Structural Features of Reconstituted Cuticular Wax Films upon Interaction with Nonionic Surfactant C12E6

Effect of Different Ethylene Oxide Addition Numbers on the Performance of Polyoxyethylene Tallow Amine as a Pesticide Emulsifier

Surfactant reduces the surface tension of liquids, resulting in improved emulsion stability, and there is great interest in pesticide additives. Ethoxylate is often used as a pesticide emulsifier. However, the degree of ethoxylation and the existence of dioxane byproducts can significantly affect the performance of emulsifiers. Here, a series of polyoxyethylene tallow amines with the addition of different numbers of ethylene oxide (EO) were synthesized and characterized. Their physical and chemical performances were measured. The ability of POEA as a surfactant to reduce water surface tension and the surface adsorption of molecules were assessed based on the static and dynamic surface tensions. The results show that the surfactant molecules preferentially form a saturated adsorption layer in solution, and the mixed-diffusion-kinetics mechanism dominates the adsorption process. With the increase of the EO addition number, the emulsifying property of POEA increases, while the wetting property gradually decreases and the contact angle increases. These results can provide a basis for the selection of pesticide additives. At the same time, the mechanism of removing dioxane by ethoxylate is described, and a simple and low-consumption method is put forward to reduce the dioxane content. It provides a new idea for the removal of dioxane.

Unraveling How Membrane Nanostructure Changes Impact the Eye Irritation of Nonionic Alkyl Ethoxylate Surfactants

Nonionic surfactants used in agri-spraying processes may cause varying degrees of corneal irritation when they come in direct contact with farmers' eyes, and the exact irritations are thought to be determined by how surfactants interact with corneal cell membranes. However, how nonionic surfactants interact with cell membranes at the molecular and nano levels remains largely unexplored. In this study, the interactions between nonionic surfactants (alkyl ethoxylate, C12Em) and lipid membranes were examined by membrane permeability measurement, quartz crystal microbalance with dissipation, dual polarization interferometry, confocal laser scanning microscopy, and neutron reflection, aiming to reveal complementary structural features at the molecular and nano levels. Apart from the extremely hydrophobic surfactant C12E2, all nonionic surfactants studied could penetrate the model cell membrane composed of a phosphocholine lipid bilayer. Nonionic surfactants with intermediate amphiphilicity (C12E6) rapidly fused into the lipid membrane and stimulated the formation of pores across the lipid bilayer, consistent with the cytoplasm leakage and fast cell necrosis observed from the cytotoxicity study of corneal cells. In comparison, while hydrophobic and hydrophilic surfactants [those with long and short ethoxylates (C12E4,12,23)] could cause mild structural alteration to the outer lipid layer of the membrane, these structural changes were insufficient to elicit large cytoplasmic leakage rapidly and instead cell death occurred over longer periods of time due to changes in the membrane permeability. These results reveal the strong link of surfactant-lipid membrane interactions to surfactant cytotoxicity and the association with amphiphilicity of nonionic surfactants.

How surfactants affect the depuration of polycyclic aromatic hydrocarbons adsorbed on the mangrove leaf surfaces: insight from an in situ method

The effects of sodium dodecyl benzene sulfonate (SDBS), polyoxyethylene (20) sorbitan monolaurate (Tween 20), and their mixtures on the depuration of anthracene (Ant) and fluoranthene (Fla) individually adsorbed on the Kandelia obovata (Ko) leaf surfaces were in situ investigated. The Ko original leaf-wax microstructures have been destroyed by SDBS, Tween 20, and their mixtures at or above their critical micelle concentration (CMC). The volatilization rate constants (k_V) of the adsorbed PAHs decreased with surfactants at or above their CMC resulting from the plasticizing effect and a decrease in the polarity of the Ko leaf-waxes induced by surfactants. Moreover, the photolysis rate constants (k_P) of the adsorbed PAHs decreased with SDBS while increased with Tween 20 and their mixtures at or above their CMC, which can be attributed to effects of surfactants on the light adsorption behavior of Ko leaf-waxes. Overall, the effects of surfactants on the depuration of the adsorbed PAHs were dependent not only on the physical-chemical properties of surfactants but also on the micro-environment of the substrates adsorbed the PAHs. These results are of great significance for further understanding the accumulation of PAHs and could expand our knowledge about the migration mechanism of PAHs from the atmosphere by mangrove leaf surface micro-zones.

