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Foamstability: The interplay between salt-, surfactant- and critical micelle concentration
Journal of Petroleum Science and Engineering 187 (2020) 106871
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Foamstability: The interplay between salt-, surfactant- and critical micelle concentration Talha Majeed a, Theis I. Sølling b, **, Muhammad Shahzad Kamal b, * a b
Politecnico di Torino, Corso Duca degli Abruzzi, 24, Torino, Italy Center for Integrative Petroleum Research, King Fahd University of Petroleum & Minerals, 31261, Dhahran, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Foam CMC AOS EOR Salinity
It is central to understand the underlying factors that affect the generation and stabilization of foam. Never theless, reports on how foam stability is impacted by surfactant concentration, electrolyte concentration, and electrolyte nature are ambiguous. This paper summarizes the foaming properties below and above the critical micelle concentration (CMC) at various electrolyte concentrations. The commonly used surfactant, alpha olefin sulfonate (AOS), was used along with NaCl in a range of concentrations. The foam height, foam volume stability, bubble count, and foam structure were studied to determine the foam behavior at given conditions. The optimum surfactant concentration for the formation of a stable micellar film was found to be just above the CMC value and the foam stability on the addition of salts is related to the surfactant concentration. There exists another critical concentration (not CMC) that is important to determine the impact of salts on the stability of the foam. The addition of NaCl does not have the same impact below and above this critical concentration. If the concentration of surfactant is below the critical concentration, addition of NaCl reduces the half-life and foam stability. However, at higher surfactant concentrations, the presence of NaCl improves the half-life and foam stability. This study clarifies the ambiguities that are present in the literature pertaining to the effect of salt on foam stability and foaming properties.
1. Introduction The use of foam in the oil and gas sector has a long history and the applications are vast (Blauer and Kohlhaas, 1974; Shosa and Schramm, 2001; Turta and Singhal, 2002). In almost all cases, the main purpose of applying foam is to manage mobility of gas in heterogeneous porous media (Bernard et al., 1980; Bond and Holbrook, 1958; Kovscek et al., 1995). Foam injection has become a central enhanced oil recovery (EOR) mechanism with the key goal of improving the volumetric sweep efficiency (Kovscek et al., 1995; Li et al., 2008). In most upstream ap plications, the stability of foam under the harsh conditions of an oil reservoir remains a major challenge. Film thinning, surfactant adsorption on rocks, film elasticity, gas diffusion, coalescence and liquid drainage due to gravity are the key parameters that impact the foam stability (Cantat et al., 2013; Weaire et al., 2001). Liquid drainage due the gravity is the most important subject of study since it affects most of the aforementioned destabilizing factors (Langevin, 2017). A good surfactant should generate plenty of
stable foam to enable its transport through porous media without rupturing. Adsorption of surfactant molecules on reservoir rock reduces surfactant concentration and limits foam generation, which in turn de � creases the propagation distance within a reservoir (Dur� an-Alvarez et al., 2016; Golub et al., 2004; Prieditis and Paulett, 1992). The exis tence of stable foam in the presence of oil is another challenge in a foam-EOR process (Mannhardt et al., 2000). The formation and stability of foam depend upon surfactant con centration, critical micelle concentration, the ability to form of black films (Common and Newton black films) and electrolyte concentration (Apaydin and Kovscek, 2001). The critical micelle concentration (CMC) is the most central parameter in a surfactant-containing solution. At CMC, the surfactant molecules start to accumulate sufficiently to form micelles (Sperandio, 1965; Williams, 1961) and these are key in defining the properties of the foam. Traditionally, a solution with a surfactant concentration above the CMC is employed to obtain optimal foaming properties (Xu et al., 2009). However, salt reduces the CMC (Farajzadeh et al., 2008; Gurkov et al., 2005) and hence affects the foamability. The
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.I. Sølling), [email protected] (M.S. Kamal). https://doi.org/10.1016/j.petrol.2019.106871 Received 9 October 2019; Received in revised form 24 December 2019; Accepted 26 December 2019 Available online 27 December 2019 0920-4105/© 2019 Published by Elsevier B.V.
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Fig. 1. Foam height (a) and maximum foam volume (b) of foam generated using AOS at various concentrations.
a minimum concentration of 35%. AOS is an anionic surfactant and was used as received. The sodium chloride (NaCl, >99%) was purchased from Ottoweg, Darmstadt, Germany. Deionized (DI) water from a Mili-Q purification system was used for the preparation of all solutions.
salt-induced decrease in CMC is appreciated by industry because it re duces the amount of surfactant needed. Formation of common and Newton black films (CBF, NBF) are other important factors that affect the stability of the foam. These films are in a lamellar (bobble wall) thickness range of 4 nm–50 nm, particularly in the presence of elec trolytes (Pugh, 1996). At this thin scale, in addition to van der Waals and electrostatic forces, short-range forces are also responsible for the for mation of such metastable films (Ruckenstein and Manciu, 2002). For thick film (>100 nm) only van der Waal and electrostatic forces contribute to film stability. However, the formation and thickness of these films depend on the type and concentration of surfactants and electrolytes (Cohen et al., 1991). Formation of common or Newton films provides extra foam stability by reducing the lamella rupturing tendency (Ruckenstein and Manciu, 2002). Several published articles reported the effect of salt addition on the foam stability. One group of researchers have found that the addition of salt increases the stability of foam (Behera et al., 2014; Varade and Ghosh, 2017; Xu et al., 2009) while others have found that the salts increase the collapse rate to destabilize the foam (Filippov et al., 2018; Vikingstad et al., 2006; Yekeen et al., 2017). Both Xu et al. and Yekeen et al. used sodium dodecyl sulfate (SDS) surfactant and NaCl salt to determine the impact of salinity on foam stability. Xu et al. reported that the addition of salt above CMC stabilizes the foam. However, Yekeen et al. found that above CMC foam stability reduced on the addition of salt. Thus, it seems, that a clear understanding of the underlying mechanisms that determine the impact of salt on the formation and stabilization of foam is necessary. In this study, we have investigated the foaming behavior of AOS in the presence and absence of NaCl at several surfactant concentrations to understand the discrepancies in the litera ture where only an effect has been considered. Here, we explore as something completely new, how opposing effects at various concentra tion regimes could be in play. We are aware that bulk foam properties are not necessarily exactly reflecting the foaming processes in an oil reservoir as pointed out in several recent studies (Hussain et al., 2019; Solbakken et al., 2014; Xiao et al., 2018) so the present study pertains primarily to the fundamentals of foam in its own right. It is, however, noticed that even leading experts in the field (Hussain et al., 2019) has made attempts to correlate bulk properties of foam with those observed in porous media.
