Microscopic probing of the size dependence in hydrophobic solvation

We report small angle x-ray scattering data demonstrating the direct experimental microscopic observation of the small-to-large crossover behavior of hydrophobic effects in hydrophobic solvation. By increasing the side chain length of amphiphilic tetraalkyl-ammonium (C(n)H(2n+1))(4)N(+) (R(4)N(+)) cations in aqueous solution we observe diffraction peaks indicating association between cations at a solute size between 4.4 and 5 A?, which show temperature dependence dominated by hydrophobic attraction. Using O K-edge x-ray absorption we show that small solutes affect hydrogen bonding in water similar to a temperature decrease, while large solutes affect water similar to a temperature increase. Molecular dynamics simulations support, and provide further insight into, the origin of the experimental observations.     

Electrical properties of dry and wet polyglycine

Real and imaginary parts of the dielectric constant of wet and dry polyglycine have been measured for several temperatures over a wide range of frequencies. A simple, rather crude model, proposed earlier for nylon 66, fits the experimental data rather well, but the blocking coefficient p is larger and adsorption effects are more important.     

Microscopic solvation: spectroscopic results vs. Monte Carlo simulations

Van-der-Waals clusters of carbazole (representing the ‘solute’) with up to 40 nitrogen or methane solvent molecules were characterized using two-color resonant two-photon ionization spectroscopy. Features in these spectra (redshift, homogeneous and heterogeneous broadening, etc.) are interpreted as being caused by various static and dynamic effects of the solvent shell surrounding the aromatic substrate. For a better understanding of such effects, Monte Carlo simulations of the clusters were performed: Statics: Using a Monte Carlo simulated annealing minimization procedure, minimum energy structures (local, probably global) were found for the various cluster species. Using a simple empirical additivity rule, spectral shifts are rationalized from these structures.Dynamics: Starting from these minimum configurations, canonical ensemble simulations were carried out in a temperature range from 0 to 35 K. Severalorder-disorder transitions were identified including:

1. orientational isomerization or ‘melting’
2. surface isomerization or decoupling
3. rigid → fluxional transitions
4. full cluster ‘melting’

We present some of our experimental results on the systems carbazole · (N₂) _n and carbazole · (CH₄) _n together with the corresponding simulation data.     

Teflon hierarchical nanopillars with dry and wet adhesive properties

The superior material properties of β‐keratin along with the hierarchical high‐aspect‐ratio structure of geckos' foot pad have enabled geckos to stick readily and rapidly to almost any surfaces in both dry and wet conditions. In this research, nonsticky fluoropolymer (Teflon AF) resembling β‐keratin rigidity and having an extremely low surface energy and dielectric constant was applied to fabricate a novel dry adhesive consisting of high‐aspect‐ratio nanopillars terminated with a “fluffy” top layer. Both the nanopillars and the terminating layer are fabricated concurrently by replica molding using a nanoporous anodic aluminum oxide membrane as the mold. These Teflon AF hierarchical nanostructures are shown to have an exceptional capacity to generate strong adhesion in both dry conditions and under water because of combined actions of van der Waals forces, electrostatic attractions, and hydrophobic effects. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Microscopic origins for the favorable solvation of carbonate ether copolymers in CO2

The strong desire for the wide use of carbon dioxide as an environmentally benign process solvent has spurred a large number of attempts to improve its solubility characteristics. Pioneering experimental work by Beckman and co-workers has pointed to carbonate ether copolymers as promising candidates for nonfluorous surfactants. It is demonstrated here that Gibbs ensemble Monte Carlo simulations (using configurational-bias and double-bridging strategies and the transferable potentials for phase equilibria force field) can be employed to accurately predict the phase equilibria of a carbonate ether copolymer with CO2. The simulations indicate that the greater accessibility of the carbonyl oxygen plays a major role for the CO2-philicity of this copolymer surfactant.     

