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Wetting behavior and mechanism of wetting agents on low-energy surface
Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
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Wetting behavior and mechanism of wetting agents on low-energy surface Hui Wang a,∗ , Chongqing Wang a , Jiangang Fu a , Guohua Gu b a School of Chemistry and Chemical Engineering, Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha, 410083 Hunan, China b School of Minerals Processing and Bioengineering, Central South University, Changsha, 410083 Hunan, China
h i g h l i g h t s
g r a p h i c a l
Adsorption of wetting agents changes the surface energy parameters of low-energy surface (LES). Hydrophobic attraction between LES and wetting agents and electrostatic force are not the major reasons of adsorption. An adsorption model is proposed. Wetting ability, adsorption intensity and desorption of wetting agent are investigated.
Adsorption of wetting agents on low-energy surface is achieved through hydration shell taken as mesophase.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 18 September 2012 Received in revised form 28 December 2012 Accepted 22 January 2013 Available online 13 February 2013 Keywords: Wetting agent Low-energy surface Wetting behavior Adsorption model Interfacial interaction Hydrogen bond
a b s t r a c t
Wetting behavior and wetting mechanism of wetting agents on low-energy surface (LES) are well investigated in this paper. Four wetting agents, namely Lignin sulfonate, Tannic acid, Methylcellulose and Triton X-100, were involved in our study. Wetting behavior was well examined through discussion of surface energy of LES, interfacial interaction and zeta potential of polymer resins. Experimental results demonstrate that adsorption of wetting agents changes the surface energy parameters of LES; interfacial free energy between wetting agent molecule and LES exhibits hydrophobic attraction between them fails to be the dominant driving force of adsorption; the effect of wetting agent on zeta potential of polymer illustrates electrostatic force is not the major factor of interfacial interaction between wetting agent molecule and LES. Based on wetting action of wetting agents, we discussed wetting mechanism of wetting agents on LES in detail, and an adsorption model was proposed. Adsorption of wetting agent on LES is achieved through hydration shell taken as mesophase, namely, hydrophobic attraction between LES and water molecule results in the formation of hydration shell, and wetting agents molecules approach to hydration shell under the effect of Lifshitz–van der Waals attraction and they adsorb on hydration shell through hydrogen bond. In terms of post-consumer plastics composed of polymer and additives, apart from hydrophobic attraction and hydrogen bond, wetting action depends also on chelating ability between wetting agent molecule and additive ions due to the chemical adsorption between wetting agent and additives. In addition, wetting ability, adsorption and selective desorption of wetting agents are well examined. © 2013 Elsevier B.V. All rights reserved.
∗ Corresponding author. Tel.: +86 731 88879616. E-mail address: [email protected] (H. Wang). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.01.063
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
1. Introduction Low-energy surface (LES) has received significant attention because of its technological importance [1–4]. Wetting behavior of LES attracts significant attention owing to its widely technological application such as plastics flotation, coating, gluing, agriculture, lithography [5–10]. In terms of plastics flotation, which has been a subject of a considerable number of researches in recent years [5,6,11], wetting behavior of flotation reagents on LES plays a central role in this field. Adsorption behavior of weak hydrophilic substances on LES in aqueous medium was reported, and some interesting conclusions were achieved, which provides insights into theoretical understanding of wetting mechanism [12]. Some strong hydrophilic substances such as Calcium lignosulphonate, Tannic acid, Methylcellulose and Triton X-100, have been used as wetting agents in flotation [12–15]. Wetting action can be achieved by adsorption of wetting agents, but wetting mechanism of wetting agents still is a subject of considerable debate [11,13,16]. Fraunholcz et al. [14,16] suggested that adsorption of wetting agent on LES of polymer stems mainly from physical adsorption, which includes hydrophobic interaction and electrostatic interaction, and intrinsic surface properties of polymer significantly affect wetting action of wetting agent. Stückrad et al. [11] proposed the model of selective adsorption and described that selective adsorption depending on molecular structure and composition. The aim of this study is to investigate wetting behavior and wetting mechanism of wetting agents on LES. Interesting conclusions on wetting behavior were achieved through discussion of surface energy and interfacial free energy. In terms of wetting mechanism of wetting agents on LES, we focus on how wetting agents adsorb on LES in this paper.
2. Materials and methods 2.1. Materials Four wetting agents are involved in this paper, namely Lignin sulfonate (LS), Tannic acid (TA), Methylcellulose (MC) and Triton X-100 (TX-100). TA, MC and TX-100 are analytical pure, LS is industrial product, and they all were used as received. The test liquids for contact-angle measurements include distilled water, glycerol, formamide, diiodomethane and ethylene glycol (all are analytical purity). The used polymer resins are polrvinyl chloride (PVC) and polystyrene (PS) and they were purchased from China Petroleum & Chemical Corporation. Post-consumer plastics were purchased from a waste plastics market (Miluo, Hunan Province, China).
2.2. Methods 2.2.1. Measurements of contact angle and zeta potential The contact angle of liquids on the solid surface was measured by a JJC-I contact angle measuring instrument (Changchun Optical Instrument Factory, China). During the process of measurement, at the distance of 3 mm above solid, the liquid was vertically, carefully dropped onto a solid surface to form a sessile drop by microinjector (2.0 mL). Make sure the droplet size was 3.5 L, diameter was 1–2 mm, and measuring time was less than 1 min. Each average value was calculated from ten measurements. All measurements were conducted at room temperature, generally 25 ◦ C. Determination of zeta potential was performed by Zeta Potential Analyzer (Brookhaven Instruments Corporation, America). KCl solution (1.0 × 10−3 mol/L) was used as the electrolytic solution.
