Crystallisation of sodium dodecyl sulphate from aqueous solution: Phase identification, crystal morphology, surface chemistry and kinetic interface roughening

The crystallisation of the surfactant sodium dodecyl sulphate (SDS) from aqueous solution, above the critical micelle concentration is investigated via in situ differential interference phase contrast microscopy under a range of solution cooling rates. It is found that variation in the cooling rate has little effect on the morphology for the range applied, producing a plate-like habit dominated by the {100} form. Nucleation studies demonstrate little correlation between metastable zone width (MSZW) and surfactant concentration with the MSZW suggesting that the ease of nucleation of SDS lies between those commonly observed for organic and inorganic systems. Weak dependence of the MSZW on cooling rate is observed. Crystal morphologies predicted using an atom-atom method implemented in the HABIT98 program (Comput. Phys. Commun. 64 (1991) 311; J. Crystal Growth 166 (1996) 78; HABIT95, Quantum Chemistry Program Exchange, Program no. 670, Bloomington, IN 47405, USA, 1996) for the four known crystal structures are presented. Comparison between these and experimental data reveal the crystallising species to be consistent with the formation of C₁₂H₂₅OSO₃^-Na ⁺·1/8H₂O with a plate-like morphology dominated by {200} and complemented by minor {110}, {202} and {111} forms. Kinetic roughening, which is found to occur at higher solution supersaturations and notably on the {202} forms, is discussed in terms of the comparative surface chemistry of the SDS habit faces. © 2003 Elsevier B.V. All rights reserved.