1 Introduction

Amphiphilic polyamides represent a class of surfactants with vast potential for applications [1, 2]. In recent years, polyamides have become one of the important polymer materials due to their high-temperature resistance, chemical resistance, wear resistance, and good toughness [3,4,5]. Despite these eminent properties, the limitations of infusibility and limited solubility restrict their practical applications. As environmentally friendly materials, water-soluble polyamides based on aliphatic polyamides are non-toxic, non-flammable, and biocompatible [6]. They have extraordinary prospects in fields such as water-based coatings [7, 8] and drug delivery [9]. Bariyanga et al. [10] synthesized a water-soluble polyamide anticancer drug capable of binding platinum, with a platinum content of up to 8% in the conjugate. Wang et al. [11] synthesized a water-soluble polyamide (PAGAP) with amphiphilic properties through melt condensation and investigated its self-assembly behavior in aqueous solution. They discovered that PAGAP could form acid-sensitive large complex micelle in aqueous solution. Nevertheless, according to our knowledge, there still exists insufficient understanding regarding the emulsifying properties and mechanisms of amphiphilic polyamides.

Emulsions, as a typical liquid–liquid dispersion system, hold enormous potential for applications [12]. Traditional emulsions, due to their thermodynamic instability [13], often require the addition of surfactants, polymers, or proteins [14,15,16] to enhance stability. Among various emulsifiers, surfactants with high molecular weight are particularly effective in stabilizing emulsions by strongly adsorbing onto the droplet surface, thus providing spatial stability [17, 18]. Ma et al. [19] designed a modified glycyrrhetinic acid surfactant (PGly) through reversible addition-fragmentation chain transfer polymerization, which can form stable oil-in-water emulsions at pH values greater than or equal to 5. Huang et al. [18] grafted hydroxyethyl cellulose onto epoxidized soybean oil (H-ESO-HEC) via ring-opening polymerization. The resulting emulsion, at a concentration of 0.4 wt.%, could maintain stable oil-in-water emulsions for 80 days; additionally, they confirmed the formation of interfacial membranes in the emulsions. Zhai et al. [12] synthesized a novel bio-based surfactant using acrylic acid epoxidized soybean oil as the raw material through free radical polymerization [20, 21] and an ultra-stable soybean oil-in-water emulsion could be obtained at a concentration of 0.3 g L−1, which can remain stable for at least 12 months.

While the long-term stability of emulsions is crucial, in certain scenarios such as emulsion polymerization and crude oil extraction, emulsions are only required for temporary use, and rapid demulsification is needed in post-application for product separation [22]. The traditional method of chemical demulsification involves the addition of demulsifiers [23, 24]. Generally, these demulsifiers exhibit higher activity compared to emulsifiers, allowing them to rapidly displace or neutralize emulsifier molecules [25], thereby reducing the interfacial tension and interfacial membrane strength of emulsion droplets and facilitating their coalescence [26, 27]. Although chemical demulsification has been widely utilized with significant efficacy, most of the demulsifiers involved are toxic [28, 29], and they tend to increase the complexity of the system, thereby increasing the difficulty of separating target substances [30]. Finding new methods for emulsion breaking is crucial. If emulsion stability can be adjusted by modulating environmental factors, the aforementioned issues can be effectively addressed. Therefore, pH-sensitive emulsifiers are ideal candidates for applications that require the alteration of properties based on environmental pH, making them highly versatile for targeted and controlled applications. For instance, in drug delivery systems [31, 32], pH-sensitive emulsifiers can stabilize drug-loaded emulsions and release the active drug in response to pH changes in different parts of the body, such as the stomach or intestines, ensuring targeted release and improved bioavailability. In wastewater treatment processes [33, 34], pH-sensitive emulsifiers are used to remove contaminants such as heavy metals and organic pollutants through flocculation and demulsification. In the food industry [35], emulsifiers such as lecithin and modified starch can stabilize food emulsions, such as salad dressings and sauces, by responding to pH changes during processing and storage. Polyamides contain a large number of amide bonds, making its structure very stable. If an amphiphilic polyamide could be synthesized, it could form stable emulsions under specific conditions, and post-use demulsification could be achieved rapidly by adjusting the pH. Such an emulsifier would possess application potential.

