Effect of sodium dodecyl sulfate concentration on the attachment between mercury and argon bubbles in aqueous solutions

Effect of Sodium Dodecyl Sulfate Concentration on the Attachment between Mercury and Argon Bubbles in Aqueous Solutions Q. DAI,* H. SASAKI,* S. USUI, *'l

AND

M. KAISHEVAt

*Research Institute of Mineral Dressing and Metallurgy, Tohoku University, Katahira, Sendai 980, Japan; and t Faculty of Chemistry, University of Sofia, Sofia 1126, Bulgaria Received November 13, 1989; accepted February 5, 1990 The attachment between mercury and argon gas bubbles was investigated at various polarization potentials E (volt vs SCE) of a mercury electrode in aqueous sodium dodecyl sulfate (SDS) solutions in the presence of 0.01 M NaCIO4. It was found that the range of E for spontaneous attachment was about - 0 . 3 ~ - 0 . 6 V when SDS concentration was less than 5 X 10 -6 M , while no spontaneous attachment was observed at SDS concentrations higher than 5 X l 0 -6 M . Under the conditions of spontaneous attachment, the Stern potentials of both mercury and bubbles were negative and the surface coverages of D S - ions on the two surfaces were less than 10% and 2%, respectively, assuming a crosssectional area of the alkyl chain of 18.5 A 2. Experimental results are discussed in terms of the heterocoagulation theory. The existence of an additional attractive hydrophobic force is suggested and an attempt is made to reveal the dependence of the hydrophobic force on the surface coverage of the surfactant. © 1990AcademicPress,Inc. INTRODUCTION

be floated. Blake and Kitchener (5) studied the contact between bubbles and methylated silica plates in aqueous electrolyte solutions and found that wetting films on the silica plates were metastable and collapsed when disturbed, indicating the existence of the effect of hydrophobic interaction. Recently, Israelachvili and Pashley (6) measured interaction forces between monolayers of cetyltrimethylammonium bromide (CTAB) adsorbed on mica plates and observed an unexpectedly strong attractive force, i.e., hydrophobic force. Several measurements of the force were reported later (7-11 ). The hydrophobic force between planar plates is now known to be one or two orders of magnitude stronger than the dispersion force and decays exponentially with distance. It has been suggested that the hydrophobic force is caused by the structural rearrangement of water molecules in regions around nonpolar surfaces (12a), yet its molecular origin remains unknown. It should be mentioned that, since all the measurements of the hydrophobic forces have been performed under the condition of monolayer adsorption

Mineral particle-bubble attachment is one of the decisive processes in flotation and can be treated as a heterocoagulation process ( 13) in which surface forces including electrical double layer forces and van der Waals forces between particles and bubbles play an important role. Though the mechanisms involved in the phenomena such as flotation, coagulation of colloidal dispersion, and stability of thin liquid films can be quantitatively analyzed on the basis of DLVO theory and heterocoagulation theory, the analyses have usually been carried out by using a theoretically ambiguous quantity like the zeta potential instead of the Stern potential which is important for calculating the electrostatic interaction (4). Precisely controlled interface potentials are essential for investigation of the flotation mechanism. On the other hand, floatability depends on the hydrophobicity of the mineral particles to To w h o m correspondence should be addressed. 30 0021-9797/90 $3.00 Copyright © 1990 by Academic Press, Inc. All fights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 139, No. t, October 1, 1990

MERCURY-BUBBLE

of ionic surfactants on mica plates, no relationship between the hydrophobic forces and coverages of the adsorption layer has been investigated, except in a recent paper ( 11 ). It is very important to investigate the effect of surface coverage of surfactants on the hydrophobic interaction for particle-bubble attachment. A model experiment using a mercury electrode proved to be useful in analyzing the colloid stability theory (13-15) because the cleanliness and smoothness of mercury surfaces are ensured and surface potential is freely changed with precision. The technique has also been applied to a mercury/aqueous pure electrolyte solution/bubble system (16), i.e., a flotation model system. In our previous paper (17), induction times of the mercury-bubble attachment have been measured in aqueous 0.05 M Na2 SO4 solution in the presence of 5 × 10 -5 M sodium dodecyl sulfate (SDS) as a function of electrode potential and the results have been discussed in terms of disjoining pressure consisting of electrostatic and van der Waals forces. Film rupture was observed at positive potentials in contrast to the fact that the disjoining pressure showed appreciable positive values (repulsion). This finding was connected with the existence of an attractive hydrophobic force. However, since this study was conducted at a particular concentration of SDS and confined polarization potentials, no quantitative analysis was available. In the present study the mercury-bubble attachment was investigated as a function of both the SDS concentration and the polarization potential of mercury. The results were discussed using the heterocoagulation theory and an attempt was made to reveal the influence of hydrophobic force on thin film stability. Furthermore, dependence of the strength of hydrophobic force on surface coverage of surfactant was examined. EXPERIMENTAL

Materials. All the materials were purified with great care in order to remove possible impurities.

