| Research Abstract |
Surface pressure (pi) - and surface potential (DELTAV) - area (A) isotherms were obtained for monolayrs of alkylammonium perfluorohexanoates of four different alkyl chains (m) on water and 4.4 M NaCl solutions as a function of molecular area of various temperatures by the Langmuir and the ionizing electrode methods. Some pi-A curves of these logn-hydro-short-fluoro chain salt showed three phase transition points. Judging from the surface potential and the apparent molar quantity, the first was assigned to the transition from the expanded phase to the condensed phase, the second from the condensed phase to another condensed phase, and the third from the latter phase to one other condensed phase. The phase transition pressure pi^<eq> explicitly changes with varying chain lengths of the hydro-carbon cation. The pi-A and DELTAV-A isotherms might suggest that the perfluorocarbon and hydrocarbon chains of alkylammonium perfluorohexanoates are aligned with the former dipping in the substrate.
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The surface potential behavior is totally positive : the present combinations of hydrocarbon and perfluorocarbon are critical. This originates from the fact that the long perfluorocarbon chain of the counterion contributes a negative change in surface potential on mono-molecular film compression owing to its electronegative fluorine atoms. This negative change cannot be expected with ionic hydrocarbon surfactants having a hydro-carbon counterion. Surfactants composed of both hydrocarbon and fluorocarbon portions should be investigated in more detail for the development of functional amphiphiles.
To obtain more generalized information concerning the two dimensional molecular interaction between fluorocarbon and hydrocarbon chains at the air/water interface, we investigated the monolayr behavior of a hydrocarbon acid, tetradecanoic acid (HC14), with five fluorocarbon fatty acids (FCn ; n=10,12,14,16,18) on a substrate of pH1.0 as a function of compositions. Data for the mixed monolayrs were analyzed in terms of the additivity rule. Judging from classification of two-dimensional phase diagrams, our phase diagrams might be classified into three types. The first is a positive azeotrope type ; the combinations of tetradecanoic acid with perfluorodecanoic acid and with perfluorododecanoic acid seem miscible with each other. The second is a completely immiscible type ; the combination of perfluorotetradecanoic acid and tetradecanoic acid. The third might be a eutectic type, to which the combinations of HC14 with FC16 and with FC18 are assigned ; they are miscible in the expanded state but immiscible in the condensed state. Furthermore, the data on the mean molecular area, the surface dipole moment, and the phase diagrams constructed enabled us to estimate themolecular orientation of the respective acids in the mixed monolayr state. Less
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