Solubility properties, pesticides

Agricultural (Ag) formulations that are commonly diluted and applied by means of spray equipment include water soluble liquids, emulsifiable concentrates, wettable powders, and flowable suspensions. The choice of which formulation to develop normally depends upon the solubility properties of the technical pesticide. Scientists often must also consider manufacturing costs, field efficacy and product toxicity. 

Pesticide Solvent. The majority of oiganic fungicides, insecticides, and herbicides (qv) are soluble in DMSO, including such difficult-to-solvate materials as the substituted ureas and carbamates (see Fungicides, agricultural Insect control technology Pesticides). Dimethyl sulfoxide forms cosolvent systems of enhanced solubility properties with many solvents (109). 

The factors which Influence the choice of formulation are pesticide physical properties (melting point, solubility, volatility), pesticide chemical properties (hydrolytic stability, thermal stability), soil application vs. foliar application, crop and cultural practice, pesticide biological properties (crop selectivity, transport), and economics. 

Vehicles that are capable uf solubilizing the pesticide, but that also favor pesticide partitioning into the stratum comeum, will promote absorption. The effect seen is thus dependent on both the inherent solubility properties of the pesticide relative to the vehicle and the relative solubility of the pesticide in the vehicle vcr.sus the stratum comeum. This makes easy generalizations very difficult. 

We have measured the absorption of radiolabeled DDT and p u athion through excised human abdominal skin using an in vitro diffusion cell procedure. The poor water solubility properties of these compounds (Table II) suggested that a nonionic surfactant (oleth 20) would be necessary in the receptor fluid. The compounds were applied to skin in an acetone vehicle at a concentration of 4 ug/cm. The surface of the skin was cleansed with acetone at 24 h to remove unabsorbed material. The absorption of the pesticides was followed for a total of 7 days until the amount of compound entering the receptor fluid was minimal (Figures 3 and 4). 

Hansch and Leo [13] described the impact of Hpophihdty on pharmacodynamic events in detailed chapters on QSAR studies of proteins and enzymes, of antitumor drugs, of central nervous system agents as well as microbial and pesticide QSAR studies. Furthermore, many reviews document the prime importance of log P as descriptors of absorption, distribution, metabolism, excretion and toxicity (ADMET) properties [5-18]. Increased lipophilicity was shown to correlate with poorer aqueous solubility, increased plasma protein binding, increased storage in tissues, and more rapid metabolism and elimination. Lipophilicity is also a highly important descriptor of blood-brain barrier (BBB) permeability [19, 20]. Last, but not least, lipophilicity plays a dominant role in toxicity prediction [21]. 

At the fundamental level of equilibrium modeling the advantages are many. The model can combine a number of compartments through simple relationship to describe a realistic environment within which chemicals can be ranked and compared. Primary compartments that chemicals will tend to migrate toward or accumulate in can be identified. The arrangement of compartments and their volumes can be selected to address specific environmental scenarios. Data requirements are minimal, if the water solubility and vapor pressure of a chemical are known, other properties can be estimated, and a reasonable estimate of partitioning characteristics can be made. This is an invaluable tool in the early evaluation of chemical, whether the model be applied to projected environmental hazard or evaluation of the behavior of a chemical in an environmental application, as with pesticides. Finally, the approach is mathematically very simple and can be handled on simple computing devices. 

Pentachlorophenol is readily soluble in most organic solvents, oils, and highly aromatic and olefinic petroleum hydrocarbons — a property that makes it compatible for inclusion in many pesticide formulations (Table 23.1 Figure 23.1). Purified PCP, however, is practically insoluble in water therefore, the readily water-soluble sodium pentachlorophen-ate salt is substituted in many industrial applications (Table 23.1) (Bevenue and Beckman 1967 USPHS 1994). 

Niche The section of the environment with which a particular property of the chemical product interacts is referred to as niche. For example, a pesticide can have as the environment the plant, the atmosphere, and the human beings. The pesticide interacts with the environment through its properties. There are different kinds of interaction depending on the niche. For example, some properties such as the contact area depend on the surfactant characteristics and the surface of the leaf. The niche is the surface of the leaf. The absorption of the pesticide depends on the characteristics of the layers, like the cuticle [25], In this case, the niche consists of the layers of the plant s leaves. Also, the diffiisivity of the active product in the layers of the plant leaves corresponds to a property that depends on the environment-product interaction. Some other pesticide properties, such as solubility of the active agent in the solvent, do not depend on the environment. 

Disulfoton is a systemic insecticide/acaricide that belongs to the organophosphate class of pesticides. Pure disulfoton is a colorless oil with low volatility and water solubility, but is readily soluble in most organic solvents (Worthing 1987). Information regarding the physical and chemical properties of disulfoton is located in Table 3-2. 

Pesticides of this category (Fig. 10, Table 3) do not ionize significantly in aqueous systems and vary widely in their chemical composition and properties (i.e., water solubility, polarity, molecular volume, and tendency to volatilization). 

The activity spectrum of a pesticidal compound is often determined by the physical properties of the compound. For example, systemic insecticides usually require both water and lipid solubility since it is necessary for them to pass through... 

The reason why this combination of principles is so powerful is that Briggs was also able to demonstrate that the key properties discussed in earlier sections which determine pesticide behaviour can be regarded as partitions. Thus solubility may be envisaged as a partition between the compound itself and water, soil adsorption can be treated as a partition... 

These properties are not unique to synthetic chemicals. Many thousands of chlorinated chemicals are produced in nature,21 and natural pesticides can bio concentrate if they are fat-soluble. Potatoes, for example, contain solanine and chaconine, which are fat-soluble, neurotoxic, natural pesticides that can be detected in the blood of all potato eaters. High levels of these potato neurotoxins have been shown to cause birth defects in rodents,22 though they have not been tested for carcinogenicity. 

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