2.2. Sample preparation A batch solution of 1% was prepared from the 35% AOS solution and left overnight. Further samples of concentrations from 0.025% to 1% were prepared by dilution of the 1% batch solution. Each sample was prepared 1 h before the actual experiment. Similarly, a 2 M batch so lution of NaCl was prepared. Stirring for 2 h at room temperature was carried out to ensure complete dissolution of the salt. Further dilutions were performed to reach the required molar concentration (0.01M–1M). 2.3. Surface tension and CMC Surface tensions were measured using an optical interfacial tensi ometer by applying the pendant drop method. All measurements were performed at 23 ̊C and enough time was given to ensure that equilibrium was established. Calibration of the apparatus was done using a small metal bead of known diameter and the surface tension of deionized water measured in this manner was 72.95 dyne/cm. This value com pares well with those that are found in the literature (Harkins and Brown, 1919; Vargaftik et al., 1983). Measurements were conducted two times for each sample and reported as the average; all the measurements were within the spread of 0.45 dyne/cm. Density values, used in the calculation of the surface tension, for each sample were first calculated with a densitometer (Anton Parr DMA 4500) at 23 ̊C. The CMC was determined by plotting surface tension measurements vs log of con centrations, the point at which the slope of isotherm changes abruptly (point of inflection) corresponds to the CMC as shown in Fig. 5. 2.4. Foamability and stability The foaming properties were determined using a Dynamic Foam Analyzer (DFA100) from KRUSS, Germany. All measurements were performed at 23 ̊C and atmospheric pressure. The solution, 50 ml, is poured with the help of a syringe in a transparent measuring column which is located between a linear LED panel and a line sensor. Air is pumped into the liquid from the bottom through a filter to produce foam. The air was injected for 20 s at a flow rate of 0.2 L/min for all experiments. The line sensor measures the light transmitted through the measuring column over its full height. The gas-phase above the foam and the liquid are transparent, while the foam column absorbs some of the emitted light. The two-phase boundaries, liquid/foam and foam/air
2. Experimental 2.1. Material The surfactant used in this study (AOS) was purchased as an aqueous solution from Al Biariq Petrochemical Ind Co Ltd (Riyadh, K.S.A) having 2
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Fig. 2. Foam volume stability with time for different AOS concentrations.
Fig. 3. Foam half-Life as a function of AOS concentration.
are detected using the differences in transmittance. The repeatability (over several days) was tested by running three replicates the minimum variation was found to be 2.2% (for the 1% AOS solution) whereas the maximum was found to be 4.1% (for the 0.25% AOS solution).
of gravitation depends on average vertical velocity (Pugh, 1996). However, this vertical drainage is more important in reasonably thick films, i.e. concentrations greater than CMC. Also, the rupturing phe nomena in a thin layer (less than 100 nm) and in a thick layer (greater than 100 nm) are different. Formation of a common-and a Newton black films gives rise to extra stability to lamina as discussed above. So, there exists an optimum concentration that gives maximum stability and above that concentration, the foam stability again starts to decrease. From these results, we can conclude that there exists an optimum con centration for stable foam which turns out to be 0.075% in this case. Behera (Behera et al., 2014) suggested that the rate of collapse in creases at higher concentrations due to a decrease in lamella elasticity which decreases with an increase in surfactant concentration and leads to fast foam collapse. Elasticity is the capacity of a film to regulate its surface tension when perturbed. High elastic films are more stable than low elastic films (Langevin, 1998; Rusanov et al., 2004). The presence of excessive surfactant molecules in solution at high concentration will result in increased adsorption of surfactant molecules at interfaces. This causes a stunted rate of film thinning and drainage of liquid. Hence, bubble coalescence is reduced and interfacial elasticity is increased because of the stabilization of foam lamellae by molecules of surfactant (Firouzi and Nguyen, 2013). But the increase in foam stability was found only up to a specific AOS concentration. Further increase in surfactant concentration increases the collapse rate of foam (Yekeen et al., 2017). Fig. 3 shows graphical representations of half-life and maximum foam volume for a clear overview of how they change with concentra tion. From these results, it can be easily seen that half-life has a direct relationship with the AOS concentration until 0.075%, and then have an inverse relation with further increase in concentration.
3. Results and discussion 3.1. Effect of AOS concentration on foamability and foam stability Fig. 1 shows the foam height and foam volume generated with respect to time for different concentrations of AOS and thus represents the foamability as a function of AOS concentration. The foam height increases with increasing surfactant concentration. It was observed that 1w% AOS gives rise to the highest initial foam height. The effect of the surfactant concentration on the ability to generate foam is inferred from the maximum foam height at a particular concentration. When more surfactant molecules are available, migration of surfactant molecules towards liquid-gas interface increases, which results in better foam generation (Bournival et al., 2014; Karakashev and Manev, 2003; Yekeen et al., 2017). It is clear from the results that the foam height is directly proportional to the AOS concentration. The same trends have been reported previously for different types of surfactants (Simjoo et al., 2013; Xu et al., 2009; Yekeen et al., 2017). Foam volume stability (FVS) is the gradient of height as a function of time. Similarly, to a height versus time, FVS plot vs time is also used to indicate the stability of the foam. However, FVS over time provides a better measure of the gradient of the plot which is valuable especially if the time interval under study is short, hence it’s a preferred measure of the stability, compared to a simple height vs time plot. Fig. 2 shows the FVS as a function of time. It is noted that the maximum stability is observed at an AOS concentration of 0.075% (just above CMC) whereas 1% AOS gives rise to minimum stability. The change from 0.075% to 1% is not linear. A similar finding was made previously by Yekeen (Yekeen et al., 2017) who found a distinct optimum concentration rather than a linear relation with concentration. The stability increases from 0.01% to reach a maximum at 0.075% and then again starts to decrease gradually to reach the lowest (and apparently constant). Wang et al. (Wang and Chen, 2013; Wang and Mulligan, 2004) related the decrease in stability of foam at higher concentrations (>CMC) with increased weight (grav itational effect) of foam because of excess molecules of surfactant at the lamella. Because of the excess of the molecules of surfactants, the impact of gravitation on the drainage of foam increases, which results in con stant liquid drainage from the film formed between adjacent bubbles, eventually rupturing the foam film to result in bubble coalescence (Baz-Rodríguez et al., 2014). The vertical drainage under the influence
3.2. Foam structure with aging Foam is comprised of bubbles of various sizes. With time, larger bubbles increase in size due to diffusion of gas from smaller adjacent bubbles and the bubbles become polyhedral from the spherical shape. The bubble growth as a function of time is called Ostwald ripening (Stevenson, 2012). Fig. 4(a) shows the images of foam captured by the camera, through the prism, attached at a height of 85 mm with a glass column. It was observed that the size of bubbles at t ¼ 0 decreased as the concentration of surfactant increased from 0.01% to 1%. This is in line with the result that the foam volume increases with surfactant concen tration as discussed in section 3.2 hence the size of the bubble reduces to produce more bubbles (compact foam) in the same area. As time passes, diffusion of gas increases the size of larger bubbles and vanishes smaller bubbles which reduce the bubble count. Due to the increase in the size of 3
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Fig. 4. Foam stability at different surfactant AOS concentrations: (a) Images of bubbles and structure of foam, (b) bubble count (the graphs overlap beyond 10 min), and (c) bubble area.
