Cuticular penetration of 2,4-D as affected by interaction between a diethylene glycol monooleate surfactant and apple leaf cuticles

Impacts of pH and sorption-desorption of ‘Pegosperse’ 100-O (PEG. 100-O; diethylene glycol monooleate, containing 15% diester) surfactant by apple (Malus pumila M.) leaf cuticles on surfactant-enhanced cuticular penetration of 2,4-D [(2,4-dichlorophenoxy)acetic acid] were studied. Glass cylinders were affixed to enzymatically isolated adaxial apple leaf cuticles after the cuticle segments had been soaked in 10 ml liter^?1 PEG 100-O solution and washed for 20 and 120 min, respectively. Quantities of [¹⁴C]2,4-D in the glass-cuticle chambers passing through the cuticles at pH values from 1 to 6 5 were determined. PEG 100-O significantly increased cuticular penetration of dissociated 2,4-D at pH 4–5; the surfactant had no effect on penetration of undissociated 2,4-D at pH 10. Surfactant-enhanced penetration of 2,4-D occurred only when the surfactant was in the cuticles, while the process of surfactant sorption-desorption alone had no effect on penetration. These results support a ‘hydrophilic channel’ hypothesis, i.e. that surfactants may create hydrophilic channels or increase the area of the channels in the cuticle and, consequently, enhance the passing of polar molecules like dissociated 2,4-D through the cuticle.     

Effects of nonionic surfactants on cuticular sorption and penetration of 2, 4-dichlorophenoxy acetic acid

Surfactants increase the uptake of some foliar-applied chemicals to a greater extent than would be expected from their effects on surface tension and spray coverage. This study of the uptake of 2, 4-D [(2, 4-dichlorophenoxy)acetic acid] evaluated the effect of surfactants on penetration through and sorption by isolated cuticles of apple leaves. [¹⁴C]2, 4-D was placed in glass chambers affixed to enzymatically isolated adaxial apple leaf cuticles after the cuticle segments had been treated with various surfactants. The same surfactant pretreatments were included in sorption studies in which cuticle segments were immersed in [¹⁴C]2, 4-D for 96 h. Quantities of 2, 4-D passing through or sorbed by the cuticle were determined. Similar experiments were conducted with unaltered cuticles and cuticles dewaxed with chloroform. The hydrophile-lipophile balance (HLB) of polyethylene-glycol-based surfactants was inversely related to the sorption of those surfactants by the cuticles and penetration of 2, 4-D. Sorption of 2, 4-D by apple leaf cuticles was unaffected by surfactant pretreatment. Dewaxed cuticle membranes showed a similar response to 2, 4-D penetration and sorption following the surfactant pretreatment.     

Variability of retention process of isoxaflutole and its diketonitrile metabolite in soil under conventional and conservation tillage

BACKGROUND: Sorption largely controls pesticide fate in soils because it influences its availability for biodegradation or transport in the soil water. In this study, variability of sorption and desorption of isoxaflutole (IFT) and its active metabolite diketonitrile (DKN) was investigated under conventional and conservation tillage. RESULTS: According to soil samples, IFT K_D values ranged from 1.4 to 3.2 L kg^?1 and DKN K_D values ranged from 0.02 to 0.17 L kg^?1. Positive correlations were found between organic carbon content and IFT and DKN sorption. IFT and DKN sorption was higher under conservation than under conventional tillage owing to higher organic carbon content. Under conservation tillage, measurements on maize and oat residues collected from the soil surface showed a greater sorption of IFT on plant residues than on soil samples, with the highest sorbed quantities measured on maize residues (K_D ≈ 45 L kg^?1). Desorption of IFT was hysteretic, and, after five consecutive desorptions, between 72 and 89% of the sorbed IFT was desorbed from soil samples. For maize residues, desorption was weak (<50% of the sorbed IFT), but, after two complementary desorptions allowing for IFT hydrolysis, DKN was released from maize residues. CONCLUSION: Owing to an increase in organic carbon in topsoil layers, sorption of IFT and DKN was enhanced under conservation tillage. Greater sorption capacities under conservation tillage could help in decreasing DKN leaching to groundwater. Copyright © 2012 Society of Chemical Industry     