Consequently, two pH-sensitive anionic polyamide surfactants (APSs), C18:1-PEA and C22:1-PEA, were synthesized in this work.

2 Materials and methods

2.1 Materials

The reagents used in the study include chlorosulfonic acid (99%, Shanghai Yien Chemical Technology Co., Ltd., CAS No.7790–94-5), polypropylene glycol bis (2-aminopropyl ether)-400 (PEA-D400, Mn = 400, Shanghai Yien Chemical Technology Co., Ltd., CAS No.9046–10-0), methyl acrylate (AR, Shanghai Yien Chemical Technology Co., Ltd., CAS No.96–33-3), methyl oleate (99%, Shanghai Boer Chemical Reagent Co., Ltd., CAS No.112–62-9), 1,4-butanediamine (98%, Meryer (Shanghai) Biochemical Technology Co., Ltd., CAS No.110–60-1), tetrabutyl titanate (AR, Meryer (Shanghai) Biochemical Technology Co., Ltd., CAS No.5593–70-4), trichloromethane (AR, Shanghai Chemical Reagent Co., Ltd., CAS No.67–66-3), methanol (AR, Sinopharm Chemical Reagent Co., Ltd., CAS No.67–56-1), N,N-dimethylformamide (DMF, ≥ 99%, Shanghai Macklin Biochemical Technology Co., Ltd., CAS No.68–12-2), methyl cis-13-docosenoate (> 90%, Aladdin Reagent (Shanghai) Co., Ltd., CAS No.1120–34-9), ethyl acetate (AR, Aladdin Reagent (Shanghai) Co., Ltd., CAS No.141–78-6), potassium carbonate (AR, Shanghai Lingfeng Chemical Reagent Co., Ltd., CAS No.584–08-7), and sodium carbonate (AR, Shanghai Lingfeng Chemical Reagent Co., Ltd., CAS No.497–19-8).

2.2 Synthetic method of APSs

The synthesis steps included sulfonation, amidation, addition, and polycondensation (Scheme 1). Fatty acid methyl ester (FAME) and chlorosulfonic acid with a molar ratio of 1:1.05 were added in a round-bottom flask and stirred at 50 °C for 5 h. Then, the sulfonated FAME was amidated using 1,4-butanediamine in a molar ratio of 1:6 and was refluxed at 140 °C for 6 h. After amidation, the mixture was washed with ethyl acetate until the residual 1,4-butanediamine was removed. Then, the amidated product and methyl methacrylate were dissolved in methanol with a molar ratio of 1:3 and were refluxed at 85 °C for 6 h. After that, the mixture was evaporated to obtain the methyl dipropionate (MD) monomer.

Scheme 1
scheme 1

Synthetic route of anionic polyamide surfactants (APSs)

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Subsequently, the MD and PEA-D400 with a molar ratio of 1:1 were added in a three-neck flask. The mixture is dissolved in N,N-dimethylformamide (DMF) stirred at 60 °C for 4 h to obtain the oligomer. After removing the DMF, the reactor was heated from 60 to 210 °C and maintained for 3 h, and then, the reactor was gradually evacuated to 40 kPa within 1 h. After 6 h, the anionic polyamide surfactants (APSs) were synthesized. To purify the APSs, the products were dissolved in methanol and precipitated in acetonitrile. The collected product was washed several times with acetonitrile and finally placed in a vacuum drying oven at 60 °C for 24 h. (More synthesis details can be seen in the supplementary material.)

2.3 Identification of products

The sulfonation, amidation, and addition reaction products were identified by liquid chromatography-mass spectrometry (LC–MS, LCMS-8045, Shimadzu, Japan) at the 0.4 mL min−1 flow rate with an injection of 10.0 µL and methanol/water as the mobile phase (an initial gradient of 80% methanol for 8 min, and 8–20 min to 100% methanol gradient for 20 min).

Special functional groups within the structure of APSs were characterized through Fourier transform infrared (FT-IR, INVENIO S + Hyperion3000, Bruker, Germany). The potassium bromide (KBr) pellet method was adopted, and the measurements were carried out in the range of 400–4000 cm−1 with an accuracy of 4 cm−1.

