Role of a Carboxylic Acid on the Crystallization, Deposition, and Gelation of Long-Chained n-Alkanes in Solution

Abstract

n-Alkanes are the primary components of solid wax formed in the wax−oil deposits in subsea oil pipelines. However, there is a wide number of other components present in these deposits that influence crystallization, deposition, and gelation. This work focuses on how a carboxylic acid, stearic acid, can influence the thermodynamics, crystallization, and gelation of two straight-chained n-alkanes, C36H74 and C32H66, in an n-alkane solvent, dodecane. Stearic acid was selected because its solubility in dodecane is very similar to C32H66, and it is known that C32H66 cocrystallizes with C36H74 and influences solution and gel properties. In spite of its similar solubility in dodecane relative to that of C32H66, stearic acid does not cocrystallize with either C32H66 or C36H74, although it influences the nucleation kinetics of the n-alkanes. Deposition experiments showed that adding stearic acid to a C36 solution does not slow deposition and that at high concentrations can actually increase the amount of deposit that forms. Stearic acid influenced gelation only if present at sufficiently high concentrations such that the n-alkanes were more soluble than stearic acid. These results are observed because stearic acid dimerizes in solution by hydrogen-bonding and associates with itself instead of n-alkanes.