Isoxazolium cationic Schiff base surfactants

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Nabel Negm

Isoxazolium cationic Schiff base surfactants

Surface and antimicrobial properties NABEL NEGM*, SALAH TAWFIK *Corresponding author Egyptian petroleum research institute Department of Petrochemicals, Nasr City, Cairo, Egypt

KEYWORDS Isoxazole; Schiff base; cationic; surface activity; antimicrobial. ABSTRACT Two series of cationic surfactants namely: 2-((benzylideneamino)amino) and 2-((4-methoxybenzylidene)amino)(2-oxo-2-alkoxy)-1,3-isoxazol-2-ium surfactants were prepared. Their structures were confirmed using elemental analysis, FTIR, and NMR spectra. The surface activities of the different surfactants showed good tendency towards adsorption at the interfaces. That was estimated from the depression of surface tension at the CMC. The different surfactants were evaluated as biocides against different strains of bacteria, fungi, yeast and sulfate reducing bacteria (SRB) using inhibition zone diameter method. The surfactants showed good antimicrobial activities against the tested microorganisms including Gram positive, Gram negative and fungi. The promising inhibition efficiency of these compounds against the sulfate reducing bacteria (SRB) facilitates them to be applicable in the petroleum field as new categories of SRB biocides.

INTRODUCTION

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mphiphiles compounds are useful as disinfectants and sanitizers in the fields of industrial and human health. Their identical long-chain moieties impart unique surface active properties and aggregation patterns (1-5), and play an important role in their antimicrobial performances (6, 7). The design and synthesis of this type of compounds have recently attracted considerable attention due to the increasing microbial resistance to common quaternary ammonium compounds and the novel biological applications emerging from their singular interactions with lipid membranes (8-14). Several contributions have reported synthetic antimicrobial agents having new active moieties such as hetero atoms (15, 16), aromatic and nonaromatic cyclic substituents (17, 18), and perfluorinated chains (19-21). Many of these compounds have good surface-active properties and good antimicrobial activities. In oilfield applications, sulfate reducing bacteria (SRB) are often problematic in water used for the recovery of petroleum or for oil drilling. SRB can form biofilms on equipment and in pipelines;

the significant problem caused by these bacteria is that they generate metabolic by-products that cause corrosion of metal surfaces. The by-products of these bacteria are consisting of high concentration of H2S gas which acidifies the medium and accelerate the corrosion of pipelines and joints (bio-corrosion). The urgent needs for biocides against SRB are necessary to prevent the damage occurred in the pipes due to the biocorrosion. Recently, several works dealt with this problem to discover effective biocides to inhibit or stop the SRB growth (22-25). Our present research aims to prepare heterocyclic cationic surfactants and studying their antimicrobial activities against bacteria and fungi, in addition to the sulfate reducing bacteria

BIOCIDES using inhibition zone diameter method. The biocidal activitysurface activity relation was discussed.

The surfactants were dissolved in bidistilled water. All the measurements were repeated three times and the surface tension values were taken as the average of these values.

MATERIALS AND METHODS

Antimicrobial activity

Chemicals

2-aminoisoxazole, benzaldehyde, anisaldehyde, alkyl bromoesters and p-toluene sulphonic acid were all analytical grade chemicals (99.9 percent purity) purshured from SIGMAALDRICH, Germany. Solvents (methanol and n-propanol) were supplied from ADWIC, Egypt.

Synthesis of 2-aminoisoxazole Schiff bases (OB) and (OA)

Equimolar amounts of 2-aminoisoxazole and benzaldehyde (B) or anisaldehyde (A) were refluxed for 8 h in the presence of methanol as a solvent and 0.01 percent of p-toluene sulphonic acid as a dehydrating agent. The reaction mixture was left to cool, and then filtered. The products were recrystallized twice from n-propanol, washed by water and dried under vacuum at 40oC to afford yellow crystals of N-benzylidene-3-amino5-methyl-1,2-oxazole (OB) and N-(4-methoxy benzylidene)-3amino-5-methyl-1,2-oxazole (OA) (Scheme 1) (26, 27).

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Antimicrobial activities of the synthesized compounds were measured in the Fermentation Biotechnology and Applied Microbiology Centre (FBAM, El-Azhar University, Cairo, Egypt) using inhibition zone technique in dimethyl formamide as a solvent, while cetyl trimethyl ammonium bromide (CTAB) as a reference biocide. The studied microorganisms were: Escherichia coli, Bacillus streptococcus, Staphylococcus auerus, Desulfomonas pigra (SRB), Candida albican. The inhibition zone diameter formed by the synthesized biocides at concentration of 1 mg ml-1 against the tested microbial strains determines their antimicrobial activities. The value of the inhibition zone diameter was the average of three measured replicates (31, 32).

