Comparison of cation adsorption by isostructural rutile and cassiterite.

Macroscopic net proton charging curves for powdered rutile and cassiterite specimens with the (110) crystal face predominant, as a function of pH in RbCl and NaCl solutions, trace SrCl(2) in NaCl, and trace ZnCl(2) in NaCl and Na Triflate solutions, are compared to corresponding molecular-level information obtained from static DFT optimizations and classical MD simulations, as well as synchrotron X-ray methods. The similarities and differences in the macroscopic charging behavior of rutile and cassiterite largely reflect the cation binding modes observed at the molecular level. Cation adsorption is primarily inner-sphere on both isostructural (110) surfaces, despite predictions that outer-sphere binding should predominate on low bulk dielectric constant oxides such as cassiterite (ε(bulk) ≈ 11). Inner-sphere adsorption is also significant for Rb(+) and Na(+) on neutral surfaces, whereas Cl(-) binding is predominately outer-sphere. As negative surface charge increases, relatively more Rb(+), Na(+), and especially Sr(2+) are bound in highly desolvated tetradentate fashion on the rutile (110) surface, largely accounting for enhanced negative charge development relative to cassiterite. Charging curves in the presence of Zn(2+) are very steep but similar for both oxides, reflective of Zn(2+) hydrolysis (and accompanying proton release) during the adsorption process, and the similar binding modes for ZnOH(+) on both surfaces. These results suggest that differences in cation adsorption between high and low bulk dielectric constant oxides are more subtly related to the relative degree of cation desolvation accompanying inner-sphere binding (i.e., more tetradentate binding on rutile), rather than distinct inner- and outer-sphere adsorption modes. Cation desolvation may be favored at the rutile (110) surface in part because inner-sphere water molecules are bound further from and less tightly than on the cassiterite (110) surface. Hence, their removal upon inner-sphere cation binding is relatively more favorable.

Modification of silicon carbide surfaces by atmospheric pressure plasma for composite applications.

In this study, we explore the use of atmospheric pressure plasmas for enhancing the adhesion of SiC surfaces using a urethane adhesive, as an alternative to grit-blasting. Surface analysis showed that He-O2 plasma treatments resulted in a hydrophilic surface mostly by producing SiOx. Four-point bending tests and bonding pull tests were carried out on control, grit-blasted, and plasma-treated surfaces. Grit-blasted samples showed enhanced bonding but also a decrease in flexural strength. Plasma treated samples did not affect the flexural strength of the material and showed an increase in bonding strength. These results suggest that atmospheric pressure plasma treatment of ceramic materials is an effective alternative to grit-blasting for adhesion enhancement.

Development of antimicrobial coatings by atmospheric pressure plasma using a guanidine-based precursor.

Antimicrobial coatings deposited onto ultra high molecular weight polyethylene (UHMWPE) films were investigated using an atmospheric pressure - plasma enhanced chemical vapor deposition (AP-PECVD) process. Varying concentrations of a guanidine-based liquid precursor, 1,1,3,3-tetramethylguanidine, were used, and different deposition conditions were studied. Attenuated total reflectance - Fourier Transform Infrared (ATR-FTIR) spectroscopy and X-ray Photoelectron Spectroscopy (XPS) were used to study the chemical structure and elemental composition of the coatings. Conformity, morphology, and coating thickness were assessed through SEM and AFM. Optimal AP-PECVD parameters were chosen and applied to deposit guanidine coatings onto woven fabrics. The coatings exhibited high antimicrobial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) based on a modified-AATCC 100 test standard, where 2-5 log reductions were achieved.

Protonation enthalpies of metal oxides from high temperature electrophoresis.

Surface protonation reactions play an important role in the behavior of mineral and colloidal systems, specifically in hydrothermal aqueous environments. However, studies addressing the reactions at the solid/liquid interface at temperatures above 100°C are scarce. In this study, newly and previously obtained high temperature electrophoresis data (up to 260°C) - zeta potentials and isoelectric points - for metal oxides, including SiO(2), SnO(2), ZrO(2), TiO(2), and Fe(3)O(4), were used in thermodynamic analysis to derive the standard enthalpies of their surface protonation. Two different approaches were used for calculating the protonation enthalpy: one is based on thermodynamic description of the 1-pKa model for surface protonation, and another one - on a combination of crystal chemistry and solvation theories which link the relative permittivity of the solid phase and the ratio of the Pauling bond strength and bond length to standard protonation enthalpy. From this analysis, two expressions relating the protonation enthalpy to the relative permittivity of the solid phase were obtained.

Electrophoretic study of the SnO2/aqueous solution interface up to 260 degrees C.

An electrophoresis cell developed in our laboratory was utilized to determine the zeta potential at the SnO(2) (cassiterite)/aqueous solution (10(-3) mol kg(-1) NaCl) interface over the temperature range from 25 to 260 degrees C. Experimental techniques and methods for the calculation of zeta potential at elevated temperature are described. From the obtained zeta potential data as a function of pH, the isoelectric points (IEPs) of SnO(2) were obtained for the first time. From these IEP values, the standard thermodynamic functions were calculated for the protonation-deprotonation equilibrium at the SnO(2) surface, using the 1-pK surface complexation model. It was found that the IEP values for SnO(2) decrease with increasing temperature, and this behavior is compared to the predicted values by the multisite complexation (MUSIC) model and other semitheoretical treatments, and were found to be in excellent agreement.

Electrophoresis system for high temperature mobility measurements of nanosize particles.

The electrophoretic mobility, which reflects the zeta potential of a solid material, is an important experimental quantity providing information about the electrical double layer at the solid/liquid interface. A new high temperature electrophoresis cell was developed suitable for electrophoretic mobility measurements of dispersed nanosize particles up to 150 degrees C and 40 bars. Amorphous silica (SiO(2)) particle size standards were used to test the particle size detection limit of the new instrument at 25, 100, and 150 degrees C and several pH values. The microscopic detection of the particles was enabled by dark-field illumination, which allowed extending the previously available capabilities and provided higher accuracy of the electrophoretic mobility data. The electrophoretic mobility measurements for SiO(2) at temperatures above 100 degrees C were reported for the first time and indicated a gradual increase in particle electrophoretic response with increasing temperature. The obtained data indicated negatively charged SiO(2) surface throughout the pH and temperature ranges studied.