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2.2.2. Calculation of surface energy and interfacial free energy The surface free energy of LES and wetting agents was calculated using the Lifshitz–van der Waals/acid–base approach [17,18]. Calculation of surface energy and interfacial free energy are demonstrated in our previous paper in detail [12,19]. 3. Results and discussion 3.1. Wetting behavior of wetting agent on LES 3.1.1. Surface energy of LES adsorbed with wetting agents TA (500 mg/L) and LS (800 mg/L) were used as wetting agents, and their static adsorption on PS and PVC resins were carried out for 10 min. Contact angles of several liquids on the processed materials were determined. Based on the data of contact angle, the surface energy parameters of the processed resins were calculated with respect to three groups of solids, i.e., distilled water–glycerol–diiodomethane (WGD), distilled water–formamide–diiodomethane (WFD), distilled water–ethanediol–diiodomethane (WED), as shown in Table 1. The results demonstrated in Table 1 show that the adsorption of wetting agents changes the surface energy parameters, significantly for Lewis base components. After the static adsorption of wetting agents on PS and PVC resins, the surface energy parameters of resins are close to that of wetting agents, which implies that adsorption of wetting agents on LES occurs in the nearly “cover” form. 3.1.2. The interfacial free energy between wetting agents and LES Based on the surface energy parameters of polymer resins and post-consumer plastics described in our previous paper [12], interfacial free energy between wetting agent and the involved materials was calculated, and the results are shown in Table 2. These conclusions can be achieved from Table 2: (1) In terms of macromolecule wetting agent whose nonpolar part is surrounded by polar one in the molecular structure, namely TA, LS, MC, there exists certain hydrophobic attraction between wetting agent and nonpolar or weakly polar polymer (such as PP, PE, PP-bar and PP-bot) whose surface energy is considerable weak. Furthermore, hydrophobic attraction is no longer dominant factor of interfacial interactions between wetting agents and surface of other polymers. (2) With respect to TX-100, which is surfactant-type wetting agent and whose nonpolar part and polar part are self-existent in the molecular structure, there exists certain hydrophobic attraction between TX-100 and LES. Similar to weak hydrophilic substances [12], the Lifshitz–van der Waals force between wetting agent and polymer is attractive. Although the Lifshitz–van der Waals attractive is very weak, it provides probability for wetting agents approaching to surface of polymer. In conclusion, Lifshitz–van der Waals attraction offers the probability for wetting agent molecule approaching to LES during the process of adsorption. If hydrophobic attraction, resulted from Table 1 The surface energy parameters of PS and PVC resins (mJ m−2 ). Polymer resins
S
SLW
S+
S−
PS (adsorbed by TA) PS (adsorbed by LS) PVC (adsorbed by TA) PVC (adsorbed by LS) PS PVC TA LS
44.57 42.03 48.17 52.35 37.03 48.33 49.88 41.38
38.00 36.84 41.45 45.81 36.58 43.41 39.24 34.24
0.16 0.12 0.18 0.21 0.010 1.73 0.47 0.26
67.43 56.14 62.77 50.96 5.07 3.50 60.26 48.96
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Table 2 The interfacial free energy between wetting agents and polymer resins or post-consumer plastics in aqueous medium (mJ m−2 ).a Free energy
GSWW
LW GSWW
Wetting agent
LS
TA
MC
TX-100
PP PE PS ABS PC PET PVC PE-bot PP-bar PVC-sho PVC-hos PVC-pac PVC-pip PVC-dec PS-lam PS-fac PET-dri ABS-tv ABS-air PC-cup PC-cd
−28.56 −28.87 −9.39 −11.81 −11.85 −15.34 −18.86 −28.17 −25.36 −9.50 −9.88 −8.54 −3.90 11.40 −1.12 −10.99 −10.05 −6.48 3.34 −16.82 0.20
−19.88 −20.30 −1.97 −5.00 −5.14 −9.54 −13.60 −19.77 −16.89 −2.91 −3.62 −2.94 1.33 16.36 4.08 −4.29 −2.65 −1.19 8.47 −11.06 5.34
−40.55 −40.91 −20.39 −22.78 −22.81 −25.47 −27.97 −40.09 −37.09 −19.46 −19.68 −18.14 −13.31 2.17 −10.50 −21.93 −21.03 −15.96 −5.99 −26.94 −9.16
−50.10 −50.30 −30.22 −31.57 −31.48 −32.51 −33.83 −49.32 −46.67 −28.09 −27.83 −25.57 −20.74 −6.48 −18.17 −30.66 −30.75 −23.19 −14.03 −33.78 −16.89
LS −2.38 −2.69 −3.26 −4.49 −4.65 −5.14 −4.54 −2.81 −2.37 −3.21 −3.61 −4.63 −5.08 −5.28 −5.10 −4.65 −3.29 −5.06 −5.12 −5.17 −5.15
AB GSWW
TA
MC
−3.20 −3.63 −4.40 −6.05 −6.27 −6.93 −6.12 −3.80 −3.19 −4.33 −4.87 −6.25 −6.85 −7.13 −6.87 −6.27 −4.43 −6.82 −6.90 −6.98 −6.95
−2.79 −3.15 −3.82 −5.26 −5.45 −6.03 −5.32 −3.30 −2.78 −3.77 −4.23 −5.43 −5.95 −6.19 −5.97 −5.45 −3.85 −5.93 −6.00 −6.07 −6.04
TX-100 −1.55 −1.75 −2.13 −2.92 −3.03 −3.35 −2.96 −1.83 −1.54 −2.09 −2.35 −3.02 −3.31 −3.44 −3.32 −3.03 −2.14 −3.30 −3.34 −3.37 −3.36
LS
TA
MC
TX-100
−26.18 −26.18 -6.13 −7.32 −7.20 −10.20 −14.32 −25.36 −22.99 −6.29 −6.27 −3.91 1.18 16.68 3.98 −6.34 −6.76 −1.42 8.44 −11.65 5.35
−16.68 −16.68 2.43 1.05 1.13 −2.61 −7.48 −15.97 −13.70 1.42 1.25 3.31 8.18 23.49 10.95 1.98 1.78 5.63 15.37 −4.08 12.29
−37.76 −37.76 −16.57 −17.52 −17.36 −19.44 −22.65 −36.79 −34.31 −15.69 −15.45 −12.71 −7.36 8.36 −4.53 −16.48 −17.18 −10.03 0.010 −20.87 −3.12
−48.55 −48.55 −28.09 −28.65 −28.45 −29.16 −30.87 −47.49 −45.13 −26.00 −25.48 −22.55 −17.43 −3.04 −14.85 −27.63 −28.61 −19.89 −10.69 −30.41 −13.53
AB AB a GSWW is the Lifshitz–van der Waals interfacial free energy between polymer resins (plastics) and wetting agents, GSWW is the Lewis acid–base interfacial free energy LW AB and GSWW . between polymer resins (plastics) and wetting agents, and GSWW is the sum of GSWW
Lewis acid–base interaction, leads to the formation of the adsorbed layer, wetting agents will have priority to adsorb on the LES that possesses larger hydrophobic attraction over the LES on which exists hydration repulsion. Obviously, it does not agree with flotation phenomenon and the results demonstrated in Table 2, and thus hydrophobic attractive force between wetting agents and surface of polymers is not the major reason that wetting agents adsorbed on LES. 3.1.3. The effect of wetting agents on zeta potential of polymer resin Fig. 1 presents the effect of wetting agents on zeta potential of PVC resin at different pH values, and Fig. 2 shows the zeta potential of PVC resin as a function of TA concentration at pH 7. As shown in Fig. 1, wetting agent does not change isoelectric point of PVC resin, and the positive value of zeta potential decreases when pH is above isoelectric point; the negative value of zeta potential reduces when pH is below isoelectric point. Thus, wetting agents does not neutralize the charges on the surface of PVC resin but shield it. Fig. 2 further reveals that adsorption of wetting agents
Fig. 1. The zeta potential of processed PVC resin as a function of pH.
