## 3 Results  
### MGS and RDA Films at the Air/Water Interface  
Figure 1 shows the surface pressure/area isocycles of MGS (left panel) and RDA (right panel). MGS displayed non-collapsibility upon compression and almost no hysteresis loop as reported elsewhere [1; 2].  
RDA had similar behavior to MGS in line with prior data on the surface properties of sesame oil and castor oil containing ophthalmic nanoemulsions [13]. As previously demonstrated [1; 2], Brewster angle microscopy (Figure 2) revealed that at low \(\pi\) (\(\leq\)15 mN/m) MGS formed heterogeneous films which consisted of dark (monolayer thin) regions and multilayer thick aggregates (bright areas).  
Figure 1: Surface pressure-area isocycles of MGS (**a**) and RDA (**b**) films. Pure MGS films are formed by the spread from chloroform solution of 20.3 micrograms (1.723 \(\times\) 10\({}^{16}\) oil phase molecules) at the air/water surface. RDA layers are formed by gentle deposition of droplets of nanoemulsions corresponding to the delivery of 1.45 \(\times\) 10\({}^{16}\) molecules oil phase (see Table 1 for details).  
Upon further compression, the thick darker regions connected to a continuous multilayer mesh while the presence of the thin brighter monolayer regions decreased, and the monolayer patches almost disappeared at very low film areas. RDA films were more homogeneous and with increasing brightness (i.e., thickness) upon compression as typical for the behavior of layers formed by surface deposition of oil-in-water emulsions [13; 20].  
The in-plane reciprocal compressibility (or rigidity) modulus \({\rm C_{s}}^{-1}\) (Figure 3) of the films was calculated via Equation (1).  
\[{\rm C_{s}}^{-1}={\rm A_{\pi}}\left(-{\rm d}\pi/{\rm d}{\rm A}\right) \tag{1}\]  
where \({\rm A_{\pi}}\) is the area at the indicated \(\pi\). The inflexion points in the \(\pi/{\rm C_{s}}^{-1}\) dependencies indicate \(\pi\) at which significant changes in the surface film structure occur upon compression. [PERSON] and [PERSON] [21] suggested that \({\rm C_{s}}^{-1}<50\) mN/m, \(100<{\rm C_{s}}^{-1}<250\) mN/m and \({\rm C_{s}}^{-1}>250\) mN/m correspond to liquid expanded, liquid condensed, and solid states of the surface layer, respectively. Both RDA and MGS had \({\rm C_{s}}^{-1}<30\) mN/m which accords to liquid expanded molecular packing density at the film/water interface.  
Figure 3: Dependence of the reciprocal compressibility (or rigidity) modulus on the surface pressure of the MGS and RDA layers. The curves are obtained by analysis with Equation (1) of the compression parts of the isocylces in Figure 1.  
Figure 2: Brewster angle images (500 \(\upmu\)m \(\times\) 300 \(\upmu\)m) of MGS and RDA films.  
The inflection at 15 mN/m of the \(\pi\)/C\({}_{\mathrm{s}}\)\({}^{-1}\) curve of MGS reflected the enclosure of the thinner regions within the MGS films and the transition to exclusively duplex architecture of the film [1,2]. The \(\pi\)/C\({}_{\mathrm{s}}\)\({}^{-1}\) curve of RDA showed more complex behavior with inflexions at 3 mN/m and 14 mN/m which probably corresponds to the redistribution of the multiple polar (Povidone, Polyoxyethylene Castor Oil 10, Polyoxyl 40 Stearate, menthol) and non-polar constituents (SO) of the formulations between the aqueous subphase and the headgroup and acyl chain regions of the layers. Similar behavior of the in-plane reciprocal compressibility is also observed for other oil-enriched ophthalmic nanoemulsions [13,20].