Evalution of sequential extractions on dry and wet sediments

A five-step sequential extraction procedure was applied on dried and wet Ballastplaat Scheldt estuary sediments. When wet (fresh) sediments were used, all sample handling up to the 3rd extraction step, inclusive, was carried out under inert atmosphere. The repeatability of the procedure was very good on dry samples. For Fe as for Mn, RSD values are lower than 4%, except for Mn in the fifth extraction step where a spread of 10% is observed. The observed RSDs for Pb are of the same order of magnitude as those for Mn. On wet samples the spread of the results is higher than on dried ones. The highest RSDs observed for Fe amount to 20%, for Mn to 15% but for Pb an RSD of up to 44% was found. Better homogenization of the solid sediment part of lyophilized sediments and different porosities of wet sediment sub-samples may be the explanation. These results also indicated that drying/oxidizing of the sediment sample causes a shift from less available/mobile metal fractions to more available/mobile fractions. The Mn and Fe oxyhydroxide spikes added to a wet sediment sample were recovered between 100±10%. The results obtained after changing the sequence of the extraction steps (multiple rotations and inversions were tested) corroborated the progressive increase in the aggressive nature of the extraction solutions in our standard scheme. Although there is also no need to change the ratio volume of extractant to amount of sediment, increasing the number of extraction repetitions in steps 1 to 3 resulted, for some of those extraction steps, in a partially modified analyte distribution. Finally the method was applied to sandy and muddy sediment cores of the Scheldt estuary and revealed clear differences between metal distributions in both types of sediment.     

Microscopic solvation of NaBO2 in water: anion photoelectron spectroscopy and ab initio calculations

We investigated the microscopic solvation of NaBO(2) in water by conducting photoelectron spectroscopy and ab initio studies on NaBO(2)(-)(H(2)O)(n) (n = 0-4) clusters. The vertical detachment energy (VDE) of NaBO(2)(-) is estimated to be 1.00 ± 0.08 eV. The photoelectron spectra of NaBO(2)(-)(H(2)O)(1) and NaBO(2)(-)(H(2)O)(2) are similar to that of bare NaBO(2)(-), except that their VDEs shift to higher electron binding energies (EBE). For the spectra of NaBO(2)(-)(H(2)O)(3) and NaBO(2)(-)(H(2)O)(4), a low EBE feature appears dramatically in addition to the features observed in the spectra of NaBO(2)(-)(H(2)O)(0-2). Our study shows that the water molecules mainly interact with the BO(2)(-) unit in NaBO(2)(-)(H(2)O)(1) and NaBO(2)(-)(H(2)O)(2) clusters to form Na-BO(2)(-)(H(2)O)(n) type structures, while in NaBO(2)(-)(H(2)O)(3) and NaBO(2)(-)(H(2)O)(4) clusters, the water molecules can interact strongly with the Na atom, therefore, the Na-BO(2)(-)(H(2)O)(n) and Na(H(2)O)(n)···BO(2)(-) types of structures coexist. That can be seen as an initial step of the transition from a contact ion pair (CIP) structure to a solvent-separated ion pair (SSIP) structure for the dissolution of NaBO(2).     

Determination of total nitrogen in atmospheric wet and dry deposition samples

A microwave-assisted persulfate oxidation method followed by ion chromatographic determination of nitrate was developed for total nitrogen determination in atmospheric wet and dry deposition samples. Various operating parameters such as oxidation reagent concentrations, microwave power, and extraction time were optimized to maximize the conversion of total nitrogen to nitrate for subsequent chemical analysis. Under optimized conditions, 0.012 M K₂S₂O₈ and 0.024 M NaOH were found to be necessary for complete digestion of wet and dry deposition samples at 400 W for 7 min using microwave. The optimized extraction method was then validated by testing different forms of organic nitrogen loaded to pre-baked filter substrates and NIST SRM 1648 (urban particulate matter), and satisfactory results were obtained. In the case of wet deposition samples, standard addition experiments were performed. The suitability of the method for real-world application was assessed by analyzing a number of wet and dry deposition samples collected in Singapore during the period of March-April 2007. The organic nitrogen content was 15% (wet) and 30% (dry) of the total nitrogen. During the study period, the estimated wet fluxes for nitrate (NO₃⁻), ammonium (NH₄⁺), organic nitrogen (ON), and total nitrogen (TN) were 16.1 ± 6.5 kg ha⁻¹ year⁻¹, 11.5 ± 5.7 kg ha⁻¹ year⁻¹, 3.8 ± 1.5 kg ha⁻¹ year⁻¹and 31.5 ± 13.2 kg ha⁻¹ year⁻¹, respectively, while the dry fluxes were 2.5 ± 0.8 kg ha⁻¹ year⁻¹, 1.4 ± 0.9 kg ha⁻¹ year⁻¹, 2.3 ± 1.4 kg ha⁻¹ year⁻¹ and 7.5 ± 2.6 kg ha⁻¹ year⁻¹, respectively.     

