(481b) Molecular Dynamics Study on the Adsorption of Cationic Surfactants on Silica Nanoparticle and Surfactant-Mediated Nanoparticle Interactions

Due to large surface area-to-volume ratio, nanoparticles (NPs) exhibit desirable characteristics, leading to numerous applications, such as enhanced oil recovery, drug delivery, and water treatment, etc. In many of these circumstances, NPs are used along with surfactants. NPs should remain dispersed to maximize their efficacy and efficiency, so the stability of NP/surfactant mixtures becomes imperative.

Negatively-charged silica NP suspension can be destabilized by adding cationic surfactant due to charge neutralization and hydrophobic effect of the hydrocarbon tail of surfactants. However, the interfacial structures of surfactant molecules on silica NPs remain elusive. Binks et al. (Langmuir 2007, 23 (7): 3626-3636) proposed that at low concentration of cationic surfactant cetyltrimethylammonium bromide (CTAB), single CTAB is adsorbed on the surface of NP with its tail pointing toward water, making NP partially-hydrophobic. Further adding CTABs will form a single layer of CTAB on NP, so the mixture of CTAB and NP become fully hydrophobic. At high CTAB concentrations, a bi-layer structure could form on the surface of NP, and the mixture becomes hydrophilic again. On the other hand, a micelle-decoration structure was proposed by Kumar et al. (Langmuir 2012, 28 (25): 9288-9297) based on small angle neutron scattering (SANS) measurements. Therefore, the interfacial structures of cationic surfactants on NPs and their effect on NP stability which are imperative to the applicability of NP/surfactant mixtures still remain unclear.

Therefore, in this work, molecular dynamics (MD) simulations are used to study the interfacial structures of CTAB on a 3 nm amorphous silica NP at various surface concentrations. As CTAB surface concentration increases, three different stages are observed based on the aggregation and adsorption morphology of CTAB. At stage I (CTAB surface concentration around 0.1 per nm²), CTAB exist as monomers floating around in the simulation box. No stable aggregation or adsorption is observed. At stage II (0.3-0.5 per nm²), CTABs aggregate as single CTAB cluster and adsorb on NP. At stage III (1-3 per nm²), multiple clusters exist with 2-3 clusters adsorbed on NP surface. Adsorbing CTAB clusters do not aggregate due to the energy barrier. Potential of mean force (PMF) calculation results between CTAB cluster/nanoparticle mixture show attractive force between them, leading to the destabilization of NP suspension.

Collectively, our study from molecular perspectives provides important insights into the interfacial structures of cationic surfactants on negatively-charged NPs and their effect on NP interactions, which are important for the rational design and optimization of NP applications.