on the surface of polymer resins have little influence on double electrode layer. The reason that the negative value of zeta potential decreases with increasing the concentration of wetting agent stems from the move of sliding surface of double electric layer, i.e., the sliding surface moves towards liquids phase, which leads to thicker stern layer and decrease of zeta potential. Combined with the effect of pH on adsorption of wetting agents on LES, it can be concluded that electrostatic force is also not the primary factor of interfacial interactions between wetting agents and LES. 3.2. Wetting mechanism of wetting agent on LES As shown above, hydrophobic attraction, electrostatic force and van der Waals force are not the dominate factors of the interfacial interactions between wetting agents and LES. Thus hydrogen bond, which possesses saturability and directivity, may be responsible for the interfacial interactions. This understanding is spurred from two aspects. On one hand, wetting agent molecule approaches to LES owing to Lifshitz–van der Waals attraction, and its numerous hydroxyls offer enough hydrogen bonds to meet the saturability. On the other hand, due to the rotation of macromolecular chain
Fig. 2. The zeta potential of PVC resin as a function of the concentration of TA.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
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Fig. 3. Comparison of Lewis acid–base interfacial free energy between polymer resins (a) or post-consumer plastics (b) and other phases in aqueous medium (The involved AB AB AB AB AB , GSW , GSWF , GSWB where GSWW is the Lewis acid–base interfacial free energy between polymer resins (plastics) and wetting interfacial free energy includes, GSWW AB AB is the Lewis acid–base interfacial free energy between polymer resins (plastics) and water, GSWF is the Lewis acid–base interfacial free energy between agents, GSW AB is the Lewis acid–base interfacial free energy between polymer resins (plastics) and bubbles). polymer resins (plastics) and frothers, and GSWB
of polymer, hydrogen donor, hydrogen atom and hydrogen acceptor can be collinear, and thus directivity of hydrogen bond can be achieved. However, simple adsorption model of hydrogen bond circumvents some problems to interpret the flotation phenomena. First, wetting agents fail to exhibit strong wetting action on the polymer resins whose hydrogen acceptors possess lone pair electrons, large electronegativity and small radius. Second, the small molecule compounds (such as tartaric acid and citric acid) do not affect the flotability of polymer resins. Therefore, wetting agent molecules do not interact indirectly with LES through hydrogen bond. 3.2.1. Interfacial free energy Based on associated data in previous literature [12], the Lewis acid–base interfacial free energy between polymer resins or postconsumer plastics and other phases in aqueous medium were calculated, as shown in Fig. 3. It is illustrated that hydration repulsion can arise even between some wetting agent molecules and LES. Moreover, hydrophobic attraction between polymer materials and bubbles or frothers is much stronger than that between polymer materials and wetting agents. In the absence of frothers and bubbles, in terms of TA, LS and MC, apart from such nonpolar or weakly polar polymer materials as PP, PE, PP-bar and PE-bot, the attraction between wetting agent molecule and another polymer material is larger than that between wetting agent molecules, which indicates that the interfacial interaction between wetting agent molecule and LES is achieved by water taken as mesophase. With respect to TX-100, taking no account of the effect of water molecule on wetting agent, wetting agent molecule can just interact directly with nonpolar or weak polar polymer materials (such as PP, PE) by weak hydrophobic attraction, while interfacial interaction between wetting agent and other polymers can be achieved directly with water molecule as mesophase. Then we studied wetting agents in detail in terms of interfacial interaction. Based on experimental data in previous literature, Lewis acid–base free energy between wetting agent and other phases in aqueous medium is shown in Fig. 4. It can be concluded that the attraction between wetting agent and water molecule is significantly larger than the attraction between other phases
(namely, polymer materials, bubbles and frothers). The adsorption of wetting agent on LES is accomplished indirectly through water molecules. Meanwhile, taking account of the interaction between wetting agent and water molecule, interfacial interaction between wetting agent and nonpolar or weakly polar polymer materials such as PP and PE, can be achieved with water molecule as mesophase.
Fig. 4. Comparison of Lewis acid–base interfacial free energy between wetting agent and other phases in aqueous medium.
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H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
Fig. 5. Adsorption model with hydration shell of wetting agents on polymer resins.
Thus, it can be concluded that, neglecting the effect of plastic additives, the wetting agent molecule adsorbs indirectly on all the polymer materials through water molecules serving as mesophase.
3.2.2. Adsorption model of wetting agents on LES 3.2.2.1. Adsorption model of wetting agents on polymer resins. As described above, such interaction forces as hydrophobic attraction, electrostatic force and van der Waals force are not the dominate factors of the interfacial interactions between wetting agents and LES. In addition, adsorption of wetting agents on LES is achieved through water molecules as a mesophase. Therefore, we propose an adsorption model with hydration shell as a mesophase to interpret the wetting mechanism of wetting agent on LES. The attraction between water and LES leads to the formation of hydration shell, and wetting agent molecule moves to LES as a result of Lifshitz–van der Waals attraction. If wetting agents are macromolecular compounds, similar to bubble in size, the hydration shell would be compressed into compact one; while wetting agents are small molecular compounds, they will approach loose area of hydration shell. The water molecules serving as the mesophase between wetting agent and LES must be the water molecules in the hydration shell. Polymer molecules make the water molecules on both sides of hydration shell locate and orient through hydrophobic attraction, i.e., hydration shell serves as the mesophase between wetting agent and LES, and thus adsorption of wetting agents on polymer resins occurs in the nearly “cover” form. The adsorption model of wetting agents on surface of polymer resins is shown in Fig. 5. According to the adsorption model, wetting action of wetting agents on LES depends on the hydrophobic attraction between water molecule and LES as well as hydrogen bond between wetting molecule and water molecule. If there is no hydrophobic
attraction, and the hydration shell would form only resulted from the Lifshitz–van der Waals attraction whose value is considerable small, wetting action of wetting agent on LES will not occur. The reasons involve two aspects: hydration shell is too thin to hold the polar groups of wetting agent molecule and thus hydrogen bond between them can not form; as the hydrogen bond between wetting molecule and water is stronger than the bond between hydration shell and LES, hydration shell will be destroyed by wetting agent, and hence LES can not be covered by wetting agent. Because hydrogen bond does not form directly between wetting agent molecule and LES, wetting agent will not have priority over polymer resins that possess a great number of hydrogen acceptors, and, inversely, strong wetting action may occur on the polymer resins (such as PVC) that there exist no or just weak hydrogen acceptors in their molecular chain. Wetting agents, especially for plastics flotation, should be strong hydrophilic and multi-hydrogen-bond macromolecular compounds. Such strong hydrophilic and multi-hydrogen-bond small-molecule compounds as tartaric acid and citric acid, cannot compress the hydration shell when they interact with LES, and then will not form stable hydration shell essential for adsorption, and thus these compounds cannot affect the flotability of polymer resins during the process of flotation.
3.2.2.2. Adsorption model of wetting agents on post-consumer plastics. Unlike polymer resins, post-consumer plastics possess a quantity of additives, which have certain impact on wetting behavior of wetting agents. For example, the interfacial interaction between wetting agents and additives plays an important role in plastics flotation system.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
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of wetting agent molecules, but results in the chemical adsorption between wetting agent and additive that makes the polymer materials (such as PVC, ABS), which are added filler, stabilizer and fire retardant, easier to wet. 3.3. Wetting ability, adsorption intensity and desorption 3.3.1. Wetting ability Similar to the hydrophilic index [12,26], we +described −elsewhere + define here wetting index as Wet + Wet , where Wet and
− Wet are Lewis acid components and Lewis base components of the surface energy parameters of wetting agents, respectively. The wetting index of TA, LS, MC, and TX-100 is 8.45, 7.51, 6.36, and 5.29, respectively. The hydrogen bond between wetting agent molecule and water molecule in the hydration shell stems from the cooperative interaction of numerous hydrophilic groups in wetting agent molecule. Because bond energy of simple hydrogen bond is very weak, the wetting ability depends largely on the number of the hydrophilic groups that can form hydrogen bond with water molecules. Thus, the number of the hydrophilic groups stands for the wetting ability of wetting agent. In addition, if hydrogen bond is taken as one type of special interfacial interaction, Lewis acid–base free energy AB between wetting agent and water can be regarded as an GWW important basis of the number of the hydrophilic groups. As shown in Fig. 5, the involved four wetting agents can be ranked (from large AB to small) as follows: TA > LS > MC > TX-100, which is consisGWW tent with the sequence of the wetting index of wetting agents. Thus, wetting index is deemed to be a significant basis of wetting ability.
Fig. 6. Physical–chemical adsorption models of LS and TA on the polymer-additive surface.