Characterizing pesticide and surfactant penetration with isolated plant cuticles

A survey is presented of the use of isolated leaf and fruit cuticles in studying foliar penetration. Isolated cuticles, which represent the prime barrier to penetration, provide a physical system with which transport studies can be conducted under well-defined and highly controlled conditions while avoiding the physiological effects of biologically active compounds on the system. Most studies with isolated cuticles have focused on sorption, desorption and infinite-dose cuticular transport of compounds in aqueous systems. Partition coefficients can be calculated for pesticides from sorption data, thereby yielding information on the solubility of the compounds in the cuticle. Permeance and diffusion coefficients can be calculated from cuticular transport studies. These transport parameters provide for a better understanding of the mechanisms of cuticular penetration. Further, they are useful in comparing both the penetration characteristics of selected compounds and permeability of cuticles. Transport systems, using isolated cuticles, may be used to quantify the effects of spray additives on pesticide penetration. A finite-dose system is described that may be useful in studying spray droplet/deposit interactions with the cuticle and, concurrently, following transcuticular penetration from droplets/deposits on the surface. These systems may prove useful in optimizing spray formulations and spray parameters, leading to more efficient pesticide development and application.     

Analysis of foliar uptake of pesticides in barley leaves: Role of epicuticular waxes and compartmentation

Uptake of pesticides into barley leaves was measured under controlled conditions. Leaves detached from plants were submerged in aqueous solutions of ¹⁴C-labelled (2,4-dichlorophenoxy)acetic acid, triadimenol, bitertanol and pentachlorophenol. Uptake was biphasic. A short (30-min) period with high rates of uptake was followed by uptake that proceeded more slowly and was steady over hours. Compartmentation of pesticides was studied by desorbing pentachlorophenol from leaves previously loaded with [¹⁴C]pentachlorophenol. From the uptake and desorption kinetics it was concluded that penetration of pesticides proceeds as follows: the compounds are first sorbed at the surface of epicuticular wax aggregates where they are in contact with the donor solutions. Solutes then diffuse through the surface wax aggregates into the cuticle. Equilibrium between donor solutions, surface wax and cuticle is established in about 30 min. After this time the amounts of solutes in these compartments no longer increase. Uptake after this time represents penetration into the leaf cells. This fraction of the pentachlorophenol is retained irreversibly, while that sorbed in wax and cutin can be desorbed again. All compounds were sorbed in cuticular waxes and partition coefficients wax/water were determined. On a mass basis only 5 to 10% of the amounts sorbed in cutin are sorbed in wax. This comparatively low solubility in wax contributes to the barrier properties of cuticular waxes. The other determinant of permeability is the very low mobility of solutes in cuticular waxes.     

Influence of some nonionic surfactants on water sorption by isolated tomato fruit cuticles in relation to cuticular penetration of glyphosate

The effects of several nonionic surfactants on [¹⁴C]glyphosate mono(isopropylammonium) diffusion across isolated tomato fruit cuticles (Lycopersicon esculentum Mill.) were compared under controlled atmospheric conditions (25°C; 65% R.H.) using a model system consisting of 1-μl droplets applied to isolated cuticles on agar blocks. Rates of diffusion for glyphosate (10 g acid equivalent litre^?1 in the applied solution) and overall amounts recovered in underlying agar blocks were influenced by the ethylene oxide (EO) chain length for a homologous nonylphenol surfactant series (10 g litre^?1). Glyphosate uptake increased with EO content, reaching an optimum at a mean of 17 EO, then decreasing below control values for surfactants with 40 EO. There was a strong influence of the hydrophobe on glyphosate penetration for different surfactants with similar mean EO content (10 EO). The primary aliphatic amine enhanced penetration the most, followed by the nonylphenol while the aliphatic alcohol showed no improvement on glyphosate transfer across cuticles. Water soprtion was greatly enhanced by a primary aliphatic amine (10 EO) and by a nonylphenol (17 EO). The aliphatic alcohol (10 EO) and a shorter-chained nonylphenol (4 EO) did not significantly enhance water sorption. Comparison of water sorption with glyphosate diffusion across cuticles suggests a strong relationship between the two. Change in solution pH over a limited range had no significant effect. Promotion of cuticular hydration by surfactants may thus play an important role in the enhancement of foliar uptake of water-soluble herbicides such as glyphosate.     

