Properties of Fluid Interfaces: Insights from Molecular Thermodynamics

Fluid interfacial properties play an important role, for example in separation processes in chemical engineering, in nucleation in propulsion systems, and also in nature. The fluid interface is a region in which properties change smoothly over a few nanometers from its liquid bulk to its gas bulk value. Fluid interfacial properties are complex and experimentally hard to examine due to the small length scale and fluctuations present at the interface (both spatial and temporal). In this contribution, results from recent studies from our group [1-11] on equilibrium properties of fluid interfaces of molecular fluids are presented that were obtained using methods based on molecular thermodynamics, namely molecular dynamics (MD) simulations, density gradient theory (DGT) + molecular-based equations of state, and (in some cases) experiments. Different types of fluid interfaces are considered, i.e. vapor-liquid, liquid-liquid, and vapor-liquid-liquid interfaces. The focus is on vapor-liquid interfaces. Both model fluids and real substance fluids were studied. The model fluids were used to study the effect of the influence of molecular interaction parameters on interfacial properties; the real substances were chosen as they are applicable for industrial processes.
This work contributes on the understanding of the fundamentals of fluid interfacial properties and progresses modelling methods. Multiple features and their interrelations are addressed: (i) general features of the monotonicity behaviour of profiles of thermodynamic properties across fluid interfaces are discussed [1] by critically revisiting statements made in the seminal monograph of Rowlinson and Widom ’Molecular Theory of Capillarity’. It is thereby shown that two non-monotonicity features can be distinguished: (a) the enrichment of components at the interface of mixtures [1,2] and (b) the oscillatory layering structure (OLS) of particles at fluid interfaces [1,3]; (ii) the enrichment, which is suspected to influence the mass transfer across interfaces [4,5], is systematically studied regarding its dependency on temperature and composition in various binary mixtures [2,6,7]; (iii) a new method is presented for determining the OLS. The results are used to confirm a link between the OLS and the Fisher-Widom line. The results are found to be in good agreement with results from the literature; (iv) the influence of long-range interactions on the monotonicity behaviour and other mixture interfacial properties is discussed [8]; (v) it is shown that conformal solution theory can be extended and applied for the modelling properties of fluid interfaces of mixtures – including the enrichment [9]. It is thereby shown that interfacial properties are directly linked to the mean interactions of the liquid phase [9]; (vi) the relation of the enrichment and the relative (Gibbs) adsorption is discussed pointing out important differences of these two closely related properties [1,2,10]. For the latter, results from both MD, DGT, and pendant drop experiments are presented [10]; (vii) the relation of interfacial properties and bulk phase equilibrium properties will be discussed using a new type of plot that combines the global phase diagram (critical lines, VLLE lines, etc.) and iso-lines for interfacial properties, which condenses the phase and interface information of a binary mixture and provides new insights into their intimate relations; (viii) it is shown that the enrichment of components at fluid interfaces can be understood as a prewetting, where a third fluid phase heterogeneously nucleates at the interface, which starts already at conditions alongside the actual three-phase equilibrium line [7,11]; (ix) an empirical enrichment model is presented that was developed using model fluid data alone using only bulk phase property descriptors. It is shown that this model can reliably predict the enrichment also for complex real substance mixtures (appr. 2000 data point from this work and retrieved from about 100 publications from the literature) [2]; (x) the relation of intersection points of profiles in the interfacial region and critical points is discussed, which can be related to the rectilinear diameter [3,6,7,11].
This work contributes to a large variety of properties and interfacial equilibrium effects of fluid interfaces. These interfacial properties are a direct result of the large gradients of the profiles of thermodynamic properties across the interface. These profiles connect the bulk values of the properties. It is therefore obvious that the interfacial properties are closely related to the respective bulk phase properties, which is demonstrated here by multiple examples (i)-(x). The properties and interfacial effects discussed in this work contribute to a picture in which the properties of the interfacial region mirror bulk properties of the system, albeit in the way of a fun-house mirror in which the bulk behavior is distorted by the strong gradients. Moreover, the interrelations of these interfacial properties and effects will be discussed as well as the question, which physical model capture which of these effects accurately. These interrelations, e.g. between the enrichment and the wetting transition as well as between different non-monotonicity effects, contribute to a better understanding of the nature of fluid interfacial properties and their relevance for different applications and conditions. Also, important new insights regarding (to date unknown and overlooked) artifacts in modelling approaches are addressed. All this demonstrates the complexity and richness of fluid interfacial properties. The complementary modelling approach using MD and DGT (and pendant drop experiments in some cases), however, provides a wealth of novel insights. Moreover, open research questions in the field of fluid interfacial properties and their modelling that should be addressed in future work are pointed out.
References
[1] S. Stephan et al.: Fluid Phase Equilib. 564 (2023) 113596.
[2] S. Stephan and H. Hasse, Int. Rev. Phys. Chem. 39, 3 (2020) 319-349.
[3] S. Stephan et al.: J. Phys. Chem. C 122 (2018) 24705.
[4] S. Stephan et al., Mol. Phys. 119, 3 (2021) e1810798.
[5] S. Stephan et al.: Chem. Eng. Trans. 69 (2018) 295.
[6] S. Stephan et al., J. Chem. Phys. 150 (2019) 174704.
[7] J. Staubach and S. Stephan, J. Chem. Phys. 157 (2022) 124702.
[8] S. Stephan and H. Hasse, Mol. Phys. 187 (2020) 1-14.
[9] S. Stephan and H. Hasse, Phys. Rev. E 101 (2020) 012802.
[10] S. Stephan et al., Fluid Phase Equilibr., 518 (2020)112583.
[11] S. Stephan and H. Hasse, Phys. Chem. Chem. Phys. 22 (2020) 12544.