Competitive Adsorption of a Multifunctional Amine and Phenol Surfactant with Ethanol on Hematite from Nonaqueous Solution

Abstract

Surfactants, which contain phenol and amine groups, are commonly used in industry to protect metallic surfaces and their efficiency depends strongly on factors such as pressure and temperature, solvent properties, and the presence of other surfactants in the system. In this work, we present a molecular simulation study of the competitive adsorption between a multi-functional phenol and amine surfactant model and ethanol at the oil/solid interface formed between iso-octane and a model hematite (α -Fe 2 O 3 ) slab. We show that the surfactant strongly adsorbs at the iso-octane/hematite interface in the absence of ethanol at moderate temperatures. As the concentration of ethanol is increased, the ethanol molecules compete effectively for the adsorption sites on the iron oxide surface. This competition drives the surfactant molecules to remain in the bulk-solution while ethanol forms an ordered and strongly coordinated layers at the oil/solid interface, despite the well-known complete miscibility of ethanol in iso-octane in bulk at standard conditions. Potential of mean force calculations show that the free energy of adsorption of the surfactant is approximately two times larger than for a single ethanol molecule, but the simulations also reveal that a single surfactant chain needs to displace up to five ethanol molecules to adsorb onto the surface. The end result is a more favorable ethanol adsorption which agrees with the experimental analysis of similar oil/iron oxide systems also reported in this work.

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

“…Until recently, a certain surface coverage of surfactant molecules on the solid surfaces was usually assumed in MD simulations of OFMs. , However, Chia et al and Jaishankar et al used MD simulations to study the adsorption of surfactants on iron oxide surfaces from hydrocarbon solvents. Chia et al studied the competitive adsorption of the fuel additive 2-[(3-(dimethylamino)­propyl)­amino]­methyl-4-dodecyl-6-methylphenol and ethanol from isooctane onto the α-Fe 2 O 3 (0001) surface at different temperatures. They showed that while the free energy of adsorption of the additive was ∼2 times larger than that for a single ethanol molecule, a single additive needs to displace up to five ethanol molecules to strongly adsorb onto the surface.…”

“…Chia et al and Jaishankar et al extracted the potential of mean force (PMF) along the reaction coordinate using umbrella sampling (US) and the weighted histogram analysis method (WHAM). , Even with modern simulation software and high performance computing (HPC) architectures, this method is extremely computationally demanding and requires hundreds of nanoseconds of simulation time to obtain converged PMFs. For example, Jaishankar et al used an overall simulation time of more than 1 μs to obtain the PMF describing the adsorption of SA onto hematite from n -hexadecane.…”

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

“…27 Until recently, a certain surface coverage of surfactant molecules molecules on the solid surfaces was usually assumed in MD simulations of OFMs. 26,27 However, Chia et al 33 and Jaishankar et al 34 used MD simulations to study the adsorption of surfactants on iron oxide surfaces from hydrocarbon solvents. Chia et al 33 studied the competitive adsorption of the fuel additive 2-[(3-(dimethylamino)propyl)amino]methyl-4-dodecyl-6-methyl-phenol and ethanol from i-octane onto the α-Fe 2 O 3 (0001) surface at different temperatures.…”

“…26,27 However, Chia et al 33 and Jaishankar et al 34 used MD simulations to study the adsorption of surfactants on iron oxide surfaces from hydrocarbon solvents. Chia et al 33 studied the competitive adsorption of the fuel additive 2-[(3-(dimethylamino)propyl)amino]methyl-4-dodecyl-6-methyl-phenol and ethanol from i-octane onto the α-Fe 2 O 3 (0001) surface at different temperatures. They showed that, while the free energy of adsorption of the additive was approximately two times larger than that for a single ethanol molecule, a single additive needs to displace up to five ethanol molecules to strongly adsorb onto the surface.…”

“…Since then, it has been used to study adsorption in many systems, such as sodium dodecyl sulphate (SDS) on graphene from water, 37 SDS on α-Al 2 O 3 nanoparticles from water, 38 and organomolybdenum friction modifiers on borondoped diamond-like-carbon (DLC) from n-heptane. 39 Chia et al 33 and Jaishankar et al 34 extracted the potential of mean force (PMF) along the reaction coordinate using umbrella sampling (US) and the weighted histogram analysis method (WHAM). 40,41 Even using modern simulation software and high performance computing (HPC) architectures, this method requires hundreds of nanoseconds of simulation time to obtain converged PMFs.…”