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Wettability of quartz controlled by UV light irradiation using an azobenzene surfactant
Colloids and Surfaces A 578 (2019) 123586
Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Wettability of quartz controlled by UV light irradiation using an azobenzene surfactant
T
⁎
Rui Donga, Xiaoming Jianga, , Chunling Haoa, Wei Xub, Huiyong Lib, Yue Chenb, Tian Xieb a b
Department of Chemistry and Chemical Engineering, Guizhou University, Guiyang, 550025, China State Key Laboratory of Efficient Utilization of Medium-Low Grade Phosphate Ore and Associated Resources (SKLE), Guiyang, 550016, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Wettability Adsorption Quartz Azobenzene Surfactant QCM-D
Solids having controllable wettabilty have received much attention due to their great importance in many industrial fields. In this paper, the quartz surfaces and particles with reversibly switchable wettability have been studied. The wettability of quartz can be controlled by UV light irradiation in the presence of an azobenzene surfactant (AZO). The adsorption behavior of AZO at the quartz-liquid interface was investigated by the measurements of the surface tension, contact angle and Zeta potential. The adsorption process was monitored by the quartz crystal microbalance with dissipation (QCM-D).The results show that AZO has a strong tendency to adsorb at the quartz–liquid interface. UV light irradiation causes the quartz-liquid interfacial tension (γsl) to decrease. The results from the QCM-D measurements show that the frequency change (Δf) decreases and the dissipation loss (ΔD) increases when AZO adsorbs onto the quartz surface. The dissipation changes are very small and the adsorption layer is rigid. After UV light irradiation, the adsorbed surfactants are desorbed from the quartz surface and the adsorption layer partly decomposes, causing the thickness of the adsorption layer to decrease.
1. Introduction Wettability is regarded as a fundamental property of a solid. Wettability of solids has many important applications in industrial fields, such as flotation, detergency, oil recovery, etc [1–3]. The ⁎
wettability of a solid is generally determined by the surface free energy, and also can be modified by surface active agents. Quartz is the common constituent of rocks. As an important industrial raw material, quartz can be widely used in the versatile products of industry. The structures of surfactants have great effect on the
Corresponding author. E-mail address: [email protected] (X. Jiang).
https://doi.org/10.1016/j.colsurfa.2019.123586 Received 23 April 2019; Received in revised form 17 June 2019; Accepted 19 June 2019 Available online 20 June 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 578 (2019) 123586
R. Dong, et al.
before test. There is a distance of 5 cm between the fiber output and the AZO solution [13–15].
wettability of quartz [4,5]. Cationic surfactants adsorb onto the quartz surface, forming a saturated adsorption layer at the quartz-liquid interface [6,7]. Zwitterionic surfactants can absorb onto the quartz surface through Lifshitz-van der Waals interaction, which is influenced by the surfactant structures [8]. Anionic surfactants adsorb onto the positively charged quartz surface (because of electrostatic attraction), and the solid surface becomes more difficult to wet by water. For the mixtures of two surfactants, anionic surfactant and cationic surfactant are reported to exhibit synergism in the mixed monolayer formation on the quartz surface, and the wettability of the quartz surface appears to be reduced [9]. In the process of flotation separation, cationic surfactants can be absorbed onto the quartz particles by the electrostatic attraction between the negatively charged particles and the positively charged ionic heads of the cationic surfactants, with their hydrocarbon chains predominantly toward the aqueous phase, making quartz particles more hydrophobic. Therefore, quartz particles exhibit large changes in their wettability and floatability [10–12]. Due to the strong electrostatic attraction, it is not easy for the adsorbed surfactants to be desorbed from quartz particles. This may show some negative effects. The wettability of quartz cannot be reversibly controlled. The adsorbed cationic surfactants may have some environmental effects (toxicity, bioconcentration). Azobenzene-containing surfactants usually show different surface properties upon UV and visible light irradiation due to the trans-cis photoisomerization, which is quite different from other conventional surfactants. As reported, the azobenzene surfactants can selectively adsorb onto the quartz particles and the floatability of particles was improved [13–15].In spite of that, understanding about the adsorption of the azobenzene surfactant at the solid-liquid interface still remains obscure. This is due to the complexity of the adsorption process. There have been few systematic studies dealing with the effect of the azobenzene surfactants on the wettability of a quartz surface under different light conditions (Scheme 1). The main purpose of this paper is to investigate the adsorption behavior of an azobenzene surfactant at the quartz-liquid interface. The influence of the surfactant adsorption on the wettability of quartz is investigated. For this purpose, the surface tension and contact angle and Zeta potential were measured. The adsorption process of the surfactant onto the surface was studied by QCM-D.
2.2. Synthesis of the azobenzene surfactant To a solution of 4-butylanilin (15.0 g, 10.0 mmol) in acetic acid (60.0 mL) were added sodium nitrite solution(6.9 g, 10.0 mmol, 200 mL H2O), nitrobenzene (5.40 g, 50 mmol) and p-aminotoluene (5.40 g, 50.0 mmol)at 0 °C. The solution was stirred in the nitrogen atmosphere for 24 h. The red product (A) was obtained by filtration and recrystallization with ethanol. To a solution of the previously obtained product (A, 1.07 g) in CCl4 (20 mL) were added N-bromosuccinimide (NBS, 0.97 g) and azobisisobutyronitrile (AIBN, 2.2 g). The solution was refluxed for 24 h.Then the solution was filtered and concentrated under reduced pressure. The compound (B) was obtained by column chromatography on silica gel. The ethanol solution of trimethylamine (30%, 50 mL) was added dropwise into a solution of the compound (B, 1.07 g) in ethanol(30 mL). The mixture was refluxed for 24 h. The obtained solution was concentrated under reduced pressure. The final product (AZO) was obtained by filtration and recrystallization with water. 1 H NMR(D2O, 400 MHz, TMS), δ(ppm): 3.00(s, 9H, CH3), 4.44(s, 2H, CH2), 7.48-7.82(m, 9H, Ar-H). 2.3. Surface tension measurements The surface tension of the surfactant solution (prepared by triply distilled water) was measured by the surface tensiometer (K100, KRÜSS Company, Germany), using the Wilhelmy plate method at 25℃.The error of the experiment was within 0.1 mN /m. 2.4. Contact angle measurements The quartz surfaces were soaked in a piranha solution for 2 h. Then the quartz surfaces were washed with water, ethanol, and ultrapure water, successively, and dried at 25℃. The contact angle measurements were performed by the Drop Shape Analyzer (DSA25, KRÜSS Company, Germany) using the sessile drop method at 25℃.The average value of the contact angle was means of five repeated measurements (measurement error < 0.3°).
