Review with 44 references of micellar catalysis in aqueous solutions containing normal micelles. Topics include conventional micellar catalysis, functional surfactants, micellar autocatalysis, emulsion polymerization, and synthesis of mesoporous aluminosilicates.
The effect of aluminate and silicate counterions on the structure of surfactant aggregates used in the preparation of mesoporous aluminosilicates were investigated using dynamic light scattering and rheological techniques. Aqueous cetyltrimethylammonium chloride (CTAC) solutions saturated with sodium aluminate, sodium silicate or tetramethylammonium silicate were studied over a wide range of CTAC concentrations (1.0-25.0 wt %), which included both spherical and rod-shaped micellar regimes. Sodium aluminate was observed to have no effect on the structure of CTAC micelles and so plays no role in aggregate formation in the prereaction stage of the production of aluminosilicates such as MCM-41. Silicate anions from sodium silicate and tetramethylammonium silicate strongly affected CTAC micelle formation, promoting the formation of flexible wormlike micelles at dilute concentrations in which elongated wormlike micelles are not entangled. The condensation of silicate ion with the tetramethylammonium cation in CTAC/tetramethylammonium silicate solution inhibited the condensation of silicate ion at the surface of the CTAC micelles. Results strongly suggest that for the two silicate sources studied, flexible CTAC wormlike micelles surrounded by silicate oligomers are formed first in the preparation of aluminosilicate materials; the subsequent intermicellar condensation of silicate oligomers then leads to the gradual arrangement of the micelles into hexagonal liquid crystalline arrays. The effect of a chloride ion (at a constant molar Cl-/CTA+ ratio of 3.98) was also investigated using sodium chloride. Unlike the aluminate and the silicate ions, a transition between the rodlike and flexible wormlike micellar regions was observed. The semidilute regime, defined as the concentration range over which the collective diffusion coefficient is described by the power law model for single chain polymer in good solvent, was observed at 11.0-21.6 wt %. The transition into the liquid crystalline regime was observed to begin at 21.6-25.0 wt %.
PET/POET (polyethylene terephthalate/polyoxyethylene terephthalate) water-soluble polymers are effective surface modifiers of polyester-containing textiles and exhibit surface activity in aqueous solutions. Adsorption of a PET/POET polymer on fabric was studied using UV-VIS and fluorescence spectroscopy. Aqueous PET/POET solutions exhibit two distinct fluorescent emission bands, 334 nm and 386 nm, and the latter band is assigned to an intramolecular excimer of terephthalate groups. The ratio of the fluorescence intensities of these two bands (I386/I334) decreases upon adsorption of the polymer onto fabrics, indicating that one of the solution structures is preferentially adsorbed. Addition of anionic surfactant such as sodium dodecylsulfate (SDS) inhibits the formation of excimers whereas nonionic surfactant does not. Effects of nonionic, anionic surfactants, as well as salt effects (Na+, Ca++) on the formation of excimers and on the adsorption behavior were investigated using both UV absorption and fluorescent emission bands, cloud point, and surface techniques. Polymer-surfactant interactions were observed predominantly at surfactant concentrations below the critical micelle concentration. The apparent cmc of SDS increased with increasing polymer concentration, and SDS interactions with the polymer can be explained by models previously used to describe interaction between polyethylene oxide and SDS.
Cell damage has been obsd. in suspension cell cultures with air sparging, esp. in the absence of any protective additives. This damage is assocd. with cells adhering to bubbles, and it has been shown that if this adhesion is prevented, cell damage is prevented. This article presents a thermodn. approach for predicting cell adhesion at the air-medium interface. With this relation it can be shown that cell-gas adhesion can be prevented by lowering the surface tension of the liq. growth medium through the addn. of surface active protective additives. The thermodn. relation describes the change in free energy as a function of the interfacial tensions between the (i) gas and liq. phases, (iii) gas and cell phases, and (iii) liq. and cell phases. Exptl. data, along with theor. and empirical equations, are used to quantify the changes in free energy that predict the process of cell-gas adhesion. The thermodn. model is nonspecific in nature and, consequently, results are equally valid for all types of cells.
Synthesis of N,N-dimethyldodecylamine N-oxide in aqueous solutions by micellar autocatalysis was investigated. Micellar autocatalysis is a novel variation of conventional micellar catalysis in which surfactant micelles catalyze the reaction by which the surfactant itself is synthesized. The lipophilic reactant, dimethyldodecylamine, was initially solubilized in micellar solutions of the amine oxide surfactant, resulting in substantially higher reaction rates. Amine conversions of 90-100% were obtained within two hours at 70oC. Effects of reactant concentrations, temperature, and initial surfactant concentration were studied. For systems with no surfactant at time zero, the system was initially an emulsion and reaction rates were low. A sharp increase in the rate was observed when enough surfactant had been produced to form micelles. Activation energy calculations indicate that enhancements of the rate were due to primarily to localized concentration of reactants in the micelle. A simple pseudophase model was used to model reactions under pseudo first-order reaction conditions.
