Rethinking the Esterquats: Synthesis, Stability, Ecotoxicity and Applications of Esterquats Incorporating Analogs of Betaine or Choline as the Cation in Their Structure

Abstract

Esterquats constitute a unique group of quaternary ammonium salts (QASs) that contain an ester bond in the structure of the cation. Despite the numerous advantages of this class of compounds, only two mini-reviews discuss the subject of esterquats: the first one (2007) briefly summarizes their types, synthesis, and structural elements required for a beneficial environmental profile and only briefly covers their applications whereas the second one only reviews the stability of selected betaine-type esterquats in aqueous solutions. The rationale for writing this review is to critically reevaluate the relevant literature and provide others with a "state-of-the-art" snapshot of choline-type esterquats and betaine-type esterquats. Hence, the first part of this survey thoroughly summarizes the most important scientific reports demonstrating effective synthesis routes leading to the formation of both types of esterquats. In the second section, the susceptibility of esterquats to hydrolysis is explained, and the influence of various factors, such as the pH, the degree of salinity, or the temperature of the solution, was subjected to thorough analysis that includes quantitative components. The next two sections refer to various aspects associated with the ecotoxicity of esterquats. Consequently, their biodegradation and toxic effects on microorganisms are extensively analyzed as crucial factors that can affect their commercialization. Then, the reported applications of esterquats are briefly discussed, including the functionalization of macromolecules, such as cotton fabric as well as their successful utilization on a commercial scale. The last section demonstrates the most essential conclusions and reported drawbacks that allow us to elucidate future recommendations regarding the development of these promising chemicals.

Keywords: biological activity; cationic surfactants; cleavable compounds; environmental safety; esters; quaternary ammonium salts.

Figure 1

The structures of the most popular esterquats: triethanolamine esterquat (TEAQ), diethanol amine esterquat (DEEDMAC), and N,N,-dimethyl-3-aminopropane-1,2-diol esterquat (HEQ).

Figure 2

Reported applications of esterquats [3,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27].

Figure 3

General formula of choline-type and betaine-type esterquats.

Figure 4

Derivatization strategies of known herbicides toward esterquats: (Strategy A) IL formation from the cholinium cation and herbicidal anion; (Strategy B) covalent modifications of the esterquat structure, in a manner similar to prodrug approach, with a formation of easily hydrolyzable ester bond; (Strategy C) a combination of a “prodrug” strategy and salt formation.

Figure 5

Structure of asymmetric gemini surfactant derived from carbonate and lidocaine moieties.

Figure 6

Left: structure of lidocaine-derived choline esterquat spontaneously self-assembling into Janus particles (A,B); middle: MDEA esterquat for delivery of cytostatic drugs and small RNA fragments (C); right: antioxidant sinapine esterquat derivative (D).

Figure 7

Structure of alkyl betainate cation paired with iodosulfuron methyl and dicamba anions.

Figure 8

Proposed alkanoylcholine and alkylbetaine transformation pathways and stoichiometry under methanogenic conditions. The colored figures are total electron densities mapped as electrostatic potential between L0.01 and 0.01 e/au³, and the structures were modified to fit their potential surfaces [7,90]. The surfaces were drawn and optimized using Avogadro 1.2.0. and its basic optimization algorithm, and the potential surfaces were generated using Jmol ver. 14.32.77 (potential surfaces −0.1–0.1). Adapted from [7].

Figure 9

Structures of choline- and betaine-derived esterquats subjected to hydrolysis studies. Letters A–K indicate cation structure, for which data were presented in Table 6.

Figure 10

Biodegradation of esterquats and their classification (numbers of compounds correspond to entries from Table 7) [3,12].

Figure 11

Structures of choline- and betaine-derived esterquats subjected to biodegradation studies.

Figure 12

Structures of different esterquats subjected to antimicrobial activity studies.

Figure 13

Structures of different esterquats subjected to toxicity studies. Letters A–D indicate cation structure, for which data was presented in Table 9.

Figure 14

Examples of compounds utilized as surfactants, emulsifiers and foam stabilizers.

Figure 15

Plot of initial foam volume and after 3 min for 0.1 wt.% surfactants at 25 °C (cations structures are detailed in Table S3 in ESI, a: Entry 66, b: Entry 67, c: Entry 68, d: Entry 69, e: Entry 70, f: Entry 71, g: Entry 72, h: Entry 73, i: Entry 74). Reprinted from J. Mol. Liq. 299, 112248, J.Wu et al. “Cationic gemini surfactants containing both amide and ester groups: Synthesis, surface properties and antibacterial activity”, Copyright 2020, with permission from Elsevier [18].

Figure 16

Examples of esterquats utilized as flocculants and fabric softeners.

Figure 17

Example of compounds utilized as pharmaceuticals and antiseptics.

Figure 18

Scheme demonstrating the cotton fabric functionalized by glycine betaine [10].

Figure 19

Examples of esterquat cations utilized as agrochemicals.

Figure 20

Phytotoxicity of alkyl betainate bromides (D, n = 1) toward white mustard, where the alkyl chain (R) starts from ethyl (1) to octadecyl (9) in comparison to sodium bromide (10). Letters above bars provide a visual representation of the statistical significance of the differences between the groups being compared [14].

Figure 21

Example of esterquats utilized as materials in electrochemistry.