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On the nature of colloidal aphrons
Colloid and Interface Science Communications 34 (2020) 100232
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On the nature of colloidal aphrons
T
Yong-li Yan , Yang Zhang, Christian-chibuike Una ⁎
College of Chemistry & Chemical Engineering, Xi'an Shiyou University, Xi'an 710065, China
ARTICLE INFO
ABSTRACT
Keywords: Colloidal aphron Foam/emulsion Formation Interfacial structure Stability
Colloidal gas (liquid) aphrons (CGAs/CLAs) were originally proposed by professor Sebba to be unique and intriguing systems with structure different from general foams or emulsions. As a consequence of their fascinating colloidal properties, a number of investigators have exploited various industrial applications. However, the nature especially the interfacial structure of this colloidal dispersion is still disputed by many researchers. Here we demonstrate the differences and similarities between colloidal aphrons and common foams/emulsions in terms of their formulation and formation, interfacial structure, stabilization and stability. Theoretical and experimental evidence indicates that the colloidal system described by Sebba is indeed identical to conventional foams or emulsions, and therefore, the specific term “aphrons” should be abandoned to eliminate current confusion. These findings are important for understanding the concepts and principles of gas/liquid and liquid/liquid dispersion systems, and would also be helpful for their wide applications.
1. Introduction The term “colloidal gas (liquid) aphrons” was first posed by Felix Sebba in the early 1970s [1–3], in order to define the specific type of gas/liquid or liquid/liquid dispersions unlike general foams or emulsions. For four decades since that time, colloidal aphrons have presented considerable potentials in many areas of industrial applications such as drilling fluid in oilfield [4,5], protein recovery and immobilization [6,7], pollution remediation of water and soil [8,9], predispersed solvent extraction [10–12] and functional material synthesis [13,14]. Despite the broad range of engineering applications, there are still disagreements over the concept and structure of colloidal aphrons. In 1980s, professor Sebba proposed a unique and intriguing model on these colloid systems in his series of publications [15–17]. In this model, as shown in Fig. 1, a striking feature is that the gas (oil) core is encapsulated into an aqueous shell stabilized by three layers of surfactants. However, an opposite viewpoint immediately appeared as suggested by Princen [18]. He claimed that the colloidal system described by Sebba had no difference from conventional foams or emulsions, and therefore, the term “aphrons” should be waived to remove this chaos, unfortunately, which cannot provide any direct evidence to clarify or invalidate Sebba's unproven postulate. Up to now, several research works have been published to take part in this discussions, but not any a conclusion has been accepted as a
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general consensus by most of the researches [19–22]. As a result, there occurs an intense puzzle on concepts of foam or emulsion, for example, the confusion among the nomenclatures of foam, microfoam and colloidal gas aphron, a similar question for the liquid/liquid dispersion concerning the terms of highly concentrated emulsion, biliquid foam and colloidal liquid aphron. To solve this puzzle, in the present contribution, combined with our series of studies, we provide insights into, for the first time, their concepts, formation, interfacial structure and stability that is necessary to understand the nature of “colloidal aphrons”. 2. Formulation and formation Through the development for four decades, the formulation of the colloidal aphrons has been achieved by anionic, cationic, non-ionic surfactants, nanoparticles and other surface active substances. For example, sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), sodium bis(2-ethylhexyl)sulfosuccinate (AOT), Tergitols and modified nano-SiO2 as foaming agent or emulsifier, the ideal aphron systems can be made [11,23–28]. Concerning colloidal liquid aphrons, the major feature on the formulation is the two surfactants used as emulsifiers, including a water-soluble surfactant and an oilsoluble surfactant, which is likely different from that of general emulsion system. However, ordinary emulsion in fact also has the use of compound emulsifier formula [29–32], indicating that there is no
Corresponding author. E-mail address: [email protected] (Y.-l. Yan).
https://doi.org/10.1016/j.colcom.2019.100232 Received 25 September 2019; Received in revised form 24 December 2019; Accepted 31 December 2019 2215-0382/ © 2020 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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Fig. 1. A structure model of colloidal gas (liquid) aphron proposed by Sebba [17].
Fig. 2. Free energy of colloidal dispersion formation.
3. Amphiphiles and arrangement
essential difference in composition between the two systems. That is to say that colloidal liquid aphrons and common emulsions usually possess almost the same ingredients to facilitate them including two immiscible liquids, and emulsifiers. As for the gas/liquid dispersions, no difference exists in formulation between colloidal gas aphrons and common foams [30,32–35]. As far as preparation of colloidal aphrons is concerned, the most often employed method is named as physical foaming/emulsification by drop rupture, such as shaking, stirring, homogenizer, ultrasonic, etc. [8,36,37]. The characteristics of aphrons including size, monodispersity and stability, are affected by various parameters in the generation process, such as the fabrication method, power input, surfactants as foaming agents or emulsifiers, some additives like as electrolytes and polymers, the dispersed phase volume ratio, and viscosity of continuous phase [38–41]. Such compositions, preparation methods and influencing factors in the formation process of aphrons are also common elements occurred in conventional foam and emulsion systems [29,33,42]. With the same composition, preparation methods and influencing factors, does it also imply that aphrons and foams/emulsions, have an identical formation mechanism? For an aphron, the free energy of the gas/liquid (bubbles in water or oil), or oil/water (liquid droplets in water or oil) colloidal dispersion is larger than that of the separated phases (gas and liquid, or oil and water), suggesting that an aphron is an unstable system from the perspective of thermodynamics (Fig. 2). The formation process of aphrons is the formation of gas/liquid of gas bubbles or oil/water of liquid droplets interface thus leading to the increase of interface area between dispersed phase and continuous phase. With the help of surface active agents adsorbed at gas/liquid or oil/water interface to form the interface film, this non-spontaneous formation process of bubbles or droplets dispersions can be realized smoothly. In principle, such a process always makes it necessary to utilize some external energy to transform the separated individual phases into a colloidal gas/liquid or liquid/liquid dispersion. No matter in what situation, an energy input must be at least higher than the positive free energy related with increased contact area of the gas/liquid or liquid/ liquid interface and adopted as ΔGInterface = γΔA, which is also confirmed by the study on the preparation and formation mechanism of aphrons [43–45]. From the above analyses, we can say, therefore, that the formation mechanism on the aphrons is exactly alike to that of conventional foams/emulsions [29,30,34].
