Investigating the behavior of catanionic surfactant Langmuir monolayers

CHED 990

Joseph D. Coppock, jdcoppock@ole.augie.edu1, Brian G. Moore1, and Mike Dennin, mdennin@uci.edu2. (1) Department of Chemistry, Augustana College, 2001 S. Summit Ave., Sioux Falls, SD 57197, (2) Department of Physics & Astronomy, University of California, Irvine, 4129 Frederick Reines Hall, Mail Code: 4575, Irvine, CA 92697
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.