Viscoelastic micellar water/CTAB/NaNO(3) solutions: rheology, SANS and cryo-TEM analysis

Aqueous micellar solutions of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and sodium nitrate (NaNO(3)) were examined using steady and dynamic rheology, small-angle neutron scattering (SANS) and cryogenic-transmission electron microscopy (cryo-TEM). Upon addition of NaNO(3), the CTAB spherical micelles transform into long, flexible wormlike micelles, conveying viscoelastic properties to the solutions. The zero-shear viscosity (eta(0)) versus NaNO(3) concentration curve exhibits a well-defined maximum. Likewise, upon increase in temperature, the viscosity decreases. Dynamic rheological data of the entangled micellar solutions can be well described by the Maxwell model. Changes in the structural parameters of the micelles with addition of NaNO(3) were inferred from SANS measurements. The intensity of scattered neutrons at the low q region was found to increase with increasing NaNO(3) concentration. This suggests an increase in size of the micelles and/or decrease of intermicellar interactions with increasing salt concentration. Analysis of the SANS data using prolate ellipsoidal structure and Yukawa form of interaction potential between micelles indicates that addition of NaNO(3) leads to a decrease in the surface charge of the ellipsoidal micelles and consequently an increase in their length. The structural transition from spherical to entangled threadlike micelles, induced by the addition of NaNO(3) to CTAB micelles is further confirmed by cryo-TEM.     

Titrations of sulphonamides in cationic micellar systems

A titrimetric determination of some sulphonamides with 0.1 M sodium hydroxide in the presence of hexadecylpyridinium chloride is described. Potentiometric titrations are slow; visual titrations are satisfactory. The pK_a shift can be interpreted in the light of general micellar behaviour.     

Phase behavior of the lecithin/water/isooctane and lecithin/water/decane systems

The isothermal pseudo-ternary-phase diagram was determined at 25 degrees C for systems composed oflecithin, water, and, as oil, either isooctane or decane. This was accomplished by a combination of polarizing microscopy, small-angle X-ray scattering, and NMR techniques. The lecithin-rich region of the phase diagram is dominated by a lamellar liquid-crystalline phase (Lalpha). For lecithin contents less than 60% and low hydration (mole ratio water/lecithin = W0 < 5.5), the system forms a viscous gel of branched cylindrical reverse micelles. With increase in the water content, the system phase separates into two phases, which is either gel in equilibrium with essentially pure isooctane (for lecithin < 25%) or a gel in equilibrium with Lalpha (for lecithin > 25%). These two-phase regions are very thin with respect to water dilution. For 8 < W0 < 54 very stable water-in-oil emulsions form. It is only after ripening for more than 1 year that the large region occupied by the emulsion reveals a complex pattern of stable phases. Moving along water dilution lines, one finds (i) the coexistence of gel, isooctane and Lalpha, (ii) equilibrium between reverse micelles and spherulites, and, finally, (iii) disconnected reverse micelles that fail to solubilize water for W0 > 54. This results in a Winsor II phase equilibrium at low lecithin content, while for lecithin > 20% the neat water is in equilibrium with a reverse hexagonal phase and an isotropic liquid-crystalline phase. The use of the decane as oil does not change the main features of the phase behavior.     

Coagulation of nanoparticles in reverse micellar systems: a Monte Carlo model

The process of formation of nanoparticles obtained by mixing two micellized, aqueous solutions has been simulated using the Monte Carlo technique. The model includes the phenomena of finite nucleation, growth via intermicellar exchange, and coagulation of nanoparticles after their formation. Using the model, an exploratory study has been conducted to analyze whether the coagulation of nanoparticles is the reason for the formation of nanoparticles whose sizes are comparable to the size of the reverse micelles. The model explains the possible mechanism of coagulation of semiconductor nanoparticles formed within reverse micelles and its effect on the evolution of their size with time. The model is predictive in nature, and the simulation results compare well with those observed experimentally.     

Volume properties of reverse micellar systems AOT/n-heptane/DMSO-water

The volume properties of reverse micellar systems bis(2-ethylhexyl) sulfosuccinate sodium salt/n-heptane/dimethyl sulfoxide-water are studied via densitometry. The presence of dimethyl sulfoxide and the increase in its amount in a dimethyl sulfoxide-water mixed solvent raise the apparent volume of the polar phase. This increase is also observed when the degree of hydration of the polar core and the temperature are raised.     