The structure information of the APSs was further elucidated by obtaining the 1H nuclear magnetic resonance (1H NMR) spectrum using a super conducting Fourier transform nuclear magnetic resonance (FT-NMR, AVANCE III400, Bruker, Switzerland) spectrometer, using deuterated methanol (CD3OD) as the solvent, which allowed for the characterization of its structure through chemical shifts.

The average molecular weight of the APSs was determined using a gel permeation chromatography (GPC, 1260, Agilent, America), with hexafluoroisopropanol (HFIP) as the solvent and polymethyl methacrylate (PMMA) as the standard, with a flow rate set to 1 mL min−1.

2.4 Thermal measurement

The thermal degradation curve of the APSs was obtained using a thermogravimetric analyzer (TGA, TGA8000, PerkinElmer, America). The measurement process was conducted on a thermal balance, heating from 35 to 600 °C at a rate of 10 °C min−1 under a nitrogen atmosphere, to assess the thermal stability of the APSs.

The melting point (Tm) of the APSs was determined using a differential scanning calorimeter (DSC, Q600, TA Instruments, America). The experiment began with heating each sample from room temperature to 250 °C at a rate of 10 °C min−1, maintaining it at 250 °C for 5 min to erase the thermal history, then cooling to 10 °C at the same rate, and subsequently reheating at a rate of 10 °C min−1 to 250 °C. The temperature was calibrated using indium.

2.5 Characterization of APSs aqueous solution

Different concentrations of APSs solutions were prepared with deionized water.

The surface tensions (SFT) of these solutions were performed at 25 °C by the plate method using a tensiometer (DCAT 21, DataPhysics, Germany), and APSs aqueous solution of each concentration was tested three times, and the measurement error for each point was set to 0.01 mN m−1.

The aggregation behavior and microstructure of the APSs in aqueous solution were observed and imaged using a transmission electron microscope (TEM, JEM-2100, Hitachi, Japan). The instrument was operated at an accelerating voltage of 200 kV. The dispersed sample was dropped onto a carbon membrane support and then completely dried at room temperature or freeze-dried for 48 h before measurements.

2.6 Preparation and characterization of emulsion

Emulsions were prepared with 50% (v/v) oil phase (10 mL), and 50% aqueous phase (10 mL) containing different APSs, followed by homogenizing at 10,000 rpm for 2.0 min using a high-speed blender (model B25, BRT Technology, China) to form emulsions. The emulsion index (EI24) was measured 24 h after preparation. The formula for calculating the EI24 is shown below:

$${EI}_{24}=\frac{\text{Height of Emulsion layer }\left(\text{cm}\right)}{\text{Total Height of Mixture }\left(\text{cm}\right)}\times 100$$
(1)

The zeta potential of the emulsion was measured using a zeta potential analyzer (Nano ZS, Malvern Instruments, Britain). The emulsion samples were diluted tenfold using deionized water before measurement with the equilibrium time of 120 s at 25 °C in triplicate.

The micrographs of emulsion droplets were recorded using an optical microscope (OPM, model CX43RF, Olympus, Japan). A small amount of emulsion was taken to be diluted with deionized water, and mixed. Then, 5 µL of emulsion was pipetted onto a hemocytometer, and covered with a coverslip. After the counting area was filled by the emulsion, put it on the stage and selected a magnification of × 10 or × 20. The focus was adjusted to capture clear photos. To minimize or eliminate errors, six images of each emulsion system were taken from different locations. Finally, Nano Measurer 1.2 software was used to measure the particle size distribution and average particle size of the emulsion.

The microstructure of the emulsion was observed and imaged using a cryo-scanning electron microscope (cryo-SEM, SU3500, Hitachi, Japan). The emulsion sample, frozen with liquid nitrogen, was fractured, sprayed, and then transferred to the stage of the SEM. The images were observed using PP3010/Quorum at − 150 °C, and imaging was conducted using an ACE600 ion sputtering instrument, a 40 mA beam current, a 10 kV accelerating voltage, and a 10 pA probe current.