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bubbles, the film thickness reduces which further increases the rate of diffusion and coalescence to make the foam unstable. Therefore, just like foam height and half-life, bubble count and average bubble size are other parameters to asses the foamability and foam stability, respec tively. Fig. 4(b) shows the bubble count with time and the results are in accordance with images i.e the bubble count increases as the concen tration increases from 0.01% to 1%. Fig. 4(c) shows the average bubble area of foam with time for the same five concentrations. Initially, 1% AOS foam shows the minimum and 0.01% shows the maximum bubble size but with time the increment in the size of bubbles is at a maximum (least stable foam) for 1% AOS foam and is minimum for 0.075% AOS foam. In Fig. 4(c), it can be seen that 0.075% shows the minimum bubble area and 1% AOS solution shows the maximum bubble area. Hence, 0.075% AOS foam has maximum stability. All concentration curves are in accordance with the half-life results obtained above: stability in creases from 0.01% to 0.075% and then again starts to decrease and reach the minimum value at 1% AOS solution. A comparison of 0.01% and 0.075% foam during the first hour of foam decay (Figs. 2 and 4) also shows that foam stability should not be merely judged by FVS and it should always be related to the foam structure. FVS is mainly defined as the fraction of foam volume remaining relative to the maximum foam volume (after foaming has been stopped). FVS is calculated by Equation (1): FVS ðtÞ ¼
Vt ðfoamÞ � 100 Vf ðfoamÞ
Table 1 Surface tension of 0.01% AOS solution at different NaCl concentration. Surface tension (mN/m)
Salt concentration (M)
56.0 32.7 30.3 29.1 28.0
0.00 0.25 0.50 0.75 1.00
AOS). The surface tension and the CMC decrease with increasing the concentration of salt. The value of CMC is reduced from 0.07% to 0.0015% with the addition of 1M NaCl. Fig. 5 represents the surface tension data for AOS and AOSþ1M NaCl solutions. Farajzadeh et al. (2008) also investigated the change in the CMC of AOS by changing the concentration of NaCl salt. According to their findings, the CMC reduces with increasing salt concentration. Similar results were obtained by (Yekeen et al., 2017) who state that a significant reduction in CMC and surface tension of SDS solution was observed with the addition of NaCl. They reported a reduction in the CMC of SDS by an order of magnitude with the addition of 1 w% NaCl to the solution. The surface tension reduces further with an increased NaCl concentration which results in lower CMC values. Chattopadhyay et al. have related the decrease of CMC and surfacetension of surfactant solution to the fact that the salt favors the transition of molecules of surfactant towards the gas-liquid interface in order to reduce the electrostatic repulsion among the charged head of surfactant molecules (Chattopadhyay and Harikumar, 2003; Xu et al., 2009). Such a shielding of repulsive force promotes the hydrophobic strength of surfactant monomers which results in the formation of micelles at lower surfactant concentrations and consequently decreasing the CMC (Muherei and Junin, 2007).
(1)
Vt is the volume at a given time and Vf is the volume of foam when foaming stopped or in other words at the end of the bubbling process. The foam volume is mainly calculated by measuring foam height. Therefore, FVS is calculated only by measuring the volume of foam (that is, a volume in a graded cylinder). However, FVS does not reflect the structure and quality of the foam (it can be wet, dry or anything in be tween). Bubble count and bubble size to a larger extent reflect the quality of the foam and therefore also provide useful information about foam stability. The two measures (volume vs bubble count and size) are not necessarily linearly related: It is oftentimes observed that foam initially decays without a significant change in the foam height but with a change in the bubble count and bubble diameter. This is related to the nature of the foam; and in the case above the foam would be referred to as being more dry. So, in conclusion, one can easily envision a scenario where foam height does not alone reflect stability. This is the case for the 0.01% experiment. Initially, the height/volume of the 0.01% foam was higher as compared to 0.075%. However, the foam generated in the 0.01% case was drier compared to 0.075% foam. As verified by the foam structure analysis in Fig. 4. Although the FVS value initially was higher for 0.01% than for 0.075%, its bubble count was much lower and its bubble diameter much higher. Both bubble count and bubble diameter show that the 0.075% foam stability was actually higher even, to begin with. This all goes to show that the 0.075% foam is more stable compared to the 0.01% foam even though FVS of the 0.01% foam initially attains a high value.
3.4. Effect of salt concentration on foam stability The effects of salt on the foamability and stability of foam are ambiguous. Some have found salts to have negative effects on stability as well as foam generation capability while others have found that salts stabilize foam or have a neutral impact. According to Behera and Varade et al. foam stability, as defined by the proportion of foam that has collapsed with time, increases with increase of salt concentration in solution (Behera et al., 2014; Varade and Ghosh, 2017). Xu et al. studied the effect of NaCl as an electrolyte on the SDS surfactant solution below and above the CMC. The addition of NaCl to an SDS solution improves the foamability and stability of foam to a certain extent. They noticed the decrease in surface tension and zeta-potential with an increase of salt concentration, which as a result reduce the surface charge of SDS mi celles. Hence, the results suggest that the stability of foam and foam ability increase in the presence of salts (Xu et al., 2009). Tan et al. studied the effect of NaCl, MgCl2, and CaCl2 on the foam generated using polypropylene glycol (PPG) (Tan et al., 2005). They suggested that the valence of the ions has a greater effect on the foamability of PPG than salt type. According to Tan et al. less foamability and foam stability in the existence of salt are because of the mitigation of electrostatic repulsion among the charged bubble surfaces. High salt concentration reduces electrostatic repulsion, and therefore foamability. Vikingstad et al. reported the effect of salt on AOS and FS500 in presence and absence of oil (Vikingstad et al., 2006). The brine concentration has little or no effect on the foam height in the absence of oil (Vikingstad et al., 2006). However, in the presence of oil, high ionic strength reduces the foam stability. Farajzadeh et al. (2008) have also analyzed the effect of salt on AOS surfactant foam. They measured the foam film thickness as a function of NaCl and AOS concentrations. They have found a relation between the film thickness and the contact angle of the meniscus with the film. They found the critical AOS and NaCl concentration for the formation of stable films and also for the formation of stable newton
3.3. CMC determination and effect of salt concentration on surface properties It was found that the surface tension of AOS solutions decreases with increasing surfactant concentration. A gradual increase in surfactant concentration gradually reduces surface tension until the CMC is reached at which point the surface tension does not decrease any further. The CMC represents the concentration at which the surfactants still are mainly segregated; at larger concentrations, the monomers aggregate to form micelles. This aggregation alters the physio-chemical properties of the surfactant-containing solution below and above CMC, hence the properties of foam also show most variability away from CMC. In the present case, the CMC was found to be 0.07 w%. The data given in Table 1 shows the effect of NaCl addition on surface tension (at 0.01% 5
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Fig. 5. Surface tension of a 0.01% AOS solution in deionized water and in 1 M salt solution.
black films. Craig et al. (1993) performed a series of experiments to demonstrate the effect of electrolytes on coalescence of foam bubbles. According to their observation, the effect on bubbles coalescence is not consistent for different type of electrolytes as some reduce the coales cence phenomena while other others do not. Hence, hydrophobic interaction with electrolytes could be the reason for the change in coa lescence phenomena due to presence of electrolytes. To investigate the effect of salinity on foaming ability, a series of experiments were performed by keeping the AOS concentration constant (0.25%) while varying the salinity. Fig. 6 shows the effect of varying salt concentration on the foamability of AOS. It can be easily noticed that the height of foam is increasing with the increase of molar concentration of salt. The experiments were only performed for 1 h to screen for the effect on foamability. Similar results were reported in our previous publication where we investigated the impact of NaCl concentration on the foam stability of four in-house synthesized surfactants. These in-house sur factants were cationic gemini surfactants containing mono phenyl ring with Br counterion (12-PhBr-12), mono phenyl ring with Cl coun terion (12-PhCl-12), biphenyl ring with Br counterion (12-BiPhBr-12), and biphenyl ring with Cl counterion (12-BiPhCl-12) (Kalam et al., 2019). For all types of surfactants regardless of the nature of spacer and counterion, the addition of salts increased the foam stability (see Fig. 7). To examine the effects of salinity on foam stability, longer time-scale experiments were performed to ensure that all effects up until the halflife (using 1M NaCl and 0.25% surfactant solution) are captured. Sur prisingly, the results show that the half-life was increased more than twofold. We propose that the presence of salt forms an electrostatic double layer (EDL) within the lamina and that the screening effect of this EDL is the main reason for the decreasing coalescence. This results in stabilizing the foam for an extended period of time because of the for mation of smaller and resistant bubbles which gives the tight packing of the liquid lamella among bubbles as also explained by Xu et al. (2009). The addition of NaCl salt decreases the gas solubility into the solution hence reduces the hydrophobic interaction which as a result increase the foam stability by suppressing the coalescence rate of bubbles (Firouzi and Nguyen, 2014). To assess the impact of surfactant concentration, a range of experi ments were conducted where the salt concentration was kept constant at 1M and AOS concentration was varied. At 0.1% AOS and 1M NaCl
Fig. 6. Foam height with time by increasing salinity at 0.25 w% AOS.
