有机电解质在胶束催化聚苯乙烯氯甲基化反应中的作用

在实施聚苯乙烯氯甲基化反应的胶束催化体系中加入四丁基溴化铵 ((Bu)₄NBr, TBAB), 研究了有机电解质TBAB对胶束催化反应的影响规律. 实验结果表明, 在非离子表面活性剂NP-10及阴离子表面活性剂SDS的胶束催化体系中, TBAB的加入使聚苯乙烯氯甲基化反应的速率明显增大, 前者尤为突出；而在阳离子表面活性剂CTAB的胶束催化体系中, TBAB的加入几乎对反应速率无促进作用. 这种结果一方面归因于加入电解质TBAB会降低SDS的临界胶束浓度, 从而增强对聚苯乙烯四氯化碳溶液的增溶能力；更主要的原因是TBAB的丁基与表面活性剂碳氢链间的疏水相互作用会使季铵离子(Bu)₄N⁺嵌入SDS的胶束之中, 结合到NP-10的胶束表面, 使SDS胶束的阴离子头基对亲核取代反应(控制步骤)的禁阻作用得以减缓, 使NP-10的胶束表面携带了正电荷, 显著促进亲核取代反应的进行, 而对于CTAB的胶束, 由于静电排斥作用, 季铵离子(Bu)₄N⁺不能接近CTAB的胶束, 故TBAB的加入对聚苯乙烯氯甲基化反应不产生作用.     

Correlating proton transfer dynamics to probe location in confined environments

The dramatic impact of differing environments on proton transfer dynamics of the photoacid HPTS prompted us to investigate these systems with two highly complementary methods: ultrafast time-resolved transient absorption and two-dimensional NMR spectroscopies. Both ultrafast time-resolved transient absorption spectroscopy and time-resolved anisotropy decays demonstrate the proton transfer dynamics depend intimately on the specific reverse micellar system. For w(0) = 10 reverse micelles formed with anionic AOT surfactant, the HPTS proton transfer dynamics are similar to dynamics in bulk aqueous solution, and the corresponding (1)H 2D NOESY NMR spectra display no cross peaks between HPTS and AOT consistent with the HPTS residing well hydrated by water in the interior of the reverse micelle water pool. In contrast, ultrafast transient absorption experiments show no evidence for HPTS photoinduced proton transfer reaction in reverse micelles formed with the cationic CTAB surfactant. In CTAB reverse micelles, clear cross peaks between HPTS and CTAB in the 2D NMR spectra show that HPTS embeds in the interface. These results indicate that the environment strongly impacts the proton transfer reaction and that complementary experimental techniques develop understanding of how location critically affects molecular responses.     

Contribution of molecular pathways in the micellar solubilization of monodisperse emulsion droplets

It is often proposed that oil solubilization in anionic and nonionic micelles proceeds by different mechanisms, with diffusion of the oil molecule thought to control the former, and the latter interfacially controlled. In order to investigate this hypothesis, the effect of aqueous phase viscosity, salt, and surfactant concentration during the solubilization process was studied. The progressive decrease in average droplet size of nearly monodisperse emulsions during solubilization in SDS or Tween 20 micellar solutions was monitored by light scattering, and the change in turbidity was measured by UV-vis spectrophotometer. The solubilization rates were analyzed using a population balance approach to calculate the mass transfer coefficients. Increasing the aqueous viscosity by adding sucrose reduced the mass transfer coefficients of n-tetradecane and n-dodecane but had a smaller effect on n-hexadecane. The strong dependence of the solubilization rate for the shorter chain length alkanes on aqueous viscosity supported a mechanism in which the oil undergoes molecular diffusion before being taken up by micelles. The dependence of the solubilization kinetics on surfactant concentration appeared consistent with this mechanism but yielded a slower micellar uptake rate than previously predicted theoretically. As the solute chain length increased in nonionic surfactant solutions, an interfacial mechanism mediated by micelles appeared to contribute substantially to the overall rate. Addition of salt only slightly increased the solubilization rate of n-hexadecane in SDS solutions and, thus, indicated a weak role of electrostatic interactions for ionic surfactants on the overall mechanism.     

