| Abstrakt: |
Precipitation of anionic surfactants, linear alkylbenzene sulfonate (LAS) and alpha olefin sulfonate (AOS), by calcium ions was studied in the presence of sodium oleate. Lather stability was determined by the Ross-Miles method, precipitation was followed by measuring the optical density (OD), and equilibrium surface tension (EST) and Fourier transform infrared (FTIR) spectroscopy were used to characterize the nature of the precipitate formed. For the 5 mM LAS-0.7 mM oleate system, lather was unstable, and the OD was high in the 2-5°FH region of calcium hardness, while at higher calcium hardness levels, lather was stable and the turbidity of solutions decreased. On the other hand, in the 5 mM AOS-0.7 mM oleate system, lather was unstable throughout the calcium hardness region studied (0-20°FH). Also, the turbidity build-up was much higher in the AOS system than in the LAS system. Analysis of the precipitates formed in these systems by FTIR spectroscopy indicated that the precipitate from the AOS system had an additional band at 1190 cm, corresponding to the sulfonate group. These results, together with the EST data, confirm that the precipitate formed in the LAS system between 2-5°FH calcium is calcium oleate, and that formed in the AOS system is likely to be calcium (AOS) oleate. It is tempting to hypothesize that the similarity of AOS and oleate in chainlength could be responsible for the coprecipitation of AOS and oleate with calcium, whereas LAS, which has a larger headgroup with a benzene ring and two smaller chains (average length is C8) is unlikely to precipitate with the oleate. [ABSTRACT FROM AUTHOR] |
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