The effect of surfactants on equilibria in aluminium(III) ion plus ofloxacin solution and adsorption of ofloxacin on aluminium-oxide

The protonation and complex formation equilibria in aluminium(III) + ofloxacin (Hoflo) solutions in the presence of either cetyltrimethylammonium bromide (CTAB, 5.0 mmol L-1), cetylpyridinium chloride (CPCL, 2.0 mmol L-1) or polyethylene glycol tert-octylphenyl ether (triton X-100, 1.0 mmol L-1) have been studied by glass-electrode potentiometric measurements in a 0.1 mol L-1 LiCl ionic medium at 298 K. In the concentration range 0.4 less than or equal to C-Al less than or equal to 1.0 mmol L-1, with a ligand to metal ratio of 1:1 to 5:1, and 3.5 less than or equal to -log h less than or equal to 7.0, a non-linear least squares treatment of the data indicate that in all studied systems the complex Al(oflo)(2+) forms as the dominating one. Its overall stability constant (log beta) is in the range 10.37-11.90 (depending on the type of surfactant), which is about 1 log unit higher than in the absence of surfactants. The formation of bis(ofloxacinato) and mixed protonated or hydrolytic complexes is largely suppressed in the presence of surfactants. The adsorption of ofloxacin on aluminium oxide was studied in neutral, acidic (0.1 mol L-1 HCl) and alkaline (0.001 mol L-1 NaOH) media; in absence and presence of ionic surfactants; sodium dodecylsulfate (SDS, 10 mmol L-1); or CTA-B (5.0 mmol L-1). The adsorption is of the Freundlich type and is higher in neutral media with no presence of surfactants, while in an acidic medium it is significantly enhanced in the presence of SDS. Both surfactants increase the adsorption in an alkaline medium. The observed phenomena were explained based on the hydrophilic/lipophilic properties of the surfactants.