Most existing thermodynamic models for calculating liquid-liquid phase separation effectively simulate bulk matter and neglect surface/interfacial effects, which can be important at the small sizes relevant in aerosol particles. This manuscript presents a model that is able to incorporate these surface/interfacial effects and evaluates four different methods of computing interfacial tensions. This manuscript represents an important contribution to the literature and I have only a few minor comments, described below:

Scientific points:

1. Line 90 gives a constraint on the value of φ, but is there any independent method of estimating φ independent of comparison with experimental values of σ^αβ, or is this essentially a purely empirical factor? Some additional discussion of this parameter would he helpful.
2. What a Guggenheim interface is, which is mentioned on lines 124 and 205, should be briefly explained.

Minor presentation points:

1. The first paragraph of the results section, lines 287 to 297 do not really seem to contain results, but rather some definitions and background information that would benefit the reader to see earlier in the manuscript. I would suggest that this section/information be moved to the introduction section.
2. In the paragraph starting on Line 320, the authors switch back and forth between referring to the different approaches by name and by equation number. For the sake of the reader, it would be helpful to stick to one consistent naming throughout the manuscript, rather than switching back and forth. Related, the shorthand ‘Butler equation’ is used on line 338 for ‘Butler equation with geometric mean activity coefficients’ (line 323), but the figure just uses ‘Geom. Mean. Act. Coeffs.’ Consistency would improve clarity.

In this manuscript, Schmedding and Zuend present a mathematical framework for predicting the surface and interfacial tensions of a droplet with liquid-liquid-vapor equilibrium, and they compare their predictions with several case studies and comparisons to literature. They provide thorough documentation of their methodology, both in the main text and in the supplement. The authors draw from classical relationships for interfacial tension, propose a new relationship, and test all for their predictive ability. This paper is an important contribution, and publication is recommended.

Some minor suggestions on presentation include:

1. On line 130 or elsewhere, it may be good to briefly describe how the partial molar area of each component is determined in the studied systems.

2. On lines 243-247, it could be better to have these equations as displayed, numbered equations on separate lines, as it could be difficult for a general reader to easily parse these in-line equations.

3. In Fig 3, Equation 15 could be referred to in the caption for the reader to be reminded of how σ^s,∗ is defined.