正戊醇对CTAB/KBr胶束体系流变性的影响

报道了正戊醇对CTAB/KBr胶束体系流变性的影响。结果表明：在0.01mol.dm^-^3CTAB(十六烷基三甲基溴化铵)溶液中,正戊醇促使CTAB/KBr胶束体系的粘度增大至一最大值,然后降低在0.08mol.dm^-^3CTAB/KBr的溶液中,正戊醇能促使该胶束体系呈现粘弹性。根据动态荧光法测定的胶束聚集对以上结果进行了解释。     

Micelles and reverse micelles in the nickel bis(2-ethylhexyl) sulfosuccinate/water/isooctane microemulsion

The ternary system Ni(2+)(AOT)(2) (nickel 2-bis[2-ethylhexyl] sulfosuccinate)/water/isooctane presents w/o and o/w microemulsions with a Winsor progression (2Phi-3Phi-2Phi), without the addition of salt; the "fish diagram" was obtained for alpha=0.5 and gamma=0.02-0.22. Using static and dynamic light scattering the micellar size, the ratio of water to surfactant, and the density of micelles for this system were estimated. In addition, the mean interfacial curvature as a function of temperature was obtained.     

Photolysis of 1,2-dihydroquinolines in micellar solutions of anionic and cationic surfactants

The kinetics of the photolysis of substituted 1,2-dihydroquinolines (DHQ) in micellar solutions was studied by steady-state and flash photolysis. The photolysis mechanism depends dramatically on the location of DHQ molecules in micelles, which is governed by the surfactant nature. In micellar solutions of the anionic surfactant sodium dodecyl sulfate (SDS), where the DHQ molecules are located in the Stern layer, the intermediate species decay kinetics follows a first-order law. When DHQ is in neutral form (pH 4–12), the rate constant of the intermediate carbocation decay increases from 25 to 198 s^?1 with an increasing concentration of DHQ in micelles. The positive micellar catalysis is caused by the acceleration of the final product formation with the DHQ molecule via proton abstraction from the intermediate cation. The formation of several types of intermediate species—carbocations in the aqueous phase and aminyl radicals in micelles—is observed in micellar solutions of the cationic surfactant cetyltrimethylammonium bromide (CTAB) due to the preferential location of DHQ molecules in the micellar core. The carbocation decays via a pseudofirst-order reaction with a rate constant close to that in the aqueous solution. The lifetime of the DHQ aminyl radicals in the micellar solutions is longer by several orders of magnitude than the lifetime observed for homogeneous solutions of hydrocarbons and alcohols.     

Quenching of fluorescence from pyrene in micellar solutions by cationic quenchers

The fluorescence from the S₁ state of pyrene solubilized within the hydrophobic region of sodium dodecyl sulfate micelles has been studied as a function of concentration of some cationic additives to the aqueous phase. At concentrations of added quencher below ca. 10^?3 mol dm^?3 the fluorescence decay follows a complex rate law; at long time, the decay is exponential with the same time constant as that of pyrene (S₁) without added quencher. The initial slope of the decay curve depends linearly on the concentration of added cation. These results are discussed in terms of the recently proposed model which requires the water-hydrocarbon interface to penetrate the micelle in the region of the probe. An analytical treatment is presented which, on the premises stated, serves to explain the observed kinetic features.     

Thermostability of Cromobacterium viscosum lipase in AOT/isooctane reverse micelle

The thermostability of Cromobacterium viscosum lipase (EC 3.1.1.3) entrapped in AOT (sodium bis-[2-ethylhexyl] sulfosuccinate) reverse micelles was increased by the addition of short-chain polyethylene glycol (PEG 400). Two different approaches were considered: (1) the determination of half-life time and (2) the mechanistic analysis of deactivation kinetics. The half-life of lipase entrapped in AOT/isooctane reverse micelles with PEG 400 at 60 degrees C was 28 h, ninefold higher than that in reverse micelles without PEG 400. The lipase entrapped in both reverse micellar systems followed a series-type deactivation mechanism involving two first-order steps. The deactivation constant for the first step at 60 degrees C in PEG containing reverse micelles was 0.055 h!1, 11-fold lower than that in reverse micelles without PEG, whereas it remained almost constant for the second step. The inactivation energy of the lipase entrapped in reverse micelles with and without PEG 400 was 88.12 and 21.97 kJ/mol, respectively.     

