SYNTHESIS OF CATIONIC SURFACTANTS AND THEIR PHYSICOCHEMICAL STUDY FOR THE PRODUCTION OF BITUMEN EMULSIONS

ABSTRACT

In this article, the esterification reaction of stearic acid and ethyl alcohol for the synthesis of cationic surfactants, the amidation reaction of the resulting ester with cationic N,N-dimethylethylenediamine, and the final reaction with benzyl bromide were studied to evaluate the reaction efficiency. It was determined that the yield of the synthesized cationic surfactant was 89%. Additionally, the hydrophilic-lipophilic balance (HLB) of the synthesized cationic surfactant was determined using Griffin’s method to assess the efficiency of direct and inverse emulsification depending on the change in bitumen concentration. Spectroscopic analyses were conducted to identify the presence of functional groups in the obtained benzyl stearamide and to detect the formed ions.

АННОТАЦИЯ

 В данной статье изучена реакция этерификации стеариновой кислоты и этилового спирта для синтеза катионных поверхностно-активных веществ, реакция амидирования полученного сложного эфира с катионным N,N-димэтилэтилендиамином, а также заключительная реакция с бензилбромидом для оценки эффективности реакции. Установлено, что выход синтезированного катионного поверхностно-активного вещества составляет 89%. Кроме того, гидрофильно-липофильный баланс (ГЛБ) синтезированного катионного ПАВ был определён методом Гриффина с целью оценки эффективности прямой и обратной эмульгации в зависимости от изменения концентрации битума. Проведены спектроскопические анализы для выявления функциональных групп в полученном бензилстеарамиде и определения образовавшихся ионов.

Keywords: Stearic acid, N,N-dimethylethylenediamine, benzyl bromide, cationic surfactant, hydrophilic-lipophilic balance, critical micelle concentration, polymer-bitumen emulsion.

Ключевые слова: Стеариновая кислота, N,N-димэтилэтилендиамин, бензилбромид, катионное поверхностно-активное вещество, гидрофильно-липофильный баланс, критическая концентрация мицеллообразования, полимер-битумная эмульсия.

Introduction. The synthesis of cationic surfactants for the formation of bitumen emulsions and the determination of the effectiveness of cationic surfactants in the formation of direct and inverse emulsions depending on the change in bitumen concentration.

Cationic surfactants (CS) are chemical compounds consisting of asymmetric parts with a positive charge. These substances exhibit active properties (surface tension, surface energy), act in the liquid phase, and demonstrate dispersion characteristics in water or other polar media. CS are widely used in the production of polymer-bitumen emulsions [1,2].

Cationic surfactants (CS) used in bitumen emulsions are primarily employed to improve the miscibility of bitumen with water and enhance its adhesion to mineral aggregates. These substances possess the necessary surface-active properties to ensure emulsion stability. CS play a crucial role in increasing bitumen stability, improving its dispersion, and facilitating better mixing with water [3,4].

Materials and Methods. The infrared spectra of the synthesized benzyl stearamide were studied using the IRAffinity-1S Fourier Transform Infrared (FTIR) Spectrophotometer (SHIMADZU).

The reaction was carried out in a three-necked flask equipped with a stirrer and a thermometer. A mixture of 14.4 g of stearic acid and 27.4 g of ethyl alcohol in a 1:1 ratio was placed into the flask. While slowly heating the mixture, 2.5 ml of 80% sulfuric acid was added dropwise. The reaction mixture in the flask was heated and stirred at atmospheric pressure for 3 hours at 80°C. After completion, the reaction mixture was cooled to room temperature (20–25°C). To neutralize the unreacted acid in the composition, a concentrated solution of sodium carbonate was used. During the neutralization process, the concentrated sodium carbonate solution was added dropwise while stirring the liquid with a glass rod until the blue litmus paper, placed in the ether phase of the mixture, turned red. The release of carbon dioxide gas resulted in foam formation. The obtained ester was separated from the lower aqueous layer using a Büchner funnel and filtered through porous filter paper. After filtration, 37.202 g of stearic acid ester was obtained, with a reaction yield of 89.0%.

The resulting esters were heated with N,N-dimethylethylenediamine for 30 minutes, forming amides, which were then reacted with benzyl bromide for 12 hours. The physicochemical and colloidal properties of the synthesized cationic surfactants were determined.

