Systematically quantifying oil–water microemulsion structures using (spin-echo) small angle neutron scattering

Maarten Mulder,Xuesong X Li,Mohammad M Nazim,Robert M Dalgliesh,Bei Tian,Marten Buijse,John Van Wunnik,Wim G Bouwman

doi:10.1016/j.colsurfa.2019.04.045

Abstract

Microemulsion systems consisting of D2O, an alkane, an anionic internal olefin sulfonate surfactant, salt and secondary butyl alcohol (SBA) as co-solvent are studied in a systematic way. In four different sample sets, either the salt content, SBA content or alkane carbon number was varied in order to study the effects of the individual compounds on the structure sizes making up the microemulsion. Using complementary small-angle neutron scattering techniques SANS and Spin-Echo SANS, it was found that the microemulsion systems exhibit the largest structures in the optimum state (domain size of d/2 =144 nm in the model by Teubner and Strey), where the structure is considered bicontinuous. In comparison, at under- and over-optimum states where the structures consist of emulsified spherical droplets, the smallest measured diameter was 2R = 44 nm. Furthermore, the structure sizes in bicontinuous microemulsions decrease exponentially (down to d/2 =15 nm for pentadecane and 5 wt% SBA) as function of both SBA content and alkane carbon number. The observed trends in structure sizes combined with the trends observed in the area per surfactant molecule, are qualitatively explained with the extended Winsor R-ratio, the HLD-NAC model and surfactant film flexibility arguments.

Highlights

Microemulsions are isotropic, thermodynamically stable dispersions consisting of water, oil and one or more types of surfactants, in which the formed structures are in the order of 100–102 nm in size [1]
A plausible explanation is that a fraction of the pore throats are similar or smaller in size as the structures the microemulsion consists of, thereby increasing the required pressure for the microemulsion to flow through the rock
Using small angle neutron scattering techniques, the microemulsion structure sizes were quantified as function of salinity, co-solvent content, and alkane carbon number

Summary

Introduction

Microemulsions are isotropic, thermodynamically stable dispersions consisting of water, oil and one or more types of surfactants, in which the formed structures are in the order of 100–102 nm in size [1] These systems are incredibly versatile, and are studied in a wide variety of research areas such as drug delivery [2], enhanced oil recovery (EOR) [3,4], the potential use of supercritical CO2 as a sustainable solvent [5], and one can find commercial applications in toiletries, cosmetics, paints and many more [6,7]. The determination of self-diffusion coefficients can be achieved with nuclear magnetic resonance (NMR) [13], or when information about the viscosity of microemulsions is required, conventional rheology can be applied. All of these characteristics are generally dependent on temperature and pressure

Methods

Results

Conclusion