Density ρ , isothermal compressibility β í , and thermal expansion coefficient α are measured with high accuracy at 15 and 25 ° C for water-n -octane-sodium dodecyl sulfate-n -pentanol microemulsions with a water/oil weight ratio varying from 0.05 to 4. The internal pressure of the microemulsions is shown to be calculable from the results of dilatometric measurements. The phenomenon of temperature inversion is revealed for the temperature coefficient of internal pressure ∆ P i / ∆ T of the microemulsions. The analysis of this coefficient makes it possible to distinguish between three regions of the structural state in the microemulsions and a continuous structural transition in the associated microemulsions.
The reactivity of primary alkylamines in cleavage of p-nitrophenyl esters of carboxylic acids in microemulsions of different structures based on cetyltrimethylammonium bromide was studied. The aminolysis rate considerably increases in going from inverse to direct microemulsions, mainly owing to the concentration of the reactants in the boundary layer.Detergent microemulsions are macroscopically uniform, thermodynamically stable, self-organizing dispersions with aqueous and hydrocarbon (oil) phases, in which the phase boundary is occupied by molecules of micelle-forming surfactants and cosurfactants. Variation of the water-to-oil volume ratio significantly alters the structure and properties of the microemulsions, with the phase inversion and rearrangement of the phase boundary [1]. Both inverse, bicontinual, and direct microemulsions can be formed, with the system as a whole remaining macrohomogeneous and optically transparent and preserving a high solubilizing capacity toward both polar and nonpolar substances. Unique solubilizing properties of microemulsions, along with exceedingly developed phase boundary surface, provide efficient contact between hydrophilic and hydrophobic reagents and allow successful use of these systems as nanoreactors for various chemical processes. With such systems, it is possible to control not only the reaction rates but even the reaction mechanisms [235].This study continues our previous investigations concerning the catalytic properties of microemulsions in nucleophilic substitutions [538]. To reveal how the structure of the reaction medium affects the rate of ester cleavage, we studied the reactions of p-nitrophenyl esters of carboxylic acids with primary aliphatic amines in microemulsions consisting of water, cetyltrimethylammonium bromide, n-butanol, and n-hexane. The content of the surfactant and cosurfactant in the microemulsions was 9.42 wt % each (molar ratio 1 : 5), which made it possible to avoid phase separation when the content of the two other components, water and hexane, was varied in a wide range. For kinetic experiments, we used the microemulsions with the water volume fraction (j) of 0.77, 0.30, and 0.14, which corresponds to the direct, bicontinual, and inverse structures, respectively [8,9].In molecular aqueous solutions, the cleavage of carboxylic acid esters in the presence of primary amines mainly occurs along two pathways: aminolysis (major reaction) and base hydrolysis (side reaction):R`NH 2 R and R`= C n H 2n+1 .The kinetic data characterizing the reaction of p-nitrophenyl acetate I and p-nitrophenyl laurate II with primary amines in microemulsions of various types are given in Table 1. It is seen from these results that, at equal concentrations of amines (C am ), the observed rate constants (k obs ) only slightly depend on the hydrophobicity of the amine. At the same time, the rate constants depend on the structure of the microemulsions, increasing in going from the inverse to direct microemulsion. In all the microemulsions studied, acetate I is cleaved by...
The influence of the external TI* "heavy" ion on the singlet and triplet excited stales of the pyrene and To'paflavine molecules solubilized in the water--heptane---sodium dodecyt sulfate--pentanol microemulsion was studied. The triplet-triplet (T--T) energy transfer in the system Try. paflavme (donor)--pyrene (aeceptor) and the sensitized phosphorescence of pyrene were first observed in the microemulsion containing TI § ions. The quenching rate constants of the excited states of the luminophores by T1 ~ ions were calculated.Key words: pyrene, Trypaflavine, microemulsion, phosphorescence, energy transfer, heavy atom.Sensitized phosphorescence of luminophores appears due to the transfer of the electron excitation energy from the triplet level of a donor molecule to the energetically lower triplet level of an acceptor molecule, t The phenomenon of sensitized phosphorescence has been observed for the first time tot the aromatic aldehyde--naphthalene system in a solid solution at a low temperature. 2 Sensitized phosphorescence at room temperature (SPRT) is also observed in liquid solutions of aromatic compounds in organic solvents containing no dissolved 0 2 .Then SPRT has been found in water-miceltar media upon the solubilization of donors and acceptors of triplet energy in direct 4-7 and reverse s.9 micelles of cationic "~,7 and anionic 5.6,8,9 surfactants. In almost all cases, diacetyl molecules served as an acceptor of the triplet energy, and the quantum phosphorescence yield for them was close to unity. This phenomenon was used for the indirect determination of polycyclic aromatic hydrocarbons (PAH). 5,6.9We have previously 1~ shown that SPRT in watermicellar solutions of anionic surfactant sodium dodecyl sulfate (SDS) can also appear in the system Trypaflavine, dye of the acridine series (donor)--PAH (acceptor)-external "heavy" atom. The -I'l + cation was used as the last atom, and its role was to increase the quantum yield of sensitized phosphorescence, The negatively charged surface of the SDS micelles binds cations of both the dye (triplet energy donor) and thallium. This provides for their approach to the PAH molecules localized in the near-surface layer of the micelle core and enables both efficient intersystem crossing and triplet enemy transfer. Such an approach allowed the selectivity of the phosphorimetric method of PAH determination to be increased. It,13 Microemulsions differ from micelles by the complex composition and larger sizes of the particles, because they contain, along with the surfactant, a nonpolar solvent and alcohol with a short hydrocarbon chain, a so-called co-surFactant, t4 Due to this, they are characterized by a higher microheterogeneity of interfaces and a higher solubilizing capacity and they better model the functioning of systems that are the basis of living matter. The specific character of microemulsions is that the variation of the content of the n0npolar solvent in the hydrocarbon core can exert a substantial effect on the localization site of luminophore molecules and, henc...
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