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Wettability of coal pitch surface by aqueous solutions of cationic Gemini surfactants
Accepted Manuscript Title: Wettability of coal pitch surface by aqueous solutions of cationic Gemini surfactants Author: Honghong Chang Haochun Zhang Zhigang Jia Xing Li Wenchao Gao WenLong Wei PII: DOI: Reference:
S0927-7757(16)30022-X http://dx.doi.org/doi:10.1016/j.colsurfa.2016.01.022 COLSUA 20403
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
26-10-2015 12-1-2016 13-1-2016
Please cite this article as: Honghong Chang, Haochun Zhang, Zhigang Jia, Xing Li, Wenchao Gao, WenLong Wei, Wettability of coal pitch surface by aqueous solutions of cationic Gemini surfactants, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.01.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Wettability of coal pitch surface by aqueous solutions of cationic Gemini surfactants Honghong Chang a,b,c, Haochun Zhang a, Zhigang Jia a, Xing Li a, Wenchao Gao a*, WenLong Wei a * a
College of Chemical and Chemistry Engineering, Taiyuan University of Technology, Taiyuan 030024, China
b Opening
Funding Supported by the Key Laboratory of Road Structure & Material Ministry of Transport, Beijing 100088,
China c Shanxi
Province Key Laboratory of Surfactant, China Research Institute of Daily Chemical Industry, Taiyuan, 030001,
China
Corresponding author at: College of Chemical and Chemistry Engineering, Taiyuan University of Technology, Taiyuan, China. Fax: +86 351 6111165. E-mail address: [email protected](W. C. Gao); [email protected](W. L. Wei).
1
Graphic Abstract Water 80
110 stage1
100
stage 2
stage 3
70
+ Coal
Coal tar
Distillation
80
60
70 60
50
Wi
40
WA
30
50 40 30
20 10 0 -7
Coal pitch Coke
2
20 -6
-5
-4
-3
-2
lg C(mmol/L)
-1
0
1
(deg)
Coal gas
2 WA(mJ/m )Wi(mN/m)
90
Highlights:
Series of Cn-8-Cn (n = 8, 10, 12, 14, 16) Gemini biquaternary ammonium salt surfactants were prepared.
Cn-8-Cn Gemini surfactants changed their wettability on coal pitch surface by changing charge and polarity.
The wettability of Gemini surfactants on coal pitch surface is the result of electrostatic interaction and van der Waals adsorption.
Abstract:The purpose of this experiment is to confirm the adsorption characteristics at liquid-gas and
CP-liquid interfaces in relation to CP wetting with a sequence of cationic Gemini surfactant solutions and to reduce the workload of screening of surfactant. A series of cationic Gemini surfactants with different hydrophobic chain length was synthesized (Cn-8-Cn, n=8, 10, 12, 14, 16). Their wettability and adsorption mechanism for coal pitch (CP) were investigated by the sessile drop analysis and electrophoresis. Within the whole range of concentration, the Zisman theory is not in accordance with the wettability on the coal tar pitch when carbon atoms of hydrophobic chains exceed 12. Meanwhile, no liner dependence was observed between adhesion tension (γlgcosθ) and surface tension (γlg). In addition, with the increase of surfactant concentration, Zeta potential on the surface of the CP changed from negative to positive and finally leveled off after a certain concentration. The corresponding concentration of zero potential was at least an order of magnitude lower than CMC. C12-8-C12 can significantly change the wettability of the CP. According to the wetting data and Zeta potential of Gemini surfactant on CP, it can be inferred that wetting was a concerted action as a result of electrostatic interaction and van der waals adsorption. Therefore, wetting process can be generally divided into three stages. Keywords: Coal pitch; Gemini surfactant; Contact angle; Wetting; Zeta potential
3
1. Introduction The researches of the solid wettability with various liquids, mainly aqueous solution of surfactants, are closely related to our daily life and industrial process. Wetting is a central point for washing, oil recovering and pulping. Understanding the wetting of solid surface by the surfactants is essential for both theoretical study and practical application. Until now, mostly wettability of solid studies focused on aqueous solution of conventional single-chain surfactants [1-7] and their blend system [8-10]. Koopal presented an overview of the quantitative calculation methods on wettability of high-energy/low-energy hydrophilic solid by the surfactant aqueous solutions and the parts of organic solvent [11]. Gemini surfactants are novel type of molecules formed by two single-chain amphiphilic moieties, which are connected at the head group level through a spacer group. Commonly, the researches on Gemini surfactants are mainly focused on its solution properties, such as CMC (critical micelle concentration) values, aggregate morphology [12-14]. However, the influence of Gemini surfactants on solid surface is relatively rare, and most of the relevant researches focused on the surface of quartz, silicon slice and polymer [15-20]. The studies disclosed that Gemini surfactant usually formed single film on the solid surface through direct adsorption or ion exchange adsorption, and then, the formation of double or multilayer film through the Gemini surfactant intermolecular interactions [15,16]. Positive charge of cationic Gemini surfactant interacted directly with negative charge (SiO-) of silica surface through electrostatic interactions [17]. In the monomer-Gemini surfactant composite system, a small amount of Gemini surfactant had a significant influence on quartz surface adsorption amount and adsorption mode [18]. The experiments of wetting on glass and mica surfaces by cationic Gemini surfactant were shown that the solid surface charge will be changed when Gemini surfactant concentration was 10 times lower than CMC [19], there is an obvious difference in wetting of PPFE and glass surface by cationic Gemini surfactant and anionic Gemini surfactant [20]. 4
According to the preliminary reports [21,22], dispersants successfully employed in coal water slurry (CWS) were no long efficient for coal pitch water slurry (CPWS), a new-fashioned slurry fuels [23-25]. The novelty of this work is found that the dispersant of CWS and CPWS does not have commonality, and cationic Gemini surfactant is regarded as excellent dispersing agents to prepare for CPWS through the further experiments. Therefore, the objective of our study is to confirm the adsorption characteristics at liquid-gas and CP-liquid interfaces in relation to CP wetting with a sequence of cationic Gemini surfactant solutions.
