Incorporation and structural arrangement of microemulsion droplets in cylindrical pores of mesoporous silica
Albert PrauseAnja HörmannViviana CristiglioGlen J. SmalesAndreas F. ThünemannMichael GradzielskiGerhard H. Findenegg
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The behaviour of microemulsion (ME) droplets in mesoporous systems is highly important for understanding the immobilisation of drugs or chemical formulations, cleaning processes or enhanced oil recovery. The loading of pores as well as the structural organisation of MEs within the pores is a relevant parameter, especially for immobilisation applications. For this reason, the uptake of microemulsions in cylindrical pores of SBA-15 was investigated via adsorption and small-angle neutron scattering (SANS). Adsorption isotherms revealed an adsorption of the microemulsion droplets based on the adsorption of surfactant as a driving force. The adapted scattering model is based on the analysis of bare SBA-15 in full contrast conditions and employs microemulsions inside of SBA-15 measured at the silica contrast matching point. Accordingly, the structural arrangement of microemulsion droplets in the pores of SBA-15 was determined in good detail. Microemulsion droplets smaller than the pore size access the pores while retaining their spherical shape and become increasingly ordered for higher loading, where the droplets arrange in a dense packing of spherical droplets in a cylindrical pore. Interestingly, microemulsion droplets larger than the pore size can easily be incorporated, but become deformed upon entering and are present as elongated rod-like structures.Keywords:
Microemulsion
The formation of water-in-carbon dioxide microemulsions with a cationic perfluoropolyether trimethylammonium acetate surfactant, PFPE−C(O)−NH−CH2−N+(CH3)3 CH3COO-, is reported over a range of temperatures (25−90 °C) and pressures (87.3−415 bar). Spherical droplets are observed by SANS with radii ranging from 16 to 36 Å for water-to-surfactant molar ratios (Wo) from 9.5 to 28. Porod analysis of the SANS data indicates an area of approximately 60 Å2/surfactant molecule at the water−CO2 interface, in reasonable agreement with the value of 72 Å2 determined from the change in the droplet radius with Wo. The CO2-phobic functionality between the surfactant headgroup and perfluoropolyether tail reduces CO2 penetration of the tails, resulting in a smaller area/surfactant than in the case of an anionic perfluoropolyether surfactant [Langmuir 1997, 13, 3934]. A relatively rigid film, with a mean film rigidity (2K + K̄) of approximately 1 kBT, along with the strong partitioning of the surfactant toward CO2 versus water, lead to the small, rigid, spherical water droplets in CO2.
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Cationic polymerization
Penetration (warfare)
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We have used light scattering to determine the interaction potential between the surfactant coated water droplets in an oil continuous microemulsion system. The forward extrapolated zero angle scattering intensities at various water volume fractions were analyzed in terms of the Carnahan–Starling hard-sphere model modified for two-body attractive interactions. It was found that interparticle attractive interaction due to the overlapping of the surfactant tails could explain the observed phase behavior with respect to changes in the carbon chain length of the oil phase, as well as the existence of the lower critical point. It is further found that the optically determined potential is consistent with the microscopic results obtained with small angle neutron scattering experiments.
Microemulsion
Small-Angle Scattering
Critical point (mathematics)
Oil droplet
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We have examined isotopically substituted concentrated emulsions and related microemulsions by small angle neutron scattering (SANS). The emulsions have 90% internal phase micron-scale water droplets in a continuous hexadecane microemulsion. The surfactants have polyisobutylene oligomer tails with acid−amide headgroups. Dilution experiments with surfactant concentration varying over a 75-fold range confirm that the oil phase component of the emulsion contains reverse spherical micelles. We have produced single phase samples of microemulsions designed to have the same composition and same high Q scattering as the oil phase within our emulsions. SANS data from these fit to a model with a compound micelle in which a core region of radius a little less than 15 Å is surrounded by a shell of ca. 20 Å thickness. There is no hexadecane in the core and no water in the shell. The overall volume percentages in the surfactant concentrated microemulsions of water, hexadecane, and surfactant are 6%, 31%, and 64%, while for the more dilute microemulsions we obtain 3%, 37%, and 60%. The dilution data show that the surfactant loading at the oil−water interface is almost independent of dilution, and at the highest concentrations only 5% of the surfactant is at the emulsion droplet interface, the rest being in the form of micelles. The headgroup area per molecule at the interface is 140 Å2 and corresponds well with that expected for a monolayer of surfactant. The aqueous−oil interface is rough, with the water−surfactant interface smoother than the very rough surfactant−oil interface.
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Hexadecane
Dilution
Aqueous two-phase system
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Microemulsions are thermodynamically stable dispersions of two otherwise non-miscible fluids—usually “oil” and “water”—that are mediated by a surfactant. The surfactant molecules have to be amphiphilic, i.e., one end of a molecule is soluble in oil and the other end in water. The surfactant forms an interface layer between oil and water in the microemulsion. The total area of this interface is basically given by the amount of surfactant, and for a given topology of the oil/water regions, it controls their dimensions.
Microemulsion
Neutron spin echo
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Mesoporous organosilica
Hexanol
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Microemulsion
Pulsed field gradient
Cationic polymerization
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Small-angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy have been used to elucidate static and dynamic structures of a microemulsion system composed of a nonionic surfactant, water, and oil. Using the contrast variation neutron scattering technique, static structure parameters are evaluated, and the molecular volume change with pressure is calculated. The bending elastic modulus increases with increasing pressure. This tendency is similar to the microemulsion system composed of anionic surfactant, water, and oil. Therefore, a universal feature of the surfactant membrane as a response to pressure is clarified, that is, the surfactant membrane becomes rigid at high pressure due to the increase of density of the hydrophobic tail of surfactant molecules and their aggregates.
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Neutron spin echo
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Microemulsion
Dispersity
Hexadecane
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Surfactant mixing in model water-in-heptane microemulsion interfaces has been investigated for blends of a cationic didodecyldimethylammonium bromide (DDAB) with poly(ethylene glycol) monododecyl ethers (C12EJ, J = 3, 4, 5, 6, 7, 8, and 23). Phase behavior studies, electrical conductivity, and contrast variation small-angle neutron scattering (SANS) have been employed to delineate the effects of systematic variation of ethylene oxide headgroup size. These mixtures are characterized by an overall surfactant concentration (0.10 mol dm-3) and mole fraction of nonionic, which was varied up 0.20. The larger ethylene oxide (EO) numbers of 5−7 and 23 lead to significant enhancements in the maximum microemulsion solubilization capacity compared to DDAB only, whereas the shortest surfactant employed, C12E3, caused a decrease in the phase stability. Microemulsion nanostructure and interfacial compositions were studied for the EO3, EO4, EO6, and EO7 systems in partial structure factor type SANS experiments, as described before for the EO5 analogue (Langmuir 1999, 15, 5271). Analysis of contrast variation SANS data showed that C12E7, C12E6, and C12E5 partition strongly into the DDAB layer. Under equivalent conditions the shorter EO chain surfactants C12E4 and C12E3 appear to adsorb much more weakly. Interfacial compositions determined by SANS have been used to rationalize trends in phase behavior and nanostructure, highlighting the importance of partitioning effects with nonionics in multicomponent mixtures of this type.
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Mole fraction
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