New aspects of the formation and growth mechanism of gold nanorods (AuNRs) during seed-mediated colloidal synthesis are revealed from the results of molecular dynamics simulation. The model systems consist of cetyltrimethylammonium bromide (CTAB) units adsorbed on low-index [Au(110), Au(100), and Au(111)] and high-index [Au(250)] gold surfaces. The CTAB units are adsorbed as adjacent cylindrical micelles when the relative number of adsorbed bromide ions is small. At later AuNR growth stages, the number of bromide ions increases as the [AuBr] species pass through the channels between the adsorbed micelles on the gold surface. Thus, the mature AuNRs have a high concentration of bromide ions at their surface, which appears to change the organization of the CTAB units on the particle surface from adsorbed micelles to a compact CTAB bilayer.

Surfactants are molecular structures with remarkable physicochemical properties and applications. Most of their characteristics are due to their ability to promote aggregation and interactions with different interfaces. The scarcity of theoretical studies dedicated to evaluating the forces involved in these interactions prompted us to propose other models capable of reproducing the experimental data in better ways. We carried out molecular dynamics (MD) simulations to obtain a model for cetyltrimethylammonium bromide (CTAB), selected from gromos54a7 force field parameters, that better describes most of its behaviors in aqueous solution (micellar structure, counterion dissociation, etc.) and its adsorption pattern on a gold surface. The parameters adopted for one of the models were able to mimic several characteristics suggested by experimental measurements of the CTAB micelles, as well their adsorption pattern on a gold surface. Indeed, this model was able to obtain quasi-spherical micelles, as well as a pattern of adjacent cylindrical micelles with alkyl chain interactions on a gold surface.Keywords: molecular dynamics, micelles, interface interaction, cetyltrimethylammonium bromide, gold IntroductionSurfactants are a class of compounds containing a polar group, charged or neutral, attached to a long hydrophobic tail.1,2 These are remarkably versatile compounds with a broad variety of important applications in the pharmaceutical, medical, and food industries and for nanomaterial synthesis.3,4 Above a certain temperature in solution (the Krafft temperature), surfactants tend to aggregate to minimize unfavorable interactions between the surfactants and the surrounding environment. 5 The minimum concentration required for surfactant aggregation is defined as the critical micellar concentration (CMC), and most of the characteristics of these aggregates are controlled by factors such as the solvent type, chemical structure of the surfactant, and solution conditions (e.g., concentration, temperature, presence of additives, and ionic strength). 6Variations of these factors yield aggregates with different morphologies such as spherical or ellipsoidal micelles, cylindrical or thread-like micelles, disk-like micelle, membranes and vesicles.7 These self-assembled structures have been characterized by a number of techniques, such as dynamic light scattering (DLS), 8 nuclear magnetic resonance (NMR), 9,10 fluorescence spectroscopy, 11 quasielastic neutron scattering (QENS), 12,13 small-angle X-ray scattering (SAXS), 14,15 and small-angle neutron scattering (SANS). 16,17 Computational simulations have also been employed to explore the structures and dynamical behaviors of micelles for different surfactants. 18Considering that no holes exist within a micelle, its radius is estimated as the maximum extension of a hydrocarbon chain and can be evaluated by using the following equation:where l max is the maximum length in nm and n C is the number of carbon atoms in the chain. 19 Indeed, under the previously mentioned conditions, ...

Understanding the role of alkylamine surfactants in the anisotropic growth mechanism of copper nanowires (CuNWs) in solutionphase synthesis has implications for tuning of the properties that can be employed in diverse, important applications. In this paper, we used molecular dynamics simulations to show that the hexadecylamine (HDA) adsorption structure can, depending on the HDA density, form a surfactant monolayer or many stacked layers parallel to the copper surface. We showed that the layers are similar on the Cu(111) and Cu(100) facets, independent of the HDA density, and that the presence of Cl − ions appears to have no significant effect on the HDA/HDA + stacked layer structure. Within the simulation time frame, Cl − ions do not pass through a regular, uniform HDA monolayer, but they can pass through a monolayer with some protonated HDA molecules (HDA + ). Our results suggest that Cl − ions not only affect the HDA/HDA + monolayer stability in CuNW synthesis, causing a disruption of the monolayer and formation of the stacked layer, but also participate in copper(II) alkylamine complex formation. Moreover, our results stress the importance of a reliable model of the surfactant layer on the metal particle, explicitly considering the protonation of part of the surfactant. Our results show a completely innovative perspective of the HDA adsorption structure and its role in the growth mechanism of CuNWs in solution-phase synthesis, allowing improvements in control of the properties and the quality of the CuNWs obtained.

Polyol synthesis allows the preparation of silver nanostructures with different well-controlled morphologies and opens a window to explore their potential in various emerging applications. Despite their success in controlling the size and shape of many noble-metal nanostructures, some fundamental aspects of this control during the synthesis remain unclear. In this paper, we used molecular dynamics (MD) simulations (three replications 1550 ns long) to represent a possible adsorption structure of poly(vinylpyrrolidone) (PVP) with an average molecular weight of 55,000 Da (PVP55) on an Ag(100) surface on ethylene glycol (EG) medium. The conditions employed in the simulations reproduce the PVP chain length and density reported experimentally. We found that the adsorbed PVP55 layer has a first polymer contact, with some degree of order, where interactions are mediated by carbonyl groups close to the Ag(100) face. In order to study the effects of the presence of AgNO 3 and NO in the medium on PVP55, three sets of simulations were run comprising (i) one PVP55 molecule in pure EG and with (ii) 0.25 M AgNO 3 or (iii) NO. These simulations showed that the PVP55 compacts in pure EG and in the presence of AgNO 3 but expands in the presence of NO. This points at new insights into the roles of the PVP adsorption layer and PVP/AgNO 3 ratio in polyol synthesis.