2. Materials and methods In the three flasks equipping with a stirrer, thermometer and condensing tube, the class of cationic Gemini surfactants, Cn-8-Cn (n=8, 10, 12, 14, 16) with (CH2)8 as spacer and different length of hydrophobic chain, was synthesized by mixing 1, 8-Dobromooctane and N, N-dimethyl amines in ethanol solvent and refluxing for a definite time. The solvent was evaporated and the residue was recrystallized from acetone several times, and then dried under vacuum. All the products are characterized using 1H NMR spectra (Bruker ADVANCE 600 spectrometer) in CDCl3, the final results of the analysis are as follows. The structural formula and abbreviations of surfactant are presented in Scheme 1. C8-8-C8, 1H-NMR(600MHz, CDCl3):δ=0.86-0.88(t, J=6.48Hz, 7.14Hz, 6H), 1.26-1.45(m, 28H), 1.73(s, 4H), 1.84(s, 4H), 3.37(s, 12H), 3.54-3.57(t, J=9.48Hz, 8.52Hz, 4H), 3.64-3.67 (t, J=8.34Hz, 8.46Hz, 4H). C10-8-C10, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89(t, J=6.9Hz, 7.14Hz, 6H), 1.26-1.45(m, 36H), 1.73(s, 4H), 1.84(s, 4H), 3.37(s, 12H) ，3.54-3.56(t, J=8.34Hz, 8.64Hz, 4H), 3.65-3.68 (m, 4H). C12-8-C12, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89 (t, J=6.84Hz, 7.14Hz, 6H), 1.25-1.46 (m, 44H), 1.72 (s, 4H) 1.85 (s, 4H), 3.36(s, 12H), 3.50-3.53(t, J=8.46Hz, 8.46Hz, 4H), 3.67-3.70(m, 4H). C14-8-C14, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89 (t, J=6.9Hz, 7.14Hz, 6H), 1.25-1.46 (m, 52H), 1.72(s, 4H), 1.85 (s, 4H), 3.36(s, 12H), 3.50-3.53(t, J=8.46Hz, 8.46Hz, 4H), 3.68-3.71(m, 4H). 5
C16-8-C16, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89 (t, J=6.84Hz, 7.14Hz, 6H), 1.25-1.47(m, 60H), 1.72(s, 4H), 1.86(s, 4H), 3.36(s, 12H), 3.50-3.53 (t, J=8.34Hz, 8.46Hz, 4H), 3.69-3.72(m, 4H). The surface tension of surfactant aqueous solutions were measured by the Wilhelmy ring method using a surface tension meter with the JK99C from Shanghai Zhongchen Digital Technic Apparatus CO., Ltd., China under 293 K; the constant temperature was maintained using an external water circulator. The platinum ring was burned after washing under alcohol flame to remove the contamination and adsorbed surfactants thoroughgoing before each experiment. The measurements were performed until constant surface tension values showed that equilibrium had been arrived. More than five continuous measurements were accomplished in all trials, and the standard deviation did not exceed 0.2 mN/m. The contact angle for aqueous solution of surfactants and water on CP slices were achieved by the sessile drop method using the SL200B from USA Kino Industry CO., Ltd, which all experiments were achieved at constant temperature 293 K; CP slice was purged several times using first acetone and then double-distilled water to remove the impurity. Within 120 seconds, a certain volume of droplets was dripped on the CP slice until contact angle values remain constant. The trials were repeated four times by placing a droplet on the new parts of the slice. Then, a new slice was placed in the platform, and the above experimental operation was repeated four times. The standard deviation of the contact angle values did not exceed 2 °. Zeta Potential of the surfactants aqueous solution on the CP particles was fulfilled by the electrophoresis technology with the JS94H from Shanghai Zhongchen digital technic apparatus CO., LTD., China. The aqueous solution of surfactants were mixed with the CP particles with a certain proportion (0.02 g/mL), oscillating under 293 K for half an hour. The measurements were implemented until constant Zeta potential values denoted that adsorption equilibrium had been arrived. The same procedure was repeated five times, and the average values are reported. The standard deviation of Zeta potential values did not exceed 0.5 mV. 6
3. Results and Discussion The surface tension values of cationic Gemini surfactant solution as a function of the logarithm of surfactant concentration are presented in Fig. 1. From Fig. 1, we can see that the surface tension decreases with the increasing concentration of cationic Gemini surfactants. Then, it remains constant for the surfactant concentration large than the CMC. According to the theoretical assumptions, Traube's rule, it states that for every extra CH2 group in a surfactant molecule, the surface activity approximately triples in the dilute aqueous solutions of surfactants, which means we can get away with using a much lower concentration of a longer chain length surfactant. However, according to the relevant published literature [26-28], the corresponding surface tension is not always complying with this law. C10-8-C10 possesses the minimum surface tension value at CMC, 28.73 mN/m, and the corresponding CMC is 2.66 mmol/L. The changes in contact angle values with the increasing concentration (lg C) of cationic Gemini surfactants are presented in Fig. 2. From Fig. 2 it can be observed that the contact angle keeps almost constant in a range of low concentration. In such a concentration, extremely low coverage of the CP surface was achieved by the Gemini surfactant molecules. With the increasing of Gemini surfactant concentration, increasing surfactants adsorption occurs with hydrophilic group are prone to the liquid-CP interface, therefore the CP surface seems to be increasingly hydrophilic, resulting in a decrease in the contact angle. Further increase in Gemini surfactant concentrations led to a further decrease in the contact angle for all studied Gemini surfactant solutions. In this situation, a bi-layer, or even multi layer likely develops with hydrophobic chains down in the first layer and hydrophilic groups out toward the solution for another layer. Fig. 2 also shows the Contact angle variation trend is similar to surface tension, which decreases with the increasing concentration of cationic Gemini surfactants, and which remains constant over the CMC. Besides, balance contact angle with the increasing of carbon chain length decreased first, and then increased. C10-8-C10 is 7
much easier to get wet on the CP surface.
