Crystal structure of silver salts of fatty acids

Abstract

Seven silver salts of even-numbered n-fatty acids were investigated by X:rays at 20 and at 78 ° C. Their unit cells were determined from powder photographs; they proved to be triclinic with two molecules of silver salt per unit cell. At 20 ° C. the unit cell varies gradually with chain length from a-4-588, b-4.016, c-20.41 A., ~-101 ° 12',/?-102 ° 28", 7-80° 4' for silver caproate, to a-4.693, b-4.120, c-50.35 A., c¢-104 ° 35',/?= 93 ° 59', 7-76° 1' for silver stearate. Changes of the unit-cell dimensions with temperature have been observed. A packing of the molecules in the unit cell is suggested. Introduction Little is known about the crystallography of silver salts of fatty acids. As we expected that soaps of heavy metals might be easier to study by X-rays than sodium soaps, since the presence of a heavy atom causes lines with high indices to appear stronger, silver soaps were chosen for our study in the hope that they might throw more light on the structure o£ sodium soaps. As we expected, the powder lines appeared sharper and the resolution of photographs better than for sodium soaps, and the powder patterns proved to be closely related to ~ and e forms of sodium soaps of Buerger, Smith, Ryer & Spike (1945). These forms were previously suspected by us to be triclinic. As we shall see later, the structure of silver soaps proved to be triclinic, giving valuable evidence in support of the trielinic structure of the above forms of sodium soaps. The intercomparison of the powder photographs of silver soaps prepared by us shows that they are all in the same form. It should be noted, however, that there exist other forms of silver soaps, as one sample of silver stearate from another source proved to be a mixture of forms different from that studied in this paper.