The effect of acids on dipalmitoyl phosphatidylcholine (DPPC) monolayers and liposomes

Dipalmitoyl phosphatidylcholine (DPPC) is the most abundant phospholipid in lung surfactant, primarily responsible for the reduction of surface tension to near 0 mN/m during expiration. In addition, DPPC liposomes are also used for drug delivery. In this study, the effects of hydrochloric acid, nitric acid and sulphuric acid on various properties of DPPC was investigated. The surface pressure versus area isotherms of DPPC monolayers at different concentrations of acid were obtained using a Langmuir–Blodgett trough. Distinct changes in the shapes of DPPC isotherms were observed in presence of acid. The lift off area for DPPC control at pH 7.4 was 58 Å2. The lift off area for DPPC was 105, 100 and 95 Å2 at concentration of 15.5 × 10−2 mol/dm3 of nitric acid, sulphuric acid and hydrochloric acid respectively. Collapse plateaus were longest in presence of nitric acid compared to hydrochloric acid and sulphuric acid. The nitric acid treated liposomes showed improved adsorption as evidenced by lower surface tension values achieved within 1 s at concentrations ranging from 3.1 × 10−6 to 15.5 × 10−4 mol/dm3 having pH 6.2–2.9. The inhibitory concentrations of nitric acid which significantly increased the average surface tension values achieved within 1 s were 3.1 × 10−3 to 15.5 × 10−2 mol/dm3 (pH 2.6–1.0). The sulphuric acid treated liposomes showed improved adsorption at concentrations of 3.1 × 10−6 to 3.1 × 10−2 mol/dm3 (pH 6.1–1.7). The adsorption was significantly inhibited at pH 1.4–0.4 achieved with sulphuric acid. The HCl-treated liposomes showed an improved adsorption at concentrations ranging from 3.1 × 10−6 to 15.5 × 10−2 mol/dm3 (pH 6.4–1.3). The average surface pressure value achieved within 1 s at inhibitory concentration of 3.1 × 10−1 mol/dm3 of HCl at a pH of 0.83 was 10.1 ± 2.2 mN/m. Acids did not affect the compressibility of DPPC. Increased leakage of calcein from DPPC liposomes at decreasing pH suggested increased permeability of DPPC liposomes under acidic conditions. The results of this study may have implications for pH dependent drug delivery in acidic conditions as well as for lung surfactant dysfunction in acid lung injury.