Surfactant Adsorption Layers: Experiments and Modeling

Abstract

During recent years, great progress has been made in understanding the adsorption of surfactants at liquid interfaces. In addition to tensiometry, new efficient methodologies have been developed, in particular interfacial selective optical methods which allow direct access to the adsorbed amounts and interfacial layer compositions. In addition to these new experimental tools, the thermodynamic description by equations of state now allows one to provide a quantitative picture of surfactant interfacial layers. This is most notable for surfactant layers at water/oil interfaces. Additional knowledge about the structure of interfacial layers was gained through different types of molecular modeling. Improved interrelationships between these three aspects are the challenges for current and future work. Particular attention must be paid to dilational interfacial rheology studies, as these mechanical quantities are much more sensitive to small changes in the interfacial composition and structure.

Interface Adsorption versus Bulk Micellization of Surfactants: Insights from Molecular Simulations

“…The pressure isotherm in Figure C represents the central result of our modeling procedure. Importantly, pressure isotherms are experimentally easily accessible, whereas the availability of reliable experimental data in the form of surfactant adsorption isotherms or EoS is still very limited …”

“…The MD data points are fitted with the polynomial Π(Γ) = k B T Γ + C 2 Γ 2 + ... + C 5 Γ 5 , where the first term corresponds to the ideal 2D gas surface pressure, whereas the higher-order terms capture the deviation from ideality, with the coefficients C 2 , ..., C 5 used as fitting parameters. At this point, we recall that the EoS cannot be compared directly to experiments of this surfactant category owing to challenging direct measurements of Γ 44 and to the rapid exchange of surfactants between interface and bulk, which prohibits indirect estimates of Γ.…”

“…Importantly, pressure isotherms are experimentally easily accessible, whereas the availability of reliable experimental data in the form of surfactant adsorption isotherms or EoS is still very limited. 44 At first glance, the presented approach may appear numerically costly. However, the computational effort can be reduced significantly with the Π-integration method, and only a few TIs are required for validation.…”

Introductory Chapter: Surfactants – Bridging Fundamental Concepts with Emerging Perspectives

“…This is especially relevant in reactions where all the reactants cannot be solubilized in a single solvent [21]. These reaction systems are complex with the presence of reactants; multiple to effectively understand the surfactant mechanisms in complex microheterogenous catalysis applications, combining experimental and modeling techniques is beneficial [23,24]. The last chapter in this book provides a unique perspective on the models that have been developed to explain the surfactant effects on reaction rates.…”

Photoswitchable arylazopyrazoles surfactants at the water-air interface: a microscopic perspective

“…Therefore, in MD simulations the preferred way is to fix the surface excess. The choice of different variables indeed hampers the comparison of experiments and simulations 20,30,31 . However, using surface sensitive techniques such as vibrational sum frequency generation (SFG) in addition to tensiometry, a direct mapping between bulk and surface concentration becomes possible, at least in relative terms.…”