Aggregation Behavior and Antimicrobial Activity of Ester-Functionalized Imidazolium- and Pyridinium-Based Ionic Liquids in Aqueous Solution

Abstract

Two series of long chain imidazolium- and pyridinium-based ionic liquids containing an ester functional group in the alkyl side chain, 3-methyl-1-alkyloxycarbonylmethylimidazolium bromides (C(n)EMeImBr) and 1-alkyloxycarbonylmethylpyridinium bromides (C(n)EPyrBr), were synthesized and their thermal stability, aggregation behavior in aqueous medium, and antimicrobial activity investigated. The introduction of an ester group decreased the thermal stability of the functionalized ILs compared to simple alkyl chain containing ILs (1-alkyl-3-methylimidazolium bromides and 1-alkylpyridinium bromides). Tensiometry, conductimetry, and spectrofluorimetry were applied to study the self-aggregation of the amphiphilic ILs in aqueous solution. The ILs investigated displayed surface activity and the characteristic chain length dependence of the micellization process of surfactants. As compared to simple alkyl chain containing ILs bearing the same hydrocarbon chain, ester-functionalized ILs possess higher adsorption efficiency (pC(20)) and significantly lower critical micelle concentration (cmc) and surface tension at the cmc (γ(cmc)), indicating that the incorporation of an ester group promotes adsorption at the air/water interface and micelle formation. The antimicrobial activity was evaluated against Gram-negative and Gram-positive bacteria and fungi. ILs containing more than eight carbon atoms in the alkyl chain showed antimicrobial activity. Their efficiency as antimicrobial agents increased with the hydrophobicity of the amphiphilic cation being the C(12) homologous the most active compounds. The incorporation of an ester group particularly increased the biological activity against fungi.

Catanionic surfactant formation from the interaction of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) and the ionic liquid 1-butyl-3-methylimidazolium octyl sulfate (bmim-octyl SO4) in aqueous solution

“…In the case of the cationic surfactant hexadecyltrimethylammonium bromide (CTAB) the effect of ILs on the surfactant cmc confirms the trend above described [8][9][10][11][12][13]. However, when IL posses surface activity as reported for long alkyl chain ionic liquids [14][15][16][17][18][19][20][21] and some short alkyl chain ILs [22,23] the ionic liquid / surfactant mixture behaves as a typical binary surfactant system [24][25][26]. In these binary systems the mixed micelle composition X 1 M as well as the  M interaction parameter quantifying the synergism can be obtained by applying the equations of the Regular Solution Theory [27].…”

Aggregation behavior of non-cytotoxic ester functionalized morpholinium based ionic liquids in aqueous media

“…Our own group has reported the aggregation behavior of a variety of SAILs including those based on amino acid based cations [15][16][17] where it has been established that the nature of ionic head group, cation and length of alkyl chain governs the aggregation behavior of SAILs. Further, a careful examination of literature regarding aggregation of SAILs reveals that the introduction of an amide [17], ether or hydroxyl [28], and ester group [29], in the molecular structure of SAILs led to marked changes in their surface as well as bulk behavior. In this regard, the hydrophobic alkyl chain of SAILs is a key moiety which can be functionalized with a variety of functional groups to achieve better aggregation behavior of SAILs.…”

“…In this regard, the hydrophobic alkyl chain of SAILs is a key moiety which can be functionalized with a variety of functional groups to achieve better aggregation behavior of SAILs. In this scenario, amide functionalized morpholinium based SAILs and ester functionalized imidazolium and pyridinium based SAILs possessing better surface active properties as compared to conventional ionic surfactants or non-functionalized SAILs have been reported [17,29]. The presence of easily cleavable functional group such as an ester group, if present in SAIL structure, has been found to increase the proclivity of SAILs towards biodegradation [30].…”

“…The presence of easily cleavable functional group such as an ester group, if present in SAIL structure, has been found to increase the proclivity of SAILs towards biodegradation [30]. Beside their physicochemical study, some of the SAILs have also been recently explored in biological applications owing to their antimicrobial nature [29] and as a gene or drug delivery carrier [31,32]. They have been found to enhance the permeability of DNA/drug through biological membrane.…”

Anti-tumor activity and cytotoxicity in vitro of novel 4,5-dialkylimidazolium surfactants

“…leads to a significant increase in surface activity thus resulting in a reduced aqueous solubility which seems to be the limiting step for the transfer of the C 15 homologues to the plasma membrane surface [27]. Therefore, the elongation of the alkyl chains exceeding a chain length of 11 carbon atoms does not promote the anti-tumor efficiency.…”