Chemical reaction-driven spreading of organic extractant on gas-water

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Chemical reaction-driven spreading of organic extractant on gas-water interface: Insight into controllable formation of gas bubble-supported organic extractant liquid membrane jie liu, Kun Huang, Wenqian Liu, and Huizhou Liu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03954 • Publication Date (Web): 18 Feb 2019 Downloaded from http://pubs.acs.org on February 21, 2019

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Chemical reaction-driven spreading of organic extractant on

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gas-water interface: Insight into controllable formation of gas

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bubble-supported organic extractant liquid membrane

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Jie Liua, Kun Huangb,c*, Wenqian Liuc, Huizhou Liuc

6 7

a. School of Environmental and Municipal Engineering, Qingdao University of

8

Technology, Qingdao 266100, P.R. China

9

b. School of Metallurgical and Ecological Engineering, University of Science &

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Technology Beijing, Beijing 100083, P.R. China

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c. CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and

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Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266100, P.R. China

13 14

* To whom correspondence should be addressed:

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Prof. Dr. Kun HUANG

16

Full postal address: School of Metallurgical and Ecological Engineering, University

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of Science & Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing

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100083, P. R. China

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E-mail: [email protected]

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TEL:

86-10-62332926;

FAX:

1

ACS Paragon Plus Environment

86-10-62332926

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ABSTRACT

2

Extraction and recovery of low-concentration valuable metals from various

3

complex aqueous solutions or industrial waste waters have attract extensive interests

4

in recent years. In our previous works, we suggested a novel technique called

5

bubbling organic liquid membrane extraction by spreading and covering an organic

6

extractant with extremely small volume on the surface of gas bubbles to form a layer

7

of gas bubble-supported organic liquid membrane for selective extraction and

8

enrichment of low-concentration targets from dilute aqueous solutions. It was found

9

that, for successfully performing the bubbling organic liquid membrane extraction, a

10

prerequisite is how to control the formation of stable organic liquid membrane

11

covered on the surface of gas bubbles. However, once the organic extractant starting

12

to spread on the surface of gas bubbles, the extraction chemical reaction at the

13

interface between organic extractant liquid membrane and rare-earth aqueous solution

14

will happen. In the present work, the spreading behavior of organic extractant P507

15

on the surface of rare-earth aqueous solutions were investigated, and was compared

16

with the behaviors on the surface of deionized water. It was revealed that the

17

spreading of organic extractant P507 on the surface of aqueous solutions containing

18

rare-earth ions was accelerated due to the occurrence of the chemical reactions at the

19

gas-water interface. The difference in spreading rate of organic extractant P507 liquid

20

droplets respectively on the surface of deionized water and on that of Er(III) aqueous

21

solutions with the increase in P507 concentration, the saponification degrees of P507

22

extractant and the pre-loading amount of Er(III) in P507 extractant revealed that the 2


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chemical reaction at the interface between the spreading P507 thin liquid membrane

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and Er(III) aqueous solution would result in the Marangoni convection along the

3

interface, which is favor of overcoming the resistance from the viscous force when

4

surface tension gradient replaces the gravity as a dominant driving force for the

5

spreading. The present work provides an experimental foundation towards

6

understanding the effect of the interfacial chemical reaction on the spreading behavior

7

of organic oil droplet on the gas-water interface.

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of our suggested new technique of bubbling organic liquid membrane extraction, and

9

to achieve a controllable generation of stable gas bubble-supported organic liquid

10

It is beneficial for the development

membrane for performing solvent extraction at large aqueous-to-oil phase ratios.

