Cari S. Dutcher

Surface Tensions of Inorganic Multicomponent Aqueous Electrolyte Solutions and Melts

A semiempirical model is presented that predicts surface tensions (σ) of aqueous electrolyte solutions and their mixtures, for concentrations ranging from infinitely dilute solution to molten salt. The model requires, at most, only two temperature-dependent terms to represent surface tensions of either pure aqueous solutions, or aqueous or molten mixtures, over the entire composition range. A relationship was found for the coefficients of the equation σ = c(1) + c(2)T (where T (K) is temperature) for molten salts in terms of ion valency and radius, melting temperature, and salt molar volume. Hypothetical liquid surface tensions can thus be estimated for electrolytes for which there are no data, or which do not exist in molten form. Surface tensions of molten (single) salts, when extrapolated to normal temperatures, were found to be consistent with data for aqueous solutions. This allowed surface tensions of very concentrated, supersaturated, aqueous solutions to be estimated. The model has been applied to the following single electrolytes over the entire concentration range, using data for aqueous solutions over the temperature range 233-523 K, and extrapolated surface tensions of molten salts and pure liquid electrolytes: HCl, HNO(3), H(2)SO(4), NaCl, NaNO(3), Na(2)SO(4), NaHSO(4), Na(2)CO(3), NaHCO(3), NaOH, NH(4)Cl, NH(4)NO(3), (NH(4))(2)SO(4), NH(4)HCO(3), NH(4)OH, KCl, KNO(3), K(2)SO(4), K(2)CO(3), KHCO(3), KOH, CaCl(2), Ca(NO(3))(2), MgCl(2), Mg(NO(3))(2), and MgSO(4). The average absolute percentage error between calculated and experimental surface tensions is 0.80% (for 2389 data points). The model extrapolates smoothly to temperatures as low as 150 K. Also, the model successfully predicts surface tensions of ternary aqueous mixtures; the effect of salt-salt interactions in these calculations was explored.

An Isotherm-Based Thermodynamic Model of Multicomponent Aqueous Solutions, Applicable Over the Entire Concentration Range

In previous studies (Dutcher et al. J. Phys. Chem. C 2011, 115, 16474-16487; 2012, 116, 1850-1864), we derived equations for the Gibbs energy, solvent and solute activities, and solute concentrations in multicomponent liquid mixtures, based upon expressions for adsorption isotherms that include arbitrary numbers of hydration layers on each solute. In this work, the long-range electrostatic interactions that dominate in dilute solutions are added to the Gibbs energy expression, thus extending the range of concentrations for which the model can be used from pure liquid solute(s) to infinite dilution in the solvent, water. An equation for the conversion of the reference state for solute activity coefficients to infinite dilution in water has been derived. A number of simplifications are identified, notably the equivalence of the sorption site parameters r and the stoichiometric coefficients of the solutes, resulting in a reduction in the number of model parameters. Solute concentrations in mixtures conform to a modified Zdanovskii-Stokes-Robinson mixing rule, and solute activity coefficients to a modified McKay-Perring relation, when the effects of the long-range (Debye-Hückel) term in the equations are taken into account. Practical applications of the equations to osmotic and activity coefficients of pure aqueous electrolyte solutions and mixtures show both satisfactory accuracy from low to high concentrations, together with a thermodynamically reasonable extrapolation (beyond the range of measurements) to extreme concentration and to the pure liquid solute(s).

Spatio-temporal mode dynamics and higher order transitions in high aspect ratio Newtonian Taylor―Couette flows

Spatial and temporal frequency dynamics were experimentally tracked via flow visualization for Newtonian fluids as a function of the inner cylinder Reynolds number (Re i ) in the flow between concentric, independently rotating cylinders with a radius ratio of 0.912 and an aspect ratio of 60.7. Eight transitions from laminar to turbulent flow were characterized in detail for a stationary outer cylinder, producing highly resolved space―time and frequency―time plots for wavy, modulated and weakly turbulent states. A previously unreported early-modulated wavy vortex flow was found in our high aspect ratio geometry both with and without the presence of a dislocation. The envelope of stability for this flow state was shown to cross into the co-rotating regime, and is present up to Re o ∼ 60, where Re o is the outer cylinder Reynolds number. This early modulation is independent of acceleration in the range 0.18 15.4], states characterized by a single distinct temporal frequency for Re * = [(1.17―1.41), (3.56―5.20), (7.85―15.4)], states with multiple temporal frequencies for Re * = [(1.41―3.56), (5.20―7.85)], and a transition from laminar to weakly turbulent vortices occurs at Re * = 5.49. All flow states are characterized by symmetry/symmetry-breaking features as well as azimuthal and axial wavenumbers.

