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Phenol Ethylene Oxide Ether Non-Ionic Surfactants and Three Water-Soluble Herbicides

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Phenol Ethylene Oxide Ether Non-Ionic Surfactants and Three Water-Soluble Herbicides Weed Res. (1966) 6, 233-242. STRUCTURE-ACTIVITY RELATIONSHIPS OF ALKYL­ PHENOL ETHYLENE OXIDE ETHER NON-IONIC SURFACTANTS AND THREE WATER-SOLUBLE HERBICIDES L. W. SMITH, C. L. FOY AND D. E. BAYER Department of Botany, University of California, Davis, California, U.S.A. Summary. Several alkylphenol ethylene oxide ether non-ionic surfactants were tested in aqueous foliar sprays with dalapon, amitrole and paraquat for their enhancement of phytocidal activity against ,Zea mays L. With three homologous series of surfactants studied (octyl-, nonyl- and laurylphenol types), the herbicide, the surfactant concentration and the hydrophilic constitution (ethylene oxide content) of the surfactant molecule all markedly influenced maximum toxicity. Smaller apparent differences in effectiveness were also attributable to the hydrophobic (alkylphenol) portion of the surfactant. The results are discussed in relation to possible cuticle-spray solution interactions and their influence on herbicide penetration. Relations entre la structure et l'activite de produits tensio-actifs non ioniques, a base d'ether d'oxyde ethylenique et d' alkylphinol, en presence de trois herbicides hydrosolubles Resume. Plusieurs produits tensio-actifs non ioniques, a base d'ether d'oxyde ethylenique et d'alkylphenol, ont ete essayes en pulverisation foliaires avec du dalapon, de l'amitrole et du paraquat, en vue de renforcer leur activite phytocide sur Zea mays L. Avec Jes trois series homologues de produits tensio-actifs etudies (types octyl-, nonyl- et laurylphenol), !'herbicide, la concentration du produit tensio-actif et la partie hydrophile (oxyde ethylen­ ique) de la molecule du produit tensio-actif, influencerent la toxicite maximum de fai;on marquee. De plus faibles differences clans l'efficacite apparente etaient egalement attribuables a la partie hydrophobe de la molecule (alkylphenol) du produit tensio-actif. Les resultats sont discutes en relation avec des interactions possibles entre la cuticule et la solution pulverisee, ainsi que leur influence sur la penetration de !'herbicide. Beziehungen der Strukturaktivitiit nichtionischer oberjliichenaktiver Stoffe vom Typ Alkylphenol- iithylen-oxyd-iither und drei wasserliislichen Herbiziden Zusammenfassung. Verschiedene nichtionische oberflachenaktive Stoffe auf der Basis von Alkylphenol-athylen-oxyd-ather wurden in wassriger Liisung mit Dalapon, Amitrol und Paraquat auf ihre Phytotoxizitat gegen ,Zea mays L. bei Blattanwendung geprtift. Bei der Untersuchung von drei homologen Serien oberflachenaktiver Stoffe (Octyl-, Nonyl- und Laurylphenol-Typen), wurde die maximale Phytotoxizitat stark durcJ?. die Konzentration des oberflachenaktiven Stoffes und die hydrophyle Konstitution (Athy­ lenoxyd-Gehalt) des oberflachenaktiven Molektils beeinflusst. Weniger augenscheinliche Unterschiede in der Wirkungsstiirke waren ausserdem auf den hydrophoben Anteil (Alkylphenol) des oberflachenaktiven Stoffes zurtickzuftihren. Miigliche Interaktionen zwischen Kutikula und Spritzfltissigkeit sowie deren Einfluss auf das Eindringen von Herbiziden wird an Hand der Ergebnisse besprochen. INTRODUCTION Many workers over the past 20 years have shown that the toxicity of foliar­ applied herbicides is enhanced by the addition of surfactants to the spray solution (Zimmerman & Hitchcock, 1942; Blackman, 1950; Freed & Mont­ gomery, 1958; Leonard, 1958; Jansen, Gentner & Shaw, 1961; McWhorter, 233 234 L. W. Smith, C. L. Foy and D. E. Bayer 1963). Such additives have been used for many years in aqueous agricultural sprays for improving formulative properties and for their ability to wet plant surfaces. However, physical measurements such as contact angle, spreading coefficient, surface tension and interfacial tension of spray solutions and visual observations on ability to wet plant surfaces have shown little correlation with the enhancement of herbicidal activity. It is generally conceded that more subtle interactions and effects are involved (Freed & Montgomery, 1958; Jansen et al., 1961; Foy & Smith, 1965). There have been several suggestions in the literature that the interactions between surfactant, herbicide and plant surface are of more importance than the surface tension lowering or wetting ability of the herbicide-surfactant solu­ tion (Freed & Montgomery, 1958; Jansen et al., 1961; Foy & Smith, 1965). Very little is known about the mode of action or structure-activity relation­ ships of surfactants in herbicidal solutions. Furmidge ( 1959a, b) working with several series of surfactants concluded that, within a series of a similar chemical structure, the phytotoxicity of the surfactant was governed by the size