Fig. 7. Effect of salinity on foam volume stability of different surfactants (Kalam et al., 2019).
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appropriate amount of) surfactants at the interface of the film. At low concentrations, the AOS molecules are surrounded by electrolytes which result in decreased film stability. However, at higher concentrations, the surplus surfactant molecules could improve film stability. In the case of higher surfactant concentrations and in the absence of electrolyte, low foam stability can be associated with a high liquid drainage rate due to increased gravitational effect as explained above. In short, it is the balance between surfactant and electrolyte concentration that decides the film stability. 4. Conclusion A systematic study was performed to examine the effect of salt on the stability of surfactant foam. The key purpose of this study was to investigate the apparently contradicting findings that are presented in the literature on foaming characteristics in the presence of salt. Following novel conclusion are drawn: � Foamability increased with increasing AOS concentration. However, the effect is more dominant below CMC and not significant at higher concentrations (>CMC); a result of two opposing effects that hitherto weren’t disclosed. � In the absence of NaCl, an optimum surfactant concentration was found that maximize the stability of foam which was slightly higher than the CMC. The decrease in stability of foam at higher surfactant concentrations is due to the greater influence of gravitational forces. � At constant salinity, the increase in AOS concentration improved foam stability. At some typical AOS concentration (0.25% in this case), the foam stability of AOS solution in presence of NaCl sur passes the foam stability of AOS solution without NaCl. � Salt can destabilize or stabilize the foam depending on the surfactant concentration; this apparently contradicting the result is due to opposing effects of letting the electrolytes surround the AOS mole cules at low vs high concentrations. A new finding that makes up for some of the ambiguity that has previously been dominating the literature.
Fig. 8. Comparison of the half-life of AOS foam with and without salts at four different surfactant concentrations.
solution, the results differ from the findings above in the sense that a lower half-life was observed for the saline solution, and this reduction in the stability becomes more prominent at AOS concentrations lower than 0.1%. Fig. 8 depicts the effect of salt addition at various AOS concen trations. From these results, it can be concluded that salts can stabilize or destabilize the foam depending on surfactant concentration in solution. Above a critical surfactant concentration, salt stabilizes the foam. However, at a lower concentration of surfactants, added salts result in destabilizing the foam. We propose a mechanism to unify these apparently contradicting findings in Fig. 9. When AOS is in demand to form the film, occupying surfactant molecules with electrolytes has a negative effect on film stability (left). Whereas when there is an AOS surplus, it was a positive effect to take the surfactants out of the equation and has a positive effect on film stability, because they will interfere with the (already
Fig. 9. Proposed mechanism of the effect of salt addition on foaming properties. 7
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� When the AOS molecules are in excess, higher stability is achieved. However, when electrolytes are in excess, AOS molecules get completely surrounded by electrolytes which prevent them from entering the micellar structure that make up the foam lamellae – yet another central and novel finding that highlights what can happen when opposing effects are in play.
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Solbakken, J.S., Skauge, A., Aarra, M.G., 2014. Foam performance in low permeability laminated sandstones. Energy Fuels 28, 803–815. https://doi.org/10.1021/ ef402020x. Sperandio, G.J., 1965. Reviews. J. Pharm. Sci. 54, 1227. https://doi.org/10.1002/ jps.2600540838. Stevenson, P., 2012. Foam Engineering, Fundamentals and Applications. Tan, S.N., Fornasiero, D., Sedev, R., Ralston, J., 2005. The role of surfactant structure on foam behaviour. Colloids Surfaces A Physicochem. Eng. Asp. 263, 233–238. https:// doi.org/10.1016/j.colsurfa.2004.12.060. Turta, A.T., Singhal, A.K., 2002. Field foam applications in enhanced oil recovery projects: screening and design aspects. J. Can. Pet. Technol. 41 https://doi.org/ 10.2118/02-10-14. Varade, S.R., Ghosh, P., 2017. Foaming in aqueous solutions of zwitterionic surfactant: effects of oil and salts. J. Dispersion Sci. Technol. 38, 1770–1784. Vargaftik, N.B., Volkov, B.N., Voljak, L.D., 1983. International tables of the surface tension of water. J. 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Xiao, S., Zeng, Y., Vavra, E.D., He, P., Puerto, M., Hirasaki, G.J., Biswal, S.L., 2018. Destabilization, propagation, and generation of surfactant-stabilized foam during crude oil displacement in heterogeneous model porous media. Langmuir 34, 739–749. https://doi.org/10.1021/acs.langmuir.7b02766. Xu, Q., Nakajima, M., Ichikawa, S., Nakamura, N., Roy, P., Okadome, H., Shiina, T., 2009. Effects of surfactant and electrolyte concentrations on bubble formation and stabilization. J. Colloid Interface Sci. 332, 208–214. https://doi.org/10.1016/j. jcis.2008.12.044. Yekeen, N., Manan, M.A., Idris, A.K., Samin, A.M., 2017. Influence of surfactant and electrolyte concentrations on surfactant Adsorption and foaming characteristics. J. Pet. Sci. Eng. 149, 612–622.