2. Experimental section
2.5. Zeta potential measurements
2.1. Materials
The Zeta potential of the quartz particles was measured at 25℃ by the Zetapotential analyzer (Brookhaven Zeta Plus). Exactly 10 mg quartz sample (＜5 μm) was added to a solution of KNO3 (1 × 10−3 molL-1). After stirring for 5 min, the aqueous solution of NaOH or HCl was added to adjust pH. The surfactant was added and conditioned for 10 min. Each result was means of five measurements (measurement error: ± 2 mV).
Quartz samples were provided by SKLE. The purity of quartz was over 90% [13–15].The sample of quartz for the Zeta potential measurement was ground in a laboratory ball mill and the size of particles was below 5 μm. The structure of AZO was identified by1H NMR spectra using the JEOL JNMECZ-400 NMR spectrometer. We used a Prizmatix device equipped with photodiodes (YL-512, Landun Photoelectricity, 365 nm) to produce UV light. The maximal power of UV light was 10 W and the light intensity was about 700 mW/ cm2. Each surfactant solution was irradiated by UV light for 30 min
2.6. Quartz crystal microbalance measurements The surfactant adsorption on the quartz surface was studied using
Scheme 1. The structure of the azobenzene surfactant (AZO). 2
Colloids and Surfaces A 578 (2019) 123586
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the quartz crystal microbalance with dissipation (QCM-D, Q-Sense E1, Biolin Scientific) at 25 °C. The sensors were AT-cut quartz crystals with a resonance frequency of 5 MHz.The quartz sensors were cleaned thoroughly and dried by N2. The mass sensitivity of the QCM-D was 2 ng/cm2. The flow rate was controlled at 100 μL/min. The adsorbed mass on the quartz crystal can be calculated by the Sauerbrey relation [16,17].
Δ m= −
CΔf n
Table 1 The physico-chemical parameters of AZO (25℃).
No UV UV
cmc mmol/L
γcmc mN/m
Гmax μmol/m2
Amn nm2
0.79 1.25
57.96 62.56
0.61 0.48
2.73 3.46
(1)
In Eq. (1), Δm is the adsorbed mass, C is the sensitivity constant, n is overtone number (n = 1, 3, 5, 7, …), and Δf is frequency changes. The dissipation loss is defined by Eq. (2)
Edissipation 1 = Q 2πEstored
Δ D=
(2)
In Eq. (2), ΔD is the dissipation loss, Q is the quality factor, Edissipation is the energy dissipated during one oscillation period, and Estored is the total energy stored in the system. ΔD provides the information about the viscoelastic properties of the adsorption layer. 3. Results and discussion 3.1. Adsorption at the air–water interface We measured the surface tension of the AZO solution, and obtained the physical and chemical parameters, including critical micelle concentration (cmc), minimum surface tension (γcmc), the saturated adsorption amount (Γmax) and the minimum area/molecule (Amin). Fig. 1 shows the surface tension (γ) vs. log surfactant molar concentration (logC) plots for AZO. The cmc value of the surfactant was determined from the break point of the γ-logC plot (Table 1). Γmax and Amin were calculated using the Gibbs adsorption equation [18].
Γmax
dγ ⎞ 1 =− × ⎜⎛ ⎟ 2.303nRT ⎝ dlogC ⎠T
Amin =
1 NA Γmax
Fig. 2. Contact angle versus the surfactant concentration.
Γmax decreases and Amin increases when the AZO solution is irradiated by UV light, due to an increase in the space hindrance of the surfactant molecule (the molecule shape changes from a linear conformation to a twisted form). 3.2. Wettability of the quartz surface Plots of the contact angle (θ) vs. the bulk phase concentration (C) are shown in Fig. 2. When the surfactant adsorbs on the quartz surface, the θ-C curve passes through one maximum. As reported, the contact angle usually reaches a maximum when the surfactant concentration increases to cmc. But for AZO, the surfactant concentration corresponding to the maximal contact angle is about 5 × 10−4molL-1 (lower than its cmc). It indicates that AZO exhibits the different adsorption behavior compared to other traditional surfactants. When the AZO solutions are irradiated by UV light, the values of contact angle are decreased, causing the wettabilty of the quartz surface to increase. The adsorption mechanism will be discussed later.
(3)
(4)
where (dγ/dlogC)T is the slope of the γ-logC plot, n = 2, R = 8.314 J mol−1 K−1, and NA is Avogadro’s number. Irradiated by UV light, the AZO solution shows higher values of cmc and γcmc, as would be expected from the increased polarity of the surfactant molecule. A similar increase was reported by Yoon and coworkers for the surfactants containing an azobenzene group [19].