11 12 13

Keywords: Spreading and Wetting; Organic extractant; Solvent extraction; Bubbling

14

organic liquid membrane extraction; Rare earths

15 16

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INTRODUCTION

2

Nowadays, extraction and recovery of low-concentration valuable metals from

3

various complex aqueous solutions or industrial waste waters have attracted extensive

4

research interests.1 In our previous works, we suggested a novel technique called

5

bubbling organic liquid membrane extraction by spreading and covering a layer of

6

organic extractant with extremely small volume on the surface of gas bubbles to form

7

a gas bubble-supported organic liquid membrane for selective extraction and

8

enrichment of low-concentration targets from dilute aqueous solutions.2 Experimental

9

results demonstrated that the suggested new technique exhibited a great potential in

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promoting the development of traditional oil-water two-phase solvent extraction to

11

treat the low-concentration diluted aqueous solutions economically. The dispersed gas

12

bubbles covered with organic extractant ascended in the extraction tower and

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contacted counter-currently with the aqueous phase. Under the action of shearing

14

force of counter-current water phase, the renewal of the oil-water interface of the

15

organic thin membrane layer on the surface of gas bubbles was very fast, so that the

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target metal ions with very low concentrations in the large volume of aqueous

17

solutions could be quickly captured and enriched into the organic liquid membrane

18

layer on the surface of gas bubbles at large aqueous-to-oil phase ratios (generally

19

above 600:1).3

20

However, it was found that, for successfully performing the bubbling organic

21

liquid membrane extraction, a prerequisite was how to control the formation of stable

22

organic liquid membrane covered on the surface of gas bubbles.4 When the gas was 4


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bubbled into the aqueous solutions through the internal gas needles of the injector, the

2

organic extractant passed through the annular gaps between the internal gas needles

3

and the external organic needles, and then met with the gas bubbles on the top end of

4

the gas needles, so that the organic extractant could be dispersed and spread on the

5

surface of gas bubbles, due to its surface activity. If the pumping speed of the gas and

6

the organic phase were kept at a certain value, and if the materials used for making

7

the internal gas and external organic needles of the injector in bubbling organic liquid

8

membrane extractor were carefully designed, the formation of stable gas

9

bubble-supported organic liquid membrane from the submerged orifice of the

10

injectors in the large-phase-ratio extractor was closely related to the wettability and

11

spreading behavior of organic extractant on the surface of gas bubbles.5 Therefore, in

12

order to achieve a controllable generation of stable gas bubble-supported organic

13

liquid membrane, it is necessary to understand what factors might bring obvious

14

influence on the spreading behavior of organic extractant and which one is the most

15

crucial.

16

Investigations in the literatures about the spreading behavior of organic liquid

17

droplets on the gas-water interface have been extensively reported.6-8 Early,

18

Robinson and Woods suggested that the spreading free energy, E, could be used to

19

judge whether an organic liquid droplet could enter the air-water interface: 9

E   aw   ow   ao

20

(1)

21

where,

aw

is the surface tension of water.

22

liquid film.

ow

is the interface tension between organic liquid film and water. When

ao

is the surface tension of organic

5


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E >0, it was thought that the organic liquid droplet could enter into the interface

2

between air and water, and to form a lens or spread to form a thin organic liquid film.

3

Since that, Harkins and Feldman studied the wetting and spreading behavior of an

4

organic liquid drop after entering the interface between air and water.10 Their works

5

indicated that whether or not the organic liquid droplet could spread on the air-water

6

interface was closely depended on the spreading coefficient, S :

7

s   aw   ow   ao

(2)

8

When S 0, the spreading of organic liquid droplet on

10

the air-water interface was possible and finally an organic lens or a thin organic liquid

11

film could be obtained on the surface of water.

12

Based on those pioneered works, the researchers combined together the free

13

energy, E, and the spreading coefficient, S, to further describe the spreading behavior

14

of organic liquid droplet on the air-water interface.11 It was found that there were

15

three types of spreading behavior of organic droplet on the air-water interface. When

16

E 98%, purchased from Xiamen Pioneer Technology Co. Ltd., was

14

used as the organic extractant. The 260# sulfonated kerosene, purchased from China

15

Petrochemical Corporation (Sinopec Group), was used as the diluent. The content of

16

various aromatic hydrocarbons in the 260# sulfonated kerosene was below 1%. The

17

stock organic solutions of P507 in kerosene with different molar concentrations of

18

P507 were prepared by respectively mixing a certain amount of P507 with kerosene at

19

different volume ratios. All of the organics were used as received, without further

20

purification. Analytical-grade (purity > 99%) of sodium hydroxide, purchased from

21

Sinopharm Chemical Reagent Co., Ltd, was dissolved into deionized water to prepare