According to the Young’s law, cosγsg－γsl)/γlg, spreading coefficient on the surface of CP can be calculated by the formula (1). S=γsg－γsl－γlg=γlg(cosθ-1)
(1)
As is shown in Fig. 3, the results show that the spreading coefficient increased rapidly at first and then leveled off with increasing concentration of cationic Gemini surfactants. Spreading coefficient is close to zero when surfactant concentration arrives at the CMC, which indicates all the five investigated Gemini surfactants can be well spread on the surface of CP at this time. As the hydrophobic chain length continues to increase, spreading coefficient showed a trend of increase and then decrease. The wetting ability of C10-8-C10 is much easier to get wet on the CP surface. Wi is a measurement of the gas on a solid surface replaced by the liquid. Wi ≤ 0 denotes that solid surfaces are able to wet by liquid, Formula is as follows: Wi=γsg－γsl= γ1gcosθ
(2)
According to the previous report for contact angle measurements of aqueous solution of surfactants and various pure liquids on the solid surface [11], an acceptable empirical method of analysis was declaimed by Zisman and coworkers. According to equation 3: cosθ=b-cγlg
(3)
where b and c are the constants. In this method, the wettability of an exactly surface by a series of experimental liquids is carried out. It is possible to describe the line relationship between cosθ and γlg of liquid, including some aqueous solution of surfactants. Dependence of the cosθ and γlg of Gemini surfactants (Cn-8-Cn, n=8, 10) are shown in Fig. 4. However, within the whole range of concentration, the Zisman theory is not in accordance with the wettability 8
on the coal tar pitch when carbon atoms of hydrophobic chains exceed 12. Since the effects of external conditions on solid-gas interface properties can be ignored in the experiment, γsg is also regarded as a fixed value. Fig. 5 reflects that the dependence of the immerse work (Wi) of surfactant solution to CP surface and surface tension (γ1g). The results showed that the Wi of Gemini surfactant (Cn-8-Cn) and surface tension are not following a linear relationship within the whole range of concentration. Therefore, Lifshitz-van der Waals adsorption does not play a leading role in the process of wetting on the CP surface by Gemini surfactant (Cn-8-Cn) solution. The Wi would be showed a whole increasing trend with increasing of the surface tension. Namely, with the increasing concentration of Gemini surfactant solution, the γsl is gradually decreased. The Gemini surfactants possess many superior features which are constructed by connecting any two different or identical conventional monomeric amphiphilic moieties with a spacer group. It is speculated that, on the basis of formed first film, CP surface was eventually formed another film structure by Gemini surfactant that the hydrophobic chains of Gemini surfactant extended to the inter film and hydrophilic tended to the liquid phase. The interaction between water molecules and solid surface increased, resulting in reducing the values of γsl. It is consistent with the experimental results. The work of adhesion (WA) of liquid to solid, which can be calculated by equation 4, and which is expressed by the reversible work required to isolate a unit area of liquid from a surface of solid. WA=γ1g+γsg－γsl
(4)
Introduce equation 4 into the Young equation, we obtain : WA=γ1g(1+cosθ)
(5)
The work of adhesion of surfactant solutions on CP, as shown in Fig. 6, which was counted by plugging the surface tension and contact angle values in equation 5. From the Fig. 6, we observed that the values of WA of aqueous solution of all studied Gemini surfactants decrease with the increasing surfactants concentration. It can 9
be found that there is a minimum value of WA in a concentration close to their CMC. While compared with the adhesion tension of all studied Gemini surfactants on the CP surface, it can be observed that WA of C10-8-C10 is lowest. Free energy of the surface measures the power of interaction for the spread liquid onto the solid surface. From the thermodynamic point of view, the molar wetting free energy of the solid can be calculated by equation 6 according to Extrand [29]. Under the constant temperature and pressure, the solid surface free energy values is lower, it is possible to get wet. The more negative it is, the more it gets wet. 2 RT 1 cos 2 cos △G= ln 3 4
(6)
where △G is the surface free energy, T is temperature in Kelvin and R is (8.314 J•mol-1•K-1) the universal gas constant. The free energies of wetting on the CP surface are more negative (ΔGCP−C8-8-C8 = −2352.39 J/mol; ΔGCP−C10-8-C10=−5568.12
J/mol;
ΔGCP−C12-8-C12=−4903.67
J/mol;
ΔGCP−C14-8-C14=−4070.83
J/mol;
ΔGCP−C16-8-C16=−2213.58 J/mol) in the company of Gemini surfactant solutions above the CMC than double distilled water (ΔGCP−water = −1129.65 J/mol). Especially C10-8-C10 can significantly changed in wetting on the CP surface. Taking about 1 mL adsorption saturated liquid which contains CP particles and uses electrophoresis to measure zeta potential of the CP surface. Experimental results are shown in Fig. 7. Fig. 7 shows that the zeta potential of CP surface with increasing Gemini surfactant concentration presented following regular pattern which varied from negative charge to positive charge and eventually tended towards stable. Adsorbed Gemini surfactant on the surface of CP significantly changes the solid surface properties, in a descendant order of the zero potential with corresponding concentration: c(C12-8-C12)
