Hydrophobic aggregation of alumina in surfactant solution

Minerals Engineering 16 (2003) 1167–1172 This article is also available online at: www.elsevier.com/locate/mineng

Hydrophobic aggregation of alumina in surfactant solution Y. Hu *, J. Dai School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China Received 1 May 2003; accepted 19 July 2003

Abstract The aggregation behavior of ultrafine alumina in surfactant solution is examined by particle size analyzer. Electrokinetic and contact angle measurements are used to discuss the adsorption mechanism of sodium dodecyl sulfate (SDS) and dodecylamine (DDA) chloride at alumina. The PZC value of alumina is 9.1. SDS and DDA made alumina surface hydrophobic, respectively at pH < PZC and pH > PZC, and hydrophobic aggregation between alumina particles take place. The interfacial interaction energies between alumina particles in solution have been obtained based on polar interfacial interaction theory and contact angle measurements. The classical DLVO theory only considering the electrostatic and van der Waals interaction fails to explain the hydrophobic aggregation of ultrafine alumina, which can be well explained in the extended DLVO theory concerning polar interfacial interaction.
2003 Published by Elsevier Ltd. Keywords: Agglomeration; Fine particle processing; Froth flotation

1. Introduction Interfacial forces are believed to play an important role in many fundamental processes in biology (Cevc, 1993; Israelachvili and Wennerstrom, 1996); in mineral industry (Yalamanchili and Miller, 1992; Hu et al., 1994, 2001); in clay swelling (Delville, 1993); in enhancing oil recovery (Schulz and Puig, 1993). Interfacial forces usually include DLVO forces and non-DLVO forces which play more major role in interfacial processes, because the non-DLVO forces commonly are as much as 100 times greater than van der Waals’ forces, and 10 or more times greater than electrostatic forces (Israelachvili and McGuiggan, 1988; van Oss et al., 1990; Israelachvili, 1982, 1991). It has been reported that there are measurable long-range attractive hydrophobic forces between two hydrophobic surfaces such as methylated silica surfaces, natural coal surfaces and solid surfaces covered with surfactants (Xu and Yoon, 1990; Rutland et al., 1992; Leong et al., 1996). The hydrophobic forcelaw between such two macroscopic hydrophobic

*

Corresponding author. Tel.: +86-731-887-9815; fax: +86-731-8871804. E-mail addresses: [email protected], [email protected] (Y. Hu). 0892-6875/$ - see front matter
2003 Published by Elsevier Ltd. doi:10.1016/j.mineng.2003.07.018

surfaces is of surprisingly long range, decaying exponentially with single or double decay functions. The various SFA and AFM have been used for the measurements of hydrophobic force curves (Israelachvili, 1992; Tsao et al., 1993; Ducker et al., 1994). The origin and nature of these forces has long been controversial. Some investigators considered it to be related to the metastability of the water film between hydrophobic surfaces. The attraction would result from the free energy gain on removing water molecules from interlayer to bulk (Claesson and Christenson, 1988); while others believe that it is entropic in origin, arising mainly from the configurational rearrangement of water molecules in the vicinity of hydrophobic surfaces (Pashley et al., 1985; Xu and Yoon, 1989). Some author proposed the formation of cavitation in the vicinity of hydrophobic surfaces (Christenson and Claesson, 1988; Yaminsky et al., 1996; Miller et al., 1999). van Oss et al. (1988, 1990) and van Oss and Good (1989) reported a polar interfacial interaction theory that is largely based on electron acceptor–electron donor (Lewis acid–base) interactions between polar substances in polar media. They used the theory to explain the mechanism of phase separation of polymers in organic media and discuss the insolubility and solubility of polymers in organic liquids. They also investigated the stability of hectorite suspensions in sodium chloride solutions based on structural

Y. Hu, J. Dai / Minerals Engineering 16 (2003) 1167–1172

force. Skvarla and Kmet (1991) used this approach to discuss the aggregation and dispersion of fine magnesite in sodium oleate solution. In present work, the hydrophobic aggregation of ultrafine alumina in surfactant has been studied by using particle size analyzer, zeta potential and contact angle measurements based on polar interfacial interactions.