22

3 mol/L NaOH aqueous solutions for saponification of P507 in kerosene solutions. 9


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The desired saponification degrees of P507 in kerosene solutions were obtained

2

according to the procedures as mentioned in previous literatures.31 The stock aqueous

3

solutions containing a certain amount of Erbium (Er) were prepared by firstly

4

dissolving analytical grade (purity > 99%) of Er2O3 (purchased from Ganzhou

5

Shunyuan Rare Earth Material Co., Ltd) using 37 wt% concentrated HCl aqueous

6

solutions, and then boiled the aqueous solutions to drive out the HCl, and then diluted

7

to the desired concentrations using deionized water. The pre-loaded P507 organic

8

solution containing a certain amount of Erbium were prepared by mixing the

9

pre-saponified P507 organic solution with the stock aqueous solutions containing a

10

known quantity of Erbium for 30 min at room temperature. After oil and aqueous

11

phase separation, the concentrations of Erbium in the loaded organic phase were

12

determined.

13

The other chemicals were of analytical grade.

14 15

Experimental Set-up.

16

Experimental set-up consisted of an injection pump (TSD01-01, Baoding

17

Leadfluid, Ltd. Co., China), a backlight (FJI-AS150150-W, Rocke TECH. Ltd. Co.,

18

China), a high-speed video camera (CP80-3-M-540, Optronis, Germany) and a high-

19

speed image recording processing system (AcutEye V3.0, Rocke TECH. Ltd. Co.,

20

China), as shown in Fig. 1.

21

10


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Figure 1. Experimental set-up

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Experimental supporting liquids for spreading of P507 organic solutions were

5

respectively the deionized water, or the Er(III) aqueous solutions prepared from its

6

original stock solution as mentioned in section 2.1. The concentrations of Er(III) in

7

the supporting aqueous solution were 30 mg/L, and the pH of the supporting Er(III)

8

aqueous solutions were 5.4. The aim to adjust the pH of the supporting Er(III)

9

aqueous solutions to 5.4 is for the comparison with the spreading on the surface of the

10

deionized water. The supporting liquids were previously poured into a Petri dish with

11

a diameter of 10 cm and a depth of 1.8 cm. The depth of Er(III) aqueous solutions or

12

deionized water in the petri dish was 1.0 cm. An injection pump was used to generate

13

stable organic droplets with a fixed volume of 5μL for per droplet. In order to

14

minimize the impact of falling velocity of the organic droplet on its spreading on the

15

surface of the tested supporting aqueous solution, the syringe needle of the injection

16

pump was bent to 90 degree, and the distance between the needle tip and the surface 11


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of the tested supporting aqueous solution in the petri dish was pre-set as 1~2 mm. The

2

high-speed video camera, equipped with Nikon micro-lens (50 mm, 1: 1.4 fixed focus

3

lens), was fixed on a tripod and connected to the high-speed image recording

4

processing system. The micro-lens of camera was located above the tested surface,

5

and the spreading behavior of organic droplets on the surface of the tested supporting

6

aqueous solution could be recorded by the high-speed image recording processing

7

system. The backlight, fixed on the shelf and kept at an angle of 30 to 45 degree with

8

the micro-lens of the camera, was used to provide a cold light source for different

9

brightness. The spreading images obtained from the high-speed camera were

10

processed by the software Image Pro Plus 6.0.

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The view-field of the high-speed video camera was 13.57 mm*13.68 mm. The

12

maximum resolution of the camera was 1696*1710 dpi. The exposure time was 2μs.

13

The image acquisition frame of the camera was 543 fps, and the storage frame rate

14

was 25 fps.

15

Similar experimental procedures could be referred to other works .32

16 17

Analysis.

