Spontaneous emulsification induced by nanoparticle surfactants.

Microemulsions, mixtures of oil, water, and surfactant, are thermodynamically stable. Unlike conventional emulsions, microemulsions form spontaneously, have a monodisperse droplet size that can be controlled by adjusting the surfactant concentration, and do not degrade with time. To make microemulsions, a judicious choice of surfactant molecules must be made, which significantly limits their potential use. Nanoparticle surfactants, on the other hand, are a promising alternative because the surface chemistry needed to make them bind to a liquid-liquid interface is both well flexible and understood. Here, we derive a thermodynamic model predicting the conditions in which nanoparticle surfactants drive spontaneous emulsification that agrees quantitatively with experiments using Noria nanoparticles. This new class of microemulsions inherits the mechanical, chemical, and optical properties of the nanoparticles used to form them, leading to novel applications.

Competitive reaction pathways for functionalization and volatilization in the heterogeneous oxidation of coronene thin films by hydroxyl radicals and ozone.

X-ray photoelectron spectroscopy (XPS) is used to monitor the heterogeneous reaction of hydroxyl radicals (OH) and ozone with thin films (∼5 Å) of coronene. Detailed elemental and functional group analysis of the XPS spectra reveals that there is a competition between the addition of oxygenated functional groups (functionalization) and the loss of material (volatilization) to the gas phase. Measurements of the film thickness and elemental composition indicate that carbon loss is as important as the formation of new oxygenated functional groups in controlling how the oxygen-to-carbon ratio (O/C) of the coronene film evolves during the surface reaction. When the O/C ratio of the film is small (∼0.1) the addition of functional groups dominates changes in film thickness, while for more oxygenated films (O/C > 0.3) carbon loss is an increasingly important reaction pathway. Decomposition of the film occurs via the loss of both carbon and oxygen atoms when the O/C ratio of the film exceeds 0.5. These results imply that chemically reduced hydrocarbons, such as primary organic aerosol, age in the atmosphere by forming new oxygenated functional groups, in contrast to oxygenated secondary organic aerosol, which decompose by a heterogeneous loss of carbon and/or oxygen.

Mechanistic insight into biopolymer induced iron oxide mineralization through quantification of molecular bonding.

Microbial production of iron (oxyhydr)oxides on polysaccharide rich biopolymers occurs on such a vast scale that it impacts the global iron cycle and has been responsible for major biogeochemical events. Yet the physiochemical controls these biopolymers exert on iron (oxyhydr)oxide formation are poorly understood. Here we used dynamic force spectroscopy to directly probe binding between complex, model and natural microbial polysaccharides and common iron (oxyhydr)oxides. Applying nucleation theory to our results demonstrates that if there is a strong attractive interaction between biopolymers and iron (oxyhydr)oxides, the biopolymers decrease the nucleation barriers, thus promoting mineral nucleation. These results are also supported by nucleation studies and density functional theory. Spectroscopic and thermogravimetric data provide insight into the subsequent growth dynamics and show that the degree and strength of water association with the polymers can explain the influence on iron (oxyhydr)oxide transformation rates. Combined, our results provide a mechanistic basis for understanding how polymer-mineral-water interactions alter iron (oxyhydr)oxides nucleation and growth dynamics and pave the way for an improved understanding of the consequences of polymer induced mineralization in natural systems.

Conservation of mass-specific metabolic rate among high- and low-elevation populations of the acridid grasshopper Xanthippus corallipes.

High-elevation populations of many grasshopper species produce small adults in response to shortened growing seasons and cooler ambient temperatures. Mass-specific metabolic rate tends to increase with elevation, and several authors have argued that this is an adaptation to accelerate development. In the present study, the relationship of thermoregulation and metabolism was investigated in adults of the acridid grasshopper Xanthippus corallipes from six populations along an elevation gradient. Thermoregulation was measured in the field, and several lines of evidence suggested that afternoon body temperatures were actively maintained within each population. Populations were found to maintain stable afternoon body temperatures that correlate negatively with elevation. Elevation had a strong negative effect on adult mass. Mass-specific metabolic rates at 35 degrees and 45 degrees C correlated positively with elevation. However, population differences in mass explained most of the variation in mass-specific metabolic rates, and when mass was used as a covariate, the effect of elevation disappeared. Mass-specific metabolic rates at afternoon field body temperatures were estimated and found not to differ among populations. Thus, differences in thermoregulation offset the effect of mass on mass-specific metabolic rate across populations, such that X. corallipes adults exhibited a common mass-specific metabolic rate in the wild, independent of large population differences in mass and ambient temperatures.