Simultaneous determination of the lipophilicity and dissociation constants of dialkyl phosphinic acids by negligible depletion hollow fiber membrane-protected liquid-phase microextraction.

Determination of the physicochemical properties, especially the lipophilicity (expressed as the logarithm of distribution coefficient, log D) and dissociation constant (pKa), is of great importance in the early stage of environmental risk assessment for an ionizable compound without these data. Currently, the log D and pKa values of dialkyl phosphinic acids (DPAs), the environmental hydrolysates of aluminum dialkyl phosphinates (ADPs) that is one class of emerging phosphorus-containing flame retardants, are not available. In this study, the log D and pKa values of three DPAs including methylethylphosphinic acid (MEPA), diethylphosphinic acid (DEPA) and methylcyclohexyl phosphinic acid (MHPA), were simultaneously determined by negligible depletion hollow fiber supported liquid phase microextraction (nd-HF-LPME) followed by ultra-performance liquid chromatography coupled with tandem mass spectrometry (UPLC-MS/MS). The pKa and log D of DPAs were determined by curve-fitting the experimental data with equations derived on the basis of the Henderson-Hasselbalch equation and compared with model calculated data. For MEPA, DEPA and MHPA, the pKa values were close and around 3, but the log Ds were strongly pH-dependent with values from -5.01 to 1.01. The log KOW of the neutral form (logKOW,HA) and ionic form (logKOW,A) were in the range of -0.67-1.02 and -3.86--1.33, respectively. The experimentally determined pKa values were highly in good agreement with ACD/pKa predicted values and the measured log KOW,HA values were closely related to KOWWIN calculated ones, suggesting ACD/pKa and KOWWIN are good alternative methods to estimate pKa and log KOW of DPAs, respectively. As far as we know, this is the first report on the pKa and log D data for DPAs, which are fundamental for the product design and evaluating the environmental behavior and effects of DPAs and ADPs.