The hydrophobicity of monolayer-protected gold nanoparticles (GNPs) is not a uniform property but rather a spatially heterogeneous phenomenon driven by complex interfacial dynamics. This study reveals that the organization of ligands on the nanoparticle surface—particularly their tendency to form bundles—plays a critical role in shaping local hydrophobicity, especially for small GNPs (<10 nm). While traditional models rely on end group chemistry or log P values to predict surface behavior, these approaches fail to capture the emergent effects arising from nanoscale curvature and cooperative ligand-ligand interactions. Using atomistic molecular dynamics simulations, we calculated local hydration free energies (L) across the surfaces of 2 nm and 6 nm diameter GNPs coated with saturated alkanethiol ligands terminating in various functional groups (CH₃, OH, COOH, CONH₂). The results show that even with chemically homogeneous monolayers, significant spatial variations in L exist due to anisotropic bundling of long alkyl chains. In 2 nm GNPs, ligands adopt a bundled configuration where methyl-rich regions at the poles exhibit low L values (high hydrophobicity), while the annular zones between poles display higher L values due to sulfur exposure and surface curvature-induced water structuring. This bundling effect is suppressed in planar SAMs, which lack curvature and allow more uniform ligand alignment. However, when the same ligands are placed on curved surfaces, the geometric constraints promote packing into directional clusters, leading to distinct hydrophobic domains. As core size increases from 2 nm to 6 nm, the degree of bundling decreases slightly, resulting in more homogeneous hydration free energy distributions. Nevertheless, all small GNPs retain some level of spatial heterogeneity, indicating that curvature itself acts as a structural driver of nonuniformity. Further investigation revealed that introducing structural disorder—via unsaturated bonds or branched methylene groups—disrupts bundle formation. These modified ligands yield more spherical, less ordered monolayers with uniformly distributed hydrophilic regions, reducing both spatial heterogeneity and overall hydrophobicity. The number of hydrophilic clusters, defined as regions with L > bulk water value (11.25 kT), increased significantly for branched and unsaturated ligands compared to their saturated counterparts.
To link these findings to real-world functionality, we simulated competitive binding between propane (hydrophobic probe) and water molecules at the interface. Occupancy maps showed that propane preferentially binds to low-L regions—corresponding to exposed methylene backbones—confirming that hydration free energy directly predicts molecular affinity. For GNPs with disordered ligands, propane occupancy decreased substantially, consistent with the loss of large hydrophobic patches.
These results underscore that the hydrophobicity of small GNPs cannot be predicted solely from end group properties.SYP Antibody Data Sheet Instead, it emerges from a dynamic interplay between ligand conformation, surface curvature, and collective self-organization.CD66A Antibody Protocol The presence of bundles creates microenvironments with vastly different wetting behaviors, which can influence protein adsorption, membrane insertion, and aggregation kinetics.PMID:35207308 Therefore, rational design of functional GNPs must account for these emergent nanoscale features—not just chemical identity.
In conclusion, this work demonstrates that surface curvature and ligand bundling are key determinants of nanoparticle hydrophobicity, particularly at the sub-10 nm scale. By integrating hydration free energy analysis with molecular binding simulations, we provide a predictive framework for engineering GNP surfaces with tailored interfacial properties. This approach enables precise control over biomolecular interactions, paving the way for advanced applications in targeted drug delivery, biosensing, and nanomedicine.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
