Quantification of Anion and Cation Uptake in Ice Ih Crystals

While ice has very low solubility for salts compared to water, small amounts of ions are doped into ice crystals. These small ion dopants can alter the fundamental physical and chemical properties of ice, such as its structure and electrical conductivity. Therefore, these results could have a direct impact on the chemical reactivity of ice and ice surfaces. Here, we examine the influence of the uptake of three salts—ammonium chloride (NH4Cl), sodium chloride (NaCl), and ammonium sulfate [(NH4)2SO4]—on ice Ih formation using capillary electrophoresis. Using both cation and anion modes, we observed and quantified the uptake of individual ions into the ice. Our results indicate that anions have a higher propensity for uptake into ice Ih crystals

Determining the Surface p\u3cem\u3eK\u3c/em\u3e\u3csub\u3ea\u3c/sub\u3e of Perfluorooctanoic Acid

Perfluorooctanoic acid (PFOA) is an environmentally prevalent and persistent organic pollutant with toxic and bioaccumulative properties. Despite the known importance of perfluorinated pollutants in the global environment, molecular-level details of the physicochemical behavior of PFOA on aqueous interfaces remain poorly understood. Here, we utilized two surface-specific techniques, vibrational sum frequency generation spectroscopy (SFG) and surface tensiometry, to investigate the pH-induced structural changes of PFOA and octanoic acid (OA) and determined the apparent pKa at the air–water surface. The SFG spectra and surface activity model were investigated over a wide range of pHs. With the surface tension measurements, the surface pKa values for OA and PFOA are determined to be 3.8 ± 0.1 and 2.2 ± 0.2, respectively. These results could provide insights into improved remediation of PFOAs and may impact climate modeling of perfluorinated alkyl chain molecules

Surface-charge-induced orientation of interfacial water suppresses heterogeneous ice nucleation on α-alumina (0001)

Surface charge is one of the surface properties of atmospheric aerosols, which has been linked to heterogeneous ice nucleation and hence cloud formation, microphysics, and optical properties. Despite the importance of surface charge for ice nucleation, many questions remain on the molecular-level mechanisms at work. Here, we combine droplet-freezing assay studies with vibrational sum frequency generation (SFG) spectroscopy to correlate interfacial water structure to surface nucleation strength. We study immersion freezing of aqueous solutions of various pHs on the atmospherically relevant aluminum oxide α-Al2O3 (0001) surface using an isolated droplet on the surface. The high-pH solutions freeze at temperatures higher than that of the low-pH solution, while the neutral pH has the highest freezing temperature. On the molecular level, the SFG spectrum of the interfacial water changes substantially upon freezing. At all pHs, crystallization leads to a reduction of intensity of the 3400 cm−1 water resonance, while the 3200 cm−1 intensity drops for low pH but increases for neutral and high pHs. We find that charge-induced surface templating suppresses nucleation, irrespective of the sign of the surface charge. Heterogeneous nucleation is most efficient for the nominally neutral surface

Exploring nanoaggregate structures of model asphaltenes using two dimensional infrared spectroscopy

Includes bibliographical references.2015 Summer.Asphaltenes have been an enigma in the scientific community; studies on the molecular masses have differed by orders of magnitude and structures have been debated between island or archipelago structures. Thus, the asphaltene community defines asphaltenes by their solubility. Asphaltenes are n-heptane-insoluble and toluene-soluble. The known nanoaggregation of asphaltenes at different timescales and concentrations causes issues to determine the molecular weight and structure of asphaltene molecules. This thesis is the first step to using two dimensional infrared (2D IR) spectroscopy to study the nanoaggregate structure of model asphaltenes. 2D IR spectroscopy is a vibrational spectroscopy that is advantageous over linear IR absorption due to the ability to spread the spectral information over two axes. The 2D IR spectra give rise to structurally sensitive cross-peaks, affording the ability to probe the structure of the nanoaggregates. The model asphaltenes used in this work are violanthrone-79 and lumogen orange, a perylene derivative. These model asphaltenes consist mostly of polycyclic aromatic hydrocarbons (PAHs), similar to asphaltenes. Violanthrone-79 and lumogen orange also have carbonyl functional groups, which provide vibrational probes. The carbonyl stretching and ring breathing vibrations are used to probe the stacked structure of the nanoaggregates. A quinone series of one, two and three ring systems was used to first study the coupling between the carbonyl stretching and ring breathing vibrational modes. The quinone series provided the foundation for the larger ring systems that emulate asphaltenes. The data from studying the stacked structure of nanoaggregate model asphaltenes can be used to reveal properties of nanoaggregate asphaltenes. This work will allow for continued study of the kinetics of nanoaggregation using 2D IR waiting time experiments for dynamic information. Thus, this work demonstrates the use of 2D IR spectroscopy, which offers femtosecond time resolution, as a viable technique for studying nanoaggregation

Molecular hydrophobicity at a macroscopically hydrophilic surface

International audienceInterfaces between water and silicates are ubiquitous and relevant for, among others, geochemistry, atmospheric chemistry, and chromatography. The molecular-level details of water organization at silica surfaces are important for a fundamental understanding of this interface. While silica is hydrophilic, weakly hydrogen-bonded OH groups have been identified at the surface of silica, characterized by a high O-H stretch vibrational frequency. Here, through a combination of experimental and theoretical surface-selective vibrational spectroscopy, we demonstrate that these OH groups originate from very weakly hydrogen-bonded water molecules at the nominally hydrophilic silica interface. The properties of these OH groups are very similar to those typically observed at hydrophobic surfaces. Molecular dynamics simulations illustrate that these weakly hydrogen-bonded water OH groups are pointing with their hydrogen atom toward local hydrophobic sites consisting of oxygen bridges of the silica. An increased density of these molecular hydrophobic sites, evident from an increase in weakly hydrogen-bonded water OH groups, correlates with an increased macroscopic contact angle