The associated research indicates that the reason tannic acid (TA) can be disincrustant, clarificant, antidote, as well as depressant of calcium and magnesium minerals, is due to the complexation between TA and various metal ions, in other words, complexation reaction between TA, serving as a multidentate ligand, and metal ions occurs [20]. Hemingway et al. [21] suggested that two neighboring phenolic hydroxyls can form stable pentacyclic chelates with metal ions in the formation of negative oxygen ions, the third phenolic hydroxyl do not react but promote dissociation of other two phenolic hydroxyls and thus facilitate the formation and stability of complex. Lignosulphonate also may interact with various metal ions [22–25]. Therefore, on the polymeradditive surface, wetting agent molecules adsorb physically on polymer through hydrophobic attraction and hydrogen bond with water as mesophase. In addition, chemical adsorption between wetting agent molecule and additive will occur. Fig. 6 is the physical–chemical adsorption model of LS and TA on the polymeradditive surface, while calcium carbonate is the filler of polymer. The existence of additives not only affects the surface energy of polymer materials and thus facilitates the physical adsorption
3.3.2. Adsorption intensity On the surface of polymer materials, physical adsorption intensity of wetting agents depends on hydrogen bond and hydrophobic attraction, while their significant parameter is Lewis acid–base free AB between wetting agent molecule and water molecule energy GSW AB between polymer and water and Lewis acid–base free energy GSW AB and GAB can molecule, respectively. Therefore, the sum of GSW WW be seen as the indicator of physical adsorption intensity of wetting agents, and the larger its absolute value of is, the stronger the adsorption of wetting agent will be. Fig. 7 shows the physical adsorption intensity of wetting agents on LES in aqueous medium. It is demonstrated that more strongly physical adsorption of one wetting agent occurs on the LES with larger hydrophilic index. 3.3.3. Desorption Comparing with chemical adsorption, the above physical adsorption between wetting agent molecule and polymer is not stable and suffers from such factors as turbulence of medium and aeration rate. Thus, the desired results cannot be achieved in traditional flotation equipments with respect to plastics flotation. In terms of flotation column in which the turbulence of medium is considerably weak, adsorption of wetting agents is also affected by bubbles. Unlike turbulence of medium, bubbles affect selectively adsorption of wetting agents, that is, wetting agent molecule desorbs selectively from LES due to the effect of bubbles. In the flotation system, as shown in Fig. 3, the attraction between LES and bubbles is significantly stronger than the attraction between LES and other phases (namely, wetting agent, frother and water), and thus the adsorption of bubbles results in the desorption of wetting agent molecule from hydration shell in the presence of bubbles. The adsorption of LS on the surface of PS and PVE resins was taken as an example. Static adsorption of PS and PVC sheets (20 × 20 mm) in LS solution (800 mg/L) was initially carried out for 10 min, and then rinsed in 1000 mL distilled water. Two stirring modes, namely stirring with a glass rod
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Fig. 7. Physical adsorption intensity of wetting agents on LES in aqueous medium.
Table 3 The surface energy parameters of the processed PS and PVC resins (mJ m−2 ). Processed resins
S
SLW
S+
S−
Unprocessed PS PS (adsorbed by LS) PS (Stirring with glass rod) PS (air agitation) Unprocessed PVC PVC (adsorbed by LS) PVC (Stirring with glass rod) PVC (air agitation)
37.03 42.03 40.11 36.86 48.33 52.35 51.05 53.39
36.58 36.84 35.63 35.82 43.41 45.81 45.37 46.35
0.010 0.12 0.089 0.029 1.73 0.21 0.16 0.25
5.07 56.14 56.37 9.37 3.50 50.96 50.49 49.50
and air agitation, were used. The surface energy parameters of the processed resins were calculated with respect to three groups of solids, i.e., distilled water–glycerol–diiodomethane (WGD), distilled water–formamide–diiodomethane (WFD), distilled water–ethanediol–diiodomethane (WED), as shown in Table 3. As demonstrated in Table 3, the adsorption of LS changes the surface energy parameters of PS and PVC resins. In addition, the adsorption of LS under stirring with glass rod is irreversible, which implies that the hydrogen bond between wetting agent and water molecule in the hydration shell is different from the hydrogen bond between wetting agent and bulk-phase water molecules, and thus hydration shell plays a significant role in the location of wetting agents on LES. In the case of air agitation, the surface of PS resins with lower hydrophilic index is affected and LS desorbs partially from PS due to the effect of bubbles, while the wetting agent adsorbed on the PVC with relatively large hydrophilic index does not suffer from bubbles. Thus, in the flotation system, the wetting agent adsorbed on LES desorbs selectively in the presence of bubbles. Schematic diagram of desorption of LS from PS is shown in Fig. 8.
4. Conclusions Four wetting agents, namely LS, TA, MC and TX-100, were involved to investigate wetting behavior and wetting mechanism of wetting agents on LES. Wetting behavior was well examined through study of surface energy of LES, interfacial interaction and zeta potential of polymer resins. Experimental results demonstrate that adsorption of wetting agents changes the surface energy parameters; interfacial free energy between wetting agent molecule and LES exhibits hydrophobic attraction between them fails to be the dominant driving force of adsorption; the effect of wetting agent on zeta potential of polymer illustrates electrostatic force is not the major factor of interfacial interaction between wetting agent molecule and LES. Based on wetting behavior of wetting agents, wetting mechanism of wetting agents on LES is well discussed, and an adsorption model was proposed. Adsorption of wetting agent on LES is achieved through hydration shell taken as mesophase, which depends mainly on hydrophobic attraction and hydrogen bond. Hydration shell plays a significant role in the location of wetting agents on LES. In detail, hydrophobic attraction between LES and water molecule results in the formation of hydration shell, wetting agents molecules approach to hydration shell under the effect of Lifshitz–van der Waals attraction, and wetting agents adsorb on hydration shell through hydrogen bond. In terms of post-consumer plastics composed of polymer and additives, apart from hydrophobic attraction and hydrogen bond, wetting action depends also on chelating ability between wetting agent molecule and additive ions due to the chemical adsorption between wetting agent and additives. In addition, wetting ability, adsorption and selective desorption are well examined. Wetting index is considered to be a significant AB and GAB can be seen basis of wetting ability. The sum of GSW WW as the indicator of physical adsorption intensity of wetting agents. Furthermore, in the flotation system, the wetting agent molecule adsorbed on LES desorbs selectively in the presence of bubbles.
Acknowledgements
Fig. 8. Schematic diagram of desorption of LS from PS.
The authors would like to gratefully acknowledge the financial support from the Project (2010CB630903) supported by the National Basic Research Program of China, and the Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education (Central South University) for the laboratory facilities, and we would like to express our sincere appreciation to the anonymous reviewers for their insightful comments, which have greatly aided us in improving the quality of the paper.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
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[14] N. Fraunholcz, W. Dalmijn, Selective wetting of polymers by surfactant adsorption during froth flotation, J. Disper. Sci. Technol. 19 (6-7) (1998) 859–873. [15] J. Shibata, et al., Flotation separation of plastics using selective depressants, Int. J. Miner. Process. 48 (3) (1996) 127–134. [16] N. Fraunholcz, W. Dalmijn, Wetting mechanisms in the flotation of plastics, Proceedings of the XX International Mineral Processing Congress, 1997. [17] C. Van Oss, M.R. Chaudhury Good, Monopolar surfaces, Adv. Colloid Interface Sci. 28 (1987) 35–64. [18] C. Van Oss, R. Good, M. Chaudhury, Additive and nonadditive surface tension components and the interpretation of contact angles, Langmuir 4 (4) (1988) 884–891. [19] H. Wang, et al., Application of dissolved air flotation on separation of waste plastics ABS and PS, Waste Manage. (2012). [20] M. Zhi-Hong, S. Zhong-Bing, L. Bi, Chemical properties and application of tanic acid, Nat. Prod. Res. Dev. 15 (1) (2003) 87–91. [21] R. Hemingway, P. Laks, in: Plant Polyphenols, Plenum Press, New York, 1992. [22] C. Le Guern, R. Conil, P. Houot, Role of calcium ions in the mechanism of action of a lignosulphonate used to modify the wettability of plastics for their separation by flotation, Miner. Eng. 13 (1) (2000) 53–63. [23] S. Pongstabodee, N. Kunachitpimol, S. Damronglerd, Combination of threestage sink–float method and selective flotation technique for separation of mixed post-consumer plastic waste, Waste Manage. 28 (3) (2008) 475–483. [24] P. Basaˇrová, et al., The influence of flotation agent concentration on the wettability and flotability of polystyrene, J. Colloid Interface Sci. 286 (1) (2005) 333–338. [25] T. Takoungsakdakun, S. Pongstabodee, Separation of mixed post-consumer PET–POM–PVC plastic waste using selective flotation, Sep. Purif. Technol. 54 (2) (2007) 248–252. [26] W.G.G. Hui, Surface energy of solid and hydrophilic/hydrophobic nature, Chemical Online 12 (2009) 1091–1096 (in Chinese).
Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Wetting behavior and mechanism of wetting agents on low-energy surface Hui Wang a,∗ , Chongqing Wang a , Jiangang Fu a , Guohua Gu b a School of Chemistry and Chemical Engineering, Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education, Central South University, Changsha, 410083 Hunan, China b School of Minerals Processing and Bioengineering, Central South University, Changsha, 410083 Hunan, China
h i g h l i g h t s
g r a p h i c a l
Adsorption of wetting agents changes the surface energy parameters of low-energy surface (LES). Hydrophobic attraction between LES and wetting agents and electrostatic force are not the major reasons of adsorption. An adsorption model is proposed. Wetting ability, adsorption intensity and desorption of wetting agent are investigated.
Adsorption of wetting agents on low-energy surface is achieved through hydration shell taken as mesophase.
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 18 September 2012 Received in revised form 28 December 2012 Accepted 22 January 2013 Available online 13 February 2013 Keywords: Wetting agent Low-energy surface Wetting behavior Adsorption model Interfacial interaction Hydrogen bond
a b s t r a c t
Wetting behavior and wetting mechanism of wetting agents on low-energy surface (LES) are well investigated in this paper. Four wetting agents, namely Lignin sulfonate, Tannic acid, Methylcellulose and Triton X-100, were involved in our study. Wetting behavior was well examined through discussion of surface energy of LES, interfacial interaction and zeta potential of polymer resins. Experimental results demonstrate that adsorption of wetting agents changes the surface energy parameters of LES; interfacial free energy between wetting agent molecule and LES exhibits hydrophobic attraction between them fails to be the dominant driving force of adsorption; the effect of wetting agent on zeta potential of polymer illustrates electrostatic force is not the major factor of interfacial interaction between wetting agent molecule and LES. Based on wetting action of wetting agents, we discussed wetting mechanism of wetting agents on LES in detail, and an adsorption model was proposed. Adsorption of wetting agent on LES is achieved through hydration shell taken as mesophase, namely, hydrophobic attraction between LES and water molecule results in the formation of hydration shell, and wetting agents molecules approach to hydration shell under the effect of Lifshitz–van der Waals attraction and they adsorb on hydration shell through hydrogen bond. In terms of post-consumer plastics composed of polymer and additives, apart from hydrophobic attraction and hydrogen bond, wetting action depends also on chelating ability between wetting agent molecule and additive ions due to the chemical adsorption between wetting agent and additives. In addition, wetting ability, adsorption and selective desorption of wetting agents are well examined. © 2013 Elsevier B.V. All rights reserved.
∗ Corresponding author. Tel.: +86 731 88879616. E-mail address: [email protected] (H. Wang). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.01.063
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
1. Introduction Low-energy surface (LES) has received significant attention because of its technological importance [1–4]. Wetting behavior of LES attracts significant attention owing to its widely technological application such as plastics flotation, coating, gluing, agriculture, lithography [5–10]. In terms of plastics flotation, which has been a subject of a considerable number of researches in recent years [5,6,11], wetting behavior of flotation reagents on LES plays a central role in this field. Adsorption behavior of weak hydrophilic substances on LES in aqueous medium was reported, and some interesting conclusions were achieved, which provides insights into theoretical understanding of wetting mechanism [12]. Some strong hydrophilic substances such as Calcium lignosulphonate, Tannic acid, Methylcellulose and Triton X-100, have been used as wetting agents in flotation [12–15]. Wetting action can be achieved by adsorption of wetting agents, but wetting mechanism of wetting agents still is a subject of considerable debate [11,13,16]. Fraunholcz et al. [14,16] suggested that adsorption of wetting agent on LES of polymer stems mainly from physical adsorption, which includes hydrophobic interaction and electrostatic interaction, and intrinsic surface properties of polymer significantly affect wetting action of wetting agent. Stückrad et al. [11] proposed the model of selective adsorption and described that selective adsorption depending on molecular structure and composition. The aim of this study is to investigate wetting behavior and wetting mechanism of wetting agents on LES. Interesting conclusions on wetting behavior were achieved through discussion of surface energy and interfacial free energy. In terms of wetting mechanism of wetting agents on LES, we focus on how wetting agents adsorb on LES in this paper.
2. Materials and methods 2.1. Materials Four wetting agents are involved in this paper, namely Lignin sulfonate (LS), Tannic acid (TA), Methylcellulose (MC) and Triton X-100 (TX-100). TA, MC and TX-100 are analytical pure, LS is industrial product, and they all were used as received. The test liquids for contact-angle measurements include distilled water, glycerol, formamide, diiodomethane and ethylene glycol (all are analytical purity). The used polymer resins are polrvinyl chloride (PVC) and polystyrene (PS) and they were purchased from China Petroleum & Chemical Corporation. Post-consumer plastics were purchased from a waste plastics market (Miluo, Hunan Province, China).
2.2. Methods 2.2.1. Measurements of contact angle and zeta potential The contact angle of liquids on the solid surface was measured by a JJC-I contact angle measuring instrument (Changchun Optical Instrument Factory, China). During the process of measurement, at the distance of 3 mm above solid, the liquid was vertically, carefully dropped onto a solid surface to form a sessile drop by microinjector (2.0 mL). Make sure the droplet size was 3.5 L, diameter was 1–2 mm, and measuring time was less than 1 min. Each average value was calculated from ten measurements. All measurements were conducted at room temperature, generally 25 ◦ C. Determination of zeta potential was performed by Zeta Potential Analyzer (Brookhaven Instruments Corporation, America). KCl solution (1.0 × 10−3 mol/L) was used as the electrolytic solution.
