Characterizing Foam Generated by CO₂-Switchable Surfactants for Underground CO₂ Storage Application

Abstract

CO₂-switchable surfactants, applicable for mitigating CO₂ geological storage efficiency challenges, offer promising control over foam stability under reservoir conditions, but their performance under extreme pressure, temperature, and salinity still needs thorough investigation. This study experimentally characterizes the performance of CO₂-switchable surfactants by evaluating their interfacial tension (IFT) reduction, foamability, and foam stability under reservoir-relevant conditions. Six surfactants, including cationic (cetyltrimethylammonium bromide (CTAB) and benzalkonium chloride (BZK)) and nonionic amine-based surfactants (N,N-Dimethyltetradecylamine, N,N-Dimethyldecylamine, and N,N-Dimethylhexylamine), were assessed using synthetic brine mimicking a depleted North Sea oil reservoir. A fractional factorial design was employed to minimize experimental runs while capturing key interactions between surfactant type, temperature, salinity, and divalent ion concentrations. Foam switchability was analyzed by alternating CO₂ and N₂ injections, and interfacial properties were measured to establish correlations between foam generation and IFT. Experimental findings demonstrate that cationic surfactants (BZK and CTAB) exhibit CO₂-switchability and moderate foam stability. Nonionic surfactants show tail length-dependent responsiveness, where D14 demonstrated the highest foamability due to its optimal hydrophilic–hydrophobic balance. IFT measurements revealed that BZK consistently maintained lower IFT values, facilitating stronger foam generation, while CTAB exhibited higher variability. The inverse correlation between IFT and foamability was observed. These insights contribute to the development of tailored surfactants for subsurface CO₂ storage applications, improving foam-based mobility control in CCS projects.

1. Introduction

Anthropogenic carbon emissions pose a significant environmental challenge, driving climate change and necessitating effective mitigation strategies [1,2]. One widely adopted approach is capturing and storing CO₂ gas in subsurface geological formations [3]. However, the efficiency of CO₂ storage is jeopardized by two primary flow disturbances: viscous fingering and gravity override [4,5,6], as visualized in Figure 1. Viscous fingering occurs when the low-viscosity CO₂ displaces more viscous reservoir fluids, leading to the formation of narrow, finger-like flow channels instead of a stable displacement front. This instability results in uneven CO₂ distribution, premature breakthrough, and inefficient pore-space utilization, ultimately diminishing overall storage capacity [7]. In contrast, gravity override arises due to the lower density of CO₂ relative to brine or oil, causing it to migrate upward within the reservoir. This vertical segregation reduces the sweep efficiency of the CO₂ flood, leaving significant portions of the reservoir underutilized and limiting the recovery of resident fluids [4,8,9]. To mitigate these inefficiencies, foam flooding using surfactants has emerged as a promising solution. Foam increases the effective viscosity of CO₂, reducing mobility contrast and suppressing viscous fingering, while also enhancing vertical conformance to counteract gravity override [10]. By stabilizing the displacement front and improving CO₂ distribution, surfactant-stabilized foam can significantly enhance storage efficiency and reservoir utilization in CO₂ sequestration projects [11].

Surface-active agents or surfactants have a major influence on CO₂-foam applications by stabilizing foam structures, reducing CO₂ mobility, and improving sweep efficiency in enhanced oil recovery (EOR) processes [13]. They are designed to lower the interfacial tension between two phases, such as oil and water, and play a significant role in applications where emulsions need to be stabilized temporarily. Aqueous foams made of carbon dioxide (CO₂) are colloidal dispersions made of CO₂ bubbles scattered across an ongoing aqueous phase [14]. The gaseous phase is dispersed within the interior phase, while the liquid phase constitutes the external phase. Illustrated in Figure 2 is the progression of foam formation, with the thin fluid sheet denoted as a lamella. A lamella can be segmented into internal gas bubbles within the foam. The “Plateau boundary” designates the region where lamellae intersect, as depicted in Figure 2. The system rapidly stratifies into two layers, with gas bubbles ascending to the surface due to density discrepancies between the medium and the bubbles. Pure liquid cannot produce froth; however, with the introduction of a surface-active substance, foaming occurs. Upon injection beneath the liquid’s surface, a gas bubble invariably ruptures instantaneously. The utilization of a diluted surfactant solution engenders a restoring force, facilitating equilibrium restoration. As the air/liquid interface expands, alterations in surface equilibrium prompt the development of a polyhedral structure, as depicted in Figure 3 [15].

