Adsorption model of pyridinium salts on quartz

Adsorption Model of Pyridinium Salts on Quartz REINHARD SCHWARZ, KLAUS HECKMANN, AND JIRI STRNAD l Institute of Physical and Macromolecular Chemistry, University of Regensburg, D-8400 Regensburg, West Germany Received February 25, 1987; accepted August 5, 1987 On the basis of adsorption and flotation measurements using a new flotation apparatus an adsorption model of pyridinium salts on quartz is presented. Depending on the character of the counterions the maximum amount of surfactant adsorbed is a bilayer or a tetralayer. Measurements of the flotation rate at various electrolyte concentrations show sharp maxima at surfactant concentrations near critical micelle concentration (CMC)/2. If the flotation rate is a function of the surface hydrophobicity, then the first adsorption layer is completed at this surfactant concentration. In the presence of bivalent counterions, adsorption bridges are formed between the second and the third layers and eventually four adsorption layers are bound to the surface. Two flotation maxima are observed in this case. The first maximum is also near the surfactant concentration CMC/2, the second one is at a concentration of 0.7 CMC. The presence of phosphate ions in solution causes the adsorption isotherms to be quite different from those measured with other counterions. The dependence of the flotation rate on phosphate concentration shows three sharp flotation maxima that do not correspond to those measured in the presence of sulfate ions. The shift of the first two maxima is explained as a chemical change of the quartz surface induced by the phosphate ions. The small third maximum may be caused by a further adsorption layer that is weakly bound to the surface. © 1988AcademicPress,Inc.

adsorption measurements using hexadecylpyridinium salts with various counterions, we believe we are able to postulate a new adsorption model not only for pyridinium salts on quartz, but perhaps for the adsorption of surfactants on mineral surfaces generally.

INTRODUCTION

Adsorption of various cationic surfactants on quartz is the subject of many papers, e.g., Refs. (1-4). Reports of flotation experiments using quartz are also not rare (5-8). In all the flotation experiments performed so far, recovery of the mineral is the only measure of floatability. The flotation kinetics therefore in many cases overlaps the adsorption kinetics of the collector on the mineral surface. The rate of mineral recovery corresponds only approximately to the flotation rate, because the concentration of the mineral suspension decreases during flotation. A new type of flotation apparatus (9, 10) makes it possible to eliminate these shortcomings, because the mineral floated is recycled until a constant flotation rate is reached. Under these steady-state conditions the flotation rate is a measure of the hydrophobieity of the mineral surface. By combining flotation and

EXPERIMENTAL

Methods. Flotation tests were performed in a new flotation apparatus. It is an adapted sinTABLE I Critical Micelle Concentrations (at 20°C) of the Surfactants Used

Surfactant

CPC1 CPH2PO4 CPBr CPNO3 CPHSO4

CIVIC (tool/liter)

9 7.3 6.6 5 2.7

× × × × ×

10-4 10-4 10-4 10-4 10 -4

Source. Reproduced, with permission, from Ref. (11).

To whom correspondence should be addressed. 50 0021-9797/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988

51

ADSORPTION OF PYRIDINIUM IONS TABLE II Adsorption Data for Pyridiniurn Salts on Quartz P~

[t~mol/m 2] at various electrolyte concentrations

Surfactant

0M

10-3 M

10-2 M

CPC1/KC1 CPBr CPNO3 CPHSO4/K2SO4 CPH2PO4/KH2PO4 CPH2PO4/K2HPO4 CPH2PO4/K3PO4

2.19 2.64 3.18 3.48

1.81

2.85

3.38

3.65 (1.8% HSO4, 98.2% SO]-) 2.43 (97.9% H2PO4) 3.67 (18.6% H2POz, 81.4% HPO4z-) 4.15 (89.5% HPO42-, 10.5% PO43-

gle-bubble Halimond's tube with recycling of the floated mineral. This enables us to measure the flotation rate under steady-state conditions. The mineral particles are floated from the fluid bed to the froth destroyer and then fall through a photometer back to the fluid bed. After a constant flotation rate is reached the stream of particles is switched to a balance

0.05 M

O.IM

pH at P ~

2.90

2.93

6.0 5.0 5.5 3.6 4.3 8.2 11.3

for a precise weight determination of the mineral floated. For adsorption measurements the concentrations of the pyridinium salts were determined spectrophotometrically at 259 nm. Critical micelle concentrations were evaluated from the change of the surface tension and/or with an optical apparatus for the determina-

I

1.0 "l

0.B

0.6 t._E L-

0.2

0

~ 0

0.2

0.4 C / CHE

0.6

0.8

1.0

1.2

FIG. 1. Adsorption isotherms in reduced coordinates: O, CPNO3; Iq, CPBr; A, CPH2PO4; O, CPHSO4; II, CPC1. Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988

