Theoretical Interpretation of pH and Salinity Effect on Oil-in-Water Emulsion Stability Based on Interfacial Chemistry and Implications for Produced Water Demulsification

Round 1

Reviewer 1 Report

1.      The language should be reviewed by one native, lots of grammar problems.

2.      The quality of all the figures should be improved.

3.      I strongly recommend the authors to add one paragraph discussing the difference between their work and the previously performed studies in literature. In other words, what is the novelty of this work? I offer the authors to revise the abstract and introduction in order to incorporate the novelty of their work.

4.      The terms of Figure 7 and Fig. 7 should be the same in section 5.2.

5.      Section 5.4 lacks experimental verification

6.      The data of interfacial tension should be supplemented in section 5.5.

The quality of English needs to be improved.

Author Response

Reviewer # 1

Reviewer remark

Comments and Suggestions for Authors

1. The language should be reviewed by one native, lots of grammar problems.

Authors’ response

We have addressed this issue by improving upon it

Reviewer remark

2. The quality of all the figures should be improved.

Authors’ response

We have responded to this review remark by checking our figures again for quality. In so doing, we realized that the font size of the legends can be reduced to make it less bulky, which can improve the quality. We have, therefore, done that.

Reviewer remark

3. I strongly recommend the authors to add one paragraph discussing the difference between their work and the previously performed studies in literature. In other words, what is the novelty of this work? I offer the authors to revise the abstract and introduction in order to incorporate the novelty of their work.

Authors’ response

We thank the learned reviewer for this constructive review remark. We have done that accordingly by adding the following text.

Addition

In the petroleum industry oil and water emulsions are encountered under field operational and transportation conditions. Consequently, depicting emulsion formation and stabilization has received the attention of operators (Sousa, Pereira, & Matos, 2022). Considering that emulsification can be naturally associated with surface active components of crude oils, identification of asphaltene contents through characterization has been used to determine emulsion formation potential (Li, et al., 2022), (Czarnecki, Tchoukov, Dabros, & Xu, 2013). Also, research findings in the area of emulsion demulsification and the dosage of demulsifier required have been published (Kokal & Al-Juraid, 1999), (Raynel, Marques, Al-Khabaz, Al-Thabet, & Oshinowo, 2021). However, considering that physicochemical processes encountered in classical emulsion systems, such as colloidal electrostatic stabilization and inter-particle interaction potential (Valadez-Pérez, Liu, & Castañeda-Priego, 2021), (Liu & Xi, 2019) in light of the electric double layer theory are applicable to oil field emulsions, electrostatic based theoretical foundations coupled with thermodynamic equilibrium constant related to ionization of surface groups of crude oils, such as asphaltenes, can equally be sued to determine emulsion stability. In this regard, the utility of electrostatic and thermodynamic parameters that result from analytical solutions to a classical model, such as the Poisson-Boltzmann mean field theory are useful literature based resources. Moreover, in the literature, classical electrostatic based theories of emulsion stability analysis have centered more on traditional emulsion systems, such as those found in the emulsion and cosmetic industries. In this paper, we particularly focus on oil in water emulsions stability in the context of classical electrostatic theory. Moreover, in the literature, studies on emulsion stability relies more on conventional emulsions found in food and biological systems, where the application of surfactants for stabilization has been central to such studies (Jin, Liu, & Hu, 2021), contrary to our course where only natural surfactants (asphaltenes) is of interest.

Reviewer remark

4. The terms of Figure 7 and Fig. 7 should be the same in section 5.2.

Authors’ response

We have corrected that.

Reviewer remark

5. Section 5.4 lacks experimental verification

Authors’ response

We thank the reviewer for this meaningful and constructive review remark. In this regard, although we have not carried out any experiment to justify this aspect of our discussion, we have added the following text from literature to justify it.

For instance, Wang et al. [149], .have demonstrated experimentally, the possibility of surfactant diffusion from the oil phase to the aqueous phase, leading to their adsorption on negatively charged fine solids in this phase. The bulk increase of surfactant concentration in line with the Stern-Grahame equation leads to experimentally measured decrease in interfacial tension. Moreover, ionic adsorption/ ionization of surface functional groups can lead to surface charge development [150]. Thus, strong coordination or reversibly adsorbed oxygen [151] or strong coordination nucleophiles (water or hydroxyl ions) can donate electrons to Silver nano particles found in commercial products, which can lead to such surface charge developments [152], and the diffusion of surfactants found in laundry waters towards such charged particles has been experimentally demonstrated to lead to agglomeration [153].

