Calibration of a Pt resistance thermometer (Netsushin) in magnetic fields and under pressure in the range of 1.5–300 K and below 1.5 GPa is presented. With the pressure medium, olefin olygomers, Daphne 7373, the pressure is continuous at its solidification and the pressure drop from 300 to 4.2 K by 0.15–0.17 GPa is constant, irrespective of the initial clamped pressure at 300 K. The applicability of the thermometer and the medium for precise study in field and pressure at low temperature is discussed.
Fluctuations within the ligand shell of a nanoparticle give rise to a significant degree of anisotropy in effective pair interactions for low grafting densities [B. Bozorgui, D. Meng, S. K. Kumar, C. Chakravarty, and A. Cacciuto, Nano Lett. 13, 2732 (2013)]. Here, we examine the corresponding fluctuation-driven anisotropy for gold nanocrystals densely passivated with short ligands. In particular, we consider gold nanocrystals capped by alkylthiols, both in vacuum and in ethane solvent at high density. As in the preceding study, we show that the anisotropy in the nanoparticle pair potential can be quantified by an angle-dependent correction term to the isotropic potential of mean force (PMF). We find that the anisotropy of the ligand shells is distance dependent, and strongly influenced by ligand interdigitation effects as well as expulsion of ligand chains from the interparticle region at short distances. Such fluctuation-driven anisotropy can be significant for alkylthiol-coated gold nanoparticles, specially for longer chain lengths, under good solvent conditions. The consequences of such anisotropy for self-assembly, specially as a function of grafting density, solvent quality and at interfaces, should provide some interesting insights in future work. Our results clearly show that an isotropic two-body PMF cannot adequately describe the thermodynamics and assembly behavior of nanoparticles in this dense grafting regime and inclusion of anisotropic effects, as well as possibly many-body interactions, is necessary. Extensions of this approach to other passivated nanoparticle systems and implications for self-assembly are considered.
Polymer ferroelectric films were prepared by dispersing TGS powder in polystyrene. Permittivity versus temperature plots exhibit a peak at around the transition temperature of TGS. The peak gets suppressed with increasing frequency and the trend continues up to microwave frequencies. We observed that the dielectric constant of films at microwave frequency stays close to the dielectric constant of polystyrene. The domain structure of TGS crystals essentially seems to affect their dielectric properties. Single domain films exhibit a higher spontaneous polarization and a higher coercive field.
The extent to which solvent-mediated effective interactions between nanoparticles can be predicted based on structure and associated thermodynamic estimators for bulk solvents and for solvation of single and pairs of nanoparticles is studied here. As a test of the approach, we analyse the strategy for creating temperature-independent solvent environments using a series of homologous chain fluids as solvents, as suggested by an experimental paper [M. I. Bodnarchuk et al., J. Am. Chem. Soc. 132, 11967 (2010)]. Our conclusions are based on molecular dynamics simulations of Au140(SC10H21)62 nanoparticles in n-alkane solvents, specifically hexane, octane, decane and dodecane, using the TraPPE-UA potential to model the alkanes and alkylthiols. The 140-atom gold core of the nanocrystal is held rigid in a truncated octahedral geometry and the gold-thiolate interaction is modeled using a Morse potential. The experimental observation was that the structural and rheological properties of n-alkane solvents are constant over a temperature range determined by equivalent solvent vapour pressures. We show that this is a consequence of the fact that long chain alkane liquids behave to a good approximation as simple liquids formed by packing of monomeric methyl/methylene units. Over the corresponding temperature range (233-361 K), the solvation environment is approximately constant at the single and pair nanoparticle levels under good solvent conditions. However, quantitative variations of the order of 10%-20% do exist in various quantities, such as molar volume of solute at infinite dilution, entropy of solvation, and onset distance for soft repulsions. In the opposite limit of a poor solvent, represented by vacuum in this study, the effective interactions between nanoparticles are no longer temperature-independent with attractive interactions increasing by up to 50% on decreasing the temperature from 361 K to 290 K, accompanied by an increase in emergent anisotropy due to correlation of mass dipoles on the two nanoparticles. One expects therefore that during self-assembly using solvent evaporation, temperature can be used as a structure-directing factor as long as good solvent conditions are maintained. It also suggests that disordered configurations may emerge as solvent quality decreases due to increasing role of short-range attractions and ligand fluctuation-driven anisotropy. The possibilities of using structural estimators of various thermodynamic quantities to analyse the interplay of ligand fluctuations and solvent quality in self-assembly as well as to design solvation environments are discussed.
A good understanding of the surface hydrophobicity of fluorinated materials is useful for their application as coating materials. The present study investigates the surface hydrophobicity of perfluoroalkyl acrylate (PFA) thin films using molecular dynamics simulations. Surface hydrophobicity is characterized by examining the contact angle of a water droplet on PFA surfaces and the cavity formation free energy in the vicinity of the surface. It is found that the calculated microscopic contact angles are in good agreement with the experimental results and partially capture the difference in the hydrophobicity of the surface arising from the variation of packing density and side chain length of PFA. The variations of cavity formation free energy in the vicinity of the surface elucidate that the surface hydrophobicity is mainly governed by the packing density rather than the chain length of PFA. The hydrophobicity generally increases with decreasing the packing density to some extent and then turns to decrease as further reducing the packing density. At higher packing density, the surface hydrophobicity slightly decreases with increasing the chain length, while at the lower packing density, the surface hydrophobicity is increased when chain length of PFA is longer than six carbons. Furthermore, we found that the influence of packing density on the surface hydrophobicity is directly related to the variation of the surface roughness and chain flexibility, that is, the surface hydrophobicity increases with increase in the surface roughness, while the chain flexibility plays a secondary role in the enhancement by affecting the stability of water staying near the interface. The study provides a significant insight into the local hydrophobicity and microscopic structure of the PFA surfaces, which would be useful for the application of surface modification.
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