18

The concentrations of Er(III) in the pre-loaded P507 organic phases were

19

determined by an Agilent 7900 ICP-MS system (Agilent Technologies, USA). The

20

saponification degrees of P507 in organic phases were determined by the acid value

21

titration processes using 0.1 mol/L NaOH aqueous solutions. The interfacial viscosity

22

and elasticity modulus of the sample organic droplets with different P507 12


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concentrations, the saponification degrees of P507 and pre-loading concentrations of

2

Er(III) were measured by the oscillating barrier method performed using a Kruss

3

K100 Mk2 tensiometer, and the surface and interface tensions of the sample organic

4

solutions respectively on the surface of deionized water and that of Er(III) aqueous

5

solutions were measured by the Wilhelmy plate method. Detailed information about

6

the apparatus and the experimental procedures for the determination of the interfacial

7

viscosity and elasticity modulus, and the surface and interface tension could be

8

referred to previous literatures.33-35 The viscosity of the sample organic solutions were

9

recorded by using an Ubbohde's viscosimeter (NDJ-8S, Shanghai, China).

10 11

Data processing.

12

The time-dependent changes in spreading area of the organic droplets with

13

different P507 concentrations, the saponification degrees of P507 extractant and the

14

pre-loading amount of Er(III) in organic solution were recorded respectively on the

15

surface of deionized water and on the surface of Er(III) aqueous solutions, as shown

16

in Fig. S1 to S6 in Supporting Information.

17

In order to modelling the spreading process of P507 organic droplets on the

18

surface of different tested supporting aqueous solutions, the time-dependent changes

19

in the spreading areas of P507 organic droplets with the increase of P507

20

concentrations, the saponification degrees of P507 extractant and the pre-loading

21

amount of Er(III) in organic solution were curve-fitted by using the classical

22

spreading area- time model, as shown in equ.(3), which was demonstrated reasonable 13


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in most cases for the description of the spreading kinetics of organic oil droplets on

2

the surface of water in previous literatures 36-38: A= kt n

3

(3)

4

In which, A is the spreading area of P507 organic droplets on the surface of the

5

tested supporting aqueous solutions. t is the spreading time. n is an empirical constant.

6

k is the empirical spreading coefficient. All the fitting equations were listed in Table

7

S1 to S6 in Supporting Information. The spreading rate of P507 organic droplets can

8

be expressed as:

dA  nkt n 1 dt

9

(4)

10 11

RESULTS

12

The effect of P507 concentrations.

13

Fig. 2 depicts the time-dependent changes in spreading rate of the P507 organic

14

droplets with the increase in the concentrations of P507 extractant, respectively on the

15

surface of deionized water (Fig.2a) and on the surface of Er(III) aqueous solutions

16

(Fig.2b).

17 18

14


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Figure 2. (a) The time-dependent changes in spreading rate of P507 organic droplet on the surface

3

of deionized water with the increase in P507 concentrations. (b) The time-dependent changes in

4

spreading rate of P507 organic droplet on the surface of Er(III) aqueous solutions with the

5

increase in P507 concentrations.

6

(The saponification degrees of P507 in the organic phases were 5%. The concentration of

7

pre-loaded Er(III) in P507 organic phases were 0 g/L. The concentrations of Er(III) in aqueous

8

solution were 30 mg/L, and the pH of the Er(III) aqueous solutions were 5.4. )

9 10

As can be seen in Fig.2, both the spreading rates of P507 organic droplets on the

11

surface of deionized water and on that of Er(III) aqueous solutions decreased quickly

12

at initial time, and the spreading rates of P507 organic droplets decreased obviously

13

with the increase of P507 concentrations. However, the spreading rates of P507

14

organic droplets containing the same concentrations of P507 on the surface of the

15

aqueous solutions of Er(III) ions was larger than that on the surface of deionized

16

water at a certain time.

17

The change in the spreading rates can be divided as two typical stages: an initial

18

fast-speed decreasing of the spreading rates with the time, and then followed a 15


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1

slow-speed spreading stage. Here, for convenience of discussion, the so-called

2

slow-speed spreading stage is defined as that in which the change in the spreading

3

rates with the time is very slowly or even not changed any more. Comparing the

4

spreading process of P507 organic droplets on the surface of deionized water and on

5

that of Er(III) aqueous solutions, the change in the transition points of the spreading

6

rates from the initial fast-speed decreasing stage to the following slow-speed

7

spreading with the increase of the P507 concentrations are depicted in the Fig. 3.