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2.2.2. Calculation of surface energy and interfacial free energy The surface free energy of LES and wetting agents was calculated using the Lifshitz–van der Waals/acid–base approach [17,18]. Calculation of surface energy and interfacial free energy are demonstrated in our previous paper in detail [12,19]. 3. Results and discussion 3.1. Wetting behavior of wetting agent on LES 3.1.1. Surface energy of LES adsorbed with wetting agents TA (500 mg/L) and LS (800 mg/L) were used as wetting agents, and their static adsorption on PS and PVC resins were carried out for 10 min. Contact angles of several liquids on the processed materials were determined. Based on the data of contact angle, the surface energy parameters of the processed resins were calculated with respect to three groups of solids, i.e., distilled water–glycerol–diiodomethane (WGD), distilled water–formamide–diiodomethane (WFD), distilled water–ethanediol–diiodomethane (WED), as shown in Table 1. The results demonstrated in Table 1 show that the adsorption of wetting agents changes the surface energy parameters, significantly for Lewis base components. After the static adsorption of wetting agents on PS and PVC resins, the surface energy parameters of resins are close to that of wetting agents, which implies that adsorption of wetting agents on LES occurs in the nearly “cover” form. 3.1.2. The interfacial free energy between wetting agents and LES Based on the surface energy parameters of polymer resins and post-consumer plastics described in our previous paper [12], interfacial free energy between wetting agent and the involved materials was calculated, and the results are shown in Table 2. These conclusions can be achieved from Table 2: (1) In terms of macromolecule wetting agent whose nonpolar part is surrounded by polar one in the molecular structure, namely TA, LS, MC, there exists certain hydrophobic attraction between wetting agent and nonpolar or weakly polar polymer (such as PP, PE, PP-bar and PP-bot) whose surface energy is considerable weak. Furthermore, hydrophobic attraction is no longer dominant factor of interfacial interactions between wetting agents and surface of other polymers. (2) With respect to TX-100, which is surfactant-type wetting agent and whose nonpolar part and polar part are self-existent in the molecular structure, there exists certain hydrophobic attraction between TX-100 and LES. Similar to weak hydrophilic substances [12], the Lifshitz–van der Waals force between wetting agent and polymer is attractive. Although the Lifshitz–van der Waals attractive is very weak, it provides probability for wetting agents approaching to surface of polymer. In conclusion, Lifshitz–van der Waals attraction offers the probability for wetting agent molecule approaching to LES during the process of adsorption. If hydrophobic attraction, resulted from Table 1 The surface energy parameters of PS and PVC resins (mJ m−2 ). Polymer resins
S
SLW
S+
S−
PS (adsorbed by TA) PS (adsorbed by LS) PVC (adsorbed by TA) PVC (adsorbed by LS) PS PVC TA LS
44.57 42.03 48.17 52.35 37.03 48.33 49.88 41.38
38.00 36.84 41.45 45.81 36.58 43.41 39.24 34.24
0.16 0.12 0.18 0.21 0.010 1.73 0.47 0.26
67.43 56.14 62.77 50.96 5.07 3.50 60.26 48.96
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H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
Table 2 The interfacial free energy between wetting agents and polymer resins or post-consumer plastics in aqueous medium (mJ m−2 ).a Free energy
GSWW
LW GSWW
Wetting agent
LS
TA
MC
TX-100
PP PE PS ABS PC PET PVC PE-bot PP-bar PVC-sho PVC-hos PVC-pac PVC-pip PVC-dec PS-lam PS-fac PET-dri ABS-tv ABS-air PC-cup PC-cd
−28.56 −28.87 −9.39 −11.81 −11.85 −15.34 −18.86 −28.17 −25.36 −9.50 −9.88 −8.54 −3.90 11.40 −1.12 −10.99 −10.05 −6.48 3.34 −16.82 0.20
−19.88 −20.30 −1.97 −5.00 −5.14 −9.54 −13.60 −19.77 −16.89 −2.91 −3.62 −2.94 1.33 16.36 4.08 −4.29 −2.65 −1.19 8.47 −11.06 5.34
−40.55 −40.91 −20.39 −22.78 −22.81 −25.47 −27.97 −40.09 −37.09 −19.46 −19.68 −18.14 −13.31 2.17 −10.50 −21.93 −21.03 −15.96 −5.99 −26.94 −9.16
−50.10 −50.30 −30.22 −31.57 −31.48 −32.51 −33.83 −49.32 −46.67 −28.09 −27.83 −25.57 −20.74 −6.48 −18.17 −30.66 −30.75 −23.19 −14.03 −33.78 −16.89
LS −2.38 −2.69 −3.26 −4.49 −4.65 −5.14 −4.54 −2.81 −2.37 −3.21 −3.61 −4.63 −5.08 −5.28 −5.10 −4.65 −3.29 −5.06 −5.12 −5.17 −5.15
AB GSWW
TA
MC
−3.20 −3.63 −4.40 −6.05 −6.27 −6.93 −6.12 −3.80 −3.19 −4.33 −4.87 −6.25 −6.85 −7.13 −6.87 −6.27 −4.43 −6.82 −6.90 −6.98 −6.95
−2.79 −3.15 −3.82 −5.26 −5.45 −6.03 −5.32 −3.30 −2.78 −3.77 −4.23 −5.43 −5.95 −6.19 −5.97 −5.45 −3.85 −5.93 −6.00 −6.07 −6.04
TX-100 −1.55 −1.75 −2.13 −2.92 −3.03 −3.35 −2.96 −1.83 −1.54 −2.09 −2.35 −3.02 −3.31 −3.44 −3.32 −3.03 −2.14 −3.30 −3.34 −3.37 −3.36
LS
TA
MC
TX-100
−26.18 −26.18 -6.13 −7.32 −7.20 −10.20 −14.32 −25.36 −22.99 −6.29 −6.27 −3.91 1.18 16.68 3.98 −6.34 −6.76 −1.42 8.44 −11.65 5.35
−16.68 −16.68 2.43 1.05 1.13 −2.61 −7.48 −15.97 −13.70 1.42 1.25 3.31 8.18 23.49 10.95 1.98 1.78 5.63 15.37 −4.08 12.29
−37.76 −37.76 −16.57 −17.52 −17.36 −19.44 −22.65 −36.79 −34.31 −15.69 −15.45 −12.71 −7.36 8.36 −4.53 −16.48 −17.18 −10.03 0.010 −20.87 −3.12
−48.55 −48.55 −28.09 −28.65 −28.45 −29.16 −30.87 −47.49 −45.13 −26.00 −25.48 −22.55 −17.43 −3.04 −14.85 −27.63 −28.61 −19.89 −10.69 −30.41 −13.53
AB AB a GSWW is the Lifshitz–van der Waals interfacial free energy between polymer resins (plastics) and wetting agents, GSWW is the Lewis acid–base interfacial free energy LW AB and GSWW . between polymer resins (plastics) and wetting agents, and GSWW is the sum of GSWW
Lewis acid–base interaction, leads to the formation of the adsorbed layer, wetting agents will have priority to adsorb on the LES that possesses larger hydrophobic attraction over the LES on which exists hydration repulsion. Obviously, it does not agree with flotation phenomenon and the results demonstrated in Table 2, and thus hydrophobic attractive force between wetting agents and surface of polymers is not the major reason that wetting agents adsorbed on LES. 3.1.3. The effect of wetting agents on zeta potential of polymer resin Fig. 1 presents the effect of wetting agents on zeta potential of PVC resin at different pH values, and Fig. 2 shows the zeta potential of PVC resin as a function of TA concentration at pH 7. As shown in Fig. 1, wetting agent does not change isoelectric point of PVC resin, and the positive value of zeta potential decreases when pH is above isoelectric point; the negative value of zeta potential reduces when pH is below isoelectric point. Thus, wetting agents does not neutralize the charges on the surface of PVC resin but shield it. Fig. 2 further reveals that adsorption of wetting agents
Fig. 1. The zeta potential of processed PVC resin as a function of pH.