Prediction of conformational characteristics and micellar solution properties of fluorocarbon surfactants

A molecular-thermodynamic theory is developed to model the micellization of fluorocarbon surfactants in aqueous solutions, by combining a molecular model that evaluates the free energy of micellization of fluorocarbon surfactant micelles with a previously developed thermodynamic framework describing the free energy of the micellar solution. In the molecular model of micellization developed, a single-chain mean-field theory is combined with an appropriate rotational isomeric state model of fluorocarbon chains to describe the packing of the fluorocarbon surfactant tails inside the micelle core. Utilizing this single-chain mean-field theory, the packing free energies of fluorocarbon surfactants are evaluated and compared with those of their hydrocarbon analogues. We find that the greater rigidity of the fluorocarbon chain promotes its packing in micellar aggregates of low curvatures, such as bilayers. In addition, the mean-field approach is utilized to predict the average conformational characteristics (specifically, the bond order parameters) of fluorocarbon and hydrocarbon surfactant tails within the micelle core, and the predictions are found to agree well with the available experimental results. The electrostatic effects in fluorocarbon ionic surfactant micelles are modeled by allowing for counterion binding onto the charged micelle surface, which accounts explicitly for the effect of the counterion type on the micellar solution properties. In addition, a theoretical formulation is developed to evaluate the free energy of micellization and the size distribution of finite disklike micelles, which often form in the case of fluorocarbon surfactants. We find that, compared to their hydrocarbon analogues, fluorocarbon surfactants exhibit a greater tendency to form cylindrical or disklike micelles, as a result of their larger molecular volume as well as due to the greater conformational rigidity of the fluorocarbon tails. The molecular-thermodynamic theory developed is then applied to several ionic fluorocarbon surfactant-electrolyte systems, including perfluoroalkanoates and perfluorosulfonates with added LiCl or NH(4)Cl, and various micellar solution properties, including critical micelle concentrations (cmc's), optimal micelle shapes, and average micelle aggregation numbers, are predicted. The predicted micellar solution properties agree reasonably well with the available experimental results.     

ESR study of photooxidation of phenothiazines in aqueous micellar solutions

The mechanism of photooxidation of phenothiazine in liquid and frozen (77 K) aqueous micellar solutions of the surfactant (sodium dodecyl sulfate) was studied by ESR. The main reaction of electrons formed by the photochemical oxidation of phenothiazine in a liquid micellar solution of the anionic surfactant is the reduction of molecular oxygen dissolved in the aqueous bulk phase. 10-Methylphenothiazine was used as a stable radical cation probe in a liquid solution. The influence of electrolytes on the photoionization yield in both frozen and liquid solutions of the surfactant was studied.     

Appearance of pure fluorocarbon micelles surveyed by fluorescence quenching of amphiphilic quinoline derivatives in fluorocarbon and hydrocarbon surfactant mixtures

A halide-sensitive fluorescence probe was utilized to evaluate the miscibility of fluorocarbon and hydrocarbon surfactants in aqueous micellar systems. The fluorescence of 6-methoxy-N-1,1,2,2-tetrahydroheptadecafluorodecylquinolinium chloride, FC10MQ, was quenched by halide ions dissociated from the surfactant. The fluorescence in micellar solutions showed an initially rapid decay. This suggests that halide ions effectively quench FC10MQ fluorescence at the micellar surface. The subsequent slow decay corresponds to the quenching of FC10MQ fluorescence in the aqueous bulk phase by the free counterions. The Stern-Volmer plots for fluorescence quenching gave a distinct break at the critical micelle concentration of the cationic surfactants. The abrupt increase in fluorescence quenching is attributed to the solubilization of the probe in the micelles. The fluorescence quenching behavior provides direct information about the immiscibility of fluorocarbon and hydrocarbon species in micelles, and the results indicate that almost pure fluorocarbon micelles appear in surfactants mixtures.     

