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Effect of Magnesium and Sodium Salts on the Interfacial Characteristics of Soybean Lecithin Dispersants Johnson Kwame Efavi, Emmanuel Nyankson, Abu Yaya, and Benjamin Agyei-Tuffour Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02862 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 17, 2017

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Industrial & Engineering Chemistry Research

Effect of Magnesium and Sodium Salts on the Interfacial Characteristics of Soybean Lecithin Dispersants Johnson Kwame Efavi, Emmanuel Nyankson,* Abu Yaya and Benjamin Agyei-Tuffour Department of Materials Science and Engineering, University of Ghana, P.O. Box LG 77, Ghana *Corresponding Author: Email: [email protected] Tel: +233 200 187 606 Abstract One of the most widely accepted oil spill response strategies is chemical dispersant application. However, since the surfactant used in their formulation may be ionic, its interfacial characteristics may be influenced by ions present in the sea. In this work, we have examined the effect of magnesium salts (MgSO4 and MgCl2) and sodium salts (NaCl, Na2SO4 and sodium benzoate) on the interfacial characteristics of hydroxylated soybean lecithin dispersant (H-PI). The oil-in-water emulsions formed with magnesium salts were more stable than those formed with sodium salts. Magnesium salts recorded the highest interfacial tension reduction and the highest dispersion effectiveness values when compared with sodium salts. These observations were attributed to (i) the Mg2+ ions interconnecting the negatively charged headgroups of H-PI at the oil-droplet-water interface, thus increasing the surface elasticity and viscosity and (ii) the smaller ionic size of Mg2+ allows for easy packing between the charged head groups of H-PI. Keywords: Magnesium Salts, Sodium Salts, Oil Spill Remediation, Soybean Lecithin, Interfacial characteristics

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Introduction Oil spills are discrete events in which oil is discharged through neglect, by accident, or with intent over a relatively short period of time and are often considered as a major occupational, environmental, and community health disaster.1 Oil spill causes great harm to vegetation, food crops, aquatic species and human inhabitant in the spilled zone hence adequate remediation procedures are adopted to minimize their effect on the ecosystem.2 Some of the most commonly used oil spill remediation strategies are in-situ burning, mechanical containment and collection, oil spill absorbents and chemical dispersant application.3 The addition of dispersants to disperse spilled oil into tiny droplets is often an efficient approach for the treatment of oil spills.4 When chemical dispersant is applied to an oil spill and sufficient energy is exerted to the dispersant-oil-seawater mixture, the slick is broken down into smaller droplets due to the reduction of the oil-water interfacial tension.5 The oil droplets diffuse both vertically and horizontally by the action of the sea waves and remain in the water column because they possess very little rising velocity.6 The increased surface area that arises from the smaller oil droplet size helps to accelerate biodegradation of the oil by bacteria action and hence reduces the environmental impact of the spilled oil.7 However, one major limitation of traditional dispersant formulations is their toxicity.8 The toxicity is associated with their indiscriminate application, and the chemical surfactants and solvents used in their formulation.9 Different dispersants have been formulated in an effort to reduce their toxicity upon application. Some of the newly formulated dispersant include the use of halloysite nanotubes as vehicles for dispersants,10 dispersant composite particles,11 and soybean in dispersant formulation.12 Soybean lecithin has been reported to be very efficient in dispersing crude oil. Its dispersion efficiencies have been reported to be higher than some of the traditionally formulated dispersants.12 It should however be noted that the dispersion efficiency of dispersant formulations is dependent on factors such as salinity, crude oil type, dispersant to oil ratio and ocean current (roughness of the sea).7b The most active agents in soybean formulated dispersant are phosphatidylcholine (PC) and phosphatidylinositol (PI). PI possesses negative charge on its head group while PC possesses both negative and positive charges on its head group.6 The ionic head group of phospholipids present in soybean lecithin formulated dispersant is expected to influence their adsorption at the 2 ACS Paragon Plus Environment

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oil-water interface. The effect of salt on the adsorption behavior of ionic surfactants at the oil droplet-water interface can be vital in the formation of stable emulsion. When ionic surfactants and salts are mixed in aqueous solution, salting-out phenomenon often occurs.13 Salting-out is the result of preferential movement of water molecules which immobilize and quench their role as solvents, from coordination shells of surfactants molecules to those of salt.14 It has been reported that, inorganic salts have significant effect on the amount of surfactant adsorbed at the fluid-fluid interface.15 Addition of inorganic salts reduces the electrostatic repulsion among the surfactants head groups. This results in a significant reduction in the surface tension. The extent of the surface tension reduction has been reported to be dependent on the valency of the cations present in the inorganic salt.15 Organic salts such as salicylate16 and tosilate17 have also been studied in ionic surfactant systems. Aside electrostatic interactions, organic salts have hydrophobic interactions with ionic salts in aqueous solutions resulting in tight packing at the oil droplet-water interface.18 There are no studies on the effect of organic and inorganic salts on the interfacial characteristics of soybean lecithin surfactant in literature. Therefore the results from this study will be helpful in predicting the efficiency of chemical dispersant application should oil spill occur. This will also be useful for pharmaceutical and detergent industries that use phospholipids from soybean lecithin. In this study, the effect of magnesium and sodium salts on the interfacial characteristics of soybean lecithin dispersant was examined. The sodium salts used in this study are sodium chloride (NaCl), sodium sulfate (Na2SO4) and sodium benzoate (Na-benzoate). The magnesium salts used are magnesium chloride (MgCl2) and magnesium sulfate (MgSO4). NaCl, Na2SO4, MgSO4 and MgCl2 are inorganic salts while Na-benzoate is an organic salt. A hydroxylated phosphatidylinositol obtained from soybean lecithin was used in this study. The extraction of phosphatidylinositol (PI) and its subsequent hydroxylation have been reported in our previous work.12 In this study, the effect of the sodium and magnesium salts on the emulsion stability and also on oil-droplet sizes were examined. The surface tension and oil (dodecane)-salt water interfacial tension were measured, and the critical micelle concentration (CMC) determined. From the surface and interfacial tension measurements, the effect of the salts on droplet coalescence was also determined. The maximum surface excess (Гmax) and the minimum surface area per surfactant molecule (Amin) were also calculated. The effect of the different salts on the dispersion effectiveness of hydroxylated phosphatidylinositol was examined using the U.S. EPA’s baffled flask test. 3 ACS Paragon Plus Environment


