Dilational properties of novel amphiphilic dendrimers at water-air and water-heptane interfaces.

In this work, a series of novel amphiphilic dendrimers taking polyamidoamine dendrimer as the core with different hydrophobic tails QPAMC(m) were synthesized and the dilational properties were studied as monolayers by dilational rheological measurements at the water-air and water-n-heptane interfaces to explore the nature of adsorption behaviors. The results showed that the maximum values of the dilational modulus seemed to have no obvious variation in a wide change of hydrophobic chain length at the surface. However, there was considerable variability in the tendency of the influence of bulk concentration on the dilational modulus at the two different interfaces. It was interestingly found that the diffusion-exchange process slowed down with the increase of alkyl chain length leading to more elastic nature of adsorption film, which was contrary to the tendencies of conventional single chain and gemini surfactants. It is reasonable to consider that, in the case of the molecule having short chain length such as QPAMC(8), the alkyl chains are too short to overlap across the headgroup, enable the intermolecular hydrophobic interaction to be predominant with increasing of surface concentration, which enhances the elasticity and shows the slowest diffusion-exchange process. Whereas, when the chain length increases to 12 or 16, the alkyl chains are long enough to act intramolecularly to form intracohesion conformation, which results in enhancing the diffusion-exchange process. In conclusion, the interfacial behaviors are dictated by the size ratio between the tail and headgroup. A reasonable model with respect to the molecular interaction was proposed on the basis of experimental data. The results of interfacial tension relaxation and dynamic light scattering (DLS) experiments, in accord with the proposed mechanism, also present the unusual tendency comparing to the traditional single or gemini surfactants.