3.3. Adsorption and wettability As reported by Lucassen-Reynders, a method has been developed to investigate the relationship between the adsorption of surfactants at the liquid-air and solid–liquid interfaces. Combining with the Gibbs adsorption equation, Young’s equation can be written as [18]:
d(γlg cosθ) dγlg
=
Гsg − Гsl Гlg
(5)
Where Гsg, Гsl, and Гlg represent the surface excess concentration of the surfactant at the solid–air, solid–liquid, and liquid–air interfaces, respectively. The slope of the plot of the adhesion tension (γlgcosθ) versus surface tension (γlg) can provide information on the surface excess of the surfactant at the solid–liquid and liquid–air interfaces, such as Гsl= Гlg(slope = 1), Гsl＞Гlg(slope＞1), Гsl＜Гlg(slope＜1). The negative slope of the plot means the wettability of a solid surface is improved by surfactants, and the positive slope indicates the wettability is decreased
Fig. 1. Surface tension versus log of the surfactant concentration. 3
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Fig. 3. The adhesion tension versus the surface tension. Fig. 4. The quartz-solution interfacial tension versus log of the surfactant concentration.
by surfactants. The plots of γlgcosθ versus γlg for AZO are shown in Fig. 3. The plots exhibit a good linear relationship, especially at the low AZO concentration. The slopes of the plots are positive. Wettability of quartz is impaired by the presence of AZO, making the quartz surface more hydrophobic. Therefore, Eq. (5) becomes [6,7]
−
d(γlg cosθ) dγlg
=
can be calculated by Gibbs adsorption equation, and the calculation method is similar to that of the liquid-gas interface [7]. The cross-sectional area of AZO was 1.59 nm2 (before UV irradiation) and 1.86 nm2 (after UV irradiation), respectively. The cross-sectional area is increased by UV irradiation, which is similar to the change of the area of the surfactant at the liquid–air interface. This is presumably because the surfactant has the conversion of trans isomers to cis isomers in the presence of UV light. UV light irradiation also causes a decrease in the values of γsl (shown in Fig. 4), because the adsorption of AZO onto the quartz surface is decreased when the photoisomerization happens. In order to investigate the adsorption of AZO at the interfaces, we show the curves of surface tension (γ), contact angle (θ) and adhesion tension (γcosθ) in Fig. 5. In the absence of UV irradiation, the curves can be divided into three stages. In the first stage, when the AZO concentration is very low (below 1 × 10−4molL-1), no obvious adsorption occurs at the gas-liquid interface, and the surface tension remains almost unchanged. On the other hand, AZO can easily absorb on the quartz surface. The hydrophobicity of the quartz surface increases and the adhesion force decreases, causing an increase in the contact angle. In the second stage, when the AZO concentration increases from1 × 10−4 to 5 × 10−4molL-1, AZO begins to adsorb rapidly at the gas-liquid interface, causing the surface tension to decrease sharply. In the same time, AZO continues to adsorb on the quartz surface, and the quartz surface becomes more hydrophobic. A saturated adsorption layer forms on the surface of quartz when the surfactant concentration is 5 × 10−4molL-1. The quartz surface has the maximum hydrophobicity, and the adhesion tension reaches its minimum. In the third stage, when AZO concentration is greater than 5 × 10−4molL-1, AZO adsorbs at the gas-liquid interface until a saturated adsorption layer forms at the interface. AZO continues to adsorb onto the quartz surface, forming a bilayer on the quartz surface. The hydrophilic head of AZO on the upper layer extends to the aqueous phase, and causes the adhesion tension to increase. In the presence of UV irradiation, the adsorption of AZO at the interfaces is similar to that of AZO without UV irradiation. It is noteworthy that AZO shows an increase in the values of adhesion tension after UV light irradiation. This is presumably because the decreased adsorption of AZO causes the hydrophobicity of the quartz surface to decrease.
Гsg − Гsl Гlg
(6)
For a quartz surface, Γsg is close to 0 because there is no solution at the solid-air interface. The slopes in Fig. 3 are very large (＞1), indicating that Гsl＞＞Гlg. The surface excess concentration at the quartz–liquid interface is much higher than that at the liquid–air interface. This phenomenon is quite different from other cationic surfactants and catanionic surfactant mixtures reported [6,7]. The slopes of these surfactants are closed to 1, which means that Гsl is almost equal to Гlg. The Lifshitz-van der Waals forces play an important role in the adsorption of these surfactants onto the quartz surface. The azobenzene surfactant has higher excess concentration at the quartz–liquid, indicating the driving force is the electrostatic attraction, which is much stronger than the Lifshitz-van der Waals interaction. The slope of a plot of γlgcosθ vs. γlg is slightly increased after UV light irradiation. The slope increases from 1.70 to 1.92.The surfactant has a stronger tendency to adsorb at the quartz–liquid interface compared to the liquid–air interface. This is because UV light irradiation improves the polarity of AZO, causing an increase in the surfactant concentration in the bulk solution. When surfactants adsorb on the solid-liquid interface, the solid-liquid interfacial tension (γsl) will increase. The solid-liquid interfacial tension has great influence on the wettability of the quartz surface. But it is well-known that experimental evaluation of γsl is very difficult. However, if we know the surface tension of a solid surface, liquid surface tension and contact angle, we can use Young equation to calculate γsl. According to the calculation method of Daniel [20], the surface tension of quartz is 75.34 mN/m. Plots of interfacial tension,γsl, vs. log of the bulk concentration in molL−1 (log C) are shown in Fig. 4. As shown in Fig. 4, the interfacial tension follows a pattern: the values of γsl increase with increasing the surfactant concentration and pass a maximum value. The hydrophobicity of quartz surface increases with an increase in the surfactant concentration. This causes an increase in the values of γsl. However, when the concentration continues to increase, the surfactant begins to form a bilayer on the quartz surface, resulting in a decrease in the value of γsl until γsl obtains the equilibrium value. The cross-sectional area of a surfactant at the solid-liquid interface 4
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Fig. 5. Concentration dependence of the adhesion data of surfactant.
