1. Introduction
In the past decades, gemini cationic surfactants containing ammonium headgroup have been extensively used for multiple oilfield applications, such as anti-corrosion in oilfield wells [
1], fracturing fluid [
2], micellar slug [
3], foam generation [
4,
5], collecting and dispersing the spilled crude oil [
6], reservoir stimulation [
7] wettability modification [
8], and enhanced oil recovery [
9,
10]. They possess important physicochemical activities including lower critical micelle concentration (CMC), higher interface/surface properties, good solubility, compatible with carbonate rocks, high thermal stability, and unique aggregation behavior as compared to their monomeric counterpart [
5,
11,
12,
13].
Gemini cationic surfactants are a comparatively new class that contains two lipophilic tails and two lipophobic headgroups chemically bonded by the spacer. The length of the surfactant tail in gemini cationic surfactants play a significant role to identify the surface properties. For example, increasing the length of the surfactant tail (≥C14) results in lowering CMC and increasing surface tension at CMC (γ
cmc) [
4,
14,
15]. However, a further increase in the surfactant tail leads to poor solubility in water [
16]. The spacer group is perhaps the most important parameter in determining solution properties of the gemini amphiphiles [
17]. The spacer group can be short (C2) [
18], long (C16) [
19], rigid (double bond or benzene ring) [
20], flexible (methylene units) [
21], lipophilic (hydrocarbon) [
22], or hydrophilic (ether linkage) [
23]. The length of the spacer significantly affects the size, shape, and morphology of the micelle, that eventually determine their physicochemical properties [
17]. The larger length of the spacer results in better surface properties and reduces adsorption of the molecule on to the reservoir rocks [
24].
Gemini surfactants can be classified as anionic, cationic, nonionic, and zwitterionic surfactants. The thermal stability and salt tolerance of gemini surfactants are the major challenges as they can precipitate in harsh reservoir conditions. Pal et al. examined the effect of the different spacers on the thermal stability of the nonionic bis (monoglyceride-1-hydrocyl-2-fattyacidester)-α-ω-alkanediether gemini surfactants using thermogravimetric analysis (TGA) and concluded that the gemini surfactant retains their structural integrity up to 448 K. However, precipitation was observed at higher NaCl concentration [
25]. Nonionic and anionic surfactants can have high retention on the carbonate rock surface. Therefore, cationic gemini surfactant can be an attractive choice for carbonate reservoirs owing to low retention of carbonate rocks. Cationic gemini surfactants are not well studied for enhanced oil recovery applications. There are only a few reports in the literature describing the thermal stability and salt tolerance of gemini cationic surfactants containing ammonium headgroup for EOR application [
26]. Recently, Wang et al. reported the synthesis of 1,3-2(lauramide propyl dimethyl ammonium chloride) isopropyl alcohol (gemini surfactant) for potential surfactant flooding material and observed a reduction in IFT value (<0.01 mN/m) upon addition of electrolytes [
5]. However, thermal stability and salt tolerance were not studied. Mao et al. revealed the heat resistance and micelle aggregation properties of the viscoelastic (VES) gemini surfactant for oilfield application [
2]. Panda and co-workers found enhancement of aqueous solubility of polycyclic aromatic hydrocarbons (PAHs) upon addition of biodegradable ester-linked dicationic geminis of different chain lengths (m-E2-m; m = 12, 14, 16) [
27]. The solubility and thermal stability of the gemini cationic surfactants in seawater (SW), which is normally the injected water, and in reservoir brine (FW) remains a big challenge. In our previous work, we observed that the gemini surfactants containing ammonium headgroup were only stable in deionized water and showed precipitation in SW and FW [
28]. In this work, we achieved excellent solubility and stability of the gemini cationic surfactants with ammonium headgroup in SW and FW by incorporating a proper number of ethoxy (EO) units between the hydrophobic tail and hydrophilic head group.
In this article, three new quaternary ammonium gemini surfactants (GS8, GS10, and GS12) having flexible larger spacer C8, C10, and C12, respectively, were synthesized and characterized with the aid of NMR, (carbon and proton), FT-IR, and MALDI-TOF MS. Special attention was paid to identify the effect of length of spacer on the surface and thermal properties of the synthesized GSs. The surfactant samples were dissolved in FW, SW, and DW, and kept in an oven for three months at 90 °C. The thermal stability of GSs was determined by thermal gravimetric analysis (TGA). Surface properties, such as CMC, surface tension at CMC (γcmc), maximum surface access (гmax), occupied surface area at the interface of air-water (Amin) were measured at different conditions of salinity and temperature. This study mainly focuses on synthesis, characterization, thermal stability, salt tolerance, and surface properties of this novel class of surfactants.