Contrasting impacts of mixed nonionic surfactant micelles on plant growth in the delivery of fungicide and herbicide

HYPOTHESIS

Nonionic alkyl ethoxylate surfactants are widely used in agrochemicals to facilitate the permeation of systemic herbicides and fungicides across the plant waxy film. Industrial grade surfactants are often highly mixed and how the mixing affects their interactions with pesticides and wax films remains largely unexplored. A better understanding could enable design of mixed nonionic surfactants for herbicides and fungicides to maximize their efficiency and reduce wastage whilst controlling their impact on plant wax films.

EXPERIMENT

In this study, nonionic surfactants with general structure n-oxyethylene glycol monododecyl ether (C₁₂E_n) were used to form surfactant mixtures with the same average ethoxylate numbers but different hydrophilic-lipophilic balance (HLB) values. Their mixed micellar systems were then used to solubilize a herbicide diuron (DN) and a fungicide cyprodinil (CP), followed by plant wax solubilization upon contact with wax films. These processes were monitored by ¹H NMR and SANS.

FINDING

Pesticide solubilization made surfactant micelles effectively more hydrophobic but subsequent wax dissolution caused pesticide release and the restoration of the micellar amphiphilicity. Nonionic surfactants with lower HLBs form larger nanoaggregates, show enhanced wettability, and have better ability to solubilize and permeate pesticides across the wax film, but may cause significant damage to plant growth. These observations help explain why herbicides applied on weeds would benefit from surfactants with lower HLB values while fungicides require surfactants with HLBs to balance between delivery efficiency and potential phytotoxicity risks.

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

Time-dependent contact angle measurements of pure water on barley leaf surfaces allow quantifying the kinetics of surfactant diffusion into the leaf. Barley leaf surfaces were sprayed with three different aqueous concentrations (0.1, 1.0 and 10%) of a monodisperse (tetraethylene glycol monododecyl ether) and a polydisperse alcohol ethoxylate (BrijL4). After 10 min, the surfactant solutions on the leaf surfaces were dry leading to surfactant coverages of 1, 10 and 63 µg cm^-2, respectively. The highest surfactant coverage (63 µg cm^-2) affected leaf physiology (photosynthesis and water loss) rapidly and irreversibly and leaves were dying within 2-6 h. These effects on leaf physiology did not occur with the lower surfactant coverages (1 and 10 µg cm^-2). Directly after spraying of 0.1 and 1.0% surfactant solution and complete drying (10 min), leaf surfaces were fully wettable for pure water and contact angles were 0°. Within 60 min (0.1% surfactant) and 6 h (1.0% surfactant), leaf surfaces were non-wettable again and contact angles of pure water were identical to control leaves. Scanning electron microscopy investigations directly performed after surfactant spraying and drying indicated that leaf surface wax crystallites were partially or fully covered by surfactants. Wax platelets with unaltered microstructure were fully visible again within 2 to 6 h after treatment with 0.1% surfactant solutions. Gas chromatographic analysis showed that surfactant amounts on leaf surfaces continuously disappeared over time. Our results indicate that surfactants, applied at realistic coverages between 1 and 10 µg cm^-2 to barley leaf surfaces, leading to total wetting (contact angles of 0°) of leaf surfaces, are rapidly taken up by the leaves. As a consequence, leaf surface non-wettability is fully reappearing. An irreversible damage of the leaf surface fine structure leading to enhanced wetting and increased foliar transpiration seems highly unlikely at low surfactant coverages of 1 µg cm^-2.

What happens when pesticides are solubilised in binary ionic/zwitterionic-nonionic mixed micelles?

HYPOTHESIS

Surfactants have been widely used as adjuvants in agri-sprays to enhance the solubility of pesticides in foliar spray deposits and their mobility through leaf cuticles. Previously, we have characterised pesticide solubilisation in nonionic surfactant micelles, but what happens when pesticides become solubilised in anionic, cationic and zwitterionic and their mixtures with nonionic surfactants remain poorly characterised.