Reviewer remark

6. The data of interfacial tension should be supplemented in section 5.5.

Authors’ response

We thank the reviewer for this review remark. Accordingly, we have added the following text to address the remark, which we hope will make our discussion more robust.

       One characteristic of smart-water flooding or low salinity enhanced water flooding (LSWF) is the concentration of injected brine, which is generally lower than those of reservoir formation brines. Rahevar et al. (2023) (Rahevar, Kakati, Kumar, Sangwai, Myers, & Al-Yaseri, 2023) have used two salinities for LSWF) oil recovery experiments. For a salinity of 0.6 M NaCl, the interfacial was 23.0 mNm^-1 while for a 0.1 M NaCl brine the interfacial tension was 21.7. mNm^-1. In the context of our paper, as found  in Fig.7 through Fig. 10, lower salinity implies higher zeta potential and higher surface charged density, which translates to lower interfacial tension as observed experimentally by Rahevar  et al. (2023)  (Rahevar, Kakati, Kumar, Sangwai, Myers, & Al-Yaseri, 2023)

Comments on the Quality of English Language

 

Reviewer remark

The quality of English needs to be improved.

Authors’ response

We thank the reviewer for this remark. We have done that accordingly.

 

Reviewer 2 Report

In this manuscript, the authors show the effect of pH and salinity on oil in water emulsion stability, analyzing by electric double layer theory. The novelty is the biggest challenge of this article. There are some requirements need to be satisfied before considering publication.

 Q1. Make sure the format of your revised manuscript (e.g., references) conforms to the journal template.

 Q2. It is recommended that the author cite similar literature and provide a brief description in the introduction to support the advancement of their theoretical understanding of the stability of oil in water emulsions.

 Q3. Does the electrostatic charges of oil-water interface completely result from the OH and carboxyl and amine groups in asphaltenes? It is recommended that the author cite recent literature.

Moderate editing of English language required

Author Response

Reviewer # 2

Comments and Suggestions for Authors

In this manuscript, the authors show the effect of pH and salinity on oil in water emulsion stability, analyzing by electric double layer theory. The novelty is the biggest challenge of this article. There are some requirements need to be satisfied before considering publication.

Reviewer Remark

 Q1. Make sure the format of your revised manuscript (e.g., references) conforms to the journal template.

Authors’ response

We thank the reviewer for this important remark. We have done that accordingly.

Reviewer Remark

 Q2. It is recommended that the author cite similar literature and provide a brief description in the introduction to support the advancement of their theoretical understanding of the stability of oil in water emulsions.

Authors’ response

We thank the learned reviewer for this this constructive review remark. We added the following text in the introductory section to address the remark.

Oil-in-water (O/W) emulsions are typically colloidal systems consisting of oil droplets dispersed in continuous aqueous media of produced oilfield brine, and stabilised by natural surfactant molecules (asphaltene), with particle sizes having mean diameters in the range of 20-500 nm (Manickam, Sivakumar, & Pang, 2020).    For large particles, the electrostatic forces in colloidal systems are comparable to gravity and van der Waals force, so it is considered while for smaller particles it is not (Xiong, Zhang, & Hui, 2022). Consequently, for such nanometric dimensions of oil droplets, the excessively higher surface to volume ratio implies electrostatic forces interactions that are well quantified within the frame work of continuum electrostatics will govern emulsion stability. At a given salinity of produced water, coupled with the amphoteric nature of surface ionisable asphaltene, the formation of the electric double layer is imminent (Tempel, 1992). Considering the fundamental structure of the electric double layer, the pH and salinity dependent electrokinetic behavior of oil droplets in such emulsion systems is governed by the potential at the surface of shear between the charged surface and the aqueous solution (Chang, 2016).Therefore, the zeta-potential measurement using electrophoretic mobility of oil droplets in such systems (Kaszuba, Corbett, Watson, & Jones, 2010) has been used as a theoretically and experimentally acceptable metric for quantifying emulsion stability, where stronger electrostatic repulsive forces inhibits coalescence and demulsification. For instance, asphalt emulsion is water-continuous dispersions of fine asphaltic cement with particle diameter in the range 1-10 μm (Yuan, Liu, Zheng, & Ma, 2021). Highway pavement preservation works employ such emulsions prepared from asphalt cement due to their lower application temperatures and versatility for a broad range of pavement restoration applications (Pinto & Buss, 2020). In the food industry, emulsions have been used to reduce transportation costs between production and the sales points, where zeta potential has been used as a guide to ensuring stability (Almeida, Larentis, & Ferraz, 2015).

