Studies of the interaction of gemini surfactants with polymers and triblock copolymers

Abstract

The interaction of two novel surfactants, known as gemini surfactants, with aqueous solutions of neutral polymers, specifically polyethylene oxide (PEO), polypropylene oxide (PPO), and polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO-PPO-PEO) triblock copolymers, have been investigated. In order to provide a basis for comparison, an investigation of the aqueous solution behaviour of two series of N,N'-bis(alkyldimethyl)-α,[omega]-alkanedianimonium dibromide surfactants was carried out. One series had a constant spacer length (s) of 3 methylene units with various alkyl chain lengths (m) of 8, 10, 12, and 16, and the other had a constant in of 12 with various s between 2 and 16 inclusive. A surfactant with in = 12 and a 'p'-xylyl ([straight phi]) spacer also was studied to assess the effect of rigidity in the spacer group on gemini interfacial properties. The results obtained for the critical micelle concentration (CMC) and head group area (a0) of the surfactants are in excellent agreement with those previously reported. The mean aggregation number of the surfactants decreases with increasing spacer chain length up to s = 8, after which the aggregation number increases. The initial decrease results from a decrease in the surface area available to a surfactant monomer as the area taken up by the spacer group is increased. Experimental apparent molar volume (AMV) data have been modeled assuming both a mass-action model (8-3-8, only) and a pseudo-phase model. The observed variation in the volume change due to micelle formation, [Delta]V[straight phi],M, is consistent with variations in ao and the CMC, and can be rationalized in terms of possible spacer conformations in the aqueous and micellar phases. Results obtained for the 12-[straight phi]-12 surfactant indicate that rigidity of the spacer has no measurable effect on the micellization process for shorter spacer lengths. The interaction of the gemini surfactants with the triblock copolymers in aqueous solution was markedly different from that typically observed in surfactant-polymer systems, and is similar in nature to a solubilization or mixed micelle formation process. The results obtained indicate that the interaction occurs primarily with the PPO segment of the triblock copolymer through a replacement of hydration water by polymer at the micellar surface. The solubility of the surfactant monomer (i.e. the CMC) may be increased through specific interactions between the surfactant and polymeric microdomains in solution. The results of a temperature dependent study indicate that the aggregation state of the copolymer in solution has a significant effect on the interaction with gemini surfactants.