adsorbed surfactant decreases, indicating that some surfactants are desorbed from the quartz surface. Fig. 8 shows the variations of the dissipation shift and the thickness of adsorption layer with time. For a rigid and thin film, the dissipation shift is 2 × 10−6 or less [21]. The dissipation changes in our experiments are very small (< 6 × 10−7), indicating that the adsorption layer of AZO is rigid. The thickness of the layer is zero before the surfactants adsorb onto the quartz surface; then a sharp increase in the thickness is observed when the quartz crystal is in contact with the AZO solution, indicating AZO is adsorbed quickly onto the quartz surface. When the AZO solution is irradiated by UV light, both the dissipation shift and the thickness of the adsorption layer appear to decrease. The UV light irradiation causes part of the adsorption layer to decompose, and both the dissipation shift and the thickness of the adsorption layer appear to decrease.
4. Conclusion Fig. 6. Variation of the Zeta potential with pH.
A cationic surfactant containing an azobenzene group was synthesized. The surface properties of the surfactant are influenced by UV light irradiation. The AZO solutions show higher values of cmc and γcmc, and Γmax decreases after the AZO solutions are irradiated by UV light. The contact angle of AZO solution on quartz surface increases with increasing the surfactant concentration, and passes through a maximum. After UV irradiation, the contact angle appears to decrease. In the presence of AZO, the wettabilty of the quartz surface is increased by UV light irradiation. The plots of γlgcosθ versus γlg show large positive slope. The surface excess concentration at the quartz–liquid interface is apparently higher than that at the liquid–air interface. The values of γsl increase with increasing the surfactant concentration and pass a maximum value. The cross-sectional area of AZO at the quartz-liquid interface is increased by UV light irradiation, The Zeta potential of quartz particles is increased when AZO is added into the solution. But the Zeta potential values decrease when the AZO solutions are irradiated by UV light. The results of QCM-D show that Δf decreases and ΔD increases when the surfactants begin to adsorb onto the quartz surface. The values of ΔD are very small. The adsorption layer is rigid. The values of Δf increase and ΔD values decrease when the AZO solution is irradiated by UV light. The results show that some surfactants are desorbed from the quartz surface, and a more rigid film forms on the quartz surface in the presence of UV irradiation. UV light irradiation causes part of the adsorption layer to decompose, and the thickness of the adsorption layer appears to decrease.
3.4. Zeta potential of the quartz particles The Zeta potential of quartz particles in the AZO solution was measured to obtain the information of the interaction between the surfactant and quartz particles. Plots of the Zeta potential vs. pH are shown in Fig. 6. In the absence of AZO, the isoelectric point of quartz was about at 2.3. Adding AZO into the solution, we find the isoelectric point increases to 4.3. It indicates that AZO has adsorbed onto the surface of quartz particles. It is interesting that the Zeta potential values decrease under UV light irradiation and the isoelectric point of quartz particles decreases from 4.3 to 2.8, indicating that the UV irradiation reduces the adsorption of AZO at the quartz particles.
3.5. Quartz crystal microbalance with dissipation The quartz crystal microbalance with dissipation (QCM-D) is regarded as a surface characterization technology with high sensitivity. QCM-D can detect the changes of the adsorbed mass and the viscoelastic property of the adsorption layer. Adsorption process of AZO onto the quartz surface was monitored by QCM-D. The variations of the frequency shift (Δf) with time are shown in Fig. 7(a). The baseline appears when the quartz crystal is immersed in water; then a sharp decrease in Δf is observed when the quartz crystal is in contact with the AZO solution, due to the adsorption of the surfactant onto the quartz surface. Finally, Δf appears to decrease slowly until AZO reaches an adsorption equilibrium. Fig. 7(b) shows the variations of the mass of the adsorbed surfactants with time. We find that the mass of the adsorbed surfactant increases with increasing the time, indicating AZO absorbs on the quartz surface. After UV light irradiation, Δf increases and the mass of the
Acknowledgment This research was sponsored by State Key Laboratory of Efficient Utilization for Low Grade Phosphate Rock and Its Associated Resources. 5
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Fig. 7. Variation of the frequency shift (Δf) and the mass of the adsorbed surfactants with time, the third overtone, C(AZO): 1.0 × 10−4mol/L.
Fig. 8. Variation of dissipation shift (ΔD) and the thickness of the adsorption layer with time, the third overtone, C(AZO): 1.0 × 10−4mol/L.
