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María Luisa Moyá - One of the best experts on this subject based on the ideXlab platform.
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water n n dimethylformamide alkyltrimethylammonium bromide Micellar Solutions thermodynamic structural and kinetic studies
Langmuir, 2005Co-Authors: María Del Mar Graciani, Maria Muñoz, And Amalia Rodriguez, María Luisa MoyáAbstract:Various amounts of N,N-dimethylformamide (DMF) with the weight percentage of DMF varying within the range 0−20, were added to aqueous Micellar Solutions of hexadecyl-, tetradecyl-, and dodecyltrimethylammonium bromides (CTAB, TTAB, and DTAB, respectively). Information about changes in the critical micelle concentrations, in the Micellar ionization degrees, in the aggregation numbers, and in the polarity of the interfacial region of micelles upon changing the weight percent of DMF was obtained through conductivity and fluorescence measurements. Surface tension measurements permitted the estimation of the Gordon parameter of the water−DMF mixtures. The thermodynamic and structural changes provoked by the addition of DMF to the cationic Micellar Solutions were evidenced through the Micellar kinetic effects observed in the reaction methyl 4-nitrobenzenesulfonate + Br-, investigated in the water−DMF cationic Micellar Solutions. The pseudophase kinetic model was adequate to quantitatively rationalize the depend...
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Micellar medium effects on the hydrolysis of phenyl chloroformate in ionic, zwitterionic, nonionic, and mixed Micellar Solutions
International Journal of Chemical Kinetics, 2002Co-Authors: María A. Muñoz, Amalia Rodríguez, María Del Mar Graciani, María Luisa MoyáAbstract:The spontaneous hydrolysis of phenyl chloroformate was studied in various anionic, nonionic, zwitterionic, and cationic aqueous Micellar Solutions, as well as in mixed anionic–nonionic Micellar Solutions. In all cases, an increase in the surfactant concentration results in a decrease in the reaction rate and Micellar effects were quantitatively explained in terms of distribution of the substrate between water and micelles and the first-order rate constants in the aqueous and Micellar pseudophases. A comparison of the kinetic data in nonionic Micellar Solutions to those in anionic and zwiterionic Micellar Solutions makes clear that charge effects of micelles is not the only factor responsible for the variations in the reaction rate. Depletion of water in the interfacial region and its different characteristics as compared to bulk water, the presence of high ionic concentration in the Stern layer of ionic micelles, and differences in the stabilization of the initial state and the transition state by hydrophobic interactions with surfactant tails can also influence reactivity. The different deceleration of the reaction observed in the various Micellar Solutions studied was discussed by considering these factors. Synergism in mixed-Micellar Solutions is shown through the kinetic data obtained in these media. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 445–451, 2002
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Kinetic Micellar effects in tetradecyltrimethylammonium bromide-pentanol Micellar Solutions.
Journal of colloid and interface science, 2002Co-Authors: Amalia Rodríguez, María A. Muñoz, María Del Mar Graciani, María Luisa MoyáAbstract:Abstract The reactions 1,1,1-trichloro-2,2-bis( p -chlorophenyl)ethane + OH − and 2-( p -nitrophenyl)ethyl bromide + OH − were studied in tetradecyltrimethylamonium bromide, TTAB, and TTAB–pentanol Micellar Solutions. The influence of changes in the surfactant concentration as well as changes in the hydroxide ion concentration on the observed rate constant was investigated. If changes in the cmc and ionization degree provoked by the presence of the different amounts of n -pentanol in the Micellar Solutions are taken into account, the experimental kinetic data can be rationalized quantitatively by using the PIE model. Assuming that the ion-exchange equilibrium constant, K OH − / Br − , for the competition between the bromide and the hydroxide ions in all TTAB and in TTAB–pentanol Micellar Solutions studied is the same, a good agreement between the theoretical and the experimental kinetic data was found in all the Micellar media for the two processes studied. This assumption was checked by experimentally determining the ion-exchange equilibrium constant K OH − / Br − in TTAB and TTAB–pentanol Micellar Solutions through a spectroscopic method, results showing that the presence of n -pentanol does not affect substantially the value of K OH − / Br − . The second-order rate constants obtained from the fittings decrease slightly when the amount of pentanol increases, being greater than that in aqueous solution. This acceleration can be explained considering that micelles accelerate the reactions in which the charge is delocalized in the transition state.
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The reaction methyl 4-nitrobenzenesulfonate + Br- in cationic and zwitterionic Micellar Solutions
Langmuir, 2002Co-Authors: María Del Mar Graciani, Amalia Rodríguez, Maria Muñoz, María Luisa MoyáAbstract:The reaction methyl 4-nitrobenzenesulfonate + Br- was studied in tetradecyltrimethylammonium bromide aqueous Solutions in the presence and absence of 0.1 M NaBr, as well as in N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SB3-14) Micellar Solutions in the presence of 0.1 M NaBr. Discussion of kinetic results in cationic Micellar Solutions was used to show some problems concerning the estimation of the second-order rate constants in the Micellar pseudophases. The errors introduced by rationalizing the kinetic Micellar effects in SB3-14 Micellar Solutions through pseudo-first-order rate constants were also shown; the contribution of the reaction occurring in the aqueous phase and the binding of the bromide ions to the sulfobetaine micelles were the cause of such errors. To complete the work, the influence of the cation coming from the added bromides on the Micellar effects observed in SB3-14 Micellar Solutions was investigated. Results give information about how cations influence the bromide assoc...