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Experimental Section Materials Soybean lecithin was purchased from Stakich Inc (Bloomfield Hills, MI) and absolute ethanol was obtained from VWR. Dodecane, hydrogen peroxide solution, sodium chloride, magnesium chloride, sodium sulfate, sodium benzoate, magnesium sulfate and acetic acid solution were obtained from Sigma-Aldrich. Sodium hydroxide pellets were obtained from Fisher Scientific. Dichloromethane was purchased from EMD Chemical Company. The crude oil used in this study was obtained from Texas Raw Crude International (Midland, TX). The major compounds present in the crude oil and the composition of the crude oil have been reported by Nyankson et al.10b Experimental Procedure Fractionating Phosphatidylinositol (PI) from Soybean Lecithin The method used in fractionating PI from soybean lecithin has been reported in our previous work.12b A summary is provided in this manuscript. Approximately 3 g of crude lecithin was weighed into a centrifuge tube, and then 9 g of ethanol was added. The tube was heated in a water bath at 60 ℃ for 60 min, stirring every 15 min during heating. After 60 min, the mixture was centrifuged and the ethanol phase poured off into a glass vial. The “solid residue” that remained in the centrifuge tube was labeled as PI (PI-enriched fraction). Hydroxylation of Phosphatidylinositol (PI) Chemical treatment of PI introduces hydroxyl groups into the PI structure. This enhances the hydrophilicity and the dispersion effectiveness. 1.5 g of PI was dispersed in 15 mL of deionized water. Glacial acetic acid (0.3 mL) and 1.5 g of 30 wt.% hydrogen peroxide solution was added and stirred at 300 rpm for 15 min. Sodium hydroxide aqueous solution (20 w/v%, 1.1 mL) was then added to neutralize the reaction mixture. The solvent was then evaporated at 40 ℃ under vacuum to obtain hydroxylated PI. The hydroxylation of the soybean phospholipid was confirmed by Fourier transformed infra-red spectroscopy and has been reported in our previous work.12a


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Solubilized Soybean Lecithin Dispersant Formulation (H-PI) The hydroxylated soybean lecithin PI was formulated by solubilizing a known amount of the hydroxylated PI in deionized water. The concentration of the formulated soybean lecithin H-PI dispersant was 0.1 𝑚𝑔⁄𝜇𝐿. Inorganic and Organic Salt Solutions Salt solution of 3.5 wt.% concentration was prepared by dissolving 35 g of the salt in 1 L of distilled water. The resulting salt solution was used as the synthetic seawater in this study. Product Characterization Emulsion Stability In the emulsion stability test, 1 mL of dodecane was added to 10 mL of the synthetic seawater (salt solution). An appropriate amount of the hydroxylated PI was added and the mixture vortexed at 3000 rpm for 3 min. The emulsion stability over a period of time was examined and images of the emulsions taken. Optical Microscopy Images Emulsions were prepared by adding 1 mL of dodecane to 10 mL of synthetic seawater. A known amount of the solubilized hydroxylated PI was then added. Emulsions were then prepared by vortex mixing at 3000 rpm for 3 min. Optical Microscopy images were taken using an optical microscope. Surface and Interfacial Tension Measurements The dynamic reduction in the surface and interfacial tension of the dodecane-salt water (synthetic seawater) was measured by the pendant drop method using a standard goniometer. For the surface tension, a known amount of the solubilized hydroxylated PI was added to a known volume of the synthetic seawater. A drop of the hydroxylated PI-synthetic seawater was quickly injected from a syringe. The dynamic surface tension at different hydroxylated PI concentrations was measured by a drop shape analysis using the DROPimage Advanced Software. For the dodecane-salt water interfacial tension, the drop of the hydroxylated PI-salt water solution was injected into dodecane. The dynamic interfacial tension was then determined. 5 ACS Paragon Plus Environment


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Dispersion Effectiveness Determination The dispersion effectiveness of the hydroxylated PI in various salt water solutions was determined using the baffled flask test. 100 𝜇𝐿 of crude oil was added to 120 mL of synthetic sea water in a baffled flask. A known volume (30 mL) of the solubilized hydroxylated PI was added directly on top of the oil, and the baffled flask was placed on an orbital shaker set at 200 rpm for 10 min. A settling time of 10 min was allowed after which 30 mL of the aqueous media was drawn and the dispersed oil was extracted with DCM and quantified with UV-vis at an absorbance difference of 300-400 nm. Results and Discussion Seawater is made up of mainly sodium, magnesium, sulfate and chlorine salts. Each of these salts may have different effect on the stability of oil-in-water emulsions. The effect of different salts on the surface and interfacial characteristics of the formulated soybean lecithin dispersant was examined by forming oil-in-water (o/w) emulsion of dodecane in synthetic sea water (in this study, synthetic seawater refers to salt-deionized water solution) using 0.03 wt.% H-PI. The 0.03 wt.% corresponds to the maximum surfactant concentration used in this study. The dispersant was formulated by solubilizing an appropriate amount of H-PI in deionized water to form 0.1 g/mL concentration. A specific volume of the formulated dispersant was added to form 1:10 v/v dodecane/synthetic seawater. The o/w emulsion was then formed by exerting a shear through a vortex mixer with 3000 rpm. The shear was exerted for 3 min after which the formed emulsion was allowed to settle. The emulsion stability was compared and pictures taken at 0 and 30 min. Figure 1 below shows the pictures of the emulsion formed with different salts using 0.03 wt.% surfactant concentration. It can be inferred from Figure 1 that the salts used in this study have different effect on the stability of the emulsions. At the initial state (t = 0), the emulsions formed were whitish in color. It can be observed that even at t = 0, the nature of the emulsions differ from salt to salt. However after 30 minutes of settling time, phase separation occurred in the emulsions formed with various salts. The extent of phase separation increased in the order Na-Benzoate > Na2SO4 > NaCl > MgCl2 > MgSO4. To have a better view of the o/w emulsions on microscopic level, optical microscopy images of the o/w emulsion formed with NaCl, Na2SO4, MgCl2, MgSO4 and Na-Benzoate were examined (Figure 2). These emulsions were formed with dodecane as the oil phase. 6 ACS Paragon Plus Environment

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Emulsion at t = 0 min (a) MgSO4

(b) MgCl2

(c) NaCl

(d) Na2SO4

(e) Na-Benzoate

Emulsion at t = 30 min

Figure 1: Images of oil-in-water (o/w) emulsions formed with 0.03 wt.% H-PI with 35 g/L (a) MgSO4, (b) MgCl2, (c) NaCl, (d) Na2SO4, and (e) Na-Benzoate.



