Synthesis and Surface Properties of Piperidinium-Based Herbicidal Ionic Liquids as a Potential Tool for Weed Control

Abstract

A series of piperidinium-based herbicidal ionic liquids (HILs) were synthesized and investigated. The designed HILs, obtained with high yields, consisted of cation 1-alkyl-1-methylpiperidinium with surface activity and a commercially available herbicidal anion: (3,6-dichloro-2-methoxy)benzoates (dicamba). The above-mentioned compounds were characterized in terms of surface activity and phytotoxicity. Preliminary results were obtained at higher wettability for all HILs when compared to the wettability of commercial Dicash, with HIL having 18 atoms in the carbon chain being the best effectiveness in wetting surfaces (weeds and crop leaves), whereby a drop of HILs with short alkyl chains (C₈-C₁₀) could not slide down a leaf. Our findings present that wettability or mobility of HILs drops varied depending on the plant species. Moreover, in this study, by zeta potential and atomic force microscopy measurements, we provide conclusive evidence to demonstrate that alkyl chain elongation plays a significant role in the evolution of surface properties of HILs.

Keywords: dicamba; herbicidal ionic liquids; sliding angle; static contact angle; zeta potential.

Scheme 1. Two-Step Synthesis of Piperidinium-Based HILs

Figure 1

Surface tension versus log concentration of the synthesized HILs at 25 °C.

Figure 2

Static CA values on the adaxial surface of leaves (more details are provided in Table S3, Supporting Information).

Figure 3

Digital microscope images for the adaxial side of leaves. The images were obtained under magnification. The bar represents 100 μm.

Figure 4

SAs of HILs on the adaxial side of the weed leaf surface and paraffin.

Figure 5

Zeta potential of aggregates of the analyzed HILs.

Figure 6

AFM results for piperidinium-based HILs deposited on mica surfaces, showing the surface coverage by piperidinium-based molecules with different alkyl chain lengths (C₉ and C₁₈). (A, E) Topography of selected areas for piperidinium-based HILs with different alkyl chains. (B, F) Five profile curves for selected round deposits. (C, G) 3D view of the test surfaces. (D, H) Topography of selected areas for piperidinium-based HILs with small surface areas.

Figure 7

AFM studies of morpholinium-based HILs (A, B) and piperidinium-based HILs (C, D) deposited on mica surfaces. (E) Height distribution of selected samples with different cations (black curve—morpholinium-based HILs, blue curve—piperidinium-based HILs). (F) Selected profile curves for topography D with round deposits.

Figure 8

Average root and shoot length for seedlings cornflower (Centaurea cyanus L.) in sand with the addition of 1-alkyl-1-methylpiperidinium (3,6-dichloro-2-methoxy)benzoates and Dicash.