₁₂H₂₅, n? C₁₄H₂₉, and n? C₁₆H₃₃) catalyze and anionic micelles of sodium dodecyl sulfate inhibit the reaction of hydroxide ion with 2-phenoxyquinoxaline (1). Inert anions such as chloride, nitrate, mesylate, and n-butanosulfonate inhibit the reaction in CTABr by competing with OH^? at the micellar surface. The overall micellar effects on rate in cationic micelles and dilute electrolyte can be treated quantitatively in terms of the pseudo-phase ion-exchange model. The determined second-order rate constants in the micellar pseudo-phase are smaller than the second-order constants in water. © 1994 John Wiley & Sons, Inc.     

Structural Behavior of Aggregate Assemblies of Cationic Surfactants and Their Mixtures with Triblock Polymers

Small angle neutron scattering (SANS) technique has been employed to study the structural aspects of micellar system of cationic surfactants viz. alkyltriphenylphosphonium bromide (C₁₂-, C₁₄-, C₁₆TPB) and hexadecylpyridinium chloride (C₁₆PyCl) with triblock polymers (L64, F68, and F127). SANS data analysis reveals the prolate ellipsoidal shape of mixed micelles and increase in the micellar size upon addition of triblock polymers (L64, F68, and F127). The influence of effective size of the head group segment on the growth of micelles of HTPB (larger head group) has also been compared with that of HPyCl (smaller head group). A proportionate micellar growth of cationic surfactants has been found with increase in the length of tail segment of cationic surfactants. The observed mixed micellar growth in mixed systems is also accounted on the basis of simultaneous increase in the hydrophobicity of both the components in the mixed system. Results from the present study enlightened the effect of variation in head group segment and hydrophobicity on the structural aspects of mixed micellar system.     

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.     

The Self-Association and Mixed Micellization of an Anionic Surfactant,Sodium Dodecyl Sulfate,and a Cationic Surfactant,Cetyltrimethylammonium Bromide: Conductometric,Dye Solubilization,and Surface Tension Studies

In the present study, we investigate the self-association and mixed micellization of an anionic surfactant, sodium dodecyl sulfate (SDS), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB). The critical micelle concentration (CMC) of SDS, CTAB, and mixed (SDS + CTAB) surfactants was measured by electrical conductivity, dye solubilization, and surface tension measurements. The surface properties (viz., C₂₀ (the surfactant concentration required to reduce the surface tension by 20 mN/m), Π_CMC (the surface pressure at the CMC), Γ_max (maximum surface excess concentration at the air/water interface), and A_min (the minimum area per surfactant molecule at the air/water interface)) of SDS, CTAB, and (SDS + CTAB) micellar/mixed micellar systems were evaluated. The thermodynamic parameters of the micellar (SDS and CTAB), and mixed micellar (SDS + CTAB) systems were evaluated.

A schematic representation of micelles and mixed micelles.     

Neutron scattering study of pentanol solubilization in sodium octanoate micelles

We extend a previous small-angle neutron scattering study of sodium octanoate (NaC₈) micelles to the ternary system sodium octanoate/pentanol/water. The use of contrast variation through selective deuteration of individual components together with explicit computation of interference effects, permits us to deduce the location of pentanol (C₅OH) in the micelles. Our main conclusion is that, although the micelles grow as (C₅OH) is solubilised, there is no concomitant variation in the NaC₈ aggregation number. At low alcohol concentrations, the C₅OH is located near the NaC₈ polar heads, while at higher concentration the -OH groups are distributed throughout the micellar core.     

Comparison of aggregation behaviors between branched and linear block polyethers: MesoDyn simulation study

The comparison of aggregation behaviors between the branched block polyether T1107 (polyether A) and linear polyether (EO)₆₀(PO)₄₀(EO)₆₀ (polyether B) in aqueous solution are investigated by the MesoDyn simulation. Polyether A forms micelles at lower concentration and has a smaller aggregation number than B. Both the polyethers show the time-dependent micellar growth behaviors. The spherical micelles appear and then change to rod-like micelles with time evolution in the 10 vol% solution of polyether A. The micellar cluster appears and changes to pseudo-spherical micelles with time evolution in the 20 vol% solution of polyether A. However, the spherical micelles appear and change to micellar cluster with time evolution in the 20 vol% polyether B solution. The shear can induce the micellar transition of both block polyethers. When the shear rate is 1?×?10⁵ s^?1, the shear can induce the sphere-to-rod transition of both polyethers at the concentration of 10 and 20 vol%. When the shear rate is lower than 1?×?10⁵ s^?1, the huge micelles and micellar clusters can be formed in the 10 and 20 vol% polyether A systems under the shear, while the huge micelles are formed and then disaggregated with the time evolution in the 20 vol% polyether B system.     