A significant degree of cell damage is observed during suspension cell culture with air sparging. Protective agents can be added to the culture medium to protect the cells from damage. It has been observed that cells tend to adhere to air-medium interfaces and cell damage is mainly due to this cell-bubble interaction; protective additives have been found to prevent this cell adhesion to the bubble surfaces. In this article, it is demonstrated that the interfacial tension between the air and medium is related to the effectiveness of the protective additives to prevent adhesion of cells to this interface. Five different types of additives (Pluronic F-68, Methocels, dextran, polyvinyl alcohol, and polyethylene glycols) were studied in an effort to determine their protective characteristics. Liquid-vapor interfacial tensions of the culture medium, with and without the additives, were measured my two different techniques (maximum bubble pressure method and Wilhelmy plate method). In addition, visualization techniques showed that in the presence of certain protective additives cells do not adhere to the bubble surface. Results obtained from these experiments indicate that the additives which rapidly lower the liquid-vapor interfacial tension of the culture medium also prevent adhesion of cells to the bubble surface. Experiments have also been conducted to determine the number of cells killed due to bubble rupture, and it was observed that this number is related to the amount of cells adhered to the bubble surface.
A review, with 27 references, of reaction engineering of particulates, encompassing a wide range of conditions (e.g., with gases, liquids, or gas-liquid mixtures) and particle sizes with an without catalysts. Four key areas were identified: ultrafast reactions, ceramic powder production, aerosol production, and colloidal systems. Important applications involve: SO2-sorbent studies in ultrafast entrained-flow reactors, ultrafast pyrolysis of heavy feedstocks (e.g., oil-sand bitumen), preparation of non-oxide ceramic powders (e.g., Si3N4, SiC, BC, etc.), Co-W-C nanophase composites, semiconductor colloids, micellar catalysts, and types of colloidal aggregates.
The solution properties of zwitterionic surfactants are shown to be strong functions of the type of negatively charged center (carboxylate vs sulfonate) and the number of methylene groups separating the charged sites. For similar structures, differences such as the higher solubility and critical micelle concentration of the betaine relative to the sulfobetaine can be explained as a direct result of the carboxylate headgroup being more hydrophilic than the sulfonate. No evidence is seen of intramolecular ion-pair formation, indicating that in a polar medium such as water, the distance between charged sites increases monotonically with increasing number of methylene units in the tether. The increasing headgroup area with tether length leads to poor foaming and aqueous solution thickening properties. The monomeric and micellar compositions of betaine surfactants as a function of pH and concentration can be determined directly from titration curves.
Fourier transform infrared spectroscopy was used to study pH effects on solutions of alkyldimethylamine oxide surfactants, CnAO (CH3(CH2)n-1(CH3)2NO with n=6, 8, 12). Results are presented for the neutral surfactant (high pH), the protonated (cationic) surfactant (low pH) and a 1:1 mixture at pH=pKa. The frequency of the Vs CH2 band is used to monitor the monomer-to-micelle transition, as well as the effects of adjusting the solution pH at a given concentration. Band assignments are presented for the pH-sensitive Va C-N-O modes. For monomer solutions of C8AO, the synthetic IR spectrum of the 1:1 mixture, constructed from a simple linear combination of the experimental spectra at high and low pH, agrees very well with the experimental spectrum, proving that monomer-monomer interactions are negligible in this particular system. Spectroscopic evidence for specific headgroup interactions between cationic and nonionic moieties in the 1:1 mixed micelle is observed.
The structures of mixed micelles of C12-14-alkyldimethylamine oxides and SDS were a function of compn. In the L1 micellar pseudophase (an isotropic water-continuous liq. phase composed of micellar aggregates), a sphere-to-rod transition driven by ion-dipole interactions between the dissimilar head groups led to synergisms in aq. soln. thickening, Ross-Miles foaming, and nonpolar oil solubilization. For example, a seven orders of magnitude increase in the zero-shear viscosity and viscoelastic properties were obsd. at a single total surfactant concn. The sphere-to-rod transition was studied by FTIR spectroscopy by examg. both the CH2 stretching for the methylene tails, and the S-O stretching for the sulfate head groups.
