In hard water, dodecyl sulfate (DS) tends to precipitate with the counterions, Ca2+ and Mg2+ forming soap scum, and is thus no longer available to participate in the cleaning action. The scum formed from the precipitation is also a problem in that it causes graying of the fabric. Solubility data of the precipitated calcium dodecyl sulfate (CDS) crystals remains quite limited. Consequently, this work has investigated the activity-based solubility product of CDS in aqueous solutions as a function of temperature, with modeling between 10 and 55°C. Two techniques of measuring the solubility were used. Below the Krafft temperature, approximately 50°C for CDS, the solutions were saturated at fixed temperatures and then analyzed using atomic-absorption spectroscopy. Above the Krafft temperature a conductivity technique was used. Plotting the conductivity of a fixed concentration solution against temperature allows the solubility to be found from a discontinuity in the derivative. The activity-based solubility product, KSP, was calculated from the experimental data based on modeling of the micellar phase in solutions for the samples which were above the critical micelle concentration. The two techniques for measuring the solubility of CDS yield consistent values of the solubility product, which can be used to model the phase boundary of CDS precipitation in solutions of sodium dodecyl sulfate (SDS) and calcium chloride as a function of temperature.
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