Water solubility, herbicides

The components in a simple penetration experiment consist of a surfactant, water-soluble herbicide, and water. Since the surfactant is at a concentration of 0.5 to 1%, it interacts with water and forms micelles. Since micelles are formed, these could solubilize some of the herbicide inside the micelle. Now we have five components, (1) water, (2) surfactant monomer, (3) surfactant micelle, (4) micelle with solubilized herbicide, and (5) an herbicide in anhydrous or hydrated form which all come in contact with the plant. Which one or more of these components has the greatest effect on the plant Before a thorough understanding of this phenomenon can be achieved, the interaction of each of these components with a plant must be investigated separately, and perhaps the plant is too complex for initial study. Perhaps a homogeneous semipermeable membrane could be used instead. 

Structure-activity relationships between several nonionic surfactants and three water-soluble herbicides have been studied (58). In general it has been shown that the herbicide, the surfactant concentration, the hydrophilic constitution (ethylene oxide content), and the hydrophobic portion of the molecule all markedly influence toxicity. 

Mackay et al. (1997) provide detailed examples of fugacity calculations to illustrate how variations in the physical and chemical properties of pesticides affect their partitioning among environmental media. Figure 3 displays the results from some of these calculations for three of the pesticides listed in Table 1. Consistent with the expectations described above, these computations predict that following their release into the hydro-logic system, the relatively water-soluble herbicide atrazine will come to reside mostly in the aqueous phase, the more hydrophobic insecticide... 

The dissipation of water-soluble herbicides such as the salts of 2,4-D, 2,2-dichloropropionic acid (dalapon), and trichloroacetic acid (TC- ) was caused largely by dilution resulting from the longitudinal dispersion of the herbicides in flowing water (26). Residues of these herbicides which enter the water during ditchbank treatment are low initially and are reduced to lower concentrations as they are carried downstream. Insignificant residue levels would remain after a water flow of 20-25 miles. 

In contrast to the above triazine pesticides, sorption of highly water-soluble herbicides, namely benazolin, bentazone, imazapyr, and triclopyr, onto hydrophobic MN-200 is small [70]. When passing the feed solution of 20)J.g/L of each component through a 0.70 mL mini-column with MN-200, the breakthrough of the herbicides occurs almost instantly, which is reflected in the low equilibrium adsorption capacity of only 0.48, 0.42, 0.26, and 1.28 mg/g for benazolin, bentazone, imazapyr, and triclopyr, respectively. 

Wang K-C, Chen S-M, Hsu J-F, Cheng S-G, Lee C- K. Simultaneous detection and quantitation of highly water-soluble herbicides in serum using ion-pair liquid chromatography-tandem mass spectrometry. J Chromatogr B 2008 876 211—8. 

Figure 10.32 Diagrammatic representation of the hydrophilic channels described by Smith et al. [269], and the effect of (a) a short ethylene oxide chain non-ionic detergent, and (b) a long-chain non-ionic detergent on the passage of a water soluble herbicide ( ) into the leaf cuticle.

The foliar penetration of water-soluble herbicides is relatively inefficient in comparison with that of lipophilic compounds, no doubt reflecting the incompatibility of the polar droplets and the nonpolar epicuticular waxes, and also the relative inefficiency of transcuticular polar transport. The choice of a suitable surfactant certainly reduces the degree of incompatibility and may also enhance passage through the membrane. 

Water-Soluble Films. Water-soluble films can be produced from such polymers as poly(vinyl alcohol) (PVOH), methylceUulose, poly(ethylene oxide), or starch (qv) (see Cellulose ethers Polyethers Vinyl polymers). Water-soluble films are used for packaging and dispensing portions of detergents, bleaches, and dyes. A principal market is disposable laundry bags for hospital use. Disposal packaging for herbicides and insecticides is an emerging use. 

Volatilization. The susceptibility of a herbicide to loss through volatilization has received much attention, due in part to the realization that herbicides in the vapor phase may be transported large distances from the point of application. Volatilization losses can be as high as 80—90% of the total applied herbicide within several days of application. The processes that control the amount of herbicide volatilized are the evaporation of the herbicide from the solution or soHd phase into the air, and dispersal and dilution of the resulting vapor into the atmosphere (250). These processes are influenced by many factors including herbicide application rate, wind velocity, temperature, soil moisture content, and the compound s sorption to soil organic and mineral surfaces. Properties of the herbicide that influence volatility include vapor pressure, water solubility, and chemical stmcture (251). 