Micellar medium effects on the hydrolysis of phenyl chloroformate in ionic,zwitterionic, nonionic,and mixed micellar solutions

The spontaneous hydrolysis of phenyl chloroformate was studied in various anionic, nonionic, zwitterionic, and cationic aqueous micellar solutions, as well as in mixed anionic–nonionic micellar solutions. In all cases, an increase in the surfactant concentration results in a decrease in the reaction rate and micellar effects were quantitatively explained in terms of distribution of the substrate between water and micelles and the first‐order rate constants in the aqueous and micellar pseudophases. A comparison of the kinetic data in nonionic micellar solutions to those in anionic and zwiterionic micellar solutions makes clear that charge effects of micelles is not the only factor responsible for the variations in the reaction rate. Depletion of water in the interfacial region and its different characteristics as compared to bulk water, the presence of high ionic concentration in the Stern layer of ionic micelles, and differences in the stabilization of the initial state and the transition state by hydrophobic interactions with surfactant tails can also influence reactivity. The different deceleration of the reaction observed in the various micellar solutions studied was discussed by considering these factors. Synergism in mixed‐micellar solutions is shown through the kinetic data obtained in these media. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 445–451, 2002     

Kinetics of alkaline fading of methyl violet in micellar solutions of surfactants: Comparing Piszkiewicz's,Berezin's,and pseudophase ion-exchange models

The micellar effect of surfactants of various types on the rate of the reaction between methyl violet and hydroxide ion is studied. The absorption spectra show that the cation of methyl violet is bound by micelles of all types at proper concentrations of surfactants. The observed rate constant in micellar systems containing nonionic Brij-35, zwitterionic 3-(dimethyldodecylammonio)-propanesulfonate, cationic cetyltrimethylammonium bromide and hydroxide surfactants is higher, whereas in solutions of the anionic surfactant sodium dodecylsulfate is lower than that one in the surfactant-free system. Piszkiewicz's, Berezin's, and pseudophase ion-exchange models of the kinetic micellar effect are used for the treatment of the dependences of the above-mentioned constants on the surfactant concentration. The values of the corresponding kinetic parameters are compared and discussed. The influence of nonionic, zwitterionic, and anionic micelles on the reaction rate is discussed on the basis of medium and concentration kinetic effects. The character of the cationic micelles effect is somewhat paradoxical. Although the observed pseudo–first-order reaction rate constant substantially increases in the presence of such micelles, the second order-rate constant in these micelles is lower than the corresponding value in surfactant-free aqueous solution. As a possible explanation, the decrease in the reactivity of the HO^– ions is proposed, owing to their electrostatic association with the cationic headgroups (“diverting effect”).     

Application of a chemical equilibrium model to thermodynamic functions of transfer of alcohols from water to aqueous surfactants

In ternary aqueous solutions, hydrophobic solutes such as alcohols tend to aggregate with surfactants to form mixed micelles. These systems can be studied by meas of the functions of transfer of hydrophobic solutes from water to aqueous solutions of surfactant. These thermodynamic functions often go through extrema in the critical micellar concentration (CMC) region of the surfactant. A simple model based on interactions between surfactant and hydrophobic solute monomers, on the distribution of the hydrophobic solute between water and the micelles and on the shift in the CMC induced by the hydrophobic solute, can simulate the magnitude and trends of the transfer functions using parameters which are mostly derived from the binary systems. In order to check the model more quantitatively, volumes and heat capacities of transfer of alcohols from water to aqueous solutions of a nonionic surfactant, octyldimethylamine oxide, were measured. A quantitative agreement was achieved with three adjustable parameters. Good fits are also obtained for the transfers to the ionic surfactants, octylamine hydrobromide and sodium dodecylsulfate. When the equilibrium displacement contribution is small, the distribution constants and the partial molar properties of the alcohols in the micellar phase agree well with the parameters obtained with similar models.     

