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

Abstract

The small macrobicyclic cage (5,12,17-trimethyl-l,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 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.

Molecular Dynamics of Novel α-Cyclodextrin Adducts Studied by 13C-NMR Relaxation

“…In the recent past the study of molecular recognition operated by low molecular weight ligands has been carried out on crown-ethers, cryptands, , cyclophanes, and calixarenes 28,29 that selectively bind ions (such as Na + , K + , Cs + , guanidinium, and UO 2 2+ ) and organic molecules (like chloroform, benzene, toluene, or fullerenes). On the other hand, very large host−guest systems are required for the study of more complicated biological complexes such as enzyme−substrate, antigen−antibody, DNA, RNA, and cell adhesion systems …”

Micellar Solutions of Octyl-18-crown-6

“…The nature and the behavior of alkyl-crown and alkyl kryptand − micellar systems are greatly affected by ion complexation performed by the polar head-group of the surfactant. This surfactant will then carry an ion and its behavior will resemble that of an ionic tenside.…”

“…This surfactant will then carry an ion and its behavior will resemble that of an ionic tenside. In this way the micellar interface charge can be controlled by simply changing the surfactant/salt molar ratio, , the pH can also be used to control the charge, especially when the complexing atom is nitrogen such as in azamacrocyclic cages. , Therefore, the addition of specific ions or the variation of the pH determines a significant change in the micellar surface charge, that is from nonionic to fully ionic micelles. , In previous papers 12,16-18 we used a macrobicyclic cage, namely the 5,12,17-trimethyl-1,5,9,12,17-pentaazobicyclo[7.5.5]nonadecane, to complex lithium counterions in lithium dodecyl sulfate solutions. Besides the reduction of the micellar charge upon complexation of Li + , the addition of the macrocyclic ligand results in a strong increment in the aggregation number and in a change of the micellar shape from spheres to elongated ellipsoids .…”

“…In our studies we have so far used the complexing properties of different ligands to encapsulate the counterions and shield them from the micellar interface, in order to evaluate the macroion−macroion and the macroion−counterion interactions. More recently, our group focused its attention on those amphiphilic compounds, such as alkyl-crown ethers and calix[ n ]arenes, that join a significant surface activity and high selectivity for complexing specific cations or including small organic molecules in host-guest systems. − ,, …”

Preparation of Polyrotaxanes and Molecular Tubes for Host—Guest Systems

“…In the recent past the study of molecular recognition, operated by relatively small ligands that selectively bind ions and organic molecules, has been the object of several works (26)(27)(28)(29)(30)(31). However, very large host-guest systems are required for the study of more complicated biological complexes such as enzyme-substrate, antigenantibody, DNA, RNA, and cell adhesion systems (22).…”