Crystallisation of Monoethanolammonium Alkylcarboxylates

Monoethanolammonium (MEA) carboxylates are common components of many household cleaning products yet have received relatively little study in literature. They can cause stability problems during product formulation, crystallising out of solution and reducing the products overall performance. A more detailed study on the crystallisation behaviour of MEA carboxylates is therefore carried out using both ex-situ and in-situ characterisation techniques. MEA carboxylates were produced from even, saturated fatty acids (C12:0 to C18:0) by cooling crystallisation and characterised by powder x-ray diffraction (PXRD), differential scanning calorimetry (DSC), and Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. Soaps are known to also form acid-soap cocrystals, hence a screen for acid-soaps was required for MEA carboxylates. Even saturated fatty acids (C12:0-C18:0) were neutralised to various degrees by MEA and the resulting crystal samples characterised by PXRD, DSC, FTIR, and Raman. The screen indicated a 1:1 acid-soap formed for all the chain lengths studied and whose formation was favoured over forming fatty acid and MEA carboxylate. Binary phase diagrams for the MEA carboxylate – fatty acid systems were generated for the chain lengths studied. Metastable zone width (MSZW) studies were carried out for MEA laurate and MEA myristate in aqueous solution, revealing non-typical dissolution and crystallisation temperature trends. Ex-situ characterisation and in-situ Raman measurements revealed the 1:1 acid-soap was forming in solution and the observed transitions were its crystallisation and dissolution. The non-typical trends were due to the relative amounts of acid-soap and MEA carboxylate changing with total solution concentration; as concentration increases, the proportion of MEA carboxylate increases leading to a lowering of dissolution temperature. In-situ simultaneous small- and wide-angle x-ray scattering studies were carried out to characterise the solution structure during crystallisation. No changes were observed on cooling into the MSZW until immediately prior to the onset of crystallisation where a broad WAXS peak at 3.3 Å appeared, suggesting pre-crystallisation ordering in solution. Samples in sealed capillaries indicated a shear-dependence by forming a different, much larger lamellar structure than observed for samples under flow.