Tricritical points in H2O–oil–amphiphile mixtures

Publisher Website
Google Scholar
Abstract
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 an A–B–C mixture that shows the phase sequence 2_→3→2̄ with rising temperature, with a second ternary mixture that shows a 2_→2̄ transition, the bar denoting in which of the two phases the amphiphile is mainly dissolved. With weakly structured solutions, that is, with short‐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 long‐chain amphiphiles, one has to add a short‐chain amphiphile for destroying the structure as a prerequisite for reaching a tcp. Insofar, our earlier presumption that with long‐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 in A–B–C’ mixtures with sufficiently short‐chain amphiphiles as C’ that separate into three phases: the amphiphile‐rich middle phase always wets the A/B interface. If a short‐chain amphiphile is added to an A–B–C mixture with a nonwetting middle phase one will, therefore, inevitably find a nonwetting→wetting transition as one approaches a tcp.