concentrations of surfactant concentration in below zero potential. To discover the mechanism accountable for adsorption of cationic Gemini surfactants Cn-8-Cn to CP surface, the relationship of contact angle, surface tension, immersion work, and adhesion work on concentration have been illustrated in Fig. 8. A similar trend of surface tension and contact angle changes was found in the experiment. Therefore, with the C12-8-C12 as an example, the wettability on CP surface was discussed in the Fig. 8. According to Fig. 7, divided from zero potential with corresponding concentration and its CMC, the wetting process can be roughly divided into three connective stages. The 1st Stage ranges from 10-5 g/L to corresponding concentration of zero potential. The concentration is too low to form adsorption film at both liquid-air and solid–liquid interfaces; Thus, γlg and γsl have substantially unchanged, which the contact angle, WA and Wi are basically unchanged. With increasing of surfactant concentration, the hydrophilic head of surfactant molecules in the presence of electrostatic interactions adsorbed on the surface of CP, hydrophobic chain extended outwards liquid phase, which formed a monomolecular adsorption layer when zeta potential reached zero. The 2nd stage ranges from the corresponding concentrations of zero potential to the CMC. With increasing of surfactant concentration, zeta potential of CP surface grew from zero, it suggestion that adsorption amount is increasing, thereby reducing to the surface tension, contact angle and Wi, and increasing WA. Under Van der Waals force, the formed of single molecular layer continues to adsorb another Gemini surfactant molecules that hydrophobic chain cross each other. Finally, CP surface formed bi-layer or multi layer adsorption film, hydrophilic group facing out of liquid phase. As a result, the wetting properties of the CP surface were completely changed. The 3rd stage reaches over its CMC. The surface tension, contact angle, WA and Wi are no longer changed at this stage. Gemini surfactants of the CP surface which formed multilayer saturated structure at last, which hydrophilic group faced outward liquid phase. So here we can get the conclusion safely, the wetting of CP surface by cationic Gemini surfactant is a result of electrostatic interaction and van der Waals interaction. 11
4. Conclusions On the basis of the surveyed values of the contact angle, surface tension and Zeta potential, the theoretical interpretation can be stated that: Surface tension variation following the same trend to contact angle, but the exact reverse is the spreading coefficients. The Wi of CP surface increases with increasing surfactant concentration up to the CMC of the surfactant solution. The WA of CP surface decreases to a minimum with increasing surfactant concentration up to the CMC, finally, it remains constant over the CMC. Cationic Gemini surfactants (Cn-8-Cn) changed the CP surface’s quantity of electric charge and electrical property which altered the wetting of CP. It can be inferred that wetting is a concerted action result of electrostatic interaction and van der waals adsorption. Wetting process can be generally divided into three phases. It is not that the lower of
surface tension is, the more easy preparing of the CPWS is. But, when the Space of cationic Gemini surfactant is the same, the performance of the slurry ability of coal pitch-water slurry is more pronounced with the increased carbon number of hydrophobic chain.
Acknowledgment This work was surpported by the National Natural Science Foundation of China (Grant No.21076135, 21206103) , the Key Laboratory of Road Structure& Material of Transport (Beijing, Opening Funding, Grant No. KF201403), the Key Project of Science and Technology of Shanxi (Grant No. 20130313026-5), the Shanxi Province Natural Science Foundation(Grant No. 2015011023) and the foundation of Shanxi Province Key Laboratory of Surfactant(Grant No. 201503).
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646–649.
15
80
Surface tension(mN/m)
70 60 50
C8-8-C8
40
C10-8-C10 C12-8-C12
30
C14-8-C14 C16-8-C16
20 10
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 1. The Curve of the surface tension of aqueous solutions of cationic Gemini surfactant vs. the logarithm of surfactant concentration.
16
80 70 60
(deg)
50
C8-8-C8 C10-8-C10
40
C12-8-C12
30
C14-8-C14 C16-8-C16
20 10
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 2. The curve of the contact angle of aqueous solutions of cationic Gemini surfactants on the CP surface vs. the logarithm of surfactant concentration.
17
Spreading coeffcient(mN/m)
5 0
C8-8-C8
-5 -10 -15 -20
C10-8-C10 C12-8-C12 C14-8-C14 C16-8-C16
-25 -30 -35 -40 -45 -50 -55
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 3. The curve of the spreading coefficient of aqueous solutions of cationic Gemini surfactants on the CP surface vs. the logarithm of surfactant concentration.
18
1.0 0.9
C8-8-C8
0.8
C10-8-C10
0.7
cos
0.6 0.5 0.4 0.3 0.2 0.1 0.0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Surface tension/(mN/m)
Fig. 4. The curve of the cosine of contact angle of aqueous solutions of cationic Gemini surfactants on CP surface vs. the surface tension.
19
40 38 36
cos
34 32 30
C8-8-C8
28
C10-8-C10
26
C12-8-C12 C14-8-C14
24 22 25
C16-8-C16 30
35
40
45
50
55
60
65
70
75
Surface tension/(mN/m) Fig. 5. The curve of the adhesion tension of aqueous solutions of cationic Gemini surfactants on the CP surface vs. the surface tension.
20
100
Work of adhension/(mJ/m2)
95 90 85 80
C8-8-C8
75
C10-8-C10
70
C12-8-C12 C14-8-C14
65
C16-8-C16
60 55 -5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 6. The curve of the work of adhesion aqueous solutions of cationic Gemini surfactants on the CP surface vs. the logarithm of surfactant concentration.
21
80
C8-8-C8 C10-8-C10
Zeta potential(mV)
60
C12-8-C12
40
C14-8-C14 C16-8-C16
20 0 -20 -40 -60
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 7. The curve of the Zeta potential of CP surface vs. the logarithm of aqueous solutions of cationic Gemini surfactant concentration.
22
110
80 stage1
100
stage 2
stage 3
70
80
60
70 60
50
Wi
40
WA
30
50 40 30
20 10 0 -7
(deg)
2 WA(mJ/m )Wi(mN/m)
90
20 -6
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 8. Dependence of the adhesion data of C12-8-C12 solution to CP surface and the logarithm of surfactant concentration.
[CnH2n+1
CH3 + N (CH2)8
CH3 + N CnH2n+1] 2Br-
.