Author contributions All authors contributed equally to the present study. Acknowledgment The authors would like to thanks College of Petroleum Engineering & Geosciences, King Fahd University of Petroleum and Minerals, Saudi Arabia for providing the research facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.petrol.2019.106871. References Apaydin, O.G., Kovscek, A.R., 2001. Surfactant concentration and end effects on foam flow in porous media. Transp. Porous Media 43, 511–536. https://doi.org/10.1023/ A:1010740811277. Baz-Rodríguez, S.A., Botello-Alvarez, J.E., Estrada-Baltazar, A., Vilchiz-Bravo, L.E., Padilla-Medina, J.A., Miranda-L� opez, R., 2014. Effect of electrolytes in aqueous solutions on oxygen transfer in gas-liquid bubble columns. Chem. Eng. Res. Des. 92, 2352–2360. https://doi.org/10.1016/j.cherd.2014.02.023. Behera, M.R., Varade, S.R., Ghosh, P., Paul, P., Negi, A.S., 2014. Foaming in micellar solutions: effects of surfactant, salt, and oil concentrations. Ind. Eng. Chem. Res. 53, 18497–18507. Bernard, G.C., Holm, L.W., Harvey, C.P., 1980. Use of surfactant to reduce CO2 mobility in oil displacement. Soc. Pet. Eng. J. 20, 281–292. https://doi.org/10.2118/8370PA. Blauer, R.E., Kohlhaas, C.A., 1974. . Bond, D.C., Holbrook, O.C., 1958. Gas Drive Oil Recovery Process. US Patent, No. 2866507 2–4. Bournival, G., Du, Z., Ata, S., Jameson, G.J., 2014. Foaming and gas dispersion properties of non-ionic surfactants in the presence of an inorganic electrolyte. Chem. Eng. Sci. 116, 536–546. https://doi.org/10.1016/j.ces.2014.05.011. Cantat, I., Cohen-Addad, S., Elias, F., Graner, F., Hohler, R., Pitois, O., Rouyer, F., SaintJalmes, A., 2013. Foams: Structure and Dynamics. Chattopadhyay, A., Harikumar, K.G., 2003. FEBS lett 1996 Chattopadhyay, 391, 1–4. Cohen, R., Koynova, R., Tenchov, B., Exerowa, D., 1991. Direct measurement of interaction forces in free thin liquid films stabilized with phosphatidylcholine. Eur. Biophys. J. 20, 203–208. https://doi.org/10.1007/BF00183456. Craig, V.S.J., Ninham, B.W., Pashley, R.M., 1993. The effect of electrolytes on bubble coalescence in water. J. Phys. Chem. 97, 10192–10197. https://doi.org/10.1021/ j100141a047. � Dur� an-Alvarez, A., Maldonado-Domínguez, M., Gonz� alez-Antonio, O., Dur� an� Valencia, C., Romero-Avila, M., Barrag� an-Aroche, F., L� opez-Ramírez, S., 2016. Experimental-theoretical approach to the adsorption mechanisms for anionic, cationic, and zwitterionic surfactants at the calcite-water interface. Langmuir 32, 2608–2616. https://doi.org/10.1021/acs.langmuir.5b04151. Farajzadeh, R., Krastev, R., Zitha, P.L.J., 2008. Foam films stabilized with alpha olefin sulfonate (AOS). Colloids Surfaces A Physicochem. Eng. Asp. 324, 35–40. https:// doi.org/10.1016/j.colsurfa.2008.03.024. Filippov, L.O.O., Javor, Z., Piriou, P., Filippova, I.V.V., 2018. Salt effect on gas dispersion in flotation column – bubble size as a function of turbulent intensity. Miner. Eng. 127, 6–14. https://doi.org/10.1016/j.mineng.2018.07.017. Firouzi, M., Nguyen, A.V., 2014. Effects of monovalent anions and cations on drainage and lifetime of foam films at different interface approach speeds q. Adv. Powder Technol. 25, 1212–1219. https://doi.org/10.1016/j.apt.2014.06.004. Firouzi, M., Nguyen, A.V., 2013. Different effects of monovalent anions and cations on the bubble coalescence and lifetime of aqueous films between air bubbles. In: Chemeca 2013: Challenging Tomorrow. Golub, T.P., Koopal, L.K., Sidorova, M.P., 2004. Adsorption of cationic surfactants on silica surface: 1. Adsorption isotherms and surface charge. Colloid J. 66, 38–43. https://doi.org/10.1023/B:COLL.0000015053.71438.fd. Gurkov, T.D., Dimitrova, D.T., Marinova, K.G., Bilke-Crause, C., Gerber, C., Ivanov, I.B., 2005. Ionic surfactants on fluid interfaces: determination of the adsorption; role of the salt and the type of the hydrophobic phase. Colloids Surfaces A Physicochem. Eng. Asp. 261, 29–38. https://doi.org/10.1016/j.colsurfa.2004.11.040. Harkins, W.D., Brown, F.E., 1919. The determination of surface tension (free surface energy), and the weight of falling drops: the surface tension of water and benzene by
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Journal of Petroleum Science and Engineering journal homepage: http://www.elsevier.com/locate/petrol
Foamstability: The interplay between salt-, surfactant- and critical micelle concentration Talha Majeed a, Theis I. Sølling b, **, Muhammad Shahzad Kamal b, * a b
Politecnico di Torino, Corso Duca degli Abruzzi, 24, Torino, Italy Center for Integrative Petroleum Research, King Fahd University of Petroleum & Minerals, 31261, Dhahran, Saudi Arabia
A R T I C L E I N F O
A B S T R A C T
Keywords: Foam CMC AOS EOR Salinity
It is central to understand the underlying factors that affect the generation and stabilization of foam. Never theless, reports on how foam stability is impacted by surfactant concentration, electrolyte concentration, and electrolyte nature are ambiguous. This paper summarizes the foaming properties below and above the critical micelle concentration (CMC) at various electrolyte concentrations. The commonly used surfactant, alpha olefin sulfonate (AOS), was used along with NaCl in a range of concentrations. The foam height, foam volume stability, bubble count, and foam structure were studied to determine the foam behavior at given conditions. The optimum surfactant concentration for the formation of a stable micellar film was found to be just above the CMC value and the foam stability on the addition of salts is related to the surfactant concentration. There exists another critical concentration (not CMC) that is important to determine the impact of salts on the stability of the foam. The addition of NaCl does not have the same impact below and above this critical concentration. If the concentration of surfactant is below the critical concentration, addition of NaCl reduces the half-life and foam stability. However, at higher surfactant concentrations, the presence of NaCl improves the half-life and foam stability. This study clarifies the ambiguities that are present in the literature pertaining to the effect of salt on foam stability and foaming properties.
1. Introduction The use of foam in the oil and gas sector has a long history and the applications are vast (Blauer and Kohlhaas, 1974; Shosa and Schramm, 2001; Turta and Singhal, 2002). In almost all cases, the main purpose of applying foam is to manage mobility of gas in heterogeneous porous media (Bernard et al., 1980; Bond and Holbrook, 1958; Kovscek et al., 1995). Foam injection has become a central enhanced oil recovery (EOR) mechanism with the key goal of improving the volumetric sweep efficiency (Kovscek et al., 1995; Li et al., 2008). In most upstream ap plications, the stability of foam under the harsh conditions of an oil reservoir remains a major challenge. Film thinning, surfactant adsorption on rocks, film elasticity, gas diffusion, coalescence and liquid drainage due to gravity are the key parameters that impact the foam stability (Cantat et al., 2013; Weaire et al., 2001). Liquid drainage due the gravity is the most important subject of study since it affects most of the aforementioned destabilizing factors (Langevin, 2017). A good surfactant should generate plenty of
stable foam to enable its transport through porous media without rupturing. Adsorption of surfactant molecules on reservoir rock reduces surfactant concentration and limits foam generation, which in turn de � creases the propagation distance within a reservoir (Dur� an-Alvarez et al., 2016; Golub et al., 2004; Prieditis and Paulett, 1992). The exis tence of stable foam in the presence of oil is another challenge in a foam-EOR process (Mannhardt et al., 2000). The formation and stability of foam depend upon surfactant con centration, critical micelle concentration, the ability to form of black films (Common and Newton black films) and electrolyte concentration (Apaydin and Kovscek, 2001). The critical micelle concentration (CMC) is the most central parameter in a surfactant-containing solution. At CMC, the surfactant molecules start to accumulate sufficiently to form micelles (Sperandio, 1965; Williams, 1961) and these are key in defining the properties of the foam. Traditionally, a solution with a surfactant concentration above the CMC is employed to obtain optimal foaming properties (Xu et al., 2009). However, salt reduces the CMC (Farajzadeh et al., 2008; Gurkov et al., 2005) and hence affects the foamability. The
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T.I. Sølling), [email protected] (M.S. Kamal). https://doi.org/10.1016/j.petrol.2019.106871 Received 9 October 2019; Received in revised form 24 December 2019; Accepted 26 December 2019 Available online 27 December 2019 0920-4105/© 2019 Published by Elsevier B.V.