In addressing the problem of emulsion stability, experimental measurement has proven to be the preferred norm (Almeida, Larentis, & Ferraz, 2015), (Bhatt, Prasa, Singh, & Panpalia, 2010), (GURPREET & SINGH*, 2018), (Costa, Basri, Shamsudin, & Basri, 2014). Considering the direct relationship between zeta potential and surface charge density, theoretical models have been developed for each of them (Yang, Shi, Wu, & Sun, 2023) and for the relationship between them (Ge & Wang, 2017) . These theoretical relationships draw on the fundamental tenets of the electric double layer theory where salinity and its dependent parameters, such as the Debye length, dielectric permittivity, and the double layer capacitance are well integrated. However, it turns out that while these models are sufficiently robust, the vast literature resources on emulsion stability have always laid emphasis on experimental designs based on theoretical foundations and a most recent determination based on Molecular Dynamics simulation (MD) (Biriukov, Fibich, & Předota, 2020). Therefore, given knowledge of produced oilfield brines and the electrokinetic parameters of asphaltenes in produced oils, theoretical models of surface charge density and zeta potential are attractive tools for the present paper and its success will motivate researchers in addition to underscoring the uniqueness of the electric double layer theory.

 Reviewer Remark

Q3. Does the electrostatic charges of oil-water interface completely result from the OH and carboxyl and amine groups in asphaltenes? It is recommended that the author cite recent literature.

Authors’ response

We thank the reviewer for this constructive review remark. We have addressed the remark using the following text.

The electrokinetic properties of crude oil draw on surface ionisable groups, namely the carboxyl and basic or amine group. The carboxyl group comes partly from naphthenic acid (Wu, Visscher, & Gates, 2019) and from asphaltenes, while the basic comes from nitrogen heteroatomic nitrogen groups. Therefore, any characterization of crude oil carboxyl groups combines the two sources. However, the basic nitrogen groups become protonated at low pH (Nolting, Aziz, Ottosson, Faubel, Hertel, & Winter, 2007), (Manga, Cayre, Biggs, & Hunter, 2018) and neutral at high pH. Considering, the imminent evolution of carbon dioxide from produced oilfield brines, the near neutral pH implies neutral basic groups, which justifies neglect of their electrostatic contribution in this paper.  Therefore, data on the surface concentration of carboxyl groups of the studied oils were used in zeta potential model for calculation

Comments on the Quality of English Language

Reviewer Remark

Moderate editing of English language required

Authors’ response

We thank the reviewer for this positive remark. We have done that accordingly.

Reviewer 3 Report

The current manuscript by Adango Miadonye and Mumuni Amadu represents a short theoretical study on asphaltene surface charge calculation in emulsion drops. The authors collect mathematical equations from various manuscripts and then construct a set of mathematical equations to calculate the surface and zeta potential of different asphaltenes. As a result of their study, they estimate ridiculous zeta potentials, e.g. -1400-1800 mV (Figures 7, 8, 9, 10), which are inconsistent with the literature data. The relevant literature data on the other hand is missing, despite the presence of 136 references. I recommend the manuscript is rejected from publication:

 

Sodium dodecyl sulfate-covered surfaces typically have a surface potential in the order of 100-200 mV (DOI: 10.1134/S1061933X12020032), which is a commonly accepted value for DLVO calculations (https://www.stevenabbott.co.uk/practical-surfactants/foam-dlvo.php). The typical adsorption density of SDS is 30.0 sq. angstrom, whereas carboxylic surfactants have slightly higher density with 22-24 sq. Angstrom and similar ionization degree (around 40%) (http://dx.doi.org/10.1016/j.cis.2012.08.003). Therefore, it is not very likely that you achieve 10 times higher zeta potentials even at a theoretical 100% ionization.

