# Interactions between Phase-Separated Liquids and Membrane Surfaces
## 6 Small Vesicles May Surround-or Act as Components of-Phase-Separated Condensates
### Vesicle Interactions with Complex Coacervates Depend on Several Physical Factors  
More recently, vesicle behavior in phase-separated systems has been investigated through interactions with droplets formed by complex coacervation. This process is an aqueous phase separation phenomenon in which a polycation and a polyanion form a dense phase driven by electrostatic interactions [88; 89] as described by Overbeek-Voorn theory [18]. The simplicity and biological relevance of such droplets have made them systems of interest as models for protocells and biomolecular condensates [80; 89]. The charge ratio of the coacervate (that is, the ratio of polycation to polyanion) impacts itsphysical properties: Coacervates with a high ratio (more polycation) have a more positive zeta potential [90; 91]. Vesicles have assembled at the surface of complex coacervates, much the same as in some ATPSs (Figure 4b). These vesicles did not fuse and did not impede exchange of RNA (a component of the coacervate) with the solution [80].  
Vesicle localization in relation to complex coacervates is dependent on several factors. A recent study indicates that vesicles have a general tendency to diffuse into a complex coacervate, as their component molecules generally contain positive, negative, and hydrophobic sections [91]. That study found that negatively-charged vesicles primarily remained at coacervate surfaces with a net positive charge, but the same did not hold for positively-charged vesicles and negatively-charged coacervate surfaces (Figure 4a,b) [91]. This finding is consistent with previous research which used negatively-charged vesicles to generate a vesicle coating on a coacervate [80; 90]. Vesicles with membranes in the gel phase rather than a liquid phase also formed a coating at the phase boundary independent of coacervate charge, presumably because they were less able to deform and so may not be able to enter the coacervate [91].  
Beyond forming a uniform, one vesicle-thick layer at the phase boundary and permeation of a coacervate, vesicles have also formed aggregates with coacervate proteins at the phase boundary or inside droplets. Negatively-charged vesicles formed aggregates at the phase boundary if they could interact with polycations in the dilute phase prior to localization (Figure 4d). Specific conditions that allowed uniformity of vesicle coating depended closely on a good \"fit\" between properties such as polyelectrolyte weight, charge density, charge ratio, and vesicle lipid composition [90]. At the interior of a coacervate, positively-charged vesicles could form fibrous aggregated structures with single-stranded oligonucleotides when the coacervate had a charge ratio less than or equal to one (that is, an equal or excess amount of polyanion relative to polycation) (Figure 4c) [91].  
Figure 4: Different observed arrangements of vesicles within or around a complex coacervate. These are illustrative examples; these spatial arrangements are not definite based only on the charge of the components present. Vesicles can (**a**) be internalized within a coacervate, (**b**) coat the surface of the coacervate, (**c**) form a fibrous internal structure with a coacervate component, and (**d**) form aggregates with a coacervate component at the coacervate surface.