Surfactant molecules and nano gold on HOPG: Experiment and theory

Abstract

Excessive surfactant molecules within the solution adhere to highly ordered pyrolytic graphite (HOPG) when a droplet containing gold nanorods evaporates, leading to the emergence of a unique coffee-ring pattern. The combination of surface hydrophobicity and evaporation of the aqueous phase results in a stick-slip motion, which enhances the convective hydrodynamics of suspended particles. This phenomenon initiates interactions that influence the deposition and flow dynamics inside the droplet. High-resolution scanning electron microscopy (HRSEM) does not provide significant insights into regions potentially linked to cetyltrimethylammonium bromide (CTAB) molecules, whereas the atomic force microscope (AFM) displays the presence of gold nanoparticles arranged by CTAB within specific CTAB patches. The layers forms with varying heights and gaps between them, indicating diverse adhesion of CTAB. The analytical focus lies on the quantitative assessment of CTAB molecules, stripe dimensions, and energy profiles influenced by concentration and the effective positioning of CTAB-coated gold nanorods in CTAB-covered regions. AFM examination reveals CTAB molecular stripes on HOPG, showing a binding energy of −20 kJ/mol with the surface, −10 kJ/mol for nitrogen bonding with HOPG, and a total binding energy (BE) of −60 kJ/mol, considering various contributing factors. Completely parallel aligned molecules (CPAM) exhibit higher binding energy than non-perfectly aligned molecules (NPAM) due to maximum Van der Waals (VdW) interactions, ideal electrostatic interactions, and minimal steric repulsion. The difference in binding energy between perfectly and non-perfectly aligned molecules is around 20kJ/mol, emphasizing the importance of molecular orientation. As temperature rises from 298 K to 348 K, the likelihood of NPAM desorption increases significantly, with the binding energy shifting from 2.5kJ/mol to 4.1kJ/mol. Temperature significantly influences the equilibrium of molecules on HOPG surfaces. The emphasis is on elucidating the alterations in energy levels during nanorod aggregation on CTAB areas compared to bare HOPG surfaces, underscoring the impact of nanorods on CTAB micelle deformation energy. Observations on the variation in CTAB desorption rates with temperature changes underscore the dynamic nature of molecular binding energy associated with surface properties, underscoring the importance of molecular configuration and energy transfers in nanoscale systems.