 

 

The values for asphaltenes in the literature for the zeta potential are typically below -60 mV with direct measurements of both zeta potential and estimation of the surface species present (doi:10.3390/molecules25051214 or https://doi.org/10.1016/S0016-2361(03)00002-4). Review articles for different asphaltenes can be found here (https://doi.org/10.1021/acs.energyfuels.0c03962), showing similar or smaller surface potentials in general.

The English language, fonts, and equations need a thorough inspection due to technical issues: missing "a" and "the", compound words missing the dashes, and multiple font mismatches after insertions (e.g. lines 460-480).

Author Response

Reviewer # 3

Comments and Suggestions for Authors

 

Reviewer Remark

The current manuscript by Adango Miadonye and Mumuni Amadu represents a short theoretical study on asphaltene surface charge calculation in emulsion drops. The authors collect mathematical equations from various manuscripts and then construct a set of mathematical equations to calculate the surface and zeta potential of different asphaltenes. As a result of their study, they estimate ridiculous zeta potentials, e.g. -1400-1800 mV (Figures 7, 8, 9, 10), which are inconsistent with the literature data. The relevant literature data on the other hand is missing, despite the presence of 136 references. I recommend the manuscript is rejected from publication:

 Authors’ response

We thank the learned reviewer for this constructive review remark. The review remark prompted to reconsider zeta potential calculations. The criticism made us review literature to add another equation for dealing with zeta potential and surface charge calculation. After addressing this remark, we have zeta potential values that are encountered in the literature for emulsion stabilized by asphaltene.

 Reviewer Remark

Sodium dodecyl sulfate-covered surfaces typically have a surface potential in the order of 100-200 mV (DOI: 10.1134/S1061933X12020032), which is a commonly accepted value for DLVO calculations (https://www.stevenabbott.co.uk/practical-surfactants/foam-dlvo.php). The typical adsorption density of SDS is 30.0 sq Angstrom, whereas carboxylic surfactants have slightly higher density with 22-24 sq. Angstrom and similar ionization degree (around 40%) (http://dx.doi.org/10.1016/j.cis.2012.08.003). Therefore, it is not very likely that you achieve 10 times higher zeta potentials even at a theoretical 100% ionization.

Authors’ response

We totally agree with the learned reviewer as far as this review remark is concerned. Obviously, by using the chemistry of crude oil for oilfield emulsions stabilized by asphaltenes, we should expect to get lower surface values of zeta potential. It is when such systems are stabilized by synthetic surfactants that excessively high values of zeta potentials are encountered.  Moreover, we mentioned we cited a reference regarding the dependence of zeta potential on surfactant. Therefore, we have addressed the remark accordingly.

 Reviewer Remark

The values for asphaltenes in the literature for the zeta potential are typically below -60 mV with direct measurements of both zeta potential and estimation of the surface species present (doi:10.3390/molecules25051214 or https://doi.org/10.1016/S0016-2361(03)00002-4). Review articles for different asphaltenes can be found here (https://doi.org/10.1021/acs.energyfuels.0c03962), showing similar or smaller surface potentials in general.

Authors’ response

This remark is absolutely true, but given the fact that we are dealing with the surface chemistry of crude oils containing asphaltenes, it is obvious that the effective zero point of charge of oil is not exactly the same as that of asphaltene. The implication is that our calculated zeta potentials will be meaningful values and not exactly equal to that of asphaltenes.

 

 

Comments on the Quality of English Language

The English language, fonts, and equations need a thorough inspection due to technical issues: missing "a" and "the", compound words missing the dashes, and multiple font mismatches after insertions (e.g. lines 460-480).

Authors’ response

Wed thank the reviewer for this review remark. We have addressed the remark accordingly.

Round 2

Reviewer 1 Report

• I approve the author's changes to the manuscript

Reviewer 2 Report

Accept in present form.

Accept in present form.

Reviewer 3 Report

The authors have addressed all my concerns regarding their manuscript: 

-They have recalculated the zeta potential and it no longer deviates 30 times from the actual experimental measurements. 

-They have extended their literature review to include instances of zeta potential measurements and the relevance of the presented study to the current state-of-the-art in crude oil emulsions.

I recommend the manuscript for publication in its current form.

There are few technical mistakes in the punctuation to be cleared.