Solvation descriptors for pesticides from the solubility of solids: diuron as an example

Solubilities of diuron in 19 organic solvents are reported. These are combined with the solubility in water and the vapour pressure of diuron to give 19 values of water–solvent partitions, P, and 19 values of gas–solvent partitions, L. Coefficients in the general solvation equations i and ii are known for all 38 of these systems. In equations i and ii, the independent variables are solute descriptors as follows: R2 is an excess molar refraction, π2H is the dipolarity/polarizability, Σα2H and Σβ2H are the overall hydrogen‐bond acidity and basicity, Vx is the McGowan characteristic volume, and log L16 is a descriptor where L16 is the solute L coefficient on hexadecane at 298 K. i,ii $$ \eqalign{ \log SP & = c + rR_{2} + {s\pi_{2}}^{H} + {a\Sigma \alpha_{2}}^{H} + {b\Sigma \beta_{2}}^{H} + \nu Vx \cr \noalign{\vskip5pt} \log SP & = c + rR_{2} + {s\pi_{2}}^{H} + {a\Sigma \alpha_{2}}^{H} + {b\Sigma \beta_{2}}^{H} + \log L^{16} \cr} $$ We estimate R2 as 1.28 and calculate Vx as 1.5992, and then solve the total set of 38 equations to yield π2H = 1.60, Σα2H = 0.57, Σβ2H = 0.70 and log L16 = 8.06 log units. These descriptors reproduce the 38 observed log P and log L values with a standard deviation of only 0.12 log units, and a test set of 13 independent log P and log L values with a standard deviation of 0.10 log units. Once the solvation descriptors for diuron are known, a range of physicochemical properties can be predicted; some examples are given. Similar analyses are reported for monuron and other substituted 3‐phenyl‐1,1‐dimethylureas. © 2000 Society of Chemical Industry