However, CO₂ foam that is only stabilized by a surfactant has some unfavorable characteristics. At high temperatures and salinity, CO₂ foam quickly breaks down when it comes into contact with crude oil [16]. Ahmed et al. demonstrated that a number of variables, including temperature, pressure, brine salinity, permeability, surfactant concentration, and foam quality, can affect foam movement [17]. With these challenges, the scientific community became interested in evaluating specific CO₂-switchable surfactants under extreme conditions and evaluating their stability [18,19,20]. Chen et al. found that, in addition to having high cloud points in brine and high ionic surfactant interfacial activities in water for foam production, the switchable ethoxylated alkyl amine surfactants also have considerable solubilities in CO₂ in the nonionic dry state for surfactant injection under 120 °C and brine salinity of 182 g/L NaCl [21]. Li et al. showed that the long-chain polyamine octadecyl dipropylene triamine had outstanding CO₂ foaming performance when subjected to high temperatures and salinities up to 160 °C and 200,000 ppm (with 1000 ppm Ca²⁺). It also showed exceptional CO₂ sensitivity for foam generation [22].

Switchable surfactants are a class of amphiphilic molecules that can undergo reversible interconversions between active and inactive forms. These surfactants can be switched on or off, allowing for the controlled stabilization or destabilization of emulsions at specific desired stages of a process [23]. Switchable surfactants offer advantages such as delayed activity until needed, reusability, and facilitated removal from the product stream by switching to the form least soluble in the relevant medium [24]. The transformation of switchable surfactants is triggered by benign gases like carbon dioxide and air [25]. CO₂ is an advantageous trigger that is inexpensive, nonhazardous, non-accumulating in the system, and easily removed, and it does not require the material to be transparent [26]. Typically, CO₂-switchable surfactants contain functional groups that can be reversibly protonated or deprotonated in the presence of CO₂, leading to alterations in their molecular structure and surface affinity [24,25]. When CO₂ is present, these surfactants undergo a chemical transformation, causing a shift in their hydrophilic–hydrophobic balance. In their activated state, CO₂-triggered switchable surfactants exhibit enhanced hydrophobicity, reducing their solubility in aqueous environments and diminishing their surface activity. Conversely, in the absence of CO₂, they revert to their original hydrophilic state, thereby restoring their surface activity and solubility [24,27]. This reversible switching mechanism enables precise control over the interfacial properties of the system, making CO₂-triggered switchable surfactants valuable for applications such as CO₂ capture and utilization, emulsion stabilization, and controlled release systems, where dynamic modulation of surfactant behavior is desired. Figure 4 demonstrates the switchability.

CO₂-responsive surfactants reversibly switch between active and inactive states in response to CO₂, and this property is advantageous in terms of mobility control in CO₂ storage applications [29]. Typically, CO₂-switchable surfactants contain functional groups such as amines or acetamidines that undergo protonation when exposed to stimuli CO₂ gas [25]. Octadecy l dipropylene triamine and some other amine-based compounds demonstrated strong CO₂ sensitivity, which aids in immediate foam generation with better tolerance to concentration, temperature, pressure, and brine water salinity [22,30]. Similarly, long-chain cationic surfactants such as UC₂₂AMPM have demonstrated reversible switching capabilities by forming wormlike micelle networks in the presence of CO₂ with enhanced foam stability and viscosity, while destabilizing upon introducing N₂ [31,32]. Other advanced formulations that are ODPTA-based surfactants, for instance, have shown improved stability under reservoir conditions. They can maintain bulk viscosity and foam persistence up to 160 °C and 10.5 MPa, more effectively than traditional surfactants like SDS [33]. A study confirmed the synergy between CO₂-soluble and nonionic surfactants that resulted in enhanced foam viscosity and stability. Also, synthesized acetamidine surfactants with varying hydrophobic tail lengths demonstrated a strong CO₂ response. Experimental results indicated that longer hydrophobic tails protonate at higher temperatures, while shorter tails are sensitive to elevated temperatures [34]. This switchability allows for enhanced influence on foam structure and volume, making such surfactants effective for dynamic reservoir conditions. Further, studies have emphasized that surfactants diluted with CO₂ perform better than conventional water dilution. In fractured carbonate reservoirs, CO₂-soluble surfactants enable faster foam propagation and stronger foam formation in fractured carbonate reservoirs, reflecting their better oil recovery [35].