Microscopic environment of metal ion controlled by the balance between preferential solvation and coordination

The preferential solvation and the coordination characterizing metal ions (Mg2+ and Zn2+) in solution, which control the microscopic environments around the metal ions, were directly observed through the mass spectrometric analysis of clusters isolated from liquid droplets.     

Correlation and prediction of partition coefficient between the gas phase and water, and the solvents dry methyl acetate, dry and wet ethyl acetate, and dry and wet butyl acetate

Experimental partition coefficient data have been compiled from the published literature for the water/methyl acetate, water/ethyl acetate and water/butyl acetate partition systems, log P data, and for the gas/methyl acetate, gas/ethyl acetate and gas/butyl acetate partition systems, log K data. Application of the Abraham solvation parameter model to the sets of partition coefficients leads to equations that correlate the log P data and log K data to 0.18 log units for the three dry alkyl acetate solvents. Slightly larger deviations were noted for solute partition into both wet ethyl acetate and wet butyl acetate. The derived correlations were validated using training set and test set analyses.     

Cadmium losses on wet and dry ashing of plant materials

Cadmium losses on wet and dry ashing were studied using tomato leaves with metabolized¹⁰⁹Cd radiotracer. The most appreciable Cd losses (up to 35%) occured when sulphuric acid was used on wet ashing due to Cd retention in the CaSO₄ precipitate formed. Only trace amounts of Cd were retained in a silicaceous precipitate resulting from wet ashing in the absence of sulphuric and hydrofluoric acids in the decomposition mixtures employed. Small Cd losses (up to 7%) were observed on dry ashing in the air in open systems at temperatures not exceeding 500°C. Cd losses increased, however, to 30% when the final ashing temperature was raised to 900°C. Incorporation of the¹⁰⁹Cd radiotracer into various parts of tomato plants and its distribution between soil and the plants are also briefly reported.     

Role of water structure in thermodynamics of nonpolar solvation

The SSOZ (site-site Ornstein-Zernike) equation with an original closure condition for liquid molecular systems is used to calculate thermodynamic functions of noble gas solvation in water. Water is modeled by two close sets of atom-atomic potential functions. The calculations indicate that the chemical solvation potential is strongly sensitive to water structure. A comparison with experiment is given. Institute of Theoretical and Experimental Biophysics, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 4, pp. 736–741, July–August, 1996. Translated by L. Smolina     

Electrolytes in a nanometer slab-confinement: ion-specific structure and solvation forces

We study the liquid structure and solvation forces of dense monovalent electrolytes (LiCl, NaCl, CsCl, and NaI) in a nanometer slab-confinement by explicit-water molecular dynamics (MD) simulations, implicit-water Monte Carlo (MC) simulations, and modified Poisson-Boltzmann (PB) theories. In order to consistently coarse-grain and to account for specific hydration effects in the implicit methods, realistic ion-ion and ion-surface pair potentials have been derived from infinite-dilution MD simulations. The electrolyte structure calculated from MC simulations is in good agreement with the corresponding MD simulations, thereby validating the coarse-graining approach. The agreement improves if a realistic, MD-derived dielectric constant is employed, which partially corrects for (water-mediated) many-body effects. Further analysis of the ionic structure and solvation pressure demonstrates that nonlocal extensions to PB (NPB) perform well for a wide parameter range when compared to MC simulations, whereas all local extensions mostly fail. A Barker-Henderson mapping of the ions onto a charged, asymmetric, and nonadditive binary hard-sphere mixture shows that the strength of structural correlations is strongly related to the magnitude and sign of the salt-specific nonadditivity. Furthermore, a grand canonical NPB analysis shows that the Donnan effect is dominated by steric correlations, whereas solvation forces and overcharging effects are mainly governed by ion-surface interactions. However, steric corrections to solvation forces are strongly repulsive for high concentrations and low surface charges, while overcharging can also be triggered by steric interactions in strongly correlated systems. Generally, we find that ion-surface and ion-ion correlations are strongly coupled and that coarse-grained methods should include both, the latter nonlocally and nonadditive (as given by our specific ionic diameters), when studying electrolytes in highly inhomogeneous situations.     

