The monolayer structure of the branched nonyl phenol oxyethylene glycols at the air-water interface

The structure of the monolayers formed by a group of monodisperse nonyl phe nol ethoxylates with parasubstituted 1-butylpentyl chains (para-(C4H9)(2)CH C6H4(OC2H4)(n)OH, abbreviated to BNPEn, n = 4, 8, and 12) at the air-water interface has been determined by surface tension measurements and neutron r eflection. The critical micellar concentration (cmc) was found to be 1.1 +/ - 0.3 x 10(-5) M for BNPE4, 4.0 +/- 0.3 x 10(-5) M for BNPE8 and 8.0 +/- 0. 3 x 10-5 M for BNPE12 and the limiting area per molecule (A(cmc)) at the cm c to be 46 +/- 3, 61 +/- 4, and 75 +/- 5 Angstrom(2), respectively. The val ues of A(cmc) are almost identical to those obtained from dodecyl ethoxylat es (C12En) for the same size of the headgroups, suggesting that A(cmc) for these nonionic surfactants is determined by the size of the headgroups and is not affected by the chemical structure of the hydrophobic chains. The th icknesses of the nonyl phenol layers projected onto the surface normal dire ction were found to be 18 +/- 3 Angstrom for BNPE4, 19 +/- 3 Angstrom for B NPE8, and 22 +/- 3 Angstrom for BNPE12. In all cases they were about twice as thick as the fully extended chain, suggesting a broad distribution of th e hydrophobic chain across the layer and hence a strong mixing of the chain with the ethoxylate groups.