Effect of molecular structure of amphiphiles on the surface pressure and electric surface potential isotherms at the air/solution interface

Surface pressure vs. concentration, π − c, and electric surface potential, ΔV − c, isotherms at free surface of aqueous solutions of selected amphiphiles were analyzed with respect to (i) size of the head group, (ii) length of paraffinic chain and (iii) presence of branched chain of the comparable number of C atoms. The Frumkin isotherm was fitted to the measured π − c dependencies and the results were examined as a function of concentration normalized with respect to the concentration of the half surface coverage, cn = c/cΘ/2, derived from the best fit Frumkin parameters. The largest variation in shape of the π vs. cn isotherms was found between amphiphiles possessing different head groups substituted with comparable paraffin chains (C8–C9). Their isotherms in the high coverage region (Θ > 50%) exhibited notably diverse slopes, dπ/d log cn, indicating significantly different surface excess of the amphiphiles. The minimum “cross-sectional areas” calculated from the fitted parameter Γ∞, as Amin = 1/NΓ∞, are slightly higher than the hard-core cross-sectional area of the most bulky part of free molecule which indicates a residual hydration of the saturated monolayers. The identical value of Amin = 27.7 Å2/molec. found for the n-alkanols series (C4–C9) is significantly greater than minimum cross-section area of hard core of the fully extended chain, ca. 21.3 Å2. On the other hand, the Amin area found for glucopyranosides (ca. 45 Å2/molec.) correlates with smaller cross-section of the “ellipsoidal” head group (ca. 40 Å2), suggesting its nearly perpendicular inclination to the interface plane in the saturated monolayer. The normalized electric surface potential isotherms, ΔV/ΔVmax vs. cn, deviate from the Frumkin Θ − cn dependencies, in particular, in the higher coverage range, Θ > 50%, which implies change of the effective dipole moment, μ⊥, and/or the local permittivity, ɛs, with Θ.