Investigations on mixed micelles of binary mixtures of zwitterionic surfactants and triblock polymers: a cyclic voltammetric study

Cyclic voltammetry has been employed to investigate the mixed micellar behavior of the binary mixtures of different zwitterionic surfactants such as 3-(N,N-dimethylhexadecylammonio)propane sulfonate (HPS), 3-(N,N-dimethyltetradecylammonio)propane sulfonate (TPS) and 3-(N,N-dimethyldodecylammonio)propane sulfonate (DPS) with three triblock polymers (L64, F127 and P65) by using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as an electroactive probe at 25 °C. Critical micellar concentration (cmc) has been determined from the plots of variation in peak current (i_p) versus the total concentration of surfactant/triblock polymer. Diffusion coefficient of the electroactive species has also been reported. The regular solution theory approximation has been used to determine various micellar parameters of ideal systems. The variation in micellar mole fraction (X₁) of the zwitterionic surfactant supports the formation of mixed micelles, which are rich in triblock polymer component in the surfactant rich region of the mixture and vice versa. The regular solution interaction parameter (β) suggests the formation of mixed micelles due to the synergistic interactions in case of HPS/TPS/DPS + F127/P65 systems and gets affected by EO/PO ratio of triblock polymers.     

Conductivity of lithium perfluorononanoate in water-poly(vinyl pyrrolidone) solutions

Conductivity measurements, focused on the counterion binding of lithium perfluorononanoate (LiPFN) micelles in pure water and in the presence of poly(vinyl pyrrolidone) (PVP), have been carried out. An abrupt decrease of the conductance of the LiPFN in pure water, due to the self aggregation of anions and to dynamic linkage of cations on the micellar surface, has been found. Analysis of the conductometric data indicates that about 50% of the stoichiometric concentration of Li⁺ interacts with the head groups of the perfluorinated anions involved in micellar assembly. Conductometric data of LiPFN-water-PVP systems reveals a favorable influence of the PVP on the micellization process modulated by the concentration and by the molecular weight of the polymer. Analysis of these data shows that in presence of PVP the degree of binding of lithium ion to the micellar assemblies linked to the polymer is smaller than in pure water. By increasing the amount of surfactant in solution up to the concentration where the polymer becomes saturated by LiPFN micelles, the binding of lithium ion in the system becomes slightly greater than that observed in LiPFN-water system. This finding can be interpreted in terms of additional binding of lithium ion to the polymer chains. Conductivity measurements carried out on LiClO₄ and KClO₄ in water + PVP support this interpretation.     

Explaining Fullerene Dispersion by using Micellar Solutions

An effective computational strategy to describe the dispersion of C₆₀ by surfactants is presented. The influence of parameters such as surfactant concentration and molecular length on the final morphology of the system is explored to explain the experimental results and to understand the incorporation of C₆₀ inside micelles. Both neutral and charged amphiphilic molecules are simulated. The long‐discussed problem of the location of fullerenes in micelles is addressed and C₆₀ is found in the hydrocarbon‐chain region of the micelles. If the available hydrophobic space increases, C₆₀ is localized in the inner part of the micellar core. Short, charged amphiphilic stabilizers are more efficient at dispersing fullerenes monomolecularly. Two different phases of C₆₀ are observed as the C₆₀/surfactant ratio varies. In the first, aggregates of C₆₀ are entrapped inside the micelles, whereas, in the second, colloidal nanoC₆₀ is formed with surfactants adsorbed on the surface.     

Self-organization of poly(methyl methacrylate) and polystyrene macromolecules functionalized by fullerene C60

Poly(methyl methacrylate) and polystyrene functionalized by fullerene C₆₀ tend to form micellar structures comprising a fullerene cluster as a core and a macromolecular shell. Films prepared from PMMA-C₆₀ and PS-C₆₀ micellar solutions are polymer matrices with fullerene-containing globular structures uniformly distributed in the polymer bulk.     

