A Comparative Study on CO2-Switchable Foams Stabilized by C22- or C18-Tailed Tertiary Amines

The CO2 aqueous foams stabilized by bioresource-derived ultra-long chain surfactants have demonstrated considerable promising application potential owing to their remarkable longevity. Nevertheless, existing research is still inadequate to establish the relationships among surfactant architecture, environmental factors, and foam properties. Herein, two cases of ultra-long chain tertiary amines with different tail lengths, N-erucamidopropyl-N,N-dimethylamine (UC22AMPM) and N-oleicamidopropyl-N,N-dimethylamine (UC18AMPM), were employed to fabricate CO2 foams. The effect of temperature, pressure and salinity on the properties of two foam systems (i.e., foamability and foam stability) was compared using a high-temperature, high-pressure visualization foam meter. The continuous phase viscosity and liquid content for both samples were characterized using rheometry and FoamScan. The results showed that the increased concentrations or pressure enhanced the properties of both foam samples, but the increased scope for UC22AMPM was more pronounced. By contrast, the foam stability for both cases was impaired with increasing salinity or temperature, but the UC18AMPM sample is more sensitive to temperature and salinity, indicating the salt and temperature resistance of UC18AMPM-CO2 foams is weaker than those of the UC22AMPM counterpart. These differences are associated with the longer hydrophobic chain of UC22AMPM, which imparts a higher viscosity and lower surface tension to foams, resisting the adverse effects of temperature and salinity.


Introduction
Carbon dioxide (CO 2 ) aqueous foams are colloidal dispersions composed of CO 2 bubbles dispersed in a continuous aqueous phase [1]. Due to their relatively low density, larger surface area and excellent fluidity, CO 2 aqueous foams have been widely used in many industrial processes and applications including the petroleum industry [2], ore flotation [3] and firefights [4]. The traditional CO 2 aqueous foams were obtained using anionic surfactants such as sodium dodecyl sulfate (SDS) [5] and alpha olefin sulfonate (AOS) [6] as foaming agents by decreasing the CO 2 -water interfacial tension (C-W IFT) and capillary forces (P c ). Unfortunately, such CO 2 aqueous foams rapidly destabilized through a combination of drainage [7,8], coalescence [7,8], and Ostwald ripening [9]. As a result, the lifetime CO 2 aqueous foams made of common surfactants do not exceed a few tens of minutes [10,11] and fail to satisfy the practical requirements. To improve foam lifetime, various foam stabilizers, including polymer [12,13], protein [14][15][16], and nanoparticles [17,18], are introduced into the aqueous foam systems against foam destabilization. In some cases, there is a demand for both the stable foam formed and controlled foam destruction. Taking

Results and Discussion
The section is organized as follows: First, the concentration of UC22AMPM and UC18AMPM is optimized by the static foam test in atmospheric pressure at 35 °C. Then, the switching behavior of UC18AMPM-CO2 aqueous foam is characterized in comparison with UC22AMPM-CO2 aqueous foam. Finally, the influence of temperature, salinity, and pressure on the performance of CO2 aqueous foams UC22AMPM and UC18AMPM is examined, respectively.

Determination of Optimum Concentration
It is known that foam properties strongly depend on the concentration of the foaming agent [31,32]. Generally, the foaming properties are referred to as foamability (the maximum volume of foam system for a certain volume of foaming agent solution after a certain time of shear effect at a certain temperature, Vmax) [33,34] and foam stability (the time taken by the volume of foam system from Vmax to a half at a certain temperature, t1/2) [33,34]. To determine the optimum concentration, the properties of UC22AMPM and UC18AMPM foams were investigated separately as a function of the concentration (0.1−0.5%) using FoamScan under atmospheric pressure at 35 C. We previously demonstrated that UC22AMPM could form stable CO2 aqueous foams but not N2 ones [21,35]. Thus, CO2 was employed as the foaming gas in this work. Figure 1A,B present the changes in Vmax and t1/2 of both aqueous foams with increasing concentration, respectively. It can be seen the Vmax for UC18AMPM was always constant at around 180 mL with increasing UC18AMPM concentration (CUC18AMPM), while the Vmax of UC22AMPM rose from 179 to 189 mL ( Figure 1A). Meanwhile, the t1/2 of both aqueous foams rose as the concentration increased ( Figure 1B). From Table 1, the t1/2 of UC22AMPM-CO2 aqueous foams improved by 2.6 fold as the concentration of UC22AMPM (CUC22AMPM) increased from 0.1% to 0.5%, higher than the increment factor of UC18AMPM-CO2 aqueous foams (1.6). These results indicated that the CUC22AMPM exerts a more prominent influence on foamability and foam stability compared to CUC18AMPM.

Results and Discussion
The section is organized as follows: First, the concentration of UC 22 AMPM and UC 18 AMPM is optimized by the static foam test in atmospheric pressure at 35 • C. Then, the switching behavior of UC 18 AMPM-CO 2 aqueous foam is characterized in comparison with UC 22 AMPM-CO 2 aqueous foam. Finally, the influence of temperature, salinity, and pressure on the performance of CO 2 aqueous foams UC 22 AMPM and UC 18 AMPM is examined, respectively.

