R. Windsor et al., Adsorption of sodium dodecyl sulfate in the presence of poly(ethylenimine)and sodium chloride studied using sum frequency vibrational spectroscopy, LANGMUIR, 17(23), 2001, pp. 7306-7312
The ordering of sodium dodecyl sulfate, SDS, at the hydrophobic solid/aqueo
us solution interface has been studied as a function of the addition of the
cationic polyelectrolyte poly(ethylenimine), PEI, and the univalent electr
olyte sodium chloride (NaCl) to solution. The nonlinear optical technique o
f sum frequency vibrational spectroscopy (SFS) was employed to determine th
e ordering of the alkyl chain of the adsorbed surfactant. Sodium chloride w
as found to promote the onset of ordered SDS adsorption at lower bulk conce
ntrations, a result interpreted primarily in terms of electrostatic shieldi
ng, with a minor contribution from salt-induced headgroup counterion conden
sation. Introduction of PEI to electrolyte-free SDS solutions was found to
promote synergistic SDS adsorption through polyelectrolyte/surfactant compl
exation. Adsorption not only occurred at lower surfactant concentrations bu
t also was more highly ordered than resulted from the addition of NaCl. The
addition of NaCl and PEI together produced cooperative complexation of SDS
and PEI and maximum ordering of the surfactant, which was independent of S
DS concentration. This result is attributed to salt-induced enhancement of
the strength of lateral surfactant alkyl chain interactions.