Predicting octanol/water partition coefficients for the SAMPL6 challenge using the SM12, SM8, and SMD solvation models

Journal of Computer-Aided Molecular Design - Blind predictions of octanol/water partition coefficients at 298 K for 11 kinase inhibitor fragment like compounds were made for the SAMPL6 challenge....     

Predicting octanol/water partition coefficients and pKa for the SAMPL7 challenge using the SM12, SM8 and SMD solvation models

Journal of Computer-Aided Molecular Design - Blind predictions of octanol/water partition coefficients and pKa at 298.15 K for 22 drug-like compounds were made for the SAMPL7 challenge....     

Withdrawal strength of fasteners in rice straw fibre-thermoplastic composites under dry and wet conditions

The withdrawal strength of nail and screw fasteners has been studied in rice straw fibre-thermoplastic composites. Two types of thermoplastic, virgin polyethylene (PE) and polypropylene (PP), were selected as separate composite matrices. Three levels of dry rice straw fibre, 45%, 60% and 75%, based on the composition by weight and passed through a 40-mesh size screen was mixed with the polymeric matrices without and with 2% (based on weight) maleic anhydride grafted polypropylene (MAPP) as coupling agent. A dry-blending method was used for compounding the materials. The 12 formulations for the polymer composites were used to prepare samples with dimensions 25 cm by 15 cm by 1 cm. The final composites were made by pressing the prepared mats between the hot plates of a compression press by employing combinations of temperature and pressure in three stages. After keeping the composites at room temperature for 15 days, the withdrawal strengths of nails and screws were measured according to BS Standard (CEN/TS 15534–1:2007) for dry composites. Withdrawal strengths were also measured after immersion of the composites for 24 h in distilled water (wet condition). The results showed that the withdrawal strength of screws is more than that of nails. Also, irrespective of the type of polymer, the percentage of rice straw fibre may significantly influence the withdrawal strength of fasteners, especially at the higher fibre to polymer ratios. Furthermore, it was found that in the wet condition the withdrawal strengths of the nail fasteners are reduced more (73.66%) than for the screws (28.9%).     

Linear solvation energy relationships of mixed micelles of sodium dodecyl sulfate and decanol: towards a better model of octanol/water partitioning.

We show that we can alter the mechanism of micelle/water partitioning by the addition of decanol as a co-surfactant to an SDS micellar solution. Linear solvation energy relationship (LSER) studies indicate that as we increase the amount of decanol added to sodium dodecyl sulfate solution, the hydrogen bond donating ability of the aqueous phase increases and the cavity term of the micellar phase increases. We obtain a better correlation with octanol/water partitioning using the mixed micelle system compared to normal micelle solution. Choosing the appropriate micelle marker is very important. Significant changes in the LSER equations can occur if a different compound is used as the micelle marker.     

Transient temperatures of wet and dry polymeric film coated specimens experiencing outdoor exposure

A mathematical model based on an “aggregated-capacity” temperature analysis is presented and validated for predicting the temperature of thin polymeric films bonded to substrate materials. This model is applicable to coatings on metallic or other substrate materials when the ratio of the thermal resistance across the substrate to the resistance across the coating to ambient is less than approximately 0.1. An analytical rather than a numerical method, was employed to circumvent formulation difficulties and calculation constraints associated with the latter approach for extremely thin coatings. Periodic measurements of ambient dry-bulb and dew-point temperature, wind speed (to estimate convection coefficients), sky temperature, precipitation, and solar irradiation are inputs for determining heat exchange at coating surfaces. In addition to convection and solar irradiation, the model accounts for the effects of infrared exchange with the surrounding sky, dew formation on coating surfaces, and precipitation along with insulation and ambient convection at the back surface of the substrate. Based on comparisons with National Institute of Standards and Technology data on specimen temperature and simultaneous weather data compiled between 2006 and 2008, the analytical model correlated well to measured temperatures, and provided a tractable, computationally efficient, and validated solution for predicting long term transient temperatures in thin-coated specimens.