2. Experimental 2.1. Materials Alumina disc (d ¼ 10 mm) was purchased from Harric Scientific Co. Ultrafine alumina powder (<1 lm, 99.99% purity) was purchased from Alfa AESA. The chemicals used in the present study include reagent grade dodecylamine (DDA) hydrochloride from ACROS Organics, analytical grade sodium laurysulfate from Fluka Chemie, glycerol (EM Science), diiodomethane (Aldrich Chem. Co.), formamide (Mallinckrodt Inc.). HCl and NaOH (AR) were used for pH modification. A Milli-Q water system (Millipore) supplied with distilled water provided high purity water with a resistivity of +18 MX and a surface tension of 72 ± 0.2 at 23 C. 2.2. Contact angle measurements The sessile drop technique was used for contact angle measurements with a NRL goniometer (Rame-Hart, Inc.). The alumina disc treated in modifier solution and dried in vacuum was placed in a rectangular glass chamber and a liquid drop was introduced onto the substrate through a microsyringe. The needle was maintained in contact with the drop. Special care was taken in these measurements to avoid vibrations of the needle and to avoid distortion of drop shape by the needle. The advancing contact angles were measured for different liquid drops with 3–4 mm drop base diameter at room temperature.

3. Results and discussion 3.1. Aggregation behavior of ultrafine alumina The particle size distribution of ultrafine alumina suspension in the presence and absence of surfactant are presented in Figs. 1 and 2. It follows from Fig. 1 that at pH 3.5–3.7 the fraction of fine particles of alumina in the suspension is decreased and the particle size of fine alumina becomes coarser by addition of sodium dodecyl sulfate (SDS) into the suspension. The average particle size of alumina suspension is 1.3 lm in deionized water, and increased to 2.14 and 2.62 lm, respectively, in the presence of 10
4 and 10
3 mol l
1 SDS solution in the pH region of 3.5–3.7. Fig. 2 shows that the particle size of fine alumina is also enhanced by addition of DDA into ultrafine alumina suspension. The average particle size of alumina suspension is greater (1.93 lm) in the presence of 10
4

60

40

pH3.5~3.7 -3 -1 KCl:10 mol.l Average size 1-1.30 micron 2-2.14 micron 3-2.62 micron

3 30 20 10

1 2

0 0

2

4 Particle size (micron)

6

8

Fig. 1. Effect of SDS on the particle size distribution of ultrafine alumina suspension.

40 pH10.2~10.5 -3 -1 KCl: 10 mol.l Average size 1-1.44 micron 2-1.93 micron

35

2.3. Zeta potential and particle size measurements

30

Distribution (%)

The zeta potential and particle size of alumina suspensions were measured by standard procedures with a Malvern Zeta Sizer III using a quartz capillary cell with a 4-mm diameter. The ultrafine alumina was added into a 100-ml beaker containing an aqueous solution of known composition. The solid concentration was about 0.02%. The suspension were agitated for 10 min and transferred to the capillary cell with a syringe, and then the zeta potential measurements were made in the electrophoretic mode and the distribution of particle size was measured in the size analyzer mode.

Deionized water -4 -1 SDS 10 mol.l -3 -1 SDS 10 mol.l

50

Distribution(%)

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25 20

1

15 10

2

5 0 0

2

4 6 Particle size (micron)

8

10

Fig. 2. Effect of DDA chloride on the particle size distribution of ultrafine alumina suspension.

