Abstract
The dynamic interfacial properties of a diblock copolymer poly((dimethylamino)ethyl methacrylate-b-methyl methacrylate) have been obtained by surface quasi-elastic light scattering. The temporal evolution of the capillary wave frequency and damping as well as the surface viscoelastic properties (surface tension, dilational modulus, and dilational viscosity) of a freshly formed air−water interface for a range of pH values (at fixed scattering vector) has been investigated. The equilibration dynamics were dependent on the solution pH, but the equilibrium state was obtained by the time the interface was 12 h old. Tensiometry showed that a surface phase transition occurs between pH 7.0 and pH 6.0. The surface light scattering data clearly showed the differences in temporal response in this same range of pH. For solutions with pH 5.0, the air−water interface is populated by unimers. For higher pH solutions, micelles are the initial species at the surface, but these disaggregate for solutions with pH 6.0 and 6.5. The disaggregation process is faster for the solution with pH 6.5, and this is associated with resonance between the capillary and dilational modes enabling energy to be transferred to the dilational mode, accelerating the disaggregation of the micelles.
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