Adsorption Layer of Potassium Dodecanoate and Diglycerol Derivative Mixtures at the Air/Water Interface Studied by...

The behavior of the air/water interface in a one-component surfactant system is studied by performing surface tension measurements and analysis based on Gibbs adsorption isotherms. However, this method is inefficient for evaluating the surface composition of surfactant-additive systems, which is important for optimizing the composition of industrial products according to their practical applications. In this study, the diglycerol derivative 2GlyPO9 with polyoxypropylene chains, developed for industrial applications, was investigated to gain insight into how its mixing with potassium dodecanoate (C11COOK) improves the foam stability. The adsorption properties at the air/water interface were examined by conducting surface tension and neutron reflectivity measurements, and the aggregation structure was studied using small-angle neutron scattering. With increasing C11COOK concentration, the surface tension decreased through the following processes: (1) both 2GlyPO9 and C11COOK were adsorbed, (2) 2GlyPO9 was replaced with C11COOK at the interface, (3) 2GlyPO9-based aggregates were formed in solution, and (4) mixed micelles were formed with C11COOK. This process changed slightly depending on the solution used. At excessive 2GlyPO9 concentrations, process (2) was less likely to occur than at lower concentrations because 2GlyPO9 formed aggregates, even in the absence of C11COOK. When C11COOK was mixed with another diglycerol derivative, 2GlyEO9, in which the hydrophobic polyoxypropylene chains of 2GlyPO9 were replaced with hydrophilic polyoxyethylene chains, process (3) was skipped because no aggregates of 2GlyEO9 were formed. Compared with the C11COOK solutions without diglycerol derivatives, the addition of 2GlyPO9 lowered the surface tension above the critical micelle concentration, whereas 2GlyEO9 did not produce this effect, although the surface excess concentration of C11COOK decreased in both cases. This detailed structural analysis conducted at the interface and in solution using neutrons provides useful insights into the mechanism of adsorption at the air/water interface, particularly in surfactant-additive systems.