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Fig. 1. Foam height (a) and maximum foam volume (b) of foam generated using AOS at various concentrations.
a minimum concentration of 35%. AOS is an anionic surfactant and was used as received. The sodium chloride (NaCl, >99%) was purchased from Ottoweg, Darmstadt, Germany. Deionized (DI) water from a Mili-Q purification system was used for the preparation of all solutions.
salt-induced decrease in CMC is appreciated by industry because it re duces the amount of surfactant needed. Formation of common and Newton black films (CBF, NBF) are other important factors that affect the stability of the foam. These films are in a lamellar (bobble wall) thickness range of 4 nm–50 nm, particularly in the presence of elec trolytes (Pugh, 1996). At this thin scale, in addition to van der Waals and electrostatic forces, short-range forces are also responsible for the for mation of such metastable films (Ruckenstein and Manciu, 2002). For thick film (>100 nm) only van der Waal and electrostatic forces contribute to film stability. However, the formation and thickness of these films depend on the type and concentration of surfactants and electrolytes (Cohen et al., 1991). Formation of common or Newton films provides extra foam stability by reducing the lamella rupturing tendency (Ruckenstein and Manciu, 2002). Several published articles reported the effect of salt addition on the foam stability. One group of researchers have found that the addition of salt increases the stability of foam (Behera et al., 2014; Varade and Ghosh, 2017; Xu et al., 2009) while others have found that the salts increase the collapse rate to destabilize the foam (Filippov et al., 2018; Vikingstad et al., 2006; Yekeen et al., 2017). Both Xu et al. and Yekeen et al. used sodium dodecyl sulfate (SDS) surfactant and NaCl salt to determine the impact of salinity on foam stability. Xu et al. reported that the addition of salt above CMC stabilizes the foam. However, Yekeen et al. found that above CMC foam stability reduced on the addition of salt. Thus, it seems, that a clear understanding of the underlying mechanisms that determine the impact of salt on the formation and stabilization of foam is necessary. In this study, we have investigated the foaming behavior of AOS in the presence and absence of NaCl at several surfactant concentrations to understand the discrepancies in the litera ture where only an effect has been considered. Here, we explore as something completely new, how opposing effects at various concentra tion regimes could be in play. We are aware that bulk foam properties are not necessarily exactly reflecting the foaming processes in an oil reservoir as pointed out in several recent studies (Hussain et al., 2019; Solbakken et al., 2014; Xiao et al., 2018) so the present study pertains primarily to the fundamentals of foam in its own right. It is, however, noticed that even leading experts in the field (Hussain et al., 2019) has made attempts to correlate bulk properties of foam with those observed in porous media.
2.2. Sample preparation A batch solution of 1% was prepared from the 35% AOS solution and left overnight. Further samples of concentrations from 0.025% to 1% were prepared by dilution of the 1% batch solution. Each sample was prepared 1 h before the actual experiment. Similarly, a 2 M batch so lution of NaCl was prepared. Stirring for 2 h at room temperature was carried out to ensure complete dissolution of the salt. Further dilutions were performed to reach the required molar concentration (0.01M–1M). 2.3. Surface tension and CMC Surface tensions were measured using an optical interfacial tensi ometer by applying the pendant drop method. All measurements were performed at 23 ̊C and enough time was given to ensure that equilibrium was established. Calibration of the apparatus was done using a small metal bead of known diameter and the surface tension of deionized water measured in this manner was 72.95 dyne/cm. This value com pares well with those that are found in the literature (Harkins and Brown, 1919; Vargaftik et al., 1983). Measurements were conducted two times for each sample and reported as the average; all the measurements were within the spread of 0.45 dyne/cm. Density values, used in the calculation of the surface tension, for each sample were first calculated with a densitometer (Anton Parr DMA 4500) at 23 ̊C. The CMC was determined by plotting surface tension measurements vs log of con centrations, the point at which the slope of isotherm changes abruptly (point of inflection) corresponds to the CMC as shown in Fig. 5. 2.4. Foamability and stability The foaming properties were determined using a Dynamic Foam Analyzer (DFA100) from KRUSS, Germany. All measurements were performed at 23 ̊C and atmospheric pressure. The solution, 50 ml, is poured with the help of a syringe in a transparent measuring column which is located between a linear LED panel and a line sensor. Air is pumped into the liquid from the bottom through a filter to produce foam. The air was injected for 20 s at a flow rate of 0.2 L/min for all experiments. The line sensor measures the light transmitted through the measuring column over its full height. The gas-phase above the foam and the liquid are transparent, while the foam column absorbs some of the emitted light. The two-phase boundaries, liquid/foam and foam/air
2. Experimental 2.1. Material The surfactant used in this study (AOS) was purchased as an aqueous solution from Al Biariq Petrochemical Ind Co Ltd (Riyadh, K.S.A) having 2
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Fig. 2. Foam volume stability with time for different AOS concentrations.
Fig. 3. Foam half-Life as a function of AOS concentration.
are detected using the differences in transmittance. The repeatability (over several days) was tested by running three replicates the minimum variation was found to be 2.2% (for the 1% AOS solution) whereas the maximum was found to be 4.1% (for the 0.25% AOS solution).
of gravitation depends on average vertical velocity (Pugh, 1996). However, this vertical drainage is more important in reasonably thick films, i.e. concentrations greater than CMC. Also, the rupturing phe nomena in a thin layer (less than 100 nm) and in a thick layer (greater than 100 nm) are different. Formation of a common-and a Newton black films gives rise to extra stability to lamina as discussed above. So, there exists an optimum concentration that gives maximum stability and above that concentration, the foam stability again starts to decrease. From these results, we can conclude that there exists an optimum con centration for stable foam which turns out to be 0.075% in this case. Behera (Behera et al., 2014) suggested that the rate of collapse in creases at higher concentrations due to a decrease in lamella elasticity which decreases with an increase in surfactant concentration and leads to fast foam collapse. Elasticity is the capacity of a film to regulate its surface tension when perturbed. High elastic films are more stable than low elastic films (Langevin, 1998; Rusanov et al., 2004). The presence of excessive surfactant molecules in solution at high concentration will result in increased adsorption of surfactant molecules at interfaces. This causes a stunted rate of film thinning and drainage of liquid. Hence, bubble coalescence is reduced and interfacial elasticity is increased because of the stabilization of foam lamellae by molecules of surfactant (Firouzi and Nguyen, 2013). But the increase in foam stability was found only up to a specific AOS concentration. Further increase in surfactant concentration increases the collapse rate of foam (Yekeen et al., 2017). Fig. 3 shows graphical representations of half-life and maximum foam volume for a clear overview of how they change with concentra tion. From these results, it can be easily seen that half-life has a direct relationship with the AOS concentration until 0.075%, and then have an inverse relation with further increase in concentration.
