Effect of Polymer Molecular Weight and Solution pH on the Surface Properties of Sodium Dodecylsulfate-Poly(Ethyleneimine) Mixtures

The effect of polymer molecular weight and solution pH on the surface properties of the anionic surfactant sodium dodecylsulfate, SDS, and a range of small linear poly­(ethyleneimine), PEI, polyelectrolytes of different molecular weights has been studied by surface tension, ST, and neutron reflectivity, NR, at the air–solution interface. The strong SDS–PEI interaction gives rise to a complex pattern of ST behavior which depends significantly on solution pH and PEI molecular weight. The ST data correlate broadly with the more direct determination of the surface adsorption and surface structure obtained using NR. At pH 3, 7, and 10, the strong SDS–PEI interaction results in a pronounced SDS adsorption at relatively low SDS and PEI concentrations, and is largely independent of pH and PEI molecular weight (for PEI molecular weights on the order of 320, 640, and 2000 Da). At pH 7 and 10, there are combinations of SDS and PEI concentrations for which surface multilayer structures form. For the PEI molecular weights of 320 and 640 Da, these surface multilayer structures are most well-developed at pH 10 and less so at pH 7. At the molecular weight of 2000 Da, they are poorly developed at both pH 7 and 10. This evolution in the surface structure with molecular weight is consistent with previous studies, where for a molecular weight of 25 000 Da no multilayer structures were observed for the linear PEI. The results show the importance with increasing polymer molecular weight of the entropic contribution due to the polymer flexibility in control of the surface multilayer formation

Effect of Architecture on the Formation of Surface Multilayer Structures at the Air–Solution Interface from Mixtures of Surfactant with Small Poly(ethyleneimine)s

The impact of ethyleneimine architecture on the adsorption behavior of mixtures of small poly­(ethyleneimines) and oligoethyleneimines (OEIs) with the anionic surfactant sodium dodecylsulfate (SDS) at the air–solution interface has been studied by surface tension (ST) and neutron reflectivity (NR). The strong surface interaction between OEI and SDS gives rise to complex surface tension behavior that has a pronounced pH dependence. The NR data provide more direct access to the surface structure and show that the patterns of ST behavior are correlated with substantial OEI/SDS adsorption and the spontaneous formation of surface multilayer structures. The regions of surface multilayer formation depend upon SDS and OEI concentrations, on the solution pH, and on the OEI architecture, linear or branched. For the linear OEIs (octaethyleneimine, linear poly­(ethyleneimine) or LPEI₈, and decaethyleneimine, LPEI₁₀) with SDS, surface multilayer formation occurs over a range of OEI and SDS concentrations at pH 7 and to a much lesser extent at pH 10, whereas at pH 3 only monolayer adsorption occurs. In contrast, for branched OEIs BPEI₈ and BPEI₁₀ surface multilayer formation occurs over a wide range of OEI and SDS concentrations at pH 3 and 7, and at pH 10, the adsorption is mainly in the form of a monolayer. The results provide important insight into how the OEI architecture and pH can be used to control and manipulate the nature of the OEI/surfactant adsorption