Investigating the Behavior of Catanionic
Surfactant Langmuir Monolayers
Joe Coppock
Department of Chemistry, Augustana College
Department
of Chemistry,
The
behavior of the catanionic system of dioctadecyldimethylammonium bromide
(DODAB) and sodium dodecyl sulfate (SDS) was investigated at the air water interface
using standard isotherm techniques and epifluorescence microscopy. The probe
NBD C12-HPC was found to associate well with the soluble surfactant,
SDS, but not with the insoluble DODAB. SDS does not form a stable monolayer on
its own as it is soluble, exhibiting no increase in surface pressure with
compression. However, using a co-spreading procedure to deposit the two
surfactants and probe together a visible homogenous monolayer was formed. Compression
revealed very condensed monolayers exhibiting a reversible folding collapse
mechanism near and above 47 mN/m, the DODAB collapse surface pressure. These
monolayers were found to exhibit some hysteresis, exhibiting a larger number of
folds at slower compressions speeds after undergoing a faster compression speed
beforehand. The number of folds was found to be dependent on compression speed
in a positive correlation. Monolayers were visible by epifluorescence
microscopy at 1:1 mole ratios and larger with respect to SDS, but not less due
to quenching of the NBD C12-HPC probe in the SDS by DODAB.