Masaki Kubo

Controlling the dispersion stability of surface-modified nanoparticles in suspensions is essential for their successful industrial application. However, the structures and interactions at the interface between the surface-modified nanoparticles and organic solvents remain poorly understood. In this study, the effects of the surface coverage and solvent species on the interfacial structure and interactions between the dodecylamine-modified Ag slab and organic solvents were investigated using all-atom molecular dynamics simulations. Toluene and methanol were selected as typical nonpolar and polar solvents, respectively. At higher surface coverages, the ligand exhibited more ordered and oriented structures. In toluene (a good solvent for dodecylamine-modified Ag), more ligands extended toward the solvent phase and more solvent molecules penetrated the ligand layer compared to methanol (a poor solvent for dodecylamine-modified Ag). As a result, the interaction between the solvent and the alkyl chain of the ligand for toluene was stronger than that for methanol. The degree of penetration and the interaction in both solvent systems varied with the surface coverage, showing a maximum in penetration and a minimum in interaction energy at 50% coverage. These findings provide insights into the design of ligands to improve the dispersion stability of surface-modified nanoparticles in organic solvents.

The industrial application of surface-modified nanoparticles requires controlling their colloidal stability by selecting suitable surface modifiers and solvents. Solvent mixtures can control the interaction between the nanoparticles and solvents, but the detailed mechanism is still unclear. In this study, molecular dynamics simulations were performed for an interface between a decanoic acid-modified Al2O3 and a binary mixture of organic solvents. Cyclohexane and ethanol were selected as a good solvent and a poor solvent for the modifier, respectively, and their volume fraction for preparation was varied. For any volume fraction of ethanol, ethanol was adsorbed onto the Al2O3 surface patches without modifiers. Cyclohexane was preferentially distributed around the modifier. The work of adhesion was calculated as a measure of interfacial affinity using thermodynamic integration. For systems with ethanol, the work of adhesion was significantly larger than for those with cyclohexane, owing to the strong interaction of the Al2O3 surface with the adsorbed ethanol; however, this result, that is, the obtained affinity, is not consistent with the experimental dispersion results of surface-modified nanoparticles. To focus on the interaction between modifiers and surrounding solvents, the work required to strip solvents from the modifier was calculated, excluding the contribution of adsorbed ethanol. The required work decreased after the ethanol fraction surpassed a specific threshold, indicating that the affinity can be related to the distribution of solvent species near the modifier layer. This work provides an evaluation method and insights into the effect of a solvent mixture on the interfacial affinity at the molecular level.

Wetting of nanofluids containing highly dispersed nanoparticles of single-nanometer size was investigated, as these nanoparticles can persist within a nanometer-scale liquid film near contact line, potentially causing significant changes in wetting characteristics. We discerned distinctive superspreading wetting, featured by temporal indices (0.29 to 0.46) in the relationship between contact radius and time. We employed a phase-shifting imaging ellipsometer to measure droplet shape, including the nanometer-scale liquid film and nanoparticle layer after drying. The liquid film shapes differed from pure liquids at micrometer-scale but not at nanometer-scale. Furthermore, surface tension measurements and substrate surface energy control contributed to unraveling these characteristics. These findings differentiated the observed superspreading wetting from the mechanisms proposed in existing studies of aqueous surfactant solutions.

Controlling the dispersion state of nanoparticles in a polymer matrix is necessary to produce polymer nanocomposites. The surface modification of nanoparticles is used to enable their dispersion in polymers. Moreover, molecular dynamics (MD) simulations are useful for revealing the interfacial properties between nanoparticles and polymers to aid in the design of materials. In this study, the effect of surface coverage, modifier length, and polymer species on the interfacial structure and affinity between surface-modified Al2O3 and polymer melts were investigated using all-atom MD simulations. Hexanoic, decanoic, and tetradecanoic acids were used as surface modifiers, and polypropylene (PP), polystyrene (PS), and poly (methyl methacrylate) (PMMA) were used as polymers. The work of adhesion Wadh and the work of immersion Wimm were selected as quantitative measures of affinity. Wadh was calculated using the phantom-wall approach, and Wimm was calculated by simply subtracting the surface tension of polymers γL from Wadh. The results showed that Wadh and Wimm were improved by surface modification with low coverage, owing to a good penetration of the polymer. The effect of modifier length on Wadh and Wimm was small. Whereas Wadh increased in the following order: PP < PS < PMMA, Wimm increased as follows: PMMA < PS < PP. Finally, the trend of Wadh and Wimm was organized using the Flory–Huggins interaction parameter χ between the modifier and the polymer. This study demonstrates that the interfacial affinity can be improved by tuning the surface coverage and modifier species depending on the polymer matrix.