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

Abstract Determination of the physicochemical properties, especially the lipophilicity (expressed as the logarithm of distribution coefficient, log D ) and dissociation constant (p K a ), is of great importance in the early stage of environmental risk assessment for an ionizable compound without these data. Currently, the log D and p K a 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 p K a 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 p K a 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 p K a values were close and around 3, but the log D s were strongly pH-dependent with values from −5.01 to 1.01. The log K OW of the neutral form (log  K OW,HA ) and ionic form (log  K OW,A ) were in the range of −0.67–1.02 and −3.86–−1.33, respectively. The experimentally determined p K a values were highly in good agreement with ACD/p K a predicted values and the measured log K OW,HA values were closely related to KOWWIN calculated ones, suggesting ACD/p K a and KOWWIN are good alternative methods to estimate p K a and log K OW of DPAs, respectively. As far as we know, this is the first report on the p K a and log D data for DPAs, which are fundamental for the product design and evaluating the environmental behavior and effects of DPAs and ADPs.