Study of molecular interactions between a phospholipidic layer and a pH-sensitive polymer using the Langmuir balance technique.

Molecular interactions between a terminally alkylated pH-sensitive N-isopropylacrylamide copolymer DODA-poly(NIPAM-co-MAA) and a monolayer of distearoylphosphatidylcholine (DSPC) at the air/water interface are investigated using the Langmuir balance technique. The compression isotherms ofthe copolymer monolayer at the air-water interface confirm that the copolymer undergoes a structural transition with a change in pH ranging from an extended coil state at neutral pH to a collapsed globular state at a pH corresponding to the pH of the polymer phase transition. Adsorption kinetics of DODA-poly(NIPAM-co-MAA) in the DSPC monolayer is analyzed using a first-order kinetics model allowing an effective interaction area Ax between DSPC and DODA-poly(NIPAM-co-MAA) molecules to be evaluated. The results clearly indicate that the interaction area increases with a decrease in pH. The results also suggest that the penetration of the DODA-poly(NIPAM-co-MAA) within the phospholipid monolayer is enhanced by a decrease in pH which causes a change in the copolymer structure and an increase in specific attractive interactions between the copolymer and the phospholipid. Therefore, the copolymer can trigger the destabilization or rupture of the phospholipidic layer through a simple variation in its structure associated with a variation in molecular interactions when coupled or inserted within the membrane. This study greatly supports the prospects of the copolymer-functionalized liposomes as stable and tunable carrier systems for in vivo applications in drug delivery.