Heterogeneous Freezing of Carbon Nanotubes: A Model System for Pore Condensation and Freezing in the Atmosphere.

Heterogeneous ice nucleation is an important mechanism for cloud formation in the upper troposphere. Recently, pores on atmospheric particles have been proposed to play a significant role in ice nucleation. To understand how ice nucleation occurs in idealized pores, we characterized the immersion freezing activity of various sizes of carbon nanotubes. Carbon nanotubes are used both as a model for pores and proxy for soot particles. We determined that carbon nanotubes with inner diameters between 2 and 3 nm exhibit the highest ice nucleation activity. Implications for the freezing behavior of porous materials and nucleation on soot particles will be discussed.

Competitive Adsorption of Acetic Acid and Water on Kaolinite.

Mineral dust is prevalent in the atmosphere as a result of emissions from natural and anthropogenic sources. As mineral dust particles undergo long-distance transport, they are exposed to trace gases and water vapor. We have characterized the interactions of acetic acid on kaolinite using diffuse reflectance infrared Fourier transform spectroscopy and molecular modeling to determine the chemisorbed species present. After the addition of acetic acid, gas-phase water was introduced to explore how water vapor competes with acetic acid for surface sites. We found that four chemisorbed acetate species are present on kaolinite after exposure to acetic acid in which acetate bonds through a monodentate, bidenatate, or bidentate bridging linkage with an aluminum atom. These species exhibit varying levels of stability after the introduction of water, indicating that water vapor affects the adsorption of organic acids. These results indicate that the type of chemisorbed species determines its stability toward competitive adsorption, which has potential implications for atmospheric composition and ice nucleation.

Ground state 14N quadrupole couplings in the microwave spectra of N,N'-dimethylaniline and 4,4'-dimethylaminobenzonitrile.

Microwave spectra of N,N'-dimethylaniline and 4,4'-dimethylaminobenzonitrile have been recorded in a pulsed supersonic jet using chirped pulse techniques. Experimental substitution structures have been determined for both molecules by detection of the spectra of all (13)C and (15)N isotopomers in natural abundance using a broadband spectrometer. Additionally, a narrowband spectrometer has been used to reveal the (14)N quadrupole splittings at high resolution, from which the orbital occupancy numbers of the amino- and cyano-nitrogen atoms have been determined. An apparent direct relationship between these values and the barriers to inversion of the amino groups is discussed.