Water Sorption Isotherms of Surfactants: A Tool To Evaluate Humectancy

Abstract

Fundamental experimental data for moisture absorption of non-ionic polydisperse surfactants with differing ethylene oxide (EO) content and variable aliphatic portions were measured at relative humidities between 0 and 95% at 25 °C. Remarkable differences in moisture absorption were observed between surfactant classes but also within one series of surfactants differing in either EO content or the long-chain aliphatic fraction. Both the EO units as well as the entire molecular structure, including also the lipophilic domain, were discussed to account for the humectant activity of surfactants. Water sorption isotherms showed an exponential shape, which was argued to be associated with the formation of a "free" water domain. These humectant properties might be relevant to the behavior of a foliar-applied spray droplet of agrochemical formulation products because the uptake of active ingredients will be enhanced as a result of deferred crystal precipitation.

Dynamics of moisture diffusion and adsorption in plant cuticles including the role of cellulose

“…1 , wax extraction only had a small effect 4 and results found elsewhere 5 agree where water travels mostly down the aqueous pathway and less down the lipophilic pathway. We note that some studies 10 , 11 have used other techniques, chemicals, pre-treatments and adjuvants, which may increase the role of the lipophilic pathway for water penetration. In Supplementary Fig.…”

“…Relative humidity (RH) has a large influence over the weight gain due to water sorption, and penetration of hydrophilic ionic agrochemicals, such as calcium chloride (CaCl 2 ), across the cuticle 3 , 6 , 10 , and so the degree of cuticle hydration is important. Humectants and non-ionic surfactants have been shown to influence the hydration of the cuticle 10 , 11 . Increasing RH can significantly increase penetration of ionic agrochemicals through plant cuticles, and the mechanisms causing this increase are hydration of the cuticle, water adsorption to the aqueous pore walls 6 , 12 and water adsorption due to hygroscopic salts that may exist both in situ on the leaf surface (Tredenick, E. C., Stuart-Williams, H. & Enge, G., 2021, Materials on plant leaf surfaces are deliquescent in a variety of environments, unpublished) and included in the spray formulation 3 .…”

“…However, we can reasonably speculate that this contribution will vary depending on the plant species, environmental conditions and growth conditions. The contribution of each pathway may also be governed by the method of cuticle isolation and chemical treatments, and in the context of agrochemical penetration, pre-treatments and applied droplet chemical composition, including surfactants 4 , 11 , 18 , 29 . One study 5 found that water transport was 2.8 times higher in the aqueous pathway than the lipophilic pathway of astomatous isolated cuticles, with significant variation between species.…”

Interaction of surfactants with barley leaf surfaces: time-dependent recovery of contact angles is due to foliar uptake of surfactants

“…This delay in reaching the plateau of the residual cuticular transpiration could have been caused by different reasons. It can be explained by the fact that the aqueous surfactants are hygroscopic and thus retain water during dehydration (Asmus et al 2016 ). Thus, drying of the applied surfactant solutions takes longer the higher the surfactant coverage is, whereas pure water evaporates within 30 min.…”

Impact of glyphosate formulations and adjuvants: effects on leaf interaction, metabolism, and control of sourgrass

“…Surfactants with low ethylene oxide (Eo) content are known to readily penetration through the cuticle and facilitate IA uptake through the lipophilic pathways of this membrane (SToCK et al, 1992;SHI et al, 2005). Adjuvants with a high Eo content increase the permeability of hydrophilic solutes (LIU, 2004;BAUR & APoNTE 2014), mainly due to their humectant activity (ASMUS et al, 2016).…”