Determination of Optimum Concentration
It is known that foam properties strongly depend on the concentration of the foaming agent [31,32]. Generally, the foaming properties are referred to as foamability (the maximum volume of foam system for a certain volume of foaming agent solution after a certain time of shear effect at a certain temperature, V max ) [33,34] and foam stability (the time taken by the volume of foam system from V max to a half at a certain temperature, t 1/2 ) [33,34]. To determine the optimum concentration, the properties of UC 22 AMPM and UC 18 AMPM foams were investigated separately as a function of the concentration (0.1-0.5%) using FoamScan under atmospheric pressure at 35 • C. We previously demonstrated that UC 22 AMPM could form stable CO 2 aqueous foams but not N 2 ones [21,35]. Thus, CO 2 was employed as the foaming gas in this work. Figure 1A,B present the changes in V max and t 1/2 of both aqueous foams with increasing concentration, respectively. It can be seen the V max for UC 18 AMPM was always constant at around 180 mL with increasing UC 18 AMPM concentration (C UC18AMPM ), while the V max of UC 22 AMPM rose from 179 to 189 mL ( Figure 1A). Meanwhile, the t 1/2 of both aqueous foams rose as the concentration increased ( Figure 1B). From Table 1, the t 1/2 of UC 22 AMPM-CO 2 aqueous foams improved by 2.6 fold as the concentration of UC 22 AMPM (C UC22AMPM ) increased from 0.1% to 0.5%, higher than the increment factor of UC 18 AMPM-CO 2 aqueous foams (~1.6). These results indicated that the C UC22AMPM exerts a more prominent influence on foamability and foam stability compared to C UC18AMPM .
The foam comprehensive index (FCI) [6], a quantitative measure to assess the foam properties, was employed to calculate the optimal concentration. The FCI can be expressed below [36]: As listed in Table 1, the FCI of UC 22 AMPM and UC 18 AMPM reached maximum values of 1,382,062 s·mL and 49,140 s·mL at a concentration of 0.5 wt.%, respectively. Typically, the value of FCI is greater, the foam properties are better [37]. Based on the FCI criterion, 0.5 wt.% as the optimal UC 22 AMPM and UC 18 AMPM concentration was used in the following experiments. Furthermore, we also concluded that the 0.5% UC 22 AMPM has superior foam properties to 0.5% UC 18 AMPM. The foam comprehensive index (FCI) [6], a quantitative measure to assess the foam properties, was employed to calculate the optimal concentration. The FCI can be expressed below [36]: As listed in Table 1, the FCI of UC22AMPM and UC18AMPM reached maximum values of 1,382,062 smL and 49,140 smL at a concentration of 0.5 wt.%, respectively. Typically, the value of FCI is greater, the foam properties are better [37]. Based on the FCI criterion, 0.5 wt.% as the optimal UC22AMPM and UC18AMPM concentration was used in the following experiments. Furthermore, we also concluded that the 0.5% UC22AMPM has superior foam properties to 0.5% UC18AMPM. To shed light on the reasons behind the difference in properties between UC22AMPM and UC18AMPM foams at their optimum concentration, the foam evolution process, liquid content () of aqueous foams (the ratio of the liquid volume to the foam volume) and continuous phase viscosity () of foam bulk phase were studied. As shown in Figure 2, the geometry of the bubble is spherical for both cases at the initial moment (30 s). For UC22AMPM foams, there was virtually no change in the bubble morphology as time progressed. In contrast, the bubbles in UC18AMPM aqueous foams evolved quickly into irregular polyhedral over time. At the 540th second, a substantial number of bubbles of UC18AMPM aqueous foams disappeared, indicative of foam bursting. In principle, the  To shed light on the reasons behind the difference in properties between UC 22 AMPM and UC 18 AMPM foams at their optimum concentration, the foam evolution process, liquid content (ϕ) of aqueous foams (the ratio of the liquid volume to the foam volume) and continuous phase viscosity (η) of foam bulk phase were studied. As shown in Figure 2, the geometry of the bubble is spherical for both cases at the initial moment (30 s). For UC 22 AMPM foams, there was virtually no change in the bubble morphology as time progressed. In contrast, the bubbles in UC 18 AMPM aqueous foams evolved quickly into irregular polyhedral over time. At the 540th second, a substantial number of bubbles of UC 18 AMPM aqueous foams disappeared, indicative of foam bursting. In principle, the bubble shape is dependent on the ϕ of the aqueous foam [8]. In the case of high ϕ in the aqueous foams, the bubbles are uniformly spherical and densely packed. Decreasing the ϕ causes bubble deformation and the formation of defined edges. Therefore, we can conclude that the ϕ of UC 22 AMPM foams remain constant for 540 s, indicative of slow drainage. In the case of UC 18 AMPM foams, the faster bubble deformation could be interpreted by the rapid lowering of ϕ, resulting from the acceleration of the drainage process. From optical visualization, we could draw a conclusion that the foam drainage process of UC 22 AMPM foams is weaker than that of UC 18 AMPM foams. aqueous foams, the bubbles are uniformly spherical and densely packed. Decreasing the  causes bubble deformation and the formation of defined edges. Therefore, we can conclude that the  of UC22AMPM foams remain constant for 540 s, indicative of slow drainage. In the case of UC18AMPM foams, the faster bubble deformation could be interpreted by the rapid lowering of , resulting from the acceleration of the drainage process. From optical visualization, we could draw a conclusion that the foam drainage process of UC22AMPM foams is weaker than that of UC18AMPM foams. The variation in the  as a function of time is shown in Figure 3A. Evidently, the  in both cases increased significantly over time during the generation process of aqueous foams, reaching a maximum liquid content (m) on completion of foaming. Comparatively speaking, the m of the 0.5% UC22AMPM-CO2 aqueous foams was about 24.7%, greater than that of the 0.5% UC18AMPM-CO2 aqueous foams (10.6%). The lower m is associated with its Vmax (145 mL), indicative of the inferior foaming ability of UC18AMPM. As is wellknown, the foamability is positively proportional to the C-W IFT () of the surfactant solution, which can be described by using the previously reported [38]: here W and A stand for external energy applied to generate the foam and the foam area created, respectively. For a fixed W, the higher the  is, the lower the Vmax will be. On the basis of a previous study by Feng et al. [39], with the identical head group, the  increases with the decrease in the hydrophobic