S. A. Tovstun

On the composition fluctuations of reverse micelles

The polydispersity of the reverse micelles is determined mainly by the fluctuations of their composition. The composition of the reverse micelle is a two-dimensional random variable whose components are the numbers of water (i) and surfactant (j) molecules. In this study the fluctuations of the composition of the reverse micelles are considered in the Gaussian approximation. It is shown that the standard deviation of the quantity w=i/j may be calculated from the dependence of the water vapor pressure above the microemulsion on the molar ratio W=[water]/[surfactant]. The estimation based on the literature data for microemulsion system sodium bis(2-ethylhexyl)sulfosuccinate/water/isooctane at 37°C in the range W=0-18 has shown that the relative standard deviation of the quantity w is about 10%. It is shown that the value of the composition fluctuations is related to the dependence of average composition on the concentration of reverse micelles at constant parameter W.

Colloidal quantum dots InP@ZnS: Inhomogeneous broadening and distribution of luminescence lifetimes

Indium phosphide colloidal quantum dots with a zinc sulfide shell, an average core diameter of 3 nm, a luminescence peak position of 600 nm, and a luminescence quantum yield up to 50% have been synthesized. By analyzing the stationary absorption and luminescence spectra in terms of the Kennard—Stepanov relationship, the values of homogeneous width and inhomogeneous broadening have been obtained, which determine the resulting width of the spectra: the corresponding full widths at half maximum (FWHM) were 31, 63, and 70 nm. From the value of inhomogeneous broadening and the sizing curve of indium phosphide, polydispersity of the synthesized particles has been estimated as 11%. Analysis of the luminescence decay kinetics has revealed three reproducible peaks with maxima near 4.35, 35 (main) and 200 ns in the lifetime distribution. It has been found that although repeated washing of the synthesized particles with methanol can decrease the quantum yield, the lifetime distribution observed remains constant, which in the context of the “blinking” effect indicates a very short luminescence decay time of the particles in the OFF-state.

What makes AOT reverse micelles spherical

It is known that sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles are spherical over a wide range of water-to-surfactant molar ratios. This contradicts the traditional concept of preferred curvature. In actual fact, this concept does not apply to the AOT monolayer because its free energy is almost a linear function of the mean curvature. To correctly predict the shape of AOT reverse micelles, it is necessary to take into account not only the curvature free energy but also the disjoining pressure arising primarily from the overlapping of the electrical double layers at the opposite sides of the water core. Based on these considerations, we develop a model to calculate the free energy of AOT reverse microemulsion. This model allows us to explain the sphericity and to calculate various thermodynamic properties: the enthalpy of solubilization, chemical potentials, polydispersity, and the phase diagrams. All results are in qualitative agreement with the available experimental data.

Estimation of confidence intervals in calculation of the distribution of luminescence lifetimes from the kinetics of multiexponential decay

A new approach to the calculation of the luminescence lifetime distribution from experimental luminescence kinetics of multiexponential decay is proposed, which is based on the Tikhonov regularization method with allowance for the estimation of confidence intervals in solving linear inverse problems under the condition of non-negativity of the solution. The proposed approach permits finding solutions with almost optimum degree of smoothness, and analyzing them for the presence of artifacts caused by the ill-posedness of the problem.

Influence of surfactant polydispersity on the structure of polyoxyethylene (5) nonylphenyl ether/cyclohexane/water reverse microemulsions

The distribution of the number of oxyethylene groups in molecules of polyoxyethylene (5) nonylphenyl ether (NP5) is shown to significantly affect the structure of NP5/cyclohexane/water reverse microemulsions. The effect is caused by the difference in solubility of NP5 components. This difference leads to fractionation: surfactant molecules dissolved in the intermicellar nonpolar medium have, on average, shorter polyoxyethylene chains than surfactant molecules forming reverse micelles. A model is proposed which takes into account the influence of this fractionation on reverse micelle size and free surfactant concentration. The validity of this model for NP5/cyclohexane/water reverse microemulsions is experimentally confirmed by dynamic light scattering. It is found that changing the degree of surfactant fractionation in the above mentioned microemulsions can result in the formation of non-spherical reverse micelles.

J-aggregation of a thiacarbocyanine dye in reverse microemulsions

J-Aggregation of the thiacarbocyanine dye 3,3′-di-(γ-sulfopropyl)-4,5,4′,5′-dibenzo-9-ethylthiacarbocyanine betaine pyridinium salt (DEC) has been studied in reverse microemulsions of sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-hexane/water and polyoxyethylene (5) nonylphenyl ether (NP5)/cyclohexane/water. In such systems, the dye occurs mainly as a trans-monomer and a J-aggregate. It has been shown that the equilibrium between these species is determined by the solubility of the trans-monomer in monolayers of surfactant molecules covering microemulsion droplets. When the total concentration of the dye in the microemulsion is less than its solubility, the equilibrium state corresponds to the presence of the dye only in the form of the trans-monomer. When the concentration is higher than the solubility, the excess dye forms J-aggregates, which gradually grow and precipitate. For the reverse microemulsions AOT/n-hexane/water, the solubility was found to be [DEC]/[AOT] = (1.45 ± 0.34) × 10−5.

Influence of alkanethiols on fluorescence blinking of InP@ZnS colloidal quantum dots

Luminescence decay curves have been measured for InP@ZnS colloidal quantum dots (CQDs) synthesized at different ratios between 1-octanethiol and 1-dodecanethiol. The luminescence lifetime distributions reflecting the ratios between the on, off, and grey states of CQDs have been calculated. With an increase in the portion of 1-octanethiol, the distribution is shifted toward the on states, which almost completely suppresses fluorescence blinking at all detection wavelengths.

Method of separation of homogeneous and inhomogeneous broadenings of absorption and luminescence spectra of colloidal quantum dots

On the basis of the universal Kennard–Stepanov–van Roosbroeck–Shockley relation, a general approach is proposed for separation of the contributions of homogeneous and inhomogeneous broadenings in the absorption and luminescence spectra of colloidal quantum dots. The applicability of the developed method is demonstrated on published experimental data.

Luminescence of Hybrid Nanostructures of InP@ZnS Colloidal Quantum Dots and meso -Tetra(3-pyridyl)porphyrin Molecules

The formation of hybrid nanostructures consisting of InP@ZnS colloidal quantum dots and mesotetra(3-pyridyl)porphyrin molecules adsorbed on the quantum dots has been studied. In such nanostructures, strong quenching of quantum dot luminescence and an increase in the emission intensity of porphyrin are observed due to nonradiative resonance energy transfer from colloidal quantum dots to porphyrin.

Photoluminescence of trans-1,2-Di(2-naphthyl)ethylene: Conformation Isomerism and Validity of the Kennard Relation

Abstracttrans-1,2-Di(2-naphthyl)ethylene (DNE) exists in liquid solutions as a mixture of three conformers which have relatively high stability in the excited state, do not convert into each other during the excited state lifetime, and thus behave as individual compounds. This behavior results in the dependence of the luminescence spectrum on the excitation wavelength and in nonexponential luminescence decay. Recently, it has been shown theoretically that the Kennard relation should hold for the excitation–emission matrix in the case of any luminophore in the thermodynamically equilibrium state independent of complexity of the luminescence properties. It has been experimentally shown in this study that the Kennard relation does hold for DNE solutions.