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.     

Dielectric behavior of aqueous micellar solutions of betaine-type surfactants

Dielectric behavior was examined for aqueous solutions of the betaine-type surfactants dodecyldimethylcarbobetaine (C(12)DCB), tetradecyldimethylcarbobetaine (C(14)DCB), cetyldimethylcarbobetaine (C(16)DCB), and oleyldimethylcarbobetaine (OleyDCB) as a function of frequency from 1.00 x 10(6) to 2.00 x 10(10) Hz (6.28 x 10(6) to 1.26 x 10(11) rad s(-1)) with changing surfactant concentration (c(D)). Rotational relaxation times (tau) of the zwitterionic headgroups of the surfactants in aqueous solutions of C(12)DCB and C(14)DCB, which form spherical micelles, are determined to be 0.26 and 0.30 ns, respectively. Values of tau for aqueous solutions of C(16)DCB and OleyDCB, which form threadlike micelles, are identical at 0.44 ns. The tau values of all micellar solutions are constant irrespective of c(D). The increase in tau with increasing alkyl chain length is assigned to an increase of molecular density at the micellar surface. The magnitude of the relaxation strength for the surfactant solutions increases in proportion to c(D) and is not so different from that of an aqueous solution of glycine betaine (GB), which has the same chemical structure as betaine-type surfactants with zwitterionic headgroups but never forms micelles. This finding suggests that the zwitterionic headgroup rotating on the micellar surface possesses a dipole moment with a magnitude essentially the same as that of GB in aqueous solutions.     

Viscoelastic behavior of aqueous threadlike micellar solutions of oleyldimethylamineoxide

The viscoelastic behavior of aqueous oleyldimethylamineoxide (ODAO) solutions was examined by varying the concentration of ODAO, c_D, and the average degree of protonation, <!>, by adding hydrogen bromide to the system. ODAO forms long threadlike micelles in aqueous solution in spite of the absence of any additives. Therefore, the aqueous ODAO system shows pronounced viscoelastic behavior caused by entanglement between threadlike micelles as highly entangled polymer systems do. The frequency dependence of the storage and loss moduli for the system is perfectly described by that of a Maxwell model possessing only one set of relaxation time, F, and strength, G_N. G_N is proportional to the square of c_D as observed in concentrated liner polymer solutions, while F decreases with increasing c_D. On the other hand, the addition of HBr to control <!> remarkably increases F when G_N is constant. This suggest that <!> alters the inner structure of the threadlike micelles; association of head groups, dimers or trimers of ODAO are formed in the micelles owing to hydrogen bonding between protonated ODAO or between protonated and unprotonated ODAO. An increase in the number density of the associated head groups in the threadlike micelles increases F. Moreover, the flexibility of the threadlike micelles in the system is not affected so much by a change in the number density of the associated head groups, since G_N corresponding to the number density of entanglements in the system is kept constant.     

Versatility of aqueous micellar solutions for self-assembled monolayers engineering

Self-assembly of aliphatic as well as aromatic thiol-terminated molecules was achieved onto a variety of gold surfaces using aqueous micellar solutions. Scanning tunneling microscopy experiments allowed us to demonstrate that the increase in the density of self-assembled monolayers (SAMs) prepared from micellar aqueous solvent compared to that prepared from ethanol directly originates from the decrease in defect density in the SAM (etch pits, domain boundaries) and not from a denser local packing of the molecules. Extending the use of such an aqueous solvent to various conjugated molecules, we report for the first time the insertion of these molecules from an aqueous solution in a dodecanethiol (DT) SAM and the ligand-exchange on the surface of DT stabilized gold nanoparticles deposited as a Langmuir-Blodgett film. Finally, we show that aqueous micellar DT solutions allow the preparation of DT SAMs on gold through a micropatterned resist mask. These results make possible the use of water to deliver molecules on a solid substrate to build molecular devices in a way compatible with lithography requirements in microelectronic processes.     

