ADSORPTION OF NONIONIC SURFACTANTS ON HYDROPHILIC SURFACES - AN EXPERIMENTAL AND THEORETICAL-STUDY ON ASSOCIATION IN THE ADSORBED LAYER

The adsorption of the nonionic polyethylene glycol monododecyl ethers C12(EO)6 and C12(EO)25 from aqueous solutions on hydrophilic surfaces was studied. The adsorption and the layer thickness of C12(EO)6 exhibit a sudden increase at a concentration close to the cmc. Neutron reflection shows that in the plateau of the adsorption isotherm a fragmented bilayer has been formed. The adsorption of C12(EO)25 increases gradually with solution concentration. The adsorbed amount and layer thickness are much higher than those for EO oligomers. The hydrophobic chain parts in the C12(EO)25 layer could not be localized exactly by neutron reflection, but association between the aliphatic chain parts must occur to obtain the measured adsorbed amounts and layer thicknesses. NMR measurements indicate that the EO segments in the adsorbed layers of C12(EO)6 and C12(EO)25 have more conformational restrictions than those in PEO layers because of the association of the aliphatic tails. For C12(EO)6 the self-consistent field lattice theory for adsorption and/or association (1D-SCFA) predicts stepwise adsorption and bilayer formation. For C12(EO)25 the adsorption and layer thickness are grossly underestimated. With an extended 2D-SCFA theory which allows for inhomogeneities perpendicular and parallel to the surface, the formation of small aggregates on the surface can be predicted. For C12(EO)25 the 2D-SCFA gives much better agreement between theory and experiment.