Add time:08/17/2019         Source:sciencedirect.com

The evaporation resistances of monolayers of the octadecanol-docosyl sulphate system have been measured by the Langmuir-Schaefer technique. Eleven different compositions spanning the entire composition range were studied. At surface pressures above 15 mN m−1 evaporation resistances of the mixed monolayers were greater than resistances calculated from the Barnes-La Mer logarithmic addition rule. At lower surface pressures the pattern was more complex, but of special note was a sharp drop in resistance from the high value of a pure octadecanol monolayer to the low values of the various mixtures.These results are discussed in terms of the energy barrier theory and excess free energies of activation (ΔG≠E) are reported. A relationship between these values and the excess free energies of mixing (ΔGE): ΔG≠E = − KΔGE, is established, provided allowance is made for the (unmeasured) contributions of the high area regions of the surface pressure-area isotherms to AGE.When the evaporation resistance data are plotted against molecular area, as suggested by the accessible area theory, all of the points fall roughly on the same curve except for the points for pure octadecanol monolayers. The high resistances of octadecanol are attributed to a low value of the evaporation coefficient.

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