The adsorption on silica wafers of a hyperbranched, high molecular weight polyethylenimine (PEI) was investigated using reflectometry. The pH and ionic strength dependence of PEI adsorption kinetics and that of the adsorbed amount were interpreted according to the complex balance of segment/segment and segment/surface site interactions. The observed adsorption properties show significant deviations from the recently studied features of polyvinylamine adsorption on cellulose coated silicas. The different behavior can be attributed to the pronounced difference in the nonelectrostatic affinity of polyamines toward the two different types of surfaces. The role of electrostatic interactions was also characterized by electrokinetic measurements. Due to the adsorption of PEI, significant charge reversal and shift in the isoelectric point of silica wafers occur. The ζ potential-pH curves show a maximum, which can be interpreted qualitatively by the adsorption characteristics of PEI. An attempt was also made to interrelate the adsorption and electrokinetic data via comparison of different estimates of the diffuse double layer charge of the PEI/silica system.

The binding isotherm of sodium dodecyl sulfate (SDS) on a hyperbranched polyethyleneimine (PEI) was determined by an equilibrium dialysis method. Dynamic light scattering, electrophoretic mobility, and coagulation kinetics measurements were also performed in order to monitor the changes in the charged nature and size of PEI/SDS complexes. The experimental binding isotherm shows that the SDS interacts with PEI in two different ways. In a first binding process, the dodecyl sulfate ions bind in monomer form to the protonated amine groups, which is accompanied by an increase of the pH. A quantitative model is presented to describe the relation between the surfactant binding and the pH change. Above a critical amount of the bound surfactant, the PEI/SDS complex molecules collapse and precipitate. After the collapse of the polyelectrolyte/surfactant molecules, the SDS adsorbs on the surface layer of the collapsed particles (causing a charge reversal). This means that the interaction of the SDS with PEI can be divided into different characteristic SDS concentration ranges. At low surfactant concentrations, the system is a thermodynamically stable solution of the polymer/surfactant complex molecules. Above this critical concentration, the system is an unstable colloid dispersion of the complex particles. At even higher surfactant concentrations, the system may be a kinetically stable dispersion of the PEI/SDS particles, depending on the method of preparation. It can be concluded that the observed mechanism of PEI-SDS interaction is different from the general characteristics of the oppositely charged linear polyelectrolytes and surfactants, where the precipitated complex dissolves in the excess surfactant due to a collective (micelle-like) polymersurfactant interaction.

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Specular neutron reflection has been used to determine the structure and composition of the mixed cationic and nonionic surfactants of hexadecyltrimethylammonium bromide, C16TAB, and hexaethylene glycol monododecyl ether, C12E6, adsorbed at the hydrophilic silica solid/water interface. Measurements have been made at two different values of pH (2.4 and 7.0), where the relative affinity for the adsorption of the two surfactants at the interface is dramatically altered. At high pH (pH = 7.0) the composition of the adsorbed bilayer is dominated by C16TAB and the structure of the bilayer is nonuniform:  that is, the surfactant layer immediately adjacent to the solid silica surface is richer in C16TAB. At the lower pH (pH = 2.4), where the affinity for the surface of C12E6 is much greater relative to that of C16TAB, the surface composition is closer to the bulk solution composition and the structure of the adsorbed bilayer is essentially uniform.

Small-angle neutron scattering has been used to study the structure and composition of mixed ionic-nonionic surfactant micelles. A comparison between two different mixed surfactant systems, sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C 12 EO 6 ) and hexadecyltrimethylammonium bromide (C 16 TAB) and C 12 EO 6 , both in 0.1 M NaCl, has been made. In the latter system, ideal mixing is observed, and in the former, departure from ideality, broadly consistent with regular solution theory, RST, is observed. The deviations from the predictions of RST are attributed to subtle changes in the packing of the two surfactants in the mixed micelles. For the SDS/C 12 EO 6 mixture, the micellar aggregation number is essentially constant with composition and concentration, whereas for the C 16 TAB/C 12 EO 6 mixture there is a marked micellar growth with increasing concentration and mole fraction of C 12 EO 6 in solution. The SANS results on structure and composition are compared with the results reported for other mixed surfactant systems and are discussed in the context of recent theoretical developments.

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