CH3
CH3
Scheme 1. The structural formula of symmetrical cationic Gemini surfactants.
23
S0927-7757(16)30022-X http://dx.doi.org/doi:10.1016/j.colsurfa.2016.01.022 COLSUA 20403
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
26-10-2015 12-1-2016 13-1-2016
Please cite this article as: Honghong Chang, Haochun Zhang, Zhigang Jia, Xing Li, Wenchao Gao, WenLong Wei, Wettability of coal pitch surface by aqueous solutions of cationic Gemini surfactants, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2016.01.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Wettability of coal pitch surface by aqueous solutions of cationic Gemini surfactants Honghong Chang a,b,c, Haochun Zhang a, Zhigang Jia a, Xing Li a, Wenchao Gao a*, WenLong Wei a * a
College of Chemical and Chemistry Engineering, Taiyuan University of Technology, Taiyuan 030024, China
b Opening
Funding Supported by the Key Laboratory of Road Structure & Material Ministry of Transport, Beijing 100088,
China c Shanxi
Province Key Laboratory of Surfactant, China Research Institute of Daily Chemical Industry, Taiyuan, 030001,
China
Corresponding author at: College of Chemical and Chemistry Engineering, Taiyuan University of Technology, Taiyuan, China. Fax: +86 351 6111165. E-mail address: [email protected](W. C. Gao); [email protected](W. L. Wei).
1
Graphic Abstract Water 80
110 stage1
100
stage 2
stage 3
70
+ Coal
Coal tar
Distillation
80
60
70 60
50
Wi
40
WA
30
50 40 30
20 10 0 -7
Coal pitch Coke
2
20 -6
-5
-4
-3
-2
lg C(mmol/L)
-1
0
1
(deg)
Coal gas
2 WA(mJ/m )Wi(mN/m)
90
Highlights:
Series of Cn-8-Cn (n = 8, 10, 12, 14, 16) Gemini biquaternary ammonium salt surfactants were prepared.
Cn-8-Cn Gemini surfactants changed their wettability on coal pitch surface by changing charge and polarity.
The wettability of Gemini surfactants on coal pitch surface is the result of electrostatic interaction and van der Waals adsorption.
Abstract:The purpose of this experiment is to confirm the adsorption characteristics at liquid-gas and
CP-liquid interfaces in relation to CP wetting with a sequence of cationic Gemini surfactant solutions and to reduce the workload of screening of surfactant. A series of cationic Gemini surfactants with different hydrophobic chain length was synthesized (Cn-8-Cn, n=8, 10, 12, 14, 16). Their wettability and adsorption mechanism for coal pitch (CP) were investigated by the sessile drop analysis and electrophoresis. Within the whole range of concentration, the Zisman theory is not in accordance with the wettability on the coal tar pitch when carbon atoms of hydrophobic chains exceed 12. Meanwhile, no liner dependence was observed between adhesion tension (γlgcosθ) and surface tension (γlg). In addition, with the increase of surfactant concentration, Zeta potential on the surface of the CP changed from negative to positive and finally leveled off after a certain concentration. The corresponding concentration of zero potential was at least an order of magnitude lower than CMC. C12-8-C12 can significantly change the wettability of the CP. According to the wetting data and Zeta potential of Gemini surfactant on CP, it can be inferred that wetting was a concerted action as a result of electrostatic interaction and van der waals adsorption. Therefore, wetting process can be generally divided into three stages. Keywords: Coal pitch; Gemini surfactant; Contact angle; Wetting; Zeta potential
3
1. Introduction The researches of the solid wettability with various liquids, mainly aqueous solution of surfactants, are closely related to our daily life and industrial process. Wetting is a central point for washing, oil recovering and pulping. Understanding the wetting of solid surface by the surfactants is essential for both theoretical study and practical application. Until now, mostly wettability of solid studies focused on aqueous solution of conventional single-chain surfactants [1-7] and their blend system [8-10]. Koopal presented an overview of the quantitative calculation methods on wettability of high-energy/low-energy hydrophilic solid by the surfactant aqueous solutions and the parts of organic solvent [11]. Gemini surfactants are novel type of molecules formed by two single-chain amphiphilic moieties, which are connected at the head group level through a spacer group. Commonly, the researches on Gemini surfactants are mainly focused on its solution properties, such as CMC (critical micelle concentration) values, aggregate morphology [12-14]. However, the influence of Gemini surfactants on solid surface is relatively rare, and most of the relevant researches focused on the surface of quartz, silicon slice and polymer [15-20]. The studies disclosed that Gemini surfactant usually formed single film on the solid surface through direct adsorption or ion exchange adsorption, and then, the formation of double or multilayer film through the Gemini surfactant intermolecular interactions [15,16]. Positive charge of cationic Gemini surfactant interacted directly with negative charge (SiO-) of silica surface through electrostatic interactions [17]. In the monomer-Gemini surfactant composite system, a small amount of Gemini surfactant had a significant influence on quartz surface adsorption amount and adsorption mode [18]. The experiments of wetting on glass and mica surfaces by cationic Gemini surfactant were shown that the solid surface charge will be changed when Gemini surfactant concentration was 10 times lower than CMC [19], there is an obvious difference in wetting of PPFE and glass surface by cationic Gemini surfactant and anionic Gemini surfactant [20]. 4
According to the preliminary reports [21,22], dispersants successfully employed in coal water slurry (CWS) were no long efficient for coal pitch water slurry (CPWS), a new-fashioned slurry fuels [23-25]. The novelty of this work is found that the dispersant of CWS and CPWS does not have commonality, and cationic Gemini surfactant is regarded as excellent dispersing agents to prepare for CPWS through the further experiments. Therefore, the objective of our study is to confirm the adsorption characteristics at liquid-gas and CP-liquid interfaces in relation to CP wetting with a sequence of cationic Gemini surfactant solutions.