RESULTS AND DISCUSSION Structures of synthesized surfactants

The chemical structures of the synthesized surfactants (OB12-18, OA12-18) were confirmed using elemental analyses (Table 1).

Table 1. Elemental analyses of the synthesized surfactants.

Scheme 1. Synthesis of 2-aminoisoxazole Schiff base derivatives: OB and OA.

Synthesis of isoxazolium Schiff base surfactants

The synthesized Schiff bases (OB and OA) (0.05 mol) were refluxed individually in the presence of 0.05 mol of different alkyl bromoesters in methanol as a solvent. The reaction mixture left overnight to precipitation of the cationic surfactants. The products were filtered off and recrystallized twice from n-propanol (22). The products were indicated as OB12-18 and OA12-18, (Scheme 2).

FTIR spectroscopy: 1640–1645 cm-1 (CH =N), 2847 and 2920 cm-1 (CH2 and CH3) respectively, and 3075 cm-1 (N+). Compounds OA12-18 showed additional absorption band at 1221 cm-1 corresponds to OCH3 group. 1H-NMR spectra in DMSO-d6 (for OB12) showed signals at: d = 0.8 ppm (t, 3H, CH3), 4.7 ppm (t, 2H, CH2O), 3.3 ppm (s, 2H, CH2 N+), and 8.5–8.7 ppm (s, 1H, N=CH). All compounds showed a characteristic signal at 1.2 ppm (m, 2nH, nCH2) with integration number (n) corresponds to the number of carbon atoms in the different alkyl chains, (n=10 for OB12 and OA12; n=12 for OB14 and OA14; n=14 for OB16 and OA16; and n=16 for OB18 and OA18).

Scheme 2. Synthesis of the different isoxazolium Schiff base

Surface and interfacial tension measurements

The measurements were carried out using Du Noüy Tensiometer with platinum ring (Krüss K6, Germany) at 25oC (28-30).

chimica oggi/Chemistry Today - vol. 30 n. 6 November/December 2012

Figure 1. Suface tension-concentration profile of 2- ((benzylideneamino)amino)-(2-oxo-2-alkoxy)-1,3-isoxazol-3-ium surfactants at 25 °C.

Surface activity

Figure 1 represents the surface tensionconcentration profiles of OB12–18 of the prepared Schiff base surfactants at 25oC. The tendency of the synthesized molecules towards adsorption at the air/water interface is indicated by the decrease of the surface tension than the distilled water (33, 34). The increase of the concentration is gradually decreases the surface tension. At a certain concentration, the surface tension stays constant which indicated as critical micelle concentration (35). This profile represents the influence of the hydrophobic chain length on the surface tension of these surfactants. Considering the homologous series (OB12–18), it is clear that the dodecyl derivatives have higher surface tension values compared to the other members. That could be attributed to the increase in the molecular hydrophobicity by increasing the length of chains (30). The cmc values extracted from the surface tension-concentration profiles of these surfactants decrease by increasing the chain length, Table 2. The terminal methoxy group has quite effect on the cmc values compared to the variation of the alkyl chain length (36). Also, Table 2. Surface the octadecyl derivatives (OB18, properties of the OA18) had the lowest CMC synthesized surfactants values (38 and 40 mmol/L) at at 25 °C. 25oC compared to the other synthesized derivatives. The change of CMC values of OB12–18 and OA12–18 surfactants by the gradual increase of the hydrophobic chains is diminutive compared to the conventional cationic surfactants (35, 37). This attributes to the high carbon content of these surfactants, which increases their hydrophobicity. The depression in surface tension values at the cmc (pcmc) for these surfactants showed that the shorter hydrophobic chain derivatives have lower pcmc. While octadecyl derivatives have higher effectiveness values. That was attributed to the high adsorption tendency and population of these derivatives at the air/water interface (23). The interfacial tension values of the synthesized isoxazol-2-ium surfactants (OB12–18 and OA12–18) were measured between aqueous surfactant solution (0.1% wt) and light paraffin oil at 25°C, (Table 2). Inspecting the interfacial tension data leads to the following conclusions:

Table 3. Antimicrobial activity of the synthesized isoxazolium Schiff base surfactants against pathogenic bacteria and fungi.

