1 research outputs found
Molecular Dynamics Simulation of the Adsorption and Aggregation of Ionic Surfactants at LiquidâSolid Interfaces
Structure
of surfactants adsorbed on solid surfaces is a key knowledge
in various technologies and applications. It is widely accepted in
the literature that the surfaceâsurfactant headgroup electrostatic
interaction is a major driving force of adsorption of ionic surfactants
on charged substrates. Our result shows that the adsorption of surfactants
as monomers is driven by both electrostatic and nonelectrostatic interactions.
Further adsorption of surfactants in aggregates is
essentially driven by the tailâtail interaction. To a great
extent, the substrateâtail interaction
determines the structures of the adsorbed surfactant aggregates. Water
and counterions influence the headgroupâsubstrate and tailâsubstrate
interactions. We investigate two vastly different surfactants and
substrates by molecular dynamics simulations: (1) SDS on alumina (SDSâAl<sub>2</sub>O<sub>3</sub>), and (2) CTAB on silica (CTABâSiO<sub>2</sub>). We study the adsorption of a single surfactant at the solid
surface by the density profiles and free energy of adsorption. In
the SDSâAl<sub>2</sub>O<sub>3</sub> system, we analyze the
free energy of adsorption on the substrate covered by aggregates of
different sizes. We examine the configurations of surfactants and
the distribution of water and ions at the liquidâsolid interface
as the number of adsorbed molecules on the substrate increases.
In the SDSâAl<sub>2</sub>O<sub>3</sub> system, the headgroup
adsorption is mediated by the Na<sup>+</sup> counterions; the adsorbed
water molecules may be displaced by the surfactant
headgroup but unlikely by the hydrocarbon tails. As a function of
the surfactant adsorption, we observe single surfactants, aggregates
of different morphologies, and bilayers. The CTABâSiO<sub>2</sub> system combines both electrostatic attraction of the surfactant
headgroup and affinity for the surfactantâs hydrocarbon tail.
At low surfactant adsorption, aggregates and single
surfactant molecules lie on the substrate; hemimicelles form at intermediate
adsorption; and micelles form at high surfactant adsorption. Our results
agree with experimental observations and indicate two different surfactant
adsorption mechanisms where the tailâtail and tailâsubstrate
interactions play a fundamental role