Stearic acid: titration curve, surface tension and formation of lipid bilayer

Stearinska kislina (C17H35COOH) je nasičena maščobna kislina in je ena najbolj razširjenih maščobnih kislin v naravi. Stearinska kislina je pomemben metabolit v človeškem telesu in kljub temu, da jo uvrščamo med nasičene maščobne kisline, ne zvišuje ravni LDL holesterola. Zaradi njenih strukturnih značilnosti jo uvrščamo med površinsko aktivne snovi (PAS). Ena od lastnosti PAS je, da v povišani koncentraciji tvori vodotopne agregate, imenovane micele. Za nekatere maščobne kisline je bilo pokazano, da pri določenih pogojih v raztopini (predvsem je pomemben pH) namesto micelov tvorijo lipidne vezikle, kar se pokaže s tem, da raztopina postane vidno motna. V diplomskem delu smo poskusili pokazati, ali je tudi stearinska kislina zmožna tvorbe lipidnih veziklov. Zaradi slabe topnosti stearinske kisline v vodi smo uporabljali raztopino njene natrijeve soli, torej natrijev stearat. Z uporabo nevtralizacijske titracije smo spremljali spremembo motnosti raztopine v odvisnosti od pH. Za tvorbo micelov ali lipidnih veziklov mora biti koncentracija raztopine PAS nad kritično micelno koncentracijo (CMC) oziroma nad tako imenovano kritično agregacijsko koncentracijo (CAC). Raztopina natrijevega stearata je bila pri sobnih pogojih v netopni obliki, in tudi ko nam je uspelo dobiti bistro raztopino, v kateri je bilo raztopljene nekaj te soli, je bila njena koncentracija pod CMC oziroma CAC. Stearinska kislina je torej pri sobni temperaturi nezmožna tvorbe lipidnih veziklov, kar pa ne pomeni, da tega ni sposobna pri višji temperaturi.Stearic acid (C17H35COOH) is a saturated fatty acid and one of the most abundant fatty acids in the environment. It is an important metabolite in the human body and, although being classified as a saturated fatty acid, does not increase the level of LDL cholesterol. Due to its structural characteristics, stearic acid is considered a surface-active agent (SAA). In high concentrations, SAAs are said to form water-soluble aggregates, called micelles. At certain concentrations (pH is particularly important), some fatty acids in solutions have been shown to form lipid vesicles instead of micelles, which is why the solution becomes visibly cloudy. In this work, we attempted to determine if stearic acid could form lipid vesicles. Due to the poor solubility of stearic acid in water, we used a solution of its sodium salt – sodium stearate. Using a neutralisation potentiometric titration, we monitored the change in turbidity of the solution as a function of its pH. The formation of micelles or lipid vesicles requires a certain concentration of the SAA in solution, which must be above the critical micelle concentration (CMC) or above the critical aggregation concentration (CAC). The sodium stearate solution was in insoluble form at room temperature, and even when we prepared a clear solution in which some of the salt had dissolved, its concentration was still below the CMC and CVC. Thus, stearic acid cannot form lipid vesicles at room temperature, but this is not necessarily true for higher temperature conditions