Predicting the influence of particle size on the glass transition temperature and viscosity of secondary organic material

Glassy relaxation in a de Vries smectic liquid crystal consisting of bent-core molecules

We report experimental investigations of a liquid crystal comprising thiophene-based achiral bent-core banana shaped molecules. The compound exhibits the following phase sequence on cooling: Isotropic (517.4 K), N (514.9 K), de Vries SmA (402 K), SmC. Practically no layer contraction was observed across the SmA to SmC transition, confirming the "de Vries" nature of the SmA phase. Interestingly, the crystallization does not occur on cooling the sample, unlike most other liquid crystals. Instead, the SmC phase undergoes a glass transition at 271 K even at a slow cooling rate. The dielectric spectroscopy studies carried out on the sample reveal the presence of a dielectric mode whose relaxation process is of the Cole-Cole type. The relaxation frequency of the mode was found to drop rapidly with decreasing temperature, confirming the glassy behavior. The variation of relaxation frequency with temperature follows the Vogel-Fulcher-Tammann equation, indicating the fragile glassy nature of the sample. This report identifies a bent-core liquid crystal exhibiting a "de Vries" SmA phase and glassy behavior at lower temperatures.

Toward a molecular understanding of the surface composition of atmospherically relevant organic particles

Many mass spectrometry methods using various ionization sources provide bulk composition of airborne particles, but little is known about the surface species that play a major role in determining their physicochemical properties that impact air quality, climate, and health. The present work shows that the composition of surface layers of atmospherically relevant submicron organic particles can be probed without the use of an external ionization source. Solid dicarboxylic acid particles are used as models, with glutaric acid being the most efficient at generating ions. Coating with small diacids or products from α-pinene ozonolysis demonstrates that ions are ejected from the surface, providing surface molecular characterization of organic particles on the fly. This unique approach provides a path forward for elucidating the role of the surface in determining chemical and physical properties of particles, including heterogeneous reactions, particle growth, water uptake, and interactions with biological systems.

The Size and Shape Effects on the Melting Point of Nanoparticles Based on the Lennard-Jones Potential Function

A model is proposed to calculate the melting points of nanoparticles based on the Lennard-Jones (L-J) potential function. The effects of the size, the shape, and the atomic volume and surface packing of the nanoparticles are considered in the model. The model, based on the L-J potential function for spherical nanoparticles, agrees with the experimental values of gold (Au) and lead (Pb) nanoparticles. The model, based on the L-J potential function, is consistent with Qi and Wang’s model that predicts the Gibbs-Thompson relation. Moreover, the model based on the non-integer L-J potential function can be used to predict the melting points Tm of nanoparticles.

Spectroscopically clean Au nanoparticles for catalytic decomposition of hydrogen peroxide

Au nanoparticles synthesized from colloidal techniques have the capability in many applications such as catalysis and sensing. Au nanoparticles function as both catalyst and highly sensitive SERS probe can be employed for sustainable and green catalytic process. However, capping ligands that are necessary to stabilize nanoparticles during synthesis are negative for catalytic activity. In this work, a simple effective mild thermal treatment to remove capping ligands meanwhile preserving the high SERS sensitivity of Au nanoparticles is reported. We show that under the optimal treatment conditions (250 °C for 2 h), 50 nm Au nanoparticles surfaces are free from any capping molecules. The catalytic activity of treated Au nanoparticles is studied through H₂O₂ decomposition, which proves that the treatment is favorable for catalytic performance improvement. A reaction intermediate during H₂O₂ decomposition is observed and identified.

Particle Size Distribution Dynamics Can Help Constrain the Phase State of Secondary Organic Aerosol

Particle phase state is a property of atmospheric aerosols that has important implications for the formation, evolution, and gas/particle partitioning of secondary organic aerosol (SOA). In this work, we use a size-resolved chemistry and microphysics model (Statistical Oxidation Model coupled to the TwO Moment Aerosol Sectional (SOM-TOMAS)), updated to include an explicit treatment of particle phase state, to constrain the bulk diffusion coefficient (D_b) of SOA produced from α-pinene ozonolysis. By leveraging data from laboratory experiments performed in the absence of a seed and under dry conditions, we find that the D_b for SOA can be constrained ((1-7) × 10^-15 cm² s^-1 in these experiments) by simultaneously reproducing the time-varying SOA mass concentrations and the evolution of the particle size distribution. Another version of our model that used the predicted SOA composition to calculate the glass-transition temperature, viscosity, and, ultimately, D_b (∼10^-15 cm² s^-1) of the SOA was able to reproduce the mass and size distribution measurements when we included oligomer formation (oligomers accounted for about a fifth of the SOA mass). Our work highlights the potential of a size-resolved SOA model to constrain the particle phase state of SOA using historical measurements of the evolution of the particle size distribution.