Crystallization of Sodium Dodecyl Sulfate-Water Micellar Solutions under Linear Cooling

Abstract

The crystallization kinetics of sodium dodecyl sulfate (SDS)-water micellar solutions, under linear cooling conditions, were experimentally investigated using optical microscopy, differential scanning calorimetry, and infrared spectroscopy. Cooling rates were systematically varied, from 0.1 to 50 °C min–1, encompassing environmental to near-“isothermal” temperature changes, between 22 and −5 °C, for a reference concentration of 20% SDS-H2O. The cooling rate was shown to determine the dominant crystal morphologies, with platelets and needles predominating at the lowest and highest rates, respectively. The results were rationalized in terms of isothermal crystallization data and the time–temperature cooling profile. Rates 0.1, 5.0, and 10 °C min–1 yield morphologies and kinetics analogous to those of isothermal quenches at the corresponding crystallization temperature window. Nontrivial deviations were observed for intermediate rates (0.5, 1.0 °C min–1), due to commensurate changes in temperature and crystallization mechanism, accompanied by solute depletion. The polythermal metastable zone width was estimated, and the non-isothermal nucleation described by the Nývlt equation, while the Avrami and Kissinger models described overall crystallization kinetics. Our measurements quantify the impact of temperature gradients in the crystallization of ubiquitous SDS micellar solutions, for a range of practically relevant profiles incurred during manufacturing and storage

Crystallisation of sodium dodecyl sulfate–water micellar solutions with structurally similar additives: counterion variation

“…The melting point trend for the group 1 dodecyl sulfate salts (the pure systems) supports this to an extent as it bears strong similarities to this mixed system: LDS < SDS ≪ KDS ≈ RbDS > CsDS, 37,47,53,54 with the last two points proving slightly anomalous compared to what was observed here. From this it could be argued that the additives affect the solubility curve on the saturation diagram, however the melting point data for this system shows that the values are largely unaffected, discussed further in the DSC: heating ramps section.…”

“…S2c). Overall most of the morphologies observed here differ to those formed during the isothermal and linear cooling studies of SDS-H 2 O, 27,47 therefore are ascribed to the additives.…”

“…All profiles exhibited a single exothermic crystallisation peak. From these curves is apparent that going down group 1 T p initially shifts towards the phase boundary of 20% SDS-H 2 O (≈19.1°C), 47 therefore crystallisation is increasingly favoured. At KDS T p reaches a maximum as a further increase in the ionic radius moves T p back to lower values.…”

“…The crystallisation of micellar SDS-H 2 O solutions was previously found to yield a range of morphologies from platelets through to needles depending on the temperature 27 and cooling rate. 47 The impact of linear cooling conditions is useful to parameterise as most formulations are likely to experience temperature variations under practically relevant conditions. With the presence of additives however T ind occurs at a variety of temperatures for a given rate, as shown above.…”

Spontaneous formation of multilamellar vesicles from aqueous micellar solutions of sodium linear alkylbenzene sulfonate (NaLAS)

“…6c. [36] Thermal reversibility. In order to assess the reversibility of the micellarto-MLV transformation, a series of thermal cycles and isothermal measurements were carried out by SANS, as shown in Fig.…”