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Addition of electrolytes resulted in a dramatic decrease in the oil droplet sizes resulting in the formation of a more stable emulsion. It can be seen from Figure 2 that, the emulsions formed with MgCl2 and MgSO4 have smaller oil droplet size as compared with that of NaCl, Na2SO4 and NaBenzoate. The droplet size distribution of the o/w emulsions is shown in Figure 3. It is obvious from Figures 2 and 3 that, the salts used in forming the emulsions affected the emulsion droplet sizes. The droplet size of the o/w formed with the salts were lower than that of the o/w emulsion formed without salts. The droplet size of the emulsion formed decreased in the order MgCl2 < MgSO4 < NaCl < Na2SO4 < Na-Benzoate < No Salt. The emulsions formed tend to coalesce with time. The o/w emulsions formed with MgCl2 and MgSO4 showed a lower tendency to coalesce with time when compared with that of NaCl, Na2SO4 and Na-Benzoate. The coalescence time was computed using a model developed by Hodgson and Woods19 using the equation: 𝜇𝑅 1.75

𝑡𝐶 = 5.202 𝛾0.75 𝐵0.25

(1)

where tc is the coalescence time in seconds (s), 𝜇 is the viscosity of aqueous phase (Pa.s) , 𝛾 is the interfacial tension (N/m), R is the radius of the oil droplet (m) and B is the modified Hamaker constant (1*10-28 Jm). The calculated coalescence times are presented in Table 1 below. Table 1: Coalescence time for emulsions formed with different salts Salt

NaCl Na2SO4 MgCl2 Na-Benzoate MgSO4

Surfactant Concentration (ug/mL) 0.15 0.3 0.15 0.3 0.15 0.3 0.15 0.3 0.15 0.3

1 g/L

Coalescence Time (s) 20 g/L

5.3 6.2 5.0 5.9 6.8 9.3 5.2 5.9 6.6 9.2


6.4 7.8 5.4 6.5 6.6 10.2 5.9 6.9 6.4 8.7

35 g/L 6.7 8.2 6.0 6.7 6.5 9.7 6.3 7.3 6.4 8.8

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(a)

(b)

(d)

(c)

(e)

(f)

Conclusion



Figure 2: Optical Microscopy images of o/w emulsions formed with (a) 35 g/L MgCl2, (b) 35 g/L MgSO4, (c) 35 g/L NaCl, (d) 35 g/L Na2SO4, (e) 35 g/L Na-Benzoate and (f) No Salt

25

Average Droplet Size (micron)

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20

15

10

5

0 No Salt

MgCl2

MgSO4

NaCl

Na2SO4

Na-Benzoate

Salts

Figure 3: Droplet size distribution of the o/w emulsions formed with different salts. The number of droplets analyzed were 34, 100, 85, 90, 85 and 50 for emulsions formed with No Salt, MgCl 2, MgSO4, NaCl, Na2SO4 and Na-Benzoate, respectively.

Generally, the coalescence time increased with increasing salt concentration depicting the effect of salt ions on the emulsion stability. The magnitude of the coalescence time can be related to the stability of the emulsions. A larger coalescence time implies that it will take a relatively longer time for smaller emulsion droplets to coalesce into larger droplets which are unstable. On the other hand a smaller coalescence time implies that it will take a relatively shorter time for smaller emulsion droplets to coalesce into larger droplets. From the calculations, it can be seen that the emulsions formed with MgCl2 and MgSO4 have the highest coalescence times and are responsible for the smaller average droplet sizes as reported in Figure 3. However, the emulsions formed with NaCl, Na2SO4 and Na-Benzoate have relatively smaller coalescence times when compared with the magnesium salts and hence recorded the largest average droplet size. The coalescence of the emulsions resulted in the formation of larger emulsion droplets leading to the reduction of the droplet density as can be seen in Figure 2. The larger droplets rose to the surface of the aqueous phase and resulted in phase separation (Figure 1). The effect of surfactant concentration on the coalescence time was also examined and the results presented in Figure 4. Generally, increasing 10 ACS Paragon Plus Environment

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the surfactant concentration resulted in an increase in coalescence time. However, for the effect of salt on the coalescence time of the droplet in air, a different observation was made. Na-Benzoate recorded the highest coalescence time at all the surfactant concentrations analyzed. On the other hand, NaCl recorded the least coalescence time at all the surfactant concentrations analyzed. As presented in Table 2, NaCl resulted in emulsion formation with relatively larger minimum area per surfactant molecule (Amin). Emulsions with relatively larger Amin are unstable and are expected to have shorter coalescence time. This implies that the nature of the continuous phase also has an effect on the emulsion stability. 4 3.8 3.6

Coalescence Time (s)

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3.4

No Salt

3.2

NaCl-35 g/L Na2SO4- 35 g/L

3

MgCl2-35 g/L

2.8

Na-Benzoate- 35 g/L 2.6

MgSO4-35 g/L

2.4 2.2 0

200

400

600

800

1000

1200

1400

Surfactant Concentration (mg/L)

Figure 4: Effect of surfactant concentration and salts on the coalescence time of oil droplet in air at 35 g/L salt concentration. The generated plots for 1 and 20 g/L salt concentrations are presented in Figures S1 and S2, respectively in the supporting information. To understand the effect of the various salts on the interfacial activity of H-PI formulated dispersant at the oil-droplet water interface, surface and interfacial tension measurements were carried out. The surface tension was measured at different salt concentrations namely 1, 20 and 35 g/L. The effect of the salt on surface tension at 1 and 35 g/L are presented in Figures 5 and 6, respectively. The surface tension measured at 20 g/L is presented in Figure S3 in the supporting information.



