Scattering Form Factors for Russian Doll Aerosol Droplet Models

Effects of molecular size and orientation on the interfacial properties and wetting behavior of water/n-alkane systems: a molecular-dynamics study

Molecular dynamics simulations (MD) are performed to study the interfacial structure/tension and wetting behavior of water/n-alkane systems (water/nC5 to water/nC16 where nCx = CxH(2x + 2)). In particular, we study complete-to-partial wetting transitions by changing the n-alkane chain length (N_C) at a constant temperature, T = 295 K. Simulations are carried out with a united-atom TraPPE model for n-alkanes and the TIP4P-2005 model of water. Simulation results are in excellent agreement with the initial spreading coefficients and contact angles calculated using experimental values of the surface and interfacial tensions. In addition, it has been determined that water/(nC5-nC7) and water/(nC8-nC16), respectively, exhibit complete and partial initial wetting modes. Simulations show that the interfacial structures of water/(nC5-nC7) are different from water/(nC8-nC16) systems. In the latter, water preferentially orients near the interface to increase the number of hydrogen bonds and the charge and mass densities. Moreover, the orientation of n-alkane molecules at water/(nC8-nC16) interfaces has a long-range persistence, resulting in layered structures that increase with N_C. In addition, simulation results of the orientational order parameter S_z show alignment behavior of the n-alkane molecules with respect to the interfaces. Simulations predict that the central segments of n-alkane are strongly packed in the interfaces while the end segments (methyl groups) form smaller peaks in the outer edge of the layer. This observation confirms the "horseshoe" or "C-shaped" structure of n-alkane molecules in the water/n-alkane interfaces. At constant temperature, the interface widths of both water and the n-alkanes decrease with increasing n-alkane molecular length. These results suggest that increasing the n-alkane chain length affects the water/n-alkane interfacial properties in a manner similar to that of cooling.

A reverse Monte Carlo algorithm to simulate two-dimensional small-angle scattering intensities

Small-angle scattering (SAS) experiments are a powerful method for studying self-assembly phenomena in nanoscopic materials because of the sensitivity of the technique to structures formed by interactions on the nanoscale. Numerous out-of-the-box options exist for analysing structures measured by SAS but many of these are underpinned by assumptions about the underlying interactions that are not always relevant for a given system. Here, a numerical algorithm based on reverse Monte Carlo simulations is described to model the intensity observed on a SAS detector as a function of the scattering vector. The model simulates a two-dimensional detector image, accounting for magnetic scattering, instrument resolution, particle polydispersity and particle collisions, while making no further assumptions about the underlying particle interactions. By simulating a two-dimensional image that can be potentially anisotropic, the algorithm is particularly useful for studying systems driven by anisotropic interactions. The final output of the algorithm is a relative particle distribution, allowing visualization of particle structures that form over long-range length scales (i.e. several hundred nanometres), along with an orientational distribution of magnetic moments. The effectiveness of the algorithm is shown by modelling a SAS experimental data set studying finite-length chains consisting of magnetic nanoparticles, which assembled in the presence of a strong magnetic field due to dipole interactions.

Confined phase separation of aqueous-organic nanodroplets

Nano-confined supercooled water occurs frequently in aqueous-organic aerosol nanodroplets that are ubiquitous in the atmosphere and in many industrial processes such as natural gas refining. The structure of these nanodroplets is important because it influences droplet growth and evaporation rates, nucleation rates, and radiative properties. We used classical molecular dynamics (MD) simulations to study the structures of binary water-butanol nanodroplets for several temperatures and droplet sizes. Water-butanol cross interactions are calculated using a Lennard-Jones (LJ) potential with non-bonded specific parameters adjusted to reproduce the experimentally observed mutual solubilities of water-butanol at 295 K. To compare with the results of the density functional theory (DFT) of aqueous-organic nanodroplets [Phys. Chem. Chem. Phys., 2006, 8, 1266-1270], we focus on T = 250 K. Our simulations show three different nanodroplet structures depending on the butanol concentration. For low concentrations, we observe a core-shell (CS) structure in which a butanol shell completely wets a water-rich core. For high concentrations, a well-mixed (WM) structure occurs as the water and the butanol become fully miscible. For intermediate concentrations of butanol, we find a distinct phase-separated Russian Doll-Shell (RDS) structure. This RDS structure consists of a roughly ellipsoidal water-rich droplet partially wetted by a well-mixed water/butanol convex lens (RD) and this lens-on-sphere structure is coated by a thin shell of butanol. We also examined the stability of our RDS structure at a higher temperature and found that at 295 K, the RDS structure had transformed into a well-mixed droplet, presumably due to the increase in the mutual solubility of water and butanol. Finally, we performed calculations using classical density functional theory for conditions that should favor the RDS structure. The results closely resembled those found using MD.

Small Angle X-ray Scattering for Nanoparticle Research

X-ray scattering is a structural characterization tool that has impacted diverse fields of study. It is unique in its ability to examine materials in real time and under realistic sample environments, enabling researchers to understand morphology at nanometer and angstrom length scales using complementary small and wide angle X-ray scattering (SAXS, WAXS), respectively. Herein, we focus on the use of SAXS to examine nanoscale particulate systems. We provide a theoretical foundation for X-ray scattering, considering both form factor and structure factor, as well as the use of correlation functions, which may be used to determine a particle's size, size distribution, shape, and organization into hierarchical structures. The theory is expanded upon with contemporary use cases. Both transmission and reflection (grazing incidence) geometries are addressed, as well as the combination of SAXS with other X-ray and non-X-ray characterization tools. We conclude with an examination of several key areas of research where X-ray scattering has played a pivotal role, including in situ nanoparticle synthesis, nanoparticle assembly, and operando studies of catalysts and energy storage materials. Throughout this review we highlight the unique capabilities of X-ray scattering for structural characterization of materials in their native environment.