Low Interfacial Free Volume of Stubby Surfactants Stabilizes Water-in-Carbon Dioxide Microemulsions

Abstract

A novel paradigm for the design of surfactants for water/CO 2 (W/C) microemulsions is presented. The paradigm focuses on the fractional free volume (FFV) available to CO 2 at the interface. The FFV is an unambiguous geometric parameter that is calculated directly from surfactant tail geometry and surface coverage. We present an analysis of recent experimental studies indicating that low FFV is a necessary, although not sufficient, condition for W/C microemulsion formation and that both microemulsion and macroemulsion stability correlate qualitatively with FFV. This correlation is understood by noting that a decrease in FFV tends to favor the factors that stabilize W/C microemulsions, namely, decreased interfacial tension, reduced overlap between tails (weakening attractive interdroplet interactions), and increased interfacial curvature. These factors are more challenging to achieve in CO 2 than in alkane solvents, implying that low FFV is especially important for W/C microemulsions.

Economical and Efficient Hybrid Surfactant with Low Fluorine Content for the Stabilisation of Water-in-CO2 Microemulsions

“…Much effort has been devoted to determine how changes in surfactant molecular structure can affect physicochemical properties of aqueous surfactant solutions and w/c phase stability [13,[18][19][20][21]. A traditional concept of molecular packing parameter (p) [22] has been widely invoked in colloid studies to explain the correlation between the molecular structure and the surfactant self-assembly.…”

Interfacial phenomena at the compressed co2-water interface

“…Moreover, united atom models do not account for the significant interaction between the CO 2 quadrupole moment and the surfactant tail-group local dipoles. This interaction plays an essential role in the solvation of species in CO 2 and is therefore expected to significantly affect the overall monolayer organization (da Rocha, et al, 2004). It is also known from experimental observation that replacing the sulfosuccinate surfactant head group by phosphate, as the tail group is kept constant, has the effect of reducing the pressure required to stabilize a W/C microemulsion of about 10 MPa.…”

“…While for the most part there seems to be a consensus regarding the use of detailed models for both solvents (water and CO 2 ), few research projects employ fully atomistic models to describe the chemistry of the surfactant (da Rocha, et al, 2002, Stone, et al, 2004. This is a surprising observation given the significant impact that subtle changes in surfactant architecture have on surfactant activity at the C|W interface , Steytler, et al, 2001.…”

“…Surfactants with a lower surface tension at the critical aggregation concentration (cac) at the airwater (A|W) interface seem to be more interfacially active at the C|W interface (Eastoe, et al, 2002), than those that provide a large free fractional volume accessible to CO 2 in the monolayer region (da Rocha, et al, 2003, Stone, et al, 2004. A detailed knowledge of the macroscopic properties and molecular level details of the binary and surfactant modified C|W interface is essential to explain these paradigms, thus providing a rational and intuitive framework for surfactant design.…”

Effect of surfactant headgroup on low-fluorine-content CO2-philic hybrid surfactants

“…The role of the surfactant chemical structure in w/c phase stability has been a central issue in CO2 studies and research has been conducted to determine how changes in surfactant molecular structure can affect physicochemical properties of surfactant aqueous solutions and w/c phase stability [13,[18][19][20][21]. A traditional concept of the molecular packing parameter (p) [22] has been widely invoked to explain the correlation between the molecular structure and the surfactant self-assembly.…”