Abstract
The crystal structures of the hydrated cationic surfactant benzethonium (Bzth) chloride, bromide, hydroxide, and citrate have been determined by X-ray diffraction analysis and compared with their structures in solution well above their critical micelle concentration. The differences in the nature of the various anions of the four Bzth-X materials lead to unique anion environments and 3-D molecular arrangements. The water molecule in the monoclinic Bzth-Cl or Bzth-Br forms is hydrogen bonded to the halides and particularly to the hydrogens of the methoxy groups of the Bzth moiety notwithstanding the weak Brønsted acidity of the methoxy hydrogens. The citrate strongly interacts with the hydrogens of the methoxy group forming an embedded anionic spherical cluster of a radius of 2.6 Å. The Bzth-OH crystallizes in a hexagonal lattice with two water molecules and reveals free water molecules forming hydrogen bonded channels through the Bzth-OH crystal along the c-axis. The distances between the cationic nitrogen and the halides are 4.04 Å and 4.20 Å, significantly longer than expected for typical van der Waals distances of 3.30 Å. The structures show weakly interacting, alternating apolar and polar layers, which run parallel to the crystallographic a-b planes or a-c planes. The Bzth-X salts were also examined in aqueous solution containing 20% (v/v) ethanol and 1.0 % (v/v) glycerol well above their critical micelle concentration by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The [1,1,1] planes for the Bzth Cl or Br, the [0,0,2] and [1,1,0] planes for the Bzth-citrate, the [2,-1,0] planes and the [0,0,1] planes for the Bzth-OH found in the crystalline phase were also present in the solution phase, accordingly, the preservation of these phases are a strong indication of periodicity in the solution phase.
Highlights
The cationic benzethonium (Bzth) moiety which is chemically a diisobutylphenoxyethoxyethyl dimethyl-benzyl ammonium compound (Fig. 1)1 and belongs to the class of quaternary ammonium salts showing exceptional strong antimicrobial, antiviral and antiseptic activities
The Bzth-X salts were examined in aqueous solution containing 20% (v/v) ethanol and 1.0 % (v/v) glycerol well above their critical micelle concentration by small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS)
The [1,1,1] planes for the Bzth Cl or Br, the [0,0,2] and [1,1,0] planes for the Bzth-citrate, the [2,1,0] planes and the [0,0,1] planes for the Bzth-OH found in the crystalline phase were present in the solution phase, the preservation of these phases are a strong indication of periodicity in the solution phase
Summary
The cationic benzethonium (Bzth) moiety which is chemically a diisobutylphenoxyethoxyethyl dimethyl-benzyl ammonium compound (Fig. 1) and belongs to the class of quaternary ammonium salts showing exceptional strong antimicrobial, antiviral and antiseptic activities. Bzth-X salts or complexes do not belong to the classical N-cationic single or double-chained quaternary lipids or surfactants, it follows a similar path as double chained or single chained N-cationic surfactants (lipids) with the accepted sequences: (i) adsorption onto the microbial cell surface; (ii) possible diffusion of the monomers through the cell wall and changes in permeabilities; (iii) disruption of the cytoplasmic membrane with simultaneous release of cytoplasmic constituents and (iv) death of the cell.15–17 This contribution presents a comprehensive study on the crystalline Bzth-X structures with X = Cl, Br, the citrate, and OH, informed about the hydration, counterion location, the conformation of the two-methylene oxide (ether) groups and the observed weak Brønsted acidity of the methoxy hydrogens.
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