chain length. One can conclude that the  of the UC18AMPM-CO2 solution is higher than that of the UC22AMPM counterpart due to its shorter alkyl chain. Thus, the UC18AMPM-CO2 solution presents poor foamability as compared with the UC22AMPM counterpart. The variation in the ϕ as a function of time is shown in Figure 3A. Evidently, the ϕ in both cases increased significantly over time during the generation process of aqueous foams, reaching a maximum liquid content (ϕ m ) on completion of foaming. Comparatively speaking, the ϕ m of the 0.5% UC 22 AMPM-CO 2 aqueous foams was about 24.7%, greater than that of the 0.5% UC 18 AMPM-CO 2 aqueous foams (10.6%). The lower ϕ m is associated with its V max (145 mL), indicative of the inferior foaming ability of UC 18 AMPM. As is well-known, the foamability is positively proportional to the C-W IFT (γ) of the surfactant solution, which can be described by using the previously reported [38]: here W and A stand for external energy applied to generate the foam and the foam area created, respectively. For a fixed W, the higher the γ is, the lower the V max will be. On the basis of a previous study by Feng et al. [39], with the identical head group, the γ increases with the decrease in the hydrophobic chain length. One can conclude that the γ of the UC 18 AMPM-CO 2 solution is higher than that of the UC 22 AMPM counterpart due to its shorter alkyl chain. Thus, the UC 18 AMPM-CO 2 solution presents poor foamability as compared with the UC 22 AMPM counterpart. Upon CO 2 sparging cease, the ϕ reduced gradually with time because of the drainage. It can be seen that the UC 18 AMPM-CO 2 aqueous foams drained in the 200s to ϕ = 0, while the ϕ of UC 22 AMPM-CO 2 aqueous foam was 20% in this period ( Figure 3A), demonstrating that the drainage from UC 18 AMPM-CO 2 aqueous foams is faster than that of UC 22 AMPM-CO 2 aqueous solution.
The rheological results demonstrated the UC 22 AMPM dispersion saturated with CO 2 attained very high values of zero-shear viscosity η o (3.75 × 10 4 mPa·s) and showed shearthinning behavior ( Figure 3B). The high magnitude of η o mirrors the presence of entangled wormlike micelles in solution [40,41]. In contrast, the η o for UC 18 AMPM samples was onlỹ 1.0 mPa·s ( Figure 3B), reflecting the absence of wormlike micelles. Numerous studies have established that drainage velocity (V) should vary inversely with the viscosity of the continuous phase (η), as the following equation [42]: where ∆P film stands for the difference in pressure between the film center and border, h f refer to the thickness of the thin film. Using Equation (3), one can conclude that the V of UC 18 AMPM-CO 2 aqueous foams is four orders of magnitude greater than that of UC 22 AMPM-CO 2 aqueous foams, consistent with our earlier conclusion ( Figure 2). The consequence of faster drainage is that the ϕ decreases rapidly, concomitant with the reduction in film thickness. The thin films tend to rupture, leading to rapid foam destruction. As a result, UC 18 AMPM-CO 2 aqueous foams show a t 1/2 of 445 s, which is much shorter relative to UC 22 AMPM-CO 2 aqueous foams (9750 s) in identical conditions.
Molecules 2023, 28, 2567 6 of 16 Upon CO2 sparging cease, the  reduced gradually with time because of the drainage. It can be seen that the UC18AMPM-CO2 aqueous foams drained in the 200s to  = 0, while the  of UC22AMPM-CO2 aqueous foam was 20% in this period ( Figure 3A), demonstrating that the drainage from UC18AMPM-CO2 aqueous foams is faster than that of UC22AMPM-CO2 aqueous solution.
The rheological results demonstrated the UC22AMPM dispersion saturated with CO2 attained very high values of zero-shear viscosity ηo (3.75 × 10 4 mPa·s) and showed shearthinning behavior ( Figure 3B). The high magnitude of ηo mirrors the presence of entangled wormlike micelles in solution [40,41]. In contrast, the ηo for UC18AMPM samples was only 1.0 mPa·s ( Figure 3B), reflecting the absence of wormlike micelles. Numerous studies have established that drainage velocity (V) should vary inversely with the viscosity of the continuous phase (), as the following equation [42]: where ΔPfilm stands for the difference in pressure between the film center and border, hf refer to the thickness of the thin film. Using Equation (3), one can conclude that the V of UC18AMPM-CO2 aqueous foams is four orders of magnitude greater than that of UC22AMPM-CO2 aqueous foams, consistent with our earlier conclusion ( Figure 2). The consequence of faster drainage is that the  decreases rapidly, concomitant with the reduction in film thickness. The thin films tend to rupture, leading to rapid foam destruction. As a result, UC18AMPM-CO2 aqueous foams show a t1/2 of 445 s, which is much shorter relative to UC22AMPM-CO2 aqueous foams (9750 s) in identical conditions. According to the aforementioned results, we attributed the differences in performance between UC22AMPM and UC18AMPM foams to their viscosity discrepancy, rooted in the different assembled structures of UC22AMPM and UC18AMPM. More specifically, UC22AMPM with 0.5% concentration can self-assemble into wormlike micelles, but UC18AMPM cannot. For the UC22AMPM system, the entangled worm-like micelles impart high viscosity to the foam continuous phase. During the foaming process, a large amount According to the aforementioned results, we attributed the differences in performance between UC 22 AMPM and UC 18 AMPM foams to their viscosity discrepancy, rooted in the different assembled structures of UC 22 AMPM and UC 18 AMPM. More specifically, UC 22 AMPM with 0.5% concentration can self-assemble into wormlike micelles, but UC 18 AMPM cannot. For the UC 22 AMPM system, the entangled worm-like micelles impart high viscosity to the foam continuous phase. During the foaming process, a large amount of liquid was transported into the foam liquid channels, forming thick foam films. The thick films would increase the thermal activation energy barrier against coalescence and Ostwald ripening. More important, the drainage is retarded by high η. Overall, high continuous phase viscosity retarded the three types of foam destabilization processes simultaneously, thereby enhancing the stability of foams. In contrast, the UC 18 AMPM behaved as a low η Newtonian fluid due to the absence of wormlike micelles, leading to the formation relatively thin foam film. Furthermore, lamellae films drained rapidly due to the low η of the aqueous phase. The consequence of faster drainage is that the foam film becomes thinner and prone to rupture, leading to foam destruction. Therefore, the UC 18 AMPM-CO 2 solution presents poor foam properties as compared with the UC 22 AMPM counterpart.

