| Abstrakt: |
Surface tension isotherms were recorded for six different but chemically closely related surfactants. The surfactants studied have three types of headgroups, tetra(ethylene oxide) (E4), penta(ethylene oxide) (E5), and maltopyranoside (Mal). For each headgroup, two hydrocarbon chain lengths were investigated, decyl (C10) and dodecyl (C12). By utilizing the Gibbs surface tension equation, surface pressure versus molecular area isotherms as well as the corresponding adsorption isotherms were generated for all six surfactants. By considering the various contributions to the free energy of the interface, theoretical surface pressure isotherms have been derived which are compared with the experimental ones in the molecular area interval especially studied here, from 60 Å2/molecule down to the molecular area obtained at the critical micelle concentration. Mainly two contributions to the free energy give rise to changes of the surface pressure with the area/molecule. The first contribution is related to restricting the hydrocarbon chain configurations, and the second one to the interactions between the polar groups and the surrounding solvent, water. It was found that the surface pressure effect of the maltoside headgroup is close to the one calculated for hard disks whereas the ethylene oxide headgroup behaves more like short polymer chains for which the Flory−Huggins theory is approximately valid. Surprisingly, we have found that the shorter carbon chain (C10) generates a higher surface pressure than the longer one (C12). |
