Characteristics of K + and Rb + as “structure-breaking” ions in dilute aqueous solution: Insights from ONIOM-XS MD simulations

Tuning Local Order in Starch Nanoparticles Exploiting Nonsolvency with "Green" Solvents

Starch is a renewable biopolymer that can be sourced from agricultural waste and used to produce nanoparticles (SNPs). In particular, amorphous SNPs have potential application in numerous fields, including the consolidation of weakened paintings in the cultural heritage preservation. Starch dissolution followed by nanoprecipitation in nonsolvents is an advantageous synthetic route, but new methodologies are needed to feasibly control the physicochemical properties of the SNPs. Here, we explored nanoprecipitation by nonsolvency using a set of "green" solvents to obtain amorphous SNPs, rather than starch nanocrystals already reported in the literature. The effect of the nonsolvent on the ordering of polymer chains in the obtained SNPs was studied. The recovery of local order (e.g., isolated V-type helices) after dissolution was shown to depend on the type of solvents used in the dissolution and precipitation steps, while long-range order (extended arrays of helices) is lost. Aqueous dispersions of the SNPs provided effective consolidation of powdery painted layers, showing that the selection of particle synthetic routes can be dictated by sustainability and scalability criteria. These "green" formulations are candidates as new consolidants in art preservation, and the possibility of tuning local order in amorphous starch assemblies might also impact fields like food chemistry, pharmaceutics, and nanocomposites, where SNPs with tunable amorphousness are more advantageous than nanocrystals.

Structure of Aqueous KNO3 Solutions by Wide-Angle X-ray Scattering and Density Functional Theory

The structure of aqueous KNO₃ solutions was studied by wide-angle X-ray scattering (WAXS) and density functional theory (DFT). The interference functions were subjected to empirical potential structure refinement (EPSR) modeling to extract the detailed hydration structure information. In aqueous KNO₃ solutions with a concentration from 0.50 to 2.72 mol·dm^-3, water molecules coordinate K⁺ with a mean coordination number (CN) from 6.6 ± 0.9 to 5.8 ± 1.2, respectively, and a K-O_W(H₂O) distance of 2.82 Å. To further describe the hydration properties of K⁺, a hydration factor (f_h) was defined based on the orientational angle between the water O-H vector and the ion-oxygen vector. The f_h value obtained for K⁺ is 0.792, 0.787, 0.766, and 0.765, and the corresponding average hydration numbers (HN) are 5.2, 5.1, 4.8, and 4.5. The reduced HN is compensated by the direct binding of oxygen atoms of NO₃^- with the average CN from 0.3 ± 0.7 to 2.6 ± 1.7, respectively, and the K-O_N(NO₃^-) distance of 2.82 Å. The average number of water molecules around NO₃^- slightly reduces from 10.5 ± 1.5 to 9.6 ± 1.7 with r_N-OW = 3.63 Å. K⁺-NO₃^- ion association is characterized by K-O_N and K-N pair correlation functions (PCFs). A K-N peak is observed at 3.81 Å as the main peak with a shoulder at 3.42 Å in g_K-N(r). This finding indicates that two occupancy sites are available for K⁺, i.e., the edge-shared bidentate (BCIP) and the corner-shared monodentate (MCIP) contact ion pairs. The structure and stability of the KNO₃(H₂O)₁₀ cluster were investigated by a DFT method and cross-checked with the results from WAXS.

Role of Counterions in the Adsorption and Micellization Behavior of 1:1 Ionic Surfactants at Fluid Interfaces─Demonstrated by the Standard Amphiphile System of Alkali Perfluoro-n-octanoates

In our latest communication, we proved experimentally that the ionic surfactant's surface excess is exclusively determined by the size of the hydrated counterion.[Lunkenheimer , Langmuir, 2017, 33, 10216-1022410.1021/acs.langmuir.7b00786]. However, at this stage of research, we were unable to decide whether this does only hold for the two or three lightest ions of lithium, sodium, and potassium, respectively. Alternatively, we could also consider the surface excess of the heavier hydrated alkali ions of potassium, rubidium, and cesium, having practically identical ion size, as being determined by the cross-sectional area of the related anionic extended chain residue. The latter assumption has represented state of art. Searching for reliable experimental results on the effect of the heavier counterions on the boundary layer, we have extended investigations to the amphiphiles' solutions of concentrations above the critical concentration of micelle formation (cmc).We provided evidence that the super-micellar solutions' equilibrium surface tension will remain constant provided the required conditions are followed. The related σ_cmc-value represents a parameter characteristic of the ionic surfactant's adsorption and micellization behavior. Evaluating the amphiphile's surface excess obtained from adsorption as a function of the related amphiphile's σ_cmc-value enables you to calculate the radius of the hydrated counterion valid in sub- and super-micellar solution likewise. The σ_cmc-value is directly proportional to the counterion's diameter concerned. Taking additionally into account the radii of naked ions known from crystal research, we succeeded in exactly discriminating the hydrated alkali ions' size from each other. There is a distinct sequence of hydration radii in absolute scale following the inequality, Li⁺ > Na⁺ > K⁺ > (NH₄)⁺ > Rb⁺ > Cs⁺. Therefore, we have to extend our model of counterion effectiveness put forward in our previous communication. It represents a general principle of the counterion effect.

Structures of ions accommodated in salty ice Ih crystals

Frozen aqueous electrolytes are ubiquitous and involved in various phenomena occurring in the natural environment. Although salts are expelled from ice during freezing of aqueous solutions, minor amounts of the constituent ions are accommodated in the crystal lattice of ice. This phenomenon was associated with the generation of the Workman-Reynolds freezing potential. Molecular simulations also confirmed the ion incorporation in the crystal lattice of ice Ih upon freezing of aqueous electrolytes and identified possible local structures of the ions. However, no experimental information is available on the structure of ions accommodated in the crystal lattice of ice Ih. In this work, we use X-ray absorption fine structure (XAFS) to study the local structures of K⁺ and Cl^- accommodated in ice Ih single crystals. Previous molecular simulations predicted that ions are trapped in the hexagonal cavities of the ice structure or replace two water molecules in the crystal lattice. Four possible configurations are considered and optimized by the calculations using ONIOM (QM/QM/QM). The results are evaluated in terms of the agreement between the experimental XAFS spectra and those simulated from the optimized structures. The spectra are most reasonably interpreted by assuming that K⁺ replaces one water molecule in the ice crystal lattice and is accommodated in a tetrahedral coordination cage. Similarly, Cl^- probably adopts the same configuration, because it explains the coordination number better than other structures, such as that assuming the replacement of two water molecules belonging to the same hexagonal planes.

The hydration structure of the heavy-alkalines Rb+ and Cs+ through molecular dynamics and X-ray absorption spectroscopy: surface clusters and eccentricity

Hydration shells around Rb⁺ and Cs⁺ are not symmetric; the cation and the 1st-shell water mass center are separated by ∼0.4 Å, and this is supported by agreement between the theoretical and experimental EXAFS spectrum.