Quaternary mixtures of water (A), an oil (B), a nonionic amphiphile (C), and an appropriately chosen fourth component offer an opportunity for searching for tricritical points (tcp) at atmospheric pressure. It is shown that for reaching a tcp, one has to couple anAndash;Bndash;Cmixture that shows the phase sequence 2lowbar;rarr;3rarr;2macr; with rising temperature, with a second ternary mixture that shows a 2lowbar;rarr;2macr; transition, the bar denoting in which of the two phases the amphiphile is mainly dissolved. With weakly structured solutions, that is, with shorthyphen;chain amphiphiles as (C) this can be done by either adding an oil with a lower carbon number, or by adding a nonaqueous polar protic solvent such as formamide. With strongly structured solutions, that is, with longhyphen;chain amphiphiles, one has to add a shorthyphen;chain amphiphile for destroying the structure as a prerequisite for reaching a tcp. Insofar, our earlier presumption that with longhyphen;chain amphiphiles, a tcp may also be reached, either by increasing their amphiphilicity or by lowering the carbon number of the oil, does not seem to apply. Experience shows that inAndash;Bndash;Crsquo;mixtures with sufficiently shorthyphen;chain amphiphiles asCrsquo;that separate into three phases: the amphiphilehyphen;rich middle phase always wets theA/Binterface. If a shorthyphen;chain amphiphile is added to anAndash;Bndash;Cmixture with a nonwetting middle phase one will, therefore, inevitably find a nonwettingrarr;wetting transition as one approaches a tcp.
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