A Comparison of the Foams Switchability
We previously demonstrated the aqueous foams stabilized by UC 22 AMPM could be turned "on" and "off" on demand through the bubbling of CO 2 or adding NH 3 ·H 2 O. It is essential to examine the switchability of the UC 18 AMPM-CO 2 foam and to make a comparison with the UC 22 AMPM ones. The pressure and temperature are constant at 3 MPa and 80 • C, respectively, to ensure that the above two compounds can be protonated again after the neutralization of NH 3 ·H 2 O.  Figure 4 depicts the parallel variations of V max and t 1/2 of both CO 2 foam systems after the alternating addition of NH 3 ·H 2 O and CO 2 . It was apparent that the t 1/2 rose or declined accordingly with the alternative introduction of CO 2 and NH 3 ·H 2 O, suggesting the foam lifetime of both foam systems can be reversibly tuned. This finding proved that CO 2 aqueous foams prepared from UC 18 AMPM feature switchability similar to UC22AMPM, resulting from their identical hydrophilic headgroups. As shown in Scheme 2, both UC 22 AMPM and UC 18 AMPM in water can be protonated into cationic surfactants after sparging CO 2 , lowering C-W IFT by adsorbing at the CO 2 /water interface and thereby promoting foam formation. Upon NH 3 ·H 2 O addition, protonated surfactant converted to a surface-inactive neutral form. Consequently, UC 22 AMPM and UC 18 AMPM would desorb from the CO 2 /water interface, disrupting the foam film and thereby leading to rapid foam destabilization.