52

SCHWARZ, HECKMANN, AND STRNAD

~.0-

3,5-

3.0-

2.5-

2.0-

..a

1.5-

~

1.0-

0.5-

0

ii,

o

012

0'.6

o.,

1.0

J

1.2

C I EHfi

FIG. 2. Adsorption isotherms with 10-2 M electrolyte: A, CPH2PO4/KHEPO4; IS],CPH2POa/K2HPO4; O, CPH2PO4/K3PO4; O, CPHSOg/K2SO4; m, CPC1/KC1.

tion of solubility curves (11), All measurements were carried out at 20°C. Materials. The quartz was of p.a. purity from Merck, Darmstadt, West Germany. The grain size used for adsorption and flotation measurements was either less than 0.063 mm or 0.16-0.20 mm; the specific BET surface area was 4.18 or 0.035 m2/g, respectively. The preparation of the surfactants used is described in (11). The abbreviation CP is used for the hexadecylpyridinium ion. RESULTS AND DISCUSSION

Adsorption and Flotation Measurements The adsorption isotherms (adsorption amount F versus log of the equilibrium concentration C) from water have the usual Journal of Colloid and Interface Science, Vol. 124, No, 1, July 1988

form. The adsorption attains saturation (Fma~) at equilibrium concentrations that approximate the critical micelle concentration (CMC). Pmaxincreases in the order CPC1 < CPHEPO4 < CPBr < CPNO3 < CPHSO4. The CMC values at 20°C for the surfactants used are given in Table I. Fmax(Table II) increases with decreasing CMC which roughly parallels the aggregating power of an anion (13) and its ability to disrupt the structure of water (14-17). The comparability of the adsorption isotherms is improved by drawing them in reduced coordinates F/Fm~xversus C/CMC (Fig. 1). In this case, the adsorption isotherms differ from each other in shape, depending on the type of counterion. The curves for CPC1, CPBr, and CPHEPO4 are practically superimposed, while the isotherms for CPNO3 and

53

ADSORPTION OF PYRIDINIUM IONS

the high electrolyte concentration and (ii) the presence of multivalent counterions in solution or both. We assume that in the first layer (ion-exchange adsorption) the adsorption of solvent molecules at the solid-liquid interface competes with the adsorption of surfactant ions, in accordance with assumptions of other authors (12). Adsorption of a second layer of surfactant with hydrophilic groups oriented toward the bulk solution is supported because of the better compensation of repulsion forces between polar heads. This compensation is increased by the high electrolyte concentration or by the presence of bivalent counterions. This is in accordance with the fact that the CMC of ionic surfactants decreases with increasing ionic strength. The results of our flotation tests with CPCI

essentially for C P H S O 4 are S-shaped. The increase in the S-shape character of the adsorption isotherms, which is connected to cooperative behavior, follows the same order of anions mentioned above. The effect of the addition of 10-2 M electrolyte on the adsorption isotherms of CPC1, C P H S O 4 , a n d CPH2PO4 on quartz is shown in Fig. 2. In this case, the adsorption isotherm of CPC1 is also S-shaped; therefore, it also shows a cooperative character. The adsorption isotherms for the systems CPHEPO4/KEHPO4, CPHEPO4/KH2PO4, and CPH2PO4/KaPO4 are quite different. The difference is discussed further in connection with flotation tests. Important adsorption data are summarized in Table II. The S shape of the adsorption isotherms in reduced coordinates is caused by (i) 35-

30-

--

25-

20-

15-

-~ 10-

5-

0

I

0.07

I

I

I

0.1

I

0.2

I

i

I

I

I

1

0.5 E I (ME

FIG. 3. Flotation rates of quartz with CPC1 at various electrolyte concentrations (M) in reduced coordinates: O, 0.1; D, 0.05; A, 0.02; O, 0.01. Journal of Colloid and Interface Science, V o l .

124, No.

1, J u l y

1988

54

SCHWARZ,

HECKMANN,

AND

STRNAD

are shown in Fig. 3. All flotation experiments The flotation rates of quartz with CPH2PO4 had to be performed at higher electrolyte con- in Fig. 5 show three sharp maxima that do not centrations in order to depress flocculation. In correspond to those for CPHSO4 or CPC1. It reduced coordinates all the flotation maxima has been noted that the adsorption isotherms coincide at C/CMC = 0.5. Under the suppo- of CPH2PO4 also differ from those for the other sition that the flotation rate is a measure of surfactants. We assume, therefore, that phosthe surface hydrophobicity, the adsorption phate ions coadsorb into the first layer and into the first layer is completed at this reduced modify the surface of quartz. concentration. Adsorption into the second layer probably begins below C/CMC = 0.5, Tentative Adsorption Model but to a small extent only. In the presence ofmonovalent counterions, The flotation rates of quartz with CPHSO4 show two sharp flotation maxima (Fig. 4), the the pyridinium salts adsorb on the surface of first one also at the reduced concentration C~ quartz as a bilayer (Fig. 6), which is attained CMC = 0.5, the second one at C/CMC ~ 0.7. at the critical micelle concentration. The secAlso included in this case is the ion-exchange ond layer, which adsorbs via hydrophobic inadsorption completed at C/CMC = 0.5. It can teractions, can be deposited when the ion-exbe seen from the adsorption isotherm of change adsorption is almost complete. In the CPHSO4 in Fig. 4 that about 25% of Fr~,x is case of quartz, this occurs at the reduced conadsorbed at the first flotation maximum and centration C/CMC = 0.5, and independent of about 75% at the second flotation maximum. the electrolyte concentration. The reduced