2.7 Demulsification experiment through pH adjustment

The emulsion was prepared at pH 11, the volume of oil and water were both 5.0 ml, and the preparation method was the same as in Sect. 2.6. Then, the prepared emulsion was added to a test tube with a stopper. The pH was adjusted by adding hydrochloric acid, followed by shaking the test tube 200 times. The test tube was then left under ambient conditions to allow for the separation of the oil and water phases. The pH of the environment after the addition of dilute hydrochloric acid can be determined by testing the aqueous solution obtained after demulsification using pH test paper. The demulsification efficiency (Ed) was calculated by determining the volume of the lower layer of water as a percentage of the total volume of the aqueous phase.

$${E}_{\text{d}}=\frac{\text{Volume of Lower Layer Water }(\text{mL})}{\text{Total Volume of Aqueous Phase }(\text{mL})}\times 100$$
(2)

The simulated oil used in the demulsification experiment was prepared by blending Daqing crude oil and kerosene in a volumetric ratio of 1:1.

The method for determining the zeta potential of emulsions was the same as described in Sect. 2.6.

3 Results and discussion

3.1 Synthesized product

The oil-based monomers were prepared through sulfonation, amidation, and addition reactions. The LC–MS data can be employed to demonstrate the successful synthesis of intermediate products and placed in the supplementary material (Fig. S9-14). According to the extracted ion chromatogram (EIC), there were no residual reactants detected in the products of the sulfonation and addition reactions. The amidation conversion rates of SFAME were 98.24% (C18) and 95.53% (C22), respectively (Table S1).

The FT-IR spectra of PEA-D400 and APSs are shown in Fig. 1a. Apparently, the peaks around 3288 cm−1 (N–H), around 1648 cm−1 (C = O), around 3077 cm−1 (C-N), and around 1547 cm−1 (C-N) are all attributed to the stretching motion of amide group. The peaks of 2967 cm−1 (CH3), 2862 cm−1 (CH3), and 1103 cm−1 (-C–O–C-) are related to the polypropylene oxide segments. The peaks of 645 cm−1 are related to the rocking vibration of the fatty acid chain. The peaks of 1169 cm−1, 1037 cm−1, and 530 cm−1 are all corresponding to the stretching motions of the sulfonic acid group.

Fig. 1
figure 1

a The FT-IR spectra of APSs and PEA-D400. b The 1H NMR spectra. c TGA spectra. d DSC spectra of APSs

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The 1H NMR spectrum of the APSs is displayed in Fig. 1b. The resonance near 8.03 ppm proves the formation of amide bonds. In addition, the chemical shift at 4.00 ppm and 2.15 ppm are assigned to protons of carbon in α and β position, respectively, concerning the amidic functionality, and 3.46 ppm (peak e) is associated with methylene of the PEA segment. Moreover, around 5.36 ppm and 5.62 ppm (peak c) are related to the protons on the unsaturated carbon. The clusters of peaks that range from 0.86 to 1.75 ppm are assigned to the protons on the alkane chains (peak k, j, i).

According to the results of GPC (Fig. S1), the weight-average molecular weights of C18:1-PEA and C22:1-PEA are 4.29 × 104 Da and 4.68 × 104 Da and have the polydispersity (PD) of 2.71 and 3.24, respectively. The high value of PD [24] is mainly due to the characteristics of condensation polymerization and the non-monodisperse oligomer PEA-D400 used in this work. Combining the results of FT-IR, 1H NMR, and GPC, it can be proved that APSs have been successfully synthesized.

3.2 The thermal properties of APSs

Thermal stability assessment of APSs employed thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA and DSC curves for APSs are displayed in Fig. 1c and d, with comprehensive details given in Table 1. Examination of the data revealed the similarity between TGA curves for the C18:1-PEA and C22:1-PEA, aside from variations in the final char yield. The initial weight loss stage corresponds to the removal of crystalline water, the second stage predominantly indicates the degradation of the polyether chain segment, and the third stage loss signifies thermal degradation of the amide bond. DSC curves showcase distinctive melting peaks, with relatively low melting temperatures of 71.56 °C and 73.87 °C for C18:1-PEA and C22:1-PEA, respectively. Longer hydrophobic branch chains may make the forces between molecules stronger, eventually leading to higher melting points.

