Jr. Lu et al., THE COMPOSITION AND STRUCTURE OF SODIUM DODECYL-SULFATE DODECANOL MIXTURES ADSORBED AT THE AIR-WATER-INTERFACE - A NEUTRON REFLECTION STUDY, Journal of colloid and interface science, 174(2), 1995, pp. 441-455
Neutron reflection has been used to study the composition of layers of
sodium dodecyl sulfate (SDS) and dodecanol mixtures adsorbed at the a
ir/water interface. Saturation of the surface by dodecanol at a 6.7 mM
SDS concentration occurs when the bulk dodecanol concentration is abo
ut four times the solubility of dodecanol in pure water, indicating th
at even below the cmc dodecanol is solubilized by SDS. At a fixed dode
canol/SDS ratio the adsorption of dodecanol passes through a maximum a
t about half the cmc as the SDS concentration is changed, dropping sha
rply, although not to zero, at the cmc. Above the cmc it is estimated
that an average of about two dodecanol molecules are incorporated into
a single micelle. The adsorption of SDS is affected by the presence o
f dodecanol. At low SDS concentrations (<1 mM) the SDS adsorption is e
nhanced but at higher concentrations it is reduced by about 10%. The f
ormer observation shows that there is a very strong attractive interac
tion between dodecanol and SDS and the overall adsorption behavior of
dodecanol and of SDS at low SDS concentrations is satisfactorily expla
ined in terms of the formation of an SDS/dodecanol complex and Langmui
r adsorption of dodecanol and the complex at the surface. As an exampl
e, at an SDS concentration of 0.0001 M, when the amount of dodecanol i
s 0.5% of the SDS, about one-third of the total surface excess of 1.9
x 10(-10) mol cm(-2) is in the form of the 1:1 complex. The structure
of the mixed layer has been investigated at a single composition and c
ompared with the structure in the absence of dodecanol. The thickness
of the dodecanol region of the layer (19 Angstrom) is slightly greater
than the length of the fully extended dodecanol molecule and the thic
kness of the SDS region increases on addition of dodecanol. Both these
effects result from an increased roughness of the layer compared with
pure SDS. The center of the dodecanol distribution is 3.5 Angstrom fu
rther away from the water than the center of the SDS distribution. Thi
s is intermediate between what would be expected for a second surfacta
nt and the value of 6.5 Angstrom observed for a nonionic additive (dod
ecane and dodecyl trimethylammonium bromide). (C) 1995 Academic Press,
Inc.