It can be seen from Figures 5, 6 and S3 that addition of salt affected the surface characteristics of the H-PI formulated dispersant. The salts resulted in a reduction of the surface tension values recorded. The effect became pronounced as the salt concentration was increased but almost levelled off at salt concentrations between 20 – 35 g/L. For 1 g/L salt concentration, a reduction in surface tension was observed at surfactant concentrations greater than 450 mg/L. It was observed that at 1 g/L NaCl salt concentration, the surface tension increased at surfactant concentrations greater than 600 mg/L. However, for 20 (Figure S3) and 35 (Figure 6) g/L salt concentration, significant reduction in surface tension was observed for some of the salts at surfactant concentrations greater than 300 mg/L. 70

65

60

Surface Tension (mN/m)

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55

No Salt NaCl

50

Na2SO4 MgCl2

45

Na-Benzoate MgSO4

40

35

30

0

200

400

600

800

1000

1200


Figure 5: The effect of 1 g/L salt on surface tension at different H-PI concentrations.


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70

65

60

Surface Tension (mN/m)

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55

No Salt NaCl

50

Na2SO4 MgCl2

45

Na-Benzoate MgSO4

40

35

30 0

200

400

600

800

1000

1200


Figure 6: The effect of 35 g/L salt on surface tension at different H-PI concentrations.

It was observed that the dynamic surface tension was measured over a period of 30 seconds and the average values were then plotted on Figures 5, 6, and S3. It can be seen that at all the salt and surfactant concentrations, the effect of Na-Benzoate, MgSO4 and Na2SO4 on the surface tension reduction was significant. We conclude that magnesium and sodium salts influenced the surface tension of the H-PI formulated dispersant. The interfacial tension at the oil-droplet water interface was also examined using dodecane as the oil phase and the various aqueous salt solutions as the continuous phase. The interfacial tension was measured at two different H-PI concentrations that is 0.015 and 0.03 wt.%. The results for 0.03 wt.% at the salt concentrations of 1 and 35 g/L are presented in Figures 7 and 8, respectively. It is obvious from Figures 7 and 8 that different salts had different effects on interfacial tension. Furthermore, the addition of salts resulted in a reduction in the interfacial tension. At all the salt concentrations analyzed, that is at 1 (Figure 7) and 35 g/L (Figure 8), the effect of MgSO4 and



MgCl2 on the reduction of the interfacial tension was more pronounced than that of NaCl, Na2SO4 and Na-Benzoate at H-PI concentrations of 0.015 and 0.03 wt.%.

25

20

Interfacial Tension (mN/m)

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No Salt 15

NaCl Na2SO4 MgCl2

10

Na-Benzoate MgSO4 5

0 0

5

10

15

20

25

30

Time (seconds)

Figure 7: Dodecane-water interfacial tension at 0.03 wt.% surfactant concentration and 1 g/L salt concentration. At a specified salt concentration, the interfacial tension decreased with increasing surfactant concentration. That is the interfacial tension values recorded at 0.03 wt.% surfactant concentrations were lower than the values recorded at 0.015 wt.%. It was observed that the extent of interfacial tension reduction became pronounced as the salt concentration increased. The interfacial tension plots at 0.015 wt.% surfactant concentration for 1 and 35 g/L salt concentration are presented in Figures S4 and S5, respectively in the supporting information. Increase in surfactant concentration results in close packed arrangement of surfactant molecules at the oil-droplets-water interface. The increased amount of surfactants at the oil-droplets-water interface is responsible for the reduction in interfacial tension.


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25

20

Interfacial Tension (mN/m)

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No Salt 15

NaCl Na2SO4 MgCl2

10

Na-Benzoate MgSO4 5

0 0

5

10

15

20

25

30

Time (seconds)

Figure 8: Dodecane-water interfacial tension at 0.03 wt.% surfactant concentration and 35 g/L salt concentration.

The reduction of the surface and interfacial tension is attributed to the arrangement of surfactants and salt ions at the oil droplets-water interface. This is also related to area occupied by the surfactants at the oil droplets-water interface. The surface excess concentration (Г) which depicts the concentration of surfactants at the oil droplet-water interface was calculated from the equation below:20 Г=

−

1 𝑑𝛾 𝑅𝑇

𝑑 ln 𝐶

(2)

where R is the universal gas constant (J/mol K), C is the surfactant concentration (mol/L), 𝛾 is the interfacial or surface tension (N/m), T is the temperature (K) and 𝑑𝛾⁄𝑑 ln 𝐶 is the slope of the plot of 𝛾 against ln C. Г is the surface excess concentration (mol/m2). The generated plot of 𝛾 against ln 𝐶 at a salt concentration of 35 g/L is presented in Figure 9. A similar plot generated for 1 and 20 g/L salt concentration are presented in Figures S6 and S7, respectively in the supporting information.



70.00 65.00 60.00

Surface Tension (γ) (mN/m)

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55.00

No Salt NaCl

50.00

Na2SO4 MgCl2

45.00

Na-Benzoate MgSO4

40.00 35.00 30.00 3.00

3.50

4.00

4.50

5.00

5.50

6.00

6.50

7.00

7.50

ln C

Figure 9: The surface tension versus natural logarithm of the surfactant concentration at 35 g/L salt concentration. The surface excess concentration at surface saturation (Г𝑚𝑎𝑥 ) was estimated from Figures 9, S6 and S7. The estimated Г𝑚𝑎𝑥 was then used to calculate the minimum area per surfactant molecule at the interface. The minimum area per surfactant molecule (Amin) at the interface was calculated from the surface excess concentration using the equation; 𝐴𝑚𝑖𝑛 = 𝑁Г

1 𝑚𝑎𝑥

(3)

In this equation, N is the Avogadro’s number. The calculated Г𝑚𝑎𝑥 and 𝐴𝑚𝑖𝑛 values at different salt and salt concentrations are presented in Table 2 and in Figure 10. At 35 g/L which is the normal concentration of seawater, it can be seen that the calculated Г𝑚𝑎𝑥 decreased in the order MgSO4 > MgCl2 > NaCl > Na2SO4 > Na-Benzoate. That is MgSO4 allowed for the maximum packing of surfactant at the interface since it recorded the highest Г𝑚𝑎𝑥 . Correspondingly 𝐴𝑚𝑖𝑛 value was smaller for MgSO4. The reverse was the case for Na-Benzoate which recorded the least Г𝑚𝑎𝑥 . This implies that salt used in forming the