Results and Discussion.

The synthesis of cationic surfactants consists of three stages. In the first stage, an esterification reaction was carried out using stearic acid and ethyl alcohol in a 1:1 molar ratio.

First Stage: The yield of this reaction is significantly influenced by temperature and reaction duration. Reaction Rate: The esterification reaction primarily accelerates at high temperatures. As the temperature increases, the kinetic energy of molecules rises, enhancing the probability of reactive molecules colliding with each other.

Second Stage: In this stage, the resulting ethyl stearate was reacted with N,N-dimethylethylenediamine, leading to the formation of the amide ester N,N-dimethyl stearamide (stearoamide). Amide Formation: The formation of amide is influenced by the removal of water from the reaction mixture, which shifts the reaction equilibrium towards product formation. Since amide synthesis requires an anhydrous environment, the reaction was conducted in a dry medium to facilitate the removal of water and promote the formation of the desired product.

The reaction was carried out in an acidic medium, using toluene as a solvent, ensuring the complete progression of the reaction. The stearic acid (or ethyl stearate) molecule undergoes protonation, and the amine groups (–NH₂) in N,N-dimethylethylenediamine (DMEDA) react with ethyl stearate. In this reaction, the nitrogen atom (N) in the amine group of DMEDA attacks the non-protonated carbonyl (C=O) group of the ethyl stearate molecule. Following the nucleophilic attack by the amine group, the ester molecule undergoes transformation, resulting in the formation of an amide (–C(NH)–) bond, with the release of a water molecule. This occurs because the formation of the amide bond leads to the cleavage of the oxygen bond in the ester.

Third Stage: Reaction with Benzyl Bromide In the third stage, the obtained N,N-dimethyl stearamide was reacted with benzyl bromide. In this reaction, the bromine atom in the benzyl group acts as a leaving group, while the free electron pair on the nitrogen atom participates in a nucleophilic substitution reaction, leading to the formation of the final product.

In the benzyl bromide (C₆H₅CH₂Br) molecule, the bromine atom readily dissociates due to its electron-withdrawing nature, making it a good leaving group. The main step of the reaction occurs with the free electron pair on the nitrogen (N) atom in the N,N-dimethyl stearamide molecule. The free electron pair on the nitrogen atom attacks the carbon (C) atom in the benzyl group of benzyl bromide through a nucleophilic substitution reaction. As a result, the bromine atom is displaced, and a new N–C (nitrogen-carbon) bond is formed. This leads to the formation of benzyl stearamide, where the benzyl group is bonded to the nitrogen atom, creating an amide linkage. Reaction Conditions and Solvent Selection The formation of benzyl stearamide was found to be highly efficient when the reaction was conducted at a temperature of 80–120°C for 9–12 hours. Under these conditions, the removal of the bromine atom from benzyl bromide and the nucleophilic attack by nitrogen are significantly enhanced, increasing the reaction efficiency. To facilitate the reaction, non-polar solvents such as toluene and chloroform were used. These solvents play a crucial role in improving the collision of reactant molecules, thereby enhancing the reaction yield.

Hydrophilic-Lipophilic Balance (HLB) Evaluation The study of various types of surfactants is crucial for evaluating coalescence, coagulation, and sedimentation processes in emulsion formation. The hydrophilic-lipophilic balance (HLB) of the synthesized cationic surfactant was determined according to Griffin’s rule [5].

The conclusion that can be drawn from the fact that the hydrophilic-lipophilic balance (HLB) of the synthesized BSA-K is 8.0 is that no coalescence of small droplets into larger ones was observed during emulsion formation. Additionally, no phase separation into layers was detected, and due to modification, the density difference between the dispersed phase and the dispersion medium was not significant. The effect of particle size and gravitational force prevented sedimentation of droplets in the dispersed phase. In bitumen emulsion formation, the HLB is crucial for optimizing the balance between the hydrophilic and lipophilic parts of surfactants. This, in turn, ensures the stability, efficiency, and performance of the emulsion. A properly selected HLB balance with suitable surfactants allows for the formation of an effective and stable bitumen emulsion.

