High-Resolution Structural Study of an Electrical Double Layer by Neutron Diffraction

Single-molecule fluorescence imaging of DNA at a potential-controlled interface

Many interfacial chemical phenomena are governed in part by electrostatic interactions between polyelectrolytes and charged surfaces; these phenomena can influence the performance of biosensors, adsorption of natural polyelectrolytes (humic substances) on soils, and production of polyelectrolyte multilayer films. In order to understand electrostatic interactions that govern these phenomena, we have investigated the behavior of a model polyelectrolyte, 15 kbp fluorescently labeled plasmid DNA, near a polarized indium tin oxide (ITO) electrode surface. The interfacial population of DNA was monitored in situ by imaging individual molecules through the transparent electrode using total-internal-reflection fluorescence microscopy. At applied potentials of +0.8 V versus Ag/AgCl, the DNA interfacial population near the ITO surface can be increased by 2 orders of magnitude relative to bulk solution. The DNA molecules attracted to the interface do not adsorb to ITO, but rather they remain mobile with a diffusion coefficient comparable to free solution. Ionic strength strongly influences the sensitivity of the interfacial population to applied potential, where the increase in the interfacial population over a +300 mV change in potential varies from 20% in 30 mM ionic strength to over 25-fold in 300 μM electrolyte. The DNA accumulation with applied potential was interpreted using a simple Boltzmann model to predict average ion concentrations in the electrical double layer and the fraction of interfacial detection volume that is influenced by applied potential. A Gouy-Chapman model was also applied to the data to account for the dependence of the ion population on distance from the electrode surface, which indicates that the net charge on DNA responsible for interactions with the polarized surface is low, on the order of one excess electron. The results are consistent with a small fraction of the DNA plasmid being resident in the double-layer and with counterions screening much of the DNA excess charge.

Monte Carlo study of the adsorption and aggregation of alkyltrimethylammonium chloride on the montmorillonite-water interface

Organically modified clays exhibit adsorption capacities for cations, anions, and nonpolar organic compounds, which make them valuable for various environmental technical applications. To improve the understanding of the adsorption processes, the molecular-scale characterization of the structures of organic aggregates assembled on the external basal surfaces of clay particles is essential. The focus of this Monte Carlo simulation study was on the effects of the surface coverage and the alkyl chain length n on the structures of alkyltrimethylammonium chloride ((C(n)TMA)Cl) aggregates assembled on the montmorillonite-water interface. We found that the amount of adsorbed C(n)TMA(+) ions is independent of the alkyl chain length and increases with the C(n)TMA(+) surface coverage. The C(n)TMA(+) ions predominantly adsorb as inner-sphere complexes; the fraction of outer-sphere adsorbed ions equals only about 10%. The conformational order of the C(n)TMA(+) alkyl chains substantially decreases with decreasing alkyl chain length. In agreement with previous experiments, the amount of C(n)TMA(+) ions that are aggregated at the mineral surface increases with increasing chain length. The maximum value of 0.66 C(n)TMA(+) adsorption complex per unit cell area of the clay surface considerably exceeds the amount of cations required to compensate the negative charge of the montmorillonite surface. Furthermore, in most of the studied systems, fractions of Na(+) surface cations remain adsorbed on montmorillonite. The resulting interfacial positive charge excess is counterbalanced by coadsorbed chloride ions forming ion pairs with both C(n)TMA(+) and Na(+).

Aggregation of alkyltrimethylammonium ions at the cleaved muscovite mica-water interface: a Monte Carlo study

The precise molecular structure of organically modified mineral surfaces is still not well understood. To establish a relation between experimental observations and underlying molecular structure, we performed Monte Carlo simulations of the aggregation behavior of alkyltrimethylammonium surfactants (C(n)TMA(+)) at the interface between C(n)TMACl solution and cleaved K(+)-muscovite. The structures were examined with regard to the influence of varying alkyl chain length n (n = 8, 12, 16) and surface coverage of C(n)TMA(+) ions. The simulation results indicate that the water film structure at the muscovite surface is considerably influenced by the adsorption of C(n)TMA(+). A fraction of the C(n)TMA(+) ions forms inner-sphere and outer-sphere adsorption complexes with nitrogen-surface distances of 3.3-3.8 and 5.5-8.4 Å, respectively. The simulated monolayer aggregates exhibit thicknesses of 31-35, 22-27, and ∼18 Å for C(16)TMA(+), C(12)TMA(+), and C(8)TMA(+), respectively. C(16)TMA(+) and C(12)TMA(+) ions form bilayer aggregates, which show a strong interdigitation of the two opposing layers composing them. The aggregate thicknesses equal 35-39 and 30-35 Å, respectively, and are in agreement with available experimental data. In contrast, the short-chained C(8)TMA(+) ions do not form bilayer aggregates. In agreement with previous experimental studies, the alkyl chains of the aggregated ions show high conformational order markedly decreasing with decreasing chain length. We suggest that the simulated structures represent C(n)TMA(+) aggregates, which are formed on muscovite during the experimentally observed initial equilibration phase characterized by the presence of inorganic ions within the aggregates.