Comparison of the Effect of External Factors on Foam Properties
It has been recognized that external factors such as temperature, pressure and salinity can significantly affect the foam properties [21]. In the following subsections, the influence of these external factors on the properties of the above two CO2 aqueous foams was investigated comparatively using an HTHP visualization foam meter.

Effect of Temperature
To examine the impact of temperature on the CO2 aqueous foams made with UC22AMPM or UC18AMPM, t1/2 and Vmax were determined in a temperature range of 25-120 C at a constant pressure of 3 MPa. As shown in Figure 5A, the Vmax of both foams systems increased slightly with the temperature elevated, meaning that the increment of temperatures improves the foaming ability.
Compared in Figure 5B are the changes in t1/2 for the above two aqueous foams systems at different temperatures. Both foam systems displayed similar evolution trends, i.e., the t1/2 diminished steeply with the elevation of temperature, demonstrating that increased temperature would deteriorate foam stability. Many studies have revealed the elevating temperature resulted in increased C-W IFT [22] and decreased  [40] at constant pressure. Therefore, the foam destabilization accelerates with increasing temperature as a consequence of the higher C-W IFT and lower , leading to poor foam stability.
Note also that the t1/2 of UC22AMPM-CO2 aqueous foams is greater than that of UC18AMPM-CO2 aqueous foams within the studied temperature scope, signifying that the CO2 aqueous foam stabilized by UC22AMPM exhibits better temperature resistance compared to UC18AMPM foams. In addition, the t1/2 of UC22AMPM-CO2 aqueous foams diminished by 5.3 fold when temperature increased from 25 to 120 C, smaller than that of the UC18AMPM-CO2 aqueous foams (9 fold), illustrating the impact of temperature on the stability of UC18AMPM-CO2 aqueous foams is more prevalent related to UC22AMPM. One explanation here could be that the Pc is higher than that of UC18AMPM due to its relatively lower . Notably, the V max of UC 22 AMPM-CO 2 foams initially remained constant and then gradually declined as the cycle number increased ( Figure 4A), demonstrating foamability weakening. By comparison, the V max of UC 18 AMPM-CO 2 foams gradually boosted as the foaming/defoaming cycle number increased ( Figure 4B), indicative of enhanced foamability. On the other hand, the t 1/2 of both CO 2 foam systems decreased as the number of foaming/defoaming cycles increased ( Figure 4A,B), indicating that foam stability deteriorated as the cycle number increased. A similar result was observed in our earlier studies, arising from the accumulation of by-products (a mixture of ammonium carbonate and bicarbonate) [21].
Molecules 2023, 28, 2567 7 of 16 tinuous phase viscosity retarded the three types of foam destabilization processes simultaneously, thereby enhancing the stability of foams. In contrast, the UC18AMPM behaved as a low  Newtonian fluid due to the absence of wormlike micelles, leading to the formation relatively thin foam film. Furthermore, lamellae films drained rapidly due to the low  of the aqueous phase. The consequence of faster drainage is that the foam film becomes thinner and prone to rupture, leading to foam destruction. Therefore, the UC18AMPM-CO2 solution presents poor foam properties as compared with the UC22AMPM counterpart.

A Comparison of the Foams Switchability
We previously demonstrated the aqueous foams stabilized by UC22AMPM could be turned "on" and "off" on demand through the bubbling of CO2 or adding NH3H2O. It is essential to examine the switchability of the UC18AMPM-CO2 foam and to make a comparison with the UC22AMPM ones. The pressure and temperature are constant at 3 MPa and 80 C, respectively, to ensure that the above two compounds can be protonated again after the neutralization of NH3H2O. Figure 4 depicts the parallel variations of Vmax and t1/2 of both CO2 foam systems after the alternating addition of NH3H2O and CO2. It was apparent that the t1/2 rose or declined accordingly with the alternative introduction of CO2 and NH3H2O, suggesting the foam lifetime of both foam systems can be reversibly tuned. This finding proved that CO2 aqueous foams prepared from UC18AMPM feature switchability similar to UC22AMPM, resulting from their identical hydrophilic headgroups. As shown in Scheme 2, both UC22AMPM and UC18AMPM in water can be protonated into cationic surfactants after sparging CO2, lowering C-W IFT by adsorbing at the CO2/water interface and thereby promoting foam formation. Upon NH3H2O addition, protonated surfactant converted to a surface-inactive neutral form. Consequently, UC22AMPM and UC18AMPM would desorb from the CO2/water interface, disrupting the foam film and thereby leading to rapid foam destabilization. Notably, the Vmax of UC22AMPM-CO2 foams initially remained constant and then gradually declined as the cycle number increased ( Figure 4A), demonstrating foamability weakening. By comparison, the Vmax of UC18AMPM-CO2 foams gradually boosted as the foaming/defoaming cycle number increased ( Figure 4B), indicative of enhanced foamability. On the other hand, the t1/2 of both CO2 foam systems decreased as the number of foam-

Comparison of the Effect of External Factors on Foam Properties
It has been recognized that external factors such as temperature, pressure and salinity can significantly affect the foam properties [21]. In the following subsections, the influence of these external factors on the properties of the above two CO 2 aqueous foams was investigated comparatively using an HTHP visualization foam meter.

Effect of Temperature
To examine the impact of temperature on the CO 2 aqueous foams made with UC 22 AMPM or UC 18 AMPM, t 1/2 and V max were determined in a temperature range of 25-120 • C at a constant pressure of 3 MPa. As shown in Figure 5A, the V max of both foams systems increased slightly with the temperature elevated, meaning that the increment of temperatures improves the foaming ability.

Effect of Pressure
As observed in Figure 6A,B, the Vmax and t1/2 for both samples increased with the increasing pressure, demonstrating that increasing pressure is conducive to foaming ability and foam stability. The finding is consistent with previous studies [21,43,44] attributed to the decrease in the C−W IFT with the pressure increasing. Specifically, high pressure enhances the interactions between CO2 and the hydrophobic tail of surfactant molecules, reducing the contact probability between CO2 and water molecules and thus generating a lower C-W IFT [44]. Clearly, a lower C-W IFT enables the foam to easier form and to mitigate the foam aging process.   Figure 5B are the changes in t 1/2 for the above two aqueous foams systems at different temperatures. Both foam systems displayed similar evolution trends, i.e., the t 1/2 diminished steeply with the elevation of temperature, demonstrating that increased temperature would deteriorate foam stability. Many studies have revealed the elevating temperature resulted in increased C-W IFT [22] and decreased η [40] at constant pressure. Therefore, the foam destabilization accelerates with increasing temperature as a consequence of the higher C-W IFT and lower η, leading to poor foam stability.