Effect of copolymer structure on micellar behavior of acrylamide-octylphenylpoly (oxyethylene) acrylate surfactant

Acrylamide-octylphenylpoly (oxyethylene) acrylate copolymer (AM-C₈PhEO_nAc) surfactant is the copolymer of acrylamide (AM) and octylphenylpoly (oxyethylene) acrylate macromonomer (C₈PhEO_nAc). The effect of the copolymer structure on the micellar behavior in aqueous solution was studied using dynamic light scattering. It has been found that the length of ethylene oxide (EO) in the branch and the content of C₈PhEO_nAc in the copolymer surfactant have great effects on the size and distribution of the micelles. For AM-C₈PhEO₇Ac copolymer, at the concentration of 5 × 10⁻⁴ g/ml, the micellar size increases with the increase of C₈PhEO₇Ac content. However, for AM-C₈PhEO₁₀Ac copolymer, the result is the opposite; the micellar size decreases with the increase of C₈PhEO₁₀Ac content. Larger C₈PhEO_nAc content leads to narrower micellar distribution. For copolymer surfactants with equal C₈PhEO_nAc content, when the concentration of copolymer solution is the same, the copolymer with longer EO length forms smaller micelles. Received: 2 February 2000 Accepted: 6 October 2000     

Interaction between two homologues of cationic surface active ionic liquids and the PEO-PPO-PEO triblock copolymers in aqueous solutions

The interaction of nonionic triblock copolymers of poly(ethyleneoxide) (PEO) and poly(propyleneoxide) (PPO) (PEO_nPPO_mPEO_n) with a series of cationic surface-active ionic liquids in aqueous solutions have been investigated. The cationic surface-active ionic liquids include 1-alkyl-3-methylimidazolium bromide (C_nmimBr, n?=?8, 10, 12, 14, 16) and N-alkyl-N-methylpyrrolidinium bromide (C_nMPB, n?=?12, 14, 16). For different polymer-surfactant systems, the critical aggregation surfactant concentration (cac), the surfactant concentration to form free micelles (C _m), and the saturation concentration of surfactant on the polymer chains (C ₂) were determined using isothermal titration microcalorimetry (ITC) and conductivity measurements. The structure of the formed aggregates depended strongly on the hydrophobicity of the surfactant and the ratio of polymer/surfactant concentration. For C₈mimBr, there were not any micelle-like surfactant?Cpolymer clusters detected in the solution, and only micelles appeared. For other surfactants, the polymer?Csurfactant aggregates were formed in the solution, which was verified by the appearance of a broad endothermic peak in the ITC thermograms. The intensity of polymer?Csurfactant interaction increased with the hydrophobicity of the surfactants and the polymers but was not affected by the surfactant headgroups.     

Growth of Gemini Surfactant Micelles Under the Influence of Additives: DLS Studies

Results are presented on the first extensive study on the influence of additives on the growth of gemini; alkanediyl α, ω - bis(dimethylcetylammonium bromide) surfactant micelles (16-s-16, with s = 5, 6); as measured by dynamic light scattering technique at 30°C. The effect of adding n-butanol, n-pentanol, n-hexanol, and n-hexylamine in the absence or presence of general ionic salt potassium bromide on 0.03 M gemini solutions were observed. The tendency for micelles to grow from spherical to rodlike structures is decisively influenced by the spacer length s. At 30°C, the micellar growth was more for s being 5, which has been interpreted in terms of short spacer having strong propensity for micellar growth. Addition of KBr plays a role in screening of the electrostatic interactions, thus promoting a change of morphology of the aggregates and giving rise to high hydrodynamic diameter (D _h ) values. The micellar growth in presence of alcohols is interpreted in terms of the formation of the gemini–alcohol mixed micelles which followed the pattern C₆OH>C₅OH>C₄OH. For equal chain length additives C₆OH and C₆NH₂, the growth was more pronounced in case of alcohol. Also, in case of C₆NH₂, the value of D _h reached to almost constancy or decreased to some extent, which is discussed in terms of its partitioning in aqueous phase. A combined presence of KBr and n-alcohols or n-hexylamine produced favorable conditions for micellar growth due to synergistic effect.     

