Exploring the Free Energy Landscape To Predict the Surfactant Adsorption Isotherm at the Nanoparticle–Water Interface

Abstract

The long-lasting stability of nanoparticle (NP) suspensions in aqueous solution is one of the main challenges in colloidal science. The addition of surfactants is generally adopted to increase the free energy barrier between NPs and hence to ensure a more stable condition avoiding the NP sedimentation. However, a tailored prediction of surfactant concentration enabling a good dispersion of NPs is still an ambitious objective. Here, we demonstrate the efficiency of coupling steered molecular dynamics (SMD) with the Langmuir theory of adsorption in the low surfactant concentration regime, to predict the adsorption isotherm of sodium-dodecyl-sulfate (SDS) on bare α-alumina NPs suspended in aqueous solution. The resulting adsorption free energy landscapes (FELs) are also investigated by tuning the percentage of SDS molecules coating the target bare NP. Our findings shed light on the competing role of enthalpic and entropic interaction contributions. On one hand, the adsorption is highly promoted by the tail–NP and tail–tail nonbonded interaction adhesion; on the other hand, our results unveil the entropic nature of water and surfactant steric effects occurring at the NP surface and preventing the adsorption. Finally, a thorough analysis on the steering works emphasizes the role of the NP curvature in the FEL of adsorption. In particular, we show that, moving from a solid infinite flat surface to a nanoscale particle, a deviation from a Markovian dynamics of adsorption occurs in close proximity to a curved solid–liquid interface. Here, both the NP curvature effect and nanoscale morphology promote a modification of the thermodynamics state of adsorption with a consequent splitting of the free energy profiles and the identification of specific sites of adsorption. The modeling framework suggested in this Article provides physical insights in the surfactant adsorption onto spherical NPs and suggests some guidelines to rationally design stable NP suspensions in aqueous solutions.

How Adsorption of Pheromones on Aerosols Controls Their Transport

“…A description of the aggregation dynamics of pheromones on the aerosol surface, unable to penetrate in the aerosol bulk, was achieved using the general framework introduced by Gibbs and refined by Tanford for the two-dimensional pseudophases created by amphiphilic molecules. ,,,, Occupancy of the surface by aerosols can be estimated with high precision as long as the gain of energy of moving the pheromone from the gas phase to the interface can be determined. Appropriated experimental and theoretical methods can lead to such results. ,− We developed here the generic theory in the case of independent pheromone molecules adsorbed on the surface of aerosols composed of pure water.…”

“…Appropriated experimental and theoretical methods can lead to such results. 27 , 45 − 48 We developed here the generic theory in the case of independent pheromone molecules adsorbed on the surface of aerosols composed of pure water.…”

Buffer Influence on the Amino Acid Silica Interaction

“…The multiscale modelling consists of calculations of energetic parameters of binding from the MD simulation and further use of these parameters in mechanistically different multicomponent Langmuir models. In a very recent work, similar multiscale modelling approach involving MD simulation and the non-cooperative Langmuir model was adopted by Angelis et al [56] to predict the adsorption of surfactant to the alumina. In this work, we extend the Langmuir model to account for multiple interacting species which is relevant in variety of physical situations including ours.…”

DNA Binding to the Silica: Cooperative Adsorption in Action

“…Although there were numerous experiments conducted in the past studying the DNA binding to silica, there is no modeling work attempting to understand the binding agent mediated DNA adsorption. It is worth mentioning that there have been numerous modeling attempts ,− to understand other small molecule adsorption to various carbon and silica surfaces. The original version of the Langmuir model (noncooperative) has been extensively used ,, in the past to understand a variety of adsorption processes.…”