Effect of Coadsorption of Electrolyte Ions on the Stability of Inner-Sphere Complexes

Coadsorption of electrolyte ions can have a marked influence on the adsorption and hence interfacial reactivity of multicharged ions but has generally been overlooked in previous density functional theory (DFT) studies. The impact of this effect is demonstrated for the DFT-based interpretation of in situ FTIR spectra of the controversial binding form of weakly adsorbed carbonate on hydrated hematite nanoparticles and ferrihydrite. Using a new methodology, we show that addition of hydronium or sodium ions in the DFT models leads to assigning the weakly adsorbed carbonate to an inner-sphere monodentate mononuclear complex. FTIR data and the “bond length−vibrational frequency” correlations established by DFT suggest that the adsorption affinity of ferrihydrite toward carbonate is lower than that of 7 nm hematite NPs. As opposed to the case of carbonate, the outer-sphere adsorption form of sulfate is demonstrated to be stable in the presence of hydronium. These findings have important implications for DFT modeling of the solid−water interfaces, which has become a major complementary tool to interpret spectroscopic and macroscopic observations of adsorption phenomena

Adsorption of Surfactants on Two Different Hydrates

The interaction between surfactants and hydrates provides insight into the role of surfactants in promoting hydrate formation. This work aims at understanding the adsorption behavior of sodium dodecyl sulfate (SDS) on cyclopentane (CP) hydrates and its derivative surfactant on tetrabutylammonium bromide (TBAB) hydrates. Cyclopentane (CP) is a hydrophobic former whereas tetrabutylammonium bromide (TBAB) is a salt that forms semiclathrate hydrates. The adsorption on these two hydrates was studied by zeta potential and pyrene fluorescence measurements. CP hydrates have a negative surface charge in the absence of SDS, and it decreases to a minimum as the SDS concentration increases from 0 to 0.17 mM. Then, it increases with further increased SDS concentration. The adsorption density of DS− on CP hydrates reaches a saturated value at 1.73 mM SDS. The micropolarity parameter of the TBAB hydrate/water interface starts to increase rapidly at 0.17 mM SDS and levels off at 1.73 mM SDS. The presence of Br− in TBAB hydrate suspensions could compete with TBADS (from association of DS− and TBA+) and DS− for the adsorption on the hydrate surface, but they have a much stronger affinity for the hydrates than does Br−. From the fluorescence measurements, it was found that the micropolarity of the hydrate/water interface is mainly dependent on the polarity of hydrate formers

Chemo-enzymatic Routes to Lipopeptides and Their Colloidal Properties

A unique chemo-enzymatic route to lipopeptides was demonstrated herein that, relative to alternative methods such as solid-phase peptide synthesis (SPPS) and microbial synthesis, is simple, efficient, and scalable. Homo- and co-oligopeptides were synthesized from amino acid ethyl esters via protease catalysis in an aqueous media, followed by chemical coupling to fatty acids to generate a library of lipopeptides. Synthesized lipopeptides were built from hydrophobic moieties with chain lengths ranging from 8 to 18 and peptides consisting of oligo­(l-Glu) or oligo­(l-Glu-<i>co</i>-l-Leu) with an average of seven to eight repeating units. The chemical structures of the lipopeptides were characterized and confirmed by NMR and matrix-assisted laser desorption/ionization (MALDI). The colloidal and interfacial properties of these lipopeptides were characterized and compared in terms of the hydrophobic chain length, oligopeptide composition, and solution pH. The results showed correlation between the interfacial activity of the lipopeptides and the hydrophobicity of the fatty acid and oligopeptide headgroup, the effects of which have been semiquantitatively described in the manuscript. Results from these studies provide insights into design principles that can be further expanded in future work to access lipopeptides from protease-catalysis with improved control over sequence and exploring a wider range of peptide and lipid compositions to further tune lipopeptide biochemical and physical properties