Kinetics of sorption-desorption of benfuracarb insecticide in mollisols

BACKGROUND: Sorption‐desorption processes govern the movement of pesticides in soil. These processes determine the potential hazard of the pesticide in a given environment for groundwater contamination and need to be investigated. RESULTS: In the present study, sorption‐desorption processes of benfuracarb were investigated using a batch method in two mollisols. The kinetics of benfuracarb sorption in mollisols conformed to two‐compartment (1 + 1) first‐order kinetics. The fast sorption rate constant was about 3 times higher for silt loam than for loam soil. However, the slow sorption rate constants were statistically similar for both soils. The concentration‐dependent sorption‐desorption isotherms of benfuracarb could not closely conform to the Freundlich isotherm in mollisols of high organic C content. The computed values of both the sorption (log K) and desorption (log K′) capacities were higher for silt loam than for loam soil. The desorption index (n′/n) values in the range 30.0–41.3 indicated poor reversibility of sorbed benfuracarb in mollisols. CONCLUSION: In view of the strong sorption of benfuracarb in mollisols with only partial desorption, the possibility of the leaching of soil‐applied benfuracarb to contaminate groundwaters appears to be low. Copyright © 2010 Society of Chemical Industry     

Influence of soil pH-sorption interactions on the carry-over of fresh and aged soil residues of imazamox

Long-term carry-over of some imidazolinone herbicides has been observed in soil that is below pH 6.5, resulting in significant damage to sugarbeet. In a field study in south-west Minnesota, imazamox concentration decreased rapidly in soil, regardless of pH. Despite similar amounts of aged soil residues of imazamox remaining at different pH levels, bioavailability differed with pH as a result of pH effects on sorption–desorption interactions. At low pH, more imazamox was sorbed than at high pH, but it was readily desorbed. At high pH, less imazamox was sorbed initially, but it did not readily desorb. Thus, after 3 months, the remaining imazamox in low-pH soil was desorbable and bioavailable, resulting in injury to oilseed rape and sugarbeet. In aged, low-pH soil, adding lime released bound imazamox, which was then degraded, resulting in less carry-over.     

Concentration-Dependent Mobility of Chlorfenvinphos in Isolated Plant Cuticles

The mobility of chlorfenvinphos in isolated pear (Pyrus communis cv. Bartlett) leaf cuticular membranes (CM) was studied as a function of concentration of chlorfenvinphos sorbed in the cuticle. Mobilities of chlorfenvinphos increased approximately 9-fold when the amount sorbed increased from 1 to 100 μg cm⁻² pear leaf cuticle. From the amounts per area, average volume fractions of chlorfenvinphos in the cuticle were calculated ranging from 2×10⁻³ to 5·1×10⁻². The increase in mobilities was steepest at the lower and levelled off at higher volume fractions. This correlation could be described for the whole range of volume fractions investigated by an equation which assumes homogeneously dispersed chlorfenvinphos. Temperature dependence of mobilities was studied at 17, 25 and 35°C and chlorfenvinphos volume fractions of 5·5×10⁻³ and 0·12, respectively. Arrhenius graphs were linear for both volume fractions, showing that cuticles did not undergo a phase transition due to the high amount of sorbed chlorfenvinphos. However, at a volume fraction of 0·12, the activation energy of diffusion, E_D, was significantly lower (83·6 kJ mol⁻¹) than at 5·5×10⁻³ (135 kJ mol⁻¹). We interpret these findings as evidence for a plasticising effect on cuticular waxes by chlorfenvinphos. So far, such an effect had been demonstrated only for certain adjuvants (ethoxylated alcohols) but not for active ingredients. Chlorfenvinphos not only increased its own mobility in pear leaf cuticles, but also that of 2,4-D in Citrus leaf cuticles. This would be expected if plasticising of waxes was the sole mechanism responsible for increased mobilities. From these data we predict that permeabilities of cuticles to chlorfenvinphos are not constant. Depending on temperature as well as types and amounts of adjuvants, rates of foliar penetration of chlorfenvinphos can be higher if its concentration in the spray liquid is increased.     

