An investigation of the dilute solutions of lauric acid and laurate ion in cationic and anionic bilayers of disc micelles

Abstract

Type II DM mesophases with disc-shaped micelles have been synthesized from decylammonium chloride and sodium decylsulphate – decanol amphiphiles with added water and electrolyte. Small amounts of guest amphiphiles have been added in the form of lauric acid or laurate ion. Specific and perdeuterated amphiphiles have been prepared and included in small quantifies in the mesophases. The deuterium magnetic resonance spectra of the magnetically aligned phases has been studied and quadrupole splitting doublets assigned to all segments of both host and guest hydrocarbon chains. The mesophases have been diluted with water over the stable range of type II DM behaviour at 34 ± 0.2 °C.The quadrupole splittings of all hydrocarbon chain segments, which reflect the degrees of order along C—D bonds in these segments, are accurately reproduced at all water contents by a knowledge of one quadrupole splitting in the chain and a simple proportionality constant which is derived for each segment of the chain. This further extends the previous results for host chains to those of guest chains studied here. The absolute degrees of order of chain segments are greatly influenced by water content because motion of the constituent disc micelles depends on the volume of interstitial water and they also vary in size as water is added. The packing in the bilayer solution of the disc micelles of the amphiphiles is therefore not a function of the interstitial water content in so far as the detailed trans/gauche motions of the hydrocarbon chains are not affected by changes in water content.The lauric acid-d23 chain in the decylammonium chloride bilayer has the same degree of order along the D—D axis and the C—D bond, confirming that the motion of the pseudo rigid chain in the bilayer is cylindrically symmetric. The degree of order profiles of hosts and guests are all quite distinct indicating that chemical identity of the head group and its anchoring in the interface is the main determinant of the motion profiles.