Hydration and Hydrogen Bond Order of Octadecanoic Acid and Octadecanol Films on Water at 21 and 1 °C.

The temperature-dependent hydration structure of long-chain fatty acids and alcohols at air-water interfaces has great significance in the fundamental interactions underlying ice nucleation in the atmosphere. We present an integrated theoretical and experimental study of the temperature-dependent vibrational structure and electric field character of the immediate hydration shells of fatty alcohol and acid headgroups. We use a combination of surface-sensitive infrared reflection-absorption spectroscopy (IRRAS), surface potentiometry, and ab initio molecular dynamics simulations to elucidate detailed molecular structures of the octadecanoic acid and octadecanol (stearic acid and stearyl alcohol) headgroup hydration shells at room temperature and near freezing. In experiments, the alcohol at high surface concentration exhibits the largest surface potential; yet we observe a strengthening of the hydrogen-bonding for the solvating water molecules near freezing for both the alcohol and the fatty acid IRRAS experiments. Results reveal that the hydration shells for both compounds screen their polar headgroup dipole moments reducing the surface potential at low surface coverages; at higher surface coverage, the polar headgroups become dehydrated, which reduces the screening, correlating to higher observed surface potential values. Lowering the temperature promotes tighter chain packing and an increase in surface potential. IRRAS reveals that the intra- and intermolecular vibrational coupling mechanisms are highly sensitive to changes in temperature. We find that intramolecular coupling dominates the vibrational relaxation pathways for interfacial water determined by comparing the H2O and the HOD spectra. Using ab initio molecular dynamics (AIMD) calculations on cluster systems of propanol + 6H2O and propionic acid + 10H2O, a spectral decomposition scheme was used to correlate the OH stretching motion with the IRRAS spectral features, revealing the effects of intra- and intermolecular coupling on the spectra. Spectra calculated with AIMD reproduce the red shift and increase in intensity observed in experimental spectra corresponding to the OH stretching region of the first solvation shell. These findings suggest that intra- and intermolecular vibrational couplings strongly impact the OH stretching region at fatty acid and fatty alcohol water interfaces. Overall, results are consistent with ice templating behavior for both the fatty acid and the alcohol, yet the surface potential signature is strongest for the fatty alcohol. These findings develop a better understanding of the complex surface potential and spectral signatures involved in ice templating.

Role of Hydration in Magnesium versus Calcium Ion Pairing with Carboxylate: Solution and the Aqueous Interface.

The binding of group II metal cations such as Ca2+ and Mg2+ has been largely categorized as electrostatic or ionic using carboxylate symmetric and asymmetric stretching frequency assignments that have been historically used with little regard for the solvation environment of aqueous solutions. However, given the importance of these cations and their binding mechanisms related to biological function and in revealing surface enrichment factors for ocean to marine aerosol transfer, it is imperative that a deeper understanding be sought to include hydration effects. Here, infrared reflection-absorption and Raman spectra for surface and solution phase carboxylate binding information, respectively, are compared against bare (unbound) carboxylate and bidentate Zn2+:carboxylate spectral signatures. Spectral non-coincidence effect analysis, temperature studies, and spectral and potential of mean force calculations result in a concise interpretation of binding motifs that include the role of mediating water molecules, that is, contact and solvent-shared ion pairs. Calcium directly binds to the carboxylate group in contact ion pairs where magnesium rarely does. Moreover, we reveal the dominance of the solvent-shared ion pair of magnesium with carboxylate at the air-water interface and in solution.

Insight into the Ionizing Surface Potential Method and Aqueous Sodium Halide Surfaces.

Complementing the microscopic picture of the surface structure of electrolyte solutions set out by previous theoretical and experimental studies, the ionizing surface potential technique offers a unique approach to quantifying the impact of aqueous inorganic ions upon the interfacial electric field of the air-aqueous interface. In this Feature Article, we review the vulnerability of theoretical and empirically derived χwater values as a normative reference for aqueous ion surface potentials. Instead, we recognize and evaluate aqueous ion surface potentials relative to well-known ionic surfactants cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). Additionally, we also explore factors that impact the magnitude of the measured surface potentials using the ionizing method, particularly in the type of reference electrode and ionizing gas environment. With potential measurements of sodium halide solutions, we show that iodide has a dominant effect on the air-aqueous electric field. Compared to chloride and bromide, iodide is directly observed with a net negatively charged surface electric field at all salt concentrations measured (0.2 to 3.0 mol/kg water). Also, above the 2 M region, bromide is observed with a net negatively charged surface. Although several scenarios contribute to this effect, it is most likely due to the surface enrichment of bromide and iodide. While the results of this study are pertinent to determining the specific interfacial reactivity of aqueous halides, these anions seldom transpire as single-halide systems in the natural environment. Therefore, we also provide an outlook on future research concerning surface potential methods and more complex aqueous electrolyte systems.

Relating Structure and Ice Nucleation of Mixed Surfactant Systems Relevant to Sea Spray Aerosol.

Ice nucleating particles (INPs) influence weather and climate by their effect on cloud phase state. Fatty alcohols present within aerosol particles confer a potentially important source of ice nucleation activity to sea spray aerosol produced in oceanic regions. However, their interactions with other aerosol components and the influence on freezing were previously largely unknown. Here, we report quantitative measurements of fatty alcohols in model sea spray aerosol and examine the relationships between the composition and structure of the surfactants and subphase in the context of these measurements. Deposited mixtures of surfactants retain the ability to nucleate ice, even in fatty acid-dominant compositions. Strong refreezing effects are also observed, where previously frozen water-surfactant samples nucleate more efficiently. Structural sources of refreezing behavior are identified as either kinetically trapped film states or three-dimensional (3D) solid surfactant particles. Salt effects are especially important for surfactant INPs, where high salt concentrations suppress freezing. A simple water uptake model suggests that surfactant-containing aerosol requires either very low salt content or kinetic trapping as solid particles to act as INPs in the atmosphere. These types of INPs could be identified through comparison of different INP instrument responses.