References [11]
[1] A.D. Gadhave, S.N. Mehdizadeh, G.G. Chase, Effect of pore size and wettability of multilayered coalescing filters on water-in-ULSD coalescence, Sep. Purif. Technol. 221 (2019) 236–248. [2] M.J. Su, S. Bai, Y. Luo, G.W. Chu, B.C. Sun, Y. Le, Controllable wettability on stainless steel substrates with highly stable coatings, Chem. Eng. Sci. 195 (2019) 791–800. [3] Y. Chen, G. Xu, J. Huang, J. Eksteen, X. Liu, Z. Zhao, Characterization of coal particles wettability in surfactant solution by using four laboratory static tests, Colloids Surf. A 567 (2019) 304–312. [4] L.N. Nwidee, M. Lebedev, A. Barifcani, M. Sarmadivaleh, S. Iglauer, Wettability alteration of oil-wet limestone using surfactant-nanoparticle formulation, J. Colloid Interface Sci. 504 (2017) 334–345. [5] P. Ghosh, K.K. Mohanty, Study of surfactant alternating gas injection (SAG) in gasflooded oil-wet low permeability carbonate rocks, Fuel 251 (2019) 260–275. [6] C. Wang, X.L. Cao, L.L. Guo, Z.Z. Xu, L. Zhang, Q.T. Gong, L. Zhang, S. Zhao, Effect of adsorption of catanionic surfactant mixtures on wettability of quartz surface, Colloids Surf. A 509 (2016) 564–573. [7] S.S. Hu, Z.H. Zhou, L. Zhang, Z.C. Xu, Q.T. Gong, Z.Q. Jin, L. Zhang, S. Zhao, Adsorption behaviors of novel betaines on the wettability of the quartz surface, Soft Matter 11 (2015) 7960–7968. [8] Z.L. Zhang, Z.Q. Wang, L. Li, Z.C. Xu, S. Zhao, J.Y. Yu, Wettability of a quartz surface in the presence of four cationic surfactants, Langmuir 26 (2010) 18834–18840. [9] J. Yu, Y.Y. Ge, X.L. Guo, W.B. Guo, The depression effect and mechanism of NSFC on dolomite in the flotation of phosphate ore, Sep. Purif. Technol. 161 (2016) 88–95. [10] R. Hakkou, M. Benzaazoua, B. Bussière, Valorization of phosphate waste rocks and
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Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Wettability of quartz controlled by UV light irradiation using an azobenzene surfactant
T
⁎
Rui Donga, Xiaoming Jianga, , Chunling Haoa, Wei Xub, Huiyong Lib, Yue Chenb, Tian Xieb a b
Department of Chemistry and Chemical Engineering, Guizhou University, Guiyang, 550025, China State Key Laboratory of Efficient Utilization of Medium-Low Grade Phosphate Ore and Associated Resources (SKLE), Guiyang, 550016, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Wettability Adsorption Quartz Azobenzene Surfactant QCM-D
Solids having controllable wettabilty have received much attention due to their great importance in many industrial fields. In this paper, the quartz surfaces and particles with reversibly switchable wettability have been studied. The wettability of quartz can be controlled by UV light irradiation in the presence of an azobenzene surfactant (AZO). The adsorption behavior of AZO at the quartz-liquid interface was investigated by the measurements of the surface tension, contact angle and Zeta potential. The adsorption process was monitored by the quartz crystal microbalance with dissipation (QCM-D).The results show that AZO has a strong tendency to adsorb at the quartz–liquid interface. UV light irradiation causes the quartz-liquid interfacial tension (γsl) to decrease. The results from the QCM-D measurements show that the frequency change (Δf) decreases and the dissipation loss (ΔD) increases when AZO adsorbs onto the quartz surface. The dissipation changes are very small and the adsorption layer is rigid. After UV light irradiation, the adsorbed surfactants are desorbed from the quartz surface and the adsorption layer partly decomposes, causing the thickness of the adsorption layer to decrease.
1. Introduction Wettability is regarded as a fundamental property of a solid. Wettability of solids has many important applications in industrial fields, such as flotation, detergency, oil recovery, etc [1–3]. The ⁎
wettability of a solid is generally determined by the surface free energy, and also can be modified by surface active agents. Quartz is the common constituent of rocks. As an important industrial raw material, quartz can be widely used in the versatile products of industry. The structures of surfactants have great effect on the
Corresponding author. E-mail address: [email protected] (X. Jiang).
https://doi.org/10.1016/j.colsurfa.2019.123586 Received 23 April 2019; Received in revised form 17 June 2019; Accepted 19 June 2019 Available online 20 June 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
Colloids and Surfaces A 578 (2019) 123586
R. Dong, et al.
before test. There is a distance of 5 cm between the fiber output and the AZO solution [13–15].
wettability of quartz [4,5]. Cationic surfactants adsorb onto the quartz surface, forming a saturated adsorption layer at the quartz-liquid interface [6,7]. Zwitterionic surfactants can absorb onto the quartz surface through Lifshitz-van der Waals interaction, which is influenced by the surfactant structures [8]. Anionic surfactants adsorb onto the positively charged quartz surface (because of electrostatic attraction), and the solid surface becomes more difficult to wet by water. For the mixtures of two surfactants, anionic surfactant and cationic surfactant are reported to exhibit synergism in the mixed monolayer formation on the quartz surface, and the wettability of the quartz surface appears to be reduced [9]. In the process of flotation separation, cationic surfactants can be absorbed onto the quartz particles by the electrostatic attraction between the negatively charged particles and the positively charged ionic heads of the cationic surfactants, with their hydrocarbon chains predominantly toward the aqueous phase, making quartz particles more hydrophobic. Therefore, quartz particles exhibit large changes in their wettability and floatability [10–12]. Due to the strong electrostatic attraction, it is not easy for the adsorbed surfactants to be desorbed from quartz particles. This may show some negative effects. The wettability of quartz cannot be reversibly controlled. The adsorbed cationic surfactants may have some environmental effects (toxicity, bioconcentration). Azobenzene-containing surfactants usually show different surface properties upon UV and visible light irradiation due to the trans-cis photoisomerization, which is quite different from other conventional surfactants. As reported, the azobenzene surfactants can selectively adsorb onto the quartz particles and the floatability of particles was improved [13–15].In spite of that, understanding about the adsorption of the azobenzene surfactant at the solid-liquid interface still remains obscure. This is due to the complexity of the adsorption process. There have been few systematic studies dealing with the effect of the azobenzene surfactants on the wettability of a quartz surface under different light conditions (Scheme 1). The main purpose of this paper is to investigate the adsorption behavior of an azobenzene surfactant at the quartz-liquid interface. The influence of the surfactant adsorption on the wettability of quartz is investigated. For this purpose, the surface tension and contact angle and Zeta potential were measured. The adsorption process of the surfactant onto the surface was studied by QCM-D.
