Membrane osmometry of aqueous micellar systems

Summary The number average micellar molecular weights,M _n , were determined with the application of membrane osmometry, for various nonionic and a cationic surfactant, in aqueous solutions. These results show that these micelles are monodisperse, as also reported by other investigators. The dependence ofM _n on temperature was also studied. It was found that both the nonionic and the cationic micelles showed an abrupt change in logM versus temperature plots. In the case of nonionics, the abrupt change was observed at approximately 30 °C lower than the cloud-point.Presented at the 25th Colloid-Meeting in Munich, October 13–15, 1971.     

Dissociation of acids in aqueous micellar systems

Incorporation in charged micelles induces large pKa shifts for a number of acids of varying type. Analysis of the measurements in terms of simple electrostatic theory is reasonably satisfactory in view of uncertainties regarding the net charge on the micelle and the exact character of its surface. The behavior of the long-chain fatty acids, whose mode of incorporation in micelles is least uncertain, confirms the suggestion of others that the effective dielectric constant at the micellar surface is quite low.     

Thermodynamic molar properties of some aqueous micellar systems

Densities and ultrasonic velocities are measured then the partial molar volumes and adiabatic compressibilities are calculated at various concentrations of aqueous solutions of sodium dodecyl sulphate , sodium cholate and sodium taurcholate. These three detergents are known to form aggregates called micelles at specific concentrations. These concentrations are given the name , critical micelle concentration (cmc). Due to the nonlinear behaviour of the thermodynamic partial molar parameters with detergent's concentration no quantitative conclusions could be drawn . However, a reasonable qualitative model is put forward to explain the results.     

Photochemical behaviour of halophenols in aqueous solution

The mechanism of photolysis by direct absorption of chloro-, bromo-and fluorophenols (XPhOH with X=Cl, Br or F) is much more dependent on the position of the halogen than on its nature. In all cases, the first step is a heterolytic C-X scission with release of the halogenated acid HX. 3-XPhOH is almost specifically converted into resorcinol. The anionic form of 2-XPhOH is transformed with a rather high quantum yield into cyclopentadiene carboxylic acids. By nanosecond laser-flash photolysis the first transient detected is a ketene which is converted into fulvene-6, 6 diol and then into the cyclopentadiene carboxylic acids. The phototransformation is about 10 times less efficient from the neutral form and not so specific. The irradiation of 4-XPhOH leads to the formation of a carbene which is the first detected transient. According to the experimental conditions, this transient reacts with oxygen producing a benzoquinone-O-oxide and subsequentlyp-benzoquinone, with water leading to hydroquinone or with another molecule of halophenol producing a halogenodihydroxybiphenyl. It was also observed that 4-chlororesorcinol behaves as both 4-ClPhOH and 2-ClPhOH. Chlorohydroquinone does not react as monohalophenols, the main photoproducts are hydroquinone and chlorobenzoquinone. This reaction is consistent with a radical mechanism. The transformation of halophenols can be sensitized by phenol and in most cases by hydroquinone. The half-life of the triplet state of hydroquinone was evaluated at 0.9μs and the intersystem crossing yield at 0.39. The sensitization of 3-FPhOH was observed with phenol but not with hydroquinone leading to the conclusion that the energy level of its triplet state lies in the range 310–350 kJ mol⁻¹. It is lower than 310 kJ for the other halophenols studied here.     

Acid—base titrations in aqueous micellar systems

Some water-insoluble carboxylic acids and amines have been titrated in aqueous surfactant solutions to demonstrate that solubilization in micelles provides a useful solvent system for certain acid—base titrations. Potentiometric titrations can be very slow because of electrode drift in these solutions, but visual titrations are convenient. Observed pK shifts are generally interpretable in terms of the influence of micellar charge on the work of proton removal.     

L-asparaginase release from Escherichia coli cells with aqueous two-phase micellar systems

A method was proposed to release and separate L-asparaginase (EC 3.5.1.1) from Escherichia coli ATCC 11303 cells with aqueous two-phase micellar systems. The systems were composed of K2HPO4 and Triton X-100. The method combines enzyme release with enzyme purification. The influence of Triton X-100 concentration, K2HPO4 concentration, and pH on the release and partition of L-asparaginase was investigated. Experimental results showed that E. coli cells treated with 9.4% (w/v) K2HPO4 and 15% (w/v) Triton X-100 at 25 degrees C for 15-20 h released nearly 80% of the enzyme. Most of the released enzyme was partitioned to the bottom phase (phosphate-rich phase). The effects of Triton X-100 concentration, K2HPO4 concentration, and pH on cloud point were also studied. Electron micrography indicated that the chemical treatment altered the inner structure of E. coli cells significantly.     

