The Mechanism of Surfactant Action

The sites and mechanisms of surfactant action have been considered by Holloway and Stock. Surfactants may act at four possible sites in treated leaves, namely, on the surface, cuticle, and epidermis or within the internal tissues. As mentioned previously, the addition of surfactants to formulations of agrochemicals invariably improves their wetting and spreading characteristics as measured by contact angle, deposit area, spread, and surface tension. Generally, however, the effect of surfactants on coverage can seldom be related to herbicide uptake or activity. 

It is possible that surfactants can aid the uptake of herbicides by increasing their water solubility and thus facilitating their partitioning from lipid to aqueous phases in the cuticle. While increases in water solubilities of certain pesticides by surfactants have been demonstrated in vitro, only small differences were evident in the solubilizing capacities of nonionic surfactants of different classes and E contents. It seems unlikely that differences between surfactants in their relative solubilizing efficiencies account for any differences in their activation efficiency.  

The moisture-absorbing capacity of certain surfactants may enhance the activation of water-soluble compounds the capacity to do so appears to depend upon surfactant chemical class, E content, and ambient relative humidity. Hydrophilic surfactants of high E content are the most hygroscopic, and under field conditions they may act as condensation nuclei just below water vapor saturation. Under these circumstances, a water droplet may re-form over the original droplet, thus enhancing uptake of the a.i. 

In penetration of the cuticle and epidermis, the mechanisms of movement involve a partition-diffusion process which involves three compartments  

an outer mainly aqueous external phase at the surface 

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The mechanism of surfactant action is uncertain. In the case of asulam, incorporation of surfactant of E6.5-8 may increase the rate of transcuticular passage by facilitating the partitioning of a.i. from droplet/deposit to cuticle waxes and from waxes to the inner, relatively aqueous phase. Alternatively, if a polar route across the cuticle does indeed exist, transport may be enhanced by prolonged retention of the a.i. in the aqueous phase of the droplet or by surfactant-induced transport via this polar route. The relatively low levels of asulam detected in the cuticle waxes tend to support either view. The beneficial effect of E4 surfactant on uptake of diflufenican... 

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