Sorption of lonisable organic compounds by field soils. Part 2: Cations,bases and zwitterions

Sorption of a range of permanent cations, bases and zwitterionic compounds was measured as a function of pH in two soil types. Pyridinium cations were more strongly sorbed (K_d 10– > 1000) than aliphatic cations (K_d < 5). At very low pH, sorption of the aliphatic cations sharply decreased, probably because they are displaced by protons. Most weak bases, including carbendazim andpyridines, were strongly sorbed (K_d 9–35) at low pH, where they would be appreciably protonated, sorption becoming much weaker at soil pH values > 6. However, an additional mechanism of sorption was observed for those zwitterions capable of chelation (e.g. picolinic acid and alanine) which gave rise to high K_d values at pH values near neutral. Inorganic phosphate was strongly sorbed (K_d > 140) save at very low pff. Glyphosate and inorganic phosphate were sorbed very strongly at pH values near to 4 (K_d > 200). The very strong sorption was attributed to ligand exchange interaction. Sorption of picolinic acid was similar when measured in water or calcium chloride solution (0.01 M). However, sorption decreased with increasing concentration of calcium chloride up to 1 M, probably because the protonated form of picolinic acid was displaced by calcium ions.     

Effects of alcohols,glycols and monodisperse ethoxylated alcohols on mobility of 2,4-D in isolated plant cuticles

Effects of n-alcohols, ethoxylated alcohols and glycols on mobility of 2,4-dichlorophenoxy acetic acid (2,4-D) in cuticular membranes (CM) isolated from bitter orange (Citrus aurantium L.) leaves were studied. 1-Heptanol, 1-octanol and 1-nonanol had the highest effects, as they increased solute (2,4-D) mobility by 25- to 30-fold. Increasing the number of carbon atoms in the alcohols decreased their effectiveness. Ethoxylation of alcohols did not increase 2,4-D mobility and effectiveness decreased with increasing ethoxylation. Free glycols had no effect on solute mobility in isolated cuticles. The results show that ethoxylation is not required for increasing solute mobility in cuticles. It is suggested that alcohols and ethoxylated alcohols are sorbed in cuticular waxes and plasticize them. The data show that alcohols and ethoxylated alcohols having between seven and ten carbon atoms are powerful accelerator adjuvants, as long as the degree of ethoxylation is not too high. Free short-chain alcohols synthesized by leaves may act as endogenous plasticizers and modulate permeabilities of cuticles, depending on environmental and growing conditions.     

Time effect on bentazone sorption and degradation in soil

Previous sorption/desorption batch experiments have indicated that bentazone is weakly sorbed by soils. In addition, field experiments have shown that 4% of the bentazone sprayed can be leached to drainage water. In order to complete bentazone characterisation, we have assessed the effect of time on its behaviour in contrasting soils. In laboratory studies, bentazone was added to three topsoils (sandy, loamy and clay soils). Bentazone degradation, sorption/desorption kinetics and isotherm measurements were carried out at different times. At 160 days after treatment, bentazone mineralisation amounts varied from 2.1% (sandy soil) to 14% (clay soil). The extractable amounts became lower (from 97% after treatment to 12% after 160 days for the clay soil) and a greater number of desorption series was needed to obtain these products. Nevertheless, at the end of the experiments, a small amount of bentazone was still extracted by water. At the same time, bound residues of bentazone reached 65% in clay soil. Statistical analysis indicated effects of both residence time and soil type on bentazone behaviour.     