3. Results and discussion 3.1. Effect of AOS concentration on foamability and foam stability Fig. 1 shows the foam height and foam volume generated with respect to time for different concentrations of AOS and thus represents the foamability as a function of AOS concentration. The foam height increases with increasing surfactant concentration. It was observed that 1w% AOS gives rise to the highest initial foam height. The effect of the surfactant concentration on the ability to generate foam is inferred from the maximum foam height at a particular concentration. When more surfactant molecules are available, migration of surfactant molecules towards liquid-gas interface increases, which results in better foam generation (Bournival et al., 2014; Karakashev and Manev, 2003; Yekeen et al., 2017). It is clear from the results that the foam height is directly proportional to the AOS concentration. The same trends have been reported previously for different types of surfactants (Simjoo et al., 2013; Xu et al., 2009; Yekeen et al., 2017). Foam volume stability (FVS) is the gradient of height as a function of time. Similarly, to a height versus time, FVS plot vs time is also used to indicate the stability of the foam. However, FVS over time provides a better measure of the gradient of the plot which is valuable especially if the time interval under study is short, hence it’s a preferred measure of the stability, compared to a simple height vs time plot. Fig. 2 shows the FVS as a function of time. It is noted that the maximum stability is observed at an AOS concentration of 0.075% (just above CMC) whereas 1% AOS gives rise to minimum stability. The change from 0.075% to 1% is not linear. A similar finding was made previously by Yekeen (Yekeen et al., 2017) who found a distinct optimum concentration rather than a linear relation with concentration. The stability increases from 0.01% to reach a maximum at 0.075% and then again starts to decrease gradually to reach the lowest (and apparently constant). Wang et al. (Wang and Chen, 2013; Wang and Mulligan, 2004) related the decrease in stability of foam at higher concentrations (>CMC) with increased weight (grav itational effect) of foam because of excess molecules of surfactant at the lamella. Because of the excess of the molecules of surfactants, the impact of gravitation on the drainage of foam increases, which results in con stant liquid drainage from the film formed between adjacent bubbles, eventually rupturing the foam film to result in bubble coalescence (Baz-Rodríguez et al., 2014). The vertical drainage under the influence
3.2. Foam structure with aging Foam is comprised of bubbles of various sizes. With time, larger bubbles increase in size due to diffusion of gas from smaller adjacent bubbles and the bubbles become polyhedral from the spherical shape. The bubble growth as a function of time is called Ostwald ripening (Stevenson, 2012). Fig. 4(a) shows the images of foam captured by the camera, through the prism, attached at a height of 85 mm with a glass column. It was observed that the size of bubbles at t ¼ 0 decreased as the concentration of surfactant increased from 0.01% to 1%. This is in line with the result that the foam volume increases with surfactant concen tration as discussed in section 3.2 hence the size of the bubble reduces to produce more bubbles (compact foam) in the same area. As time passes, diffusion of gas increases the size of larger bubbles and vanishes smaller bubbles which reduce the bubble count. Due to the increase in the size of 3
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Fig. 4. Foam stability at different surfactant AOS concentrations: (a) Images of bubbles and structure of foam, (b) bubble count (the graphs overlap beyond 10 min), and (c) bubble area.
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bubbles, the film thickness reduces which further increases the rate of diffusion and coalescence to make the foam unstable. Therefore, just like foam height and half-life, bubble count and average bubble size are other parameters to asses the foamability and foam stability, respec tively. Fig. 4(b) shows the bubble count with time and the results are in accordance with images i.e the bubble count increases as the concen tration increases from 0.01% to 1%. Fig. 4(c) shows the average bubble area of foam with time for the same five concentrations. Initially, 1% AOS foam shows the minimum and 0.01% shows the maximum bubble size but with time the increment in the size of bubbles is at a maximum (least stable foam) for 1% AOS foam and is minimum for 0.075% AOS foam. In Fig. 4(c), it can be seen that 0.075% shows the minimum bubble area and 1% AOS solution shows the maximum bubble area. Hence, 0.075% AOS foam has maximum stability. All concentration curves are in accordance with the half-life results obtained above: stability in creases from 0.01% to 0.075% and then again starts to decrease and reach the minimum value at 1% AOS solution. A comparison of 0.01% and 0.075% foam during the first hour of foam decay (Figs. 2 and 4) also shows that foam stability should not be merely judged by FVS and it should always be related to the foam structure. FVS is mainly defined as the fraction of foam volume remaining relative to the maximum foam volume (after foaming has been stopped). FVS is calculated by Equation (1): FVS ðtÞ ¼
Vt ðfoamÞ � 100 Vf ðfoamÞ
Table 1 Surface tension of 0.01% AOS solution at different NaCl concentration. Surface tension (mN/m)
Salt concentration (M)
56.0 32.7 30.3 29.1 28.0
0.00 0.25 0.50 0.75 1.00
AOS). The surface tension and the CMC decrease with increasing the concentration of salt. The value of CMC is reduced from 0.07% to 0.0015% with the addition of 1M NaCl. Fig. 5 represents the surface tension data for AOS and AOSþ1M NaCl solutions. Farajzadeh et al. (2008) also investigated the change in the CMC of AOS by changing the concentration of NaCl salt. According to their findings, the CMC reduces with increasing salt concentration. Similar results were obtained by (Yekeen et al., 2017) who state that a significant reduction in CMC and surface tension of SDS solution was observed with the addition of NaCl. They reported a reduction in the CMC of SDS by an order of magnitude with the addition of 1 w% NaCl to the solution. The surface tension reduces further with an increased NaCl concentration which results in lower CMC values. Chattopadhyay et al. have related the decrease of CMC and surfacetension of surfactant solution to the fact that the salt favors the transition of molecules of surfactant towards the gas-liquid interface in order to reduce the electrostatic repulsion among the charged head of surfactant molecules (Chattopadhyay and Harikumar, 2003; Xu et al., 2009). Such a shielding of repulsive force promotes the hydrophobic strength of surfactant monomers which results in the formation of micelles at lower surfactant concentrations and consequently decreasing the CMC (Muherei and Junin, 2007).
(1)
Vt is the volume at a given time and Vf is the volume of foam when foaming stopped or in other words at the end of the bubbling process. The foam volume is mainly calculated by measuring foam height. Therefore, FVS is calculated only by measuring the volume of foam (that is, a volume in a graded cylinder). However, FVS does not reflect the structure and quality of the foam (it can be wet, dry or anything in be tween). Bubble count and bubble size to a larger extent reflect the quality of the foam and therefore also provide useful information about foam stability. The two measures (volume vs bubble count and size) are not necessarily linearly related: It is oftentimes observed that foam initially decays without a significant change in the foam height but with a change in the bubble count and bubble diameter. This is related to the nature of the foam; and in the case above the foam would be referred to as being more dry. So, in conclusion, one can easily envision a scenario where foam height does not alone reflect stability. This is the case for the 0.01% experiment. Initially, the height/volume of the 0.01% foam was higher as compared to 0.075%. However, the foam generated in the 0.01% case was drier compared to 0.075% foam. As verified by the foam structure analysis in Fig. 4. Although the FVS value initially was higher for 0.01% than for 0.075%, its bubble count was much lower and its bubble diameter much higher. Both bubble count and bubble diameter show that the 0.075% foam stability was actually higher even, to begin with. This all goes to show that the 0.075% foam is more stable compared to the 0.01% foam even though FVS of the 0.01% foam initially attains a high value.