3.1. On the aphron's model Based on the definition of aphrons, one of the substantive features is its particle size being < 100 μm [17]. This system was initially described by Sebba and termed microfoams/biliquid foams due to the minute size of the bubbles/droplets. The bubbles/droplets are so small that they exhibit some colloidal properties, the name, therefore, has since been changed to colloidal gas/liquid aphrons (CGAs/CLAs), by which name they are now usually designated. By comparison, the conventional foam/emulsion according to the particle size can be divided into microfoam with particle size of < 100 μm and macrofoam with particle size beyond 100 μm [32,33,46], microemulsion (droplets < 100 nm) and macroemulsion (droplets > 100 nm) [29,32,45], which covers the whole particle size of aphrons. Therefore, the foam/emulsion system within a specific particle size range also shows colloidal performance like aphrons. Moreover, the flow behavior dependent on the size of the aphrons themselves can be an important feature. This size dimension (< 100 μm) of the bubbles/droplets, which endows them with colloidal characteristics, creates a system which flows as easily as water. It can be seen that the aphrons, with the same size, presents a very similar flow characteristic to ordinary foams/emulsions. The second characteristic of aphrons is associated with its unique interface structure formed from arrangements of amphiphiles adsorbed at interface [17]. The structure model raised by Sebba was based on the series of experiments concerning the coalescence behavior of colloidal gas/liquid aphrons. In contrast to conventional foams/emulsions in which a surfactant monolayer separates the gas/oil core from the bulk liquid phase, colloidal aphrons are made of a gas/oil core surrounded by three surfactant layers as displayed in Fig. 1. The three surfactant layers form an inner and an outer film. The inner film contains surfactant monolayer with their hydrophobic tail orienting towards the gas/oil phase. The outer film consists of a surfactant bilayer which is hydrophilic outward that makes the colloidal aphron compatible with the surrounding aqueous phase. Sebba argued that the dramatic delay in coalescence and better stability of aphrons in comparison with general foams/emulsions are attributed to their multilayered film. In the case of aphrons, the barrier to coalescence is enhanced distinctly. Instead of two monolayers of surfactants, one on each bubble/droplet, there are now three thin films that must be ruptured before coalescence 2
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can occur. For the unique interface structure of aphrons, it has also inspired scientists to explore and verify the correctness of the model. Sebba originally investigated the structure of polyaphrons (CLAs) using fluorescent dyes and provided indirect evidence for the presence of a shell, though the structure of the shell has remained to be unclear [17]. Once the concept of aphrons was put forward, it was immediately opposed by Princen [16,18], who was a very excellent expert in field of colloid dispersion, especially in emulsions. He pointed out that the paper lacks originality, except for the coined name used by the authors to denote what was widely known as HIPREs and believed it was meaningless to replace highly concentrated emulsions with the term ‘polyaphrons’. Unfortunately, Princen also did not give any experimental data on the structure of the CLA interface to support his argument. In view of this debate, several scientific teams have also launched studies on the interface structure of the aphrons. For CGAs, the first investigation was in 1990 and conducted by Amiri and Woodburn [47]. They attempted to analyze the structure of CGAs via evaluating the shell thickness of amphiphilic molecules and measuring the aphron rise velocity. Their experimental results revealed that the film thickness of CGAs generated from the cationic surfactant CTAB with 750 nm is equal to 350 molecules of amphiphile distributed in neighboring layers. While Bredwell and Worden determined the film thickness to range from 200 nm to 300 nm prepared from non-ionic surfactant Tween-20, according to the gas diffusion from the inner cavity of the CGA bubble to the liquid bulk [48]. These research data to a certain extent support the Sebba's model involving a multilayered interface of aphrons. In year of 2000, Julie Varley group carried out investigations on the structural characteristics of CGAs utilizing freeze fracture transmission electron microscopy (FF-TEM) and X-ray diffraction (XRD) methods [20]. Experimental results indicate that the total interface has an average thickness of 96 nm suggesting that about 80 surfactant AOT molecules could fill in the interface. Therefore, they demonstrated that CGA dispersions have different interfacial structure from general foams, and further claimed that these differences could be as a powerful evidence for the presence of surfactant multilayers. Nonetheless, a considerable uncertainty was also noticed in terms of the precise number of layers. With regard to CLAs, the structure of this liquid/liquid dispersion was first studied by Stuckey using such test methods as cryogenic transmission electron microscopy (cryo-TEM), differential scanning calorimetry (DSC), and light scattering [19]. All the experimental data revealed distinct differences between CLAs and conventional emulsions. Nevertheless, there was not any direct evidence on the interfacial structure of CLAs to support Sebba's model. Srivastava undertook a variety of experimental tools, including FFTEM, DSC, SAXS (small angle X-ray scattering) and ITC (isothermal titration calorimetry), to characterize the interfacial structure of CLA systems [49]. DSC results suggested the presence of a separate phase, apart from the aqueous and organic phases present in CLA dispersions. FF-TEM demonstrated the unique lamellar or liquid crystalline bilayers surrounding the organic phase in CLA dispersions. These layers are formed by the interaction between the two surfactants (Tween 80 and Tergitol 15-S-3) used in CLA formulation, which is responsible for the stabilization of these systems. In Stuart Hicks' PhD thesis, the Small-Angle Neutron Scattering experiments (SANS) were adopted in an effort to explore the underlying structure of CLAs dispersions [50]. The conclusion from these experiments were that instead of a bilayer of surfactant, and a soapy shell of contained water, as proposed by Sebba, the droplets are stabilized by a single surfactant monolayer. Such results are comparable with investigations on HIPREs, that there is nothing particularly special in the structure of biliquid foams. However the phase behavior results, from SANS in tandem with the surfactant solution and the biliquid foam, depicted that there was more to these systems than simply a single
monolayer stabilizing the system. These results are consistent with large oil droplets in equilibrium with excess swollen micelles, which disappear as the surfactant concentration is reduced. To gain insight into this debate, in our laboratory, the microstructure of CLAs composed of SDS/polyoxyethylene 3 dodecyl ether/ndecane/water was also studied employing series of characterization methods such as FF-TEM, DSC, SAXS and polarizing microscopy [22,51,52]. The comprehensive results demonstrated that the CLAs (O/ W) comprised of oil droplets in micron size surrounded by an aqueous “soapy shell”, and vice verse for W/O type of CLAs, and there occurred supramolecular structures such as micelles or microemulsions within interfacial film or bulk phase. The interfacial shell of CLAs is equivalent to that of CGAs, with an approximate thickness of 0.3 μm to 0.4 μm. It was suggested that the microstructure of CLAs was similar to that of HIPREs, with two-phase structure and existence of the surfactant aggregates in these biphases. On the nature and structure of CLAs, this conclusion seems to be completely different from Sebba's opinion. 3.2. Arrangement of amphiphilic molecules at interface For the single-component surfactants, two types of interface structures are often formed by adsorption behavior [32,53], which is affected by the characteristics and structure of the amphiphile, ionic strength, as well as temperature in the solution system. The first is a monolayer at gas/liquid or liquid/liquid interface formed by the singlecomponent surfactants, as displayed in Fig. 3A. The second structure is these single-component surfactants adsorbed and self-assembled at interface to form lamellar micelles or lamellar liquid crystals with multilayer model, as presented in Fig. 3B. These multilayer structures in thin films have been frequently encountered in dispersion systems with relatively high concentration of surfactant. Liquid crystals or bilayer phases appear in these systems when the concentration increases from a dilute solution to a saturated state. Evidently, varied kinds of molecular aggregates, in addition to micelles, bilayer and liquid crystal, can be established in foam/emulsion films [32,33,54–56]. The multilayer microstructure formed from single-component surfactants played an important role in stabilizing foam/emulsion lamella [29,32,54–57]. Such structures incurs significant changes in the features of the system, such as rigidity, structure units and composition fluctuation. Either a monolayer or a multilayer with ordered aggregates, these two interface structures often occur in foam/emulsion systems. It is clear that the second structure of the foam/emulsion system is not different from the essence of aphrons with three-layer interface structure [58–61]. Mixed surfactant systems, mainly composed of ionic-, nonionic-, and zwitterionic-surfactants, are often used to prepare stable foam/emulsion systems [32,53]. In such systems, a variety of interfacial structures could be formed. The first interfacial structure is the simultaneous adsorption at the gas/liquid or liquid/liquid interface, forming a monolayer comprised of mixed surfactants, displaying synergistic effects [31,62,63], as elucidated in Fig. 3C. This monolayer formed from mixed surfactants, except for reduced interface energy, may exhibit a stronger mechanical strength than a monolayer made of the single-composition surfactants. For the synergistic effects, the closed molecule packing and improved lateral interaction between hydrocarbon chains could induce a dramatic against to the mobility of surfactant molecules at the interface and the drop in the coalescence behavior. Such an effect has often been associated with variation on the interfacial viscoelasticity. This interface structure can occur both in the foam/emulsion system and the aphron dispersion system, without any difference. For the preparation of emulsions, a hydrophilic and a lipophilic surfactant are usually chosen as a mixed surfactant component recipe. Along with the different preparation methods, the adsorption behavior of these two types of surfactant displays different arrangements at interface [56,64]. If the two surfactants are adsorbed simultaneously at the oil/water interface, a single-molecular layer of mixed surfactants 3
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Fig. 3. Arrangement of amphiphilic molecules at the interface of foam/emulsion dispersions.