Effects of surfactants on water permeability of isolated plant cuticles and on the composition of their cuticular waxes

Treating the outer surfaces of isolated cuticles of Seville orange (Citrus aurantium L.) and pear (Pyrus communis L. cv. Bartlett) leaves with a number of nonionic (polyoxyethylene) surfactants increased their permeability to water by factors ranging from 4.1 to 14.7 and from 7.2 to 152.4, respectively. However, sodium dodecylsulphate, an anionic surfactant, had little effect on water permeability. In both species the major aliphatic constituents were n-alkanes, 1-alkanols and n-alkyl esters. None of the surfactant treatments altered the amounts or composition of waxes in the cuticles used for transport measurements. The reasons for the apparent absence of solubilization or dissolution of cuticular waxes by nonionic surfactants are discussed and a hypothesis is presented for the mechanism by which activator adjuvants may increase the permeability of plant cuticles.     

Mobilities of Organic Compounds in Reconstituted Cuticular Wax of Barley Leaves: Effects of Monodisperse Alcohol Ethoxylates on Diffusion of Pentachlorophenol and Tetracosanoic Acid

Effects of monodisperse alcohol ethoxylates on mobilities of ¹⁴C-labelled pentachlorophenol (PCP) and tetracosanoic acid (C₂₄AC) in reconstituted cuticular wax of barley leaves were measured. Depending on the respective alcohol ethoxylate investigated, the diffusion coefficient (D) of PCP in barley wax was increased by factors ranging from 3·3 to 19·6, whereas D of C₂₄AC, was increased by factors varying between 22 and 315. In order to analyse the relationship between the concentration of surfactants in the wax and their effects on D, the amounts of alcohol ethoxylates dissolved in the wax at equilibrium with external concentrations well above the critical micelle concentration (CMC) were determined. Wax/water partition coefficients (K_ww) of the alcohol ethoxylates were about one order of magnitude lower than cuticle/water partition coefficients (K_cw), which is a consequence of the semi-crystalline structure of the wax compared with amorphous cutin. Correlations between effects on D and maximum amounts of alcohol ethoxylates dissolved in the wax were obtained indicating an unspecific wax/surfactant interaction. This was solely dependent on the amount of surfactant sorbed to the wax, leading to increased mobilities of pesticides in the wax. Applying ESR-spectroscopy, which gave an insight into the molecular structure of the wax, supported this interpretation of an unspecific plasticising effect of the alcohol ethoxylates on the molecular structure of the wax. The results obtained in this study are in good accordance with the results obtained in a recent study investigating the effects of the same group of alcohol ethoxylates on mobilities of pesticides in isolated, but intact, cuticular membranes of Citrus. This demonstrates that the investigation of isolated and subsequently reconstituted cuticular wax is a useful model system analysing the mechanisms of the surfactant interaction with the transport-limiting barrier of plant cuticles.     

Sorption of aged dicamba residues in soil

The effect of aging (residence time in soil) on dicamba (3,6-dichloro-2-methoxybenzoic acid) and a major metabolite, 3,6-dichlorosalicylic acid (3,6-DCSA) sorption was determined in an unamended and a carbon-amended sandy loam and in a silt loam soil. During the incubation, sequential solvent extraction with 0.01 M calcium chloride solution and aqueous acetonitrile + hydrochloric acid was used to determine the solution and sorbed concentrations of dicamba and 3,6-DSCA, and sorption coefficients were calculated. Dicamba was weakly sorbed to soil (Kd < 0.7). In contrast to some other classes of pesticides, sorption of dicamba did not significantly increase with aging, at least not until < 15% of the applied dicamba remained. 3,6-DSCA was strongly sorbed to soil (Kd > 8) and the Kd-a value increased by a factor of 2-6 during a 28-day aging period. Addition of a carbon source to the soil had minimal effect on the strength of sorption of aged dicamba. However, it did appear to decrease 3,6-DSCA availability to soil micro-organisms; once formed 3,6-DSCA was not further mineralized. While it appears that sorption can be well characterized for weakly sorbed pesticides using the batch equilibration method with freshly treated soils, this procedure may not be adequate for more strongly sorbed pesticides and their degradates.     