Sphere-to-rod transitions of nonionic surfactant micelles in aqueous solution modeled by molecular dynamics simulations

Control of the size and agglomeration of micellar systems is important for pharmaceutical applications such as drug delivery. Although shape-related transitions in surfactant solutions are studied experimentally, their molecular mechanisms are still not well understood. In this study, we use coarse-grained molecular dynamics simulations to describe micellar assemblies of pentaethylene glycol monododecyl ether (C(12)E(5)) in aqueous solution at different concentrations. The obtained size and aggregation numbers of the aggregates formed are in very good agreement with the available experimental data. Importantly, increase of the concentration leads to a second critical micelle concentration where a transition to rod-like aggregates is observed. This transition is quantified in terms of shape anisotropy, together with a detailed structural analysis of the micelles as a function of aggregation number.     

Kinetics of Simultaneous Adsorption of Poly(vinylpyrrolidone) and Sodium Dodecyl Sulfate on Alumina Particles

The effect of Cibacron Blue 3GA (CB) and bovine serum albumin (BSA) as guest molecules on the microstructure of reversed micelles has been investigated with electrical conductivity measurements. CB as an affinity ligand was directly introduced to reversed micelles formed with a cationic surfactant, cetyltrimethylammonium bromide. The anionic CB has electrostatic interactions with the cationic surfactant and also has a strong binding affinity to BSA. The conductivity of reversed micellar systems increases gradually with the increase of temperature either with or without the addition of CB. The conductivity of reversed micellar systems increases with the addition of tributyl phosphate to the organic phase. No electrical percolation appears with an increase of temperature or water concentration. The conductivity of reversed micellar systems decreases with the addition of CB and decreases further with the addition of both CB and BSA. The conductivity of the organic phase is 3 orders of magnitude lower than that of the aqueous phase under the same CB concentration, which indicates that CB is probably confined to the closed microdomains of reversed micellar systems. The conductivity behavior of reversed micelles has not shown much difference with the methods used for the addition of CB either by the injection method or by phase transfer. Copyright 2000 Academic Press.     

Preparation of substituted ionic carbohydrate polymers and their interactions with ionic surfactants

The formation of micelles of hexadecyltrimethylammonium chloride (CTAC) and sodium dodecylsulfate (SDS) in aqueous solutions containing charged polysaccharides was studied by steady-state and time-resolved fluorescence measurements using pyrene as a photophysical probe. Micropolarity studies using the I₁/I₃ ratio of the vibronic emission bands of pyrene and the behaviour of the I_E/I_M ratio between the excimer and monomer emissions show the formation of hydrophobic domains. The interactions between the polyelectrolytes and surfactants of opposite charge lead to the formation of induced pre-micelles at surfactant concentrations lower than the critical micellar concentration (cmc) of the surfactants. At similar concentrations, the I_E/I_M ratio shows a peak. This aggregation process is assumed to be due to electrostatic attractions. At higher surfactant concentrations, near the critical micellar concentration, micelles with the same properties as those found in pure aqueous solution are formed. On the other hand, systems containing polyelectrolytes and surfactants of the same charge do not show this behaviour at low concentrations. The presence of long alkyl chains bound to the polyelectrolytes also induces the formation of free micelles at concentrations somewhat below the aqueous cmc.     

Structure and rheology of reverse micelles in dipentaerythrityl tri-(12-hydroxystearate)/oil systems

We report the formation of reverse rod-like micelles and their rheological properties in novel nonionic surfactant, dipentaerythrityl tri-(12-hydroxystearate) (designated as WO-6)/oil systems without external water addition. Small-angle X-ray scattering (SAXS) was used to investigate the structure of the micelles and their flow properties were studied by rheological measurements. We found that WO-6 spontaneously self-assembles into reverse micelles in a variety of organic solvents at ambient conditions, their structure depending on solvent molecular architecture, surfactant concentration, and temperature. Rod-like micelles with a maximum length of ca. 12 nm and a cross section diameter of ca. 2 nm were observed in cyclohexane. When cyclohexane was replaced with a linear chain octane, the length and the cross section diameter were simultaneously increased. With a further increase of hydrocarbon chain length of solvent oils from octane to hexadecane, the rod-like micelles grew axially, keeping the cross section diameter (ca. 3 nm) virtually constant. Increasing surfactant concentration also favored one-dimensional micellar growth. On the other hand, micelles shrunk with the rise of temperature, which is similar to a rod-to-sphere transition, and is essentially the opposite temperature dependence to that often observed in aqueous micellar systems. A structural picture drawn by SAXS is well supported by rheology; the relative (zero-shear) viscosity of the WO-6/oil systems was found to be markedly greater than that expected for a dispersion of spherical particles due to the elongated micellar structure, despite quantitative inconsistency with semi-empirically predicted values for rigid rod-like particles.     

