Sodium octadecyl sulfate isotherms

Figure 1. Surface pressure-area isotherms for sodium octadecyl sulfate at the air-0.01 M sodium chloride solution interface. 0 A 5°C, O 20°C.

Figure 7. Surface pressure-area isotherms for an equimolar mixture of sodium octadecyl sulfate and octadecyltrimethylammonium bromide at the n-heptane-O.OOlM sodium chloride interface. ...

Using surface pressure (u)-area (A) isotherms for sodium octadecyl sulfate (C18 sulfate) and octadecyl trimethylam-monium bromide (C18 TAB) monolayers spread at the air/water (A/W) and n-heptane/water (O/W) interfaces, we show that the Davies-Guastalla and Davies surface equations of state are not followed over a wide range of temperature and salt concentration. We attribute the discrepancies to Davies incorrect estimate of the chain-cohesion term at the A/W interface, the inadequacy of the electrical contribution to n at all but the highest A studied, and a possible chain-cohesion term at the O/W interface. 

We describe in a companion paper (33) the salient features of our procedures for measuring low surface pressures, and we provide n-A isotherms for octadecyl trimethylammonium bromide (Ci8 TAB) and sodium octadecyl sulfate (Ci8 sulfate) monolayers spread at A/W and O/W interfaces. Here we extend some of these data to higher A using, in some instances, some of our earlier reported measurements. We have also taken from literature the few reported isotherms for dilute charged monolayers and discuss them in the light of our results and the predictions of Equations 7 and 8. 

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