is exactly the same as that of conventional foams/emulsions system. In order to improve the stability of this kind of dispersion system, the preparation of CGA/CLA is also conducted by use of surfactant aggregation structure such as micelle, lamellar micelle or liquid crystal, the addition of polymer thickener, gelation, nanoparticles and other measures [70–73]. Like conventional foams/emulsions, various structured film is formed at the bubble or droplet interface of aphrons to improve its stability [55,74–78]. The component units of the structured film can be diverse amphiphilic association structures such as spherical micelles, lamellar micelles, or network-like micelles (Fig. 4A), and colloidal particle halos (Fig. 4B).
may be established, and their interface structure is similar to that shown in Fig. 3C. It has been believed that the existence of two surfactant species, one with water-solubility and another with oil-solubility, could considerably improve the stability of an emulsion system. This impact has been illuminated using two possible mechanisms, one for the synergistic effect leading to low oil/water interfacial tensions, and another for the adsorption of surfactant complexes to enhance the film strength to the oil/water interface. The second arrangement of the two amphiphiles is that the adsorption layer of water-soluble surfactants covers on the interface of the oil droplet, and the adsorption layer of oil-soluble surfactants covers on the surface of the water droplet, which is the structural characteristic of the classical O/W/O or W/O/W multiple emulsions [35,65,66], as exhibited in Fig. 3D. This structure is basically the same as the interfacial structure of colloidal liquid aphrons. Through the above analysis, it can be concluded that the interface structure of aphron is exactly the same with that of the ordinary foam/ emulsion [32,54,67,68].
4.2. Stability mechanism The unique interface structure of aphrons does not exist, therefore, there is no basis for the idea that it has a different stability mechanism from general foam/emulsion system. Like the ordinary foam/emulsion system, the aphron dispersion also involves three stability mechanisms, such as liquid drainage, bubble coarsening and film rupture. As a thermodynamically unstable system, an aphron will dissolve over time at a rate which relies on the height of energy barriers between the aphron dispersions and separated phases. The height of the energy barrier is mainly fixed by physicochemical behaviors which arrest the droplets from getting together and coalescence, including repulsive hydrodynamic and colloidal interactions between droplets. Aiming at understanding the stability mechanism of aphron, the research is often focused on its drainage behavior. Stuckey [19] reported a first-order kinetic model based on the decay behavior of CLAs, and found that the decay-life displayed a negative correlation trend with the size of CLAs. In our laboratory, Yan [79] depicted the liquid
4. Stabilization and stability 4.1. Stabilization As analyzed above, the structure of the aphron does not exhibit any difference from that of ordinary foam/emulsion, therefore, its stability naturally falls in the scope of these ordinary gas/liquid or liquid/liquid dispersion systems, ranging from minutes to hours and even days. On the basis of our researches and the documented literatures, we can see that the stability of the aphron [4,7,9,12,44,51] has no advantage over the common foam/emulsion dispersions [64,69]. From these we can infer that the stabilization mechanism of aphrons 4
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Fig. 4. Stabilization of foam/emulsion dispersions with structured interface. 0.40
160
0.36
C
140
0.32
Drainage Vt /mL
0.28 120
0.24
100
0.20
80
0.16 0.12
D
60
0.08 40
0.04
20
0.00
0
At the same time, the stability behavior of CLAs was also determined by conductivity analysis and further represented with a sigmoidal equation in our previous studies [44]. The CLA evolution process displays two distinct stages including film drainage and film rupture driven by two independent mechanisms, respectively. Such drainage behaviors on the colloidal gas/liquid aphrons is often observed in conventional foam/emulsion systems [84–87].
Rate of drainage dv/dt / mL/s
180
5. Conclusions In summary, based on the analyses of formation process, interface structure and stability mechanism, the essence of aphron has been found identical to the general foam/emulsion dispersion system [88–92]. Therefore, we suggest to cancel the specific term of “aphron” in order to avoid unnecessary confusion in the field of colloidal dispersion. We believe that the analysis in the present investigation will establish a better understanding on the nature (in terms of formation, interface structure and stability) of foams, emulsions, and other dispersed systems.
-0.04 0
200
400
600
800
1000 1200 1400 1600
Time t /s Fig. 5. Liquid drainage behavior from CGAs. D, first stage; C, second stage.
drainage profiles from CGAs with a kinetic model. It was found that the drainage behavior of CGAs could be well simulated by Sigmoidal eq. (1).
V = Vmax
Kn
tn + tn
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(1)
where V denotes the volume of drained liquid, Vmax presents the maximum volume of drained liquid, t is the drainage time and K refers to the half-life (t1/2) of the drainage behavior. This kinetic model was also successfully confirmed by means of the Arrhenius equation. Based on the analysis of the liquid drainage velocity, two distinct stages of liquid drainage from CGAs dispersion were first proposed, as illustrated in Fig. 5. The first stage of drainage is driven by gravity and accounts for the removal of > 90% of the total liquid (D region). The second stage occurred in the lamellae and plateau borders, is attributed to capillary pressure and plateau border suction (C region). The mechanism analysis of CGA drainage has also been reported by other researchers [80–83]. In Singhal's report, the drainage curve is exactly the same as our findings. The difference is that gravity-driven drainage is divided into two stages [80]. In the analysis of Moshkelani, the first stage is defined as the conductivity of the continuous phase, and the last two stages of drainage process should be identical to our theory [81]. From these we can see that the mechanisms of the above three-stage phases are essentially the same as the two-stage theory.
