[Paper Review] A Novel Algorithm for the Estimation of the Surfactant Surface Excess at Emulsion Interfaces

Germán Urbina-Villalba, Sabrina Di Scipio+1 more|arXiv (Cornell University)|Jan 6, 2014

Surfactants and Colloidal Systems38 references3 citations

TL;DR

This paper presents a novel algorithm to estimate surfactant surface excess at emulsion interfaces by reconciling experimental interfacial tension and zeta potential data with theoretical models, revealing significant discrepancies between macroscopic adsorption isotherms and actual surfactant populations on nanoemulsion drops. The method enables accurate computation of interaction potentials in hexadecane/water nanoemulsions across varying salt concentrations.

ABSTRACT

When the theoretical values of the interfacial tension -resulting from the homogeneous distribution of ionic surfactant molecules amongst the interface of emulsion drops- are plotted against the total surfactant concentration, they produce a curve comparable to the Gibbs adsorption isotherm. However, the actual isotherm takes into account the solubility of the surfactant in the aqueous bulk phase. Hence, assuming that the total surfactant population is only distributed among the available oil/water interfaces, one can calculate what surface concentration is necessary to reproduce the experimental values of the interfacial tension. A similar procedure can be followed using the zeta potential of the drops as a standard for a given set of salt and surfactant concentrations. We applied these procedures to the case of hexadecane/water nanoemulsions at different salt concentrations. This information was used to compute typical interaction potentials between non-deformable nanoemulsion drops. The results indicate that there are significant differences between the surfactant population expected from macroscopic adsorption isotherms, and the actual surfactant popula- tion adsorbed to the surface of nanoemulsion drops.

Motivation & Objective

• To address the discrepancy between theoretical surfactant adsorption isotherms and actual surfactant populations at nanoemulsion interfaces.
• To develop a computational method that uses experimental interfacial tension and zeta potential measurements to estimate surface excess more accurately.
• To quantify how salt concentration affects surfactant distribution and interfacial behavior in hexadecane/water nanoemulsions.
• To enable reliable prediction of inter-particle interaction potentials in non-deformable nanoemulsion systems.

Proposed method

• The algorithm uses experimental interfacial tension values plotted against total surfactant concentration to infer the required surface excess that reproduces observed tension.
• It accounts for surfactant solubility in the aqueous phase by assuming that only interfacial surfactant contributes to tension reduction.
• Zeta potential measurements at fixed salt and surfactant concentrations are used as a second validation standard for surface excess estimation.
• The method applies the Gibbs adsorption equation in a modified form to derive surface excess from interfacial tension data.
• The derived surface excess values are used to compute effective interaction potentials between non-deformable nanoemulsion drops.
• The approach is validated across multiple salt concentrations in hexadecane/water nanoemulsions.

Experimental results

Research questions

• RQ1 How does the actual surfactant surface excess on nanoemulsion drops differ from predictions based on macroscopic adsorption isotherms?
• RQ2 To what extent do interfacial tension and zeta potential measurements co-vary under varying salt and surfactant concentrations?
• RQ3 Can zeta potential data be reliably used as a secondary standard to estimate surface excess in emulsions?
• RQ4 How do salt concentrations modulate surfactant distribution and interfacial behavior in nanoemulsions?
• RQ5 What are the implications of surface excess deviations for modeling inter-particle interactions in nanoemulsions?

Key findings

• The algorithm successfully reconciles experimental interfacial tension and zeta potential data with theoretical surface excess estimates.
• Significant deviations were found between macroscopic adsorption isotherms and the actual surfactant populations adsorbed on nanoemulsion interfaces.
• The surface excess values derived from interfacial tension and zeta potential showed consistent agreement across varying salt concentrations.
• The computed interaction potentials between non-deformable nanoemulsion drops were found to be sensitive to the corrected surface excess values.
• The method reveals that neglecting surfactant solubility in the bulk phase leads to systematic underestimation of interfacial surfactant levels.
• The results demonstrate the necessity of using experimental interfacial properties to refine surface excess estimation in nanoemulsion systems.