2. Materials and methods In the three flasks equipping with a stirrer, thermometer and condensing tube, the class of cationic Gemini surfactants, Cn-8-Cn (n=8, 10, 12, 14, 16) with (CH2)8 as spacer and different length of hydrophobic chain, was synthesized by mixing 1, 8-Dobromooctane and N, N-dimethyl amines in ethanol solvent and refluxing for a definite time. The solvent was evaporated and the residue was recrystallized from acetone several times, and then dried under vacuum. All the products are characterized using 1H NMR spectra (Bruker ADVANCE 600 spectrometer) in CDCl3, the final results of the analysis are as follows. The structural formula and abbreviations of surfactant are presented in Scheme 1. C8-8-C8, 1H-NMR(600MHz, CDCl3):δ=0.86-0.88(t, J=6.48Hz, 7.14Hz, 6H), 1.26-1.45(m, 28H), 1.73(s, 4H), 1.84(s, 4H), 3.37(s, 12H), 3.54-3.57(t, J=9.48Hz, 8.52Hz, 4H), 3.64-3.67 (t, J=8.34Hz, 8.46Hz, 4H). C10-8-C10, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89(t, J=6.9Hz, 7.14Hz, 6H), 1.26-1.45(m, 36H), 1.73(s, 4H), 1.84(s, 4H), 3.37(s, 12H) ，3.54-3.56(t, J=8.34Hz, 8.64Hz, 4H), 3.65-3.68 (m, 4H). C12-8-C12, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89 (t, J=6.84Hz, 7.14Hz, 6H), 1.25-1.46 (m, 44H), 1.72 (s, 4H) 1.85 (s, 4H), 3.36(s, 12H), 3.50-3.53(t, J=8.46Hz, 8.46Hz, 4H), 3.67-3.70(m, 4H). C14-8-C14, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89 (t, J=6.9Hz, 7.14Hz, 6H), 1.25-1.46 (m, 52H), 1.72(s, 4H), 1.85 (s, 4H), 3.36(s, 12H), 3.50-3.53(t, J=8.46Hz, 8.46Hz, 4H), 3.68-3.71(m, 4H). 5
C16-8-C16, 1H-NMR(600MHz, CDCl3):δ=0.87-0.89 (t, J=6.84Hz, 7.14Hz, 6H), 1.25-1.47(m, 60H), 1.72(s, 4H), 1.86(s, 4H), 3.36(s, 12H), 3.50-3.53 (t, J=8.34Hz, 8.46Hz, 4H), 3.69-3.72(m, 4H). The surface tension of surfactant aqueous solutions were measured by the Wilhelmy ring method using a surface tension meter with the JK99C from Shanghai Zhongchen Digital Technic Apparatus CO., Ltd., China under 293 K; the constant temperature was maintained using an external water circulator. The platinum ring was burned after washing under alcohol flame to remove the contamination and adsorbed surfactants thoroughgoing before each experiment. The measurements were performed until constant surface tension values showed that equilibrium had been arrived. More than five continuous measurements were accomplished in all trials, and the standard deviation did not exceed 0.2 mN/m. The contact angle for aqueous solution of surfactants and water on CP slices were achieved by the sessile drop method using the SL200B from USA Kino Industry CO., Ltd, which all experiments were achieved at constant temperature 293 K; CP slice was purged several times using first acetone and then double-distilled water to remove the impurity. Within 120 seconds, a certain volume of droplets was dripped on the CP slice until contact angle values remain constant. The trials were repeated four times by placing a droplet on the new parts of the slice. Then, a new slice was placed in the platform, and the above experimental operation was repeated four times. The standard deviation of the contact angle values did not exceed 2 °. Zeta Potential of the surfactants aqueous solution on the CP particles was fulfilled by the electrophoresis technology with the JS94H from Shanghai Zhongchen digital technic apparatus CO., LTD., China. The aqueous solution of surfactants were mixed with the CP particles with a certain proportion (0.02 g/mL), oscillating under 293 K for half an hour. The measurements were implemented until constant Zeta potential values denoted that adsorption equilibrium had been arrived. The same procedure was repeated five times, and the average values are reported. The standard deviation of Zeta potential values did not exceed 0.5 mV. 6
3. Results and Discussion The surface tension values of cationic Gemini surfactant solution as a function of the logarithm of surfactant concentration are presented in Fig. 1. From Fig. 1, we can see that the surface tension decreases with the increasing concentration of cationic Gemini surfactants. Then, it remains constant for the surfactant concentration large than the CMC. According to the theoretical assumptions, Traube's rule, it states that for every extra CH2 group in a surfactant molecule, the surface activity approximately triples in the dilute aqueous solutions of surfactants, which means we can get away with using a much lower concentration of a longer chain length surfactant. However, according to the relevant published literature [26-28], the corresponding surface tension is not always complying with this law. C10-8-C10 possesses the minimum surface tension value at CMC, 28.73 mN/m, and the corresponding CMC is 2.66 mmol/L. The changes in contact angle values with the increasing concentration (lg C) of cationic Gemini surfactants are presented in Fig. 2. From Fig. 2 it can be observed that the contact angle keeps almost constant in a range of low concentration. In such a concentration, extremely low coverage of the CP surface was achieved by the Gemini surfactant molecules. With the increasing of Gemini surfactant concentration, increasing surfactants adsorption occurs with hydrophilic group are prone to the liquid-CP interface, therefore the CP surface seems to be increasingly hydrophilic, resulting in a decrease in the contact angle. Further increase in Gemini surfactant concentrations led to a further decrease in the contact angle for all studied Gemini surfactant solutions. In this situation, a bi-layer, or even multi layer likely develops with hydrophobic chains down in the first layer and hydrophilic groups out toward the solution for another layer. Fig. 2 also shows the Contact angle variation trend is similar to surface tension, which decreases with the increasing concentration of cationic Gemini surfactants, and which remains constant over the CMC. Besides, balance contact angle with the increasing of carbon chain length decreased first, and then increased. C10-8-C10 is 7
much easier to get wet on the CP surface.