2.2. Synthesis of the azobenzene surfactant To a solution of 4-butylanilin (15.0 g, 10.0 mmol) in acetic acid (60.0 mL) were added sodium nitrite solution(6.9 g, 10.0 mmol, 200 mL H2O), nitrobenzene (5.40 g, 50 mmol) and p-aminotoluene (5.40 g, 50.0 mmol)at 0 °C. The solution was stirred in the nitrogen atmosphere for 24 h. The red product (A) was obtained by filtration and recrystallization with ethanol. To a solution of the previously obtained product (A, 1.07 g) in CCl4 (20 mL) were added N-bromosuccinimide (NBS, 0.97 g) and azobisisobutyronitrile (AIBN, 2.2 g). The solution was refluxed for 24 h.Then the solution was filtered and concentrated under reduced pressure. The compound (B) was obtained by column chromatography on silica gel. The ethanol solution of trimethylamine (30%, 50 mL) was added dropwise into a solution of the compound (B, 1.07 g) in ethanol(30 mL). The mixture was refluxed for 24 h. The obtained solution was concentrated under reduced pressure. The final product (AZO) was obtained by filtration and recrystallization with water. 1 H NMR(D2O, 400 MHz, TMS), δ(ppm): 3.00(s, 9H, CH3), 4.44(s, 2H, CH2), 7.48-7.82(m, 9H, Ar-H). 2.3. Surface tension measurements The surface tension of the surfactant solution (prepared by triply distilled water) was measured by the surface tensiometer (K100, KRÜSS Company, Germany), using the Wilhelmy plate method at 25℃.The error of the experiment was within 0.1 mN /m. 2.4. Contact angle measurements The quartz surfaces were soaked in a piranha solution for 2 h. Then the quartz surfaces were washed with water, ethanol, and ultrapure water, successively, and dried at 25℃. The contact angle measurements were performed by the Drop Shape Analyzer (DSA25, KRÜSS Company, Germany) using the sessile drop method at 25℃.The average value of the contact angle was means of five repeated measurements (measurement error < 0.3°).
2. Experimental section
2.5. Zeta potential measurements
2.1. Materials
The Zeta potential of the quartz particles was measured at 25℃ by the Zetapotential analyzer (Brookhaven Zeta Plus). Exactly 10 mg quartz sample (＜5 μm) was added to a solution of KNO3 (1 × 10−3 molL-1). After stirring for 5 min, the aqueous solution of NaOH or HCl was added to adjust pH. The surfactant was added and conditioned for 10 min. Each result was means of five measurements (measurement error: ± 2 mV).
Quartz samples were provided by SKLE. The purity of quartz was over 90% [13–15].The sample of quartz for the Zeta potential measurement was ground in a laboratory ball mill and the size of particles was below 5 μm. The structure of AZO was identified by1H NMR spectra using the JEOL JNMECZ-400 NMR spectrometer. We used a Prizmatix device equipped with photodiodes (YL-512, Landun Photoelectricity, 365 nm) to produce UV light. The maximal power of UV light was 10 W and the light intensity was about 700 mW/ cm2. Each surfactant solution was irradiated by UV light for 30 min
2.6. Quartz crystal microbalance measurements The surfactant adsorption on the quartz surface was studied using
Scheme 1. The structure of the azobenzene surfactant (AZO). 2
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the quartz crystal microbalance with dissipation (QCM-D, Q-Sense E1, Biolin Scientific) at 25 °C. The sensors were AT-cut quartz crystals with a resonance frequency of 5 MHz.The quartz sensors were cleaned thoroughly and dried by N2. The mass sensitivity of the QCM-D was 2 ng/cm2. The flow rate was controlled at 100 μL/min. The adsorbed mass on the quartz crystal can be calculated by the Sauerbrey relation [16,17].
Δ m= −
CΔf n
Table 1 The physico-chemical parameters of AZO (25℃).
No UV UV
cmc mmol/L
γcmc mN/m
Гmax μmol/m2
Amn nm2
0.79 1.25
57.96 62.56
0.61 0.48
2.73 3.46
(1)
In Eq. (1), Δm is the adsorbed mass, C is the sensitivity constant, n is overtone number (n = 1, 3, 5, 7, …), and Δf is frequency changes. The dissipation loss is defined by Eq. (2)
Edissipation 1 = Q 2πEstored
Δ D=
(2)
In Eq. (2), ΔD is the dissipation loss, Q is the quality factor, Edissipation is the energy dissipated during one oscillation period, and Estored is the total energy stored in the system. ΔD provides the information about the viscoelastic properties of the adsorption layer. 3. Results and discussion 3.1. Adsorption at the air–water interface We measured the surface tension of the AZO solution, and obtained the physical and chemical parameters, including critical micelle concentration (cmc), minimum surface tension (γcmc), the saturated adsorption amount (Γmax) and the minimum area/molecule (Amin). Fig. 1 shows the surface tension (γ) vs. log surfactant molar concentration (logC) plots for AZO. The cmc value of the surfactant was determined from the break point of the γ-logC plot (Table 1). Γmax and Amin were calculated using the Gibbs adsorption equation [18].
Γmax
dγ ⎞ 1 =− × ⎜⎛ ⎟ 2.303nRT ⎝ dlogC ⎠T
Amin =
1 NA Γmax
Fig. 2. Contact angle versus the surfactant concentration.