on the surface of polymer resins have little influence on double electrode layer. The reason that the negative value of zeta potential decreases with increasing the concentration of wetting agent stems from the move of sliding surface of double electric layer, i.e., the sliding surface moves towards liquids phase, which leads to thicker stern layer and decrease of zeta potential. Combined with the effect of pH on adsorption of wetting agents on LES, it can be concluded that electrostatic force is also not the primary factor of interfacial interactions between wetting agents and LES. 3.2. Wetting mechanism of wetting agent on LES As shown above, hydrophobic attraction, electrostatic force and van der Waals force are not the dominate factors of the interfacial interactions between wetting agents and LES. Thus hydrogen bond, which possesses saturability and directivity, may be responsible for the interfacial interactions. This understanding is spurred from two aspects. On one hand, wetting agent molecule approaches to LES owing to Lifshitz–van der Waals attraction, and its numerous hydroxyls offer enough hydrogen bonds to meet the saturability. On the other hand, due to the rotation of macromolecular chain
Fig. 2. The zeta potential of PVC resin as a function of the concentration of TA.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
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Fig. 3. Comparison of Lewis acid–base interfacial free energy between polymer resins (a) or post-consumer plastics (b) and other phases in aqueous medium (The involved AB AB AB AB AB , GSW , GSWF , GSWB where GSWW is the Lewis acid–base interfacial free energy between polymer resins (plastics) and wetting interfacial free energy includes, GSWW AB AB is the Lewis acid–base interfacial free energy between polymer resins (plastics) and water, GSWF is the Lewis acid–base interfacial free energy between agents, GSW AB is the Lewis acid–base interfacial free energy between polymer resins (plastics) and bubbles). polymer resins (plastics) and frothers, and GSWB
of polymer, hydrogen donor, hydrogen atom and hydrogen acceptor can be collinear, and thus directivity of hydrogen bond can be achieved. However, simple adsorption model of hydrogen bond circumvents some problems to interpret the flotation phenomena. First, wetting agents fail to exhibit strong wetting action on the polymer resins whose hydrogen acceptors possess lone pair electrons, large electronegativity and small radius. Second, the small molecule compounds (such as tartaric acid and citric acid) do not affect the flotability of polymer resins. Therefore, wetting agent molecules do not interact indirectly with LES through hydrogen bond. 3.2.1. Interfacial free energy Based on associated data in previous literature [12], the Lewis acid–base interfacial free energy between polymer resins or postconsumer plastics and other phases in aqueous medium were calculated, as shown in Fig. 3. It is illustrated that hydration repulsion can arise even between some wetting agent molecules and LES. Moreover, hydrophobic attraction between polymer materials and bubbles or frothers is much stronger than that between polymer materials and wetting agents. In the absence of frothers and bubbles, in terms of TA, LS and MC, apart from such nonpolar or weakly polar polymer materials as PP, PE, PP-bar and PE-bot, the attraction between wetting agent molecule and another polymer material is larger than that between wetting agent molecules, which indicates that the interfacial interaction between wetting agent molecule and LES is achieved by water taken as mesophase. With respect to TX-100, taking no account of the effect of water molecule on wetting agent, wetting agent molecule can just interact directly with nonpolar or weak polar polymer materials (such as PP, PE) by weak hydrophobic attraction, while interfacial interaction between wetting agent and other polymers can be achieved directly with water molecule as mesophase. Then we studied wetting agents in detail in terms of interfacial interaction. Based on experimental data in previous literature, Lewis acid–base free energy between wetting agent and other phases in aqueous medium is shown in Fig. 4. It can be concluded that the attraction between wetting agent and water molecule is significantly larger than the attraction between other phases
(namely, polymer materials, bubbles and frothers). The adsorption of wetting agent on LES is accomplished indirectly through water molecules. Meanwhile, taking account of the interaction between wetting agent and water molecule, interfacial interaction between wetting agent and nonpolar or weakly polar polymer materials such as PP and PE, can be achieved with water molecule as mesophase.
Fig. 4. Comparison of Lewis acid–base interfacial free energy between wetting agent and other phases in aqueous medium.
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H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
Fig. 5. Adsorption model with hydration shell of wetting agents on polymer resins.
Thus, it can be concluded that, neglecting the effect of plastic additives, the wetting agent molecule adsorbs indirectly on all the polymer materials through water molecules serving as mesophase.
3.2.2. Adsorption model of wetting agents on LES 3.2.2.1. Adsorption model of wetting agents on polymer resins. As described above, such interaction forces as hydrophobic attraction, electrostatic force and van der Waals force are not the dominate factors of the interfacial interactions between wetting agents and LES. In addition, adsorption of wetting agents on LES is achieved through water molecules as a mesophase. Therefore, we propose an adsorption model with hydration shell as a mesophase to interpret the wetting mechanism of wetting agent on LES. The attraction between water and LES leads to the formation of hydration shell, and wetting agent molecule moves to LES as a result of Lifshitz–van der Waals attraction. If wetting agents are macromolecular compounds, similar to bubble in size, the hydration shell would be compressed into compact one; while wetting agents are small molecular compounds, they will approach loose area of hydration shell. The water molecules serving as the mesophase between wetting agent and LES must be the water molecules in the hydration shell. Polymer molecules make the water molecules on both sides of hydration shell locate and orient through hydrophobic attraction, i.e., hydration shell serves as the mesophase between wetting agent and LES, and thus adsorption of wetting agents on polymer resins occurs in the nearly “cover” form. The adsorption model of wetting agents on surface of polymer resins is shown in Fig. 5. According to the adsorption model, wetting action of wetting agents on LES depends on the hydrophobic attraction between water molecule and LES as well as hydrogen bond between wetting molecule and water molecule. If there is no hydrophobic
attraction, and the hydration shell would form only resulted from the Lifshitz–van der Waals attraction whose value is considerable small, wetting action of wetting agent on LES will not occur. The reasons involve two aspects: hydration shell is too thin to hold the polar groups of wetting agent molecule and thus hydrogen bond between them can not form; as the hydrogen bond between wetting molecule and water is stronger than the bond between hydration shell and LES, hydration shell will be destroyed by wetting agent, and hence LES can not be covered by wetting agent. Because hydrogen bond does not form directly between wetting agent molecule and LES, wetting agent will not have priority over polymer resins that possess a great number of hydrogen acceptors, and, inversely, strong wetting action may occur on the polymer resins (such as PVC) that there exist no or just weak hydrogen acceptors in their molecular chain. Wetting agents, especially for plastics flotation, should be strong hydrophilic and multi-hydrogen-bond macromolecular compounds. Such strong hydrophilic and multi-hydrogen-bond small-molecule compounds as tartaric acid and citric acid, cannot compress the hydration shell when they interact with LES, and then will not form stable hydration shell essential for adsorption, and thus these compounds cannot affect the flotability of polymer resins during the process of flotation.
3.2.2.2. Adsorption model of wetting agents on post-consumer plastics. Unlike polymer resins, post-consumer plastics possess a quantity of additives, which have certain impact on wetting behavior of wetting agents. For example, the interfacial interaction between wetting agents and additives plays an important role in plastics flotation system.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
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of wetting agent molecules, but results in the chemical adsorption between wetting agent and additive that makes the polymer materials (such as PVC, ABS), which are added filler, stabilizer and fire retardant, easier to wet. 3.3. Wetting ability, adsorption intensity and desorption 3.3.1. Wetting ability Similar to the hydrophilic index [12,26], we +described −elsewhere + define here wetting index as Wet + Wet , where Wet and
− Wet are Lewis acid components and Lewis base components of the surface energy parameters of wetting agents, respectively. The wetting index of TA, LS, MC, and TX-100 is 8.45, 7.51, 6.36, and 5.29, respectively. The hydrogen bond between wetting agent molecule and water molecule in the hydration shell stems from the cooperative interaction of numerous hydrophilic groups in wetting agent molecule. Because bond energy of simple hydrogen bond is very weak, the wetting ability depends largely on the number of the hydrophilic groups that can form hydrogen bond with water molecules. Thus, the number of the hydrophilic groups stands for the wetting ability of wetting agent. In addition, if hydrogen bond is taken as one type of special interfacial interaction, Lewis acid–base free energy AB between wetting agent and water can be regarded as an GWW important basis of the number of the hydrophilic groups. As shown in Fig. 5, the involved four wetting agents can be ranked (from large AB to small) as follows: TA > LS > MC > TX-100, which is consisGWW tent with the sequence of the wetting index of wetting agents. Thus, wetting index is deemed to be a significant basis of wetting ability.