Polydisperse ethoxylated fatty alcohol surfactants as accelerators of cuticular penetration. 1. Effects of ethoxy chain length and the size of the penetrants

The effects of polydisperse ethoxylated fatty alcohol (EFA) surfactants on the penetration of six organic compounds varying in size (molar volumes, 107–282 cm³ mol^-1) and lipophilicity (log K_ow 0·8–6·5) were investigated using astomatous isolated cuticular membranes (CM) of Citrus and pear leaves. Mobilities of model compounds in CM were measured by unilateral desorption from the outer surface (UDOS). Rate constants (k*) obtained in these experiments are directly proportional to diffusion coefficients and, in the absence of EFA, k* values decreased by a factor of 52 when molar volumes increased only 2·64-fold. Under UDOS conditions using micellar surfactant solutions as desorption media, surfactants are sorbed in the CM and the volume fractions sorbed were found to decrease from approximately 0·062 to 0·018 when the average number of ethoxy groups (nE) increased from 5 to 17. In the presence of the EFA surfactants in the CM, solute mobilities increased markedly though this effect diminished with increasing nE. Surfactants with nE=17 affected solute mobilities only marginally. Surfactant effects on solute mobility increased with the size of the solutes leading to almost identical mobilities of the model compounds. With the current range of our model compounds, lipophilicity increased with increasing molar volumes, though evidence is presented showing that the mobilities of solutes depend on their molar volumes while lipophilicity has no effect. Effects of micellar aqueous solutions of polydisperse surfactants on solute mobilities followed the pattern observed with monodisperse ones. © 1997 SCI     

Desorption of diuron and isoproturon from undispersed clay loam soil

Studies were conducted to investigate the desorption of diuron and isoproturon adsorbed on undispersed clay loam soil, and the influence of residence time in soil on desorption. The soil was treated at 0·6 or 3 mg kg^-1, at 70% moisture content and in the presence of sodium azide to prevent degradation. Measurement of herbicide concentrations in soil solution sampled by means of glass microfibre filters showed that adsorption mainly occurred for one day but long-term sorption proceeded for >two weeks. After a one-day or three-week residence time, soil solution was partly replaced (28%). Measurement of concentrations in solution showed rapid desorption, with equilibria being achieved within 1 h (diuron) or a few hours (isoproturon). After 16 successive desorptions done at 30-min or 12-h intervals, equilibration times tended to be longer. For the short residence time, desorption and long-term sorption could occur simultaneously and equilibration might be faster. Residence time had no significant effect on desorption kinetics nor on the small hysteresis observed for diuron. The aging effect, involving long-term sorption only, decreased the proportion of diuron removed from the soil by successive desorptions but, for isoproturon, desorption frequency and desorption kinetics were more important. © 1997 SCI     

Finite dose diffusion studies: I. Characterizing cuticular penetration in a model system using NAA and isolated tomato fruit cuticles

