Structuro-kinetic studies of the crystallisation of straight chain surfactants and homologues mixtures

• Main Author: Van Gelder, Richard N. M. R.
• Published: University of Strathclyde 1998
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.665988

An investigation was undertaken into the crystallisation process of the natural surfactants sodium laurate, myristate and palmitate. A multi-pass zone refiner was modified for purification of fatty acids, the starting materials in soap manufacture. It was found that purified lauric and myristic acid transform spontaneously in the solid state over a period of days from the C (=a) form to the A-super (=y) form, which was previously unknown. Low levels of impurities were found to inhibit this phase transformation. This was explained in terms of molecular rearrangement necessary for this transition. The nucleation behaviour from the isotropic micellar phase of single and mixed surfactant systems was studied using a specially developed turbidity probe. A two step nucleation and dissolution process was found to occur for some surfactant mixtures. Since this was not observed during static in-situ XRD experiments, it was postulated that during crystallisation above a certain degree of agitation segregation occurs of the two surfactants present in the mixture. Based on induction time measurements the size of the critical nucleus was estimated. The results indicated that the shorter the chainlength the more molecules are required to form a stable nucleus. The crystallisation process of single and mixed surfactant systems from the isotropic micellar phase, the hexagonal and the lamellar liquid crystalline phases was investigated by in-situ XRD using synchrotron radiation. A detailed transformation mechanism for the crystallisation process from the liquid crystalline phases was proposed based on epitaxial relations between all ordered phases of the following sequence: lamellar, tetragonal mesh, hexagonal mesh, (deformed hexagonal for surfactant mixtures with a chainlength difference of two CH2 groups), hexagonal, isotropic micellar, solid phase. Crystallisation of soap from ethanol-water mixtures revealed the ethanol to be incorporated into the micelles and to affect thresulting solid state structure above a certain ethanol content.