Penetration of trifluoperazine (TFPeZ), a phenothiazine tranquilizer, into mixed monolayers of dipalmitoyl lecithin (DPL) and cholesterol (CH) was studied by measurement of surface pressure (F)-mean molecular area (A^^
-34) curves. The amount of penetration (or adsorption) by TFPeZ was calculated by the modified Gibbs adsorption equation. Adsorption isotherms at a constant A^^
-34 showed a type of Langmuir adsorption. Saturated amount of adsorption (Γ
∞2) and Langmuir constant (k) were obtained by Langmuir plots. Correlation between those parameters and A^^
-34 at various mixing ratios of DPL and CH were discussed. With decreasing A^^
-34, Γ
∞2 decreased in any DPL-CH mixed monolayer. The relation between Γ
∞2 and composition of the monolayer at a constant A^^
-34 corresponded well with the state of mixed monolayer, i.e. Γ
∞2 is small in expanded monolayer rich in DPL (X
DPL>X
CH), and increases with decreasing X
DPL down to X
DPL=0.5. This increase corresponds to changes of the mixed monolayer state, from liquid expanded to solid condensed state. Below X
DPL=0.5, in which the mixed monolayer is fully solid, Γ
∞2 level is independent of the monolayer composition. Miscibility of penetrated TFPeZ with insoluble components, DPL and CH, was also discussed.
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