Pyrene-N,N-dimethylaniline heteroexcimer systems in aqueous micellar solutions

Strong quenching of fluoroscence of pyrene by N,N-dimethylaniline as well as N,N-dimethylaniline sulfonate has been observed in aqueous micellar solutions. Photoinduced electron transfer from the anilines to excited pyrene near the micelle-water interface has been demonstrated by means of laser photolysis studies.     

Extraction of the antimicrobial peptide cerein 8A by aqueous two-phase systems and aqueous two-phase micellar systems

Cerein 8A is an antimicrobial peptide with potential application against food spoilage and pathogenic bacteria. The partitioning of cerein 8A was investigated in two liquid-liquid extraction systems that are considered promising for bioseparation and purification purposes. Aqueous two-phase systems (ATPSs) were prepared with polyethylene glycol (PEG) and inorganic salts, and the addition of NaCl was investigated in this system. The best results concerning partition coefficients (K (b)) were obtained with PEG?+?ammonium sulphate, and K (b) value significantly increases when NaCl was added. Cerein 8A was effectively extracted into the micelle-rich phase in a 4% Triton X-114 medium. Recovery yield was higher for ATPS compared to micellar systems. Cerein 8A can be isolated from a crude suspension containing the bioactive molecule by ATPSs. Successful implementation of peptide partitioning represents an important step towards developing a low-cost effective separation method for cerein 8A.     

Chemiluminescence of bis(2,4,6-trichlorophenyl) oxalate in aqueous micellar systems

The chemiluminescent reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) in aqueous micellar systems is compared with the reaction in a mixture of acetonitrile and aqueous phosphate buffer. The chemiluminescence was studied in batch experiments with perylene as the fluorophore. The oxidation of TCPO produced the same intensity of chemiluminescence in the buffered acetonitrile as in Arkopal N-300 micelles, the best micellar system. The solubility of TCPO in an aqueous micellar system is greater than that in the acetonitrile/aqueous buffer (80:20, v/v), but TCPO is less stable in the former system.     

Thermal analysis of aqueous micellar solutions

A micro differential temperature scanning calorimeter was used to characterize the structural changes between different types of micelles in aqueous solutions of ionic surfactants: anionic — sodium dodecylsulfate (SDS) — and cationic — hexadecyltrimethyl ammonium bromide (CTAB). Moreover, this technique allowed to confirm the existence of peculiar types of complexes between surfactants and selected solutes. In SDS solutions containing polyethylene glycols (PEG), the presence of complexes formed by small micelles adsorbed along the chains of the polymers was evidenced in the case of long enough polymer chains. In CTAB-phenol solutions, due to strong interactions between the polar heads of surfactant and phenol, molecular complexes of a composition of 1:1 molar ratio have been characterized. Depending on the ratio [phenol]/[CTAB], the rheological behaviour was found to change from fluid to viscoelastic and gel-like solutions, owing to the growth of elongated rod-like micelles. With entangled worm-like micelles, the important role of kinetics to reach the thermodynamic equilibria was shown.     

Fluorescence quenching mechanisms of aromatic hydrocarbon-dicyanobenzene and-n.n-dimethylaniline systems in aqueous micellar solutions

Fluorescence quenching of pyrene and several other aromatic hydrocarbons by dicyanobenzenes as well as N,N-dimethylaniline has been observed in aqueous micellar solutions. The experimental results have been explained by means of equations for fluorescence intensity and decay curve derived on the basis of a simplified model.     

Ring-closing metathesis in aqueous micellar medium

Underwater exploration: The ring-closing metathesis of N,N-diallyltosylamine (DATs) and diallyldiethyl malonate has been studied in aqueous micellar medium, at room temperature, in the presence of four different gemini cationic surfactants and various ruthenium catalysts. For the first time, the adsorption mechanisms and the reaction steps involved in this heterogeneous catalytic process were elucidated.     