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emulsions affected the extent of the surfactant packing at the interface which influenced the surface and interfacial tension as already observed. Table 2: Calculated Г𝑚𝑎𝑥 and 𝐴𝑚𝑖𝑛 values at different salts and salt concentrations Concentration of Salt (g/L) No Salt 1 20 35 1 20 35 1 20 35 1 20 35 1 20 35

CMC (mg/L) 600 MgSO4 750 600 600 Na-Benzoate 600 450 450 MgCl2 450 450 450 Na2SO4 900 600 600 NaCl 900 600 600

Гmax * 106 (mol/m2)

Amin (nm2)

2.235

0.74

2.805 2.987 3.349

0.59 0.56 0.5

2.842 2.236 2.428

0.58 0.74 0.68

2.805 2.807 2.840

0.59 0.59 0.58

2.839 2.936 2.555

0.58 0.57 0.65

3.013 2.642 2.642

0.55 0.63 0.63

A low Amin and a high Гmax is characteristic of dispersants that possess high interfacial activity.21 It is therefore not surprising when the emulsion formed with magnesium salts recorded a relatively lower critical micelle concentration (CMC) (Table 2). At lower salt concentrations, addition of sodium salt with the exception of Na-benzoate resulted in an increase in the CMC. This confirms that salt ions have an effect on the stability of o/w emulsions formed with H-PI.



0.8

0 g/L

1 g/L

20 g/L

35 g/L

0.7 0.6

Amin (nm2)

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Page 18 of 29

0.5 0.4 0.3 0.2 0.1

0 MgSO4

Na-Benzoate

MgCl2

Na2SO4

NaCl

Salts

Figure 10: Minimum area per Surfactant Molecule as a Function of Salt Concentration From Figure 10 above, it can be seen that the Amin varies with salt concentration but the variation is not regular. As an example, while the Amin decreased with increasing concentration for MgSO4, the Amin increased when the NaCl concentration was increased from 1 g/L to 20 g/L and the value remained the same even at 35 g/L. The effect of salt concentration on Amin can be attributed to the salting-out effect of the various salts. Salting-out is the result of preferential movement of water molecules which immobilize and quench their role as solvents, from coordination shells of surfactants molecules to those of salt. Salting-out influences surfactant packing at the oil-water interface which in turn influences Гmax and Amin. For a given surfactant, the salting-out phenomenon has been reported to be dependent on the type and concentration of salt used in the emulsion formation. The trend observed for the variation of emulsion stability with salt concentration is reported to be different from salt to salt

13a, 14

. It is therefore expected that the

variation of Amin with salt concentration will vary from salt to salt and hence the irregularity observed in Figure 10 since emulsion stability is dependent on Amin. A discussion on the variation of Amin with the various salt types examined and other observations has been presented below.


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It is obvious from the result presented above that surface and interfacial characteristics of the soybean formulated dispersant (H-PI) is dependent on the nature of the salt used in formulating the emulsions. What accounted for this observation? A molecular level discussion for this observation is provided here. Soybean lecithin is a complex mixture of phospholipids. The most common phospholipids present in

soybean

lecithin

are

phosphatidylcholine

(PC),

phosphotidylinositol

(PI)

and

phosphatidylethanolamine (PE).22 These components can be obtained by fractionating lecithin with absolute ethanol. This results in ethanol soluble component (PC) and ethanol insoluble component (PI). The unique surface active properties of soybean lecithin is attributed to its phospholipid content.23 Current studies conducted by Nyankson et al. concluded that PC and PI are effective in remediating spilled crude oil.12b It has also been reported that, the effectiveness of PC and PI in remediating crude oil can be enhanced through chemical hydroxylation.12a The surfactant efficiency in lowering the surface and interfacial tension is related to the thermodynamic transfer from bulk to interface.20 The structure of soybean PI is presented in Figure 11 below. The possible structure of its hydroxylated counterpart has already been reported by Nyankson et al.12a It is obvious from Figure 11 that soybean PI surfactant is ionic. Hence its transfer and subsequent adsorption at the interface is expected to be influenced by the presence of electrolytes (salts).

Figure 11: Chemical Structure of Soybean PI.12a Reprinted in part with permission from Nyankson, E.; Demir, M.; Gonen, M.; Gupta, R. B., Interfacially Active Hydroxylated Soybean Lecithin Dispersant for Crude Oil Spill Remediation. ACS Sustainable Chemistry & Engineering 2016, 4 (4), 2056-2067. 2016. ACS Publications. The effect of the salts on the adsorption of PI at the oil-water interface is expected to differ for all salts. The natural sea water has a higher content of chlorine, sodium, magnesium and sulfate ions14 19 ACS Paragon Plus Environment


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Page 20 of 29

and hence our choice of the salts examined in this study. It can be seen that the salts ions influenced the adsorption of the ionic H-PI at the oil-droplets water interface. At 35 g/L salt concentration, MgSO4 and Na-Benzoate were more effective in reducing the surface tension than NaCl, Na2SO4 and MgCl2. However, the magnesium salts were more effective in reducing the interfacial tension than the sodium salts (Figures 8 and 9). It has been postulated that when ionic surfactant molecules are adsorbed at the interface, the counterions are arranged in the form of diffused layer near them. A possibility therefore exist for the ionic interaction between the adsorbed head groups and the counterions.24 This interaction is known as counterion binding. This phenomena is responsible for the stabilization of o/w emulsions when salts are present.25 With soybean H-PI possessing a charged head group, it is expected that the electrostatic repulsion between the charged head groups will reduce the amount of surfactants adsorbed at the oil-droplets water interface (Figure 12). The polar head groups of H-PI in water repel one another due to mutual charge repulsion.15 If the charge is large, the repulsion is great and micelles do not form. Hence the reduced surfactant at the interface which hinders the reduction in surface and interfacial tension can be attributed to increased surfactant diffusion from the interface into the bulk continuous phase. The surfactants at the oil-water interface is depleted and this results in the coalescence of smaller oil droplets into larger ones. On the other hand the decreased surface and interfacial tension upon addition of the various salts can be attributed to increased surfactant adsorption at the oil-droplet water interface. The counter ion phenomena reduced the electrostatic repulsion among the charged head groups (Figure 12).