Enthalpies of mixing of some nitriles aqueous solutions with dodecylsurfactants micellar solutions

The enthalpies of mixing of some n-nitriles (from acetonitrile to valeronitrile) aqueous solutions with dodecyltzimethylammonium bromide, sodium dodecylsulfate and dodecyltzimethylammonium oxide micellar solutions were determined. The measurements were performed by systematically changing the surfactant concentration at a given solute concentration. The experimental enthalpies were rationalized in terms of the standard enthalpy of transfer of solute from the aqueous to the micellar phase and of the distribution constant between the two phase. Information on the effect of the nature of the surfactant on the standard thermodynamic quantities of transfer(G _t ^o , H _t ^o , TS _t ^o ) is reported. The present data are compared to those previously reported for primary alcohols and the solubilizing properties shown by the different types of micelles are discussed.     

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.     

十二烷基聚氧乙烯硫酸钠胶束体系的流变性质研究

用流变学方法研究了无机电解质KBr存在时,阴离子表面活性剂十二烷基聚氧乙烯(3)硫酸钠(SDES)水溶液中胶束的生长和结构。通过测量体系的稳态剪切粘度(η)和应力(σ)关系,得到零剪切粘度(η0)、复合粘度(|η^*|)、动态模量[储能模量(G')和损耗模量(G")、平台模量(G0)、结构松驰时间(τ)等流变学参数,并应用Cox-Merz规则和Cole-Cole图,发现在SDES/KBr体系中可以形成蠕虫状胶束网络结构,体系为假塑性流体,偏离Maxwell模型,具有非线性粘弹性,没有单一的结构松驰时间。     

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.     

Solubility and hydrolytic stability of indomethacin in aqueous micellar solutions

Russian Chemical Bulletin - The solubility of the indomethacin drug in aqueous micellar solutions of a series of dicationic surfactants was characterized by spectrophotometry. The use of these...     

Paclitaxel solubility in aqueous dispersions and mixed micellar solutions of lecithin

The aim of this study was to find a biocompatible, lecithin-based carrier for paclitaxel (PTX) suitable for intravenous infusion and ensuring a soluble PTX concentration of 100 mg/100 ml or higher for at least 24 h. Aqueous dispersions of egg or soya lecithin (water-lecithin dispersions, WLD), mixed micellar (MM) solutions of egg lecithin and sodium deoxycholate, and formulations containing lecithin plus the co-surfactants and co-solvents poloxamer, polysorbate, Span, benzalkonium chloride, and macrogol were investigated. Amorphous PTX was prepared by lyophilization. PTX co-lyophilized with surfactants was also studied. Unlike crystalline PTX, the drug in an amorphous form is easily soluble in 1-5% (w/w) WLD or in MM. The highest solubility (up to 570 mg/100 ml) was achieved in 5% WLD. Dissolved PTX precipitated from all tested formulations over 24 h. Despite this, concentrations of dissolved PTX of 100 mg/100 ml or higher were observed after 24 h in 5% egg WLD, 1-5% soya WLD, and in 5% MM (lecithin : deoxycholate ratio 1 : 1 w/w). When four different batches of 5% egg WLD were prepared, containing PTX in clinically relevant concentration of 100 mg/100 ml, no precipitation of PTX was observed within 24 h and this formulation is the most promising candidate for further in vivo studies. Neither additional surfactants nor co-lyophilization increased PTX solubility in the lecithin-based carriers. The use of parenteral emulsions as solvents for the co-lyophilized PTX also failed to increase the solubility of the drug up to the target concentration.     

