Study of the pH-Induced Self-Assembly of Conjugated Linoleic Acid/Conjugated Linoleate In An Aqueous Solution

Conjugated linoleic acid (CLA) was prepared by alkali isomerization from linoleic acid (LA) with KOH as catalyst and ethylene glycol as solvent. When w(KOH):w(LA):v(ethylene glycol)=1:2:3 at 170 ^oC for 4 h, the yield of CLA was 85 % and purity of CLA was as high as 95 %. The chemical structure of the surfactant was confirmed by FTIR, GC, GC-MS. Depending on the pH, conjugated linoleic acid/conjugated linoleate self-assembled into different types of aggregates. CLA can self-assemble to form vesicles by adjusting the pH (pH 8.2-9.2) which similar to the pKa of CLA, whereas at higher pH, the vesicles were transformed into micelles. The critical micelle concentration (cmc), surface tension at the cmc (γ_cmc), the maximum surface excess concentration at the air/water interface (Γ_max) and the minimum area per surfacatant molecule at the air/water interface (A_min) of conjugated linoleic acid/conjugated linoleate were 0.197 mmol•L^-1, 29.26 mN•m^-1, 5.96×10⁶ mol•m^-2 and 0.278 nm² at pH 9.2 in borate buffer solution with excess NaCl, while the cmc, γ_cmc,  Γ_max and A_min were 0.344 mmol•L^-1, 35.66 mN•m^-1, 3.29×10⁶ mol•m^-2 and 0.505 nm² at pH 13 in 0.1 mol•L^-1 NaOH aqueous solution. Subsequent, the different types of conjugated linoleic acid/conjugated linoleate self-assembly aggregates were cross-linked to fix the microstructers of the self-assembly aggregates.