Effect of lithium encapsulation by a macrocyclic aza cage in micellar solutions of lithium dodecyl sulfate

The small macrobicyclic cage (5,12,17-trimethyl-1,5,9,12,17-pentaazobicyclo[7.5.5]nonadecane, CESTO), which can selectively encapsulate lithium ions, has been studied in the presence of micellar solutions of 0.037 M lithium dodecyl sulfate (LDS). The CESTO cage behaves as a fairly strong base in the first protonation step (log K1=11.83 where K1 is the equilibrium constant) and exhibits two different behaviors in LDS micellar solutions. Surface tension, electron spin echo modulation (ESEM), 7Li NMR and small-angle neutron scattering (SANS) measurements show that the macrocyclic cage at about pH 10.2 behaves as a bulkier counterion, while at about pH 12.7 it complexes selectively lithium counterions at the micellar surface. Furthermore, ESEM studies show that CESTO encapsulates lithium ions at the micellar surface by reducing the amount of water at the surfactant polar head groups. SANS results show that, at about pH 12.5, LDS molecules form micelles of quasi-spherical shape with an average aggregation number of 90. The effective ionization of the micelles is about 20%. The addition of CESTO leads to lithium ion complexation with a drastic reduction of the micellar surface charge, and of the hydration number. This is accompanied by the growth of the aggregation number and the micelles become more elongated.