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 (pK), is of great importance in the early stage of environmental risk assessment for an ionizable compound without these data. Currently, the log D and pK 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 pK 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 pK 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 pK values were close and around 3, but the log Ds were strongly pH-dependent with values from -5.01 to 1.01. The log K of the neutral form (logK) and ionic form (logK) were in the range of -0.67-1.02 and -3.86--1.33, respectively. The experimentally determined pK values were highly in good agreement with ACD/pK predicted values and the measured log K values were closely related to KOWWIN calculated ones, suggesting ACD/pK and KOWWIN are good alternative methods to estimate pK and log K of DPAs, respectively. As far as we know, this is the first report on the pK and log D data for DPAs, which are fundamental for the product design and evaluating the environmental behavior and effects of DPAs and ADPs.

Sulfur and Phosphorus Oxyacid Radicals

“… a Experimental p K a data as follows: H 3 PO 4 , 49 HPO(OH) 2 , P(OH) 3 , P(OH) 2 (OEt), HPO(OH)(OEt), P(OH)(OEt) 2 and HPO(OEt) 2, 50 PO(OH)(Et) 2 , 51 PO(OH)( t Bu) 2 , 52 H 2 SO 4 and HSO 3 Me, 53 H 2 SO 3 , 54 HSO 3 F, 55 HSO 3 Et, 56 and HSO 3 CF 3 ,. 57 …”