Γmax decreases and Amin increases when the AZO solution is irradiated by UV light, due to an increase in the space hindrance of the surfactant molecule (the molecule shape changes from a linear conformation to a twisted form). 3.2. Wettability of the quartz surface Plots of the contact angle (θ) vs. the bulk phase concentration (C) are shown in Fig. 2. When the surfactant adsorbs on the quartz surface, the θ-C curve passes through one maximum. As reported, the contact angle usually reaches a maximum when the surfactant concentration increases to cmc. But for AZO, the surfactant concentration corresponding to the maximal contact angle is about 5 × 10−4molL-1 (lower than its cmc). It indicates that AZO exhibits the different adsorption behavior compared to other traditional surfactants. When the AZO solutions are irradiated by UV light, the values of contact angle are decreased, causing the wettabilty of the quartz surface to increase. The adsorption mechanism will be discussed later.
(3)
(4)
where (dγ/dlogC)T is the slope of the γ-logC plot, n = 2, R = 8.314 J mol−1 K−1, and NA is Avogadro’s number. Irradiated by UV light, the AZO solution shows higher values of cmc and γcmc, as would be expected from the increased polarity of the surfactant molecule. A similar increase was reported by Yoon and coworkers for the surfactants containing an azobenzene group [19].
3.3. Adsorption and wettability As reported by Lucassen-Reynders, a method has been developed to investigate the relationship between the adsorption of surfactants at the liquid-air and solid–liquid interfaces. Combining with the Gibbs adsorption equation, Young’s equation can be written as [18]:
d(γlg cosθ) dγlg
=
Гsg − Гsl Гlg
(5)
Where Гsg, Гsl, and Гlg represent the surface excess concentration of the surfactant at the solid–air, solid–liquid, and liquid–air interfaces, respectively. The slope of the plot of the adhesion tension (γlgcosθ) versus surface tension (γlg) can provide information on the surface excess of the surfactant at the solid–liquid and liquid–air interfaces, such as Гsl= Гlg(slope = 1), Гsl＞Гlg(slope＞1), Гsl＜Гlg(slope＜1). The negative slope of the plot means the wettability of a solid surface is improved by surfactants, and the positive slope indicates the wettability is decreased
Fig. 1. Surface tension versus log of the surfactant concentration. 3
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Fig. 3. The adhesion tension versus the surface tension. Fig. 4. The quartz-solution interfacial tension versus log of the surfactant concentration.
by surfactants. The plots of γlgcosθ versus γlg for AZO are shown in Fig. 3. The plots exhibit a good linear relationship, especially at the low AZO concentration. The slopes of the plots are positive. Wettability of quartz is impaired by the presence of AZO, making the quartz surface more hydrophobic. Therefore, Eq. (5) becomes [6,7]
−
d(γlg cosθ) dγlg
=
can be calculated by Gibbs adsorption equation, and the calculation method is similar to that of the liquid-gas interface [7]. The cross-sectional area of AZO was 1.59 nm2 (before UV irradiation) and 1.86 nm2 (after UV irradiation), respectively. The cross-sectional area is increased by UV irradiation, which is similar to the change of the area of the surfactant at the liquid–air interface. This is presumably because the surfactant has the conversion of trans isomers to cis isomers in the presence of UV light. UV light irradiation also causes a decrease in the values of γsl (shown in Fig. 4), because the adsorption of AZO onto the quartz surface is decreased when the photoisomerization happens. In order to investigate the adsorption of AZO at the interfaces, we show the curves of surface tension (γ), contact angle (θ) and adhesion tension (γcosθ) in Fig. 5. In the absence of UV irradiation, the curves can be divided into three stages. In the first stage, when the AZO concentration is very low (below 1 × 10−4molL-1), no obvious adsorption occurs at the gas-liquid interface, and the surface tension remains almost unchanged. On the other hand, AZO can easily absorb on the quartz surface. The hydrophobicity of the quartz surface increases and the adhesion force decreases, causing an increase in the contact angle. In the second stage, when the AZO concentration increases from1 × 10−4 to 5 × 10−4molL-1, AZO begins to adsorb rapidly at the gas-liquid interface, causing the surface tension to decrease sharply. In the same time, AZO continues to adsorb on the quartz surface, and the quartz surface becomes more hydrophobic. A saturated adsorption layer forms on the surface of quartz when the surfactant concentration is 5 × 10−4molL-1. The quartz surface has the maximum hydrophobicity, and the adhesion tension reaches its minimum. In the third stage, when AZO concentration is greater than 5 × 10−4molL-1, AZO adsorbs at the gas-liquid interface until a saturated adsorption layer forms at the interface. AZO continues to adsorb onto the quartz surface, forming a bilayer on the quartz surface. The hydrophilic head of AZO on the upper layer extends to the aqueous phase, and causes the adhesion tension to increase. In the presence of UV irradiation, the adsorption of AZO at the interfaces is similar to that of AZO without UV irradiation. It is noteworthy that AZO shows an increase in the values of adhesion tension after UV light irradiation. This is presumably because the decreased adsorption of AZO causes the hydrophobicity of the quartz surface to decrease.
