The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

The Microscopic and Molecular Strucure of Precursing Thin

Films of Surfactant Solutions on Silicon Oxide/Silicon Surfaces

by William R. Birch



In this thesis, we use x-ray and neutron reflectivity to probe the molecular and microscopic structure of surfactant monolayers on silicon oxide/silicon surfaces.  A cationic surfactant, cetyltrimethylammonium bromide (CTAB), and an anionic surfactant, sodium dodecyl sulfate (SDS), are examined.  We study the structure of these monolayers both after they are deposited from receding menisci of bulk surfactant solutions and as they exist in precursing films when reconnected to the bulk menisci.  The self-assembly of the surfactant molecules at both the solid/liquid and liquid/vapor interfaces is driven by hydrophobicity; while at the solid surface, it is strongly influenced by the charge interaction between the surfactant head groups and the solid surface.  Our x-ray reflectivity measurements allow us to deduce the fine details of the molecular structure of the as-deposited monolayers.  On hydration, the CTAB monolayer does not desorb from the substrate nor does it restructure.  In contrast, the SDS monolayer desorbs, forming a film with thickness on the order of 100 Å.  The packing density of the SDS monolayer changes as the film evolves.  The location and quantity of the water of hydration in the surfactant monolayer is found by combining x-ray and neutron reflectivity.  Thus, our work reveals new information on the self-assembly mechanisms governing the deposition of soluble surfactants from solution.  Further, we see the modification of the self-assembly due to the confined environment of the precursing film and have gained new insight into the microscopic mechanisms governing the wetting of oxide surfaces by surfactant solutions.