Direct Measurement of pH in Individual Particles via Raman Microspectroscopy and Variation in Acidity with Relative Humidity.

Atmospheric aerosol acidity is an important characteristic of aqueous particles, which has been linked to the formation of secondary organic aerosol by catalyzing reactions of oxidized organic compounds that have partitioned to the particle phase. However, aerosol acidity is difficult to measure and traditionally estimated using indirect methods or assumptions based on composition. Ongoing disagreements between experiments and thermodynamic models of particle acidity necessitate improved fundamental understanding of pH and ion behavior in high ionic strength atmospheric particles. Herein, Raman microspectroscopy was used to determine the pH of individual particles (H2SO4+MgSO4) based on sulfate and bisulfate concentrations determined from νs(SO4(2-)) and νs(HSO4(-)), the acid dissociation constant, and activity coefficients from extended Debye-Hückel calculations. Shifts in pH and peak positions of νs(SO4(2-)) and νs(HSO4(-)) were observed as a function of relative humidity. These results indicate the potential for direct spectroscopic determination of pH in individual particles and the need to improve fundamental understanding of ion behavior in atmospheric particles.

Organic component vapor pressures and hygroscopicities of aqueous aerosol measured by optical tweezers

Measurements of the hygroscopic response of aerosol and the particle-to-gas partitioning of semivolatile organic compounds are crucial for providing more accurate descriptions of the compositional and size distributions of atmospheric aerosol. Concurrent measurements of particle size and composition (inferred from refractive index) are reported here using optical tweezers to isolate and probe individual aerosol droplets over extended timeframes. The measurements are shown to allow accurate retrievals of component vapor pressures and hygroscopic response through examining correlated variations in size and composition for binary droplets containing water and a single organic component. Measurements are reported for a homologous series of dicarboxylic acids, maleic acid, citric acid, glycerol, or 1,2,6-hexanetriol. An assessment of the inherent uncertainties in such measurements when measuring only particle size is provided to confirm the value of such a correlational approach. We also show that the method of molar refraction provides an accurate characterization of the compositional dependence of the refractive index of the solutions. In this method, the density of the pure liquid solute is the largest uncertainty and must be either known or inferred from subsaturated measurements with an error of <±2.5% to discriminate between different thermodynamic treatments.

Elastic secondary flows in sharp 90 degree micro-bends: A comparison of PEO and DNA solutions

The growth or absence of elastic secondary flows is documented for flows of dilute and semi-dilute polymer solutions in sharp 90° micro-bends in channels of rectangular cross-section; secondary flows are not present for Newtonian flows under similar conditions. Flow visualization shows that a vortex is present in the inner, upstream corner of the bend and grows with increasing Reynolds (Re) and Weissenberg (Wi) numbers for flows of shear-thinning, semi-dilute polymeric solutions containing λ-DNA (9.9×10−7<Re<3.1×10−2, 0.42<Wi<126) or high molecular weight poly(ethylene) oxide (PEO) (3.5×10−4<Re<4.7×10−3, 1.8<Wi<17.7). Rheological differences, likely due to differences in the flexibility of DNA and PEO, influence the degree of vortex enhancement with increasing Wi. The vortex is absent for flow of a dilute, non-shear-thinning PEO solution over a large Re and Wi range (3.3×10−4<Re<1.6×10−2, 1.1<Wi<52.8) that includes conditions where vortices are observed for the semi-dilute, shear-thinning solutions. Hence...

Effects of moderate elasticity on the stability of co- and counter-rotating Taylor-Couette flows

Recently, we explored the effects of weak fluid elasticity (El ≪ 1) on the stability of co- and counter-rotating Taylor–Couette (TC) flows [Dutcher and Muller, J. Rheol. 55(6), 1271–1295 (2011)], where accessible flow states were primarily governed by the dominant inertial forces, yet modified by the weaker elastic forces. Here, the study of the inertial–elastic TC problem is expanded to El near unity, illuminating the effects of increasing the elastic forces on the inertially driven instabilities. A polyethylene oxide solution was carefully chosen to have optimal rheological properties and exhibit limited shear and oxidative degradation. The sequence of transitions to turbulence found here is notably different from that observed previously for either Newtonian or low-elasticity fluids. As El approaches order 1, laminar and turbulent flows are separated by only two coherent flow states: Standing vortices and disordered rotating standing waves. In contrast to our experiments at lower El, we also observe flow state hysteresis. In addition, the final turbulent flow state was not turbulent Taylor vortices (TTV) as seen with Newtonian and weakly elastic fluids, but rather a state we refer to as elastically dominated turbulence, which occurs at a significantly lower Reynolds number than TTV. Stability mappings involving rotation of the outer cylinder show that the flow states and transitions depend on the amount of counter- or co-rotation. As the degree of counter rotation increased, greater deviations from Newtonian and low El behavior were found, due to the presence of a nodal surface that changes the characteristic length scale of the flow.

