Polymorphism of Alkyl Sulphate Soaps

Abstract

ALKYL sulphate soaps NaSO₄C_nH_{2n + 1}, like the fatty acids and alcohols, may be expected to have a tendency to join together the hydrophylic groups of neighbouring molecules. In the crystalline state, this tendency will lead to sheets of double molecules with stretched chains more or less perpendicular to the sheets. We have measured the corresponding long spacing, using the classical method of copper Kα reflexion, at a thin layer of soap on a glass slide, the distance plate to axis in the Muller camera being 10 cm. During the investigation, we saw a paper by Rawlings and Lingafelter¹, who made a complete determination of the unit cells. They found different modifications, the α-form with the smallest water content being the most stable. We have also found this α-modification. Moreover, we succeeded in making all our substances crystallize completely in this modification. This proves the chemical homogeneity (within some per cent) of our substances and their identity with Rawlings and Lingafelter's substances. As we have, however, also found a number of rather stable modifications not mentioned by them, we give here a short account of our procedure.