Ion exchange in reverse micelles

Modulation of probe-genomic DNA interaction within the confined interior of a reverse micelle: Is the bulk-like properties of water truly achieved in large reverse micelles?

The prime motivation of the present study is to explore the effect of reverse micellar confinement on the binding interaction of an anthracene-based probe 9-methyl anthroate with herring-sperm DNA. The structural modification of the genomic DNA from its native B-form to the non-native C-form and subsequently to the condensed Ψ-form as a function of the level of hydration (W₀, defined as [water] / [surfactant]) of the reverse micellar core is found to reveal a remarkable regulatory role on the stability of the stacking interaction (intercalation) of the probe within the DNA helix; the interaction being progressively stabilized at higher W₀. Particularly, a close perusal of the dynamical aspects of the interaction is found to be counter-intuitive to the popular notion of the properties of the confined water within the reverse micelles typically approaching bulk-like properties at sufficiently high hydration levels (W₀ > 10).

Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality

Surface charging phenomena in nonpolar dispersions are exploited in a wide range of industrial applications, but their mechanistic understanding lags far behind. We investigate the surface charging of a variety of polymer particles with different surface functionality in alkane solutions of a custom-synthesized and purified polyisobutylene succinimide (PIBS) polyamine surfactant and a related commercial surfactant mixture commonly used to control particle charge. We find that the observed electrophoretic particle mobility cannot be explained exclusively by donor-acceptor interactions between surface functional groups and surfactant polar moieties. Our results instead suggest an interplay of multiple charging pathways, which likely include the competitive adsorption of ions generated among inverse micelles in the solution bulk. We discuss possible factors affecting the competitive adsorption of micellar ions, such as the chemical nature of the particle bulk material and the size asymmetry between inverse micelles of opposite charge.

Mechanisms of Particle Charging by Surfactants in Nonpolar Dispersions

Electric charging of colloidal particles in nonpolar solvents plays a crucial role for many industrial applications and products, including rubbers, engine oils, toners, or electronic displays. Although disfavored by the low solvent permittivity, particle charging can be induced by added surfactants, even nonionic ones, but the underlying mechanism is poorly understood, and neither the magnitude nor the sign of charge can generally be predicted from the particle and surfactant properties. The conclusiveness of scientific studies has been limited partly by a traditional focus on few surfactant types with many differences in their chemical structure and often poorly defined composition. Here we investigate the surface charging of poly(methyl methacrylate) particles dispersed in hexane-based solutions of three purified polyisobutylene succinimide polyamine surfactants with "subtle" structural variations. We precisely vary the surfactant chemistry by replacing only a single electronegative atom located at a fixed position within the polar headgroup. Electrophoresis reveals that these small differences between the surfactants lead to qualitatively different particle charging. In the respective particle-free surfactant solutions we also find potentially telling differences in the size of the surfactant aggregates (inverse micelles), the residual water content, and the electric solution conductivity as well as indications for a significant size difference between oppositely charged inverse micelles of the most hygroscopic surfactant. An analysis that accounts for the acid/base properties of all constituents suggests that the observed particle charging is better described by asymmetric adsorption of charged inverse micelles from the liquid bulk than by charge creation at the particle surface. Intramicellar acid-base interaction and intermicellar surfactant exchange help rationalize the formation of micellar ions pairs with size asymmetry.

Difference in screening effect of alkali metal counterions on H-AOT-based W/O microemulsion formation