Co-solubilization of the Hydrophobic Drugs Carbamazepine and Nifedipine in Aqueous Nonionic Surfactant Media

Co-solubilization of the hydrophobic drugs Carbamezipine (CBZ) and Nifedipine (NFD) by micellar solutions at 25 °C, using two series of polyoxyethylene based nonionic surfactants, was measured and compared. The first series is composed of surfactants with a 12 carbon (C₁₂) hydrophobic chain while the second series had 16 carbon (C₁₆) hydrophobic chains. Experimental results were obtained for solubilization and co-solubilization of CBZ and NFD within the micelles at saturation and quantification was done in terms of the molar solubilization ratio and the micelle–water partition coefficient employing spectrophotometric and tensiometric techniques. The extent of micellar solubilization of CBZ is much greater than NFD. The C₁₂ series of surfactants exhibit higher solubilization capacities for CBZ than the C₁₆ series while the reverse is the case for NFD. Co-solubilization results showed competitive solubilization of the drugs. A synergistic effect on the solubilization of NFD was observed in the presence of CBZ in Brij30 and Brij56 surfactant systems while, in the remaining surfactants, the solubility of NFD was slightly reduced. Since the surfactants used in the present study are either nontoxic or have minimal toxicity, it is expected that they can be employed as drug delivery vehicles for co-administration of the two drugs in vivo. Both from industrial and research points of view, this paper reports a comprehensive study for co-solubilization of differently structured drugs in micellar media.     

The strong influence of alkyl tail length on the aggregation and viscoelasticity of carboxylate gemini surfactants with a p-dibenzenediol spacer in aqueous solution

This paper reports a study on the aggregation and rheological behavior of the family of O, O’-bis(sodium 2-alkylcarboxylate)-p-dibenzenediol (referred to as C_m?₂C_m, m?=?10, 12, 14, respectively) in aqueous solution using dynamic light scattering, ¹H NMR and rheology measurements. The results showed that all three surfactants formed large network-like aggregates at low concentrations. However, C₁₀?₂C₁₀ formed small compact micelles simultaneously but neither C₁₂?₂C₁₂ nor C₁₄?₂C₁₄ did. These network-like aggregates were transformed into the wormlike micelles with increasing the surfactant concentration. The length of alkyl tails was found to strongly affect the viscoelasticity of wormlike micellar solutions. From C₁₀?₂C₁₀, C₁₂?₂C₁₂ to C₁₄?₂C₁₄ in turn, the system developed rapidly from the viscous fluid to typically viscoelastic solution and then to a solid-like gel. The scaling exponents of the concentration dependence of both zero-shear viscosity (η ₀) and plateau elastic modulus (G′_∞) greatly exceeded the theoretic predictions, showing fast micellar growth and strong entanglements between the wormlike micelles. For C₁₄?₂C₁₄ that had the longest alkyl tails in this series, the wormlike micelles formed at 140?mmol L^?1 were quite long and the micellar reptation dominated over the scission and recombination. This system yielded a viscosity as high as 2.20?×?10⁴ Pa?s at 25 °C.     

Aggregation processes in self-associating polymer systems: Computer simulation study of micelles in the superstrong segregation regime

We present the results of extensive molecular dynamics and Monte Carlo studies of the self-organization in the solution of short polymer chains with strongly attracting head groups at their end. The formation of micelles (multiplets) is studied in detail. Both two dimensional (2d) and three-dimensional (3d) systems are considered. The off-lattice and lattice models under study incorporate physical factors which control micelle structure and growth in the so-called superstrong segregation regime. These factors include (i) conformational effects associated with short-range excluded-volume interaction between the tails of flexible-chain molecules and (ii) very strong attraction of head groups. Our computer simulations of 3d micelles, constructed a priori from chains with strong attraction of head groups (with the characteristic energy ≈ 10 k_BT), show that size and shape of the micellar core depends crucially on the radius r_c of the interaction of head-groups. If the value of r_c is comparable with chain length, then micelles of nearly spherical shape emerges. The decrease of r_c can induce a sharp polymorphic transition from the micellar core which is spheric in shape to a disk-like (bilayer-shaped) aggregate. Such molecular organization differs from the commonly held notion of a radially symmetric micellar core. On the other hand, these findings fall into line with a recent theory of the super strong segregation regime. When the starting configuration is a random one (i.e., no micelles were a priori formed) the type of final microstructures, emerging as a result of micellization in the superstrong segregation regime, also depends essentially on the radius of head-head attraction. In the case of three-dimensional systems and/or short range attractive potentials we always obtain many small spherically shaped aggregates which, once formed at initial stages of micellization, remain stable for all time scales. Such a behavior is due to both the strong head-head attraction and the screening (repulsive) action of micellar shells creating insurmountable potential barriers. As a result, we deal with kinetically “frozen-in” microstructures which are not reversible and cannot exchange molecules with one another. In dense systems, we observe the formation of a (quasi) periodic pattern of alternating microdomains.     