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Colloid and Interface Science Communications journal homepage: www.elsevier.com/locate/colcom
On the nature of colloidal aphrons
T
Yong-li Yan , Yang Zhang, Christian-chibuike Una ⁎
College of Chemistry & Chemical Engineering, Xi'an Shiyou University, Xi'an 710065, China
ARTICLE INFO
ABSTRACT
Keywords: Colloidal aphron Foam/emulsion Formation Interfacial structure Stability
Colloidal gas (liquid) aphrons (CGAs/CLAs) were originally proposed by professor Sebba to be unique and intriguing systems with structure different from general foams or emulsions. As a consequence of their fascinating colloidal properties, a number of investigators have exploited various industrial applications. However, the nature especially the interfacial structure of this colloidal dispersion is still disputed by many researchers. Here we demonstrate the differences and similarities between colloidal aphrons and common foams/emulsions in terms of their formulation and formation, interfacial structure, stabilization and stability. Theoretical and experimental evidence indicates that the colloidal system described by Sebba is indeed identical to conventional foams or emulsions, and therefore, the specific term “aphrons” should be abandoned to eliminate current confusion. These findings are important for understanding the concepts and principles of gas/liquid and liquid/liquid dispersion systems, and would also be helpful for their wide applications.
1. Introduction The term “colloidal gas (liquid) aphrons” was first posed by Felix Sebba in the early 1970s [1–3], in order to define the specific type of gas/liquid or liquid/liquid dispersions unlike general foams or emulsions. For four decades since that time, colloidal aphrons have presented considerable potentials in many areas of industrial applications such as drilling fluid in oilfield [4,5], protein recovery and immobilization [6,7], pollution remediation of water and soil [8,9], predispersed solvent extraction [10–12] and functional material synthesis [13,14]. Despite the broad range of engineering applications, there are still disagreements over the concept and structure of colloidal aphrons. In 1980s, professor Sebba proposed a unique and intriguing model on these colloid systems in his series of publications [15–17]. In this model, as shown in Fig. 1, a striking feature is that the gas (oil) core is encapsulated into an aqueous shell stabilized by three layers of surfactants. However, an opposite viewpoint immediately appeared as suggested by Princen [18]. He claimed that the colloidal system described by Sebba had no difference from conventional foams or emulsions, and therefore, the term “aphrons” should be waived to remove this chaos, unfortunately, which cannot provide any direct evidence to clarify or invalidate Sebba's unproven postulate. Up to now, several research works have been published to take part in this discussions, but not any a conclusion has been accepted as a
⁎
general consensus by most of the researches [19–22]. As a result, there occurs an intense puzzle on concepts of foam or emulsion, for example, the confusion among the nomenclatures of foam, microfoam and colloidal gas aphron, a similar question for the liquid/liquid dispersion concerning the terms of highly concentrated emulsion, biliquid foam and colloidal liquid aphron. To solve this puzzle, in the present contribution, combined with our series of studies, we provide insights into, for the first time, their concepts, formation, interfacial structure and stability that is necessary to understand the nature of “colloidal aphrons”. 2. Formulation and formation Through the development for four decades, the formulation of the colloidal aphrons has been achieved by anionic, cationic, non-ionic surfactants, nanoparticles and other surface active substances. For example, sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB), sodium bis(2-ethylhexyl)sulfosuccinate (AOT), Tergitols and modified nano-SiO2 as foaming agent or emulsifier, the ideal aphron systems can be made [11,23–28]. Concerning colloidal liquid aphrons, the major feature on the formulation is the two surfactants used as emulsifiers, including a water-soluble surfactant and an oilsoluble surfactant, which is likely different from that of general emulsion system. However, ordinary emulsion in fact also has the use of compound emulsifier formula [29–32], indicating that there is no
Corresponding author. E-mail address: [email protected] (Y.-l. Yan).
https://doi.org/10.1016/j.colcom.2019.100232 Received 25 September 2019; Received in revised form 24 December 2019; Accepted 31 December 2019 2215-0382/ © 2020 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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Fig. 1. A structure model of colloidal gas (liquid) aphron proposed by Sebba [17].
Fig. 2. Free energy of colloidal dispersion formation.
3. Amphiphiles and arrangement
essential difference in composition between the two systems. That is to say that colloidal liquid aphrons and common emulsions usually possess almost the same ingredients to facilitate them including two immiscible liquids, and emulsifiers. As for the gas/liquid dispersions, no difference exists in formulation between colloidal gas aphrons and common foams [30,32–35]. As far as preparation of colloidal aphrons is concerned, the most often employed method is named as physical foaming/emulsification by drop rupture, such as shaking, stirring, homogenizer, ultrasonic, etc. [8,36,37]. The characteristics of aphrons including size, monodispersity and stability, are affected by various parameters in the generation process, such as the fabrication method, power input, surfactants as foaming agents or emulsifiers, some additives like as electrolytes and polymers, the dispersed phase volume ratio, and viscosity of continuous phase [38–41]. Such compositions, preparation methods and influencing factors in the formation process of aphrons are also common elements occurred in conventional foam and emulsion systems [29,33,42]. With the same composition, preparation methods and influencing factors, does it also imply that aphrons and foams/emulsions, have an identical formation mechanism? For an aphron, the free energy of the gas/liquid (bubbles in water or oil), or oil/water (liquid droplets in water or oil) colloidal dispersion is larger than that of the separated phases (gas and liquid, or oil and water), suggesting that an aphron is an unstable system from the perspective of thermodynamics (Fig. 2). The formation process of aphrons is the formation of gas/liquid of gas bubbles or oil/water of liquid droplets interface thus leading to the increase of interface area between dispersed phase and continuous phase. With the help of surface active agents adsorbed at gas/liquid or oil/water interface to form the interface film, this non-spontaneous formation process of bubbles or droplets dispersions can be realized smoothly. In principle, such a process always makes it necessary to utilize some external energy to transform the separated individual phases into a colloidal gas/liquid or liquid/liquid dispersion. No matter in what situation, an energy input must be at least higher than the positive free energy related with increased contact area of the gas/liquid or liquid/ liquid interface and adopted as ΔGInterface = γΔA, which is also confirmed by the study on the preparation and formation mechanism of aphrons [43–45]. From the above analyses, we can say, therefore, that the formation mechanism on the aphrons is exactly alike to that of conventional foams/emulsions [29,30,34].