The purpose of this study was to estimate the screening of electrostatic repulsions between the polar headgroups of AOT(-) by alkali metal counterions and to explore the relationships between the screening effect and the phase behavior of H-AOT-based W/O microemulsions. The screening effect was evaluated by means of critical micelle concentration (CMC) data using the pyrene 1:3 ratio method with aqueous solutions containing M-AOT (where M(+) = Li(+), Na(+), K(+), Rb(+) and Cs(+)) to form normal micelles, and by counterion binding constants, determined from plots of CMC versus counterion concentration. The order of the screening effect was found to be K(+) approximately = Rb(+) > Cs(+) > Na(+) > Li(+). Interestingly, the order does not follow the hydration size dependence of the alkali metal counterions. An aqueous MOH solution containing a given concentration/H-AOT/isooctane was emulsified at a water content (w(0) = [water]/[H-AOT]) of 10 to produce H-AOT-based W/O microemulsions. The phase behavior and size variation were investigated by FT-IR and DLS measurements. The emulsified mixture separates into two phases at lower MOH concentration due to an insufficient screening effect. When the concentration is increased to a level sufficient to intensify the screening effect, W/O microemulsions are formed without phase separation at lower KOH and RbOH concentrations compared to CsOH. A period of standing after the emulsification and a higher concentration of NaOH compared to KOH, RbOH, and CsOH are required to form W/O microemulsions. W/O microemulsions are not formed in the case of LiOH. These results indicate that the formation of a W/O microemulsion with H-AOT is strongly correlated with the order of the screening effect. A possible cause for the difference in the screening effect is proposed based on hydration of the polar headgroups and counterions, as evidenced by FT-IR spectral data, i.e., symmetrical sulfonate stretching and O-H stretching.

1H NMR studies of aerosol-OT reverse micelles with alkali and magnesium counterions: preparation and analysis of MAOTs

Simple procedures and characterization of a series of well-defined precursors are described for preparation of a unique microenvironment in nanoreactors, reverse micelles. The Na(+), K(+), Rb(+), Cs(+), and Mg(2+) surfactants were prepared using liquid-liquid ion exchange using chloride and nitrate salts. The surfactants were characterized using (1)H NMR spectroscopy and a variety of other techniques. (1)H NMR spectroscopy was found to be a sensitive probe for characterization of the size of the nanoreactor as well as its water content. (1)H NMR spectra can be used for detailed characterization of reactions in confined environments when counterion effects are likely to be important. (1)H NMR spectroscopy revealed two separate peaks corresponding to water in Mg(AOT)2 samples; one peak arises from water coordinated to the Mg(2+) ion while the other peak arises from bulk water. The two water signals arise directly from the slow exchange of the water coordinated to Mg(2+) in these microemulsions with water in the water pool, and provide an opportunity to study hydration of Mg(2+). This work thus extends the potential use of MAOT microemulsions for applications such as in green chemistry.

Study and modeling of iron hydroxide nanoparticle uptake by AOT (w/o) microemulsions

Control over nanoparticle size is a key factor which labels a given preparation technique successful. When organic reactions are mediated by ultradispersed catalysts, the concentration of the colloidal nanoparticle catalysts and their stability become key factors as well. In this study, variables affecting iron hydroxide nanoparticle size, stability, and maximum possible colloidal concentration in AOT/water/isooctane microemulsions were investigated. Iron hydroxide was prepared in single microemulsions by first solubilizing iron chloride powder in the water pools, followed by addition of aqueous NaOH. Upon addition of NaOH, Fe(OH)3 nanoparticles stabilized in the water pools formed in addition to bulk precipitate of Fe(OH)3. The time-invariant concentration of the stabilized Fe(OH)3 is defined as the nanoparticle uptake, and it corresponds to the maximum possible concentration of the colloidal nanoparticles. The effect of the following variables on the nanoparticle uptake and size distribution was investigated: mixing time; surfactant concentration; water to surfactant mole ratio; and the initial concentration of the precursor salt. At 300 rpm of mixing a constant uptake of iron hydroxide nanoparticles was achieved in about 2 h and further mixing had limited effect on the nanoparticle uptake and particle size. An optimum R was found for which a maximum nanoparticle uptake was obtained. Nanoparticle uptake increased linearly with the surfactant concentration and displayed a power function with the initial concentrations of the precursor salt. The surface area/g of the nanoparticles was much higher than literature values, however, following a trend opposite to that of the nanoparticle uptake. The surface area/unit volume of the microemulsion, on the other hand, followed the same trend as the nanoparticle uptake. The particle size increased as R and/or the surfactant concentration increased. A mathematical model based on correlations for water uptake by Winsor type II microemulsions accurately accounted for the effect of the aforementioned variables on the nanoparticle uptake.

