Engineering Chemistry, ISSN: 2813-6535, Vols. Engineering Chemistry Vol. 9, pp 53-60

doi: 10.4028/p-MxwzR2

© 2025 The Author(s). Published by Trans Tech Publications Ltd, Switzerland.

Submitted:
2024-11-14

Revised:
2025-01-26

Accepted:
2025-01-27

Online:
2025-03-10

Synthesis and Investigation of Surface Tension Properties of Fatty Pyrimidinium betaines

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Malki Fatiha , Missoum Amina and Hamza Kahina

¹Laboratoire de recherche sur les produits bioactifs et valorisation de la biomasse, Ecole Normale Supérieure, BP 92, Kouba, Algiers, Algeria

Keywords: amphoteric surfactant, pyrimidinium betaine, synthesis, surface activity, critical micelle concentration (CMC).

Abstract

A novel series of fatty betaines containing a pyrimidine ring with different alky chain lengths were synthesized by condensation of fatty -diphenylalkylamidines with a specific highly reactive malonic ester derivative under mild conditions. The chemical structures of products were characterized by common spectroscopic analyses (FTIR, mass spectra, NMR, and NMR spectrometry). Equilibrium surface tension and conductivity as a function of concentration of dodecyl pyrimidinum betaine ( betaine with a 12 -carbon alkyl chain) in ethanolic solutions were measured and the critical micelle concentration (CMC) was determined at . The results showed that the critical micelle concentration (CMC) of dodecyl pyrimidinum betaine is and the corresponding surface tension is respectively. These results revealed that the synthesized fatty pyrimidinium betaines are efficient amphoteric surfactants making them promising candidates for applications in a very large number of areas.

Introduction

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Amphoteric surfactants possess long hydrophobic chain with positively and negatively charged hydrophilic head groups [1]. Due to the various properties demonstrated in the amphoteric surfactants (remarkable interfacial activity, lower CMC, solubility, less harm to the skin, less toxic and more biodegradable-depending on the polar headgroups), they have a lot of different uses in cosmetics, enhanced oil recovery, electrochemistry, nanoscience, polymer chemistry, etc ... [2-3].
In recent decades, amphoteric surfactants have attracted considerable interest in academic and industrial research, due to their unique properties such as: solubility, micellization and surface tension [4].

Betaine amphoteric surfactants contain both a positively charged group and a negatively charged group within the same molecule. Betaine- type amphoteric surfactants have been shown to exhibit antimicrobial activity against various microorganisms [5]. They have a variety of applications in biological research, medicine, pharmacy and other scientific fields [6-8]. Betaine derivatives are widely used in personal care products, detergents, as antistatic agents and are applied in enhanced oil recovery [9]. An important factor determining the properties of betaine surfactants is their chemical structure. The zwitterionic form of betaine exhibits a much lower CMC and a much higher surface activity [10], favoring the formation of micelles and a greater surfactant activity at the air-water interface [11].

Betaines containing a pyrimidine ring are a special class of mesoionic compounds [12]. Mesoionic pyrimidinium betaines have been of a great interest for their particular structure [13] and their chemical stability [14]. They also possess various bioactivities [15, 16] which have raised much interest as biologically active compounds [17].

In recent years, many studies have been conducted on the effect of modifying hydrophobic groups, hydrocarbon chain length and types of hydrophilic head group [18]. Several works have focused on the synthesis of imidazolinium-based fatty surfactants and have studied their surface activity [19, 20, 21]. No studies on the surface tension of alkyl pyrimidinium betaines were reported.

In the present work, a series of novel betaines containing a pyrimidine ring with different alkyl chain lengths were synthesized, and characterized by FTIR, NMR, NMR and mass
spectrometry. In addition, the surface tension of the prepared compounds was investigated using both tensiometric and conductometric techniques [22]. The critical micelle concentration (CMC) and the corresponding surface tension ( ) were measured at .

Materials and Methods

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Materials. All of the chemicals and reagents used were of analytical grade and used directly without purification. Chemicals were purchased from Sigma-Aldrich, Merck, Fluka and Prolabo and were of analytical grade purity. Triethylamine was purified by distillation and the solvents were dried using conventional techniques. Thin layer chromatographic analysis, using Aluminum plates precoated with silica gel was used to monitor the reaction's progress and the chromatograms were visualized using UV at 254 . Melting points were measured in a capillary tube, using a Büchi 540 type digital apparatus. UV-Visible absorption spectra were carried out using a Shimadzu 160 double-beam spectrophotometer. FTIR spectra were recorded as KBr pellets on a Jasco system NMR and NMR spectra were acquired on a Bruker AC 200 instrument at 300 and 75 MHz respectively. Electron impact mass spectra (EIMS) were performed on a Nermag R10-10 type apparatus.