This study presents a new approach by integrating interfacial tension (IFT) measurements with foam generation and structure analysis, which aimed to investigate the relationship between interfacial properties and foamability in CO₂-responsive surfactants. It explores the performance of both cationic (e.g., benzalkonium chloride and cetyltrimethylammonium bromide) and nonionic surfactants with varying hydrophobic tail lengths (N,N-Dimethyltetradecylamine (D14), N,N-Dimethyldecylamine (D10), and N,N-Dimethylhexylamine (D6)) under varying levels of temperatures and salinities. Complementing earlier investigations, this study evaluates the CO₂ switching behavior of these five surfactants of different types and compares them to a conventional non-switchable surfactant, sorbitan monooleate ethoxylated (Tween 80), which gives a baseline for the investigation. Anionic surfactants were deliberately excluded due to their poor compatibility with high-salinity brines, particularly their susceptibility to precipitation in the presence of divalent cations such as Ca²⁺ and Mg²⁺. Finally, this work offers valuable design principles for formulating efficient CO₂ foams by linking interfacial properties to foam dynamics, addressing the main challenges of CO₂ handling in carbon storage and enhanced oil recovery applications.

2. Materials and Methods

2.1. Brine

To replicate real reservoir conditions, synthetic brine was prepared with a specific composition mimicking the saline environment of one of the Danish North Sea depleted oil reservoirs. The preparation involved dissolving a precise mixture of monovalent and divalent salts, including sodium chloride (NaCl), sodium sulfate (Na₂SO₄), potassium chloride (KCl), magnesium chloride (MgCl₂·6H₂O), calcium chloride (CaCl₂·2H₂O), strontium chloride (SrCl₂·6H₂O), and barium chloride (BaCl₂·2H₂O), in ultrapure deionized water. These salts, purchased from Sigma-Aldrich, Darmstadt, Germany, with a minimum purity of 99%, were carefully weighed using gravimetric methods and mixed under controlled ambient conditions to ensure homogeneity.

Table 1. Brine composition.

Species	Concentration (mol/kgw)
Na+	6.15 × 10−1
K+	5.50 × 10−3
Mg2+	1.20 × 10−2
Ca2+	9.06 × 10−3
Sr2+	5.81 × 10−4
SO42−	3.47 × 10−3
Cl−	6.57 × 10−1
Ba2+	2.19 × 10−5
Total dissolved solid (TDS)—ppm	38,575
Ionic strength (IS)—mol/kgw	6.89 × 10−1

represents the composition of the brine.

Viscosity and density of the brine at different temperatures and atmospheric pressure were measured by Anton Paar AMV 200 and DMA 4100, Ostfildern-Scharnhausen, Germany, respectively, and are reported in Figure 5.

2.2. Surfactants

In this study, we examined six surfactants, including sorbitan monooleate ethoxylated (Tween 80), cetyltrimethylammonium bromide (CTAB), benzalkonium chloride (BZK), N,N-Dimethyltetradecylamine (D14), N,N-Dimethyldecylamine (D10), and N,N-Dimethylhexylamine (D6). All surfactants were purchased from Sigma-Aldrich, Darmstadt, Germany. Tween 80 is not a CO₂-switchable surfactant and is used to serve as a baseline for comparison. CTAB and BZK are cationic surfactants, while D14, D10, and D6 are nonionic surfactants. These nonionic surfactants are tertiary amines with different alkyl chain lengths; the longest one is D14, which contains a tetradecyl (C₁₄H₂₉) group attached to the nitrogen, and the shortest one is D6, which contains a hexyl (C₆H₁₃) group attached to the nitrogen. The longer the chain, the more hydrophobic and more soluble in water the component [26,36].

Table 2. Summary of the surfactants used in this study.

	CAS No.	Molecular Formula	Molecular Weight (g/mol)	Appearance	Density (g/cc)	Boiling Point (C)	Solubility in Water
Tween 80	9005-65-6	C64H124O26	1309.67	Amber viscous liquid	1.07	-	Very soluble
CTAB	57-09-0	C19H42N.Br	364.53	Hygroscopic white powder	0.5	-	55 g/L
BZK	63449-41-2	Variable	~340–400	Pale yellow/white powder	-	197	Very soluble
D14	112-75-4	C16H35N	241.46	Light yellow liquid	0.795	302	Slightly soluble
D10	1120-24-7	C12H27N	185.35	Transparent yellow liquid	0.778	234	Slightly soluble
D6	4385-04-0	C8H19N	129.24	Colorless liquid	0.744	146–150	Slightly soluble

summarizes the properties of these surfactants, and below, there is a description of each one. Additional information, including the molecular structure of these surfactants, can be found in Appendix A.

2.3. Experimental Setup

2.3.1. Interfacial Tension (IFT) by Pendant Drop

IFT is defined as the force that exists between two immiscible fluids. It can be quantified and shows the relationship between the force acting on a surface and the distance per unit length (F/L), where the force is acting normal to the surface of the liquid with units as dynes/cm, J/m², or mN/m [37]. The IFT experimental setup, provided by Kruss-Scientific, with a maximum working temperature and pressure of 100 °C and 500 bars, respectively, is used in this study, making it possible to measure interfacial tension for two-phase systems (immiscible) using the pendant drop method [38]. The experimental setup primarily consists of three main components. The high-pressure visualization cell is equipped with a needle with a diameter of 1.588 mm and a fluid circuit and feeding system, including pumps and high-pressure cylinders for the fluids.

