Ideal (Langmuir-type surface and electric properties of tert- isopropylphosphine oxide at the air/water interface

Abstract

The surface pressure (π) and electric surface potential (ΔV) of tert-isopropylphosphine oxide (i-Pr₃PO) were measured at the air/water interface. Solutions matching the criterion of “surface-chemical purity” were investigated. Measurable effects on π and ΔV are revealed at 1 x 10^-4 mol/dm³ i-Pr₃PO and extend throughout three concentration decades. The initial rise of the surface potential with the concentration is several times steeper compared to that of the surface tension. The dependence of the surface tension on the concentration of i-Pr₃PO is well described by a Langmuir-type isotherm. Furthermore, the relation between ΔV and the surface concentration (Γ_L) calculated using the Langmuir isotherm is linear. Such consistency with the Langmuir-type adsorption throughout the entire surface activity range is an uncommon feature among soluble surfactants. The ideal surface properties of i-Pr₃PO are mainly due to the ellipsoidal hard-sphere-like structure of the molecule delimited by the isopropyl chain, which determines the closest packing density of the adsorbate. This is justified by the minimum cross-sectional area of about 80 Ų/molecule adsorbed, which corresponds to a sphere delimited by the length of the fully extended isopropyl chain (5 Å). The other reason for the ideal surface behavior is the compatibility of the dimensions of an i-Pr₃PO molecule with an average water cluster (of 13 H₂O). The slope (ε₀ dΔV/dΓ_L) determines the effective dipole moment (per molecule) μ⊥/ε_s= 1.2 ± 0.1 D. This relatively high value of μ⊥/ε_s is a result of the dipole moments of the highly polar ≡P = O head group and of six —CH₃ terminal groups. Owing to the radial distribution of the terminal groups around the head group, the effective dipole moment is practically independent of the molecule’s conformation at the surface.