Гsg − Гsl Гlg
(6)
For a quartz surface, Γsg is close to 0 because there is no solution at the solid-air interface. The slopes in Fig. 3 are very large (＞1), indicating that Гsl＞＞Гlg. The surface excess concentration at the quartz–liquid interface is much higher than that at the liquid–air interface. This phenomenon is quite different from other cationic surfactants and catanionic surfactant mixtures reported [6,7]. The slopes of these surfactants are closed to 1, which means that Гsl is almost equal to Гlg. The Lifshitz-van der Waals forces play an important role in the adsorption of these surfactants onto the quartz surface. The azobenzene surfactant has higher excess concentration at the quartz–liquid, indicating the driving force is the electrostatic attraction, which is much stronger than the Lifshitz-van der Waals interaction. The slope of a plot of γlgcosθ vs. γlg is slightly increased after UV light irradiation. The slope increases from 1.70 to 1.92.The surfactant has a stronger tendency to adsorb at the quartz–liquid interface compared to the liquid–air interface. This is because UV light irradiation improves the polarity of AZO, causing an increase in the surfactant concentration in the bulk solution. When surfactants adsorb on the solid-liquid interface, the solid-liquid interfacial tension (γsl) will increase. The solid-liquid interfacial tension has great influence on the wettability of the quartz surface. But it is well-known that experimental evaluation of γsl is very difficult. However, if we know the surface tension of a solid surface, liquid surface tension and contact angle, we can use Young equation to calculate γsl. According to the calculation method of Daniel [20], the surface tension of quartz is 75.34 mN/m. Plots of interfacial tension,γsl, vs. log of the bulk concentration in molL−1 (log C) are shown in Fig. 4. As shown in Fig. 4, the interfacial tension follows a pattern: the values of γsl increase with increasing the surfactant concentration and pass a maximum value. The hydrophobicity of quartz surface increases with an increase in the surfactant concentration. This causes an increase in the values of γsl. However, when the concentration continues to increase, the surfactant begins to form a bilayer on the quartz surface, resulting in a decrease in the value of γsl until γsl obtains the equilibrium value. The cross-sectional area of a surfactant at the solid-liquid interface 4
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Fig. 5. Concentration dependence of the adhesion data of surfactant.
adsorbed surfactant decreases, indicating that some surfactants are desorbed from the quartz surface. Fig. 8 shows the variations of the dissipation shift and the thickness of adsorption layer with time. For a rigid and thin film, the dissipation shift is 2 × 10−6 or less [21]. The dissipation changes in our experiments are very small (< 6 × 10−7), indicating that the adsorption layer of AZO is rigid. The thickness of the layer is zero before the surfactants adsorb onto the quartz surface; then a sharp increase in the thickness is observed when the quartz crystal is in contact with the AZO solution, indicating AZO is adsorbed quickly onto the quartz surface. When the AZO solution is irradiated by UV light, both the dissipation shift and the thickness of the adsorption layer appear to decrease. The UV light irradiation causes part of the adsorption layer to decompose, and both the dissipation shift and the thickness of the adsorption layer appear to decrease.
4. Conclusion Fig. 6. Variation of the Zeta potential with pH.
A cationic surfactant containing an azobenzene group was synthesized. The surface properties of the surfactant are influenced by UV light irradiation. The AZO solutions show higher values of cmc and γcmc, and Γmax decreases after the AZO solutions are irradiated by UV light. The contact angle of AZO solution on quartz surface increases with increasing the surfactant concentration, and passes through a maximum. After UV irradiation, the contact angle appears to decrease. In the presence of AZO, the wettabilty of the quartz surface is increased by UV light irradiation. The plots of γlgcosθ versus γlg show large positive slope. The surface excess concentration at the quartz–liquid interface is apparently higher than that at the liquid–air interface. The values of γsl increase with increasing the surfactant concentration and pass a maximum value. The cross-sectional area of AZO at the quartz-liquid interface is increased by UV light irradiation, The Zeta potential of quartz particles is increased when AZO is added into the solution. But the Zeta potential values decrease when the AZO solutions are irradiated by UV light. The results of QCM-D show that Δf decreases and ΔD increases when the surfactants begin to adsorb onto the quartz surface. The values of ΔD are very small. The adsorption layer is rigid. The values of Δf increase and ΔD values decrease when the AZO solution is irradiated by UV light. The results show that some surfactants are desorbed from the quartz surface, and a more rigid film forms on the quartz surface in the presence of UV irradiation. UV light irradiation causes part of the adsorption layer to decompose, and the thickness of the adsorption layer appears to decrease.
3.4. Zeta potential of the quartz particles The Zeta potential of quartz particles in the AZO solution was measured to obtain the information of the interaction between the surfactant and quartz particles. Plots of the Zeta potential vs. pH are shown in Fig. 6. In the absence of AZO, the isoelectric point of quartz was about at 2.3. Adding AZO into the solution, we find the isoelectric point increases to 4.3. It indicates that AZO has adsorbed onto the surface of quartz particles. It is interesting that the Zeta potential values decrease under UV light irradiation and the isoelectric point of quartz particles decreases from 4.3 to 2.8, indicating that the UV irradiation reduces the adsorption of AZO at the quartz particles.
3.5. Quartz crystal microbalance with dissipation The quartz crystal microbalance with dissipation (QCM-D) is regarded as a surface characterization technology with high sensitivity. QCM-D can detect the changes of the adsorbed mass and the viscoelastic property of the adsorption layer. Adsorption process of AZO onto the quartz surface was monitored by QCM-D. The variations of the frequency shift (Δf) with time are shown in Fig. 7(a). The baseline appears when the quartz crystal is immersed in water; then a sharp decrease in Δf is observed when the quartz crystal is in contact with the AZO solution, due to the adsorption of the surfactant onto the quartz surface. Finally, Δf appears to decrease slowly until AZO reaches an adsorption equilibrium. Fig. 7(b) shows the variations of the mass of the adsorbed surfactants with time. We find that the mass of the adsorbed surfactant increases with increasing the time, indicating AZO absorbs on the quartz surface. After UV light irradiation, Δf increases and the mass of the
Acknowledgment This research was sponsored by State Key Laboratory of Efficient Utilization for Low Grade Phosphate Rock and Its Associated Resources. 5
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Fig. 7. Variation of the frequency shift (Δf) and the mass of the adsorbed surfactants with time, the third overtone, C(AZO): 1.0 × 10−4mol/L.
Fig. 8. Variation of dissipation shift (ΔD) and the thickness of the adsorption layer with time, the third overtone, C(AZO): 1.0 × 10−4mol/L.
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