Occurrence of shear thickening in aqueous micellar solutions of CTAB with some added organic counterions

 In this experimental work we carefully investigate the influence of some organic counterions (having similarities): sodium salicylate (NaSal), sodium tosylate (NaTos) and sodium benzoate (NaBz) on the rheological properties of two aqueous solutions (0.1 and 0.05 M) of cetyltrimethylammonium bromide (CTAB). Here we are particularly interested in the occurrence of the shear thickening effect corresponding to shear induced structures (SIS). All the rheological measurements presented in this work are realized with the same geometrical device (plan-cone) with controlled imposed shear stress. Conditions of occurrence and evolutions of the characteristics of the obtained shear thickening are given. Received: 30 June 1997 Accepted: 20 October 1997     

Temperature-dependent solvation dynamics of water in sodium bis(2-ethylhexyl)sulfosuccinate/isooctane reverse micelles

In this paper, for the first time, we report a detailed study of the temperature-dependent solvation dynamics of a probe fluorophore, coumarin-500, in AOT/isooctane reverse micelles (RMs) with varying degrees of hydration (w0) of 5, 10, and 20 at four different temperatures, 293, 313, 328, and 343 K. The average solvation time constant becomes faster with the increase in w0 values at a particular temperature. The solvation dynamics of a RM with a fixed w0 value also becomes faster with the increase in temperature. The observed temperature-induced faster solvation dynamics is associated with a transition of bound- to free-type water molecules, and the corresponding activation energy value for the w0 = 5 system has been found to be 3.4 kcal mol-1, whereas for the latter two systems, it is approximately 5 kcal mol-1. Dynamic light scattering measurements indicate an insignificant change in size with temperature for RMs with w0 = 5 and 10, whereas for a w0 = 20 system, the hydrodynamic diameter increases with temperature. Time-resolved fluorescence anisotropy studies reveal a decrease in the rotational restriction on the probe with increasing temperature for all systems. Wobbling-in-cone analysis of the anisotropy data also supports this finding.     

Dielectric studies of polymer-water interactions and water organization in PEO/water systems

We report the results of detailed investigations of polymer-water interactions and of the organization of water in the poly(ethylene oxide) (PEO)/water system by dielectric techniques. They include thermally stimulated depolarization currents (TSDC) techniques in the temperature range of 77-300 K and broadband dielectric relaxation spectroscopy (DRS) techniques of frequencies, 5 Hz-10 GHz, and temperatures, 173-300 K. The water content h was varied between 0 and 1.21 (grams of water per gram of dry PEO). The secondary γ mechanism of PEO and the reorientation of absorbed water molecules were extensively studied. The γ mechanism was found to be plasticized up to water contents of about 0.20. The reorientation of water molecules was studied in three different experiments and frequency/temperature regions. The results suggest strong interactions in the PEO/water system and indicate the presence of a separate water phase at high water contents. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1545–1560, 1997     

Fluorescence study of drug-carrier interactions in CTAB/PBS buffer model systems

The well-known cationic surfactant hexadecyltrimethylammonium bromide (CTAB) was used as a model carrier to study drug-carrier interactions with fluorescence probes (5-hexadecanoylaminofluorescein (HAF) and 2,10-bis-(3-aminopropyloxy)dibenzo[a,j]perylene-8,16-dione (NIR 628) by applying ensemble as well as single molecule fluorescence techniques. The impact of the probes on the micelle parameters (critical micelle concentration, average aggregation number, hydrodynamic radius) was investigated under physiological conditions. In the presence of additional electrolytes, such as buffer, the critical micelle concentration decreased by a factor of about 10. In contrast, no influence of the probes on the critical micelle concentration and on average aggregation number was observed. The results show that HAF does not affect the characteristics of CTAB micelles. Analyzing fluorescence correlation spectroscopy data and time-resolved anisotropy decays in terms of the "two-step" in combination with the "wobbling-in-cone" model, it was proven that HAF and NIR 628 are differently associated with the micelles. Based on ensemble and single molecule fluorescence experiments, intra- and intermicellar energy transfer process between the two dyes were probed and characterized.     

Interface of AOT/Brij mixed reverse micellar systems: conductometric and spectrophotometric investigations

Solubilization and conductivity studies are carried out with AOT/Brijs (Brij-30, Brij-35, Brij-52, Brij-56, Brij-58, Brij-72, Brij-76, Brij-78)/isooctane/water mixed reverse micellar systems. Replacement of AOT molecules with large head group Brij molecules (Brij-30, Brij-35, Brij-56, Brij-58, Brij-76, Brij-78) decreases the solubilization capacity, whereas those with smaller polar head groups (Brij-52 and Brij-72) increases it. The former blends assist the conductance percolation whereas the latter retard it. An attempt has been taken to obtain more insight on the interfacial composition of the mixed interface with the help of spectrophotometric studies using 7-hydroxycoumarin as the fluorophore. The results obtained from the solubilization and conductometric studies have been correlated with those obtained from the spectroscopic studies.     

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.     

