Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air–water interface

Abstract

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air–water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca2+ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca2+·RCO2−]+·(H2O)n=0to12 and RCO2−·(H2O)n=0to14 cluster ions toward the features observed at the air–water interface. Surprisingly, not only does stepwise hydration of the RCO2− anion and the [Ca2+·RCO2−]+ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca2+·RCO2−]+·(H2O)n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation or deprotonation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C–C bond that connects the head group to the hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.

Interactions of Na+ Cations with a Highly Charged Fatty Acid Langmuir Monolayer: Molecular Description of the Phase Transition

“…In this regard, AIMD simulations could provide a useful insight into possible contact ion pairing configurations. [72][73] When compared with theoretical models, the experimental results are only consistent with the Gouy Chapman predictions at low charge densities (surface charge > ~-0.1 C/m 2 ). At higher surface charges, the experimental data is in good agreement with an MPB model that accounts for the finite size of the counterion.…”

Binding of Divalent Cations to Aqueous Acetate: Molecular Simulations Guided by Raman Spectroscopy

“…[61][62][63][64] In a first simulation (S1), the cation in pure water is progressively decoupled from its environment (ΔG1); in a second simulation (S2), the cation in a specific ion pair geometry with acetate is progressively decoupled from its environment (ΔG2). This was done first turning off the electrostatic interactions between the cation and its environment, before turning off the van der Waals interactions, using soft-core potentials to improve convergence, for a total of 24 (Table S2). The free-energy contribution of this restraint needs to be carefully accounted for both in the coupled and the uncoupled state.…”

“…It is, therefore, crucial to properly benchmark the interaction parameters against experimental data. 11,12 Prior metal carboxylate binding constant compilations 13 and individual studies include experimental potentiometric, [14][15][16][17][18] NMR, 19 vibrational spectroscopic, [20][21][22] and surface sum frequency 23,24 measurements, as well as theoretical classical 25 and quantum, 26 calculations. The reported acetate binding constants for a given cation often vary by up to an order of magnitude, and are not always in agreement regarding the relative binding strengths of the above three cations (although studies that have included all three cations generally agree that Zn 2+ binds more strongly to Ac − than either Mg 2+ or Ca 2+ ).…”

Binding of Divalent Cations to Aqueous Acetate: Molecular Simulations Guided by Raman Spectroscopy

“…Prior metal carboxylate binding constant compilations 12 and individual studies include experimental potentiometric, [13][14][15][16][17] NMR, 18 vibrational spectroscopic, [19][20][21] and surface sum frequency 22,23 measurements, as well as theoretical classical 24 and quantum, 25 calculations. In the case of acetate, the range of previously reported binding constants consistently point to weak pairing with divalent cations.…”