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Study of the bromide oxidation by bromate in zwitterionic Micellar Solutions
International Journal of Chemical Kinetics, 2000Co-Authors: Amalia Rodríguez, María A. Muñoz, María Del Mar Graciani, María Luisa MoyáAbstract:The redox reaction Br− + BrO3− has been studied in aqueous zwitterionic Micellar Solutions of N-tetradecyl-N, N-dimethyl-3-ammonio-1-propanesulfonate, SB3-14, and N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate, SB3-16. A simple expression for the observed rate constant, kobs, based on the pseudophase model, could explain the influences of changes in the surfactant concentration on kobs. The kinetic effect of added NaClO4 on the reaction rate in SB3-14 Micellar Solutions has also been studied. They were rationalized by considering the binding of the perchlorate anions to the sulfobetaine micelles and their competition with the reactive bromide ions for the Micellar surface. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 388–394, 2000
P Thiyagarajan - One of the best experts on this subject based on the ideXlab platform.
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small angle neutron scattering and viscosity studies of ctab nasal viscoelastic Micellar Solutions
Journal of Physical Chemistry B, 1998Co-Authors: V Aswal K And, P. S. Goyal, P ThiyagarajanAbstract:Micellar Solutions of cationic surfactant cetyltrimethylammonium bromide (CTAB) in the presence of sodium salicylate (NaSal) show a viscoelastic behavior. Small-angle neutron-scattering (SANS) and viscosity studies from CTAB/NaSal Micellar Solutions are reported. Zero-shear viscosity of these Solutions as a function of NaSal concentration (Cs), at all the four measured surfactant concentrations (Cd) of 12.5. 25, 50 and 100 mM, show a double-peak behavior. The effect of Cs/Cd on two viscosity maxima and a minimum has been examined, and the scaling relations are obtained. SANS experiments have been carried out at different NaSal concentrations, beyond the first viscosity maximum for two surfactant concentrations of 25 and 100 mM. It is found that micelles are rigid rods and their exponential length distribution shows that they behave as living polymers beyond the first viscosity maximum. The Micellar structure does not change in the living polymer regime, whereas viscosity varies with increase in NaSal conc...
P. S. Goyal - One of the best experts on this subject based on the ideXlab platform.
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Selective counterion condensation in ionic Micellar Solutions.
Physical Review E, 2003Co-Authors: Vinod K. Aswal, P. S. GoyalAbstract:Small-angle neutron scattering experiments have been carried out on Micellar Solutions of cationic surfactants of cetyltrimethylammonium bromide ~CTABr! and chloride ~CTACl! in the presence of varying concentrations of salts KBr and KCl. In these systems, while the size of micelles strongly increases with the addition of KBr, the effect of addition of KCl in comparison is much less pronounced. It is found that in equimolar surfactant to salt Micellar Solutions of CTABr/KCl and CTACl/KBr, the Micellar sizes are larger in CTACl/KBr than those in CTABr/KCl. The measurements have been done for different equimolar surfactant to salt concentrations and at different temperatures. We explain these results in terms of selective counterion condensation on the micelles. That is, while the condensation of Cl 2 counterions on the CTABr micelles in CTABr/KCl takes place around the condensed Br 2 counterions of CTABr, the Cl 2 counterions of CTACl in CTACl/KBr are replaced by Br 2 counterions of the salt. Similar results have also been obtained on Micellar Solutions of anionic surfactants of sodium dodecyl sulfate and lithium dodecyl sulfate in the presence of salts LiBr and NaBr, respectively.
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small angle neutron scattering and viscosity studies of ctab nasal viscoelastic Micellar Solutions
Journal of Physical Chemistry B, 1998Co-Authors: V Aswal K And, P. S. Goyal, P ThiyagarajanAbstract:Micellar Solutions of cationic surfactant cetyltrimethylammonium bromide (CTAB) in the presence of sodium salicylate (NaSal) show a viscoelastic behavior. Small-angle neutron-scattering (SANS) and viscosity studies from CTAB/NaSal Micellar Solutions are reported. Zero-shear viscosity of these Solutions as a function of NaSal concentration (Cs), at all the four measured surfactant concentrations (Cd) of 12.5. 25, 50 and 100 mM, show a double-peak behavior. The effect of Cs/Cd on two viscosity maxima and a minimum has been examined, and the scaling relations are obtained. SANS experiments have been carried out at different NaSal concentrations, beyond the first viscosity maximum for two surfactant concentrations of 25 and 100 mM. It is found that micelles are rigid rods and their exponential length distribution shows that they behave as living polymers beyond the first viscosity maximum. The Micellar structure does not change in the living polymer regime, whereas viscosity varies with increase in NaSal conc...
Gareth H Mckinley - One of the best experts on this subject based on the ideXlab platform.