Compared in
Note also that the t 1/2 of UC 22 AMPM-CO 2 aqueous foams is greater than that of UC 18 AMPM-CO 2 aqueous foams within the studied temperature scope, signifying that the CO 2 aqueous foam stabilized by UC 22 AMPM exhibits better temperature resistance compared to UC 18 AMPM foams. In addition, the t 1/2 of UC 22 AMPM-CO 2 aqueous foams diminished by 5.3 fold when temperature increased from 25 to 120 • C, smaller than that of the UC 18 AMPM-CO 2 aqueous foams (~9 fold), illustrating the impact of temperature on the stability of UC 18 AMPM-CO 2 aqueous foams is more prevalent related to UC 22 AMPM. One explanation here could be that the P c is higher than that of UC 18 AMPM due to its relatively lower ϕ.

Effect of Pressure
As observed in Figure 6A,B, the V max and t 1/2 for both samples increased with the increasing pressure, demonstrating that increasing pressure is conducive to foaming ability and foam stability. The finding is consistent with previous studies [21,43,44] attributed to the decrease in the C−W IFT with the pressure increasing. Specifically, high pressure enhances the interactions between CO 2 and the hydrophobic tail of surfactant molecules, reducing the contact probability between CO 2 and water molecules and thus generating a lower C-W IFT [44]. Clearly, a lower C-W IFT enables the foam to easier form and to mitigate the foam aging process.

Effect of Pressure
As observed in Figure 6A,B, the Vmax and t1/2 for both samples increased with the increasing pressure, demonstrating that increasing pressure is conducive to foaming ability and foam stability. The finding is consistent with previous studies [21,43,44] attributed to the decrease in the C−W IFT with the pressure increasing. Specifically, high pressure enhances the interactions between CO2 and the hydrophobic tail of surfactant molecules, reducing the contact probability between CO2 and water molecules and thus generating a lower C-W IFT [44]. Clearly, a lower C-W IFT enables the foam to easier form and to mitigate the foam aging process.  Interestingly, the increased scope of V max of both foams showed a similar variation tendency with increasing pressure. The V max for UC 22 AMPM-CO 2 aqueous foam increased from 70 to 230 mL at the tested pressures scope; the UC 18 AMPM-CO 2 aqueous foam increased from 54 and 150 mL under identical conditions. Their V max increased by approximately three times, suggesting the effect of pressure on the foaming ability of both compounds is identical. Instead, the t 1/2 for UC 22 AMPM-CO 2 aqueous foam increased from 3200 and 12,400 s, showing a faint increase; while the t 1/2 of UC 18 AMPM-CO 2 aqueous foam underwent a slight increase from 1000 to 2200 s. The growth fold of t 1/2 for UC 22 AMPM-CO 2 aqueous foam is around 3.9, higher than that of UC 18 AMPM ones (2.2). These results highlighted that pressure is more prominent in enhancing the stability of UC 22 AMPM-CO 2 aqueous foam compared with that of UC 18 AMPM-CO 2 aqueous foam.

Effect of Salinity
Inorganic salts have been found to modulate the surface activities [45], altering the properties of the surfactant-stabilized foam [43]. Hence, a common sodium chloride (NaCl) was used as representative inorganic salt to add the above two foam systems to clarify the effect of salt on the properties of UC 22 AMPM and UC 18 AMPM CO 2 aqueous foams.
As depicted in Figure 7A, the V max of both foams samples increased initially and then maintained constant with increasing NaCl concentration. For example, the UC 18 AMPM foam expanded from 151 and 175 mL when NaCl concentration increased from 0 to 1 wt.%; while the UC 22 AMPM foam slightly grew from 189 and 199 mL by increasing NaCl concentration from 0 to 0.5 wt.%. This means that the addition of a small amount of NaCl is beneficial for foamability. A plausible explanation could be that the addition of NaCl enhanced the adsorption of surfactant molecules at the C-W interface as a result of the charge neutralization, leading to the reducing C-W IFT, and thereby improving foaming ability [46]. Thereafter, the V max of both samples remained virtually constant with a further increase in NaCl concentration. We believe that electrostatic repulsions between surfactants are sufficiently shielded at high NaCl content (≥1.0 wt.%). In this scenario, the surfactants were saturated in CO 2 /water interfaces, and the C-W IFT achieved a minimum value. Consequently, high NaCl concentrations have a negligible effect on foamability. 023, 28, 2567 11 of 16