8

9 10

Figure 3. (a) The change in the transition points of the spreading rates of P507 organic droplet on

11

the surface of deionized water with increasing the P507 concentrations. (b) The change in the

12

transition points of the spreading rates of P507 organic droplet on the surface of Er(III) aqueous

13

solutions with increasing the P507 concentrations.

14

(The saponification degrees of P507 in the organic phases were 5%. The concentration of

15

pre-loaded Er(III) in P507 organic phases were 0 g/L. The concentrations of Er(III) in aqueous

16

solution were 30 mg/L, and the pH of the Er(III) aqueous solutions were 5.4.)

17 18

The transition points of the spreading rates from the initial fast-speed decreasing 16


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to the following slow-speed spreading are defined as the intersections of the two

2

straight lines, respectively corresponding to the linear fitting of those points in the

3

initial fast-speed decreasing stage and those in the final slow-speed decreasing stage

4

of the spreading rate-time curve. (See Fig.S7 in Supporting Information for how to

5

obtain the linear fitting of the two straight lines. The intersections of the two straight

6

lines are depicted as the O points.) The time corresponding to the O point is defined

7

as the time To for the transition point of the spreading rates. (See Fig.S7 in Supporting

8

Information for how to obtain the time To.)

9

As depicted in Fig. 3, the time To for the transition points of the spreading rates

10

decreased not only with the increase of P507 concentration, but also was linearly

11

negatively correlated with the concentration of P507 in the organic droplet, whether

12

on the surface of deionized water or on that of the Er(III) aqueous solutions. As for

13

the spreading of oil droplets with the same P507 concentrations, the To values for the

14

transition points of the spreading rates on the surface of deionized water were always

15

shorter than that on the surface of the Er(III) aqueous solutions. It means that, if an

16

extraction reaction occurred with the spreading of the P507 organic droplet, it might

17

result in the time to reach the transition points of the spreading rates increased

18

appreciably.

19 20

The effect of the saponification degrees of P507 extractant.

21

Fig. 4(a) and 5(a) depict the time-dependent changes in spreading rates of the

22

organic droplets with the increase in the saponification degrees of P507 extractant, 17


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1

respectively on the surface of deionized water and on the surface of Er(III) aqueous

2

solutions.

3

4 5

Figure 4. (a) The time-dependent changes in spreading rates of P507 organic droplet on the

6

surface of deionized water with increasing the saponification degrees of P507. (b) The change in

7

the transition points of the spreading rates of P507 organic droplet on the surface of deionized

8

water with increasing the saponification degrees of P507.

9

(The P507 concentrations in the organic phase were 1.5 mol/L. The concentration of pre-loaded

10

Er(III) in P507 organic phase were 0 g/L.)

11

12 13

Figure 5. (a) The time-dependent changes in spreading rates of P507 organic droplet on the 18


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surface of Er(III) aqueous solutions with increasing the saponification degrees of P507. (b) The

2

change in the transition points of the spreading rates of P507 organic droplet on the surface of

3

Er(III) aqueous solutions with increasing the saponification degrees of P507.

4

(The P507 concentrations in the organic phase were 1.5 mol/L. The concentration of pre-loaded

5

Er(III) in P507 organic phase were 0 g/L. The concentrations of Er(III) in aqueous solution were

6

30 mg/L, and the pH of the Er(III) aqueous solutions were 5.4.)

7 8

As can be seen in Fig. 4(a) and Fig. 5(a), the changing trend of the spreading

9

rates with the increase in the saponification degrees of P507 organic droplets on the

10

surface of deionized water are similar with that on the surface of Er(III) aqueous

11

solutions. Within the initial 0.4 seconds, the spreading rates decreased appreciably,

12

then both of them reached a stable spreading at slow speed. In addition, both the

13

spreading rates of P507 organic droplets on the surface of deionized water and on that

14

of Er(III) aqueous solutions decreased with the increase in the P507 saponification

15

degrees. Comparing to the spreading on the surface of deionized water, the spreading

16

rates on the surface of Er(III) aqueous solutions were more quicker, even the

17

saponification degrees of P507 were the same. The curve of the spreading of P507

18

sample without saponification treatment is located within the range of saponification

19

degrees between 5% to 8%. The deviation of the spreading from the changing trend of

20

other samples with increasing saponification degrees might be attributed to a quick

21

decrease in the pH values of the supporting liquids, due to the ionization and release

22

out of hydrogen ions from the active groups in P507 molecules in the aqueous 19


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1

solutions under neutral pHs5, 39. Therefore, the resistance for the spreading of P507

2

organic solutions increased.