3.1. On the aphron's model Based on the definition of aphrons, one of the substantive features is its particle size being < 100 μm [17]. This system was initially described by Sebba and termed microfoams/biliquid foams due to the minute size of the bubbles/droplets. The bubbles/droplets are so small that they exhibit some colloidal properties, the name, therefore, has since been changed to colloidal gas/liquid aphrons (CGAs/CLAs), by which name they are now usually designated. By comparison, the conventional foam/emulsion according to the particle size can be divided into microfoam with particle size of < 100 μm and macrofoam with particle size beyond 100 μm [32,33,46], microemulsion (droplets < 100 nm) and macroemulsion (droplets > 100 nm) [29,32,45], which covers the whole particle size of aphrons. Therefore, the foam/emulsion system within a specific particle size range also shows colloidal performance like aphrons. Moreover, the flow behavior dependent on the size of the aphrons themselves can be an important feature. This size dimension (< 100 μm) of the bubbles/droplets, which endows them with colloidal characteristics, creates a system which flows as easily as water. It can be seen that the aphrons, with the same size, presents a very similar flow characteristic to ordinary foams/emulsions. The second characteristic of aphrons is associated with its unique interface structure formed from arrangements of amphiphiles adsorbed at interface [17]. The structure model raised by Sebba was based on the series of experiments concerning the coalescence behavior of colloidal gas/liquid aphrons. In contrast to conventional foams/emulsions in which a surfactant monolayer separates the gas/oil core from the bulk liquid phase, colloidal aphrons are made of a gas/oil core surrounded by three surfactant layers as displayed in Fig. 1. The three surfactant layers form an inner and an outer film. The inner film contains surfactant monolayer with their hydrophobic tail orienting towards the gas/oil phase. The outer film consists of a surfactant bilayer which is hydrophilic outward that makes the colloidal aphron compatible with the surrounding aqueous phase. Sebba argued that the dramatic delay in coalescence and better stability of aphrons in comparison with general foams/emulsions are attributed to their multilayered film. In the case of aphrons, the barrier to coalescence is enhanced distinctly. Instead of two monolayers of surfactants, one on each bubble/droplet, there are now three thin films that must be ruptured before coalescence 2
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can occur. For the unique interface structure of aphrons, it has also inspired scientists to explore and verify the correctness of the model. Sebba originally investigated the structure of polyaphrons (CLAs) using fluorescent dyes and provided indirect evidence for the presence of a shell, though the structure of the shell has remained to be unclear [17]. Once the concept of aphrons was put forward, it was immediately opposed by Princen [16,18], who was a very excellent expert in field of colloid dispersion, especially in emulsions. He pointed out that the paper lacks originality, except for the coined name used by the authors to denote what was widely known as HIPREs and believed it was meaningless to replace highly concentrated emulsions with the term ‘polyaphrons’. Unfortunately, Princen also did not give any experimental data on the structure of the CLA interface to support his argument. In view of this debate, several scientific teams have also launched studies on the interface structure of the aphrons. For CGAs, the first investigation was in 1990 and conducted by Amiri and Woodburn [47]. They attempted to analyze the structure of CGAs via evaluating the shell thickness of amphiphilic molecules and measuring the aphron rise velocity. Their experimental results revealed that the film thickness of CGAs generated from the cationic surfactant CTAB with 750 nm is equal to 350 molecules of amphiphile distributed in neighboring layers. While Bredwell and Worden determined the film thickness to range from 200 nm to 300 nm prepared from non-ionic surfactant Tween-20, according to the gas diffusion from the inner cavity of the CGA bubble to the liquid bulk [48]. These research data to a certain extent support the Sebba's model involving a multilayered interface of aphrons. In year of 2000, Julie Varley group carried out investigations on the structural characteristics of CGAs utilizing freeze fracture transmission electron microscopy (FF-TEM) and X-ray diffraction (XRD) methods [20]. Experimental results indicate that the total interface has an average thickness of 96 nm suggesting that about 80 surfactant AOT molecules could fill in the interface. Therefore, they demonstrated that CGA dispersions have different interfacial structure from general foams, and further claimed that these differences could be as a powerful evidence for the presence of surfactant multilayers. Nonetheless, a considerable uncertainty was also noticed in terms of the precise number of layers. With regard to CLAs, the structure of this liquid/liquid dispersion was first studied by Stuckey using such test methods as cryogenic transmission electron microscopy (cryo-TEM), differential scanning calorimetry (DSC), and light scattering [19]. All the experimental data revealed distinct differences between CLAs and conventional emulsions. Nevertheless, there was not any direct evidence on the interfacial structure of CLAs to support Sebba's model. Srivastava undertook a variety of experimental tools, including FFTEM, DSC, SAXS (small angle X-ray scattering) and ITC (isothermal titration calorimetry), to characterize the interfacial structure of CLA systems [49]. DSC results suggested the presence of a separate phase, apart from the aqueous and organic phases present in CLA dispersions. FF-TEM demonstrated the unique lamellar or liquid crystalline bilayers surrounding the organic phase in CLA dispersions. These layers are formed by the interaction between the two surfactants (Tween 80 and Tergitol 15-S-3) used in CLA formulation, which is responsible for the stabilization of these systems. In Stuart Hicks' PhD thesis, the Small-Angle Neutron Scattering experiments (SANS) were adopted in an effort to explore the underlying structure of CLAs dispersions [50]. The conclusion from these experiments were that instead of a bilayer of surfactant, and a soapy shell of contained water, as proposed by Sebba, the droplets are stabilized by a single surfactant monolayer. Such results are comparable with investigations on HIPREs, that there is nothing particularly special in the structure of biliquid foams. However the phase behavior results, from SANS in tandem with the surfactant solution and the biliquid foam, depicted that there was more to these systems than simply a single
monolayer stabilizing the system. These results are consistent with large oil droplets in equilibrium with excess swollen micelles, which disappear as the surfactant concentration is reduced. To gain insight into this debate, in our laboratory, the microstructure of CLAs composed of SDS/polyoxyethylene 3 dodecyl ether/ndecane/water was also studied employing series of characterization methods such as FF-TEM, DSC, SAXS and polarizing microscopy [22,51,52]. The comprehensive results demonstrated that the CLAs (O/ W) comprised of oil droplets in micron size surrounded by an aqueous “soapy shell”, and vice verse for W/O type of CLAs, and there occurred supramolecular structures such as micelles or microemulsions within interfacial film or bulk phase. The interfacial shell of CLAs is equivalent to that of CGAs, with an approximate thickness of 0.3 μm to 0.4 μm. It was suggested that the microstructure of CLAs was similar to that of HIPREs, with two-phase structure and existence of the surfactant aggregates in these biphases. On the nature and structure of CLAs, this conclusion seems to be completely different from Sebba's opinion. 3.2. Arrangement of amphiphilic molecules at interface For the single-component surfactants, two types of interface structures are often formed by adsorption behavior [32,53], which is affected by the characteristics and structure of the amphiphile, ionic strength, as well as temperature in the solution system. The first is a monolayer at gas/liquid or liquid/liquid interface formed by the singlecomponent surfactants, as displayed in Fig. 3A. The second structure is these single-component surfactants adsorbed and self-assembled at interface to form lamellar micelles or lamellar liquid crystals with multilayer model, as presented in Fig. 3B. These multilayer structures in thin films have been frequently encountered in dispersion systems with relatively high concentration of surfactant. Liquid crystals or bilayer phases appear in these systems when the concentration increases from a dilute solution to a saturated state. Evidently, varied kinds of molecular aggregates, in addition to micelles, bilayer and liquid crystal, can be established in foam/emulsion films [32,33,54–56]. The multilayer microstructure formed from single-component surfactants played an important role in stabilizing foam/emulsion lamella [29,32,54–57]. Such structures incurs significant changes in the features of the system, such as rigidity, structure units and composition fluctuation. Either a monolayer or a multilayer with ordered aggregates, these two interface structures often occur in foam/emulsion systems. It is clear that the second structure of the foam/emulsion system is not different from the essence of aphrons with three-layer interface structure [58–61]. Mixed surfactant systems, mainly composed of ionic-, nonionic-, and zwitterionic-surfactants, are often used to prepare stable foam/emulsion systems [32,53]. In such systems, a variety of interfacial structures could be formed. The first interfacial structure is the simultaneous adsorption at the gas/liquid or liquid/liquid interface, forming a monolayer comprised of mixed surfactants, displaying synergistic effects [31,62,63], as elucidated in Fig. 3C. This monolayer formed from mixed surfactants, except for reduced interface energy, may exhibit a stronger mechanical strength than a monolayer made of the single-composition surfactants. For the synergistic effects, the closed molecule packing and improved lateral interaction between hydrocarbon chains could induce a dramatic against to the mobility of surfactant molecules at the interface and the drop in the coalescence behavior. Such an effect has often been associated with variation on the interfacial viscoelasticity. This interface structure can occur both in the foam/emulsion system and the aphron dispersion system, without any difference. For the preparation of emulsions, a hydrophilic and a lipophilic surfactant are usually chosen as a mixed surfactant component recipe. Along with the different preparation methods, the adsorption behavior of these two types of surfactant displays different arrangements at interface [56,64]. If the two surfactants are adsorbed simultaneously at the oil/water interface, a single-molecular layer of mixed surfactants 3
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Fig. 3. Arrangement of amphiphilic molecules at the interface of foam/emulsion dispersions.
