Conformability Of The Frumkin /langmuir Model
CONFORMABILITY OF the Frumkin /Langmuir MODEL
to ADSORPTION OF amphiphiles at THE AIR/WATER INTErface – RELATION TO MOLECULAR STRUCTURE
AND THERMODYNAMICS
W. Barzyk1, B. Jachimska1, P. Warszyński1,
A. Pomianowski1, K. Lunkenheimer 2
1Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,
Niezapominajek ul. 8, 30-239 Krakow, Poland
2Max-Planck-Institut für Kolloid- und Grenzflächenforschung, Potsdam
Changes in course of the surface pressure (p) vs. concentration (c) isotherms of simple amphiphiles ("surface-chemically" pure) at the air/solution interface are analysed in respect to amphiphile’s molecular structure. The studied surface active species differed in: a) the head group structure at comparable paraffinic chain length, C8-C10 (n-decanoic acid, n-decanol, n-octyl- and n-decyl-b-D-glucopyranosides), b) length of paraffinic chain (n-alcanols of the carbon number: C4, C6, C8, C9), or c) by possessing the branched chain (tert. iso-propyl phosphine oxide). The adsorption isotherms were determined using de Noüy ring method. The general Frumkin/Langmuir isotherm was fitted to the results, obtaining a good conformability of the model with a slight deviation appearing in the initial concentration range. It is discussed with respect to the measurement precision and reproducibility.
Values of the best fit Frumkin parameters (the – maximum surface excess, the Hs – enthalpy of surface mixing, and the b – referred to as the adsorption equilibrium constant) are analysed in respect to their conformability to the Traube’s rule. Change of the standard free energy of adsorption, – DG0Q/2, and of the standard entropy, DS0Q/2 – referred to the half surface coverage, Q/2, is evaluated from the surface tension results. The values are compared to cross-sections of hard core diameters of different amphiphiles (Chem3D software). Courses of the adsorption isotherms are conveniently compared by using the normalized concentration scale with respect to the half surface coverage concentration: cnorm = c/cQ/2.
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