Surfactant solutions confined in homogeneous and Janus-like slits

Abstract

We study the behavior of aqueous surfactant solutions in the bulk phase and in slit-like pores by molecular dynamics. Adsorption and self-assembly of nonionic surfactants C$_7$E$_3$ that mimic alkyl poly(ethylene oxide) molecules are investigated. We consider pores with the same walls and Janus-like slits. The individual walls are inert, hydrophilic, or hydrophobic. We focus on the morphology of the surfactant solution confined in different slits. The influence of a pore type and its width is discussed. The aggregative adsorption of surfactants was found. Our simulations show that in slits surfactants assemble into structures that do not occur in the bulk phases.

Figures (9)

• Figure 1: (Colour online) Example of equilibrium configuration of aqueous C$_7$E$_3$ bulk solution. The red spheres correspond to the polar segments E, the green spheres represent the non-polar segments C, and the blue spheres are for the solvent (W). For greater clarity of the drawing, the solvent molecules are reduced.
• Figure 2: (Colour online) Examples of equilibrium configuration of aqueous C$_7$E$_3$ solution confined in homogeneous slits with inert walls (S1) and different pore widths $H^*=$ 15 (a), 10 (b) and 5 (c). The red spheres correspond to the polar segments E, the green spheres represent the non-polar segments C, the blue spheres are the solvent W, and the gray spheres are for the inert surface (S1). For greater clarity of the drawing, the solvent molecules were omitted.
• Figure 3: (Colour online) The segment density profiles along the z-axis for C$_7$E$_3$ confined in homogeneous slits with different walls and different pore widths $H^*=$ 15 (a), 10 (b) and 5 (c). The densities of segments E (C) are plotted as solid lines (dashed lines). The profiles for slits with: the inert walls (S1) are black, the hydrophilic walls (S2) are red and the hydrophobic walls (S3) are green. The insets show the fragments of the density profiles with greater accuracy.
• Figure 4: (Colour online) Examples of equilibrium configuration of aqueous C$_7$E$_3$ solution confined in homogeneous slits with hydrophilic walls (S2) and different pore widths $H^*=$ 15 (a), 10 (b) and 5 (c). The red spheres correspond to the polar segments E, the green spheres represent the non-polar segments C, the blue spheres are the solvent W, and the brown spheres are for the inert surface (S2). For greater clarity of the drawing, the solvent molecules were omitted.
• Figure 5: (Colour online) Examples of equilibrium configuration of aqueous C$_7$E$_3$ solution confined in homogeneous slits with hydrophobic walls (S3) and different pore widths $H^*=$ 15 (a), 10 (b) and 5 (c). The red spheres correspond to the polar segments E, the green spheres represent the non-polar segments C, the blue spheres are the solvent W, and the pea spheres are for the inert surface (S3). For greater clarity of the drawing, the solvent molecules were omitted.