Adsorption and synergism in superspreading of trisiloxane surfactant-N-alkyl pyrrolidinone mixtures on polyethylene substrates

To investigate the mechanism(s) of synergism in superspreading, surface tensions, contact angles and adsorption isotherms of surfactant aqueous solutions have been measured. It was found that the adsorption of trisiloxane surfactant(L77) from its aqueous solution onto powered polyethylene can be remarkably enhanced by the addition of certain N-alkyl pyrrolidinones and that there is a significant attractive interaction between the surfactant molecules at the polyethylene/aqueous solution interface. The reductions in interfacial pressure at the liquid/air, solid/liquid and solid/air interfaces caused by the surfactant mixtures have been evaluated by use of the Gibbs equation. The surface tension at the liquid/air interface is always increased in the mixtures, indicating that there is no synergism at this interface upon the addition of N-alkyl pyrrolidinones. However, the interfacial pressure at the solid/liquid interface is increased when L77 and the proper N-alkyl pyrrolidinone are mixed at a certain ratio, indicating that the mixtures can show a synergistic effect at this interface. Compared with the changes at the liquid/air interface and the solid/liquid interface, the change at the solid/air interface is insignificant. The change in the spreading coefficients on polyethylene film of aqueous solutions of L77, when mixed with the N-alkyl pyrrolidinone is in almost the same order as that of their enhancement of the spreading.; In order to investigate the cause of the changes in interfacial tensions at the various interfaces produced by replacement of L77 by N-alkyl pyrrolidinone, interaction parameters (beta) of L77 with the different pyrrolidinones at the various interfaces were determined. bsSL for all mixtures was between 0 and -1, indicating that the interaction at the air/aqueous solution interfaces is very weak. However, the values of bsSL were between -2.7 and -6.7 for the mixtures with those N-alkyl-pyrrolidinones that produce enhancement of the superspreading of aqueous solution of L77 on polyethylene, indicating a significant attractive interaction with L77 at the polyethylene/aqueous solution interface. The order of increasing negative bsSL values is L77-C2, 6P>L77-C6P >L77-C8P>L77-CHP>L77-C4P, which is exactly the same as the order of increasing enhancement of superspreading. This attractive interaction results in a remarkable enhancement of adsorption of L77 onto polyethylene from the aqueous solution of the mixtures.