Monolayer/Bilayer Equilibrium of Phospholipids in Gel or Liquid States: Interfacial Adsorption via Monomer or Liposome Diffusion?

Phospholipids (PLs) are widely used in the pharma industry and a better understanding
of their behavior under different conditions is helpful for applications such as their use as medical
transporters. The transition temperature T$_m$ affects the lipid conformation and the interfacial tension
between perfluoroperhydrophenanthrene (PFP) and an aqueous suspension of 1,2-dipalmitoyl-sn-
glycero-3-phosphatidylcholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC), as
well as a mixture of these PLs with cholesterol. Interfacial tensions were measured with the Du Noüy
ring at quasi-equilibrium; the area per molecule was calculated according to the Gibbsian approach
and a time-dependent tension gradient. Results show that the time tε to reach quasi-equilibrium
was shorter when the temperature was above T$_m$, indicating a faster adsorption process (t$_{ε,DPPC,36^◦ C}$
= 48 h, (t$_{ε,DPPC,48 ^◦ C}$ = 24 h) for PL in the liquid crystalline state than in the gel state (T < Tm). In
addition, concentration-dependent results of the interfacial tension revealed that above the respective
T$_m$ and at all concentrations c > 0.1 mM, the average minimum interfacial tension for DPPC and DSPC
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(14.1 mN/m and 15.3 mN/m) does not differ significantly between those two lipids. Equilibrium
between monolayers and bilayers shows that for T < T$_m$, surface pressures ∏ ≈ 31 mN/m are reached
while for T > T$_m$, ∏ ≈ 41 mN/m. Mixtures with cholesterol only reach ∏ ≤ 31 mN/m T$_m$, with no
significant difference between the two PLs. The higher interfacial tension of the mixture indicates
stabilization of the liposomal conformation in the aqueous phase by the addition of cholesterol. The
high diffusion coefficients show that adsorption is mainly based on liposomes.