Infrared Spectroscopic Study of the Crystalline Phases of Sodium Dodecyl Sulfate

Abstract

Four hydrate forms of sodium dodecyl sulfate (SDS), the 1/8 hydrate, the hemihydrate, the monohydrate, and the dihydrate, and the Rb+ and Mg2+ salts of dodecyl sulfate (RbDS and MgDS) were synthesized under conditions previously used to form crystals for X-ray crystallography. Correlations were made between the IR transmission spectra and crystal structures. IR spectra of 10−75 wt % slurries of DS salts in water were examined at the same concentration and after the same thermal treatments as previously described for “coagel phases” of SDS. The “coagels” of SDS were shown to consist of suspensions of well-characterized crystalline hydrate phases or of metastable solids in the process of conversion to one of the hydrates described; the term “coagel” should be rejected in favor of “crystalline phase”. The crystalline phases of SDS formed under “coagelization” included the hemihydrate, the monohydrate, and the dihydrate. Only the dihydrate was stable (two different forms, interconverted at 19.6 °C) in contact with aqueous solutions at temperatures below 55 °C and compositions below 87.8 wt %, as predicted in the literature. The other hydrates slowly interconverted to the dihydrate when in contact with aqueous solutions, even below 0 °C. The interconversion often required >7 days under the conditions favoring formation of a single hydrate, but did occur in <2 h near 5 °C. The practice of prediction of order in the packing of alkyl chains based on the position of the 2920 cm-1 band was criticized; SDS monohydrate, having one all-trans molecule and one chain with one gauche bond, presented the band at 2915.7 cm-1; the hemihydrate, however, with all-trans bonds presented the band at 2923.2 cm-1. The relative absorbances of the 2850 and 2920 cm-1 peaks in SDS changed drastically with crystallinity and crystal identity. Three of the unit cells (methylene local symmetries, MLS) of the packed all-trans alkyl chains occurring in these phases were novel. The A2/m structures (of SDS mono- and hemihydrate) have been predicted but not previously observed. The MLS of RbDS required four chains to describe the methylene unit cell but is otherwise similar to “hybrid” cells, while the nonintegral chain-to-chain axial offset in the MgDS structure was unique.