Eur. Phys. J. B (2015) 88: 261
https://doi.org/10.1140/epjb/e2015-60543-1

Regular Article

The stability and interfacial properties of functionalized silica nanoparticles dispersed in brine studied by molecular dynamics

¹ Centro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC (UFABC), Santo André, SP, Brazil
² Universidade Estadual de Ponta Grossa (UEPG), Ponta Grossa, PR, Brazil
³ Universidade Tecnológica Federal do Paraná (UTFPR), Cornélio Procópio, PR, Brazil
⁴ Instituto de Física, Universidade de São Paulo (IF-USP), São Paulo, SP, Brazil

^a e-mail: cmiranda@if.usp.br

Received: 4 July 2015
Received in final form: 7 September 2015
Published online: 14 October 2015

Abstract

The charge accumulation and surface tension of overall neutral functionalized silica nanoparticles (NPs) dispersed in brine (NaCl and CaCl₂) were studied using large scale fully atomistic molecular dynamics. Sulphonic (SA) and ethylene-glycol (EG) functional groups have been incorporated in the NP surface respectively, covering both the hydrophobic and hydrophilic characters. For the latter, groups with one (EG) and two (PEG2) monomers were considered. The ion distribution in electrolyte aqueous solution and its accumulation around NPs were monitored for different salt concentrations (from 0.05 up to 1 wt%), and temperature (300 and 350 K) at 1 atm. At certain conditions, the ion accumulation surrounding the overall neutral NPs leads to a formation of electrical double layers (EDL). Compared with the monovalent ions (NaCl), the accumulation of divalent ions (CaCl₂) was found to be more pronounced and the thickness of the EDL around the NPs is more compact. According to the functional group attached, the EDL width also reduces going from EG, to PEG2, to SA. Our simulations suggest that the EDL formation, its narrowing, the large variation of the interface tension, followed by a steep increase in ion mobility are conditions which may precede instability of functionalized NPs dispersion in brine.