Search

Hybrid organic−inorganic nanocomposites containing uniform distributions of metal nanoparticles have been prepared by mixing a preformed nanoparticle colloid with the precursors of a ureasil, prior to the sol−gel transition. These nanocomposites possess not only high optical quality and optical features dictated by the size and shape of the nanoparticle dopants but also a high degree of flexibility, which can largely enhance the range of applications in practical devices. The deposition of a uniform silica shell on the nanoparticle surface prior to the sol−gel transition was found to be required to maintain the colloidal stability during the process and, thus, to retain the optical properties in the final nanocomposite material. This method can be readily extended to other materials, such as semiconductor and magnetic nanoparticles.

The basic hydrolysis of crystal violet (CV) in mixed systems consisting of β-cyclodextrin (β-CD) and a micelle-forming surfactant, cetyltrimethylammonium chloride (CTACl), has been studied. β-CD was found to catalyze the basic hydrolysis of CV through the interaction of its hydroxyl group, in its deprotonated form, with the carbocation in the complexed substrate. The addition of small amounts of CTACl, with [CTACl] below the critical micelle concentration, to β-CD solutions does not have an effect upon the observed rate constant for the basic hydrolysis of CV. This behavior is different from that observed for the alkaline hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide and nitrophenyl acetates in mixed β-CD/cationic surfactant systems. The proposed mechanism allows us to explain the experimental results on the basis of the high percentage of uncomplexed β-CD in equilibrium with the micellar system, the low CV concentration, and the high value for the binding constant of CV by β-CD.

The kinetics of the transfer of the nitroso group from N-methyl-N-nitroso-p-toluenesulfonamide to secondary alkylamines (dimethylamine, methylethylamine, methylbutylamine, methylhexylamine, methyloctylamine, and methyldodecylamine) was studied using a wide variety of water/AOT/isooctane microemulsions as reaction media. The diverse kinetic behavior of these amines can be explained quantitatively on the basis of a single model, taking into account the distribution of the amine between the aqueous and isooctane phases and their mutual interface (surfactant film); the reaction always takes place at the surfactant film. The reactivities of the amines are discussed in comparison with the reactivity observed in water.

A study was carried out on the basic hydrolysis of m-nitrophenylacetate in mixed systems made up of trimethylalkylammonium micelles and α- or β-cyclodextrin (CD). The surfactants used enabled us to vary the length of the hydrocarbon chain between 8 and 18 carbon atoms. In all cases the existence of a significant concentration of uncomplexed CD was observable in equilibrium with the micellar system. Contrary to expectations, the concentration of uncomplexed CD increased together with the hydrocarbon chain of the surfactant. This result was interpreted as a consequence of the balance between the complexation processes of the surfactant monomers by the CD and its autoassociation to form micelles. We have found the existence of a linear dependence between the percentage of uncomplexed CD and the inverse of the critical micellar concentration of the surfactant, indicating that, as the tendency toward micellization increases, so too does the percentage of the uncomplexed CD. The results obtained indicate that the concentration of uncomplexed α-CD in equilibrium with the micellar system is always lower than that of β-CD, which has been interpreted as a consequence of the greater tendency of the α-CD to form CD-surfactant complexes with a stoichiometry of 2:1. One of the main consequences of this study is that the critical micellar concentration of the mixed CD-surfactant systems cannot be used directly to obtain the stoichiometry of the CD-surfactant complexes, since the micellization occurs without the complexation ability of the CD having been saturated.

The kinetics of the transfer of the nitroso group from N-methyl-N-nitroso-p-toluenesulfonamide to each of three secondary amines (piperazine, N-methylbenzylamine, and morpholine) was studied using a wide variety of water/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/sodium dodecyl sulfate/isooctane quaternary microemulsions as reaction media. The diverse kinetic behavior of these amines can be explained quantitatively on the basis of a single model taking into account the distribution of the amine between the aqueous and isooctane phases and their mutual interface (surfactant film); the reaction always takes place at the surfactant film. The reactivities of the amines are discussed in comparison with the behavior observed in water and in AOT/isooctane/water ternary microemulsions.

The effects of temperature and cryptand complex concentration upon the conductivity of the system sodium bis(2-ethylhexyl) sulfosuccinate + 2,2,4-trimethylpentane + water have been studied. The cryptand complexes (potential phase-transfer catalysts) used in the ternary systems were 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, 4,7,13,16,21-pentaoxa-1,10-diazabicyclo[8.8.5]tricosane, and 4,7,13,18-tetraoxa-1,10-diazabicyclo[8.5.5]eicosane.

We have analyzed the different catalytic effects of surfactant aggregates upon the rate-determining hydroxide ion induced deprotonation reaction of 1. Vesicles are more effective catalysts than micelles, most likely providing a more apolar microenvironment at the substrate binding sites. We suggest that this leads to a catalytic reaction involving less strongly hydrated hydroxide ions. In the case of DODAB and DODAC vesicles, binding of cholesterol to the bilayer further increases the catalytic efficiency.

