CRYSTAL PHASES OF DODECYL SULFATES OBTAINED FROM AQUEOUS-SOLUTIONS - STRUCTURE OF THE HEXAAQUAMAGNESIUM SALT

Hexaaquamagnesium dodecyl sulfate, [Mg(H2O)6][C12H25O4S]2, M(r) = 663.2, monoclinic, A2, a = 6.2860 (4), b = 7.0234 (6), c = 42.086 (3) angstrom, beta = 92.915 (5)-degrees, V = 1855.7 (2) angstrom 3, Z = 4, D(x) = 1.19 g cm-3, lambda-(Cu K-alpha) = 1.5418 angstrom, mu-(Cu K-alpha) = 18.9 cm-1, F(000) = 724, room temperature, R = 0.074 for 1614 independent reflections with I greater-than-or-equal-to 1.5-sigma-(I). The crystal packing is characterized by a lamellar structure where polar and apolar regions alternate. In the polar region each magnesium ion is hydrated by six water molecules octahedrally coordinated and each water molecule forms two hydrogen bonds with different O atoms of the adjacent polar heads. Therefore, the Coulombic interactions in the polar region are partially shielded by the water molecules surrounding the cation. In contrast with previously determined crystal packings of Na and Rb dodecyl sulfates, in the apolar region the hydrocarbon chains of opposite bilayer halves interdigitate completely and form a common matrix. The intermolecular contacts among the C atoms of the adjacent hydrocarbon chains show, however, that weak van der Waals interactions contribute to the packing in this region.