(197bj) Relationship between Nanoscale Structure and Affinity for Organic-Modified Inorganic Solid/Organic Solvent Interface

Nanomaterials including inorganic nanoparticles in liquids or polymers have been actively researched as highly-functional materials. For example, nanofluids are expected to be applied to coolant and conductive nano-ink, and polymer nanocomposites are expected to be applied to automobile parts, thermal interface materials, and optical thin films. To control the properties of these materials, it is important to control the dispersion state and spatial distribution of nanoparticles in host materials. Surface modification of nanoparticles with organic molecules is one of the effective technologies for controlling an affinity between nanoparticles and dispersed media, i.e., the dispersibility of nanoparticles. However, a design guideline for suitable surface modification is not well established because it is difficult to directly measure the affinity between surface-modified nanoparticles and dispersed media through experiments with high accuracy. In this study, molecular dynamics (MD) simulations were conducted for the interface between organic-modified inorganic solid and organic solvent to evaluate the interfacial structure and affinity.

All-atom MD simulations were performed for organic-modified inorganic solid substrate/organic solvent interfaces to simulate a part of a nanoparticle surface. The inorganic solid was α-Al₂O₃ (0001), the modifier was carboxylic acid with different chain lengths, and the solvent was n-alkane, also with different chain lengths. The surface coverage of modifier was 100% (4.8 molecules/nm²) and 50%. The simulation systems were equilibrated at 300 K and 1 MPa. Work of adhesion was selected as a measure of interfacial affinity, and calculated by using the phantom-wall method, which is a kind of thermodynamic integration method. All simulations were conducted by using LAMMPS.

The effect of modifier chain length and solvent chain length on the work of adhesion was investigated. For the surface coverage of 100%, the work of adhesion remained almost unchanged because the solvent did not penetrate the modifier layer. For the surface coverage of 50%, the work of adhesion was maximum in the case of decanoic acid modifier. The longer the solvent chain length was, the larger the work of adhesion was. To evaluate the degree of solvent penetration to the modifier layer, the overlap parameter was calculated by using the density profile at the interface. The work of adhesion for various carboxylic acid-modified Al₂O₃/n-alkane systems was mostly correlated with the overlap parameter. Therefore, a high interfacial affinity can be achieved by appropriately selecting the modifier and the surface coverage to increase solvent penetration.