The Solvation of Mg2+ with Gas-Phase Clusters Composed of Alcohol Molecules

The Solvation of Ca2+ with Gas Phase Clusters of Alcohol Molecules

A comprehensive examination of how the identity of an alcohol molecule can change the behavior of a solvated, alkaline earth dication has been undertaken. The metal dication of Ca²⁺ has been clustered with a range of different alcohols to form [Ca(ROH)_n]²⁺ complexes, where n lies in the range 2-20. Following collisional activation via electron capture from nitrogen gas, complexes for n in the range 2-6 exhibit a switch in reaction product as a function of n. For low values, solvated CaOH⁺ is the dominant fragment, but as n increases beyond 4, this is displaced by the appearance of solvated CaOR⁺. A separate study of unimolecular metastable decay by [Ca(ROH)_n]²⁺ complexes found evidence of charge separation to form CaOH⁺(ROH)_n-1 + R⁺. For two isomers of butanol, the n = 3 complexes were found to follow parallel, but different metastable pathways: one leading to the appearance of CaOH⁺ and another that resulted in proton abstraction to form ROH₂⁺. These differences have been attributed to the precursor complexes adopting geometries where one ROH molecule occupies a secondary solvation shell. Comparisons were made with a previous study of magnesium complexes; [Mg(ROH)_n]²⁺ show that the difference in second ionization energy Mg⁺ (15.09 eV) as opposed to Ca⁺ (11.88 eV) influences behavior. A complex between Ca²⁺ and 1-chloroethanol is shown to favor the formation of CaCl⁺ as opposed to CaOH⁺ as a unimolecular charge separation product, which is attributed to differences in bond energy in the precursor molecule.

In vitro degradation kinetics of pure PLA and Mg/PLA composite: Effects of immersion temperature and compression stress

The effects of the immersion temperature and compression stress on the in vitro degradation behavior of pure poly-lactic acid (pure-PLA) and PLA-based composite unidirectionally reinforced with micro-arc oxidized magnesium alloy wires (Mg/PLA or MAO-MAWs/PLA) are investigated. The degradation kinetics of pure-PLA and the PLA matrix in MAO-MAWs/PLA exhibit an Arrhenius-type behavior. For the composite, the synergic degradation of MAO-MAWs maintains a steady pH and mitigates the degradation of PLA matrix during immersion. However, the external compression stress decreases the activation energy (E_a) and pre-exponential factor (k₀) consequently increasing the degradation rate of PLA. Under a compression stress of 1MPa, E_a and k₀ of pure PLA are 57.54kJ/mol and 9.74×10⁷day^-1, respectively, but 65.5kJ/mol and 9.81×10⁸day^-1 for the PLA matrix in the composite. Accelerated tests are conducted in rising immersion temperature in order to shorten the experimental time. Our analysis indicates there are well-defined relationships between the bending strength of the specimens and the PLA molecular weight during immersion, which are independent of the degradation temperature and external compression stress. Finally, a numerical model is established to elucidate the relationship of bending strength, the PLA molecular weight, activation energy, immersion time and temperature.

STATEMENT OF SIGNIFICANCE

We systematically evaluate the effects of compression stress and temperature on the degradation properties of two materials: (pure-PLA) and MAO-MAWs/PLA (or Mg/PLA). The initial in vitro degradation kinetics of the unstressed or stressed pure-PLA and MAO-MAWs/PLA composite is confirmed to be Arrhenius-like. MAO-MAWs and external compression stress would influence the degradation activation energy (E_a) and pre-exponential factor (k₀) of PLA, and we noticed there is a linear relationship between E_a and ln k₀. Thereafter, we noticed that Mg²⁺, not H⁺, plays a significant role on the mitigation of the PLA degradation and external compression stress brings the molecular structure change of PLA. Finally, we proposed a model to predict the bending strength of the specimens versus immersion time at different immersion temperatures. This fundamental study could provide some scientific basis in our understanding for the evaluations and biomedical applications of these biodegradable materials.

Proton transfer reactions for methanol and water containing manganese ion complexes

Under considerations in the current study are reactions of the type [Mn(LOH)(2)](2+) → [Mn(LO)](+) + LOH(2)(+), where the ligand LOH represents water or/and methanol. Preferential proton transfer reactions and loss of any ligand fragments are discussed in the light of ligand polarizability, dipole moment, dissociation energy, proton affinity, differences in ligand-ion ionization energy, and ion radii. The results indicate the proton affinity and dissociation energy of the O-H bond are more important for the overall proton transfer reaction than differences in the first ionization energy of the ligand and the second ionization energy of the metal ion.