Temperature dependence of viscosity for sucrose laurate/water micellar systems

The viscous behavior of sucrose laurate aqueous systems of high hydrophilic-lipophilic balance up to a 50% (wt) surfactant concentration at temperatures between 5°C and 60°C has been studied. Systems up to a 45% (wt) surfactant concentration show Newtonian behavior. The influence of temperature was studied using the activated diffusive relaxation model described by Goodwin. A maximum specific viscosity that appears at lower temperature as sucrose laurate concentration increases can be observed. These results are related to the micellar growth of the sucrose laurate aggregates as temperature rises. More concentrated systems show complex viscous response. Thus, a limit viscosity at low shear rates and a shear-thinning behavior after a critical shear rate are observed. Limit viscosity decreases and critical shear rate increases as temperature rises. This behavior is related to the threshold micelle concentration for entanglement of rod-like micelles.Nomenclature A Parameter of the equation that relatesE and temperature - B Pre-exponential factor of the Arrhenius equation - C Sucrose ester concentration (kg · m^–3) - CMC Critical micelle concentration - E Activation energy for long-range diffusive motion (Goodwin model) - E _a Activation energy of the viscous flow (Arrhenius equation) - E ₀ Parameter of the equation that relatesE and temperature - HLB Hydrophilic/lipophilic balance of the sucrose ester - J Constant that depends on the aqueous phase viscosity and mean micellar radius - k Boltzmann's constant - k ₁ Parameter of the Goodwin equation - k ₂ Parameter of the Goodwin equation - q _rel Contribution of the hydrodynamic interactions - R _e External radius of the sensor system - R _i Inner radius of the sensor system - T Temperature - T _max Temperature at the maximum viscosity - Newtonian viscosity - _i Intrinsic viscosity - _rel Relative viscosity = _solution/_water - _red Reduced viscosity = _sp/C - _sp Specific viscosity = _rel – 1 - ₀ Zero-shear-rate viscosity     

Determination of neutral and cationic herbicides in water by micellar electrokinetic capillary chromatography

The separation and determination of two s-triazines and two quats in water samples by MEKC was described. The influence of pH, type and concentration of buffer, concentration and type of surfactant, organic modifier and the added salt on the separation of the two neutral and the two cationic herbicides was studied. Simazine and atrazine (neutral compounds at the working pH) were little influenced by the chemical variables, while the cationic paraquat and diquat were more influenced. In the optimisation on the separation voltage, the total concentration of salt in the run buffer and the repeatability of the separation were taken into consideration. The composition of dissolution, in which the analytes were dissolved, was also studied.A solid-phase extraction method to retain and to elute the four analytes in a single step was also developed. Recoveries between 80 and 95% and R.S.D. between 6 and 10% was obtained in the analysis of a well-water sample spiked with 2 and 5 ppb of triazines and quats, respectively. Detection limits were between 0.6 and 1.9 ppb.     

Determination of the acid dissociation constant of bromocresol green and cresol red in water/AOT/isooctane reverse micelles by multiple linear regression and extended principal component analysis

The pKa of 3',3",5',5"tetrabromo-m-cresolsulfonephtalein (Bromocresol Green) and o-cresolsulphonephtalein (Cresol Red) was spectrophotometrically measured in a water/AOT/isooctane microemulsion in the presence of a series of buffers carrying different charges at different water/surfactant ratios. Extended Principal Component Analysis was used for a precise determination of the apparent pKa and of the spectra of the acid and base forms of the dye. The apparent pKa of dyes in water-in-oil microemulsions depends on the charge of the acid and base forms of the buffers present in the water pool. Combination with multiple linear regression increases the precision. Results are discussed taking into account the profile of the electrostatic potential in the water pool and the possible partition of the indicator between the aqueous core and the surfactant. The pKa corrected for these effects are independent of w0 and are close to the value of the pKa in bulk water. On the basis of a tentative hypothesis it is possible to calculate the true pKa of the buffer in the pool.     

Phase behaviors of Gemini cationic surfactants/n-butanol/water systems

Phase diagrams for ternary system of the Gemini cationic surfactants, N,N-long chain alkyl-2-hydroxyl-N,N,N,N-tetramethyl diammonium dichloride (G_nCl₂) with butanol and water have been drawn based on experimental data at 25 °C. The phase diagrams show that L phase and different liquid crystalline phases are existent in the ternary system at different components. Electric conductivity of the L phase has been studied. Small-angle X-ray scattering (SAXS), ²H (deuterium) quadrupolar splitting (²H NMR) and the polarizing-light microscope were employed to confirm the characteristic texture structures and the microstructure of three different liquid crystalline phases.