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.     

Local velocity measurements in the shear-thickening transition of dilute micellar solutions of surfactants

Local velocimetry and rheometric measurements are performed on three dilute micellar solutions which undergo the shear-thickening transition. The three surfactants, namely, alkyltrimethyl ammonium bromide (C(n)TAB), all belong to the same family and only differ by the length of the aliphatic chain. Simultaneous ultrasonic velocimetry and rheometry recordings provide convincing evidence for a heterogeneous flow in the shear-thickening domain. A detailed analysis allows us to demonstrate surprisingly similar evolutions of the wall slip magnitude and of the apparent viscosity as well as subtle differences between the three systems. Together with the velocimetry results, the direct observation of the flow in the vorticity-velocity plane reveals that the shear-thickening transition is associated with the emergence of a three-dimensional unstable flow.     

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.     

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.     

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.     

Thermodynamic studies on the interaction of some ureas and salts with micellar triton-X-100 in aqueous solutions

We have determined the enthalpies of solution in the micellar state (ΔH_s) for Triton-X-100 in 1 m aqueous solutions of urea, 1,3-dimethyl urea, tetramethyl urea, sodium chloride and calcium chloride at 298.15 K and 308.15 K. These results were used to evaluate the heat capacities of solution (ΔC_p,s) for Triton-X-100 micelles in these solvent systems. It has been observed that ΔC_p,s values of micellar Triton-X-100 decreases drastically upon transfer from water to these solutions but is positive in all cases. Thus, the heat capacities of transfer of Triton-X-100 micelles (ΔC_p,tr) are negative in all the systems studied. A comparison of the effect of non-electrolytes (ureas) and electrolytes (salts) on the micelle has been presented. The results have been discussed in terms of the relative water-structure-disrupting tendencies of the ureas and the salts.     

Cryo-TEM studies of worm-like micellar solutions

Solutions of worm-like micelles display a rich rheological behavior that makes them useful as, for example, drag-reducing agents or viscosity enhancers. The properties of these solutions depend on the morphology and interactions between the micelles, both of which can be tuned by changing solution conditions. Although there has been extensive theoretical study of these solutions, there are often conflicting explanations, or no explanation, of an observed trend. Application of cryogenic transmission electron microscopy (cryo-TEM) for the direct visualization of the micelles can help correlate microstructure to rheology. Of particular interest is the cause of a maximum in viscosity as a function of increasing surfactant or salt concentration. Several studies support the theory of a transition from linear to branched micelles, while other studies report no change in microstructure or no connection between structure evolution and changes in viscosity. More systematic and thorough studies that combine cryo-TEM with other experimental techniques are needed.     

EQCM studies on polypyrrole in aqueous solutions

Electrochemical quartz crystal microbalance (EQCM) studies revealed that the electrochemical properties of polypyrrole (PPY) in aqueous solutions are greatly dependent on the solution pH. A PPY film immersed in an aqueous KCl solution shows redox processes which involve not only anion transport but also cation transport if the solution pH is greater than 3–4, whereas at lower pH values only anion transport prevails as has been widely recognized. It has been found from comparative studies using poly(N-methylpyrrole) that protonation and deprotonation at nitrogen atoms of PPY play a key role in the observed pH-dependent electrochemical behavior. Deprotonated PPY allows transient incorporation of hydrated cations upon reduction in weak alkaline aqueous solutions, followed by slow ejection of the incorporated cations when conventional electrolytes such as KCl and NaClO₄ are used, but not in NH₄Cl solution. Discussion on the cation transport in PPY is made in terms of deprotonation and protonation of nitrogen atoms of pyrrole subunits in PPY.     

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...     

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.     

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.     

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.     

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.     

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.     

Thermodynamic properties of ibuprofen sodium salt in aqueous/urea micellar solutions at 298.15 K

Thermodynamic, surface and micellar properties of anti-inflammatory drug sodium 2-(4-isobutylphenyl) propionate (sodium salt of ibuprofen (NaIBF)) in aqueous/urea solution were studied by surface tension measurements at 298.15 K in the presence of anionic surfactant sodium dodecylsulfate (SDS). Critical micelle concentration (cmc), surface tension at cmc (γcmc), maximum Gibbs surface excess (Γ_max), minimum surface area per surfactant molecule at the air/water interface (A _min) etc. were determined in pure water as well as in aqueous urea solution. The theories of Clint, Rosen and Rubingh have been applied to describe the interactions between these amphiphiles at the interface and in the micellar solution. Various thermodynamic parameters have been calculated and discussed in detail.     

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.     

Evaporation studies on sodium dehydrocholate aqueous solutions

The evaporation through the air/solution interface of sodium dehydrocholate (NaDHC) aqueous solutions was studied by surface tension and microbalance measurements. The evaporation rate was related to the aggregation processes in the bulk, but not with the adsorption monolayer compactness. Except for very dilute solutions the air/solution interface was saturated by NaDHC molecules, giving rise to a strongly nonideal monolayer. At very low concentration, the adsorbed monolayer behaved as an ideal two-dimensional gas. The results were in agreement with previous research results.     

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。     

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