Orientational Behavior and Vibrational Response of Glycine at Aqueous Interfaces

Self-Assembled Oligomers Facilitate Amino Acid-Driven CO2 Capture at the Air-Aqueous Interface

Direct air capture of CO2 using amino acid absorbents, such as glycine or sarcosine, is constrained by the relatively slow mass transfer of CO2 through the air-aqueous interface. Our recent study showed a marked improvement in CO2 capture by introducing CO2-permeable oligo-dimethylsiloxane (ODMS-MIM+) oligomers with cationic (imidazolium, MIM+) headgroups. In this work, we have employed all-atom molecular dynamics simulations in combination with subensemble analysis using network theory to provide a detailed molecular picture of the behavior of CO2 and the glycinate anions (Gly-) at the ODMS-MIM+ decorated air-aqueous interfaces. We show that the cationic head groups of the surfactants enhance the concentration and lifetime of Gly- in the interfacial region, while ODMS tails promote the physisorption of CO2 in the interfacial region. Together, these two factors increase the effective region of contact and the probability of interactions between CO2 and Gly- compared to that of the pure air-aqueous interface. The fundamental insights gained in this work establish essential foundations for developing hybrid systems with oligomer-decorated interfaces to maximize the overall CO2 capture rates.

Observation of Electrostatically Driven Surface Adsorption in Mixed Surfactant Systems

We employed heterodyne-detected vibrational sum-frequency generation (HD-VSFG) spectroscopy to obtain a molecular-level understanding of the interaction between the anionic surfactant sodium dodecyl ammonium sulfate (SDS) and the cationic surfactant dodecyltrimethylammonium bromide (DTAB). We observed that these surfactants show a strong cooperative effect on their adsorption to the water-air interface. Even at bulk concentrations 1000 times lower than the critical micelle concentrations of SDS and DTAB, a nearly complete surface surfactant layer is observed when both surfactants are present. This strong enhancement of the surface concentrations of DS- and DTA+ can be quantitatively explained from the favorable Coulomb interaction of the oppositely charged headgroups of DS- and DTA+ and the electrostatic interactions with their counterions. The HD-VSFG results are complemented by a modified Langmuir adsorption model in which we include the free energy associated with the electrostatic interactions of the surfactant ions and their counterions.