3

Fig. 4(b) and 5(b) give the change in the time To for the transition points of the

4

spreading rates from a rapid decrease to a slow-speed spreading with increasing the

5

saponification degrees of P507. Notably, the change in To was negatively correlated

6

with the saponification degrees of P507, whether the spreading was occurred on the

7

surface of deionized water or on that of Er(III) aqueous solutions. To became shorter

8

with increasing the P507 saponification degrees. However, the time To for the

9

transition points of the spreading on the surface of Er(III) aqueous solutions were

10

longer than that on the surface of deionized water.

11 12

The effect of the pre-loading amount of rare earths in organic phase.

13

Fig. 6(a) and 7(a) respectively depict the time-dependent changes in spreading

14

rates of the organic droplets with the increase in the pre-loading amount of Er(III) in

15

organic phase, respectively on the surface of deionized water and on the surface of

16

Er(III) aqueous solutions.

17

20


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Langmuir

1 2


3

surface of deionized water with increasing the pre-loaded concentrations of Er(III) in P507

4

extractant. (b) The change in the transition points of the spreading rates of P507 organic droplet on

5

the surface of deionized water with increasing the pre-loaded concentrations of Er(III) in P507

6

extractant .

7

(The P507 concentrations in the organic phases were 1.5 mol/L. The saponification degrees of

8

P507 in the organic phases were 5%.)

9

10 11


12

surface of Er(III) aqueous solutions with increasing the pre-loaded concentrations of Er(III) in

13

P507 extractant. (b) The change in the transition points of the spreading rates of P507 organic 21


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

droplet on the surface of Er(III) aqueous solutions with increasing the pre-loaded concentrations

2

of Er(III) in P507 extractant.

3

(The P507 concentrations in the organic phase were 1.5 mol/L. The saponification degrees of

4

P507 in the organic phases were 5%. The concentrations of Er(III) in aqueous solution were 30

5

mg/L, and the pH of the Er(III) aqueous solutions were 5.4.)

6 7

As can be seen in the Fig. 6(a) and Fig. 7(a), both the rates of the spreading of

8

P507 organic droplets on the surface of deionized water and on that of Er(III)

9

aqueous solutions decrease significantly with the time expanding, and then reaches a

10

slow spreading rate. The spreading rates of P507 organic droplets decreased with the

11

increase in the pre-loaded Erbium concentrations in the organic phases. However,

12

comparing to the changes in spreading rates on the surface of deionized water, the

13

change in spreading rates on the surface of Er(III) aqueous solutions were more

14

quicker, if the pre-loaded concentrations of Er(III) in P507 extractant were the same.

15

Fig. 6(b) and 7(b) depict the change in the time To for the transition points of the

16

spreading rates with increasing the pre-loaded concentrations of Er(III) in P507

17

extractant. Obviously, the change in To was negatively correlated with the pre-loading

18

amount of Er(III) in P507 organic droplets. The time To decreased with the increase of

19

the pre-loading Er(III) concentration in P507 organic droplets. However, all of the To

20

for the transition points of the spreading on the surface of Er(III) aqueous solutions

21

were longer than that on the surface of deionized water.

22 22


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Langmuir

1

DISCUSSION

2

Previous experimental results indicated that the spreading rates decreased

3

quickly before the time To, and then reached a stable slow-speed spreading after To.