EXPERIMENTS

To facilitate characterisations by SANS and NMR, we used nonionic surfactant hexaethylene glycol monododecyl ether (C₁₂E₆), anionic sodium dodecylsulphate (SDS), cationic dodecyltrimethylammonium bromide (DTAB) and zwitterionic dodecylphosphocholine (C₁₂PC) as model adjuvant systems to solubilise 3 pesticides, Cyprodinil (CP), Azoxystrobin (AZ) and Difenoconazole (DF), representing different structural features. The investigation focused on the influence of solubilisates in driving changes to the micellar nanostructures in the absence or presence of electrolytes. NMR and NOESY were applied to investigate the solubility and location of each pesticide in the micelles. SANS was used to reveal subtle changes to the micellar structures due to pesticide solubilisation with and without electrolytes.

FINDINGS

Unlike nonionic surfactants, the ionic and zwitterionic surfactant micellar structures remain unchanged upon pesticide solubilisation. Electrolytes slightly elongate the ionic surfactant micelles but have no effect on nonionic and zwitterionic surfactants. Pesticide solubilisation could alter the structures of the binary mixtures of ionic/zwitterionic and ionic/nonionic micelles by causing elongation, shell shrinkage and dehydration, with the exact alteration being determined by the molar ratio in the mixture.

How does substrate hydrophobicity affect the morphological features of reconstituted wax films and their interactions with nonionic surfactant and pesticide?

HYPOTHESIS

Surfactants are widely used in agri-sprays to improve pesticide efficiency, but the mechanism underlying their interactions with the surface wax film on plants remains poorly understood. To facilitate physical characterisations, we have reconstituted wheat cuticular wax films onto an optically flat silicon substrate with and without octadecyltrimethoxysilane modification to control surface hydrophobicity.

EXPERIMENTS

Imaging techniques including scanning electron microscopy (SEM) unravelled morphological features of the reconstituted wax films similar to those on leaves, showing little impact from the different substrates used. Neutron reflection (NR) established that reconstituted wax films were comprised of an underlying wax film decorated with top surface wax protrusions, a common feature irrespective of substrate hydrophobicity and highly consistent with what was observed from natural wax films. NR measurements, with the help of isotopic H/D substitutions to modify the scattering contributions of the wax and solvent, revealed different wax regimes within the wax films, illustrating the impact of surface hydrophilicity on the nanostructures within the wax films.

FINDINGS

It was observed from both spectroscopic ellipsometry and NR measurements that wax films formed on the hydrophobic substrate were more robust and durable against attack by nonionic surfactant C₁₂E₆ solubilised with pesticide Cyprodinil (CP) than films coated on the bare hydrophilic silica. Thus, the former could be a more feasible model for studying the wax-surfactant-pesticide interactions.

Agricultural adjuvants may impair leaf transpiration and photosynthetic activity

Adjuvants such as surfactants are commonly incorporated into agrochemical formulations to enhance the biological efficiency of foliar sprays by improving the wetting behavior of the spray and/or the penetration of the active ingredients into the leaf tissues. Penetration accelerating adjuvants are known to increase the cuticular permeability and may alter the cuticular barrier to water loss. However, none or very little emphasis has been given to the impacts of adjuvants on crop water balance or drought tolerance, a very important factor affecting crop performance under water scarcity. Two model crops with strongly varying leaf traits, kohlrabi (Brassica oleracea) and apple (Malus domestica) seedlings were grown in controlled environments. Three adjuvants with varying solubility in the cuticle, i.e. octanol-water partition coefficients (logKow) were selected: rapeseed methyl ester (RME) and the surfactants alkyl polyglycoside (APG) and polyoxyethylated tallow amine (POEA). The higher the logKow of the adjuvant, the stronger was the increase of minimum epidermal conductance (g_min, an essential parameter describing plant drought tolerance). However, such effects depended on the physio-chemical properties of the leaf surface. In comparison to kohlrabi, the adjuvant effects on g_min of apple leaves were relatively weak. The increase of g_min was associated with a decrease in contact angle and with an alteration of the wax microstructure. Furthermore, POEA affected photochemical efficiency of kohlrabi leaves. Some adjuvants could have a temporal influence on transpirational water loss and g_min. At repeated applications, they might alter the effective water use and possibly reduce drought tolerance of some horticultural crops.