Fluctuation-driven anisotropy in effective pair interactions between nanoparticles: Thiolated gold nanoparticles in ethane

Abstract

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.

Colloidal Stability of Apolar Nanoparticles: The Role of Particle Size and Ligand Shell Structure

“…Previous simulation studies of the interaction between small gold nanoparticles in solvents were limited to the high-temperature regime where the particles repel one another. [26][27][28] We stress that the present discussion is relevant to the interaction between nanoparticles in good solvents such as decane. In vacuum, by contrast, the interaction between the particles is attractive irrespective of ligand conformation (see Fig.…”

Nonadditivity of nanoparticle interactions

“…Besides the variety of intermolecular interactions mentioned above, the latter ones can also originate from fluctuations of the ligands (38), analogous to the London dispersion force. Based on MD simulations, the London-like interactions from dynamic reconfiguration of the surface layer may contribute as much as 6k B T to the total interaction potential (113). Interparticle forces at the nanoscale can, in fact, be dominated by collective ligand alignment that organizes nearby solvent molecules (114).…”

Perspective: Outstanding theoretical questions in polymer-nanoparticle hybrids

“…In this context, studies by Bozorgui et al, 141 Lane and Grest, 110 and Chakravarty et al 151 clearly show that polymer chains do not "uniformly" cover the NP, even in the hypothetical limit where the graft points are randomly distributed on the surface (Fig. 2).…”