Materials
UC18AMPM and UC22AMPM were synthesized according to our previously-reported procedure [39] and confirmed by proton nuclear magnetic resonance spectroscopy ( 1 H NMR, Figures 8 and 9). CO2 (≥99.998%) was purchased from Jinnengda Gas Company (Chengdu, China) and was used as received. Sodium chloride (NaCl, 99%, GC) and NH3H2O (25 Vol.%) were purchased from Chengdu Kelong Chemical Factory Co., Ltd. (China). CD3Cl (≥98% deuterium content) used for 1 H NMR analysis was obtained from Sigma-Aldrich (Shanghai, China). The deionized water with a resistivity of 18.25 MΩcm used throughout this study was prepared from a quartz water purification system (UPH-I-10T, Chengdu Ultra-pure Technology Co., Ltd., Chengdu, China).  Compared in Figure 7B is the t 1/2 for two cases of CO 2 aqueous foams as a function of NaCl concentration. Overall, the t 1/2 of the UC 22 AMPM foam samples showed a downtrend at the tested NaCl concentrations, manifesting that the addition of NaCl undermined the foam stability of UC 22 AMPM. This can be interpreted with the fact that the additional NaCl causes a transformation from linear to branched micelles, leading to a decrease in η [47,48]. Upon the decrease in η, the foam aging process would speed up, leading to rapid foam destruction. As for CO 2 aqueous foams made from UC 18 AMPM, t 1/2 gradually increased and then remain unchanged with increasing salinity. We also attributed this enhanced t 1/2 to the fact that the presence of NaCl enhances the adsorption density of surfactant molecules on the CO 2 /water interface through electrostatic screening, enhancing the strength of foam lamella and therefore resisting gas diffusion between bubbles.
It is also noteworthy that the t 1/2 of UC 22 AMPM-CO 2 aqueous foams is higher than that of UC 18 AMPM-CO 2 aqueous foams within the studied salinity scope, signifying that the CO 2 aqueous foam stabilized by UC 22 AMPM exhibits better salt tolerance compared to UC 18 AMPM foams.

Preparation of Foaming Solution
A concentrated parent dispersion was prepared by adding designed amounts of surfactant samples (UC22AMPM or UC18AMPM) and deionized water to a sealed Schott-Duran bottle equipped with a magnetic bar inside. Next, the resulting mixture was stirred at 60 C for at least 10 min, yielding low-viscosity emulsion-like dispersion. Remarkably, the dispersion concentration was calibrated by adding water to compensate for the water evaporation during the agitation process. The parent dispersions were cooled to room temperature. Then, the dispersions with desired concentration were obtained by diluting the concentrated parent dispersion with deionized water or brine.

Evaluation of Aqueous Foams at Atmospheric Pressure
The FoamScan setup (Figure 10, TECLIS, Lyon, France), which combines image analysis and conductivity measurements to monitor foam properties, was employed to characterize the foam properties of two types of ultra-long chain tertiary amines (i.e., UC22AMPM and UC18AMPM). Briefly, 60 mL of dispersions were placed in the glass column with a porous glass filter (pore diameter 0.2 mm) and heated to the desired temper-

Preparation of Foaming Solution
A concentrated parent dispersion was prepared by adding designed amounts of surfactant samples (UC 22 AMPM or UC 18 AMPM) and deionized water to a sealed Schott-Duran bottle equipped with a magnetic bar inside. Next, the resulting mixture was stirred at 60 • C for at least 10 min, yielding low-viscosity emulsion-like dispersion. Remarkably, the dispersion concentration was calibrated by adding water to compensate for the water evaporation during the agitation process. The parent dispersions were cooled to room temperature. Then, the dispersions with desired concentration were obtained by diluting the concentrated parent dispersion with deionized water or brine.

Evaluation of Aqueous Foams at Atmospheric Pressure
The FoamScan setup (Figure 10, TECLIS, Lyon, France), which combines image analysis and conductivity measurements to monitor foam properties, was employed to characterize the foam properties of two types of ultra-long chain tertiary amines (i.e., UC 22 AMPM and UC 18 AMPM). Briefly, 60 mL of dispersions were placed in the glass column with a porous glass filter (pore diameter 0.2 mm) and heated to the desired temperature by an embedded electric heating system. The pressure of the chamber was fixed at atmospheric pressure. Afterward, aqueous foams were formed by bubbling CO 2 for two minutes. The CO 2 flow rate is constant at 100 mL/min by mass flow meters. The foam volume and liquid content were measured by five pairs of electrodes located along the glass column. The bubbles evolution was captured by the CCD (charge-coupled device) camera after the gas flow stopped.

Evaluation of Aqueous Foams at High Pressure
Given that the FoamScan cannot perform at high pressure, the foam properties under high-pressure conditions were evaluated by an HTHP visualization foam meter (Jiangsu Hongbo Machinery Manufacturing Co., Ltd., Haian China). A detailed description of the HTHP visualization foam meter and operating procedures have been reported in our previous work [6−8]. Firstly, 100 mL dispersions were pumped into the visual chamber and heated to the desired temperature by an embedded electric heating system. The CO2 was then bubbled into the chamber to achieve the desired pressure. Afterward, the surfactant dispersions and CO2 were vigorously stirred at 1100 rpm for 3 min. Once agitation ceased, the Vmax and t1/2 were recorded by observing the foam height. All values were measured three times per experiment, and the average value was taken as the final result.