BEHAVIOR OF ANIONIC SURFACTANT MICELLES IN THE PRESENCE OF Al3+ AND Ca2+

The estimation of the C-potential of ionic surfactant micelles may be useful for the study of adsorption of solutes onto the micellar surface, which causes a reduction of the net electrostatic charge. This work presents results on the variation of ζ-potential of alkylsulfate and fatty carboxylate micelles with the bulk concentrations of Al³⁺ and Ca²⁺ cations. Combined with results from the literature about the effect of micellar surfactant concentration on reducing surfactant precipitation in the presence of polyvalent cations, these allow to conclude that micelles of anionic surfactants will have a higher chance of electroneutralization of their surface charge by adsorbing cations if the end functional group of the surfactant is smaller.     

Self-assembly of mixed anionic and nonionic surfactants in aqueous solution

We present the phase diagram and the microstructure of the binary surfactant mixture of AOT and C(12)E(4) in D(2)O as characterized by surface tension and small angle neutron scattering. The micellar region is considerably extended in composition and concentration compared to that observed for the pure surfactant systems, and two types of aggregates are formed. Spherical micelles are present for AOT-rich composition, whereas cylindrical micelles with a mean length between 80 and 300 ? are present in the nonionic-rich region. The size of the micelles depends on both concentration and molar ratio of the surfactant mixtures. At higher concentration, a swollen lamellar phase is formed, where electrostatic repulsions dominate over the Helfrich interaction in the mixed bilayers. At intermediate concentrations, a mixed micellar/lamellar phase exists.     

Concentration and medium micellar kinetic effects caused by morphological transitions

The reaction methyl naphthalene-2-sulfonate + Br(-) was investigated in several alkanediyl-α-ω-bis(dodecyldimethylammonium) bromide, 12-s-12,2Br(-) (with s = 2, 3, 4, 5, 6, 8, 10, 12), micellar solutions in the absence and in the presence of various additives. The additives were 1,2-propylene glycol, which remains in the bulk phase, N-decyl N-methylglucamide, MEGA10, which forms mixed micelles with the dimeric surfactants, and 1-butanol, which distributes between the aqueous and micellar phases. Information about the micellar reaction media was obtained by using conductivity and fluorescence measurements. In all cases, with the exception of water-1,2-prop 12-5-12,2Br(-) micellar solutions, with 30% weight percentage of the organic solvent, a sphere-to-rod transition takes place upon increasing surfactant concentration. In order to quantitatively explain the experimental data within the whole surfactant concentration range, a kinetic equation based on the pseudophase kinetic model was considered, together with the decrease in the micellar ionization degree accompanying micellar growth. However, theoretical predictions did not agree with the experimental kinetic data for surfactant concentrations above the morphological transition. An empirical kinetic equation was proposed in order to explain the data. It contains a parameter b which is assumed to account for the medium micellar kinetic effects caused by the morphological transition. The use of this empirical equation permits the quantitative rationalization of the kinetic micellar effects in the whole surfactant concentration range.     

