Quantifying the Counterion-Specific Effect on Surfactant Adsorption Using Modeling, Simulation, and Experiments

Abstract

Ionic surfactants behave differently in the presence of various counterions, which plays an important role in many scientific and engineering processes. Previous work has shown that the counterion-specific surface tension can be reproduced with classical adsorption models, but the underlying origin of this effect has not been explained. In this paper, we extend our previously developed adsorption model to account for the specific counterion adsorption. This model can accurately predict the surface tension of surfactant solutions like sodium dodecyl sulfate (SDS) in the presence of the monovalent salts LiCl, NaCl, KCl, and CsCl. The predicted surface excess and surface potential are validated by corresponding sum-frequency generation (SFG) spectroscopy experiments. We also used molecular dynamic (MD) simulation to explain the origin of the counterion-specific effect for surfactant behavior. Our study shows that for SDS, binding of the counterion to both the headgroup and a few CH2 fragments close to the surfactant head contributes to the counterion-specific effect. In general, SDS behaves like a large ion, and it prefers to bind with large counterions such as Cs+, which is consistent with Collins’s law of matching water affinity. Therefore, large counterions enhance the surface adsorption and lower the surface tension the most.

The electrostatic origins of specific ion effects: quantifying the Hofmeister series for anions

“…Despite their ubiquity and importance, no reliable theory exists for quantitatively predicting SIE, although progress is being made 2,[36][37][38][39][40][41][42][43][44] . Furthermore, alterations or even reversals in SIE have been observed by changing the solute 45 , solvent 46 , pH 47 , temperature 48 , counterion 49 or concentration 50 .…”

From Surface Tension to Molecular Distribution: Modeling Surfactant Adsorption at the Air–Water Interface

“…Also, the model predictions agree very well with the SFG and MD results. 21 SDS behaves like a large ion, and it prefers to bind to large counterions that enhance the surface adsorption and lower the surface tension the most. The same equation in our previous work (eq 4 in section 4.1) was used for this model, and the model calibration procedures are similar except for one modification: the Gaussian form of the adsorption energy now applies to both surfactant and counterions.…”

“…This simplification is reasonable because the counterion adsorption is mostly caused by binding to the surfactant, as shown in our MD simulation. 21 The experimental and predicted surface tension values (using the MPB model) of 1 mM SDS in the presence of LiCl, NaCl, KCl, and CsCl are shown in Figure 7(left). Adding salts to the surfactant solution decreases the surface tension, and the surface tension is strongly counterion-specific: the Li + ion affects the surface tension of SDS the least while the Cs + ion reduces it the most.…”

“…This is why CsCl results in a higher surface excess and a lower surface tension. 21 The SFG spectra at the air/water interface formed by 1 mM SDS in the presence of LiCl and CsCl at different concentrations are shown in Figure 8. Briefly, the height of organic peaks from 2800 to 3000 cm −1 is proportional to the number of surfactant molecules at the interface.…”

Temperature-Responsive Hydrogel Carrier for Reducing Adsorption Loss of Petroleum Sulfonates

“…The crude oil in these reservoirs is concentrated deep inside, and there is almost no crude oil near the injection well . Surfactants are always adsorbed in large quantities on the rock surface near the injection well and cannot drive the crude oil deeper within the reservoir. , Notably, the temperature of an oil reservoir increases with depth such that the temperature inside the oil reservoir is typically tens of degrees higher than the temperature at the surface. Our hydrogel carrier (XLK), composed of xanthan gum (XG), locust bean gum (LBG), KCl, and water, changes from a gel state to a sol state at higher temperatures.…”