The basic hydrolysis of o-, m-, and p-nitrophenyl acetates (NPA) in mixed systems consisting of β-cyclodextrin (β-CD) and a surfactant&sbd;sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium hydroxide (TTAOH), or tetradecyltrimethylammonium bromide (TTABr)&sbd;has been studied. β-CD was found to catalyze the basic hydrolysis of NPA via the interaction of its hydroxyl group, in deprotonated form, with the carbonyl group in the complexed substrate. In the presence of a surfactant, the rate of the reaction is initially decreased. This inhibitory effect is the result of the surfactant monomers being complexed by β-CD and the NPA being in turn displaced from the aqueous medium, which cancels the catalytic effect of the cyclodextrin. Once micellization starts, the system behaves like a typical micellar system in the hydrolysis of a hydrophobic substrate. Plots of the ratios of pseudo-first-order rate constants in micelle−CD mixed systems and in micelles&sbd;%@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%SDS+CD%@sxx@%%@mx@% /%@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%SDS%@sxx@%%@mx@% and %@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%TTAOH+CD%@sxx@%%@mx@% /%@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%TTAOH%@sxx@%%@mx@% &sbd;versus surfactant concentration revealed the ratio to be greater than unity even at high surfactant concentrations, which is consistent with a CD-catalyzed pathway. This result is in turn consistent with the presence of a substantial concentration of free CD in equilibrium with the micellar systems. The constancy of %@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%SDS+CD%@sxx@%%@mx@% /%@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%SDS%@sxx@%%@mx@% and %@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%TTAOH+CD%@sxx@%%@mx@% /%@mt;sys@%%@ital@%k%@rsf@%%@sx@%obs%@be@%TTAOH%@sxx@%%@mx@%[S_EL2;space] at surfactant concentrations above the critical micelle concentration suggests the absence of interactions between CD and the micellar system once the latter has been established.

The effect of the presence of alkylamines (n-propylamine, n-butylamine, n-pentylamine, n-octylamine, n-decylamine, n-laurylamine, i-butylamine, i-pentylamine, neopentylamine, sec-butylamine, tert-butylamine, methyl-n-octylamine, dimethylamine, dipropylamine, di-i-butylamine, dimethyl-n-laurylamine, dimethyl-n-octylamine, triethylamine, triethylamine, and diethylmethylamine) on the variation of electrical conductivity with the temperature of the ternary systems sodium bis(2-ethylhexyl)sulfosuccinate (AOT) + 2,2,4-trimethylpentane + water has been studied. Also, the effect of other amines (N-methylbenzylamine, dibenzylamine, diphenylamine, morpholine, piperazine, piperidine, and pyrrolidine) on the variation of electrical conductivity with the temperature of these ternary systems has been studied.

The effect of temperature on the conductivity of ternary systems sodium bis(2-ethylhexyl)sulfosuccinate + 2,2,4-trimethylpentane + water has been studied. The effect of LiCl, LiBr, LiClO4, KCl, RbCl, CsCl, NH4Cl, and CaCl2 has also been studied.

The effect of the temperature on the conductivity of ternary systems sodium bis(2-ethylhexyl)sulfosuccinate + 2,2,4-trimethylpentane + water has been studied. Also the effect exercised by the presence of sodium hydroxide and hydrochloric acid on the conductivity of these ternary systems has been analyzed.

The alkaline hydrolysis of m-nitrophenyl acetate (m-NPA) was studied in sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium (TTAX; X = Br^- or HO^-) micellar systems containing β-cyclodextrin, CD. CD catalyzes the alkaline hydrolysis of m-NPA through reaction of the complexed substrate with the hydroxyl group of the cyclodextrin. The presence of aggregates at concentrations below the cmc exert an inhibitory effect. Above their cmc's, SDS or TTAX micelles exhibit the typical inhibitory and catalytic effects, respectively, on the alkaline hydrolysis of hydrophobic substrates. This behavior is explained in terms of two competing equilibria involving complexation of the substrate and the surfactant monomers by CD. The kinetic model combines the pseudophase model, including ion exchange, with a competitive binding model for the CD-catalyzed hydrolysis of m-NPA. In this model, counterions do not bind to the surfactant ion in the surfactant·CD complex and neither CD nor the surfactant·CD complex bind to micelles. Competitive complexation by CD and self-association (micellization) of the surfactant monomers results in the presence of a substantial amount of free (uncomplexed) CD. Once micelles are formed, no effect of cyclodextrin addition is observed on their properties.

The kinetics of the basic hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide were studied in media containing sodium dodecyl sulfate (SDS) or tetradecyltrimethylammonium bromide (TTABr) micelles and β-cyclodextrin (CD). Under the experimental conditions, [NaOH] = 0.17 M, all CD will have been deprotonated; thus, binding constants apply to the CD anion. The results have been interpreted in terms of a pseudophase model that takes into account the formation of both CD−surfactant and CD−substrate complexes and also, for TTABr systems, the exchange of Br^- and OH^- ions between the micellar and aqueous pseudophases. The presence of CD has no effect on existing SDS or TTABr micelles but raises the cmc:  complexation of surfactant by cyclodextrin makes the cmc dependent on CD concentration because the cmc is now the sum of the concentrations of free and complexed surfactant when micelles begin to form; increasing [CD] reduces the former quantity but increases the latter to a greater extent. At surfactant concentrations above the cmc, competition between the micellization and complexation processes leads to the existence of a significant concentration of free cyclodextrin.

The acid and alkaline hydrolyses of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in the presence of cationic vesicles of the surfactant dioctadecyldimethylammonium chloride (DODAC) were studied. The ion-exchange pseudophase model adapted to vesicle systems was used to fit the experimental results. An association constant for MNTS with the vesicles of 380 ± 30 M^-1 was determined by kinetic and spectrophotometric methods. The rate constant for the alkaline hydrolysis of MNTS in the vesicular pseudophase was also determined, and its value (0.0084 M^-1 s^-1) was found to be about one-tenth that in water and slightly smaller than that in CTABr or TTABr micelles. This intrinsically decreased reactivity is related to the different polarities in these three media.