Spectral analysis confirmed the presence of vibrational regions corresponding to all functional groups in its composition. The obtained spectra are presented in the following figures.

The spectra showed absorption regions at 720.36-730.4 cm⁻¹, corresponding to the CH₂ δ_asym bending vibrations of the sp³ hybridized saturated alkyl group present in stearic acid. The absorption at 806.25-846.75 cm⁻¹ corresponds to the benzene ring in the benzyl molecule, while the peak at 1000.98 cm⁻¹ represents the δ Car-H aromatic ring and hydrogen-bonded vibrations in a long-chain structure. Weakly active CH₂ groups appear in the range of 1244.10-13751.23 cm⁻¹. The absorption at 1400.82 cm⁻¹ corresponds to the νC-N stretching of the N,N-dimethylethylenediamine derivatives bonded with ethyl stearate. The peaks at 1456.26 cm⁻¹ (δ C-H) and 1496.42 cm⁻¹ (ν benzene group) confirm the presence of aromatic structures. The absorption at 1597.04 cm⁻¹ corresponds to δN-H bending vibrations of the secondary amide group, while the peak at 1637.19-1739.79 cm⁻¹ is attributed to the νC=O stretching of the carbonyl group.

Figure 1. Infrared (IR) Spectrum of Benzyl Stearamide

The presence of methyl group C-H bonds is confirmed by the absorption at 2857.52 cm⁻¹ (νCH₃). The peak at 2926.56 cm⁻¹ corresponds to the benzyl group and the tertiary nitrogen group bonded to the N,N-dimethylethylenediamine molecule. Finally, the absorption at 3411.67 cm⁻¹, with medium intensity, is attributed to the νN-H stretching of the secondary amide. These peaks confirm the presence of all functional groups characteristic of the synthesized benzyl stearamide.

The chromatographic mass spectrum of the synthesized benzyl stearamide (Figure 2) reveals ion fragments that correspond to the molecular mass and fragmentation pattern of the compound.

Figure 2. Ions Formed in the Mass Spectrum of Benzyl Stearamide

The molecular ion peak of benzyl stearamide was 431.0. Below is the chromatogram-mass spectrum of the fragment ions formed from the molecular ion of the initial benzyl stearamide. Additionally, the spectrum showed the formation of fragment ions with masses: m/z = 368.0, m/z = 354.0, m/z = 323.0, m/z = 282.0, m/z = 267.0, m/z = 249.0, m/z = 240.0, m/z = 226.0, m/z = 179.0, m/z = 164.0, m/z = 135.0, m/z = 104.0, m/z = 99.0, m/z = 91.0, m/z = 78.0, m/z = 50.0. This indicates that various fragment ions corresponding to the molecular ion of benzyl stearamide were formed.

In the second pathway, the elimination of an ethanal molecule resulted in the formation of the m/z = 183 ion. Complex esters tend to eliminate a neutral molecule and undergo skeletal rearrangement. Considering that the benzyl molecular ion contains an aromatic ring and a nitrogen atom attached to it, the molecular ion undergoes rearrangement in the third pathway by eliminating a neutral CO₂ molecule. The release of CO₂ proves the possible presence of an unsaturated N-H group in the molecular ion. As a result of this rearrangement, a stable benzene molecule is formed.

Figure 3. Ions Formed in Mass Spectroscopy of Cationic Surfactants

In the chromatographic mass spectrum, the fragment ions formed from the molecular ion also generate smaller ion fragments. In two different pathways, the m/z = 76 phenyl radical fragment ion undergoes acetylene elimination, leading to the formation of the m/z = 50 cyclobutadienyl fragment ion, which can be observed in the spectrum.

Conclusion.

The cationic surfactants responsible for the formation of rapid-breaking bitumen emulsions were synthesized in three stages, and the kinetic analysis of each stage was studied. It was determined that the reaction yield of the synthesized cationic benzyl stearamide surfactant was 89.0%, and its hydrophilic-lipophilic balance (HLB) was found to be 8.0. The study confirmed that the synthesized surfactant enables the formation of an emulsion with bitumen in both the dispersed phase and the dispersion medium. Furthermore, spectroscopic analyses of the synthesized cationic surfactants revealed the presence of all functional groups and formed ions, indicating the success of the synthesis process.

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