Lastly, the setup is equipped with a data acquisition and imaging system with a camera that makes it possible to monitor and take dynamic images of the droplet, and the IFT is measured using a Drop Shape Analyzer. Figure 6 represents a schematic of the experimental setup used for interfacial tension measurements in this study.

Three levels of temperature were set at different stages (from ambient, 45 °C, and 70 °C max), and the pressure was maintained at max 100 bar throughout the experiment. The injected liquid generates a bubble at the tip of the needle, and the camera captures images of the drop over the course of the experiment.

Firstly, two high-pressure volume cylinders, one with brine and the other with CO₂, are equilibrated, and the test temperature on the temperature controller is set until it reaches the preset temperature. The bulk fluid of lighter density (CO₂) is pressurized to fill the cell at 100 bar. Then, stable drops are made with the drop fluid (brine mixed with surfactant) at the tip of the needle. The drop images are captured during the test period. The measurement time was set at 120 s with a frequency of 1 measurement per second.

2.3.2. Foam Analyzer

The FoamScan^TM Cylindrical Double-Walled Jacket by Teclis Scientific, Rhône, France, was used to investigate foam generation, stability, bubble size, and bubble distribution under controlled conditions. Through a porous distributor filter, CO₂ or N₂ gas is introduced into a surfactant solution to generate foam. FoamScan™ was set on fixed parameters to reduce errors and to obtain comparable results. A 60 mL surfactant solution was injected into the 240 mL testing tube using a disposable syringe, providing sufficient space for foam expansion. The system operated under 1 bar pressure, with temperatures ranging from 22 °C to 77 °C, using a double-walled glass cylinder to ensure uniform heating and controlled environmental conditions. The flow rate was fixed at 220 mL per minute, and the pressure was constant at 1 bar. The foam-generating procedure is set to generate foam until 100 mL or 120 s of generating, and then bubbling stops, and stability is evaluated. The foam column is shown in Figure 7.

Throughout the experiment, real-time monitoring was conducted using conductance probes, a temperature probe, optical prisms, and electronic controls. Two cameras were utilized: Camera 1 captured the overall foam formation, while Camera 2 focused on individual bubbles to analyze structural characteristics. The software continuously tracked foam behavior by recording foam volume over time and conductivity changes in both the liquid and foam phases. These measurements provided valuable insights into foam growth, decay, and stability. To further evaluate the bubble structure, Teclis Scientific’s Cell Size Analyzer^TM (CSA) version 6.7.1.397 software processed images captured by Camera 2. The software extracted statistical data, including mean bubble radius, bubble count, and bubble area distribution, offering a detailed understanding of foam behavior under different conditions.

By integrating precise control over temperature, pressure, and gas flow, this setup provided a comprehensive evaluation of foam dynamics, allowing for a thorough assessment of surfactant performance in various environmental conditions.

2.4. Experimental Design

Using Jump, an experimental design was performed considering five factors of interest in this experimental work, namely surfactant type, temperature of the solution, salinity, and content of three divalent ions: calcium, magnesium, and sulfate. To start, the number of experiments needed to investigate the effect of the main factors, and all levels of interaction go beyond the available resources. To overcome this issue, many procedures are taken to reduce the number of needed experiments. First, surfactant concentration is kept at the CMC level. Secondly, salinity levels are limited by changing levels of the mentioned divalent ions and salinity calculated at each of their level combinations. Third, factors are set to be continuous variables between high and low preset values to avoid discrete values that dictate more experiments. Fourth, factor screening is performed through a fractional factorial design to test second-degree level interaction to evaluate the interaction surface and locate potential areas for further screening. It is possible to reduce the number of experiments through fractional factorial design, where higher-order interactions between factors are neglected, and the design is reduced to lower-order interactions [39]. This design is based on creating subgroups of experiments, projection of higher-order interactions by investigating lower-order and sequential experimentation. Finally, three center runs are introduced to set a threshold for variations. Adopting this design reduced the number of experiments to 14 randomized runs of different level combinations, as shown in

Table 3. Experimental design (DOE) for each surfactant.