4

At the beginning of spreading, the gravity force is the dominant driving force to

5

induce the spreading. The organic droplets must overcome the deformation resistance

6

and the interfacial friction of the substrate liquid to spread and then gradually form a

7

thin oil film on the surface of water.40 In this stage, the inertia force is initially a

8

predominant resisting force, and then the viscous drag gradually replaces the inertia to

9

become a main retarding force. The later would become more and more obvious with

10

the increase of the spreading area of the organic droplet.15 Therefore, the spreading

11

rates of organic droplets decreased quickly at first and then slow down. The spreading

12

of the organic droplet results in the deformation of the organic droplet quickly from a

13

sphere to a flat organic lens on the supporting fluid.41

14

After the time T0, the organic lens spread further, and finally a thin precursor oil

15

film, even a monolayer, formed on the surface of the supporting substrate aqueous

16

solutions. In this stage, the surface tension gradient gradually replaces the gravity to

17

become the dominant driving force for spreading, although the viscous force is still a

18

main resisting force. The spreading rates of P507 organic droplet depended mainly on

19

the wettability of P507 oil membrane on the surface of the supporting substrate

20

aqueous solutions. Therefore, the spreading rates of organic droplets would not

21

change any more and reached a stable spreading rate.

22

As for the reasons why the spreading rates decreased with the increase of P507 23


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

concentration, the saponification degree of P507 extractant and the pre-loaded Erbium

2

concentration, we thought it might be attribute to the change in the rheological

3

characteristics of P507 organic liquid membrane spreading on the surface of the

4

supporting substrate aqueous solutions.42 During the process of spreading, the ratios

5

of the viscosity to elasticity of P507 organic droplets reflect the damping capacity of

6

the droplets spreading on the surface of the supporting substrate aqueous

7

solutions.43,44 Figure 8 depicts the changes in the ratios of viscosity to elasticity of

8

P507 organic droplets with the increase in the concentration of P507, the

9

saponification degree of P507 extractant and the pre-loaded Erbium concentration.

10 11

Figure 8. The changes in the ratios of viscosity to elasticity of P507 organic droplets with (a) the

12

concentration of P507, (b) the saponification degree of P507 extractant, and (c) the pre-loaded

13

Erbium concentration in the organic phase. 24


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Langmuir

1 2

As can be seen in the Fig. 8, the ratios of viscosity to elasticity (See Table S7 to

3

S9 in Supporting Information for how to calculate the ratios of viscosity to elasticity)

4

of P507 organic droplets increased with increasing the concentration of P507, the

5

saponification degree of P507 extractant, and the pre-loaded Erbium concentration in

6

the organic phase. It means that the damping capacity of P507 organic droplets

7

spreading on the surface of the supporting substrate aqueous solutions increased. The

8

increasing in damping capacity of organic droplets would lead to the increase in the

9

frictional resistance between the P507 organic liquid membrane and the supporting

10

substrate aqueous solutions.45 Therefore, the spreading rates of P507 organic droplets

11

decreased.

12

Experimental results indicated that the time To would become shorter with the

13

increase in P507 concentration, the saponification degree of P507 extractant and the

14

pre-loaded Erbium concentration. It might be attributed to the increase in viscosity of

15

P507 droplet with the increase of above three factors (The change in the viscosities of

16

the organic phases with the increase in P507 concentration, the saponification degree

17

of P507 extractant and the pre-loaded Erbium concentration are listed in Table S10 to

18

S12 in Supporting Information). The viscous force is a main resisting force before

19

time To, therefore, the increase in the viscosity of P507 droplet would result in the

20

decrease in spreading rate becoming quickly. Thus, the time To would become shorter.

21

In order to explain the differences in spreading rates of P507 organic droplets on

22

the surface of deionized water and on that of Er(III) aqueous solution, a theoretic 25


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

calculation about the changes in spreading coefficient S of organic droplets on the

2

surface of different supporting liquids with the increase in P507 concentration, the

3

saponification degree of P507 extractant, and the pre-loaded erbium concentration in

4

the organic phase were performed, and the results were listed in Fig. 9. All of the

5

calculations about the spreading coefficient S were based on the experimental data

6

listed in Table S13 to S18 in Supporting Information.10

7

8 9

Figure 9. The changes in spreading coefficient S of P507 organic droplets with (a) the

10

concentration of P507, (b) the saponification degree of P507 extractant, and (c) the pre-loaded

11

Erbium concentration in the organic phase.