Characterization of Switchability of Aqueous Foams
To examine the switchability of aqueous foams produced from UC22AMPM and UC18AMPM, the CO2 and NH3·H 2O (25 vol.%) were used as triggers to ''switch'' foam on and off. First, at a 3 MPa CO2 atmosphere, the aqueous foams were generated by the agitation of 100 mL of UC22AMPM and UC18AMPM aqueous dispersion at 1020 rpm for 3 min using an HTHP visualization foam meter, respectively. Subsequently, the appropriate amount of NH3·H 2O was introduced to the CO2 aqueous foam system, during which the foaming and defoaming processes were tracked. This operation was repeated five times, and each cycle was separated by 10 min. All measurements were performed at 80 C.

Rheological Test of Foaming Solution
The rheological measurements of the foaming solution were carried out on a Physica MCR 302 (Anton Paar, Graz, Austria) rotational rheometer equipped with a concentric cylinder geometry CC27. At atmospheric pressure, CO2 was first bubbled into the sample at a flow rate of 200 ± 1 mL/min for 2 min. Then, 16 mL of previously gas-treated sample was introduced to the measuring cell and thermostatically incubated at the desired tem-

Evaluation of Aqueous Foams at High Pressure
Given that the FoamScan cannot perform at high pressure, the foam properties under high-pressure conditions were evaluated by an HTHP visualization foam meter (Jiangsu Hongbo Machinery Manufacturing Co., Ltd., Haian China). A detailed description of the HTHP visualization foam meter and operating procedures have been reported in our previous work [6][7][8]. Firstly, 100 mL dispersions were pumped into the visual chamber and heated to the desired temperature by an embedded electric heating system. The CO 2 was then bubbled into the chamber to achieve the desired pressure. Afterward, the surfactant dispersions and CO 2 were vigorously stirred at 1100 rpm for 3 min. Once agitation ceased, the V max and t 1/2 were recorded by observing the foam height. All values were measured three times per experiment, and the average value was taken as the final result.

Characterization of Switchability of Aqueous Foams
To examine the switchability of aqueous foams produced from UC 22 AMPM and UC 18 AMPM, the CO 2 and NH 3 ·H 2 O (25 vol.%) were used as triggers to "switch" foam on and off. First, at a 3 MPa CO 2 atmosphere, the aqueous foams were generated by the agitation of 100 mL of UC 22 AMPM and UC 18 AMPM aqueous dispersion at 1020 rpm for 3 min using an HTHP visualization foam meter, respectively. Subsequently, the appropriate amount of NH 3 ·H 2 O was introduced to the CO 2 aqueous foam system, during which the foaming and defoaming processes were tracked. This operation was repeated five times, and each cycle was separated by 10 min. All measurements were performed at 80 • C.

Rheological Test of Foaming Solution
The rheological measurements of the foaming solution were carried out on a Physica MCR 302 (Anton Paar, Graz, Austria) rotational rheometer equipped with a concentric cylinder geometry CC27. At atmospheric pressure, CO 2 was first bubbled into the sample at a flow rate of 200 ± 1 mL/min for 2 min. Then, 16 mL of previously gas-treated sample was introduced to the measuring cell and thermostatically incubated at the desired temperature for 20 min prior to experimentation. A solvent trap was used to reduce water evaporation in the experiments. For all experiments, flow curves were registered in a stress-controlled mode, and the data were acquired by the software Rheoplus TM. The temperature was finely controlled by a Peltier temperature control device.

Conclusions
In this work, we investigate comparatively the properties of CO 2 foams stabilized by UC 22 AMPM and UC 18 AMPAM and examined the evolution trend of foam properties concerning variation in external factors (i.e., temperature, pressure and salinity). The results showed that CO 2 aqueous foams prepared from UC 18 AMPM exhibited similar switching properties to UC 22 AMPM, arising from their identical tertiary amine headgroups. However, due to the relatively long hydrophobic chain, UC 22 AMPM molecules self-assembled into wormlike micelles, but UC 18 AMPM cannot. The entanglement of these wormlike micelles into a transient network imparts high viscosity to the continuous phase of foam. During the foaming process, a large amount of liquid was transported into the foam liquid channels, forming the thicker foam film. Meanwhile, the high continuous phase viscosity of the foam system decelerates lamellae drainage. With lower drainage, the lamella remained thicker. The thicker films would enhance foam strength as well as hinder gas diffusion, arresting coalescence and Ostwald ripening, thereby enhancing the foam's lifetime. On the contrary, the viscosity of the UC 18 AMPM sample decreased to~1.0 mPa·s because of the absence of wormlike micelles. The lower viscosity accelerated the drainage process, weakening the strength of the foam film. The reduced strength and thickness of foam film, in turn, led to the bursting of bubbles. As a result, UC 22 AMPM foam displayed better foaming ability and foam stability compared to UC 18 AMPAM foam under identical concentrations. More importantly, for UC 22 AMPM-CO 2 foam, the positive influence derived from pressure and concentration on its foam properties is much more pronounced than those of its UC 18 AMPM counterpart. Compared with UC 18 AMPM-CO 2 foam, the salinity and temperature had a relatively weak negative effect on the properties of UC 22 AMPM-CO 2 foam. In summary, this comparative study advances mechanistic insights into the role of surfactant architecture in foam properties, as well as establishes macroscopic links among foam properties, surfactant structure and environmental factors, promoting the development of such foam systems.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds including UC 22 AMPM and UC 18 AMPM, are available from the authors.