Run	Temp. (°C)	Salinity (ppm)	CaCl2	MgCl2	NaSO4
9	45	40,340	Mid—3.28%	Mid—6.03%	Mid—1.23%
1	22	36,460	High—6.57%	High—12.06%	Low—0.62%
2	70	36,480	High—6.57%	High—12.06%	High—2.46%
3	22	36,410	Low—1.64%	High—12.06%	High—2.46%
4	70	36,190	Low—1.64%	Low—3.02%	High—2.46%
5	22	36,250	High—6.57%	Low—3.02%	Low—0.62%
6	70	36,250	High—6.57%	Low—3.02%	High—2.46%
12	45	40,330	Mid—3.28%	Mid—6.03%	Mid—1.23%
7	70	36,240	High—6.57%	Low—3.02%	Low—0.62%
8	70	36,400	Low—1.64%	High—12.06%	Low 0.62%
10	22	36,180	Low—1.64%	Low—3.02%	Low—0.62%
11	22	36,470	High—6.57%	High—12.06%	High—2.46%
14	22	36,180	Low—1.64%	Low—3.02%	High—2.46%
13	45	40,340	Mid—3.28%	Mid—6.03%	Mid—1.23%

.

This shows three levels of temperatures, calculated salinity for each experiment, and three levels of divalent ion concentrations; note that the percentage (%) means out of the total brine mixture for that experiment. This study employs a randomized design methodology, wherein runs are meticulously conducted in the sequence outlined in the table provided. Notably, the design incorporates three central runs strategically placed to meticulously evaluate the significance of errors, ensuring robustness and reliability in our analysis of each surfactant. It is important to note that even though the absorption of CO₂ may lead to a decrease in pH due to carbonic acid formation, and that will impact the activation of surfactants [18,40,41,42], pH levels were not explicitly monitored or controlled during the experiments. Since salinity is not significantly different in all 14 experiments, pH changes within the same surfactant system are assumed to remain relatively constant. Differences in CO₂ solubility and corresponding pH behavior between different surfactants were acknowledged but were considered outside the scope of this study.

To ensure the accuracy and reliability of the experimental data, center runs were repeated three times as an important part of the design. These center runs were used to evaluate the variability and consistency of the results under the same conditions; all test parameters were set at a medium level. The foam volume curves, or foam lifespan curves, show minor variations against the objective of the investigation, which is to observe the behavior and response of the surfactant systems to CO₂ stimuli with a reasonable precision level. Detailed graphical results for all six surfactants tested are provided in Appendix B. Notably, interfacial tension (IFT) measurements showed minor errors for the surfactants BZK (±0.3 mN/m) and CTAB (±0.2 mN/m). These small deviations are also considered acceptable due to the inherent complexity of the setup and the challenges associated with maintaining consistent droplet size during IFT measurements. Overall, observed data variations from the repeated center runs confirm the robustness of the experimental approach. Despite the fact that there were minor instrumental or setup-induced inconsistencies, confidence in the trends and insights drawn from the study is maintained.

3. Results and Discussions

3.1. CO₂ Responsiveness

This study focuses on surfactants that respond to external stimuli, such as CO₂ and N₂ gases. Specifically, certain surfactants exhibit switchable properties through the protonation of amine groups, which transition from hydrophobic to hydrophilic when exposed to CO₂. Consequently, anionic surfactants are excluded from this analysis, as they lack protonatable amine groups to interact with CO₂ and are already negatively charged, preventing the CO₂/N₂ switchability cycle.

The analysis includes six surfactants: one baseline surfactant, two cationic surfactants, and three nonionic surfactants with varying tail lengths. The full results of the foam characterization experiments for these surfactants, based on the DOE table, are provided in Appendix B as Figure A7, Figure A8, Figure A9, Figure A10, Figure A11 and Figure A12.

Figure 8 demonstrates the behavior of Tween 80 when subjected to alternating CO₂ and N₂ injections. The chart clearly shows minimal differences between the gas alternations, indicating that this surfactant is not CO₂-switchable. This behavior is typical for standard, non-switchable surfactants. As expected, the maximum foam volume remains nearly constant at approximately 102 mL. This is also demonstrated in other studies where Tween 80 foam is found to be unstable even at higher concentrations [43].

Next, cationic surfactants are the dominant type that responds to CO₂. The primary reason these surfactants react to CO₂ is their structure, which includes amine and amidine functional groups that govern their switchability. Figure 9 and Figure 10 illustrate the foam volume for the two cationic surfactants, benzalkonium chloride (BZK) and hexadecyltrimethylammonium bromide (CTAB), under alternating CO₂ and N₂ gas injection. Both surfactants show a significant drop in foam volume when nitrogen is injected, compared to CO₂, indicating their CO₂ responsiveness.

CTAB maintains its foam volume for approximately 200 s, while BZK only sustains it for 61 s. This difference is due to the higher hydrophobicity of BZK, which results in stronger interactions with organic phases. Additionally, BZK has limited intrinsic CO₂-switchability, as it lacks the CO₂-responsive functional groups (e.g., amidine or tertiary amines) found in CTAB.