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pressure driven flow of wormlike Micellar Solutions in rectilinear microchannels
Journal of Non-newtonian Fluid Mechanics, 2011Co-Authors: Michael Cromer, Pamela L Cook, Gareth H MckinleyAbstract:Abstract In this paper the inhomogeneous response of the (two species) VCM model (Vasquez et al., A network scission model for wormlike Micellar Solutions. I. Model formulation and homogeneous flow predictions, J. Non-Newtonian Fluid Mech. 144 (2007) 122–139) is examined in steady rectilinear pressure-driven flow through a planar channel. This microstructural network model incorporates elastically active network connections that break and reform mimicking the behavior of concentrated wormlike Micellar Solutions. The constitutive model, which includes non-local effects arising from Brownian motion and from the coupling between the stress and the microstructure (finite length worms), consists of a set of coupled nonlinear partial differential equations describing the two Micellar species (a long species ‘A’ and a shorter species ‘B’) which relax due to reptative and Rouse-like mechanisms as well as rupture of the long Micellar chains. In pressure-driven flow, the velocity profile predicted by the VCM model deviates from the regular parabolic profile expected for a Newtonian fluid and exhibits a complex spatial structure. An apparent slip layer develops near the wall as a consequence of the microstructural boundary conditions and the shear-induced diffusion and rupture of the Micellar species. Above a critical pressure drop, the flow exhibits shear banding with a high shear rate band located near the channel walls. This pressure-driven shear banding transition or ‘spurt’ has been observed experimentally in macroscopic and microscopic channel flow experiments. The detailed structure of the shear banding profiles and the resulting flow curves predicted by the model depend on the magnitude of the dimensionless diffusion parameter. For small channel dimensions, the Solutions exhibit ‘non-local’ effects that are consistent with very recent experiments in microfluidic geometries (Masselon et al., Influence of boundary conditions and confinement on non local effects in flows of wormlike Micellar systems, Phys. Rev. E 81 (2010) 021502).
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wormlike Micellar Solutions ii comparison between experimental data and scission model predictions
Journal of Rheology, 2010Co-Authors: Christopher J Pipe, Nahn Ju Kim, Paula A Vasquez, L P Cook, Gareth H MckinleyAbstract:Although many constitutive models for wormlike Micellar Solutions have been proposed, few quantitative comparisons have been made with detailed rheological measurements. The majority of comparative studies focus on the linear viscoelastic properties of Micellar Solutions, which are well described by monoexponential Maxwell-like behavior. In the present work we compare the predictions of a prototypical two-species reptation-reaction model [developed in Part 1, Vasquez et al., “A network scission model for wormlike Micellar Solutions: I. Model formulation and viscometric flow predictions,” J. Non-Newtonian Fluid Mech. 144(2–3), 122–139 (2007)] with rheological measurements performed using a concentrated cetyl pyridinium chloride/sodium salicylate (CPyCl/NaSal) solution in a range of steady and transient shear flows. The model captures the continuous rupture and reformation of the long entangled chains that form a physically entangled viscoelastic network and the enhanced breakage rates that occur during imp...
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extensional flow of wormlike Micellar Solutions
Chemical Engineering Science, 2009Co-Authors: Michael Cromer, Pamela L Cook, Gareth H MckinleyAbstract:Abstract We consider the inhomogeneous extensional response of a new constitutive model, the VCM model [Vasquez, et al., 2007. A network scission model for wormlike Micellar Solutions I: model formulation and homogeneous flow predictions. J. Non-Newt. Fluid Mech. 144, 122–139] that has been developed to describe concentrated Solutions of wormlike micelles. The time dependent numerical analysis is carried out in a simplified slender filament formulation appropriate for transient elongational flows of complex fluids. The simulations show that, beyond a critical extension rate, elongating filaments of a Micellar fluid described by the VCM model exhibit a dramatic and sudden rupture event as a result of the scission of the entangled wormlike chains. The computations capture many of the features of the high-speed rupture process observed experimentally [Bhardwaj, et al., 2007. Filament stretching and capillary breakup extensional rheometry measurements of viscoelastic wormlike micelle Solutions. J. Rheol. 51, 693–719] in filament stretching experiments with wormlike micelle Solutions. The highly localized rupture predicted by the VCM model and the corresponding evolution in the tensile force within the filament is contrasted with the familiar and more gradual necking responses predicted by the upper convected Maxwell and Giesekus models under equivalent kinematic boundary conditions.
Shi-qing Wang - One of the best experts on this subject based on the ideXlab platform.
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Hydrodynamic theory of Micellar Solutions : suppression of Micellar growth by interactions
Macromolecules, 1992Co-Authors: Shi-qing WangAbstract:A hydrodynamic theory of rigid-rod Micellar Solutions is presented. The previously omitted intermicelle interactions are incorporated by generalizing the Onsager theory for nematic liquid crystals to account for polydispersity and self-assembly in the presence of flow. The Micellar size distribution is derived as a function of shear rate in the form of a highly nonlinear integral equation. It is found that interMicellar repulsion postpones the Micellar growth in flow to higher shear rates.
