The study of Langmuir
monolayers incorporating biomimetic and bioactive
substances plays an important role today in assessing the properties
and quality of the molecular films for potential biomedical applications.
Here, miscibility of binary and ternary monolayers of phospholipid
(dioleoyl phosphatidylcholine, DOPC), immunosuppressant (cyclosporine
A, CsA), and antioxidant (lauryl gallate, LG) of varying molar fractions
was analyzed by means of the Langmuir technique coupled with a surface
potential (ΔV) module at the air–water
interface. The surface pressure–area per molecule (π–A) isotherms provided information on the physical state
of the films at a given surface pressure, the monolayer packing and
ordering, and the type and strength of intermolecular interactions.
Surface potential–area (ΔV–A) isotherms revealed the molecular orientation changes
at the interface upon compression. In addition, the apparent dipole
moment of the monolayer-forming molecules was determined from the
surface potential isotherms. The obtained results indicated that the
film compression provoked subsequent changes of CsA conformation and/or
orientation, conferring better affinity for the hydrocarbon environment.
The mutual interactions between the components were analyzed here
in terms of the excess and total Gibbs energy of mixing, whose values
depended on the stoichiometry of the mixed films. The strongest attraction,
thus the highest thermodynamic stability, was found for a DOPC–CsA–LG
mixture with a 1:1:2 molar ratio. Based on these results, a molecular
model for the organization of the molecules within the Langmuir film
was proposed. Through this model, we elucidated the significant role
of LG in improving the miscibility of CsA in the model DOPC membrane
and thus in increasing the stability of self-assembled monolayers
by noncovalent interactions, such as H-bonds and Lifshitz–van
der Waals forces. The above 1:1:2 combination of three components
is revealed as the most promising film composition for the modification
of implant device surfaces to improve their biocompatibility. Further
insight into mechanisms concerning drug–membrane interactions
at the molecular level is provided, which results in great importance
for biocoating design and development as well as for drug release
at target sites.