Neutron reflection study of a double-chained sugar surfactant

Neutron reflection and surface tension have been used to investigate the structural features exhibited at the air-liquid interface by an adsorbed layer of the sugar-based nonionic surfactant 7,7-bis[(1,2,3,4,5-pentahydroxyhexanamido)methyl]-n-tridecane. It consists of two glucamide headgroups and two C6H13 alkyl chains, i.e., (C6H13)(2)C(CH2NHCO(CHOH)(4)CH2OH)2, abbreviated to di(C-6-Glu). Isotopic substitution was used to distinguish the chains of the surfactant from the headgroup and solvent distributions. The critical micelle concentration (CMC) was found to be (1.9 +/- 0.1) x 10(-3) M using surface tension measurements, and the latter also indicated that the surface properties were not affected by isotopic substitution. The specular reflection of neutrons has been used to determine the surface concentrations of skeletally deuterated di(C-6-Glu) over the concentration range CMC/400 to 2CMC. The neutron measurements showed the area per molecule A at the CMC to be 66 +/- 3 Angstrom 2, as compared with 60 +/- 10 Angstrom(2) from surface tension measurements. The layer structure has been determined using three different isotopic contrasts at concentrations of CMC and CMC/400. The fit of a two-layer model shows that the total monolayer thickness decreases from 17.5 +/- 1.5 Angstrom at the CMC to 14.5 +/- 1.5 Angstrom at CMC/400, respectively. The kinematic approximation was used to fit the neutron data and an X-ray reflection profile. The most remarkable feature of the structure obtained is the apparent smoothness of the layer. The calculated roughness of the layer at the CMC is 9.7 Angstrom(2) which corresponds exactly to the value expected for capillary waves using the model of a pure liquid. At the CMC the molecule as a whole appears to be tilted about 35-40 degrees away from the surface normal, but at CMC/400 the hydrocarbon chains are approximately normal to the surface while the gluconamide groups are strongly tilted.