3.4. Effect of salt concentration on foam stability The effects of salt on the foamability and stability of foam are ambiguous. Some have found salts to have negative effects on stability as well as foam generation capability while others have found that salts stabilize foam or have a neutral impact. According to Behera and Varade et al. foam stability, as defined by the proportion of foam that has collapsed with time, increases with increase of salt concentration in solution (Behera et al., 2014; Varade and Ghosh, 2017). Xu et al. studied the effect of NaCl as an electrolyte on the SDS surfactant solution below and above the CMC. The addition of NaCl to an SDS solution improves the foamability and stability of foam to a certain extent. They noticed the decrease in surface tension and zeta-potential with an increase of salt concentration, which as a result reduce the surface charge of SDS mi celles. Hence, the results suggest that the stability of foam and foam ability increase in the presence of salts (Xu et al., 2009). Tan et al. studied the effect of NaCl, MgCl2, and CaCl2 on the foam generated using polypropylene glycol (PPG) (Tan et al., 2005). They suggested that the valence of the ions has a greater effect on the foamability of PPG than salt type. According to Tan et al. less foamability and foam stability in the existence of salt are because of the mitigation of electrostatic repulsion among the charged bubble surfaces. High salt concentration reduces electrostatic repulsion, and therefore foamability. Vikingstad et al. reported the effect of salt on AOS and FS500 in presence and absence of oil (Vikingstad et al., 2006). The brine concentration has little or no effect on the foam height in the absence of oil (Vikingstad et al., 2006). However, in the presence of oil, high ionic strength reduces the foam stability. Farajzadeh et al. (2008) have also analyzed the effect of salt on AOS surfactant foam. They measured the foam film thickness as a function of NaCl and AOS concentrations. They have found a relation between the film thickness and the contact angle of the meniscus with the film. They found the critical AOS and NaCl concentration for the formation of stable films and also for the formation of stable newton
3.3. CMC determination and effect of salt concentration on surface properties It was found that the surface tension of AOS solutions decreases with increasing surfactant concentration. A gradual increase in surfactant concentration gradually reduces surface tension until the CMC is reached at which point the surface tension does not decrease any further. The CMC represents the concentration at which the surfactants still are mainly segregated; at larger concentrations, the monomers aggregate to form micelles. This aggregation alters the physio-chemical properties of the surfactant-containing solution below and above CMC, hence the properties of foam also show most variability away from CMC. In the present case, the CMC was found to be 0.07 w%. The data given in Table 1 shows the effect of NaCl addition on surface tension (at 0.01% 5
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Fig. 5. Surface tension of a 0.01% AOS solution in deionized water and in 1 M salt solution.
black films. Craig et al. (1993) performed a series of experiments to demonstrate the effect of electrolytes on coalescence of foam bubbles. According to their observation, the effect on bubbles coalescence is not consistent for different type of electrolytes as some reduce the coales cence phenomena while other others do not. Hence, hydrophobic interaction with electrolytes could be the reason for the change in coa lescence phenomena due to presence of electrolytes. To investigate the effect of salinity on foaming ability, a series of experiments were performed by keeping the AOS concentration constant (0.25%) while varying the salinity. Fig. 6 shows the effect of varying salt concentration on the foamability of AOS. It can be easily noticed that the height of foam is increasing with the increase of molar concentration of salt. The experiments were only performed for 1 h to screen for the effect on foamability. Similar results were reported in our previous publication where we investigated the impact of NaCl concentration on the foam stability of four in-house synthesized surfactants. These in-house sur factants were cationic gemini surfactants containing mono phenyl ring with Br counterion (12-PhBr-12), mono phenyl ring with Cl coun terion (12-PhCl-12), biphenyl ring with Br counterion (12-BiPhBr-12), and biphenyl ring with Cl counterion (12-BiPhCl-12) (Kalam et al., 2019). For all types of surfactants regardless of the nature of spacer and counterion, the addition of salts increased the foam stability (see Fig. 7). To examine the effects of salinity on foam stability, longer time-scale experiments were performed to ensure that all effects up until the halflife (using 1M NaCl and 0.25% surfactant solution) are captured. Sur prisingly, the results show that the half-life was increased more than twofold. We propose that the presence of salt forms an electrostatic double layer (EDL) within the lamina and that the screening effect of this EDL is the main reason for the decreasing coalescence. This results in stabilizing the foam for an extended period of time because of the for mation of smaller and resistant bubbles which gives the tight packing of the liquid lamella among bubbles as also explained by Xu et al. (2009). The addition of NaCl salt decreases the gas solubility into the solution hence reduces the hydrophobic interaction which as a result increase the foam stability by suppressing the coalescence rate of bubbles (Firouzi and Nguyen, 2014). To assess the impact of surfactant concentration, a range of experi ments were conducted where the salt concentration was kept constant at 1M and AOS concentration was varied. At 0.1% AOS and 1M NaCl
Fig. 6. Foam height with time by increasing salinity at 0.25 w% AOS.
Fig. 7. Effect of salinity on foam volume stability of different surfactants (Kalam et al., 2019).
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appropriate amount of) surfactants at the interface of the film. At low concentrations, the AOS molecules are surrounded by electrolytes which result in decreased film stability. However, at higher concentrations, the surplus surfactant molecules could improve film stability. In the case of higher surfactant concentrations and in the absence of electrolyte, low foam stability can be associated with a high liquid drainage rate due to increased gravitational effect as explained above. In short, it is the balance between surfactant and electrolyte concentration that decides the film stability. 4. Conclusion A systematic study was performed to examine the effect of salt on the stability of surfactant foam. The key purpose of this study was to investigate the apparently contradicting findings that are presented in the literature on foaming characteristics in the presence of salt. Following novel conclusion are drawn: � Foamability increased with increasing AOS concentration. However, the effect is more dominant below CMC and not significant at higher concentrations (>CMC); a result of two opposing effects that hitherto weren’t disclosed. � In the absence of NaCl, an optimum surfactant concentration was found that maximize the stability of foam which was slightly higher than the CMC. The decrease in stability of foam at higher surfactant concentrations is due to the greater influence of gravitational forces. � At constant salinity, the increase in AOS concentration improved foam stability. At some typical AOS concentration (0.25% in this case), the foam stability of AOS solution in presence of NaCl sur passes the foam stability of AOS solution without NaCl. � Salt can destabilize or stabilize the foam depending on the surfactant concentration; this apparently contradicting the result is due to opposing effects of letting the electrolytes surround the AOS mole cules at low vs high concentrations. A new finding that makes up for some of the ambiguity that has previously been dominating the literature.
Fig. 8. Comparison of the half-life of AOS foam with and without salts at four different surfactant concentrations.
solution, the results differ from the findings above in the sense that a lower half-life was observed for the saline solution, and this reduction in the stability becomes more prominent at AOS concentrations lower than 0.1%. Fig. 8 depicts the effect of salt addition at various AOS concen trations. From these results, it can be concluded that salts can stabilize or destabilize the foam depending on surfactant concentration in solution. Above a critical surfactant concentration, salt stabilizes the foam. However, at a lower concentration of surfactants, added salts result in destabilizing the foam. We propose a mechanism to unify these apparently contradicting findings in Fig. 9. When AOS is in demand to form the film, occupying surfactant molecules with electrolytes has a negative effect on film stability (left). Whereas when there is an AOS surplus, it was a positive effect to take the surfactants out of the equation and has a positive effect on film stability, because they will interfere with the (already
Fig. 9. Proposed mechanism of the effect of salt addition on foaming properties. 7
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� When the AOS molecules are in excess, higher stability is achieved. However, when electrolytes are in excess, AOS molecules get completely surrounded by electrolytes which prevent them from entering the micellar structure that make up the foam lamellae – yet another central and novel finding that highlights what can happen when opposing effects are in play.
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