Exploring parameter space effects on structure-property relationships of surfactants at liquid-liquid interfaces

Abstract

The ubiquitous use of surfactants in commercial and industrial applications has led to many experimental, theoretical, and simulation based studies. These efforts seek to provide a molecular level understanding of the effects on structuring behavior and the corresponding impacts on observable properties (e.g., interfacial tension). With such physical detail, targeted system design can be improved over typical techniques of observational trends and phenomenological correlations by taking advantage of predictive system response. This research provides a systematic study of part of the broad parameter space effects on equilibrium microstructure and interfacial properties of amphiphiles at a liquid-liquid interface using the interfacial statistical associating fluid theory density functional theory as a molecular model for the system from the bulk to the interface. Insights into the molecular level physics and thermodynamics governing the system behavior are discussed as they relate to both predictions qualitatively consistent with experimental observations and extensions beyond currently available studies.

Hydrophobic and hydrophilic interactions in aqueous mixtures of alcohols at a hydrophobic surface

“…For surfactants, increasing the size of the hydrophobic tail length (equivalent to going from methanol to propanol) increases the effectiveness of surfactant in reducing the interfacial tension. 60 Comparison of the structure of 1-and 2-propanol is interesting because they both have the same size of hydrophobic group but in 1-propanol the hydrophilic segment is an end segment while in 2-propanol it is a middle segment. At low concentrations, the two hydrophobic segments of 1-propanol can pack the surface well keeping the hydrogen bonding network far from the surface.…”

“…It is known that a higher bulk concentration of two tail surfactants (which has a structure like 2-propanol) is required to achieve the same interfacial tension as a one tail surfactant (which has a structure like 1-propanol). 60,61 Another way to look at how structuring affects the properties is to see the interfacial tension varying with surface mole fraction. We define the surface mole fraction of the alcohols as the average mole fraction of the alcohol in the first three alcohol molecular layers next to the hydrophobic surface (∼10 Å).…”

Modeling micelle formation and interfacial properties with iSAFT classical density functional theory

“…Surfactants, being amphiphilic, are modeled as a chain built of oil-like segments and water-like segments. 27,33,44 The comparison of vapor pressures and saturated liquid densities between results from iSAFT and experiments is shown in Figure 3. For the segments in the head of surfactants, the water diameter and Lennard-Jones energy are used.…”

“…DFT has shown its strength in modeling inhomogeneous and complex fluids and excellent agreement with molecular simulations and experiments for a variety of systems, including the phase behavior of associating fluids under confinement, 65,66 the behavior of polymer brushes, [17][18][19][20][21] the phase behavior and structure of block copolymers, [22][23][24] the interfacial properties of oil/water systems, 25,26 and the impact of surfactant architecture on interfacial properties. 27 The theory can be computationally more efficient than molecular simulations since density fields rather than trajectories of individual molecules are calculated, and the method takes advantage of system symmetry. More importantly, due to the limitation of computing resources, molecular simulation cannot model true thermodynamic equilibrium between surfactant monomers and aggregates.…”

Extensions of the SAFT model for complex association in the bulk and interface

“…In this section, we will focus on inhomogeneous systems with the presence of surfactants and the application of interfacial Statistical Associating Fluid Theory or iSAFT [25,68] for such systems. iSAFT has been applied to block copolymers [69,70], polymer brushes [71][72][73], associating molecules [74][75][76][77][78], and surfactants [79,80]. Here we consider micelle formation.…”