Surface pKa of octanoic, nonanoic, and decanoic fatty acids at the air-water interface: applications to atmospheric aerosol chemistry.

There exists large uncertainty in the literature as to the pKa of medium-chain fatty acids at the air-water interface. Via surface tension titration, the surface-pKa values of octanoic (C8), nonanoic (C9), and decanoic (C10) fatty acids are determined to be 4.9, 5.8, and 6.4, respectively. The surface-pKa determined with surface tension differs from the bulk value obtained during a standard acid-base titration. Near the surface-pKa of the C8 and C9 systems, surface tension minima are observed and are attributed to the formation of surface-active acid-soap complexes. The direction of the titration is shown to affect the surface-pKa of the C9 system, as the value shifts to 5.2 with NaOH titrant due to a higher concentration of Na+ ions at pH values close to the surface-pKa. As the reactivity and climate-relevant properties of sea spray aerosols (SSA) are partially dictated by the charge and surface activity of the organics at the aerosol-atmosphere interface, the results presented here on SSA-identified C8-C10 fatty acids can be used to better predict the health and climate impact of particles with significant concentrations of medium-chain fatty acids.

Sodium-carboxylate contact ion pair formation induces stabilization of palmitic acid monolayers at high pH.

Sea spray aerosols (SSA) are known to have an organic coating that is mainly composed of fatty acids. In this study, the effect of pH and salt on the stability and organization of a palmitic acid (PA) monolayer is investigated by surface vibrational spectroscopy and molecular dynamics simulations. Results indicate that alkyl chain packing becomes more disordered as the carboxylic headgroup becomes deprotonated. This is associated with packing mismatch of charged and neutral species as charged headgroups penetrate deeper into the solution phase. At pH 10.7, when the monolayer is ∼99% deprotonated, palmitate (PA-) molecules desorb and solubilize into the bulk solution where there is spectroscopic evidence for aggregate formation. Yet, addition of 100 mM NaCl to the bulk solution is found to drive PA- molecules to the aqueous surface. Free energy calculations show that PA- molecules become stabilized within the interface with increasing NaCl concentration. Formation of contact -COO-:Na+ pairs alters the hydration state of PA- headgroups, thus increasing the surface propensity. As salts are highly concentrated in SSA, these results suggest that deprotonated fatty acids may be found at the air-aqueous interface of aerosol particles due to sea salt's role in surface stabilization.

Using fixed-node diffusion Monte Carlo to investigate the effects of rotation-vibration coupling in highly fluxional asymmetric top molecules: application to H2D+.

A fixed-node diffusion Monte Carlo approach for obtaining the energies and wave functions of the rotationally excited states of asymmetric top molecules that undergo large amplitude, zero-point vibrational motions is reported. The nodal surfaces required to introduce rotational excitation into the diffusion Monte Carlo calculations are obtained from the roots of the asymmetric top rigid rotor wave functions calculated using the system's zero-point, vibrationally averaged rotational constants. Using H(2)D(+) as a model system, the overall accuracy of the methodology is tested by comparing to the results of converged variational calculations. The ability of the fixed-node diffusion Monte Carlo approach to provide insights into the nature and strength of the rotation-vibration coupling present in the rotationally excited states of highly fluxional asymmetric tops is discussed. Finally, the sensitivity of the methodology to the details of its implementation, such as the choice of embedding scheme, is explored.

Unraveling rotation-vibration mixing in highly fluxional molecules using diffusion Monte Carlo: applications to H3+ and H3O+.

A thorough examination of the use of fixed-node diffusion Monte Carlo for the study of rotation-vibration mixing in systems that undergo large amplitude vibrational motions is reported. Using H(3)(+) as a model system, the overall accuracy of the method is tested by comparing the results of these calculations with those from converged variational calculations. The effects of the presence of a large amplitude inversion mode on rotation-vibration mixing are considered by comparing the H(3)(+) results with those for H(3)O(+). Finally, analysis of the results of the fixed-node diffusion Monte Carlo calculations performed in different nodal regions is found to provide clear indications of when some of the methodology's underlying assumptions are breaking down as well as provide physical insights into the form of the rotation-vibration coupling that is most likely responsible.