ATTACHMENT

31

Sodium dodecyl sulfate, a commercial reagent, was recrystallized twice from ethanol before it was used as an anionic surfactant. In order to simplify the analysis of experimental results NaC104 was chosen as a supporting electrolyte due to its 1-1 type, low specific adsorption on mercury (18), higher solubility in water than NaF, and hence ease of its recrystallization. NaCIO4 of guaranteed reagent grade was recrystallized once from distilled water and then dried in an electric oven at 250°C for 1 day. A high-purity argon gas was used to produce bubbles. Distilled water used throughout the experiments was prepared as follows. Deionized and double-distilled water was redistilled over alkaline permanganate in an all-Pyrex distillation unit and finally filtered through Millipore filter (0.05 urn) under nitrogen atmosphere. The water thus treated had a pH value of about 5.7-6.0 and a specific conductance of about 0.85-1 /~S cm -1 . Mercury of guaranteed reagent grade was purified by the procedure described below. Mercury was aerated first in a ferric ammonium sulfate solution (Fe2 (SO4)3 (NH4) 50424H20:H2SOa:H20 = 10 g:50 m h l 0 0 0 ml) and again in a dilute nitric acid solution (3% HNO3) each for 1 day. The mercury was then well washed with distilled water and dried in a desiccator. Finally distillation was carded out and the mercury was kept in a bottle filled with nitrogen gas. Apparatus and method. Figure 1 shows schematically the experimental setup for measuring mercury-bubble attachment. The apparatus and the measurement method are almost the same as described in Ref. (16) except that the cell was constructed with Pyrex glass and the mercury electrode was modified to be of stationary type. The mercury drop, 13, could be prevented from growing by turning off the cock, 11, attached on the capillary (inner diameter: 2 m m ) . The mercury-bubble approach was carded out by using a micrometer device, 17. The polarization potentials of mercury were applied by a potentiometer, 8, Journal of ColloM and Interface Science, Vol. 139, No. 1, October 1, 1990

32

DAI ET AL.

m m . The internal diameter of the capillary tip, 0.037 m m , was calibrated with 0.05 M Na2SO4 solution in which the interfacial tension is known to be 426.2 dyne cm -~ at the electrocapillary m a x i m u m ( E C M ) at 25°C (19). The surface tension of SDS solutions was also measured by means of a surface tensiometer (+0.2 dyne cm -j ) using the Wilhelmy Plate method for the purpose of calculating the adsorption density of organic ions and the FIG. 1. Schematic drawing of apparatus for measuring surface charge density at the gas/liquid ( G / mercury-bubble attachment. (1) Ar gas cylinder. (2) Valve. (3) Humidifier. (4) Electrometer. (5) SCE. (6) Vi- L) interface. All the experiments were performed in SDS nyl tube filled with 1 M NH4NO3 solution. (7) I M NH4NO3-agar bridge. (8) Potentiometer. (9) Reference solutions in the presence of 0.01 M NaC104 electrode. (10) Gas disperser. ( 11) Stopcock. ( 12) Ar bub- at 25 ___ 1°C. The solutions were purged with ble. (13) Mercury drop. (14) Pt gauze. (15) Aqueous so- argon for at least 1 h and the experiments were lution. (16) Mercury pool. ( 17) Micrometer device. conducted under argon atmosphere. In the preliminary attachment experiment, with respect to a saturated calomel electrode it was observed that instantaneous attachment (SCE), 5. In the following, the polarization occurred only at concentrations of SDS below potential E is given in terms of the potential 1 × 10-5 M , and in such dilute solutions the vs SCE (volt vs SCE). The Pyrex glass cell ( 50 adsorption of dodecyl sulfate ions ( D S - ) are × 50 × 60 m m ) containing solutions was thought to be time dependent. To confirm this, placed in a transparent thermostating cell differential capacities were measured against through which the attachment behavior was time in several SDS solutions at E = - 0 . 6 5 observed by means of the microscope of a V. Figure 2 shows two of the results from which the time for the equilibrium adsorption cathetometer from a horizontal direction. The differential capacities at the m e r c u r y / being reached was estimated to be about 30 liquid ( M / L ) interface were measured to de- min. For this reason, each datum was taken termine the orientation of the adsorbate by 30 min after the surface of mercury or the employing a setup which was essentially iden- bubble was renewed. tical to that previously reported (14) except that a stationary mercury electrode (SME) and a three-electrode system for measuring the poO.01M NaCl04 larization potential of mercury were used. The E =-0.65 V AC frequency utilized was 1000 Hz. [ SDS], M o lx10 "6 In order to obtain the adsorption density of • 5x10-6 organic ions and the surface charge density at u the M / L interface, electrocapillary curve measurements were made on the SME by the ~ 8 capillary electrometer method: the head of the mercury pool was first adjusted to bring the 10 20 30 lower meniscus of mercury to the reference Time. rain point in the capillary (0.25 m m from the tip) FIG. 2. Variation of the differential capacities (SME) and then the height of the mercury head and with the time elapsed in 0.01 M NaCIO4aqueous solution the depth the tip dipped in the solution were in the presenceof SDS at a polarizationpotential of-0.65 read with a cathetometer to a precision of 0.05 V (volt vs SCE). SME: stationary mercury electrode. .