Fig. 6. Physical–chemical adsorption models of LS and TA on the polymer-additive surface.
The associated research indicates that the reason tannic acid (TA) can be disincrustant, clarificant, antidote, as well as depressant of calcium and magnesium minerals, is due to the complexation between TA and various metal ions, in other words, complexation reaction between TA, serving as a multidentate ligand, and metal ions occurs [20]. Hemingway et al. [21] suggested that two neighboring phenolic hydroxyls can form stable pentacyclic chelates with metal ions in the formation of negative oxygen ions, the third phenolic hydroxyl do not react but promote dissociation of other two phenolic hydroxyls and thus facilitate the formation and stability of complex. Lignosulphonate also may interact with various metal ions [22–25]. Therefore, on the polymeradditive surface, wetting agent molecules adsorb physically on polymer through hydrophobic attraction and hydrogen bond with water as mesophase. In addition, chemical adsorption between wetting agent molecule and additive will occur. Fig. 6 is the physical–chemical adsorption model of LS and TA on the polymeradditive surface, while calcium carbonate is the filler of polymer. The existence of additives not only affects the surface energy of polymer materials and thus facilitates the physical adsorption
3.3.2. Adsorption intensity On the surface of polymer materials, physical adsorption intensity of wetting agents depends on hydrogen bond and hydrophobic attraction, while their significant parameter is Lewis acid–base free AB between wetting agent molecule and water molecule energy GSW AB between polymer and water and Lewis acid–base free energy GSW AB and GAB can molecule, respectively. Therefore, the sum of GSW WW be seen as the indicator of physical adsorption intensity of wetting agents, and the larger its absolute value of is, the stronger the adsorption of wetting agent will be. Fig. 7 shows the physical adsorption intensity of wetting agents on LES in aqueous medium. It is demonstrated that more strongly physical adsorption of one wetting agent occurs on the LES with larger hydrophilic index. 3.3.3. Desorption Comparing with chemical adsorption, the above physical adsorption between wetting agent molecule and polymer is not stable and suffers from such factors as turbulence of medium and aeration rate. Thus, the desired results cannot be achieved in traditional flotation equipments with respect to plastics flotation. In terms of flotation column in which the turbulence of medium is considerably weak, adsorption of wetting agents is also affected by bubbles. Unlike turbulence of medium, bubbles affect selectively adsorption of wetting agents, that is, wetting agent molecule desorbs selectively from LES due to the effect of bubbles. In the flotation system, as shown in Fig. 3, the attraction between LES and bubbles is significantly stronger than the attraction between LES and other phases (namely, wetting agent, frother and water), and thus the adsorption of bubbles results in the desorption of wetting agent molecule from hydration shell in the presence of bubbles. The adsorption of LS on the surface of PS and PVE resins was taken as an example. Static adsorption of PS and PVC sheets (20 × 20 mm) in LS solution (800 mg/L) was initially carried out for 10 min, and then rinsed in 1000 mL distilled water. Two stirring modes, namely stirring with a glass rod
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H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
Fig. 7. Physical adsorption intensity of wetting agents on LES in aqueous medium.
Table 3 The surface energy parameters of the processed PS and PVC resins (mJ m−2 ). Processed resins
S
SLW
S+
S−
Unprocessed PS PS (adsorbed by LS) PS (Stirring with glass rod) PS (air agitation) Unprocessed PVC PVC (adsorbed by LS) PVC (Stirring with glass rod) PVC (air agitation)
37.03 42.03 40.11 36.86 48.33 52.35 51.05 53.39
36.58 36.84 35.63 35.82 43.41 45.81 45.37 46.35
0.010 0.12 0.089 0.029 1.73 0.21 0.16 0.25
5.07 56.14 56.37 9.37 3.50 50.96 50.49 49.50
and air agitation, were used. The surface energy parameters of the processed resins were calculated with respect to three groups of solids, i.e., distilled water–glycerol–diiodomethane (WGD), distilled water–formamide–diiodomethane (WFD), distilled water–ethanediol–diiodomethane (WED), as shown in Table 3. As demonstrated in Table 3, the adsorption of LS changes the surface energy parameters of PS and PVC resins. In addition, the adsorption of LS under stirring with glass rod is irreversible, which implies that the hydrogen bond between wetting agent and water molecule in the hydration shell is different from the hydrogen bond between wetting agent and bulk-phase water molecules, and thus hydration shell plays a significant role in the location of wetting agents on LES. In the case of air agitation, the surface of PS resins with lower hydrophilic index is affected and LS desorbs partially from PS due to the effect of bubbles, while the wetting agent adsorbed on the PVC with relatively large hydrophilic index does not suffer from bubbles. Thus, in the flotation system, the wetting agent adsorbed on LES desorbs selectively in the presence of bubbles. Schematic diagram of desorption of LS from PS is shown in Fig. 8.
4. Conclusions Four wetting agents, namely LS, TA, MC and TX-100, were involved to investigate wetting behavior and wetting mechanism of wetting agents on LES. Wetting behavior was well examined through study of surface energy of LES, interfacial interaction and zeta potential of polymer resins. Experimental results demonstrate that adsorption of wetting agents changes the surface energy parameters; interfacial free energy between wetting agent molecule and LES exhibits hydrophobic attraction between them fails to be the dominant driving force of adsorption; the effect of wetting agent on zeta potential of polymer illustrates electrostatic force is not the major factor of interfacial interaction between wetting agent molecule and LES. Based on wetting behavior of wetting agents, wetting mechanism of wetting agents on LES is well discussed, and an adsorption model was proposed. Adsorption of wetting agent on LES is achieved through hydration shell taken as mesophase, which depends mainly on hydrophobic attraction and hydrogen bond. Hydration shell plays a significant role in the location of wetting agents on LES. In detail, hydrophobic attraction between LES and water molecule results in the formation of hydration shell, wetting agents molecules approach to hydration shell under the effect of Lifshitz–van der Waals attraction, and wetting agents adsorb on hydration shell through hydrogen bond. In terms of post-consumer plastics composed of polymer and additives, apart from hydrophobic attraction and hydrogen bond, wetting action depends also on chelating ability between wetting agent molecule and additive ions due to the chemical adsorption between wetting agent and additives. In addition, wetting ability, adsorption and selective desorption are well examined. Wetting index is considered to be a significant AB and GAB can be seen basis of wetting ability. The sum of GSW WW as the indicator of physical adsorption intensity of wetting agents. Furthermore, in the flotation system, the wetting agent molecule adsorbed on LES desorbs selectively in the presence of bubbles.
Acknowledgements
Fig. 8. Schematic diagram of desorption of LS from PS.
The authors would like to gratefully acknowledge the financial support from the Project (2010CB630903) supported by the National Basic Research Program of China, and the Key Laboratory of Resources Chemistry of Nonferrous Metals, Ministry of Education (Central South University) for the laboratory facilities, and we would like to express our sincere appreciation to the anonymous reviewers for their insightful comments, which have greatly aided us in improving the quality of the paper.
H. Wang et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 424 (2013) 10–17
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