There are at least 22 chemical families of organic herbicides. Even a cursory treatment of the chemistry of these materials would be extensive. Herbicides of limited toxicity (Treflan, Atrazine) as well as extremely toxic ones (Paraquat. Dinoseb) are in use in many parts of the world. They range from water soluble to insoluble. The detailed chemistry of each should be determined prior to handling. 

Emulsions containing a water-solubile herbicide salt and a herbicide soluble in an organic solvent are prepared by a mixture of water, dioctyl phthalate, and isophorone. In this case ethoxylated poly ary lphenol phosphate neutralized with triethanolamine and ethoxylated acid phosphoric acid esters is used as emulsifier [231],... 

Before leaving the subject of polarity and in relation to uptake and distribution, mention should be made of weak acids and bases. The complicating factor here is that they exist in solution in different forms, the balance between which is dependent on pH. The different forms have different polarities, and thus different values. In other words, the values measured are pH-dependent. Take, for example, the plant growth regulator herbicide 2,4-D. This is often formulated as the sodium or potassium salt, which has high water solubility. When dissolved in water, however, the following equilibrium is established ... 

Ureides (e.g., diuron, linuron) and triazines (e.g., atrazine, simazine, ametryne) all act as inhibitors of photosynthesis and are applied to soil (see Figure 14.1 for structures). They are toxic to seedling weeds, which they can absorb from the soil. Some of them (e.g., simazine) have very low water solubility and, consequently, are persistent and relatively immobile in soil (see Chapter 4, Section 4.3, which also mentions the question of depth selection when these soil-acting herbicides are used for selective weed control). 

Owing to its low water solubility and high octanol/water partition coefficients, dinitroaniline herbicides adsorb and bind to soil macromolecules and show minimal leaching potential. Dinitroanilines herbicides show good soil residue activities with soil half-lives ranging from 30 days for benfluralin and oryzalin to 6-7 months for trifluralin. Al-Dealkylation (aerobic conditions) and reduction of the nitro group to an amino moiety (anaerobic conditions) have been reported as major soil degradation pathways. 

The diphenyl ether herbicides are nonvolatile compounds, generally very lipophilic and insoluble in water. Solubility in water and octanol-water partition coefficients (logXow) of the various diphenyl ether herbicides range from 120mgL (acifluorfen) to 0.16 mg (oxyfluorfen) and from 2.9 (fomesafen) to 5.4 (acifluorfen), respectively. Diphenyl ether herbicides are stable in an acidic or alkaline condition, but some compounds are gradually degraded under the sunlight. ... 

Trigard 75 WP is a new insecticide with a unique mode of action and a unique triazine structure. It is a solid formulated as a wettable powder and is packaged in water-soluble bags. The active ingredient in Trigard has the common name cyromazine. Cyromazine is a triazine, but, unlike the well-known triazine herbicides, this compound has insecticidal properties and no herbicidal activity. Cyromazine has low mammalian toxicity and low vapor pressure. It is hydrophilic, so dermal penetration is expected to be... 

Transport of the herbicides by vapor diffusion on moist soil was shown to be directly related to vapor pressure and inversely related to water solubility. Transport of the herbicides by leaching was shown to be inversely related to the Freundlich adsorption coefficient which in turn was directly related to the octanol/water partition coefficient and inversely related to water solubility (16). 

Dacthal is a widely used pre-emergent herbicide that is applied to many crops for the control of annual weeds. Dacthal is typically applied to agricultural soils at 6-14kg ha [214], In the soil environment, Dacthal transforms to mono- and diacid-metabolites that are more water soluble than the parent herbicide [215-217]. In eastern Oregon, where Dacthal is applied to onions, the diacid metabolite is the principal form of Dacthal detected in groundwater obtained from domestic wells [218, 219],... 

It is seen from Table 16.4, for example, that TCA a phenoxyacetic acid type of herbicide has a solubility exceeding 500000pg L 1 while chlorotoluron, a phenyl urea herbicide has a water solubility of 70mg L-1. Thus, due to rainfall over a period of time the concentration of TCA in soil will reduce considerably more rapidly than that of chlorotoluron. 

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