The switching mechanisms for nonionic surfactants are based on changes in hydrogen bonding and polarity. In the continuous CO₂ phase, these surfactants become hydrophilic due to enhanced interactions with dissolved carbon dioxide, whereas in the N₂ phase, the effect is reversed. However, the degree of responsiveness to CO₂ depends on the tail length, as the solubility of these surfactants is influenced by the length of their hydrophobic tails. Shorter tails primarily rely on headgroup surface interactions for solubility changes, while longer tails exhibit greater CO₂ responsiveness due to their direct interaction with CO₂ molecules.

D6 and D10 are tertiary amine-based surfactants with shorter tail lengths. D6, with its shorter hexyl (-C₆H₁₃) tail, mainly relies on the protonation–deprotonation of its -N (CH₃)₂ headgroup for CO₂/N₂ switching. Figure 11 shows that D6 exhibits almost no response to CO₂/N₂ phases, generating only a few milliliters of foam in CO₂, and none in N₂. In contrast, D10, with its longer decyl (-C₁₀H₂₁) tail, demonstrates stronger hydrophobic interactions and a higher responsiveness to CO₂, as shown in Figure 12. More foam volume is observed with D10, although there is no notable responsiveness to N₂.

Finally, surfactants with medium tail lengths tend to exhibit more pronounced switchability, as they strike a balance between hydrophilicity and hydrophobicity, resulting in an enhanced CO₂ response. The interaction between CO₂ and the surfactant’s headgroup can significantly alter its solubility and self-assembly properties.

N,N-Dimethyltetradecylamine (D14) is a surfactant with a medium-length tetradecyl (-C₁₄H₂₉) tail, which is more hydrophobic than both D6 and D10. Figure 13 demonstrates its CO₂/N₂ responsiveness, which arises from both headgroup protonation and hydrophobic interactions with CO₂. This results in a significantly higher foam volume compared to surfactants with shorter tails. The longer hydrophobic tail of D14 enhances its affinity for CO₂, as CO₂ preferentially dissolves in nonpolar regions, disrupting molecular packing and influencing self-assembly behavior.

3.2. Interfacial Tension (IFT)

The IFT measurement results for both BZK and CTAB are presented in Figure 14 using the DOE in Table 3. The results obtained for these two switchable cationic surfactants show different trends in terms of their IFT values, with BZK generally recording lower and more consistent IFT values than CTAB under the same conditions.

The lowest IFT values for BZK were obtained from Experiments 8 and 9, with respective temperatures of 70 °C and 45 °C, with each recording a similar IFT value of 2.5 mN/m when both experiments were conducted at total salinities of 36.40% and 40.3%, respectively.

Experiment 9, which recorded one of the two lowest IFTs, was a central run with medium concentrations of Mg²⁺, Ca²⁺, and SO₄²⁻ ions. On the contrary, the maximum IFT value is 5.5 mN/m, and this was recorded in Experiment 14 with low Ca²⁺ and Mg²⁺ ions and high sulfate ion concentrations at ambient temperature. The same trend was observed in the literature, emphasizing the role of divalent ions on the IFT value in the presence of CO₂ [44]. Overall, the average IFT values for BZK for all the experiments were lower compared to those obtained for CTAB. Thus, BZK showed a better performance and lower IFT compared to CTAB under similar ionic conditions, and the results seem consistent with values between 3 and 4 mN/m, as can be seen from most of the experiments, except in Experiment 9, with the highest value of 5.5 mN/m.

Higher IFT values were obtained for CTAB, with a maximum IFT value of 18.3 mN/m for Experiment 5. This is where the concentrations of Mg²⁺ and SO₄²⁻ were low, combined with a high concentration of Ca²⁺ at ambient temperature. Moreover, a minimum IFT value of 5.6 mN/m was recorded for CTAB in Experiment 8, which is at an increased temperature of 70 °C.

This lowest value of 5.6 mN/m for CTAB is slightly higher than the highest (max) value of 5.5 mN/m obtained for BZK. It can be observed that CTAB worked much better to decrease the IFT at a temperature of 70 °C in Experiment 8 when the concentration of Ca²⁺ and SO₄²⁻ was low and the concentration of Mg²⁺ was high.