Protons in Non-ionic Aqueous Reverse Micelles

Using molecular dynamics techniques, we investigate the solvation of an excess proton within an aqueous reverse micelle in vacuo, with the neutral surfactant diethylene glycol monodecyl ether [CH3(CH2)11(OC2H4)2OH]. The simulation experiments were performed using a multistate empirical valence bond Hamiltonian model. Our results show that the stable solvation environments for the excess proton are located in the water-surfactant interface and that its first solvation shell is composed exclusively by water molecules. The relative prevalence of Eigen- versus Zundel-like solvation structures is investigated; compared to bulk results, Zundel-like structures in micelles become somewhat more stable. Characteristic times for the proton translocation jumps have been computed using population relaxation time correlation functions. The micellar rate for proton transfer is approximately 40x smaller than that found in bulk water at ambient conditions. Differences in the computed rates are examined in terms of the hydrogen-bond connectivity involving the first solvation shell of the excess charge with the rest of the micellar environment. Simulation results would indicate that proton transfers are correlated with rare episodes during which the HB connectivity between the first and second solvation shells suffers profound modifications.

Relaxation and Short Time Dynamics of Bulk Liquids and Fluids Confined in Spherical Cavities and Slit Pores

The density of states for bulk and confined fluids have been modeled using a recently proposed gamma distribution (Krishnan, S. H.; Ayappa, K. G. J. Chem. Phys. 2004, 121, 3197). The gamma distribution results in a closed form analytical expression for the velocity autocorrelation function and the relaxation time of the fluid. The two parameters of the gamma distribution are related analytically to the second and fourth frequency moments of the fluid using short time expansions. The predictions by the proposed gamma model are compared with the velocity autocorrelation functions obtained using the theory of instantaneous normal modes (INMs) and from molecular dynamics simulations. The model is applied to a bulk soft sphere liquid and fluids confined in a spherical cavity and slit-shaped pores. The gamma model is able to capture the resulting changes in relaxation time due to changes in density and temperature extremely well for both the bulk liquid and confined inhomogeneous fluid situations. In all cases, the predictions by the gamma model are superior to those obtained from the INM theory. In the case of the fluid confined in a slit pore, the loadings were obtained from a grand canonical Monte Carlo simulation where the pore is equilibrated with a bulk fluid. This is similar to a confinement situation in a surface force apparatus. The predicted relaxation times vs pore widths from the gamma model are seen to accurately capture the oscillations due to formation and disruption of layers within the slit pore.

The Effect of the Counterion on Water Mobility in Reverse Micelles Studied by Molecular Dynamics Simulations

In this study, mobility and structure of water molecules in Aerosol OT (bis(2-ethylhexyl) sulfosuccinate, AOT) reverse micelles with water content w0 = 5 and Na+, K+, Cs+ counterions have been explored with molecular dynamics (MD) simulations. Using the Faeder/Ladanyi model (J. Phys. Chem. B, 2000, 104, 1033) of the reverse micelle interior, MD simulations were performed to calculate the self-intermediate scattering function, FS(Q,t), for water hydrogen atoms that could be measured in a quasielastic neutron scattering experiment. Separate intermediate scattering functions FRS(Q,t) and FCMS(Q,t) were determined for rotational and translational motion. We find that the decay of FCMS(Q,t) is nonexponential and our analysis of the MD data indicates that this behavior arises from decreased water mobility for molecules close to the interface and from confinement-induced restrictions on the range of translational displacements. Rotational relaxation also exhibits nonexponential decay, which is consistent with relatively rapid restricted rotation and slower rotational relaxation over the full angular range. Rotational relaxation is anisotropic, with the O-H bond short-time rotational mobility considerably higher than that of the molecular dipole. This behavior is related to the decreased density of water-water hydrogen bonds in the vicinity of the interface compared to core or bulk water. We find that the interfacial mobility of water molecules is quite different for the three counterion types, but that the core mobility exhibits weak counterion dependence. Differences in interfacial mobility are strongly correlated with structural features, especially ion-water coordination, and the extent of disruption by the counterions of the water hydrogen bond network.