Glass Transition Temperatures of Individual Submicrometer Atmospheric Particles: Direct Measurement via Heated Atomic Force Microscopy Probe.

The phase (solid, semisolid, or liquid) of atmospheric aerosols is central to their ability to take up water or undergo heterogeneous reactions. In recent years, the unexpected prevalence of viscous organic particles has been shown through field measurements and global atmospheric modeling. The aerosol phase has been predicted using glass transition temperatures (Tg), which were estimated based on molecular weight, oxygen:carbon ratio, and chemical formulae of organic species present in atmospheric particles via studies of bulk materials. However, at the most important sizes for cloud nucleation (∼50-500 nm), particles are complex mixtures of numerous organic species, inorganic salts, and water with substantial particle-to-particle variability. To date, direct measurements of Tg have not been feasible for individual atmospheric particles. Herein, nanothermal analysis (NanoTA), which uses a resistively heated atomic force microscopy (AFM) probe, is combined with AFM photothermal infrared (AFM-PTIR) spectroscopy to determine the Tg and composition of individual particles down to 76 nm in diameter at ambient temperature and pressure. Laboratory-generated proxies for organic aerosol (sucrose, ouabain, raffinose, and maltoheptaose) had similar Tg values to bulk Tg values measured with differential scanning calorimetry (DSC) and the Tg predictions used in atmospheric models. Laboratory-generated phase-separated particles and ambient particles were analyzed with NanoTA + AFM-PTIR showing intraparticle variation in composition and Tg. These results demonstrate the potential for NanoTA + AFM-PTIR to increase our understanding of viscosity within submicrometer atmospheric particles with complex phases, morphologies, and compositions, which will enable improved modeling of aerosol impacts on clouds and climate.

Using A Spin-Coater to Capture Adhesive Species during Polydopamine Thin-Film Fabrication.

Spin-coating was evaluated as a technique to study events that occur during polydopamine (PDA) thin-film formation. The reaction variants studied included type of oxidant, dopamine (DA) concentration, pH, adhesion time prior to spin, substrate chemistry, and notably, DA solution aging time. A strong oxidant, sodium periodate (SP), and a weak oxidant, atmospheric oxygen were chosen. It was found that reactions in solution were much faster and produced much thicker PDA films with SP than with oxygen. PDA thickness correlated positively with DA concentration, SP solution pH, and adhesion time. DA oxidation and aggregation is a dynamic process, which is reflected in the DA aging-time parameter. PDA film thickness reached a maximum value as DA solution aged. Color photography, UV-vis spectroscopy, and dynamic light scattering indicated that the optimal DA aging time for PDA adhesion is the result of the evolution of PDA particle size and chemistry over time. The capture of the optimal aging-time window was identified as the critical parameter for preparing PDA films with continuity and appreciable thickness. When these conditions were applied in a modified dip-coating method, comparable PDA films were fabricated as those obtained from spin-coating. Native silicon wafers (SiO2) as well as wafers that were modified with polydimethylsiloxane (PDMS) and amine-containing polydimethylsiloxane (PADMS) were chosen to represent a wide range of substrates with different substrate-PDA interactions. The main effect of substrate structural difference was on PDA film morphology. "Island" morphologies were obtained on PDMS where only hydrophobic interactions are responsible for PDA adhesion, while "speck" morphologies were observed on SiO2 and PADMS. The stabilities of the fabricated PDA films were tested in 0.1 M HCl and DMSO. The SP-derived PDA films exhibited very little mass loss compared to those fabricated using either the conventional dip-coating method or oxygen as an oxidant. Choosing a strong oxidant, understanding the DA reaction dynamics, and taking advantage of the optimal DA aging time are important in the fabrication of stable PDA films on a variety of substrates.