(a)

Electrostatic repulsion between charged head groups

(b)

After some time

Oil droplet

Electrostatic repulsion between charged head groups results in the desorption of H-PI from the interface

_

_ _

Sea water

Oil Oil droplet droplet

Desorption of H-PI surfactants from the interface results in an increase in interfacial tension

_

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_

Page 21 of 29

H-PI SURFACTANT

Figure 12: (a) The adsorption and (b) desorption of H-PI surfactant at the oil-droplet water interface. The reduction of electrostatic repulsion between the charged head groups has two effects. Firstly, it allows close packing of the charged head groups at the oil-droplets water interface and this results in higher value of the surface excess. As can be seen in Table 2 and in Figure 10, with the exception of sodium benzoate, the maximum surface excess generally increased (for 20 g/L and 35 g/L salt concentration) after the addition of salt. An increase in the maximum surface excess resulted in a reduction of the area occupied by surfactant molecule depicting that more of the surfactants were closely packed at the oil-droplet water interface. This is evident from the significant reduction in the interfacial and surface tension upon the addition of the various salts examined. Secondly, it has been reported that counterion binding result in the reduction of double layer repulsion and this results in increased maximum surface excess.26 With increase in the amount of surfactant at the interface, the tails of the surfactant molecules associate with each other through hydrophobic interactions. This resulted in an increased interfacial viscosity. Increase in the interations among the surfactant tails also results in a decrease in surface diffusivity of the surfactant molecules.27 The H-PI molecules remained at the interface (Figure 13) and as a result the emulsions remained stable. This explanation is confirmed by the calculated Гmax and Amin values presented in Table 2.



The close packed surfactant at the oil droplets water interface resulted in the reduction of the interfacial tension and this resulted in the formation of stable oil-in-water emulsion. A much stable oil in water emulsion is characterized by smaller oil droplets. As already mentioned, the emulsions formed with the salts resulted in smaller oil droplets which were more stable (Figures 1,2 and 3). Electrostatic repulsion between charged head groups is reduced by the counterion

+

Sea water

The counterion binding reduces the electrostatic repulsion between the charged head groups. This allows the surfactants to be adsorbed at the oil-droplets interface for a long time resulting in a significant reduction in the oil-water interfacial tension and stable o/w emulsions.

+ +

+ +

+

Oil droplet

+

+

+

+ + +

+

_

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Page 22 of 29

H-PI surfactant

+ +

-

+

+

Ions from salt -

Figure 13: Effect of salt ions (counterion) on the adsorption of (H-PI) surfactants at the oil dropletswater interface. It was observed that the effect of the various salts on the reduction of the surface and interfacial tensions differs. As an example, salts with divalent counterions (Mg2+) were more effective in reducing the interfacial tension than salts with monovalent ion (Na+). This observation can be attributed to the stabilization role of the Mg2+ to H-PI head groups in the adsorption layer due to their larger electric charge. In the Stern layer, Mg2+ neutralizes the charge of two H-PI ionic molecules thus decreasing the electrostatic repulsion between the charged head groups of H-PI. Furthermore, the ionic radius of Mg2+ (0.072 nm) is smaller than Na+ (0.102 nm) hence it is expected that more of the Mg2+ will be packed in between the H-PI head groups at the oil-dropletwater interface than Na+. As a result, closed-packed adsorption of H-PI at the oil droplet-water interface is facilitated, increasing the density of the adsorption layer and its elasticity. This subsequently “solidify” the oil droplet surfaces28 resulting in the formation of a much stable emulsion (Figure 14). This is also responsible for the variation of Amin with salt concentration observed in Table 2 and Figure 10 since the different salts influenced the emulsion stability differently at the various salt concentrations examined. Since the interfacial and surface tension of 22 ACS Paragon Plus Environment

Page 23 of 29

NaCl/ MgCl2, MgCl2/MgSO4, NaCl/Na2SO4 and MgSO4/Na2SO4 differ, it can be hypothesized that the coions (SO42-, Cl-) and the continuous phase also affected the emulsion stability by the various salts.

Mg2+ ions interconnect the negatively

Mg2+ ions interconnect the negatively charged headgroups of H-PI at the oil droplets-water interface

charged headgroups of the H-PI at the oil Sea water

droplet-water interface. This results in the stabilization of the o/w emulsions.

__

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Oil droplet

+ +

Mg2+ ions

-

Coions

-

H-PI surfactant

Figure 15: Emulsion stability by Mg2+ ions Figure 14: Possible Mechanism for the adsorption of Mg2+ ions at the oil droplet-water interface. The practical application of this work was examined using the U.S. EPA’s baffled flask test. This is an approved procedure used in examining the effectiveness of a formulated dispersant in dispersing crude oil. In this case since the same formulated dispersant was used under similar experimental conditions, the baffled flask test was used to examine the effect of magnesium and sodium salts on dispersion effectiveness. Hence the higher the dispersion effectiveness, the stronger the effect of salt in reducing the interfacial tension. The dispersion effectiveness obtained from the different salts are presented in Figure 15. Dispersion effectiveness values obtained from the baffled flask test confirmed that indeed, salts with magnesium ions resulted in the formation of much stable emulsions. At surfactant-to-oil ratio (SOR) of 37.5 mg/g, dispersion effectiveness values of 82.9, 71.4, 53.4, 48.8 and 32.8 % were obtained for MgSO4, MgCl2, Na2SO4, NaCl and Na-Benzoate, respectively. This trend is in line with the observations made in the emulsion stability and interfacial tension analysis.



90 80

Dispersion Effectiveness (%)

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Page 24 of 29

70 60 50 40 30 20 10 0 NaCl

Na-Benzoate

Na2SO4

MgCl2

MgSO4

Salt

Figure 15: Dispersion Effectiveness of H-PI in different salts (35 g/L) at Surfactant-to-Oil ratio (SOR) of 37 mg/g

It is therefore obvious that the salts used in the formation of the emulsion affected the emulsion stability. In general, the effect of the inorganic salts in reducing the surface and interfacial tension was very significant than the organic salt. The salt concentration in seawater is usually around 35 g/L, however the study was conducted at other salt concentrations of 20 g/L and 1 g/L. The study was conducted at lower salinity (20 g/L and 1 g/L) to examine scenarios where oil spill occurs on seas with lower salinity such as the Baltic sea and also on surface fresh water (for onshore oil drilling and crude oil transportation). The results from the study have shown that interfacial characteristics of hydroxylated soybean lecithin dispersant (H-PI) is dependent on the type of salt and also on salt concentration. This study will help in predicting the possible outcome of oil spill response for the case where soybean lecithin dispersant is applied.

