Abstract
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. |