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Titration of Monolayers

E. D. Goddard ; Lever Brothers Company, Edgewater, New Jersey, U.S.A.

Puni tekst: engleski pdf 19.178 Kb

str. 13-30

preuzimanja: 62



The early work on the effect of pH on ionizable materials at
aqueous interfaces and the consequent development of the concept
of an effective pH in the interface region, which differs from tnat
in aqueous bulk phase, are outlined. Properties as a function of pH
of monolayers of a variety of surfactant types, viz., weak and
nonelectrolytes (fatty acids, amines, amine oxides, alcohols, ethers),
intermediate strength electrolytes (alkyl phosphates), strong electrolytes
(alkyl sulfates and alkane sulfonates, both as spread a:nd
adsorbed monolayers) and two zwitterionic materials, are presented
in some detail. Use is made of the approaches of Schulman and
Hughes and of Davies for analysis of the results which are shown
generally to be consistent with a shift towards weakness of the
monolayer electrolyte because of the charge developed during its
ionization; exceptions are the zwitterionic materials. Points of
interest are (a) the need to invoke acid-soap formation to explain
the surface potential- pH data of close-packed fatty acid monolayers;
(b) a prevalent tendency for most of the anionic or nonionic
monolayers to form oxonium ions at \'ery low pH; (c) the relatively
expanded nature of the uncharged form of monolayers of the highly
dipolar amine oxide compared to its ionized form; (d) definite indications of »weakness« in both spread and adsorbed films of long
chain sulfates and alkane sulfonates. A brief section on the influence
of pH on polyelectrolyte monolayers is included.

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