Y. Hu, J. Dai / Minerals Engineering 16 (2003) 1167–1172 50 40

Zeta potential (mV)

30 20

KCl: 10 -3 mol.l -1

10 0 -10 -20 -30

Deionized water SDS: 10 -4 mol.l -1 DDA: 10 -4 mol.l -1

-40 -50

0

2

4

6

8

10

12

14

pH

Fig. 3. Zeta potential of alumina as a function of pH.

mol l
1 DDA than that (1.4 lm) in absence of DDA at pH 10.2–10.4. These results indicated that the hydrophobic aggregation takes place between alumina particles in alumina suspensions by addition of 10
4 mol l
1 DDA and 10
4 – 10
3 mol l
1 SDS. 3.2. Electrokinetic behavior of alumina The zeta potential of alumina in the absence and presence of surfactant is given in Fig. 3. Alumina is positively charged in pure water until pH 9.1, at which charge reversal occurs. Addition of 10
4 mol l
1 SDS made the positive zeta potential of alumina become negative for pH values greater than pH 2. With an increase in the concentration of SDS, the zeta potential of alumina becomes more negative. DDA has almost no effect on the zeta potential of alumina at pH < 9. The zeta potential of alumina changes from negative to positive in the presence of 10
4 mol l
1 DDA at pH > 9 and reverses to negative again at pH > 10.5, above which the precipitation of neutral amine molecule is dominant (Hu and Wang, 1990). These results show that the electrostatic adsorption is the dominant mechanism for SDS and DDA adsorption on alumina. The higher negative zeta potential at pH 3.5 in SDS solution and high positive zeta potential at pH 10.2 in DDA solution for alumina will result in stronger electrostatic repulsive interactions at these two pH values, which cannot explain the aggregation behavior of alumina shown in Figs. 1 and 2. 3.3. Wettability and interfacial interaction energies According to van Oss’s surface thermodynamic approach, the surface energy of a solid as well as the solid–liquid interfacial energy is determined by two components. The first is the apolar (Lifshitz–van der Waals, LW) component cLW and the second is the polar

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(Lewis acid–base, AB) component cAB . van Oss et al. (1987a,b, 1988, 1990) and van Oss and Good (1989) considered the polar interactions as electron acceptor– electron donor interactions, or Lewis acid–base (AB) interactions, which were essentially asymmetrical and could only be satisfactory treated by taking that asymmetry into account. They designated the symbol c
i to express the parameter of the polar component (cAB i ) of the surface tension of compound i, c
i to the electron donor or proton acceptor, and cþ to express the electron i acceptor or proton donor parameter of the cAB and i derived an equation qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi þ LW
cL ð1 þ cos hÞ ¼ 2 c
cLW cþ ð1Þ S cL S cL þ S cL þ Thus, by contact angle (h) measurements with three different liquids (of which two must be polar) with þ
known cLW L , cL , cL values, using Eq. (1) three times, the þ LW values cS , cS , and c
S of any solid can be determined. Similarly, by contact angle measurements of a liquid on þ various solids (of which two must be polar) the cLW L , cL ,
and cL can be determined. It is always necessary to determine (or to know) the value of cL . The interfacial interaction energy parameters were obtained by using the following relationships. For the LW interactions between similar solid surfaces (1) in a liquid (3) qffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffi2 LW DG131 ¼
2
cLW cLW ð2Þ L S and for the AB interactions between similar solid surface (1) in a liquid (3). qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffi AB þ


cþ c
DG131 ¼
4 cþ cþ L cL þ S cL S cS
S cL
ð3Þ When DAB 131 < 0 defines the hydrophobic AB or hydrophobic structural interactions. When DGAB 131 > 0 defines the hydrophilic AB or hydrophilic structural interactions. Table 1 presents the average contact angles for alumina surfaces in surfactant solutions. In DDA cationic surfactant solution, the water contact angles with alumina at pH > 10.2 are increased and thereby hydrophobicity enhanced. In anionic surfactant solutions, the water contact angle with alumina is gradually increased at pH < 9 with the increase in concentration of SDS enhancing the hydrophobicity of alumina surface. The measured different contact angles of other liquids with alumina surface will give the different values for the components of surface energy of alumina. The values for the components of surface energy of alumina corresponding the solution conditions in Table 1 are calculated on the basis of Eq. (1) and presented in Table 2. The energy parameters of interactions between alumina