According to the Young’s law, cosγsg－γsl)/γlg, spreading coefficient on the surface of CP can be calculated by the formula (1). S=γsg－γsl－γlg=γlg(cosθ-1)
(1)
As is shown in Fig. 3, the results show that the spreading coefficient increased rapidly at first and then leveled off with increasing concentration of cationic Gemini surfactants. Spreading coefficient is close to zero when surfactant concentration arrives at the CMC, which indicates all the five investigated Gemini surfactants can be well spread on the surface of CP at this time. As the hydrophobic chain length continues to increase, spreading coefficient showed a trend of increase and then decrease. The wetting ability of C10-8-C10 is much easier to get wet on the CP surface. Wi is a measurement of the gas on a solid surface replaced by the liquid. Wi ≤ 0 denotes that solid surfaces are able to wet by liquid, Formula is as follows: Wi=γsg－γsl= γ1gcosθ
(2)
According to the previous report for contact angle measurements of aqueous solution of surfactants and various pure liquids on the solid surface [11], an acceptable empirical method of analysis was declaimed by Zisman and coworkers. According to equation 3: cosθ=b-cγlg
(3)
where b and c are the constants. In this method, the wettability of an exactly surface by a series of experimental liquids is carried out. It is possible to describe the line relationship between cosθ and γlg of liquid, including some aqueous solution of surfactants. Dependence of the cosθ and γlg of Gemini surfactants (Cn-8-Cn, n=8, 10) are shown in Fig. 4. However, within the whole range of concentration, the Zisman theory is not in accordance with the wettability 8
on the coal tar pitch when carbon atoms of hydrophobic chains exceed 12. Since the effects of external conditions on solid-gas interface properties can be ignored in the experiment, γsg is also regarded as a fixed value. Fig. 5 reflects that the dependence of the immerse work (Wi) of surfactant solution to CP surface and surface tension (γ1g). The results showed that the Wi of Gemini surfactant (Cn-8-Cn) and surface tension are not following a linear relationship within the whole range of concentration. Therefore, Lifshitz-van der Waals adsorption does not play a leading role in the process of wetting on the CP surface by Gemini surfactant (Cn-8-Cn) solution. The Wi would be showed a whole increasing trend with increasing of the surface tension. Namely, with the increasing concentration of Gemini surfactant solution, the γsl is gradually decreased. The Gemini surfactants possess many superior features which are constructed by connecting any two different or identical conventional monomeric amphiphilic moieties with a spacer group. It is speculated that, on the basis of formed first film, CP surface was eventually formed another film structure by Gemini surfactant that the hydrophobic chains of Gemini surfactant extended to the inter film and hydrophilic tended to the liquid phase. The interaction between water molecules and solid surface increased, resulting in reducing the values of γsl. It is consistent with the experimental results. The work of adhesion (WA) of liquid to solid, which can be calculated by equation 4, and which is expressed by the reversible work required to isolate a unit area of liquid from a surface of solid. WA=γ1g+γsg－γsl
(4)
Introduce equation 4 into the Young equation, we obtain : WA=γ1g(1+cosθ)
(5)
The work of adhesion of surfactant solutions on CP, as shown in Fig. 6, which was counted by plugging the surface tension and contact angle values in equation 5. From the Fig. 6, we observed that the values of WA of aqueous solution of all studied Gemini surfactants decrease with the increasing surfactants concentration. It can 9
be found that there is a minimum value of WA in a concentration close to their CMC. While compared with the adhesion tension of all studied Gemini surfactants on the CP surface, it can be observed that WA of C10-8-C10 is lowest. Free energy of the surface measures the power of interaction for the spread liquid onto the solid surface. From the thermodynamic point of view, the molar wetting free energy of the solid can be calculated by equation 6 according to Extrand [29]. Under the constant temperature and pressure, the solid surface free energy values is lower, it is possible to get wet. The more negative it is, the more it gets wet. 2 RT 1 cos 2 cos △G= ln 3 4
(6)
where △G is the surface free energy, T is temperature in Kelvin and R is (8.314 J•mol-1•K-1) the universal gas constant. The free energies of wetting on the CP surface are more negative (ΔGCP−C8-8-C8 = −2352.39 J/mol; ΔGCP−C10-8-C10=−5568.12
J/mol;
ΔGCP−C12-8-C12=−4903.67
J/mol;
ΔGCP−C14-8-C14=−4070.83
J/mol;
ΔGCP−C16-8-C16=−2213.58 J/mol) in the company of Gemini surfactant solutions above the CMC than double distilled water (ΔGCP−water = −1129.65 J/mol). Especially C10-8-C10 can significantly changed in wetting on the CP surface. Taking about 1 mL adsorption saturated liquid which contains CP particles and uses electrophoresis to measure zeta potential of the CP surface. Experimental results are shown in Fig. 7. Fig. 7 shows that the zeta potential of CP surface with increasing Gemini surfactant concentration presented following regular pattern which varied from negative charge to positive charge and eventually tended towards stable. Adsorbed Gemini surfactant on the surface of CP significantly changes the solid surface properties, in a descendant order of the zero potential with corresponding concentration: c(C12-8-C12)
