Neutron reflectometry and surface tensiometry have been used to study the composition and structure of a mixed monolayer of dodecane and the nonionic surfactant pentaethylene glycol monododecyl ether (C 12 E 5 ) and the structure of the C 12 E 5 on its own at the surface of aqueous solutions of C 12 E 5 . Partially labeling of the surfactant with deuterium gave the surface normal distributions of the ethylene glycol and hydrocarbon chains of the surfactant and of the water, and these results are compared with those from other members of the C 12 E m series. The C 12 E 5 layer has a higher capacity for oil than that of the comparable hydrocarbon chain length cationic surfactant, dodecyl trimethylammonium bromide (C 12 TAB), and the structure of the composite layer is also different. The separation of the centers of the surfactant chain and dodecane distributions is only 3 Å in the C 12 E 5 system in comparison with 6.5 Å for C 12 TAB. In this respect, the C 12 E 5 /dodecane system more closely resembles the sodium dodecyl sulfate/dodecanol system, and this is attributed to a favorable interaction between the dodecane and the ethylene glycol chain, which encourages a closer approach of the oil to the water surface. The hydrocarbon chain in the C 12 E m series where m is 6 or less is tilted at about 40°a way from the surface normal. Upon incorporation of dodecane into the C 12 E 5 layer, the hydrocarbon chain changes to an almost perpendicular orientation.

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