Location and Average Alignment of Alkylpyridinium Ions in Cationic Nematic Lyomesophases

Abstract

Deuterium quadrupole splittings from the aromatic ring of a series of deuterium labeled n-alkylpyridinium ions with linear n-alkyl groups from Ci to Cie have been measured using *12 345H-NMR spectroscopy. These splittings were used to calculate the two order parameters describing the average alignment of the individual pyridinium rings, and these values were used to determine the location and preferred orientation of the ions when contained in either of the lyotropic liquid crystals of this study. Two cationic mesophases, prepared from either hexadecylpyridinium chloride or tetradecyltrimethylammonium bromide, were used as the host systems. The smaller alkylpyridinium ions (C1-C5) were found predominantely in the interstitial water. The intermediate size ions (Ce-C10), which were distributed more effectively into the micelle, have a disruptive effect in the mesophase, probably a result of modifications in the degree of alignment of the micelle, possibly because of modifications of its internal dynamics. Differences in the symmetry properties of the positively charged headgroups were found to impose restrictions on the motion of the dissolved alkylpyridinium ions.

Solubilization of N-Alkylpyridinium Ions in Anionic Nematic Lyomesophases

“…It has also been recently observed that addition of methylsalicylic acid and hydroxybenzoic acid to micellar solutions of cetyltrimethylammonium bromide causes spherical micelles to undergo a phase transition to wormlike micelles . In a previous study, we have observed that intermediate chain length N -alkylpyridinium ions (C 5 to C 10 ) have a disruptive effect on the integrity of the interface of lyotropic liquid crystals made from hexadecylpyridinium chloride and tetradecyltrimethylammonium bromide …”

“…Synthesis of N -Alkylpyridinium Salts. The synthesis of the pyridinium salts, their purification, and characterization have been previously described . All these compounds are permanently stored under vacuum over phosphorus pentoxide, due to their high hygroscopicity, particularly the short chain derivatives (C 1 to C 10 ).…”

Average Orientation of Benzyl-d5 Alkyl Ethers in Cationic Nematic Lyomesophases

“…An inspection of this table shows that the magnitudes of the calculated order parameters are greater for the TDTMABr mesophase. They are also bigger than the corresponding values of the pyridinium analogs, particularly the shorter alkyl chain length derivatives . This is expected for more hydrophobic molecules.…”

“…This effect experiences a small decrease when the alkyl chain length of the ether increases. As may be seen from Table , the quadrupole splitting of water in HDPyCl is always larger than that in TDTMABr, possibly as a consequence of the strong hydrophilic character of the pyridinium head group …”

“… − The use of 2 H-NMR quadrupole splittings is possibly one of the simplest methods to estimate molecular order parameters and, from these, to infer the average orientation, and sometimes the location, of guest molecules dissolved in anisotropic solutions. Recently, using 2 H-NMR spectroscopy, we have measured the two order parameters that completely describe the average orientation and have been able to infer the location of the aromatic ring in a series of linear N -alkylpyridinium- d 5 ions, dissolved in cationic and anionic nematic lyotropic liquid crystals. , All the mesophases studied by us were type II systems . We found that addition of small amounts of pyridinium ions to liquid crystal solutions made from hexadecylpyridinium chloride (HDPyCl), tetradecyltrimethylammonium bromide (TDTMABr), and cesium decyl sulfate (CsDS) had an appreciable effect on the mobility of these interfaces.…”