Intercalation of cations in crystalline anionic surfactants

Abstract

Crystal structures of sodium, potassium and cesium salts of anionic surfactants, methyl 2-sulfoalkanoates, were analyzed by X-ray methods to characterize physico-chemical properties of the solid state in relation to counter ions. The crystal contains racemic molecules with the stereogenic β carbon atom. The crystals have a bilayer structure with the interdigitated alkyl chain of anions, while the cations and water molecules are intercalated between the layers. These crystals have different thermal stability indicated by the decrease in melting temperature in the order of potassium, cesium and sodium salts. The crystals of sodium, potassium and cesium salts contain two, one and one water molecules, respectively. The space group is Pbca for all of these crystals having the same type of crystal packing of anions regardless of the different cations. The crystal packing of the potassium salts is not significantly affected by the alkyl chain length, except for the difference in the c dimension. The energetic difference of the crystal structures was analyzed by molecular mechanics calculations using X-ray coordinates. The thermal stability of the crystal is related to the crystal structure, especially to the packing of cations and sulfonato groups between the layers. The potassium ion contributes more to the thermal stabilization of the crystal than the sodium and cesium ions because of more effective contact with the sulfonato groups by less coordination with the water molecule and by acquired electrostatic potential. The close packing of ionic layers observed in the crystals of potassium salts causes dense packing of the alkyl chains which stabilizes the crystal packing.