Monolayers on water

Abstract

Studies of erucic acid monolayers on aqueous copper chloride solutions, as a function of surface pressure, area per molecule, contact potential, pH (0 to 5), and copper ion concentration (0 to 3.3 × 10 −3 molar) have revealed the following relationship between the copper content of the film, the hydrogen ion concentration ( H b ), and the copper ion concentration ( C b ) in the subsolution: K 1 K b ≃ (Y/C b ) exp (− 2ϵψ/kT) . Here K 1 is the equilibrium constant for the reaction 2R + Cu ++ ⇄: CuR 2 + 2H + , where R represents an erucic acid molecule; K b is the equilibrium constant for the specific adsorption of Cu ++ at the interface; ϵ is the charge of an electron; k is the Boltzmann factor; T is the temperature; and φ is the electrical potential at the interface. rmY = ( N 2 N)(H b 2 4N)(0.5 − N 2 /N) 2 , where N 2 is the moles/cm. 2 of CuR 2 , and N is the moles/cm 2 of initially spread erucic acid (at pH b = 5.25, Y / C b ∼ 63). Monolayer equilibration with respect to Cu ++ causes a decrease in the average area per molecule which can be expressed by the relationship N 2 N ≈ (A 1 − A)(2A 1 − A 2 ) . Here N 2 / N is equivalent to the ratio of Cu ++ to erucic acid in the film, A 1 and A 2 are the area per molecule of R and CuR 2 , respectively (at a surface pressure of 5 dynes/cm.), and A is the average area of the mixture per molecule of initially spread erucic acid. On pure water, erucic acid monolayers collapse very slowly (∼0.0013 cm./sec. at a surface pressure of 19 dynes/cm.). With Cu ++ in solution the rate of collapse appears to be proportional to the CuR 2 content of the film with a maximum (∼0.8 cm./sec.) when the film is predominantly CuR 2 .