Ionic Surfactants at Air/Water and Oil/Water Interfaces: A Comparison Based on Molecular Dynamics Simulations

Abstract

Ionic surfactants are known to build up higher interfacial pressures at oil/water interfaces than at air/water interfaces for the same surfactant bulk concentration. Here, we systematically investigate this effect through atomistic molecular dynamics (MD) simulations of surfactant-loaded air/water and oil/water interfaces. Two prototypical ionic surfactants, C12TAB and sodium dodecyl sulfate (SDS), are studied and found to give consistent results, which are also robust with respect to variations in the simulation force field. The simulations reproduce the experimental interfacial pressure data on a semiquantitative level and reveal that the influence of oil on the surfactants’ in-plane distribution is a major contribution to the observed effect, albeit insufficient to be the sole reason. The simulations are further analyzed with regard to surfactant/oil cooperative/competitive effects that have been invoked recently as an explanation. However, the interfacial orientation of oil molecules, a presumable indicator for such behavior, is found to display changes only for high levels of surfactant coverage.

Effect of Surfactants on the Molecular Structure of the Buried Oil/Water Interface

“…Based on these observations,t he increased ordering of alkyl chains of the interfacial oil molecules at the buried oil/water interface, owing to the intercalation of hydrophobic tails of the surfactant, seems to be aprerequisite for asignificant change of the IFT between the oil and aqueous phase.This conclusion is consistent with the observations by Müller et al that SDS clustering reduces the interfacial tension at the oil/water interface by changing the organization of the oil molecules. [29] Fainerman et al also explained the change in interfacial tension at the oil/water interface with the increased ordering of both ionic surfactant and alkane molecules,f ar below the CMC value.T hey proposed at wo-step model that describes the adsorption of surfactants at the oil/water interface;Initial cooperative effects enhance the surfactant adsorption at the oil/water interface compared to the water/air interface, whereas,a th igher surfactant concentrations,c ompetitive Figure 6. Changes of the interfacial tension between MCT oil and water with increasing surfactant concentration.…”

“…Theo bserved increased degree of ordering in the interfacial oil molecules with increasing the surfactant concentration can be explained by the intercalation of aliphatic chains of surfactant molecules between the chains of interfacial MCT oil molecules (see Figure 3), which decreases the available area per molecule at the buried oil/water interface and increases the vdW interactions.T his conclusion is consistent with quantum chemical calculations and thermodynamic evaluations on long alkane chains of mixed monolayers of surfactants,c onfirming cooperative interactions between surfactant and alkane molecules at the buried oil/ water interface. [27] Other studies also indicate that the presence of surfactant molecules at the oil/water interface alters the organization of oil molecules at the buried oil/water interface.F or instance,r ecent SFG experiments [28] and molecular dynamic studies [29] revealed that in the absence of surfactants,oil molecules lie rather parallel with respect to the oil/water interface,w hereas surfactant adsorption to this interface changes the alignment of the oil molecules to amore perpendicular orientation.…”

Einfluss von Tensiden auf die molekulare Struktur der Öl/Wasser‐Grenzfläche

“…Diese Schlussfol- Organisation der Ölmoleküle aufgrund von zunehmender Clusterbildung von SDS-Molekülen an der Öl/Wasser-GF korrelierten. [29] Fainerman et al erklärten die ¾nderung der IFT an der Öl/Wasser-GF ebenfalls mit der zunehmenden Ordnung eines ionischen Te nsids und eines Alkans (weit unterhalb der CMC). Sie schlugen ein Zwei-Schritt-Modell vor,welches die Adsorption des Te nsids an der Öl/Wasser-GF beschreibt.…”

“…[27] Weitere Studien legen nahe,d ass die Anwesenheit von Te nsiden an der Öl/Wasser-GF die Organisation der Ölmoleküle an der Öl/Wasser-GF verändert. So haben jüngste SFG-Experimente [28] und moleküldynamische Studien [29] gezeigt, dass die Ölmoleküle in Abwesenheit von Te nsiden eher parallel zur Öl/Wasser-GF liegen, während eine Tensidadsorption an der GF zu einer verstärkt senkrechten Ausrichtung der Ölmoleküle führt.…”

Surface Activity and Efficiency of Cat-Anionic Surfactant Mixtures

“…This holds in aqueous solutions, even though it is possible to achieve very low γ values in micro-emulsions and at water-oil interfaces. The reason for this is inherent to the very nature of surfactants, saturating air-water, or water-oil, interfaces ( Chanda and Bandyopadhyay, 2006 ; Müller et al, 2021 ), but still retaining a finite area. Such behavior is controlled by solubility, packing at interfaces, film elasticity, the orientation of polar part(s) toward the bulk, etc.…”