Table 1 Thermo properties of the APSs
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3.3 The surface activity of APSs

The surface tension of APSs at different concentrations (pH 7) is shown in Fig. 2, the CMCs of C18:1-PEA and C22:1-PEA are 0.012 g L−1 (2.80 × 10−7 mol L−1) and 0.069 g L−1 (1.47 × 10−6 mol L−1) at 25 °C, respectively. Based on three currently reported surfactants with high molecular weight, the measured CMC values were 9.07 × 10−6 mol L−1 [12], 7.22 × 10−7 mol L−1 [17], and 9.35 × 10−7 mol L−1 [36]. In comparison, the CMC value of C18:1-PEA was significantly lower than those of the aforementioned surfactants. This indicates that in practical applications, C18:1-PEA can achieve the same effect at a much lower dosage. More surface activity parameters are shown in Table 2. (The surface tension of APSs at different concentrations at pH 11 is shown in Fig. 8a and discussed in Sect. 3.4.)

Fig. 2
figure 2

The surface tension of APSs at different concentrations (pH 7)

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Table 2 The surface activity parameters of APSs solution
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3.4 The performance of emulsion stabilized by APSs

Emulsion stability is closely related to the pH of the aqueous solution and the concentration of surfactants [12, 19]. Liquid paraffin is a liquid hydrocarbon mixture that is commonly employed as an oil phase in emulsions [37, 38]. The emulsion index of C18:1-PEA and C22:1-PEA on liquid paraffin is shown in Fig. 3 at varying pH. Notably, both high-molecular-weight emulsifiers exhibit suboptimal emulsification at lower pH (pH < 11). However, the emulsion index (EI24) tends toward a stable value when the pH is equal to or greater than 11. Consequently, pH 11 was chosen for evaluating the emulsifying capability of both emulsifiers. Fundamentally, surface activity tests were conducted on APSs aqueous solution at pH 11 (Fig. 8a). Apart from the increase in surface tension compared to neutral conditions, CMC remained largely unchanged. It is observed that, after 24 h, stable liquid paraffin-in-water emulsions can be obtained when the concentrations of C18:1-PEA and C22:1-PEA are higher than 0.3 g L−1 and 0.2 g L−1, respectively (Fig. S3), both of which are higher than CMC (pH 11). The appearance of stable liquid paraffin-in-water emulsions at different concentrations (0.1, 0.2, 0.3, 0.6, 1.0, 2.0, 3.0, 6.0 g L−1) of APSs 1 week after emulsion preparation was captured in Fig. 4, which indicates a minimal alteration in emulsion appearance 1 week after preparation, with no demulsification signs. The optical micrographs and their corresponding particle sizes of stable liquid paraffin-in-water emulsions prepared with 0.3 g L−1 and 3 g L−1 (pH 11) of C18:1-PEA and C22:1-PEA after 1 week are illustrated in Fig. 5. It can be observed that with the increase in concentration, there is a noticeable decrease in the droplet sizes of the emulsion (Fig. S4-7). Zeta potentials at 3.0 g L−1 (pH 11) on the oil-in-water emulsions, diluted tenfold with deionized water 1 week after preparation, indicated that zeta potentials of − 54.27 mV and − 56.70 mV for emulsions were prepared by C18:1-PEA and C22:1-PEA, respectively. Since surface potential cannot be directly determined, calculations of electrostatic interactions are typically substituted with zeta potential. Zeta potential represents the potential difference between the continuous phase and the fluid stability layer adhering to dispersed particles [39, 40]. According to the double layer theory, at least approximately 30 mV of surface potential is required to prevent flocculation and coalescence between emulsion droplets [41, 42]. All these results indicate that the liquid paraffin-in-water emulsion stabilized by APSs exhibits good stability at pH 11. (The temperature and salt resistance performance of the oil-in-water emulsions can be seen in Fig. S8.)

