Abstract
It has been known for some time that poly(dimethylsiloxanes) (PDMS) can form monolayers at the air-water interface. In this paper we show that formation of mixed monolayers of PDMS with fatty acids, di- and triglycerides is also possible. At the air-water interface silicone can interact and increase the retention of the fatty acids, which otherwise easily desorb into the aqueous subphase. Unlike fatty acids, long acyl chain tri- and diglycerides did not desorb into the aqueous subphase and their interaction with silicone was manifested by a condensing effect and a change in the collapse pressure of the mixed monolayers. Geotrichum candidum lipase (GCL) showed a similar level of binding to silicone monolayer as to an air-water interface. At low surface pressure (7.5 mN/m) the lipase hydrolyzed dicaprin in mixed 1,2- sn-dicaprin-PDMS monolayers at a rate 1.5-fold lower than the desorption rate of the capric acid from mixed monolayers of capric acid and PDMS. Compression of the dicaprin-PDMS monolayer to 20 mN/m led to a 15-fold increase in lipase activity (70% of the activity observed for monolayer of pure dicaprin compressed to the same surface pressure), indicating an increase in dicaprin's surface concentration and its accessibility to the enzyme. The lipase activity profile determined titrimetrically against varying amounts of glycerides in mixed glyceride-PDMS emulsions (at constant 5% oil-phase concentration) resembles the lipase activity-surface pressure profile obtained with the monolayer method when using pure glycerides. This suggests that the location of glycerides in condensed mixed monolayers with PDMS should be similar to that in mixed glyceride-PDMS emulsions. By changing the molar fraction of tristearin and tripalmitin in the mixture with PDMS, it was possible to regulate the quality of the interface in emulsions and to optimize the enzymatic hydrolysis of these solid triglycerides.
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