Synthesis. The synthesis of pyrimidinium betaine surfactants involves three steps. The first step consists of the synthesis of fatty '-diphenyalkylamidines 3a-e, from the corresponding alkylanilides 2a-e, themselves prepared from the homologous carboxylic acids 1a-e [23] (Scheme 1). The second step includes the synthesis of bispentachlorophenyl ester of phenyl malonic acid by the action of pentachlorophenol on phenyl malonic acid 4 [24] (Scheme 2). The third step involves the synthesis of fatty pyrimidinium betaines 6a-e from the condensation of fatty amidine derivatives 3a with malonic ester derivative 5, according to a previously reported procedure [23,25,26] (Scheme 3). The synthesis pathways of the reagents 3a-e and are as follows :

Scheme 1. Synthesis route of fatty N, N'- diphenylalkylamidines 3a-e

Figure 1. Scheme 1. Synthesis route of fatty N, N'- diphenylalkylamidines 3a-e

Scheme 2. Preparation of bispentachlorophenyl ester of phenyl malonic acid 5

Figure 2. Scheme 2. Preparation of bispentachlorophenyl ester of phenyl malonic acid 5

General procedure for the synthesis of fatty chain pyrimidinium betaines 6a-e. Fatty chain pyrimidinium betaines 6a-e were synthesized according to the Dvortsak procedure [25] by reacting 1 mmol of fatty N,N'-alkylamidines 3a-e with 1 mmol of the bis pentachlorophenyl ester of phenyl malonic acid 5 in diethyl ether [26] under stirring at room temperature (Scheme 3), To the milky suspension obtained, 2 mmol of triethylamine was added. Less than a minute after this last addition a white solid precipitated. The entire system was then stirred for 30 min . The products were isolated by filtration. Their purification after several successive extractions afforded powdery white solids [26]. The purity of the synthesized fatty betaines was confirmed by TLC technique.

Scheme 3. Synthetic pathway of fatty pyrimidinium betaines 6a-e

Figure 3. Scheme 3. Synthetic pathway of fatty pyrimidinium betaines 6a-e

Measurements. In this work our study focused on a pyrimidinium betaine with a 12 -carbon alkyl chain 6b (dodecyl pyrimidinium betaine BT12) as an example.

Surface tension measurements. A series of ethanolic solutions of dodecyl pyrimidinium betaine surfactant BT12 were prepared at different concentrations from to . The surface tension was measured at using a Krüss GmbH , K 6 tensiometer by the ring tear-off method. All measurements were repeated three times and the average value was taken [9, 27, 28].

Conductivity measurements. The conductivity of ethanolic solutions of dodecyl pyrimidinium betaine surfactant BT12 was measured by a HI 8633 conductivity meter. The conductivity measurements were carried out at different concentrations at [22,29].

Results and Discussion

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Characterization of synthesized compounds. Betaines with fatty alkyl chain 6a-e containing a pyrimidine moiety were realized according to a previously reported procedure [23, 26]. As presented in Table 1, the different alkyl betaines were isolated in low to moderate yields, .

Table 1. Yields of synthesized fatty alkyl pyrimidinium betaines 6a-e

Betaine | R | Aspect | Yield (%)
6a | C₉H₁₉ | white solid | 39
6b | C₁₁H₂₃ | white solid | 45
6c | C₁₃H₂₇ | white solid | 31
6d | C₁₅H₃₁ | white solid | 30
6e | C₁₇H₃₅ | white solid | 35

The fatty pyrimidinium betaines 6a-e were also characterized by the spectral analyses and their different spectra were found to be identical. As an example, UV-Visible, FTIR, MS, H NMR and C NMR spectral results of dodecyl pyrimidinium betaine are compiled in Table 2.