Isotherm-based thermodynamic model for electrolyte and nonelectrolyte solutions incorporating long- and short-range electrostatic interactions.

The activities of solutes and solvents in solutions govern numerous physical phenomena in a wide range of practical applications. In prior work, we used statistical mechanics and multilayer adsorption isotherms to develop a transformative model for capturing thermodynamic properties of multicomponent aqueous solutions over the entire concentration range (Dutcher et al. J. Phys. Chem. 2011, 2012, 2013). That model needed only a few adsorption energy values to represent the solution thermodynamics of each solute. In the current work, we posit that the adsorption energies are due to dipole-dipole electrostatic forces in solute-solvent and solvent-solvent interactions. This hypothesis was tested in aqueous solutions on (a) 37 1:1 electrolytes, over a range of cation sizes, from H(+) to tetrabutylammonium, for common anions including Cl(-), Br(-), I(-), NO3(-), OH(-), ClO4(-), and (b) 20 water-soluble organic molecules including alcohols and polyols. For both electrolytes and organic solutions, the energies of adsorption can be calculated with the dipole moments of the solvent, molecular size of the solvent and solute, and the solvent-solvent and solvent-solute intermolecular bond lengths. Many of these physical properties are available in the literature, with the exception of the solute-solvent intermolecular bond lengths. For those, predictive correlations developed here enable estimation of solute and solvent solution activities for which there are little or no activity data.

Electrolyte-dependent aggregation of colloidal particles near electrodes in oscillatory electric fields

Colloidal particles adjacent to electrodes have been observed to exhibit drastically different aggregation behavior depending on the identity of the suspending electrolyte. For example, particles suspended in potassium chloride aggregate laterally near the electrode upon application of a low-frequency (∼100 Hz) oscillatory electric field, but the same particles suspended in potassium hydroxide are instead observed to separate. Previous work has interpreted the particle aggregation or separation in terms of various types of electrically induced fluid flow around the particle, but the details remain poorly understood. Here we present experimental evidence that the aggregation rate is highly correlated to both the particle zeta potential and the electric field amplitude, both of which depend on the electrolyte type. Measurement of the aggregation rate in 26 unique electrolyte-particle combinations demonstrates that the aggregation rate decreases with increasing zeta potential magnitude (i.e., particles with a large zeta potential tended to separate regardless of sign). Likewise, direct measurements of the oscillatory electric field in different electrolytes revealed that the aggregation rate was negatively correlated with solution conductivity and thus positively correlated with the field strength. We tested the experimentally measured aggregation rates against a previously developed point dipole model and found that the model fails to capture the observed electrolyte dependence. The results point to the need for more detailed modeling to capture the effect of electrolyte on the zeta potential and solution conductivity to predict fluid flow around colloids near electrodes.

Effects of weak elasticity on the stability of high Reynolds number co- and counter-rotating Taylor-Couette flows

This study examines the impact of dilute polymer solutions on the unique isolated secondary flows between concentric, rotating cylinders, namely Taylor-Couette (TC) flow. We mapped the stability of flow states using Newtonian and dilute polyethylene oxide (PEO) solutions over the Reynolds number range of − 100 < Reo < 500 and 0 < Rei < O(103), where subscripts ‘o’ and ‘i’ refer to outer and inner cylinders, respectively. Elasticity number (El) of the PEO fluids, defined as the ratio of elastic to inertial forces, ranges from O(10− 4) to O(10− 2). This work expands on previous studies by (a) significantly expanding the range of Rei, Reo, and El examined, (b) use of a consistent, conservative protocol for reaching flow states, and (c) rheological characterization of the solutions via shear and capillary breakup extensional rheometry. Using spectral analysis of flow visualization of the r-z or z-θ planes, we find the effect of El on the critical conditions for laminar and chaotic axisymmetric and nonaxisymmetric flow states is nonmonotonic and mode-dependent, with greater modification of higher order transitions involving small-scale features. While the critical conditions are modified by low El for all transitions, the flow states vary from those for Newtonian fluids at higher Rei and for the more elastic fluids.