is exactly the same as that of conventional foams/emulsions system. In order to improve the stability of this kind of dispersion system, the preparation of CGA/CLA is also conducted by use of surfactant aggregation structure such as micelle, lamellar micelle or liquid crystal, the addition of polymer thickener, gelation, nanoparticles and other measures [70–73]. Like conventional foams/emulsions, various structured film is formed at the bubble or droplet interface of aphrons to improve its stability [55,74–78]. The component units of the structured film can be diverse amphiphilic association structures such as spherical micelles, lamellar micelles, or network-like micelles (Fig. 4A), and colloidal particle halos (Fig. 4B).
may be established, and their interface structure is similar to that shown in Fig. 3C. It has been believed that the existence of two surfactant species, one with water-solubility and another with oil-solubility, could considerably improve the stability of an emulsion system. This impact has been illuminated using two possible mechanisms, one for the synergistic effect leading to low oil/water interfacial tensions, and another for the adsorption of surfactant complexes to enhance the film strength to the oil/water interface. The second arrangement of the two amphiphiles is that the adsorption layer of water-soluble surfactants covers on the interface of the oil droplet, and the adsorption layer of oil-soluble surfactants covers on the surface of the water droplet, which is the structural characteristic of the classical O/W/O or W/O/W multiple emulsions [35,65,66], as exhibited in Fig. 3D. This structure is basically the same as the interfacial structure of colloidal liquid aphrons. Through the above analysis, it can be concluded that the interface structure of aphron is exactly the same with that of the ordinary foam/ emulsion [32,54,67,68].
4.2. Stability mechanism The unique interface structure of aphrons does not exist, therefore, there is no basis for the idea that it has a different stability mechanism from general foam/emulsion system. Like the ordinary foam/emulsion system, the aphron dispersion also involves three stability mechanisms, such as liquid drainage, bubble coarsening and film rupture. As a thermodynamically unstable system, an aphron will dissolve over time at a rate which relies on the height of energy barriers between the aphron dispersions and separated phases. The height of the energy barrier is mainly fixed by physicochemical behaviors which arrest the droplets from getting together and coalescence, including repulsive hydrodynamic and colloidal interactions between droplets. Aiming at understanding the stability mechanism of aphron, the research is often focused on its drainage behavior. Stuckey [19] reported a first-order kinetic model based on the decay behavior of CLAs, and found that the decay-life displayed a negative correlation trend with the size of CLAs. In our laboratory, Yan [79] depicted the liquid
4. Stabilization and stability 4.1. Stabilization As analyzed above, the structure of the aphron does not exhibit any difference from that of ordinary foam/emulsion, therefore, its stability naturally falls in the scope of these ordinary gas/liquid or liquid/liquid dispersion systems, ranging from minutes to hours and even days. On the basis of our researches and the documented literatures, we can see that the stability of the aphron [4,7,9,12,44,51] has no advantage over the common foam/emulsion dispersions [64,69]. From these we can infer that the stabilization mechanism of aphrons 4
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Fig. 4. Stabilization of foam/emulsion dispersions with structured interface. 0.40
160
0.36
C
140
0.32
Drainage Vt /mL
0.28 120
0.24
100
0.20
80
0.16 0.12
D
60
0.08 40
0.04
20
0.00
0
At the same time, the stability behavior of CLAs was also determined by conductivity analysis and further represented with a sigmoidal equation in our previous studies [44]. The CLA evolution process displays two distinct stages including film drainage and film rupture driven by two independent mechanisms, respectively. Such drainage behaviors on the colloidal gas/liquid aphrons is often observed in conventional foam/emulsion systems [84–87].
Rate of drainage dv/dt / mL/s
180
5. Conclusions In summary, based on the analyses of formation process, interface structure and stability mechanism, the essence of aphron has been found identical to the general foam/emulsion dispersion system [88–92]. Therefore, we suggest to cancel the specific term of “aphron” in order to avoid unnecessary confusion in the field of colloidal dispersion. We believe that the analysis in the present investigation will establish a better understanding on the nature (in terms of formation, interface structure and stability) of foams, emulsions, and other dispersed systems.
-0.04 0
200
400
600
800
1000 1200 1400 1600
Time t /s Fig. 5. Liquid drainage behavior from CGAs. D, first stage; C, second stage.
drainage profiles from CGAs with a kinetic model. It was found that the drainage behavior of CGAs could be well simulated by Sigmoidal eq. (1).
V = Vmax
Kn
tn + tn
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
(1)
where V denotes the volume of drained liquid, Vmax presents the maximum volume of drained liquid, t is the drainage time and K refers to the half-life (t1/2) of the drainage behavior. This kinetic model was also successfully confirmed by means of the Arrhenius equation. Based on the analysis of the liquid drainage velocity, two distinct stages of liquid drainage from CGAs dispersion were first proposed, as illustrated in Fig. 5. The first stage of drainage is driven by gravity and accounts for the removal of > 90% of the total liquid (D region). The second stage occurred in the lamellae and plateau borders, is attributed to capillary pressure and plateau border suction (C region). The mechanism analysis of CGA drainage has also been reported by other researchers [80–83]. In Singhal's report, the drainage curve is exactly the same as our findings. The difference is that gravity-driven drainage is divided into two stages [80]. In the analysis of Moshkelani, the first stage is defined as the conductivity of the continuous phase, and the last two stages of drainage process should be identical to our theory [81]. From these we can see that the mechanisms of the above three-stage phases are essentially the same as the two-stage theory.
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