Surfactant Adsorption Isotherms: A Review

Abstract

The need to minimize surfactant adsorption on rock surfaces has been a challenge for surfactant-based, chemicalenhanced oil recovery (cEOR) techniques. Modeling of adsorption experimental data is very useful in estimating the extent of adsorption and, hence, optimizing the process. This paper presents a mini-review of surfactant adsorption isotherms, focusing on theories of adsorption and the most frequently used adsorption isotherm models. Two-step and four-region adsorption theories are well-known, with the former representing adsorption in two steps, while the latter distinguishes four regions in the adsorption isotherm. Langmuir and Freundlich are two-parameter adsorption isotherms that are widely used in cEOR studies. The Langmuir isotherm is applied to monolayer adsorption on homogeneous sites, whereas the Freundlich isotherm suites are applied to multilayer adsorption on heterogeneous sites. Some more complex adsorption isotherms are also discussed in this paper, such as Redlich− Peterson and Sips isotherms, both involve three parameters. This paper will help select and apply a suitable adsorption isotherm to experimental data.

Adsorption of Phosphate and Nitrate Ions on Oxidic Substrates Prepared with a Variable-Charge Lithological Material

“…The Langmuir isotherm is a valid theoretical model for adsorption in a monolayer on a completely homogeneous surface with a finite number of identical and equivalent sites of adsorption and with a negligible interaction between the molecules. The non-linear and linear form of the Langmuir isotherm are described in Equations ( 2) and (3) [25][26][27][28]:…”

“…The Freundlich adsorption isotherm is an empirical model describing the reversible and non-ideal adsorption process. It can be used for multilayer adsorption on heterogeneous sites, assuming non-uniform sites and adsorption energies [8,[25][26][27][28]. The mathematical model of the Freundlich adsorption isotherm is represented by Equation (4), and its linearized form by Equation ( 5):…”

“…The active sites for the adsorption are not equivalent; therefore, active sites with less energetic requirement are occupied first, etc. However, energetic heterogeneity in such a kind of substrate is expected; the topology and chemical composition of the surface also show non-homogeneity [27,28,46]. The Freundlich and Langmuir models pursue the same objective; however, the Langmuir model has limitations when used to describe the adsorption process on nonhomogeneous surfaces [25][26][27]41].…”

“…Furthermore, the repulsion increases between the adsorbed molecules, which at the same time causes the mobility of the molecules through the surface, and different places can be occupied. As a result, physisorbed layers can be formed over the chemisorbed layer [26][27][28]45,46].…”

Observation of strong synergy in the interfacial water response of binary ionic and non-ionic surfactant mixtures

“…We nd that SDS and C 12 E 6 have a strong synergistic effect on the structure of the near-surface water layers and that the properties of binary aqueous solutions of ionic and non-ionic surfactants cannot be described with a conventional Langmuir adsorption model 33,34 . We rationalize the synergy of SDS and C 12 E 6 using molecular dynamics simulations that show that the presence of C 12 E 6 at the water surface serves as an attractive sink for DS − ions, and that the uptake of DS − in the C 12 E 6 layer is accompanied by large changes in the hydrogen-bond network of the water layers near the surface.…”