The Interaction between Nonionic Dendrimers and Surfactants-Electromotive Force and Microcalorimetry Studies

The interaction between the dendrimers poly(1,4-diaminobutane) DABn (n = 8, 16, 32, 64) and poly(pyrrolidone-1,4-diaminobutane) DABnPy (n = 8, 16, 32) with the surfactants sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTAB), and hexaethyleneglycol monododecyl ether (C12EO6) have been studied using electromotive force (EMF) measurements involving a surfactant-selective electrode and also isothermal titration microcalorimetry (ITC). The EMF data shows apparent binding of SDS to all the dendrimers at SDS concentrations lower than 10-5 mol dm-3. As more SDS is added, the binding process continues until the dendrimer becomes fully saturated with bound SDS, at which point the EMF and ITC data for SDS solutions with and without the dendrimers merge. At low SDS concentrations the binding mechanism is a noncooperative process driven by hydrophobic interactions between the hydrocarbon chains in the cavity of the dendrimer and also electrostatic attraction between the surfactant headgroups and the mildly cationic nitrogen atoms in the inner core of the dendrimer. As binding proceeds, there is a gradual transition to a cooperative binding process in which micellar-type bound SDS aggregates are formed on the dendrimers. This continues until the dendrimer can no longer bind any further surfactant which signals the occurrence of free regular SDS micelles in solution. The binding between SDS micelles and the dendrimers is driven by primary electrostatic interactions which also promote stable micellar bound SDS aggregates. C12EO6 shows a limited amount of binding to some of the dendrimers, although the binding is noncooperative, driven by hydrophobic interactions between the surfactant and the internal cavity of the dendrimer. TTABr was only found to show a very small interaction with DAB16 at a high dendrimer concentration