Effect of Tetraalkylammonium Chlorides on Foaming Properties of Sodium Dodecyl Sulfate Solutions

The effect of tetraalkylammonium chlorides (TCnAC for n ) 1, 2, 3, and 4) on the micellar stability of sodium dodecyl sulfate (SDS) solutions was investigated and related to dynamic interfacial properties, such as foamability and foam stability as well as surface viscosity and surface tension. The slow relaxation time τ2, which is directly related to the micellar stability, was determined using the pressure-jump technique. It was found that upon increasing the concentration of TCnAC, the micellar stability increases, due to the ionic interactions between the oppositely charged TCnAC and the SDS headgroup. Beyond a critical concentration (depending on the alkyl chain length of TCnAC), the micellar stability decreases again, which is attributed to the disruption of the molecular packing in the micelles as well as at the air/water interface. The location of a maximum in the micellar stability corresponds to maxima in foam stability and surface viscosity and to minima in foamability and surface tension. These maxima and minima shift to lower concentrations as the alkyl chain length of TCnAC is increased. This indicates that alkylammonium chlorides with longer chain length are more effective in decreasing the molecular packing of SDS in micelles as well as adsorbed monolayers.