3.3. Foam Generation and Interfacial Tension

Foam generation and interfacial tension (IFT) are closely interconnected concepts in surface chemistry. This analysis was conducted to explore the relationship between IFT and foam generation rate (foam volume/time). The experimental results, illustrated in Figure 15, generally demonstrate an inverse correlation where lower IFT values are associated with higher foam generation rates. While this general trend is evident, the data also display a degree of variability, with several outlier points that deviate from the expected pattern. These outliers highlight the organic nature of the experimental data that reflects the inherent complexity of interfacial phenomena and the dynamic behavior of foam systems. It is important to note that such deviations are not uncommon in surface chemistry studies, where multiple factors can influence the results, such as temperature variation, surfactant concentrations, measurement errors, or even microscopic impurities. These outliers may also stem from minor experimental errors or limitations in the measurement techniques, which could affect the consistency of both IFT values and foam volume quantification. Despite these outliers, BZK and CTAB were studied in detail. The amphiphilic structure of BZK allows it to readily adsorb at the CO₂–brine interface, effectively reducing interfacial tension. As shown in Figure 15, BZK consistently produced more foam under lower IFT conditions. This is likely due to CO₂’s solubility in the brine phase, which can influence the structural characteristics of the interfacial film formed by BZK. The reduced IFT enhances BZK’s ability to stabilize foam by enabling the formation of smaller, more persistent bubbles, even in the presence of experimental variability.

CTAB also demonstrates significant effects on interfacial tension. The interaction of CO₂ with CTAB leads to a reduction in IFT, like BZK, as CO₂ effectively alters the properties of the brine phase, further promoting foam formation. Figure 16 illustrates a similar trend for CTAB, where higher foam volumes were generally observed at lower IFT values. However, as with BZK, the data points for CTAB also include a few noticeable outliers.

The lower interfacial tension between the gas and liquid phases facilitates the formation of smaller, more stable bubbles, which is an important factor for effective foam generation. Some outliers were observed in the data, but these deviations are considered part of the natural complexity of foam systems and surface chemistry experiments. Such outliers may result from experimental limitations or transient physicochemical interactions that are difficult to fully control, yet they do not diminish the overall consistency of the observed trend. The ability of BZK and CTAB to reduce IFT in the presence of CO₂ significantly enhances their performance in foam flooding applications. These findings reinforce that foam stability and generation are strongly influenced by the interfacial properties of surfactants.

3.4. Number of Bubbles and Mean Bubble Radius

CSA bubble analysis was performed by fixing a foam image in 30 s for all experiments. This means that the image that was captured at a time of 30 s for all experiments for both surfactants and is considered for CSA analysis, including mean bubble radius and number of bubbles.

The analysis for CTAB reveals a fluctuating trend in both the number of bubbles and the mean bubble radius across different experiments. The number of bubbles varies significantly, with the highest bubble number observed in Experiments 7 and 13, with a number of bubbles of about 500, while the lowest occurs in Experiments 10 and 11. Similarly, the mean bubble radius demonstrates notable fluctuations, peaking at approximately 0.18 mm in Experiment 10 and reaching its lowest value of about 0.06 mm in Experiment 13. This variation suggests that different experimental conditions or surfactant interactions may be influencing bubble formation and stability.

As illustrated in Figure 17, there is an inverse relationship between the number of bubbles and the mean bubble radius, indicating that as the bubble number increases, the average size of individual bubbles decreases. This is a characteristic behavior of surfactant-stabilized foams, where higher bubble numbers correspond to the formation of smaller bubbles due to efficient surface stabilization, limited coalescence, and increased surfactant adsorption at the bubble interface. Previous studies on CTAB demonstrated that variations in head group type, degree of substitution, and tail length significantly affect CO₂–water interfacial properties. Specifically, longer hydrophobic tails and smaller head groups were found to enhance adsorption at the interface and reduce interfacial tension [45].

BZK also shows a similar trend with bubble size and radius, but with a relatively small mean radius ranging from 0.046 to 0.057 mm in Figure 18.

Generally, the results for BZK suggest that there is no significant variation, as the standard deviation is ±0.003, which is too low.

3.5. Effects of Temperature and Salinity

The effect of temperature and salinity on the performance of the examined surfactants was analyzed using p-values generated from statistical tests.

Table 4. Summary of the statistical runs.

Factor	p-Value
	BZK	CTAB	D14	Tween 80
Temperature	0.0012	<0.0001	0.0176	0.6498
CaCl2	0.4053	0.1547	0.8617	0.8138
MgCl2	0.8897	0.324	0.4351	0.1581
Na2SO4	0.4162	0.0895	0.5305	0.6568

and Figure 19 indicate that temperature has the greatest influence by showing statistical significance on p-values of 0.0012, <0.0001, and 0.0176 for BZK, CTAB, and D14, respectively. However, there is not enough statistical evidence shown for the temperature on Tween 80. Clearly, cationic CO₂-switchable surfactants are more sensitive to thermal changes compared to conventional surfactants. The higher responsiveness of BZK and CTAB to temperature may be attributed to thermodynamically driven alterations in surface activity and micelle formation. On the other hand, CaCl₂, MgCl₂, and Na₂SO₄ showed no statistically significant influence across most surfactants. However, Na₂SO₄ displayed a borderline effect on CTAB with a p-value of 0.0895, which can be attributed to ionic interactions with cationic head groups. Tween 80, a baseline, showed the least responsiveness to any factor, as expected, due to its inert behavior under changes in conditions. This supports the CO₂-switchable surfactants’ desired responsiveness for the earlier-mentioned applications. Generally, BZK and CTAB are suitable in temperature-dependent, stimuli-responsive systems while D14 poses a moderate alternative.