The pseudophase separation model is used to describe pH and concentration effects on dimethyldodecylamine oxide (DDAO) solutions. If the protonated and neutral species are treated as separate surfactants, an amine oxide surfactant can be modeled thermodynamically as a binary mixture, the composition of which is varied by adjusting the solution pH. With the Gibbs-Duhem equation written for the micelle pseudophase, activities of each surfactant species at concentrations greater than the critical micelle concentration (cmc) can be calculated directly from experimental titration curves. It is not necessary to introduce an "apparent" pKa for the surfactant in micellar form.
The heat of mixing single-component micelles to form mixed micelles was measured by isoperibol calorimetry. Results are presented for 10 binary surfactant mixtures: 2 nonionic/nonionic systems, 1 cationic/cationic system, 5 cationic/nonionic systems, and 2 anionic/nonionic systems. The heat of mixing was exothermic for all systems except for mixtures of cationic surfactant with a nonionic alkylphosphine oxide, for which endothermic heats were observed. The heats of mixing for anionic/nonionic systems were much more exothermic than for the other mixtures studied. These results indicate that the heat of mixing is strongly dependent on electrostatic interactions and the structure of the surfactants involved. Comparison of the heat of mixing data with the excess Gibbs free energy of mixing, obtained from mixture cmc measurements, suggests that the relative contribution of enthalpic and entropic effects to the nonideal behavior of mixed micelle formation may be quite different for different types of systems.
Previous work has shown that an electrostatic model can be used to accurately predict the fractional counterion binding on mixed ionic/nonionic micelles. New results show that this model can be successfully applied to a wide variety of surfactant mixtures. In particular, it is shown that mixed micelles containing nonethoxylated nonionic surfactants such as phosphine oxides, amine oxides, and sulfoxides at appropriate pH conditions exhibit binding behavior similar to that of mixtures containing polyethoxylates. The following ionic surfactants were studied in mixtures with a polydisperse nonylphenol polyethoxylate as the nonionic surfactant: dodecylpyridinium chloride, hexadecylpyridinium chloride, sodium dodecyl sulfate, and sodium octylbenzenesulfonate. The electrostatic model describes the binding for these systems very well. The following nonionic surfactants were studied in mixtures with the cationic surfactant hexadecylpyridinium chloride: a monodisperse dodecyl polyethoxylate alcohol, two polydisperse nonylphenol polyethoxylates, dodecyldimethyphosphine oxide, dodecyldimethylamine oxide, and decyl methyl sulfoxide. The electrostatic model accurately describes the binding on all systems, although the model is slightly less accurate for the sulfoxide mixtures. These results are consistent with the concept that ionic/nonionic surfactant interactions and nonidealities of mixing in micelles are primarily of electrostatic origin, with specific chemical interactions being of secondary importance.
A new method of calculating the composition of mixed micelles in equilibrium with monomer of known composition is proposed. This technique applies the Gibbs-Duhem equation to the mixed micelle, which is treated as a pseudophase. The method needs only critical micellization concentration (cmc) data as a function of monomer composition, but is limited to binary surfactant systems. The proposed methodology is applied to cationic/cationic, cationic/nonionic, and anionic/nonionic surfactant pairs. The calculated monomer-micelle equilibrium is very similar to that from ideal solution theory for the cationic/cationic system and to the much-used regular solution theory for the nonideal systems.
Critical micelle concentrations were measured as a function of composition for three binary ionic/nonionic surfactant mixtures. These systems exhibit large negative deviations from ideality. Two models based on electrostatic considerations alone were developed to describe mixed micellar nonidealities. One model considers the micelle pseudophase to consist of surfactant components only, while the other also includes bound counterions. Both models give a priori predictions of mixture behavior and give excellent agreement with experimental data. These results indicate that the factors giving rise to the thermodynamic nonidealities are primarily electrostatic in nature.
Fractional counterion bindings were measured on micelles composed of mixtures of anionic/nonionic, cationic/nonionic, anionic/anionic, and cationic/cationic surfactants. Surfactants used were sodium decyl sulfate, sodium decyl sulfonate, hexadecylpyridinium chloride, hexadecyltrimethylammonium chloride, and a nonylphenol polyethoxylate. For ionic/nonionic mixed micelles, the fractional counterion binding varied monotonically between the value for the pure ionic micelle and zero, as the nonionic surfactant content of the micelle increases. For ionic/ionic mixed micelles, the binding varies monotonically between the values of the pure ionic micelles as composition varies. A localized adsorption model and a mobile adsorption model were developed to describe the binding. Both models were based on electrostatic considerations and both described the binding data very well. On physical grounds, the localized adsorption model is preferred, particularly for ionic/nonionic mixed micelles composed mainly of nonionic surfactant. These results indicate that electrostatic considerations are the dominant cause of the thermodynamic nonideality of the ionic/nonionic mixed micelle. Counterion bindings on mixed ionic/nonionic micelles vary little with temperature, a feature in common with micelles composed of a single ionic surfactant.