# The High Interfacial Activity of Betaine Surfactants Triggered by Nonionic Surfactant: The Vacancy Size Matching Mechanism of Hydrophobic Groups

## TL;DR

This study explores how combining betaine surfactants with nonionic surfactants can drastically reduce interfacial tension, which is important for oil recovery.

## Contribution

The paper introduces a vacancy size matching mechanism that explains how nonionic surfactants enhance betaine surfactant performance.

## Key findings

- Span80 forms a compact interfacial film with betaine surfactants, reducing interfacial tension to ultra-low values.
- Tween80 has minimal impact on betaine surfactant interfacial tension due to lack of aqueous-side vacancies.
- The study offers insights for selecting betaine surfactants in enhanced oil recovery processes.

## Abstract

Alkyl sulfobetaine shows a strong advantage in the compounding of surfactants due to the defects in the size matching of hydrophilic and hydrophobic groups. The interfacial tensions (IFTs) of alkyl sulfobetaine (ASB) and xylene-substituted alkyl sulfobetaine (XSB) with oil-soluble (Span80) and water-soluble (Tween80) nonionic surfactants on a series of n-alkanes were studied using a spinning drop tensiometer to investigate the mechanism of IFT between nonionic and betaine surfactants. The two betaine surfactants’ IFTs are considerably impacted differently by Span80 and Tween80. The results demonstrate that Span80, through mixed adsorption with ASB and XSB, can create a relatively compacted interfacial film at the n-alkanes–water interface. The equilibrium IFT can be reduced to ultra-low values of 5.7 × 10−3 mN/m at ideal concentrations by tuning the fit between the size of the nonionic surfactant and the size of the oil-side vacancies of the betaine surfactant. Nevertheless, Tween80 has minimal effect on the IFT of betaine surfactants, and the betaine surfactant has no vacancies on the aqueous side. The present study provides significant research implications for screening betaine surfactants and their potential application in enhanced oil recovery (EOR) processes.