Sodium octadecyl sulfate surface pressure-area

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. 

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