Molecular dynamics insights into energy barrier modulation by thiol-mixed co-surfactants in surfactant-mediated gold nanocrystal growth

How do thiol-based co-surfactants influence micelle morphology and, in turn, regulate gold nucleation dynamics? To answer this question, we investigated a ternary surfactant system comprising cetyltrimethylammonium bromide (CTAB), oleylamine (OLA), and hexadecanethiol (HT) using all-atom molecular dynamics simulations at different molar ratios. Among the compositions studied, a CTAB/OLA/HT ratio of 3:1:1 leads to the well-defined and stable cylindrical micelle, creating a confined environment conducive to anisotropic gold nucleation. This geometry promoted directional growth, leading to the formation of an elliptical gold nucleate, with the alignment of surfactant tails guiding preferential nucleation along a single axis. To track the structural evolution during nucleation, we monitored micelle behaviour as gold atoms were incorporated over time. Initially dispersed randomly in solution, gold atoms progressively aggregated into larger nucleates that penetrated the micelle core. Their incorporation caused a gradual decrease in the surfactant order parameter, indicating a transition towards bilayer-like organization and highlighting the micelle’s adaptive response to nucleate growth. The free energy barrier associated with the migration of a gold nucleate from the micelle interior to its surface was quantified using umbrella sampling combined with the Weighted Histogram Analysis Method (WHAM). The inclusion of thiol molecules significantly lowered this barrier to 2.83 ± 0.12 kcal/mol, demonstrating a more favourable pathway for nucleation and growth within the micelle.