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Table 1 Advancing contact angle of the various polar and apolar liquids on a-alumina disc at different solution conditions Solution conditions

Average contact angles (h) pH

W

GL

FM

DM

Deionized water

10–10.2 3.2–3.4

0 0

23 23.7

8.5 10

40.3 40

SDS: mol l
1 5 · 10
5 10
4 10
3 3 · 10
3

3.4–3.6 3.5–3.7 3.6–3.8 3.5–3.7

38.8 47.5 60.5 65

34.2 39.5 57.5 62

19 23.5 47.6

34 35 52 55.3

DDA: mol l
1 10
5 10
4

10–10.2 10–10.2

59 81

55 80

45 72

40 58

Note: W––deionized water; GL––glycerol; FM––formamide; DM––diiodomethane.

Table 2 The values of components of surface energies of a-alumina at different solution conditions Solution conditions

Surface energy c (mJ m
2 ) pH

pffiffiffiffiffiffiffiffi cLW S

pffiffiffiffiffi cþ S

pffiffiffiffiffi c
S

10–10.2 3.2–3.4

5.45 5.84

1.97 1.64

7.45 7.28

SDS mol l
1 5 · 10
5 10
4 10
3 3 · 10
3

3.4–3.6 3.5–3.7 3.6–3.8 3.6–3.8

5.9 5.9 5.25 5.12

1.71 1.64 1.22 1.12

5.66 4.98 4.68 4.44

DDA mol l
1 10
5 10
4

10–10.2 10–10.2

5.8 5.08

0.99 0.14

4.56 3.5

Deionized water

particles at corresponding conditions were calculated based on Eqs. (2) and (3) and shown in Table 3. 3.4. Interaction forces between alumina In order to explain the aggregation behavior of ultrafine alumina in various solutions, we can calculate the total interaction forces between the particles under corresponding conditions. For two similar substances, electrostatic force between two spheres of radius R is (Hiemenz, 1977; Israelachvili, 1991) FE ¼ 2pea RjW20 expðjDÞ

ð4Þ

where D is interaction distance; W0 is the surface potential which is usually approximately substituted by the zeta potential; ea is the absolute dielectric constant of dispersion medium, 78.5 · 8.854 · 10
12 C2 J
1 m
1 j
1 is the Debye length. For two spheres of radius R, Lifshitz–van der Waals interactions are (van Oss et al., 1990; Israelachvili, 1991)  2 D0 Fw ¼ pRDGLW ð5Þ 131 D where D0 is the minimum equilibrium contact distance between particles, D0 ¼ 0:158 nm (Israelachvili and McGuiggan, 1988) or 0.163 nm (van Oss et al., 1990);

Table 3 The energy parameters of interactions between alumina particles at different conditions System Particle (1)/Water(3)/Particle (1)

Conditions Reagent (mol l
1 )