A finite dose diffusion system was employed to study cuticular penetration of 2‐(1‐naphthyl) [1−¹⁴C]acetic acid (NAA) from simulated spray droplets through enzymatically isolated tomato fruit cuticles (Lycopersicon esculentum Mill cv Pik Red). Isolated cuticles were mounted on diffusion half‐cells with the cell wall surfaces facing a 20 mM citric acid receiver solution (pH 3.2, volume 2.9 ml, prepared with deionized water). A 5‐µl donor droplet containing NAA at 100 µM in 20 mM citric acid buffer (pH 3.2) was applied to the outer surface. Penetration was monitored by repeated sampling of the receiver solution. NAA penetration was characterized by (1) an initial lag phase of about 2.3 h, (2) a phase of nearly constant maximum rate of penetration averaging 6.3% of applied NAA h⁻¹ (equivalent to 0.032 nmol h⁻¹) and (3) a plateau phase approaching an asymptote at 81.2% of applied NAA (equivalent to 0.406 nmol) at 120 h. Within 1 h after application droplets appeared dry on visual inspection. Immediately after droplet drying, 7.0% of the applied NAA was sorbed to the cuticle, but only 0.5% penetrated into the receiver solution, indicating that penetration occurred almost exclusively from the apparently dry deposit. At 120 h, 5.2% of the NAA applied was associated with the deposit and 4.3% with the cuticle. The distribution of maximum rates of penetration was log‐normal, but penetration at 120 h followed a normal distribution. Cuticle thickness (estimated 5–25 µm) had no significant effect on NAA penetration. Maximum rates of penetration through pepper fruit and citrus and ficus leaf cuticles were 4.9‐, 2.6‐ and 0.1‐times that through tomato fruit cuticles. At 120 h, penetration averaged 85.5, 79.5 and 34.7% for pepper, citrus and ficus cuticles, respectively. Extracting epicuticular and embedded waxes increased NAA penetration rates through tomato fruit cuticle more than three‐fold, but had little effect on penetration at 120 h (71.0 vs 87.7% for cuticular vs dewaxed cuticular membranes). The maximum penetration rate and total penetration were found to be useful parameters in describing the penetration time‐course. © 2000 Society of Chemical Industry     

Sorption of ionisable organic compounds by field soils. Part 1: Acids

Sorption of different classes of weak organic acids was measured using soils with a range of pH values, taken from long-term field experiments that had received different amounts of lime. Non-ionisable compounds were used to demonstrate that the soils of different pH used in the experiments have similar sorptive properties. Values of the sorption coefficients for chloride ion were negative at all pHs except one. Sorption of moderately polar, monobasic, weak acids was weak in acidic soils and very weak in neutral and alkaline soils where they were predominantly dissociated. A lipophilic weak acid was strongly sorbed even at high pH. A model is presented which estimates soil/water distribution coefficients, at any soil pH, from lipophilicity and pK_a of the acid and organic matter content of the soil. The model was derived using sorption measured for substituted phenoxyacetic acids. Sorption values calculated using the model were compared with values measured for chlorsulfur on and showed useful agreement. Dibasic acids were strongly sorbed, probably by the mechanism of ligand exchange, if they were chelating agents with potential to form 5- or 6-membered rings with an acceptor atom. Phenylphosphonic acid was strongly sorbed, being a strong monodentate ligand.     

预测农药植物角质层-水分配系数的LSER模型

植物角质层-水分配系数(Kcw)对评价农药的渗透及残留具有重要意义。通过实验方法难以高效且经济地测定多种农药的Kcw值,因此有必要发展一种快速预测农药Kcw值的模型。作者收集了23种农药在不同植物中的64个log Kcw实测值,构建了预测农药log Kcw值的线性溶解能关系(liner solvation energy relationship,LSER)模型。所建模型具有良好的拟合度(R^2adj,tra=0.79,RMSEtra=0.38)、稳健性(Q^2BOOT=0.78)和预测能力(Q^2ext=0.81)。该模型可用于预测含有-X(Cl,F,I)、>N-C(O)-NH2、-OCH2COOH、-NH-、-NH2、>C=O、-O-C(O)-NH-、-CN、-S-及-S(O)(O)-等官能团的农药的log Kcw值。机理分析结果表明:疏水相互作用(平均相对贡献率为48%)和n/π电子对相互作用(平均相对贡献率9%)对农药进入植物角质层为正贡献,而氢键受体相互作用(平均相对贡献率26%)和极性相互作用(平均相对贡献率17%)对农药进入角质层为负贡献。本研究构建的pp-LFER模型可用于预测新农药的log Kcw值,并且有助于理解农药在植物角质层与水相间分配的相互作用机制。