The adsorption of a series of nonionic surfactants,
C
n
H2
n
+1(OC2H4)
m
OH
(C
n
E
m
), from aqueous
solution
onto a self-assembled monolayer (SAM) before and after chemical
modification has been studied by neutron
reflection. On a SAM consisting of a layer of undecenyl
trichlorosilane with the ethylenic group at the
aqueous interface all the surfactants adsorbed to form a monolayer
whose thickness compared with the
length of the molecule suggests that the molecules are tilted at angles
in the region of 60° away from the
surface normal. There was some penetration of the SAM layer by the
alkyl chains of the surfactants.
When the SAM was hydroxylated, the pattern of adsorption was found
to be quite different and, for all
the surfactants, the layer was only about 4 Å thick with a segmental
surface density constant within
experimental error, and there was no penetration of the SAM. The
structure of this layer is attributed
to hydrogen bonding between the ethylene glycol groups in the
surfactant and the OH groups on the surface
of the SAM. For comparison the adsorption of the same set of
surfactants was studied on the bare silica
surface. In all cases the adsorption was in the form of a
defective bilayer, quite different again from the
adsorption pattern on the two SAMs.
Spun-cast layers of polystyrene have been transferred to large silicon crystals as substrates for neutron
reflectivity studies and to prisms for infrared-attenuated total reflection measurements of surfactant
adsorption. This paper describes an investigation of the adsorption from aqueous solutions of sodium
dodecyl sulfate (SDS) to this interface of aqueous solution and polystyrene. The structure and amount of
adsorbate is described and discussed. The adsorption isotherm shows that both the thickness and volume
fraction of the SDS layer increase to plateau values above the critical micelle concentration (cmc) where
the area per molecule is approximately 42 Å2. Above the cmc, there is a decrease in the total amount of
adsorbed material unless SDS of very high purity is used. This unusual behavior is explained by the
presence of dodecan-1-ol from hydrolysis of SDS which is highly surface active but can be solubilized in
micelles of SDS.
We have used neutron reflection to study the effects of pH and added electrolyte on the
structure of a weak polyelectrolyte brush anchored by means of a hydrophobic block to a hydrophobic
surface. The copolymer was poly(2-(dimethylamino)ethyl methacrylate-block-methyl methacrylate) (poly(DMAEMA-b-MMA)) containing 70% DMAEMA and having a molecular weight of 10000. The MMA block
and the hydrophobic octadecyltrichlorosilane (OTS) layer were deuterated in order to highlight the
DMAEMA fraction of the layer. The MMA block was found to bind strongly and irreversibly to a self-assembled monolayer of octadecyl trichlorosilane (OTS) on silica. The copolymer distribution could be
described in terms of two uniform layers, a thin layer next to the surface containing all the MMA block
and variable amounts of the DMAEMA residues with a copolymer volume fraction of about 0.8 and a
more diffuse block corresponding to the polyelectrolyte brush. The presence of DMAEMA residues in the
thin layer and the variation of this amount with pH and added electrolyte show that, near the anchor
layer of the MMA block, the DMAEMA block is not ionized, in agreement with the model of a grafted
weak polyelectrolyte proposed by Israels et al.
The technique of neutron reflection has been used to investigate the adsorption of alpha-enriched gelatin from aqueous solution onto spun polystyrene substrates. Neutron reflection can provide information about the distribution of material perpendicular to an interface as well as total adsorbed amounts. The adsorbed layers were found to have maximum density at the surface, decaying with distance into solution. The adsorbed amount, layer thickness, and density were all seen to increase with solution concentration. Temperature was found to have little effect on adsorption. Thicker, less dense layers were observed at high pH and thinner, denser layers were observed at low pH, but the total adsorbed amount did not change significantly. The presence of sodium chloride had little effect on the adsorbed layers. The results are discussed in the context of other studies and the known amino acid sequence of alpha-gelatin.
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