1- The short chain derivatives have high interfacial tension compared to the long chain derivatives, 2- The synthesized hexadecyl and octadecyl derivatives showed the lowest interfacial tension values of the two series (5, 4 mN/m for OB16-18 and 7, 3 mN/m for OA16–18). This materializes their applicability in the interfacial applications including: corrosion inhibition, antimicrobial agents and phase transfer processes.

3- Increasing the alkyl chain length from 12 to 18 methylene group decreases the interfacial tension values considerably which ascribed to the increasing of the repulsion between these chains and the polar phase. Hence the chains are differentially pumped to the interface and the interfacial tension consequently decreases to lower values.

Antimicrobial activity against bacteria, fungi, and sulfate reducing bacteria (SRB)

It is known that cationic surfactants have pathogenic effects against the microorganisms including bacteria, fungi and yeast. The presence of hetero atoms (N, P, and S) in the chemical structures of these surfactants increases their pathogenic effect. The synthesized cationic Schiff base surfactants were evaluated for their pathogenity toward Gram-negative and Gram-positive bacterial strains at concentration of 1 mg/ ml (Table 3). The values of the inhibition zone diameters are ranged between 13 and 19 mm, which is considered good antimicrobial activity

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compared with the reference used (CTABr: cetyltrimethyl ammonium bromide, 12.3 mm). It is clear that the antimicrobial activities of the synthesized cationic surfactants (OB12–18 and OA12–18) against the studied microorganisms are increased gradually by increasing their hydrophobic chain length. The most efficient biocides are those containing octadecyl chains (OB18, OA18). That behaviour is depending on their surface activities. Increasing the hydrophobic chain length increases the adsorption tendency of the biocide molecules at the interfaces. The action mode of the cationic biocides on the microorganisms is originally focused on the adsorption tendency of these cationics on the cellular membranes (23, 38). In Gram-positive bacteria, the adsorption is occurred in the lipoteichonic acid layer which is characterized by the charged nature and the ability to interact with the positively charged molecules. While in the Gram-negative bacteria, the lipid layer is the target of the biocide molecules. That can explain the natural resistance of some bacterial genera towards cationic biocides. This adsorption disturbs the selective permeability of these membranes, which causes severe disturbance of the biological reactions inside the cells due to the diffusion of several compounds from the environment due to the lake of the selective permeability (38). Also, the presence of the halogen atoms (Br-) as counter ions increases the potent action when penetrated into the cells. From the data in Table 3, the action mode of the synthesized biocides is seemed to be identical in case of fungi and bacteria. Also, the results of the antifungal activity obtained from the biological study showed promising features of the tested biocides against the pathogenic fungal strain (Candida albicans). An important factor that influences the biocidal activity of the different biocides is the net charge on their molecules. Several studies showed that the electrostatic interactions play a key role in the action of cationic biocides, and that a decrease in the charge density of the cationic compounds results in a reduction in adsorption and biocidal efficiency (3841). Comparing the inhibition values of the synthesized cationic surfactants with the classically antimicrobial surfactant (CTAB) showed their relatively higher biocidal activity owing to their relatively lower inhibition zone values. The relatively high biocidal activity of the synthesized surfactants is accounted to the high average charge on their molecules. In case of CTAB, the head group has one positive charge that locates on the nitrogen atom (N+) (38). On the contrary, conjugation of electrons over the isoxazol-2-ium nucleus sharply increases the charged centres and consequently increases the average charge of the

molecules. That increases their tendency towards adsorption in addition to the biocidal activity. The SRB bacteria are producing H2S gas due to the reduction of sulfur compounds in oil. That increases the acidity of the medium which causes corrosion to the pipelines. That type of corrosion is called bio-corrosion. To decrease the effect of the bio corrosion, biocides should be used to inhibit the growth of the SRB bacteria and consequently the production of H2S gas. The synthesized cationic surfactants were applied as biocides against sulfate reducing bacteria (Desulfomonas pigra). Table 3 showed imperative results due to their relatively high efficiency against this type of bacteria. It is clear from data listed in Table 3 that the gradual increase of the alkyl chain length of the biocides increases their biocidal activities against SRB.

CONCLUSION The synthesized phosphate containing nonionic surfactants bearing Schiff base groups showed good surface activity in their solutions and at the interfaces. The different surfactants showed high potency against the Gram-positive and Gram– negative bacterial strains. The antibacterial activity of these compounds against sulfur reducing bacteria showed moderate to high efficacy.

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