Ca/Na montmorillonite: structure, forces and swelling properties

Ca/Na montmorillonite and natural Wyoming bentonite (MX-80) have been studied experimentally and theoretically. For a clay system in equilibrium with pure water, Monte Carlo simulations predict a large swelling when the clay counterions are monovalent, while in presence of divalent counterions a limited swelling is obtained with an aqueous layer between the clay platelets of about 10 A. This latter result is in excellent agreement with X-ray scattering data, while dialysis experiments give a significantly larger swelling for Ca montmorillonite in pure water. Obviously, there is one "intra-lamellar" and a second "extra-lamellar" swelling. Montmorillonite in contact with a salt reservoir containing both Na(+) and Ca(2+) counterions will only show a modest swelling unless the Na(+) concentration in the bulk is several orders of magnitude larger than the Ca(2+) concentration. The limited swelling of clay in presence of divalent counterions is a consequence of ion-ion correlations, which reduce the entropic repulsion as well as give rise to an attractive component in the total osmotic pressure. Ion-ion correlations also favor divalent counterions in a situation with a competition with monovalent ones. A more fundamental result of ion-ion correlations is that the osmotic pressure as a function of clay sheet separation becomes nonmonotonic, which indicates the possibility of a phase separation into a concentrated and a dilute clay phase, which would correspond to the "extra-lamellar" swelling found in dialysis experiments. This idea also finds support in the X-ray scattering spectra, where sometimes two peaks corresponding to different lamellar spacings appear.

Zn2+ and Sr2+ adsorption at the TiO2 (110)–electrolyte interface: Influence of ionic strength, coverage, and anions

The X-ray standing wave technique was used to probe the sensitivity of Zn2+ and Sr2+ ion adsorption to changes in both the adsorbed ion coverage and the background electrolyte species and concentrations at the rutile (alpha-TiO2) (110)-aqueous interface. Measurements were made with various background electrolytes (NaCl, NaTr, RbCl, NaBr) at concentrations as high as 1 m. The results demonstrate that Zn2+ and Sr2+ reside primarily in the condensed layer and that the ion heights above the Ti-O surface plane are insensitive to ionic strength and the choice of background electrolyte (with <0.1 A changes over the full compositional range). The lack of any specific anion coadsorption upon probing with Br-, coupled with the insensitivity of Zn2+ and Sr2+ cation heights to changes in the background electrolyte, implies that anions do not play a significant role in the adsorption of these divalent metal ions to the rutile (110) surface. Absolute ion coverage measurements for Zn2+ and Sr2+ show a maximum Stern-layer coverage of approximately 0.5 monolayer, with no significant variation in height as a function of Stern-layer coverage. These observations are discussed in the context of Gouy-Chapman-Stern models of the electrical double layer developed from macroscopic sorption and pH-titration studies of rutile powder suspensions. Direct comparison between these experimental observations and the MUltiSIte Complexation (MUSIC) model predictions of cation surface coverage as a function of ionic strength revealed good agreement between measured and predicted surface coverages with no adjustable parameters.

Gas-Phase Ion−Molecule Reactions of Metal−Carbide Cations MCn+ (M=Y and La; n=2, 4, and 6) with Benzene and Cyclohexane Investigated by FTICR Mass Spectrometry and DFT Calculations

Yttrium- and lanthanum-carbide cluster cations YC(n)(+) and LaC(n)(+) (n = 2, 4, and 6) are generated by laser ablation of carbonaceous material containing Y(2)O(3) or La(2)O(3). YC(2)(+), YC(4)(+), LaC(2)(+), LaC(4)(+), and LaC(6)(+) are selected to undergo gas-phase ion-molecule reactions with benzene and cyclohexane. The FTICR mass spectrometry study shows that the reactions of YC(2)(+) and LaC(2)(+) with benzene produce three main series of cluster ions. They are in the form of M(C(6)H(4))(C(6)H(6))(n)(+), M(C(8)H(4))(C(6)H(6))(n)(+), and M(C(8)H(6))(C(6)H(6))(m)(+) (M = Y and La; n = 0-3; m = 0-2). For YC(4)(+), LaC(4)(+), and LaC(6)(+), benzene addition products in the form of MC(n)(C(6)H(6))(m)(+) (M = Y and La; n = 4, 6; m = 1, 2) are observed. In the reaction with cyclohexane, all the metal-carbide cluster ions are observed to form metal-benzene complexes M(C(6)H(6))(n)(+) (M = Y and La; n= 1-3). Collision-induced-dissociation experiments were performed on the major reaction product ions, and the different levels of energy required for the fragmentation suggest that both covalent bonding and weak electrostatic interaction exist in these organometallic complexes. Several major product ions were calculated using DFT theory, and their ground-state geometries and energies were obtained.

Impact of organic solvents on the resolution of synthetic peptides by capillary electrophoresis

The effect of variations in the concentrations of different organic solvents, including acetonitrile, methanol, ethanol, propanol and isopropanol, with aqueous buffer electrolytes of defined composition and pH on the electroosmotic flow velocity, v(EOF), of uncoated fused silica capillaries and on the electrophoretic mobility, mu(e), of synthetic peptides in high-performance capillary electrophoresis (HPCE) has been systematically investigated. In these experiments, the volume fractions of the organic solvent in the aqueous buffer electrolyte were changed from psi = 0.0 to 0.80. The addition of these organic solvents to the aqueous buffer electrolyte reduced the electroosmotic flow (EOF) of the system, but to significantly different extents. For the protic solvents as the alkyl chain of the alcohol increased, at the same volume fraction the greater was the influence on the electroosmotic flow. However, for the aprotic solvent, acetonitrile, the EOF did not change substantially as the volume fraction was varied. The electrophoretic mobility of synthetic peptides under the different buffer electrolyte conditions showed similar trends, confirming that the content and type of the organic modifier can be rationally employed to subtly manipulate the separation selectivity of synthetic peptides. These results, therefore, provide fundamental insight into the experimental options that can be used to maximise resolution of synthetic peptides in HPCE with aqueous buffer-organic solvent mixtures as well as a basis to select optimal binary or ternary buffer electrolyte compositions for the analysis of peptides when hyphenated techniques, such as HPCE-electrospray ionisation mass spectrometry (ESI-MS), are contemplated for the analysis of peptide samples of low abundance as can often be experienced in proteomic investigations.