Conclusions The present study demonstrates the effect of salt ions on the interfacial characteristics of soybean lecithin formulated dispersant. The results from the study showed that, different salts affect the


Page 25 of 29

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stability of emulsions formed with H-PI differently. The emulsions formed with magnesium containing salts were very stable compared to that of sodium with organic sodium benzoate salts forming the most unstable emulsion. The magnesium containing salts also recorded the least droplet diameter and the highest emulsion coalescence time. Addition of the salts resulted in a significant reduction of the surface and interfacial tension values. That is the interfacial and surface characteristics of the soybean lecithin dispersant was improved by the addition of salts. However, it was observed that, the values of the surface and interfacial tension recorded were dependent on the concentration and type of salt. Magnesium salts (MgCl2 and MgSO4) reduced significantly the interfacial tension when compared with the sodium salts (NaCl, Na2SO4 and sodium benzoate). A decrease in the interfacial tension upon the addition of salts was attributed to close packing of the H-PI surfactants at the oil droplets-water interface. This was confirmed by calculating Amin and Гmax. At salt concentration of 35 g/L, magnesium containing salts recorded the highest Гmax and the least Amin. The Гmax and Amin values obtained for MgSO4 were 3.349*10-6 mol/m2 and 0.5 nm2, respectively. A slightly different values of 2.840*10-6 mol/m2 for Гmax and 0.55 nm2 for Amin were reported for MgCl2.The CMC decreased with increasing salt concentration with the magnesium salt recording a relatively lower CMC. We hypothesized that, the Mg2+ interconnect the H-PI polar headgroups at the oil droplet water interface resulting in the reduction of the electrostatic repulsion between the charged head groups. In addition more of Mg2+ will be packed at the interface due to its smaller ionic radius (0.072 nm). These subsequently resulted in closed packed arrangement of H-PI at the oil droplets-water interface and hence the improved interfacial activity observed for MgSO4 and MgCl2. The practical effect of the salts on oil spill remediation using the baffled flask test confirmed that the salts have different effect on the dispersion effectiveness of H-PI formulated dispersant with MgSO4, MgCl2, NaCl, Na2SO4 and sodium benzoate recording a dispersion effectiveness of 82.9, 71.4, 53.4, 48.8 and 32.8 %, respectively. It can be concluded that, the different salts affect the interfacial characteristics of H-PI differently with the magnesium salts used in this study being more effective in reducing the interfacial tension than the sodium salts. The results from this study suggest that oil spill respond planners should take into consideration the salts present in the marine oil spill environment during chemical dispersant application.



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Page 26 of 29

Supporting Information The following Figures are available: Effect of surfactant concentration and salts on the coalescence time of oil droplets (for 1 and 20 g/L salt concentrations), effect of salts on surface tension (for 20 g/L salt concentration), dodecane-water interfacial tension (for 1 and 35 g/L salt concentrations), and surface tension versus natural logarithm of surfactant concentration (for 1 and 20 g/L salt concentrations). Reference (1). (a) Etkin, D. S. Analysis of oil spill trends in the United States and worldwide, International Oil Spill Conference 2001, 1291, American Petroleum Institute; (b) Goldstein, B. D.; Osofsky, H. J.; Lichtveld, M. Y., The Gulf oil spill. N. Engl. J. Med. 2011, 364, 1334. (2). (a) De Jong, E., The effect of a crude oil spill on cereals. Environ. Pollut., Ser. A, 1980, 22, 187; (b) Hjermann, D. Ø.; Melsom, A.; Dingsør, G. E.; Durant, J. M.; Eikeset, A. M.; Røed, L. P.; Ottersen, G.; Storvik, G.; Stenseth, N. C., Fish and oil in the Lofoten–Barents Sea system: synoptic review of the effect of oil spills on fish populations. Mar. Ecol. : Prog. Ser. 2007, 339, 283; (c) Pennings, S. C.; McCall, B. D.; Hooper-Bui, L., Effects of oil spills on terrestrial arthropods in coastal wetlands. BioScience 2014, 64, 789; (d) Alford, J. B.; Peterson, M. S.; Green, C. C., Impacts of Oil Spill Disasters on Marine Habitats and Fisheries in North America. CRC Press; New York, 2014; (e) Atlas, R. M., Petroleum biodegradation and oil spill bioremediation. Mar. Pollut. Bull. 1995, 31, 178. (3). (a) Nyankson, E.; Rodene, D.; Gupta, R. B., Advancements in crude oil spill remediation research after the Deepwater Horizon oil spill. Water, Air, Soil Pollut. 2016, 227, 29; (b) Nyankson, E. Smart Dispersant Formulations for Reduced Environmental Impact of Crude Oil Spills. Auburn University, 2015. (4). Venkataraman, P.; Tang, J.; Frenkel, E.; McPherson, G. L.; He, J.; Raghavan, S. R.; Kolesnichenko, V.; Bose, A.; John, V. T., Attachment of a hydrophobically modified biopolymer at the oil–water interface in the treatment of oil spills. ACS Appl. Mater. interfaces 2013, 5, 3572. (5). Sorial, G. A.; Venosa, A. D.; Koran, K. M.; Holder, E.; King, D. W., Oil spill dispersant effectiveness protocol. II: Performance of revised protocol. J. Environ. Eng.(Reston, VA, U.S.) 2004, 130, 1085. (6). Lewis, A.; Byford, D.; Laskey, P. The significance of dispersed oil droplet size in determining dispersant effectiveness under various conditions, International Oil Spill Conference 1985, 433, American Petroleum Institute:. (7). (a) Swannell, R. P.; Daniel, F. Effect of dispersants on oil biodegradation under simulated marine conditions, International Oil Spill Conference 1999, 169, American Petroleum Institute; (b) Chapman, H.; Purnell, K.; Law, R. J.; Kirby, M. F., The use of chemical dispersants to combat oil spills at sea: A review of practice and research needs in Europe. Mar. Pollut. Bull. 2007, 54, 827. (8). (a) Almeda, R.; Hyatt, C.; Buskey, E. J., Toxicity of dispersant Corexit 9500A and crude oil to marine microzooplankton. Ecotoxicol. Environmental Saf. 2014, 106, 76; (b) Swedmark, M.; Granmo, Å.; Kollberg, S., Effects of oil dispersants and oil emulsions on marine animals. Water Res. 1973, 7, 1649.