Fig. 3
figure 3

Emulsion index (EI24) of APSs to liquid paraffin at different pH (7, 8, 9, 10, 11, 12, and 13). a C18:1-PEA. b C22:1-PEA

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Fig. 4
figure 4

Physical appearance of the liquid paraffin-in-water emulsions stabilized by a C18:1-PEA and b C22:1-PEA at different concentrations (pH 11) taken at 7 days after preparation; the concentrations from left to right are 0.1, 0.2, 0.3, 0.6, 1.0, 2.0, 3.0, and 6.0 g L−1, respectively

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Fig. 5
figure 5

Optical micrographs and particle size of liquid paraffin-in-water emulsion stabilized by C18:1-PEA at a, e 0.3 g L−1 and b, f 3 g L−1 and C22:1-PEA at c, g 0.3 g L−1 and d, h 3 g L−1 taken at 7 days after preparation

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To further verify the emulsifying capacity of APSs in practical applications, emulsions were formed using APSs at a concentration of 3 g L−1 (pH 11), with canola oil, peanut oil, motor oil, and n-hexadecane as the oil phase. The emulsion index (EI24) of the emulsions is shown in Fig. 6a. The EI24 of vegetable oils and n-hexadecane were both around 60%, while the emulsification effect for motor oil was more pronounced, with an EI24 exceeding 70%. Furthermore, under the same conditions, the emulsifying capacity of sodium dodecyl sulfate (SDS) was tested and compared with APSs’. SDS has been confirmed [43] as a surfactant with excellent emulsifying abilities, commonly employed in cosmetics, detergents, and other industries. The results indicated that the emulsifying capability of APSs for vegetable oil and n-hexadecane was equivalent to SDS, while it was superior to SDS for motor oil. In addition, the physical appearance of the emulsions was taken after 100 days of storage. As shown in Fig. 6b and c, no significant demulsification was observed during this period. These results suggest that APSs demonstrate good emulsifying capabilities for canola oil, peanut oil, n-hexadecane, and motor oil at pH 11.

Fig. 6
figure 6

a Emulsification index (EI24) of emulsion stabilized by APSs prepared from different oils at 3.0 g L−1 (pH 11), and physical appearance of emulsion stabilized by b C18:1-PEA and c C22:1-PEA prepared from different oils at 3.0 g L−1 (pH 11) taken at 100 days after preparation; the oils from left to right are canola oil, peanut oil, n-hexadecane, and motor oil

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3.5 The effect of pH on emulsion stability

To validate the impact of pH on the stability of alkaline emulsions, dilute hydrochloric acid was added to the alkaline emulsion system. After thorough mixing, the demulsification efficiency within 120 min is recorded and depicted in Fig. 7a and d. It is evident that as the pH decreases, the stability of the emulsion declines, a fact supported by the results of the zeta potential (Fig. 7b and e), where the absolute value of the zeta potential of the emulsions decreases with decreasing pH. At pH 9, both emulsions exhibit a demulsification efficiency of over 80% within 120 min. Moreover, when the pH is adjusted to 8, the liquid paraffin-in-water emulsion can almost completely destabilize within 30 min (Fig. 7c), while for the simulated oil, pH adjustment to 7.5 achieved complete destabilization within 30 min (Fig. 7f). Chemical agents with significant emulsifying capabilities can effectively enhance the yield of tertiary oil recovery [44, 45]. However, emulsified crude oil often exhibits increased viscosity, making it difficult to transport through pipelines and leading to increased costs [46]. Additionally, water commonly contains dissolved salts that can cause corrosion and equipment malfunction during distillation [47]. Similarly, the demulsification after emulsion polymerization can directly affect the properties of the target substance [48]. Adding demulsifiers is one of the most efficient methods for demulsification. However, these demulsifiers increase the difficulty of separating target substances. In recent years, pH-sensitive emulsifiers have garnered attention from researchers and are expected to achieve demulsification without relying on demulsifiers [19, 28]. As described previously, emulsions stabilized by C18:1-PEA under alkaline conditions can achieve rapid demulsification by lowering the pH value, which makes it have great application potential in the fields of emulsion polymerization and oil extraction.

