Pressure-Responsive, Surfactant-Free CO₂-Based Nanostructured Fluids

Abstract

Microemulsions are extensively used in advanced material and chemical processing. However, considerable amounts of surfactant are needed for their formulation, which is a drawback due to both economic and ecological reasons. Here, we describe the nanostructuration of recently discovered surfactant-free, carbon dioxide (CO2)-based microemulsion-like systems in a water/organic-solvent/CO2 pressurized ternary mixture. “Water-rich” nanodomains embedded into a “water-depleted” matrix have been observed and characterized by the combination of Raman spectroscopy, molecular dynamics simulations, and small-angle neutron scattering. These single-phase fluids show a reversible, pressure-responsive nanostructuration; the “water-rich” nanodomains at a given pressure can be instantaneously degraded/expanded by increasing/decreasing the pressure, resulting in a reversible, rapid, and homogeneous mixing/demixing of their content. This pressure-triggered responsiveness, together with other inherent features of these fluids, such as the absence of any contaminant in the ternary mixture (e.g., surfactant), their spontaneous formation, and their solvation capability (enabling the dissolution of both hydrophobic and hydrophilic molecules), make them appealing complex fluid systems to be used in molecular material processing and in chemical engineering.

Hydrogen Bond Networks in Binary Mixtures of Water and Organic Solvents

“…The Raman shift of the centroid ν̅ OH (centroid position) of the OH stretching vibration is computed as generally described in ref (54)In eq 3, we consider the molar OH Raman spectrum of the mixture s (ν̃ S ) = s mix OH (ν̃ S ) for the computation of its centroid position ν̅ OH = ν̅ mix OH .We consider the partial molar spectra of the constituents of the mixture s (ν̃ S ) = s̅ i ( j ) OH (ν̃ S ) for the computation of their centroid position ν̅ OH = ν̅ i ( j ) OH .We consider the ideal molar mixture spectra s (ν̃ S ) = s mix OH,ideal (ν̃ S ) for the computation of ν̅ OH = ν̅ mix OH,ideal .…”

A fast and remote screening method for sub-micro-structuration in pressurized mixtures containing water and carbon dioxide

“…Some of us were able to show in the ternary systems CO 2 /acetone/water and CO 2 /acetonitrile/water that the mixture organizes into domains with high water concentration and others with small water concentration leading to an overall smaller effective distances between the water molecules in the water-concentrated domains. [6,18] These regimes can be referred to as "water-concentrated" and "water-depleted" (or "acetonitrile/CO 2 -concentrated") sub-microdomains", respectively. The highest water concentration where the structuration starts is defined at the beginning of a decreasing vwater value.…”

“…In sub-micro-structured fluid systems, such as in microemulsions [1][2][3][4][5] or other non-surfactant containing systems [6][7][8][9][10][11], the characteristic size of structural heterogeneities is smaller than the wavelengths of visible light. Thus, the structured system itself appears to be optically completely transparent.…”

“…The identification of sub-micro-structures in fluid systems in MCS is based on an approach that has already been applied in a similar manner in high-pressure variable volume view cells (HPVVVC) [6,18] It is based on the analysis of the shape of the symmetric stretching vibration Raman band of water. Thus it works in systems that contain water, or other compounds that develop hydrogen bonds, such as alcohols or ammonia.…”

“…It is known that the nanostructuration of this system is tunable via temperature. [6] What is special and what provides firstly the high degree of flexibility and secondly the fast screening possibility, both with respect to the variation of the operational conditions, is that the analyzed mixture is continuously conveyed through a fused silica microcapillary at elevated pressure [22,23]. The general advantages of using continuously flow-through fused silica microcapillaries instead of conventional large-volumes-containing high-pressure view cells, as we used previously [6,18], are:…”