2. Materials and Methods
2.1. Materials
Glycolic acid ethoxylate lauryl ether (average M
n ~ 690), NaF (≥99%), 3-(dimethylamino)-1-propylamine (99%), 1,8-dibromooctane (98%), 1,10-dibromodecane (97%), 1,12-dibromododecane (98%), aluminum oxide (99.99%), were acquired from Aldrich company. The salts, such as CaCl
2, Na
2SO
4, MgCl
2, NaHCO
3, NaCl, were used for the preparation of FW and SW. The concentration of each salt is given in
Table 1 and all these salts were purchased from Panreac.
2.2. Elucidation of Chemical Structure
The structure characterization tools including NMR (carbon and proton), FT-IR, MALDI-TOF MS were used to confirm the chemical structure of all the synthesized gemini surfactants (GS8, GS10, and GS12). 13C and 1H/NMR spectra were recorded on a Jeol 1500 spectrometer (Jeol, Tokyo, Japan). The FT-IR data was recorded on a 16F Perkin-Elmer FT-IR spectrometer (Perkin-Elmer, Waltham, MA, USA). MALDI-TOF MS spectra of the GSs were recorded on a Bruker SolariX XR instrument (Bruker, Billerica, MA, USA) in a matrix of Dithranol in dichloromethane.
2.3. Solubility Experiments
The gemini amphiphiles (GS8, GS10, GS12) (10 wt % each) were dissolved in FW, SW, DW, and kept in an oven for three months. The temperature of the oven was set at 90 °C and the solubility was visually observed with elapsed time.
2.4. Thermal Gravimetric Analysis
SDT Q600 machine from TA instrument was used for the TGA measurement (New Castle, DE, USA). The experiment was done with a fixed heating rate (20 °C/min) and fixed nitrogen flow (100 mL/min), and the temperature interval was 30 °C to 500 °C.
2.5. Surface Tension Experiments
Surface tension values for GS8, GS10, and GS12 were investigated with the aid of a force tensiometer (Sigma 702, Biolin Scientific, Gothenburg, Sweden) using the Wilhelmy plate technique. The experiments were conducted at 30 ± 0.1 °C and 60 ± 0.1 °C. Wilhelmy plate was washed using DW and burnt on a blue flame before every experiment. The surface tension of DW was examined as a benchmark.
2.6. Synthesis
Synthesis of GS8
The GS8 was synthesized as depicted in
Scheme 1. Glycolic acid ethoxylate lauryl ether (6) (average Mn ~ 690) (10 g, 14.49 mmol) and 3-(dimethylamino)-1-propylamine (5) (2.96 g, 28.99 mmol) were refluxed for 6 h at 160 °C in a 250 mL round bottom flask using sodium fluoride (0.06 g, 1.45 mmol) as a catalyst [
29]. An extra 3-(dimethylamino)-1-propylamine (2.22 g, 21.74 mmol) was introduced in order to convert all the glycolic acid into amide intermediate (4) and the reaction progressed an extra four hours. Eventually, the unreacted 3-(dimethylamino)-1-propylamine was extracted and the solid sodium fluoride was removed to acquire intermediate 4. In the second step, intermediate 4 (10.0 g, 12.92 mmol) was refluxed with 1,8-dibromooctane (1.41 g, 5.17 mmol) in anhydrous ethanol (5 mL) for 48 h at 80 °C. Finally, the crude material was refined by column chromatography with ethanol (mobile phase) to acquired GS8 as a thick oil [
30].
GS10 and GS12 were synthesized in the exact same manner as GS8.
4. Conclusions
Environmentally friendly gemini surfactants are comparatively new materials for oilfield application. In this report, three quaternary ammonium gemini surfactants (GS8, GS10, GS12) with the same functionalities, except for the length of the spacer, were synthesized and characterized by FT-IR, 13C NMR, 1H NMR, and MALDI-TOF MS. The effect of flexible larger spacers (C8, C10, and C12) on the thermal and surface properties was studied. According to TGA analysis, the GSs showed almost similar thermograms and exhibited higher decomposition temperature (227 °C) compared to existing oilfield temperature (≥90 °C). The GSs exhibited excellent solubility in normal and saline water and the aqueous solutions of GS8, GS10, GS12 stayed clear for up to three months at 90 °C without precipitation or phase separation. It was noticed that the increase in spacer length shifts the CMC and γcmc to lower values in the order of GS8 ˃ GS10 ˃ GS12. The larger spacer in GS12 stimulates micelle formation at the water-micelle interface and creates a more closely packed micelle structure which leads to a lower CMC. The effect of salinity and temperature on micellization was also investigated through surface tension measurements. A significant decrease in CMC and γcmc was observed in the presence of salts and at high temperature. The IFT, rheology, and foam analysis are proceeding in our laboratories.