Separation of phenols from aqueous micellar solutions by cloud point extraction

The cloud point technique was used to recover phenol, 4-methylphenol, and 4-nitrophenol from aqueous solutions using oxyethylated methyl dodecanoates as nonionic surfactants. Oxyethylated methyl dodecanoates are convenient nonionic surfactants for such separations. Their cloud points can be easily modified by a change in surfactant hydrophilicity or by the addition of a second nonionic surfactant and/or an electrolyte. The use of the hydrophile lipophile balance is preferred to model the cloud point of oxyethylated methyl dodecanoates and their mixtures with other surfactants. The composition of the surfactant-rich phase depends on electrolyte type and the overheating. The phase can contain only 5-15% of water. Recovery of phenols changes in the order 4-nitrophenol >4-methylphenol > phenol and is increased in the presence of sodium chloride. The presence of salting-out electrolytes is preferred both to decrease the cloud point and to increase the efficiency of extraction.     

Rheo-SANS studies on shear-thickening/thinning in aqueous rodlike micellar solutions

Shear-induced thickening/thinning phenomena of aqueous rodlike micellar solutions of cetyltrimethylammonium bromide (CTAB) and sodium p-toluene sulfonate (NapTS) were investigated by means of simultaneous measurements of rheology and small-angle neutron scattering (SANS), the so-called Rheo-SANS. The aqueous CTAB/NapTS solutions were classified into five different categories dependent on their flow behavior and micellar structure. By increasing salt concentration and/or shear rates, the micelles underwent morphological transition from (i) spherical or short rodlike micelles to (ii) long rodlike micelles without entanglements, followed by (iii) those with entanglements. These transitions were recognized as changes in flow behavior from Newtonian to shear-thickening and shear-thinning flow, respectively. In the latter two cases, anisotropic SANS patterns appeared around these critical shear rates. The physical meaning of the anisotropic SANS patterns accompanied by shear-thickening flow behavior is discussed in conjunction with other shear-thickening systems.     

Structure and interactions in micellar solutions: molecular simulations of pluronic L64 aqueous solutions

Coarse-grained, implicit solvent molecular simulations have been conducted to investigate the structure and interactions of L64 Pluronic micelles in aqueous solutions. Simulations of an L64 solution beginning with monodisperse micelles (aggregation number Nagg = 40 chains) resulted in a narrow Gaussian distribution of Nagg centered around 40. While not fully equilibrated, this distribution supports the supposition that L64 micelles with Nagg = 40 are representative of the conditions considered and model employed. Detailed analysis of intramicellar monomers distribution and micelle shapes revealed that L64 micelles have a scalene ellipsoidal shape. Additional simulations of solutions containing 125 micelles constrained to have Nagg = 40 at polymer volume fractions of 0.024 and 0.110 were performed to study micelle-micelle structure factor, single micelle form factor, and total scattering intensity. The ability of various models utilized in analysis of scattering profiles in micellar solutions to describe the structure of the model L64 solutions was investigated. Investigation of the potential of mean force between two micelles reveals that the interactions between micelles are repulsive but on a length scale smaller than the mean micelle diameter, indicating that the micellar shape fluctuations are important in determining intermicellar interactions.     

Effect of counterion on thermodynamic micellar properties of tetradecylpyridinium in aqueous solutions

Electrical conductivity of aqueous solutions of tetradecylpyridinium bromide and chloride has been measured as a function of surfactant molal concentration and temperature. From the molal dependence of conductivity, the critical micelle concentration and the micellar ionization degree were estimated. The temperature dependence of these parameters has been used for calculating the thermodynamic parameters related with the micellization process by using the classical charged pseudophase separation model. The effect of the counterion on the conventional thermodynamic potentials of micellization such as standard Gibbs free energy, enthalpy and entropy has also been a matter of study. Finally, the occurrence of the enthalpy–entropy compensation phenomenon was verified and the relevant parameters discussed.     

The reactivity of phosphonic esters in aqueous micellar solutions of cationic surfactants

The micellar effect of cationic surfactants in alkaline hydrolysis of O-alkyl-O-aryl-chloromethyl phosphonates involves a positive contribution of concentrating the reagents and a negative effect of the micellar environment due to a loss in the activation entropy. The reactivity of substrates in micelles depends on both electronic and hydrophobic characteristics of substituents in the aryl group.     