Table 2. UV-Visible, IR, and NMR spectral results of dodecyl pyrimidinium betaine

Dodecyl pyrimidinium Betaine 6b (BT12). | Chemical formula | Mol. Mass. (g/mol) | m.p. (°C) | UV-Visible (dioxane) λmax (nm) | IR (KBr) ν̃ (cm⁻¹) | ¹H NMR (CDCl₃) δ (ppm) | ¹³H NMR (CDCl₃) δ (ppm) | EI Mass
(6-oxo-2-dodecyl,1,3,5-tripheny-1,6-dihydropyrimidin-3-ium-4-olate) | C₃₃H₃₈N₂O₂ | 494 | 124–128 | 352 | 3055 (C-H, Ar.), 1647 (C=O), 1597 (C=C, Ar.) | 7.91–7.14 (15H, Ar.), 2.41–0.78 (23H, aliph.) | 161.42 (C-14,16), 159.00 (C-12), 136.27-125.76 (C, Ar.), 32.77–14.11 (C, aliph.) | 495[M+1]⁺

Surface Activity

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Surface tension measurements. Measuring surface tension allows us to know the surface free energy per unit area at the air-liquid interface. This data is necssary to establish the CMC and study the micellization and adsorption properties [22]. The surface tension of dodecyl pyrimidinium betaine surfactant BT12 as a function of concentration at was determined and plotted in Figure 1.

Fig. 1. Variation of surface tension as a function of the concentration of the pyrimidinium betaine surfactant BT12 in ethanolic solution at

Fig. 1. Variation of surface tension as a function of the concentration of the pyrimidinium betaine surfactant BT12 in ethanolic solution at

Fig. 2. Determination of CMC value by tensiometric technique

Fig. 2. Determination of CMC value by tensiometric technique

As illustrated in the figures above, the surface tension values decrease rapidly with increasing surfactant concentration due to the adsorption of molecules at the air-liquid interface. Then a distinct break point appears, known as critical micelle concentration (CMC) [29]. At the CMC values the surfactant begins to form micelles to decrease the electrostatic energies of the system [20]. The CMC and surface tension at CMC ( ) values were estimated from the breakpoints of these plots. After the breaking point, the surface tension remains almost constant with increasing BT12 concentrations [18]. This is due to the saturation of the air-liquid interface with BT12 molecules [28] . Figure 1 and 2 show that and CMC values of BT12 are and respectively. The results obtained reveal that the synthesized pyrimidinium betaine surfactant has a great ability to reduce the surface tension of the system [28]. It appears that the length of the alkyl chain of the fatty pyrimidinium betaine studied has a significant effect on the reduction of surface tension [19,29].
In general, the CMC value of surfactants is a sign of surface properties. The corresponding surface tension value indicates the ability of surfactants to reduce surface tensions and, therefore, the CMC indicates the effectiveness [9]. This means that BT12 with a long alkyl chain has excellent micellization capacity at low concentrations.

Conductivity measurements. The variations of conductivity against concentration of BT12 at are shown in Figure 3.

Fig. 3. Dependence of conductivity on the concentration of the pyrimidinium betaine surfactant BT12 in ethanolic solution at .

Fig. 3. Dependence of conductivity on the concentration of the pyrimidinium betaine surfactant BT12 in ethanolic solution at .

Fig. 4. Determination of CMC value by conductometric technique

Fig. 4. Determination of CMC value by conductometric technique

As showed in Figures 3 and 4, the experimental results correspond in two straight lines with a significant change in direction at the breakpoint [22,30]. The conductivity of the solution increases rapidly in the pre-micellar region. This is due to the increase in the concentration of current carriers which increases with increasing surfactant concentration. Regarding the conductivity measurements, CMC value was obtained at the point of intersection of the linear parts of the conductivity versus concentration plots [28]. As depicted in Figure 4 the value of CMC obtained is .

By comparing the results obtained using both tensiometric and conductometric techniques we concluded that the CMC values are very close.

Summary

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In the present work, a series of new fatty betaines containing a pyrimidine ring were prepared and their chemical structures were characterized using FTIR, NMR, NMR and mass spectra. The surface activity of dodecyl pyrimidinium betaine surfactant in ethanolique solutions at was investigated by surface tension and conductivity measurements. The synthesized surfactant has proved satisfactory surface properties. This work has demonstrated that it is possible to obtain amphoteric surfactant with appropriate surface properties based on fatty acids, available and inexpensive products. The synthesized compounds could therefore have wide potential applications. They can be used in several industrial application such as laundry detergent, corrosion/rust inhibitor and lubricating emulsion.

Acknowledgements

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Authors are grateful for the financial support from the Algerian Ministry of Higher Education and Scientific Research (Projet de Recherche Formation-Universitaire. PRFU B00L01EN160220180003). Authors deeply thank the researchers of Laboratory of organic synthesis, university of science and technology, Houari-Boumédiène (USTHB), Algiers, Algeria, for kindly providing a tensiometer. We are also grateful to Doctor Michel Baltas for the NMR and mass spectra performed in the Laboratory of Synthesis and Physiochemistry of Molecules of Biological Interest, UMR 5068, University Paul Sabatier Toulouse, France.

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