Energy parameters (mJ m
2 ) DGLW 131

DGAB 131

Al2 O3 /W/Al2 O3 pH: 3.5–3.7

Deionized water SDS: 10
4 10
3

)2.742 )3.031 )0.675

30.417 )0.955 )5.668

Al2 O3 /W/Al2 O3 pH: 10.2

Deionized water DDA: 10
4

)2.513 )0.338

32.27 )30.442

Y. Hu, J. Dai / Minerals Engineering 16 (2003) 1167–1172 4

ð7Þ ð8Þ

Based on the classic DLVO theory, the sum of the electrical double-layer forces and van der Waals forces are responsible for the colloidal stability. At pH 3.5–3.7, DLVO force profiles of interactions between alumina particles shown in Fig. 4 indicate a small potential barrier in the absence and presence of SDS. The force barrier is even higher in 10
3 mol l
1 SDS solution than in deionized water. It appears that alumina may be aggregated both in the absence and presence of SDS at this pH range, and even flocculated easier in pure water than in 10
3 mol l
1 SDS solution based on DLVO theory. It is obvious deviated the experimental results in Fig. 1. EDLVO force profiles shown in Fig. 4 demonstrate a sharp repulsive force between alumina particles in pure water and the strong attractive forces between alumina particles in the presence of SDS, which is greater in 10
3 mol l
1 than in 10
4 mol l
1 SDS solution. It explains well the aggregation and dispersion behaviors of alumina suspension in surfactant solution and in deionized water shown in Fig. 1. Fig. 5 illustrates the interaction forces between alumina particles at pH 10.2–10.5. DLVO force profiles predict a small force barrier between alumina particles both in absence and presence of 10
4 mol l
1 DDA. It

2

-2

pH10.2~10.5 KCl: 10-3mol.l -1

-4

1,2:DDA 10-4 mol.l -1 3,4: Deionized water

-8

0

10

20 Separation (nm)

30

40

Fig. 5. DLVO and the extended DLVO interaction force profiles for alumina/DDA system.

indicated that the aggregation or dispersion of ultrafine alumina at pH 10.2–10.4 in absence and presence of DDA will be almost same if based on DLVO theory. It cannot explain the different aggregation/dispersion behavior of alumina at pH 10 in deionized water and in DDA solution shown in Fig. 2. The EDLVO forces profiles, however, exhibit a evident repulsion in absence of DDA and a strong attraction in presence of 10
4 mol l
1 DDA at pH 10.2–10.4, which suggests that hydrophobic aggregation takes place between alumina particles adsorbed DDA. These results provide the evidence that EDLVO theory can explain the aggregation behavior of ultrafine particles in surfactant solution and the dispersion behavior in deionized water. In collector solution, alumina is rendered very hydrophobic. The strong hydrophobic attractive forces (polar interfacial interaction) between ultrafine alumina particles make these particles aggregate in the suspensions.

4. Conclusion pH3.5~3.7 -3 -1 KCl: 10 mol.l

5

2

1

6

F/R (mN/m)

4

-6

¼ FLW þ FE þ FAB

0 4

-1

--- DLVO forces __

3

EDLVO forces

-2 1

-3

1,2: SDS 10 -3 mol.l -1 3,4: SDS 10 -4 mol.l -1 5,6: Deionized water

-4 -5

0

1

Total extended DLVO forces are FTED

--- DLVO forces __ EDLVO forces

2 2

where h0 is the decay length, usually h0 ¼ 1–10 nm for hydrophobic systems (Pashley et al., 1985; Rabinovich and Yoon, 1994). Total DLVO forces are FTD ¼ FLW þ FE

3

ð6Þ F/R (mN/m)

Polar interfacial interaction forces are   D0
D AB FAB ¼ 2pRDG131 exp h0

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0

10

20 Separation (nm)

30

40

Fig. 4. DLVO and the extended DLVO interaction force profiles for alumina/SDS system.

The PZC of alumina is 9.1. SDS changed the zeta potential of alumina from positive to negative and decreased its PZC from 9.1 to 2. DDA chloride changed the zeta potential of alumina from negative to positive at pH 9–10.5. The aggregation between alumina particles was observed in SDS solution at pH 3.5 and in DDA solution at pH 10. The surfactant SDS and DDA made the alumina surface hydrophobic at corresponding pH values. The classical DLVO theory cannot describe the aggregation behavior. According to the extended DLVO theory, the aggregation behavior is attributed to the hydrophobic interactions between alumina particles adsorbed surfactant. The hydrophobic interaction forces are reasonably calculated based on van Oss’ polar interfacial interaction theory.

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