Surface activities of tertiary amine local anesthetics at air/water interface in the presence and absence of phospholipid monolayers

Abstract

Adsorption of procaine and tetracaine to the dipalmitoyl phosphatidylcholine monolayers at the air/water interface is analyzed in terms of two types of interaction: (1) between the phospholipid molecules and the ligand molecules, and (2) among the ligand molecules themselves. The presence of the phospholipid monolayer increases the surface concentration of the anesthetics. The interaction energy, ω AB, between the phospholipid molecules and the anesthetic molecules at the interface accounts for this excess adsorption. The values were −2.95 kT for procaine and −2.99 kT for tetracaine where k is the Boltzmann constant and T = 298 K. The adsorption of the local anesthetics to the interface was cooperative. The interaction energy, ω AA, between the anesthetics molecules on the surface determines the cooperativity. The values were −0.056 kT for procaine and −0.397 kT for tetracaine, where T = 298 K. This parameter determines the slope of the curve plotted relating the surface concentration (Γ) and the logarithm of the bulk concentration (log C). When | ω AA/ kT| ⩾ 1, the adsorption follows the phase-transition. A parameter K A, which is related to the difference of the free energy of anesthetics between the surface and the bulk molecules, locates the take-off point of the adsorption curve at the log C axis. The values were 2.15 · 10 3 for procaine and 7.00 · 10 3 for tetracaine.