EFFECT OF ALKALI-METAL IONS ON GEL FORMATION IN THE 12 HYDROXYSTEARIC ACID SOYBEAN OIL SYSTEM

It has been known that 12-hydroxystearic acid (12HSA) forms a thermoreversible gel with a fibrous associated network in organic solvents. Mechanically durable gels were induced by the addition of small amounts of alkali metal ions, which can be supplied in the form of alkali metal salts of 12HSA (abbreviated as 12HS-M), even at a lower concentration range of gelling agent than with 12HSA alone. The structure of the networks was investigated by infrared spectrometry (IR), circular dichroism (CD) and scanning electron microscopy (SEM). The experiments were carried out over a total concentration range of 0.1 approximate to 4.0 wt/vol% of 12HSA and 12HS-M in soybean oil. 12HS-Na showed more effective gel forming power than any other alkali metal ions. The molar ratio of 12HSA/12HS-Na for obtaining optimal gel forming effect was 100:1. The intensity of the absorption bands of the IR spectra, due to the hydrogen-bonded hydroxyl groups, remained constant regardless of the amount of added 12HS-Na. A remarkable molar ellipticity decrease was observed with an increase in the concentration of 12HS-Na, although the maximum wavelength of the CD spectrum, originating from the chirality of 12HSA, remained constant. By SEM observation, linearly extended fibrils were observed with the addition of 12HS-Na. These results suggest that the mechanical strengthening of the gel by alkali metal ions is due to alteration of the dispersed state of the helically grown fibrous crystals of 12HSA.