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(9). Makkar, R.; Cameotra, S., An update on the use of unconventional substrates for biosurfactant production and their new applications. Appl. Microbiol. Biotechnol. 2002, 58, 428. (10). (a) Owoseni, O.; Nyankson, E.; Zhang, Y.; Adams, S. J.; He, J.; McPherson, G. L.; Bose, A.; Gupta, R. B.; John, V. T., Release of surfactant cargo from interfacially-active halloysite clay nanotubes for oil spill remediation. Langmuir 2014, 30, 13533; (b) Nyankson, E.; Olasehinde, O.; John, V. T.; Gupta, R. B., Surfactant-loaded halloysite clay nanotube dispersants for crude oil spill remediation. Ind. Eng. Chem. Res. 2015, 54, 9328; (c) Owoseni, O.; Nyankson, E.; Zhang, Y.; Adams, D. J.; He, J.; Spinu, L.; McPherson, G. L.; Bose, A.; Gupta, R. B.; John, V. T., Interfacial adsorption and surfactant release characteristics of magnetically functionalized halloysite nanotubes for responsive emulsions. J. Colloid Interface Sci. 2016, 463, 288. (11). Nyankson, E.; Ober, C. A.; DeCuir, M. J.; Gupta, R. B., Comparison of the Effectiveness of solid and solubilized dioctyl sodium sulfosuccinate (DOSS) on oil dispersion using the baffled flask test, for crude oil spill applications. Ind. Eng. Chem. Res. 2014, 53, 11862. (12). (a) Nyankson, E.; Demir, M.; Gonen, M.; Gupta, R. B., Interfacially Active Hydroxylated Soybean Lecithin Dispersant for Crude Oil Spill Remediation. ACS Sustainable Chem. Eng. 2016, 4, 2056; (b) Nyankson, E.; DeCuir, M. J.; Gupta, R. B., Soybean lecithin as a dispersant for crude oil spills. ACS Sustainable Chem. Eng. 2015, 3, 920. (13). (a) Mukerjee, P.; Chan, C. C., Effects of High Salt Concentrations on the Micellization of Octyl Glucoside: Salting-Out of Monomers and Electrolyte Effects on the Micelle− Water Interfacial Tension1. Langmuir 2002, 18, 5375; (b) Wattebled, L.; Laschewsky, A., Effects of organic salt additives on the behavior of dimeric (“Gemini”) surfactants in aqueous solution. Langmuir 2007, 23, 10044. (14). Grover, P. K.; Ryall, R. L., Critical appraisal of salting-out and its implications for chemical and biological sciences. Chem. Rev. (Washington, DC, U.S.) 2005, 105, 1. (15). Giribabu, K.; Reddy, M.; Ghosh, P., Coalescence of air bubbles in surfactant solutions: role of salts containing mono-, di-, and trivalent ions. Chem. Eng. Commun. 2007, 195, 336. (16). (a) Shikata, T.; Hirata, H.; Kotaka, T., Micelle formation of detergent molecules in aqueous media. 3. Viscoelastic properties of aqueous cetyltrimethylammonium bromide-salicylic acid solutions. Langmuir 1989, 5, 398; (b) Hassan, P.; Yakhmi, J., Growth of cationic micelles in the presence of organic additives. Langmuir 2000, 16, 7187. (17). Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L., Electrolyte effects on the cationic micelle catalyzed decarboxylation of 6-nitrobenzisoxazole-3-carboxylate anion. J. Am. Chem. Soc. 1973, 95, 3262. (18). Shikata, T.; Imai, S.-i.; Morishima, Y., Self-diffusion of constituent cationic surfactants in threadlike micelles. Langmuir 1998, 14, 2020. (19). Hodgson, T.; Woods, D., The effect of surfactants on the coalescence of a drop at an interface. II. J. Colloid Interface Sci. 1969, 30, 429. (20). Myers, D., Surfactant science and technology. John Wiley & Sons: New Jersey, 2005. (21). Asadov, Z. H.; Rahimov, R. A.; Salamova, N. V., Synthesis of animal fats ethylolamides, ethylolamide phosphates and their petroleum-collecting and dispersing properties. J. Am. Oil Chem. Soc. 2012, 89, 505. (22). Kjellin, M.; Johansson, I., Surfactants from renewable resources. John Wiley & Sons: United Kingdom, 2010. (23). (a) Wu, Y.; Wang, T., Soybean lecithin fractionation and functionality. J. Am. Oil Chem. Soc. 2003, 80, 319; (b) Szuhaj, B. F., Lecithins: sources, manufacture & uses. The American Oil Chemists Society: Illinois, 1989. 27 ACS Paragon Plus Environment


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(24). Kalinin, V.; Radke, C., An ion-binding model for ionic surfactant adsorption at aqueousfluid interfaces. Colloids Surf., A 1996, 114, 337. (25). Gurkov, T. D.; Dimitrova, D. T.; Marinova, K. G.; Bilke-Crause, C.; Gerber, C.; Ivanov, I. B., Ionic surfactants on fluid interfaces: determination of the adsorption; role of the salt and the type of the hydrophobic phase. Colloids Surf., A 2005, 261, 29. (26). Ghosh, P.; Juvekar, V., Analysis of the drop rest phenomenon. Chem. Eng. Res. Des. 2002, 80, 715. (27). Ghosh, P., Coalescence of air bubbles at air–water interface. Chem. Eng. Res. Des. 2004, 82, 849. (28). Angarska, J.; Tachev, K.; Ivanov, I.; Mehreteab, A.; Brose, G., Effect of magnesium ions on the properties of foam films stabilized with sodium dodecyl sulfate. J. Colloid Interface Sci. 1997, 195, 316.


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