Furthermore, Figure 20 shows one of the diagrams that illustrate factor-to-factor interactions or second-degree interactions between temperature, CaCl₂, MgCl₂, and Na₂SO₄ on the foam flow rate for the BZK. The second-degree interaction profiles for other surfactants are reported in Appendix B as Figure A13, Figure A14 and Figure A15. The response levels were set at −1 (low), 0 (center), and 1 (high) for each factor. The interaction plots for BZK, CTAB, and D14 indicate that temperature interacts with all other factors influencing foam generation rate. Lower foam generation rates were consistently observed at the lower temperature level (−1), which confirms the dependence on thermal conditions. This behavior suggests that at lower temperatures, these surfactants experienced a reduction in surface activity and micellization efficiency, leading to diminished foam formation. Also, the interactions between temperature and salt concentration further influenced the foam generation rate, especially for BZK and CTAB. The presence of CaCl₂ and MgCl₂ appeared to enhance foamability at elevated temperatures because of electrostatic interactions and surfactant aggregation. The same effect of temperature was observed for nonionic surfactant D14. In contrast, Tween 80, the baseline conventional surfactant, demonstrated weak second-degree interaction effects across all factors. This suggests that Tween 80 remains relatively stable under temperature or salt concentration changes. However, MgCl₂ had a distinct influence that could affect all its interactions with other factors. This suggests that Mg²⁺ ions play a unique role in modifying the foam properties of Tween 80, altering its solubility or interfacial behavior. Though there is no notable effect of sodium sulfate (Na₂SO₄), some studies showed that high concentrations can lead to high IFT values. Anions like sulfate disrupt the structure and tend to gather at the interface, while dissolved cations increase ionic strength and make the interface more hydrated [46].

4. Conclusions

In this study, we have investigated the potential of CO₂-switchable surfactants to optimize foam behavior for CO₂ storage applications. By leveraging their reversible switching properties, CO₂-switchable surfactants offer a promising approach to improving CO₂ injection efficiency, mitigating gas channeling, and increasing overall storage security. This contributes to the broader field of carbon capture and storage (CCS) by providing a viable alternative to conventional surfactants with enhanced tunability and environmental adaptability.

Key conclusions from this study include the following:

• Foam stability and CO₂ responsiveness: All CO₂-switchable surfactants tested in this study showed lower stability compared to regular surfactants (Tween 80). This instability is attributed to their responsiveness to CO₂; when CO₂ injection ceases, the CO₂ saturation in the foam medium decreases, leading to foam collapse due to the open-column system in the foam analyzer.
• Promising foamability: Despite stability challenges, CO₂-switchable surfactants demonstrated promising foamability, indicating their potential for foam-based CO₂ storage applications.
• Interfacial tension (IFT) influence: We established a strong correlation between IFT, foam generation flow rate, and foam mean bubble size. A decrease in IFT leads to an increase in foam generation flow rate and a reduction in mean bubble size, thereby improving foam efficiency.
• Effect of alkyl-group length: The alkyl (tail) group in amine-based surfactants plays a crucial role in their performance. Surfactants with longer alkyl chains exhibited better foamability, highlighting the importance of surfactants’ molecular structure in foam optimization.
• Stability against salinity but sensitivity to temperature: The surfactants tested in this study demonstrated stability under high salinity and in the presence of divalent ions. However, elevated temperatures negatively affected their performance, necessitating further research to enhance their thermal stability.
• Future research directions: While this study primarily focused on the effect of the tail group, future work should investigate the role of different head groups in surfactant performance. Understanding these interactions will aid in designing optimal surfactant structures for use in harsh reservoir conditions and improving underground CO₂ storage efficiency.

This study highlights the potential of CO₂-switchable surfactants in optimizing foam behavior for CO₂ storage, paving the way for their integration into large-scale CCS operations. By addressing key challenges related to foam stability and tunability, these surfactants can enhance the efficiency and security of CO₂ sequestration, ultimately contributing to global efforts to mitigate climate change. Continued research and field trials will be crucial in validating their practical applicability and unlocking their full potential in carbon storage technologies.