Journal of Colloid and Interface Science, Vol. 139, No. 1, October 1. 1990

.

.

.

i

'

MERCURY-BUBBLE ATTACHMENT RESULTS

33

' ' I ....

I ' ' ' ' I ' 'i

'1

'

2

The behavior of mercury-bubble attachment in SDS solutions in the presence of 0.01 M NaC104 is shown in Fig. 3. The two solid lines represent critical potentials between which mercury and bubble attach to each other instantly when the two surfaces are brought to contact, i.e., induction time being zero. The dashed line indicates the point of zero charge (PZC). The way to determine the PZC is described later in this paper. It is evident that the potential range of instantaneous attachment is in the vicinity of the PZC with a little wider range of positive polarization potentials. This may be explained by considering the fact that the H a m a k e r constant of merc u r y / w a t e r / b u b b l e is known to be negative (16); i.e., the van der Waals force between the mercury and bubble is repulsive. Bubble surfaces in SDS solutions are negatively charged because of the preferential adsorption of anions, D S - ions. According to the heterocoagulation theory (20-22), the mercury must be positively or less negatively charged so as to make the double layer force attractive enough to overcome van der Waals repulsion. However, no explanation is available for the fact that there exists a positive critical potential of mercury beyond which spontaneous attachment becomes impossible. A similar result was reported (16) in m e r c u r y / K F aqueous solution/argon bubble system and this behavior was attributed to the retarded drainage of the

>.

-0.7

u~

-0.6

'-~

-0.4

•-~

-0.2

,

,

r

0.0

,

A t t a c h m e i ~

!

g 3so

[SDSI,M ,~

30(3

1

0

2. 3. 4. 5. 6. ?

1x10-6 lx10-5 5x10-5 lx10-4 5xlO-4 lxlO-3 l

250'

, ,,,

t,

~

,

Q0 -Q5 -t0 -1.5 PolarizQtion potentiol vs. 5CE,V

FIG. 4. Electrocapillary curves measured on SME for varying SDS concentrations in 0.01 M NaCIO4 aqueous solution. Data points are omitted for clarity. Arrow represents the PZC in base solution.

intervening thin aqueous film due to the enhanced surface viscosity as a result of the high concentration of F - ions near positively polarized mercury surfaces. Figure 4 shows the electrocapillary curves measured on SME in various SDS concentrations in the presence of 0.01 M NaC104, in which experimental points are omitted for clarity. The solid lines were obtained by the ninth degree least-squares fits to the data. The M / L interfacial tension decreases with increase of SDS concentration, and hence, with increase of the adsorption of D S - ions. In deep negative polarization potentials all the curves tend to coincide with that of base solution, indicating the desorption of DS ions from the mercury surface. The interfacial tension ("YM/L) in Fig. 4 was plotted With respect to the natural logarithm of SDS concentration (C,) and the Gibbs adsorption equation

Repulsion

N Z c~ -0.1 ~.

~-----

Repulsion

~OC

FM/L = -(1/RT)(dTM/L/dln Cs)E

Hg-Ar 0.01M NGCIO 4 1

2

3

4

5

6

SDS concentrotion, xlO6M

FIG. 3. Critical polarization potential for mercurybubble attachment as a function of SDS concentration in the presence of 0.01 M NaCIO4.

[1]

was applied to calculate the adsorption density of DS ions at the M / L interface, FS4/L, for each polarization potential E, where R is the gas constant and T the absolute temperature. (dTM/L/d In Cs) in Eq. [1] was obtained by Journal of Colloid and Interface Science

Vol.