30

I I

-1.0

i

-:2o. ~-~'~

/

" ":

0.B

0.6 t--E L-

0.t~ 3

10-

0.2

5-

0

i 0.2

i

@

adsorphon

0

flofafion

q

,

O.L~

0,6

J 0.8

i

i 1.0

[ I CMC lOG. 4. F l o t a t i o n r a t e s o f q u a r t z w i t h C P H S O 4 .

Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988

1.2

ADSORPTION OF PYRIDINIUM IONS

55

30-

0.8

20

06

15-

t._ t_ 0.t~ oc10 _ m o

0.2

i

c• ,

i

J

0.06

• o j

adsorption

flotation

J

0.1

l

0.2

~

r

,

0.5 [ / CMC

FIG. 5, Flotation rates of quartz with CPH~PO4.

concentration at which the flotation maxim u m occurs is only a property of the mineral surface. The presence of phosphate ions causes a modification of the surface due to coadsorption into the first adsorption layer. Such a modified surface reaches its highest hydro-

@

@

I

@

I

I

(D

@

I

lOG. 6. Adsorption of a bilayer,

I

®

phobicity at reduced concentrations other than the one at which the pure quartz surface does. The bi- and trivalent counterions create adsorption bridges, so that a second or third hilayer is reached at the CMC (Fig. 7). The third and fifth layers cause the second and third flotation maxima, respectively. The surface of quartz is certainly heterogeneous. The negative charge can accumulate on edges or lattice dislocations. Therefore, there are different surface concentrations of surfactants on various patches of the surface. The admicelle hypothesis (18) maintains that surfactant aggregation producing a bilayered structure (admicelle) occurs on a given patch of a heterogeneous surface at a certain critical admicelle concentration (CAC). But the very sharp breakdown in the flotation rate can only be explained by a sudden hydrophobic-to-hydrophilic phase transition on the surface. The heterogeneity could explain the finding that Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988

SCHWARZ, HECKMANN, AND STRNAD

56

®

® '

®

®

@......_.

®

s,......

®

@

FIG. 7. Adsorption of a tetralayer.

the mean area per surfactant ion in one layer is about 180 A2 in the case ofa tetralayer. The surfactant adsorbed on the edges might be responsible for the flotation rate. The ion-exchange adsorption (at least on the places responsible for the flotation) must be completed to make hydrophobic interaction possible. REFERENCES 1. Gaudin, A. M., and Decker, T. G., J. Colloid Interface Sci. 24, 151 (1967).

Journal of Colloid and Interface Science, Vol. 124, No. 1, July 1988

2. Schubert, H., and Baldauf, H., Tenside Detergents 4, 151 (1967). 3. Smith, R. W., Trans. AIME 226, 427 (1963). 4. Somasundaran, P., Healy, T. W., and Fuerstenau, D. W., J. Phys. Chem. 68, 3562 (1964). 5. Chibowski, F., and Holysz, L., J. Colloid Interface Sci. 112, 15 (1986). 6. Fuerstenau, D. W., Healy, T. W., and Somasundaran, P., Trans. AIME 229, 321 (1964). 7. Schubert, H., and Schneider, W., in "Proceedings, Mineral. Process. Congr., 8th, Leningrad, 1968," Vol. 2, p. 315. 8. Somasundaran, P., and Lin, I. J., Trans. AIME 254, 181 (1973). 9. Heckmann, K., and Schwarz, R., Chem. Ing. Technol. 58, 396 (1986). 10. Schwarz, R., thesis, University of Regensburg, 1986. 11. Heckmann, K., Schwarz, R., and Strnad, J., J. Colloid Interface Sci. 120, 114 (1987). 12. Tamamushi, B., and Tamaki, K., in "Proceedings, International Congress on Surface Activity, 2nd, London, 1957," Vol. 3, p. 449. 13. Anacker, E. W., and Ghose, H. M., J. Amer. Chem. Soc. 90, 3161 (1968). 14. Bingham, E. C., J. Phys. Chem. 45, 885 (1941). 15. Frank, H. S., and Evans, M. W., J. Chem. Phys. 13, 507 (1945). 16. Kaminski, M., Discuss. FaradaySoc. 24, 171 (1957). 17. Jones, G., and Dole, M., J. Amer. Chem. Soc. 51, 2950 (1929). 18. Harwell, J. H., Hoskins, J. C., Schechter, R. S., and Wade, W. H., Langmuir 1, 251 (1985).