The methods of determination of critical micellar concentrations of the amphiphilic systems in aqueous medium

Science is the attempt to make the chaotic diversity of our sense-experience to a logically unified system of thought. A sound theoretical basis and a rational physicochemical understanding based on experimental findings lead to a proper understanding of a topic. Colloid chemistry strengthened its footing by the extensive studies on gold sols by Michael Faraday in 1850s, however, the term “colloid” was coined by Thomas Graham in 1861. Historically, the Stone Age paintings in the Lascaux cave in France and the written records of Egyptian pharaohs were produced with stabilized colloid pigments. The part of colloid science dealing with amphiphilic molecules is so diverse on its own that it is dealt in a separate branch called the “association colloid”. A key development in the study of association colloids was the observation by James McBain that the osmotic pressure of salts of alkali metal fatty acid displayed a pronounced break in the concentration beyond a specific characteristic concentration, after which the osmotic coefficient remained almost constant (Evans and Wennerstrom, 1994). McBain attributed this fact to the self-association of these molecules among themselves (McBain, 1913, McBain, 1944) in solution. Here, we are providing a molecular as well as thermodynamic approach toward the micellization process.     

Two-photon absorption in aqueous micellar solutions

Addition of the surfactant sodium dodecyl sulfate at concentrations above the critical micelle concentration increases the fluorescence quantum yield and the two-photon absorption cross-section of charged [2.2]paracyclophane chromophores containing pairs of D-pi-D chromophores. The resulting spectra in the micellar solutions are very similar to those obtained for neutral isostructural analogues in toluene. The measured etadelta values are 1300 GM for 1C and 1920 GM for 2C, which are comparable or larger to those in toluene. These results highlight possible misleading interpretation of two-photon-induced emission for evaluating the concentration of labeled substrates used in two-photon microscopy and provide guidelines for designing molecular structures with optimized two-photon action cross-sections in water.     

Viscosity of aqueous micellar solutions of surfactants

The viscosity of aqueous micellar solutions of two long-chain surfactants, C₂₄H₄₃N₂ClO and C₂₁H₃₈NCl, is studied in a concentration range of 10^?4?C10^?2 mol/L and a temperature range of 20?C40°C. It is established that, in the region of critical micelle concentration, the viscosity is a nonmonotonic function of concentration and has minima and maxima.     

Reactions in micellar systems

The notion of "green chemistry" has encouraged even synthetic organic chemists to include water as a solvent. Incredible selectivities and activities can be achieved through the addition of amphiphiles with a defined structure. The morphology of supramolecular assemblies or associates formed by surfactants vary according to the temperature and concentration. As a rule, reactions are typically conducted using simple spherical aggregates, that is, micelles in the nanometer range. The strong polarity gradient present between the hydrophilic surface and the hydrophobic core of the micelle means that both nonpolar and polar reagents can be solubilized. This solubilization results in reactants becoming more concentrated within the micelle than in the surrounding water phase and leads to an acceleration of the reaction and causes selective effects. The kinetic treatment of reactions in micellar systems can be accomplished by considering them as microheterogeneous two-phase systems.     

Molecular recognition with micellar and micelle-like aggregates in aqueous media

In this article, attention is directed to molecular recognition by micellar aggregates made with ionic surfactants involving directed interactions of substrates. Particular emphasis is placed on chiral recognition of enantiomers by hydrogen bonding functionalities incorporated in hydrophobic micellar interior. Hydrophobic properties within micelles, the ordering of their polar headgroups containing chiral functionalities essential for the recognition and the cessation of micellar kinetic association-dissociation with polymerization and immobilization of the surfactants on the support are discussed.     

Conductometric and fluorometric investigations on the mixed micellar systems of cationic surfactants in aqueous media

Micellar properties of binary mixtures of hexadecyldiethylethanolammonium bromide surfactant with tetradecyldimethylammonium, trimethylammonium, triphenylphosphonium, diethylethanolammonium, and pyridinium bromide surfactants have been characterized employing conductometric and fluorescence techniques. The critical micelle concentration (cmc*) and the degree of counter-ion binding values (delta) of the binary systems were determined from the conductivity measurements. The results were analyzed in light of various existing theories to calculate micellar composition, activity coefficients, and the interaction parameter (beta). Partial contribution of each surfactant, cmc1*, cmc2*, to the overall cmc* value was also evaluated. Aggregation numbers and micropolarity of the mixed micelles were determined from fluorescence measurements. The results were discussed in terms of synergetic interactions in these systems on the basis of the head group/head group and tail/tail interactions and the counter-ion binding.     

Computer simulations of micellar systems

Works published during the last decade devoted to simulation of micellar solutions of different surfactants are reviewed. The main attention is focused on studying the local structure of spherical and cylindrical micelles and their surface layers in solutions, as well as problems relevant to self-aggregation kinetics.