Clouding phenomenon and SANS studies on tetra-n-butylammonium dodecylsulfate micellar solutions in the absence and presence of salts

Clouding phenomenon in aqueous micellar solutions of an anionic surfactant tetra-n-butylammonium dodecylsulfate (TBADS) has been observed as a function of surfactant concentration. Small-angle neutron scattering (SANS) experiments in these systems show clustering of micelles as the temperature approaches the cloud point (CP). The individual micelles and the clusters of micelles coexist at CP. The clustering of micelles depends on the surfactant concentration and temperature. It is proposed that clustering is due to depletion of H-bonded water present around the butyl chains at the micellar surface. This is associated with entropy gain which is considered to be the major thermodynamic factor related to micellar aggregation. The structures (clusters) that emerge depend on the relative lengths of the alkyl chains of the counterion and can be tuned by the temperature.     

Thermodynamic Characteristics of the Micellization in Droplet and Quasi-Droplet Models of Surfactant Molecular Aggregates with Account of Experimental Data on Equilibrium Micelle Distribution

Formuals for the thermodynamic characteristics of micellization in the droplet and quasi-droplet models of surfactant molecular aggregates are derived. These formulas account for the experimental data on the mean size of micelles and average statistical scatter of their sizes in the equilibrium state. These formulas cover critical micellization concentration corresponded to the onset of surfactant accumulation in micelles and higher (than CMC) concentrations at which micelles incorporate noticeable or even the largest portion of surfactant in micellar solution. Analytical dependence of thermodynamic characteristics of micellization on the initial parameters of droplet and quasi-droplet models of molecular aggregates at critical micellization concentration is disclosed.     

Boltzmann distributions and slow relaxation in systems with spherical and cylindrical micelles

Equilibrium and nonequilibrium distributions of molecular aggregates in a solution of a nonionic surfactant are investigated at the total surfactant concentration above the second critical micelle concentration (CMC2). The investigation is not limited by the choice of a specific micellar model. Expressions for the direct and reverse fluxes of molecular aggregates over the potential humps of the aggregation work are derived. These aggregation work humps set up activation barriers for the formation of spherical and cylindrical micelles. With the aid of the expressions for molecular aggregate fluxes, a set of two kinetic equations of micellization is derived. This set, along with the material balance equation, describes the molecular mechanism of the slow relaxation of micellar solution above the CMC2. A realistic situation has been analyzed when the CMC2 exceeds the first critical micelle concentration, CMC1, by an order of magnitude, and the total surfactant concentration varies within the range lying markedly above the CMC2 but not by more than 2 orders of magnitude. For such conditions, an equation relating the parameters of the aggregation work of a cylindrical micelle to the observable ratio of the total surfactant concentration and the monomer concentration is found for an equilibrium solution. For the same conditions, but in the nonequilibrium state of materially isolated surfactant solution, a closed set of linearized relaxation equations for total concentrations of spherical and cylindrical micelles is derived. These equations determine the time development of two modes of slow relaxation in micellar solutions markedly above the CMC2. Solving the set of equations yields two rates and two times of slow relaxation.     

Counterion condensation and release in micellar solutions

Counterion condensation and release in micellar solutions are investigated by direct measurement of counterion concentration with ion-selective electrode. Monte Carlo simulations based on the cell model are also performed to analyze the experimental results. The degree of counterion condensation is indicated by the concentration ratio of counterions in the bulk to the total ionic surfactant added, alpha< or =1. The ionic surfactant is completely dissociated below the critical micelle concentration (cmc). However, as cmc is exceeded, the free counterion ratio alpha declines with increasing the surfactant concentration and approaches an asymptotic value owing to counterion condensation to the surface of the highly charged micelles. Micelle formation leads to much stronger electrostatic attraction between the counterion and the highly charged sphere in comparison to the attraction of single surfactant ion with its counterion. A simple model is developed to obtain the true degree of ionization, which agrees with our Monte Carlo results. Upon addition of neutral polymer or monovalent salts, some of the surfactant counterions are released to the bulk. The former is due to the decrease of the intrinsic charge (smaller aggregation number) and the degree of ionization is increased. The latter is attributed to competitive counterion condensation, which follows the Hefmeister series. This consequence indicates that the specific ion effect plays an important role next to the electrostatic attraction.