Dediazoniation of 1-naphthalenediazonium tetrafluoroborate in aqueous acid and in micellar solutions

We have measured the rates and product yields of dediazoniation of 1-naphthalenediazonium (1ND) tetrafluoroborate in the presence and absence of sodium dodecyl sulfate (SDS) micellar aggregates by employing a combination of UV–vis spectroscopy and high-performance liquid chromatography (HPLC) measurements. Kinetic data were obtained by a derivatization procedure with product yields were determined by HPLC. HPLC chromatograms show that in aqueous acid and in micellar solutions only one dediazoniation product is formed in significant quantities, 1-naphthol (NOH), and the observed rate constants (k_obs) are the same when 1ND loss is monitored spectrometrically and when NOH formation is monitored by HPLC. Activation parameters were obtained both in the presence and absence of SDS micellar aggregates. In both the systems, the enthalpies of activation are high and the entropies of activation are positive. The enthalpy of activation in the absence of SDS is very similar to that in the presence of SDS micelles, but the entropy of activation is lower by a factor of 4. As a consequence, SDS micelles speed up the thermal decomposition of 1ND and increase k_obs by a factor of 1.5 when [SDS] = 0.02 M. In contrast, results obtained in the presence of complexing systems such as crown ethers and polyethers show significant stabilization of the parent arenediazonium ions. Kinetic and HPLC data are consistent with the heterolytic D_N + A_N mechanism that involves the rate-determining fragmentation of the arenediazonium ion into a very reactive phenyl cation that reacts competitively with available nucleophiles. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 301–309, 2008     

Acid-base properties and nucleophilicity of o-aminomethylphenols in aqueous micellar solutions and microemulsions

The pK _a values and constants of tautomeric equilibrium of several o-aminomethylphenols with different hydrophilic-lipophilic ability were measured in aqueous micellar solutions and in direct microemulsions based on cetyltrimethylammonium bromide. The kinetics of hydrolysis of p-nitrophenyl acetate at different pH and concentrations of aminomethylphenol and surfactant was studied. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1724–1729, October, 2006     

Cd(II) determination in the presence of aqueous micellar solutions

The equilibrium constants and molar absorptivities for the fast formation of a 1:3 complex between cadmium(II) (Cd(II)) and dithizonate anion, in the presence of cationic and non-ionic surfactants, allowed a simple and fast spectrophotometric determination of total cadmium. Indeed, the molar absorptivities of the Cd(II)-dithizone (Dz) complex formed in the presence of the neutral Triton X-100 and cationic cetyltrimethylammonium bromide (CTAB) surfactants are almost twice the value observed in the standard method and the maxima of absorption are shifted by about 40 nm when compared with the standard method. Clearly, the use of neutral and cationic surfactants promotes a higher value of the molar absorptivities of the complex, resulting in an increase in the sensitivity of the method. Application of the method to the desorption of Cd(II) ions from clays is illustrated.     

A kinetic method to estimate dissociation degrees of micellar aggregates in TTAB–alcohol aqueous micellar solutions

The reactions of dehydrochlorination of 1,1‐trichloro‐2,2‐bis(p‐chlorophenyl)ethane, DDT, and 1,1‐dichloro‐2,2‐bis(p‐chlorophenyl)ethane, DDD, with hydroxide ions were studied in various TTAB–alcohol (TTAB = tetradecyltrimethylammonium bromide) aqueous micellar solutions as a function of alcohol content. The alcohols used were heptanol, hexanol, pentanol, butanol, isobutanol, tert butanol, propanol and isopropanol. Kinetic data show that the dissociation degree of the micelles is the main factor controlling reactivity in all the TTAB–alcohol micellar solutions. This fact permits the development of a kinetic method in order to estimate the dissociation degree of the micellar aggregates present in the alcohol–TTAB aqueous micellar solutions. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 204–209, 2000