The effect of electrolyte on the micellar charge and hydration of a polyoxyethylene sulphate type of detergent in aqueous solution

Summary Measurements of the electrophoretic mobilities of the micelles of the anionic detergent C₁₆H₃₃(OCH₂ · CH₂)₇OSO₃Na have been made in aqueous sodium chloride solutions over the concentration range 0-0.01 M and used to calculate the magnitude of the electroviscous effect and the micellar charge in these solutions. The micellar hydration, as assessed from viscosity data, is shown to decrease as the salt concentration is increased and this is thought to be a consequence of the lower micellar charge in the more concentrated salt solutions which allows a greater contraction of the ethylene oxide chains resulting in a loss of water mechanically trapped by the micelles.     

Catalyse micellaire—I: Etude de la réactivite en milieu micellaire a l'aide de reactions compétitives

The limitations of macroscopic models of micellar solutions are revealed by thermodynamic and kinetic studies of micellar catalysis and in fact the properties of such media are essentially dependent on the microscopic organization. The micellar effects of CTAB [C₁₆H₃₃N⁺(CH₃)₃Br^?] and SDS (C₁₂H₂₅SO₄^?Na⁺) on the competitive reactions (S_N1, S_N2 and E2) of 1-bromo-2-phenyl propane in basic medium and on the alkaline and neutral hydrolysis of α-phénylallylic esters has been studied. The specific effect of the micellar microenvironment is shown. The results are interpreted in terms of an enhancement of the nucleophilicity of the hydroxide ion and a diminution of the nucleophilic and electrophilic properties of water. The variation of the catalytic effect of cationic micelles with the concentration of the nucleophilic reagent is discussed.     

A viscometric study of tuning micellar morphology by organic additives

The micellar morphology in aqueous 0.2 M sodium dodecyl sulfate (SDS) solutions has been studied in the simultaneous presence of organic salts (anilinium hydrochloride, AHC; ortho-toluidine hydrochloride, oTHC; para-toluidine hydrochloride, pTHC) and aliphatic alcohols (n-butanol, C₄OH; n-pentanol, C₅OH; n-hexanol, C₆OH; n-heptanol, C₇OH), aliphatic amines (n-butylamine, C₄NH₂; n-pentylamine, C₅NH₂; n-hexylamine, C₆NH₂; n-heptylamine, C₇NH₂), or hydrocarbons (n-hexane, C₆H; n-heptane, C₇H) by viscosity measurements under Newtonian flow conditions at 30 °C. Addition of alcohols and amines causes micellar growth which is found to be dependent upon chain length of the additive and nature of organic salt counterion. It is observed that amines are more effective in increasing the viscosity of the system if added in pure 0.2 M SDS solution, while SDS + pTHC system was found versatile when alcohols were added to this system. The increased viscosity or the micellar growth is explained in terms of the site of solubilization of the respective additive and the interaction of the additive with micellar surface and salt counterion present in the head group region. Hydrocarbons are nearly ineffective in inducing micellar growth and can be used as ‘micellar destroyer’ for the grown micelles. The additives used are found effective in tuning the environment of the micelle which is reflected in viscosity behavior. This work may find use in micellar ultrafiltration as well as in mimicking the natural cell, which has several aspects common with the micelle.     

Cholesterol Binding to Simple Micelles in Aqueous Bile-Salt–Cholesterol Solutions

The true thermodynamic activity (A_T) of cholesterol (Ch) in aqueous solutions containing taurocholate (TC)–Ch was determined by employing a direct assay of a 1 × 2-cm silicone polymer film with 0.025 cm thickness. Using theA_Tdata, information on the nature of micellar species present in the TC–Ch system, and employing a binding-site model previously developed for tauroursodeoxycholate (TUDC)–Ch and taurochenodeoxycholate (TCDC)–Ch systems, it appeared that the Ch-binding affinity for simple bile-salt micelles corresponds precisely with the order of hydrophobicity, TUDC < TC < TCDC. Further, although simple TC micelles and simple TCDC micelles have similar binding capacities, the first Ch binding to a simple TC micelle may not significantly facilitate the second Ch binding, as occurs in simple TCDC micelles. For TUDC–Ch, TC–Ch, and TCDC–Ch systems, the concentration of bound simple micelles increased with increasingA_Tvalues, whereas the unbound simple micelle concentration decreased proportionally. These results provide insights into the possible influence of bile-salt species on Ch-binding to simple micelles in bile-salt–Ch solutions.