Partitioning of chloromethanes between aqueous and surfactant micellar phases

Abstract

The partition constants for three volatile organic chemicals (VOCs)—methylene chloride, chloroform and carbon tetrachloride—between aqueous and surfactant micellar phases were determined using an equilibrium partitioning method. The surfactants used were sodium dodecylbenzene sulfonate (DDBS), sodium dodecylsulfate (DDS) and hexadecyltrimethyl ammonium bromide (HTAB). The vapor concentration of the VOCs remained unchanged until the critical micellar concentration (CMC) of the surfactant was reached. Above the CMC the vapor concentration decreased linearly with increasing surfactant concentration in the aqueous phase. The partition constants ( K m) were observed to increase with increasing ionic strength of the solution. The partition constants also increased with increasing hydrophobicity of the VOCs. A pseudo-two-phase model was used to determine the K m values and was found to fit the experimental data well. Cationic micelles of HTAB gave larger K m values than anionic micelles of DDS and DDBS surfactants. K m values also increased with increasing temperature. From a thermodynamic treatment of the equilibrium process the values of the partition constants for transfer of VOCs from an ideal vapor phase to the micellar phase were calculated. These values were found to fall in between the values for aliphatic hydrocarbons and alcohols.