Adsorption dynamics of quaternary-ammonium-salt–based gemini and trimeric surfactants with different spacer structures at air/water interface

Abstract

The dynamic surface tensions of gemini surfactants with spacers containing cyclic structures such as diethylene and triethylene chains, and atoms such as oxygen and nitrogen, and of linear- and star-type trimeric surfactants with spacers having propylene or hexylene chains between the quaternary ammonium groups (for the linear-type) and propylene or hexylene chains between the quaternary ammonium group and the central amino group (for the star-type) were measured using the maximum bubble pressure method. The effects of the molecular structures and surfactant concentration on the maximum rate of reduction in the surface tension as well as the diffusion coefficient were elucidated. The gemini surfactants containing cyclic structures in the spacer exhibited slower interfacial adsorption, which was ascribed to their rigid spacer structure. The longer the spacer chain length of the gemini surfactants, the greater the molecular flexibility and hence the faster the adsorption at the interface. Thus, the diffusion and adsorption of gemini surfactants are affected by the rigidity and hydrophobicity of the spacer, respectively. However, the adsorption of the trimeric surfactants at the air/water interface (which resulted in a maximum rate of reduction in the surface tension of 0.0007–0.004 mN m–1 s–1) was lower than that of the gemini surfactants (which resulted in a maximum rate of reduction in the surface tension of 0.0009–0.9 mN m–1 s–1). This was ascribed to the greater bulkiness of trimeric structures compared with those of the gemini surfactants. The star-type trimeric surfactants adsorbed faster at the air/water interface than the linear-type trimeric surfactants because the former were flexible despite their bulky spacer structure. For the linear- and star-type trimeric structures, the diffusion rate increased with the spacer chain length because of the resulting increase in the molecular flexibility. The linear-type surfactants were adsorbed faster at the air/water interface as their spacer chain length was increased, whereas the star-type surfactants were adsorbed more slowly because of their molecular bulkiness.