concentrations of surfactant concentration in below zero potential. To discover the mechanism accountable for adsorption of cationic Gemini surfactants Cn-8-Cn to CP surface, the relationship of contact angle, surface tension, immersion work, and adhesion work on concentration have been illustrated in Fig. 8. A similar trend of surface tension and contact angle changes was found in the experiment. Therefore, with the C12-8-C12 as an example, the wettability on CP surface was discussed in the Fig. 8. According to Fig. 7, divided from zero potential with corresponding concentration and its CMC, the wetting process can be roughly divided into three connective stages. The 1st Stage ranges from 10-5 g/L to corresponding concentration of zero potential. The concentration is too low to form adsorption film at both liquid-air and solid–liquid interfaces; Thus, γlg and γsl have substantially unchanged, which the contact angle, WA and Wi are basically unchanged. With increasing of surfactant concentration, the hydrophilic head of surfactant molecules in the presence of electrostatic interactions adsorbed on the surface of CP, hydrophobic chain extended outwards liquid phase, which formed a monomolecular adsorption layer when zeta potential reached zero. The 2nd stage ranges from the corresponding concentrations of zero potential to the CMC. With increasing of surfactant concentration, zeta potential of CP surface grew from zero, it suggestion that adsorption amount is increasing, thereby reducing to the surface tension, contact angle and Wi, and increasing WA. Under Van der Waals force, the formed of single molecular layer continues to adsorb another Gemini surfactant molecules that hydrophobic chain cross each other. Finally, CP surface formed bi-layer or multi layer adsorption film, hydrophilic group facing out of liquid phase. As a result, the wetting properties of the CP surface were completely changed. The 3rd stage reaches over its CMC. The surface tension, contact angle, WA and Wi are no longer changed at this stage. Gemini surfactants of the CP surface which formed multilayer saturated structure at last, which hydrophilic group faced outward liquid phase. So here we can get the conclusion safely, the wetting of CP surface by cationic Gemini surfactant is a result of electrostatic interaction and van der Waals interaction. 11
4. Conclusions On the basis of the surveyed values of the contact angle, surface tension and Zeta potential, the theoretical interpretation can be stated that: Surface tension variation following the same trend to contact angle, but the exact reverse is the spreading coefficients. The Wi of CP surface increases with increasing surfactant concentration up to the CMC of the surfactant solution. The WA of CP surface decreases to a minimum with increasing surfactant concentration up to the CMC, finally, it remains constant over the CMC. Cationic Gemini surfactants (Cn-8-Cn) changed the CP surface’s quantity of electric charge and electrical property which altered the wetting of CP. It can be inferred that wetting is a concerted action result of electrostatic interaction and van der waals adsorption. Wetting process can be generally divided into three phases. It is not that the lower of
surface tension is, the more easy preparing of the CPWS is. But, when the Space of cationic Gemini surfactant is the same, the performance of the slurry ability of coal pitch-water slurry is more pronounced with the increased carbon number of hydrophobic chain.
Acknowledgment This work was surpported by the National Natural Science Foundation of China (Grant No.21076135, 21206103) , the Key Laboratory of Road Structure& Material of Transport (Beijing, Opening Funding, Grant No. KF201403), the Key Project of Science and Technology of Shanxi (Grant No. 20130313026-5), the Shanxi Province Natural Science Foundation(Grant No. 2015011023) and the foundation of Shanxi Province Key Laboratory of Surfactant(Grant No. 201503).
12
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646–649.
15
80
Surface tension(mN/m)
70 60 50
C8-8-C8
40
C10-8-C10 C12-8-C12
30
C14-8-C14 C16-8-C16
20 10
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 1. The Curve of the surface tension of aqueous solutions of cationic Gemini surfactant vs. the logarithm of surfactant concentration.
16
80 70 60
(deg)
50
C8-8-C8 C10-8-C10
40
C12-8-C12
30
C14-8-C14 C16-8-C16
20 10
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 2. The curve of the contact angle of aqueous solutions of cationic Gemini surfactants on the CP surface vs. the logarithm of surfactant concentration.
17
Spreading coeffcient(mN/m)
5 0
C8-8-C8
-5 -10 -15 -20
C10-8-C10 C12-8-C12 C14-8-C14 C16-8-C16
-25 -30 -35 -40 -45 -50 -55
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 3. The curve of the spreading coefficient of aqueous solutions of cationic Gemini surfactants on the CP surface vs. the logarithm of surfactant concentration.
18
1.0 0.9
C8-8-C8
0.8
C10-8-C10
0.7
cos
0.6 0.5 0.4 0.3 0.2 0.1 0.0 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
Surface tension/(mN/m)
Fig. 4. The curve of the cosine of contact angle of aqueous solutions of cationic Gemini surfactants on CP surface vs. the surface tension.
19
40 38 36
cos
34 32 30
C8-8-C8
28
C10-8-C10
26
C12-8-C12 C14-8-C14
24 22 25
C16-8-C16 30
35
40
45
50
55
60
65
70
75
Surface tension/(mN/m) Fig. 5. The curve of the adhesion tension of aqueous solutions of cationic Gemini surfactants on the CP surface vs. the surface tension.
20
100
Work of adhension/(mJ/m2)
95 90 85 80
C8-8-C8
75
C10-8-C10
70
C12-8-C12 C14-8-C14
65
C16-8-C16
60 55 -5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 6. The curve of the work of adhesion aqueous solutions of cationic Gemini surfactants on the CP surface vs. the logarithm of surfactant concentration.
21
80
C8-8-C8 C10-8-C10
Zeta potential(mV)
60
C12-8-C12
40
C14-8-C14 C16-8-C16
20 0 -20 -40 -60
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 7. The curve of the Zeta potential of CP surface vs. the logarithm of aqueous solutions of cationic Gemini surfactant concentration.
22
110
80 stage1
100
stage 2
stage 3
70
80
60
70 60
50
Wi
40
WA
30
50 40 30
20 10 0 -7
(deg)
2 WA(mJ/m )Wi(mN/m)
90
20 -6
-5
-4
-3
-2
-1
0
1
lg C(mmol/L) Fig. 8. Dependence of the adhesion data of C12-8-C12 solution to CP surface and the logarithm of surfactant concentration.
[CnH2n+1
CH3 + N (CH2)8
CH3 + N CnH2n+1] 2Br-
.
CH3
CH3
Scheme 1. The structural formula of symmetrical cationic Gemini surfactants.
23