12 13 26


Page 26 of 36

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Langmuir

1

As can be seen in Fig. 9, the spreading coefficient S of P507 organic droplets on

2

the surface of Er(III) aqueous solution are larger than that on the surface of deionized

3

water. According to Svitova’s work, the transient precursor film radius obeys a simple

4

power scaling law in time elapsed t with the time exponent n (=3/4 theoretically

5

predicted).46 The increase in the spreading coefficient S would enhance the driving

6

force for the spreading. Therefore, the wettability of P507 organic droplets on the

7

surface of Er(III) aqueous solution was accelerated. In fact, the increase in the

8

wettability of P507 organic droplets on the surface of Er(III) aqueous solution might

9

be attributed to the occurrence of a chemical reaction at the interface between P507

10

oil membrane and Er(III) aqueous solution. The Marangoni convection was triggered

11

and therefore promote the mass transferring along the interface, which is favor of

12

overcoming the resistance from the viscous force when surface tension gradient

13

replaces the gravity as a dominant driving force for spreading. Thus, the spreading

14

rate of P507 oil membrane on the surface of Er(III) aqueous solution was enhanced.

15

Correspondingly, the time T0 for the spreading on the surface of Er(III) aqueous

16

solution was also shorten, compared to the spreading on the surface of deionized

17

water.

18 19

CONCLUSION

20

The present work demonstrated that the spreading of organic extractant P507 on

21

the surface of aqueous solutions containing rare-earth ions was accelerated due to the

22

occurrence of the chemical reactions at the gas-water interface. The difference in 27


Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

spreading rate of organic extractant P507 liquid droplets respectively on the surface of

2

deionized water and on that of Er(III) aqueous solutions with the increase in P507

3

concentration, the saponification degrees of P507 extractant and the pre-loading

4

amount of Er(III) in P507 extractant revealed that the chemical reaction at the

5

interface between the spreading P507 thin liquid membrane and Er(III) aqueous

6

solution would result in the Marangoni convection along the interface, which is favor

7

of overcoming the resistance from the viscous force when surface tension gradient

8

replaces the gravity as a dominant driving force for the spreading. The ratios of

9

viscosity to elasticity in organic droplets play a crucial role during the process of the

10

spreading, and have a significant impact on the formation of stable organic liquid film

11

on the surface of gas-water interface.

12

The present work provides an experimental foundation towards understanding

13

the effect of the interfacial chemical reaction on the spreading behavior of organic oil

14

droplet on the gas-water interface. It is beneficial for the development of our

15

suggested new technique of bubbling organic liquid membrane extraction, and to

16

achieve a controllable generation of stable gas bubble-supported organic liquid

17

membrane for performing solvent extraction at large aqueous-to-oil phase ratios.

18 19

Supporting Information

20

Details of the time-dependent changes in spreading area of the organic droplets with

21


22

pre-loading amount of Er(III) in organic solutions respectively on the surface of 28


Page 28 of 36

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Langmuir

1

deionized water and on the surface of Er(III) aqueous solutions; the schematic

2

diagram about how to obtain the time To for the transition point of the spreading rates;

3

the fitting equations of the spreading areas of the organic droplets with different P507

4

concentrations, the saponification degrees of P507 extractant and the pre-loading

5

amount of Er(III) in organic solution respectively on the surface of deionized water

6

and on the surface of Er(III) aqueous solutions; the elasticity modulus and viscous

7

modulus, and the viscosity of the organic phase with different P507 concentrations,

8

the saponification degrees of P507 extractant and the pre-loading amount of Er(III) in

9

organic solution; the surface tension and interfacial tension of the organic phase with

10


11

pre-loading amount of Er(III) in organic solution respectively on the surface of

12

deionized water and on the surface of Er(III) aqueous solutions.

13 14

NOTES

15

The authors declare no competing financial interest.

16 17

ACKNOWLEDGEMENTS

18

This work was financially supported by the National Natural Science Foundation of

19

China (Nos. 51574213, 21606248, 51074150), the Key Research and Development

20

Program of Shandong Province(2018GSF117028), and the Key Project of Chinese

21

National Programs for Fundamental Research and Development (973 Programs Nos.

22

2012CBA01203). 29


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Page 30 of 36

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TOC graphic

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4 5 6 7 8 9 10 11 12

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Chemical reaction-driven spreading of organic extractant on gas-water

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