of the molecule. The smaller molecules were more phytotoxic. Jansen ( 1964) was able to show that differences in the phytotoxicity of herbicide-surfactant mixtures were associated with variations in both the hydro­ philic and hydrophobic portions of the surfactant molecule. Maximal toxicity within a homologous group also varied with the herbicide and the plant species used. Nonionic surfactants of the type used in this study (alkylphenol-polyoxy­ ethylene glycol ethers) consist of an alkylphenol hydrophobe and an ethylene oxide (EO) hydrophile. The number of EO moieties in the molecule can be varied to give different properties to the surfactant which include modifications in solubility, molecular weight, surface tension and 'hydrophilic-lipophilic balance' (H.L.B.). · The reaction products formed during the manufacture of such materials are not pure compounds and the number of moles ofEO per molecule varies around a Poisson distribution. The average number is used to denote the EO content of the surfactant. The main purpose of this study was to evaluate a series of herbicide solutions containing alkylphenoloxyethylene glycol ether surfactants that varied in the average number of moles of EO and the length of the alkyl chain, thus allowing a study to be made of the influence of the EO content, the length of the hydro­ carbon chain and other physical-chemical properties of these surfactants on the activity of certain herbicide solutions. METHODS AND MATERIALS Suifactants and herbicides studied . A series oflaurylphenol, nonylphenol and octylphenol polyoxyethylene glycol ether type surfactants of the general structure shown in Fig. 1 were obtained to carry out these experiments. Three foliar-applied, water-soluble herbicides, dalapon (2,2-dichloropro­ pionic acid), amitrole (3-amino-1,2,4-triazole) and paraquat (1,1 '-dimethyl- Suifactants and Herbicides 235 4,4'-bipyridylium dichloride) were used to evaluate the surfactants. Previous workers have shown them to respond to the addition of surfactants (Jansen et al., 1961; Foy & Smith, 1965). The average content of the EO in the surfactant molecule varied from 6, 9, 12 to 15 moles for the laurylphenols (products 12-P-6, 12-P-9, 12-P-12 and 12-P-15, respectively); 4, 7, 9-10, 10·5, 13, 20 and 40 moles for the nonylphenols (NP-14, NP-27, N-101, NPX, NP-33, NP-40 and NP-44, respectively); 1, 5, 7-8, 9-10, 13, 16, 20 and 30 moles for the octylphenols (X-15, X-45, X-114, X-100, X-102, C,,H,.,,Oo CCH 2CH2o>, H Fm. 1. General structure of an alkylphenolpolyoxyethylene glycol ether surfactant. X-165, X-205 and X-305, respectively). In all trials, the herbicides were used at rates which had been determined previously to give a 50 % reduction in fresh weight of corn when mixed with 1 g/1 of Tween 20 surfactant (containing polyoxyethylene sorbitan monolaurate). These rates were dalapon 10 lb/ac, amitrole 5 lb/ac, and paraquat 1/64 lb/ac. The first series of tests was carried out to compare 5 g/1 and 0·005 M solutions of surfactants alone and in combination with the herbicides. In the second series, the surfactants were used at concentrations of 1 and 10 g/1 which correspond to 0 ·33 lb/ac and 3 ·3 lb/ac, respectively. A third series of tests compared the effect of surfactant concentration of three nonylphenol surfactants on the toxicity of each of the herbicides. Biological evaluation of suifactant-herbicide solutions Corn (Zea mays L., var. De Kalb 414) was grown under standard glasshouse conditions in 4-in. diameter pots. The original seeding rate was six seeds per pot, and thinning of plants to four per pot was carried out 10-12 days after planting. Growth was maintained at a vigorous level by fertilizing with nutrient solution (Hoagland & Amon, 1950) and a nitrogen-phosphorus fertilizer. Before spraying, the plants were ranked for uniform growth according to replica­ tion; thus a uniform plant population was provided for each replication. The corn was usually between 35 and 40 cm tall at this time. The herbicidal solutions were applied using an experimental spraying table as described by Shaw & Swanson ( 1952). All treatments were applied in water in a total volume of 40 gal/ac. The soil of the pots was covered with vermiculite which was removed after the spray deposit had dried. This procedure prevented any herbicide or surfactant from reaching the soil. Growth reduction and toxic effects are thus solely attributable to foliar absorption. Plant response or toxicity of the herbicide-surfactant solutions was assessed " by visual observations of symptoms and by weighing the fresh green growth of the above-ground portions of the plant. The phytotoxicity of the surfactant­ herbicide mixtures is reported in terms of a toxicity index value, which was calculated by expressing the fresh weight of the plant as a percentage of the un­ treated control and subtracting this value from 100. B* 236
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