Dynamics and Energetics of Droplet Aggregation in Percolating AOT Water-in-Oil Microemulsions

Abstract

This work reports on the dynamics of L2-microemulsions stabilized by Aerosol OT. Time-resolved luminescence quenching measurements using the probe Tb(pda)3 3- show the existence of clusters in water-in-oil (w/o) microemulsions. The fast exchange appearing over several microeseconds is attributed to intracluster quenching, whereas the slow exchange on the millisecond time scale corresponds to intercluster exchange. The fast exchange is decelerated when the temperature is increased and is related to a temperature-induced cluster growth. The slow exchange, conversely, is strongly accelerated within the one-phase region. Below the percolation threshold, the corresponding rate constant obeys an Arrhenius relation. The activation energies increase with the droplet size. In the percolation domain, strong deviations from linearity in the Arrhenius plots occur, which are interpreted by a kinetic scheme considering the limited quenching reaction rate and aggregate collisions disturbing the exchange transition state. Enthalpy−entropy compensation is established from the intercepts of the Arrhenius plots. The activation entropy is discussed in terms of the clustering entropy due to an aggregation equilibrium prior to exchange, yielding ΔS cl in very good agreement with results from other methods. ΔS cl increases with the droplet size and the solvent hydrocarbon length. The percolation phenomenon is discussed as an entropy-driven droplet aggregation accompanied by a facilitated exchange.

Water solubilization capacity of mixed reverse micelles: Effect of surfactant component, the nature of the oil, and electrolyte concentration

“…This phenomenon is called percolation of conductance [69]. Percolation signifies the increase in droplet size, attractive interaction among droplets and exchange rate of materials between the droplets [70][71][72]. In the present investigation, conductance studies have been carried out for mixed surfactant reverse micellar systems wherein solubilization maxima (ω 0,max or ω max ) are observed.…”

Energy Gap Dependence of the Nonradiative Decay Rate Constant of 1-Anilino-8-naphthalene Sulfonate in Reverse Micelles

“…The exchange of water and/or AOT molecules between the micelles (i.e., percolation) might be one of the reasons for the temperature effects. 30 Non-reproducible results were also obtained when a micellar solution was deaerated directly by purging in an inert-gas stream, probably due to evaporation of water in the sample, as this would lead to a change in w 0 . Sample preparation in a globe box under deaerated conditions is recommended for obtaining reproducible spectroscopic data.…”

“…Indeed, reverse AOT micelles have been studied by various techniques such as fluorescence spectroscopy [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] , NMR 22,23 , ESR 24,25 , IR [26][27][28] , and so on. [29][30][31][32] These studies demonstrated that polarity [8][9][10] , viscosity 11 , pH 12 , and other properties 30,32 of the water pool in the micelles were different from those in the bulk state and were dependent on the w 0 value. In addition to such characteristics, since reverse AOT micelles have been utilized widely as both separation and reaction media 33 , further extensive efforts should be devoted to understand their chemical and physical characteristics.…”

Physicochemical studies on microemulsions

“…This can be achieved in two ways: either by adding water to a constant mole ratio of amphiphile (ionic surfactant + cosurfactant) and oil (normally linear aliphatic hydrocarbon oil) or by increasing temperature at "volume percolation" and the second is called "temperature percolation." The said phenomenon of conductance percolation has been amply studied [4][5][6][7][8][9][10][11][12][13][14][15][16][17] with Aerosol-OT (AOT) as the surfactant with hydrocarbon oils for the amphiphile does not require a cosurfactant (normally lower alkanols and amines) for microemulsification, and relatively simple to interpret. Other ionic surfactants need nonionic surfactants and cosurfactants in combination to augment the process [18a,18b].…”

“…Other ionic surfactants need nonionic surfactants and cosurfactants in combination to augment the process [18a,18b]. A literature survey reveals that temperature-induced percolation [4][5][6][7][8][9][10][11][12][13][14][15][16][17] has been studied more frequently than volumeinduced percolation [5,[17][18][19][20][21][22][23][24] using all kinds of aliphatic hydrocarbon oils. The studies with nonaliphatic hydrocarbon oils [25,26] are much lower in number.…”