Fig. 7
figure 7

The corresponding demulsification efficiency and zeta potential of a, b liquid paraffin and d, e simulated oil-in-water emulsion prepared by C18:1-PEA at different pH values; visible observation of demulsification of c liquid paraffin and f simulated oil-in-water emulsions formed by C18:1-PEA at pH 8 and 7.5, respectively

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3.6 Probable mechanism of pH-sensitive emulsion

The pH-sensitive emulsion phenomenon can be attributed to several key factors. Firstly, in an alkaline environment, with the increase in the concentration of hydroxide ions in the continuous phase, the sulfonic acid groups in the emulsifier molecules exist in the -SO3− form in the solution. Concurrently, hydrogen atoms in the hydroxide ions interact with the highly electronegative ether oxygen atoms through hydrogen bonding. This interaction results in APS molecules acquiring additional negative charge, thereby enhancing electrostatic repulsion and reinforcing stability between emulsion droplets. Moreover, the TEM image (Fig. 8b) revealed the presence of numerous complex micelles in 3 g L−1 APS (pH 11) aqueous solution, which can be supported by Wang et al.’s research [11]. Based on Fig. 8b, the average particle size of the formed micelles is 31.97 nm. According to the steric stabilization mechanism [49], the dispersion of micelles in the continuous phase leads to spatial hindrance, significantly reducing the attraction between oil droplets and thereby enhancing emulsion stability. This mechanism was further corroborated by Cryo-SEM images (Fig. 8c and d), which showed many micelles dispersed in the continuous phase and formed a network structure, improving emulsion stability. A similar network structure surrounding emulsion droplets stabilized by polymeric compounds was reported by Xu et al. [50] and Yu et al. [51] utilizing cryo-SEM. However, as the pH decreases, the concentration of hydroxide ions binding to ether oxygen atoms decreases as well, resulting in diminished electrostatic repulsion. Concurrently, the abundance of hydrated hydrogen ions in water gradually increases, leading to the formation of hydrogen bonds between hydrogen atoms in hydrated hydrogen ions and oxygen atoms in sulfonate groups and ether oxygen atoms. The aggregation phenomenon is evident in the Cryo-SEM images of emulsions formed under pH 10, as illustrated in Fig. 8e and f. In comparison to pH 11, although the presence of a network structure can still be observed, it is significantly reduced. The decline in electrostatic repulsion and the aggregation of APS molecules collectively result in decreased emulsion stability and the initiation of coalescence between emulsion droplets, which is consistent with the results in Sects. 3.4 and 3.5, in which emulsion stability and zeta potential absolute value were higher under a higher pH. In contrast, they were lower under a lower pH. This also corresponds to the phenomenon in Fig. S2 where the solution becomes clearer as the pH value increases. In fact, during the synthesis process, when another two APSs with similar structures were synthesized and PEA-D400 was substituted by 1,4-butanediamine, the amphiphilic polyamides obtained did not exhibit pH-sensitive characteristics (Scheme S1, Fig. S15-16). Therefore, the introduction of the polyether structure may be the crucial element contributing to the pH-sensitive characteristics of amphiphilic polyamides in the emulsion.

Fig. 8
figure 8

a The surface tension of APSs at different concentrations (pH 11). b TEM image of C18:1- PEA at 3.0 g L−1 (pH 11). Cryo-SEM images of emulsions prepared by C18:1-PEA at pH 11 (c, d) and pH 10 (e, f)

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4 Conclusions

In summary, two novel pH-sensitive polyamide surfactants, APSs, were prepared from FAME through sulfonation, amidation, addition, and polycondensation and identified by FT-IR, 1H NMR, and GPC. The weight-average molecular weights of C18:1-PEA and C22:1-PEA are 4.29 × 104 Da and 4.68 × 104 Da, respectively. When the pH is equal to or greater than 11, stable liquid paraffin-in-water emulsions can be formed with lower concentrations of APSs and exhibits excellent emulsifying capability for peanut oil, canola oil, motor oil, and n-hexadecane, maintaining stable emulsions for at least 100 days. Moreover, the stable liquid paraffin or simulated oil-in-water emulsions formed under alkaline conditions by the C18:1-PEA can be completely destabilized at lower pH values in a short time. The pH-sensitive behavior of the emulsions could be attributed to the introduction of the polyether structure. This work provides another route for pH-sensitive emulsifiers; their emulsifying ability could be regulated by adjusting solution pH.