139, No.

I, O c l o b e r

I, 1990

34

DAI

derivation of the sixth degree regression curves of'YM/L- - In Cs plots. The adsorption density, FM/L, was calculated as a function of potential E for various concentrations of SDS and the results are shown in Fig. 5, in which surface coverage of DS- is also shown. The surface coverage is expressed by assuming that the hydrocarbon chain of DS- has a cross-sectional area of 18.5 ,~2 (23), the value of solid paraffins. The region enclosed by the dashed line represents where the attachment with no induction time take place. It is seen that within this region the percentage of the mercury surface covered by DS- ions is less than about 10%. The charge density crsdue to the specifically adsorbed DS- ions at the Stern layer of the M / L interface may be obtained by the relation ~rs = - - F F M / L ,

ET AL. -200 r , , . . . .

~ ....

~

-150 -lOO

131~1,M

5°I'

/

t/

lO0

'~ ,g

II

II

-

',n

j

~'

¢/

~

/y

• 3,1o-5 o

~,,o-5

• ?x10-6

~-6

0.0 -05 -1.0 Polarization potential vs. SCE,V FIG. 6. Stern potential o f Hg as a function o f the polarization potential ( v o l t vs SCE ) at various SDS concentrations in 0.01 M NaCIO4 aqueous solution. A r r o w represents the PZC in base solution.

[2]

where F is the Faraday constant. By applying the Lipmann equation [ 3 ] the surface charge density go can be obtained at a given SDS concentration (24), i.e., [3]

gO = - ( d ' Y M I L / d g ) ~ , ,

where # is the chemical potential of SDS. It is shown in Fig. 4 that the PZC shifts toward the cathodic polarization potential due to the increase in the specifically adsorbed anions DSas SDS concentration increases, but in the present study the PZC was found to be virtually constant (-0.46 ~ -0.48 V vs SCE) for Cs ~< 1 × 10 -5 M, a concentration above

which the spontaneous attachment becomes impossible. Once as and ~r0are known, the Stern potential, ffs, for the M / L interface may be obtained using the Gouy-Chapman equation --ad = as + gO = ( 2 N A C e ~ k T / 1 0 0 0 7 r )

1/2

× sinh(ve~bs/2kT),

[4]

where NA is Avogadro's number, Ce the total concentration of the electrolytes in molarity, E the dielectric constant of the solution, k the Boltzmann constant, v the valence of the ions, and e the elementary charge. Figure 6 shows ~bsof mercury as a function of the polarization potential of mercury for , , , , , , , 7 25 various SDS concentrations together with that % 20 HglSolution ~ ~ S1'M t in the base solution (0.01 M NaC104 solu[SD 20 ~n" j. O01MNQCIO4 tion). The Stern potentials tend to coincide 9 t.5 × • txllTs 15 "6 with ~bs of base solution as the mercury elec= 7x10-6 ~ t.o trode is polarized in deep negative potentials ._~ as a result of desorption of DS- ions due to g os 2x10-6 5 the electrostatic repulsion. It is seen that, except for extremely positive polarization poT ~'''~ ~'~ ' O0 0.0 -02 -0.4 -0.6 -fib a2 tentials, in the positive polarization branch the Polarization potential vs. SCE,V lOG.5. Adsorptiondensitiesand surfacecoveragesof Stern potential shows a negative sign and inSDSat Hg/0.01MaqueousNaCIO4solutioninterfacefor creases its negative value with increasing SDS concentration, indicating a strong affinity of variousconcentrationsofSDS. r

~

,

Journal of Colloid and Interface Science,

'

Vol. 139, No.

' ~ 0

1, O c t o b e r

1, 1 9 9 0

MERCURY-BUBBLE ATTACHMENT D S - ions toward the mercury surface. The region enclosed by the dashed line represents where the mercury and bubble attached with no induction time. It is seen from Fig. 6 that a maximum negative Stern potential for possible spontaneous attachment reaches as high as about - 140 mV. The adsorption density of D S - ions at the bubble surface can be obtained in the same way as in the case of M / L interface, i.e., by applying the Gibbs equation (Eq. [l]) to the surface tension versus the logarithm of the SDS concentration curve given in Fig. 7. The adsorption isotherm of SDS at the surface of aqueous 0.01 M NaC104 solution is shown in Fig. 8. The adsorption density remains a low value ( ~ 1 X 10 Jl mol cm 2) and nearly unchanged in the range of SDS concentration less than 6 × 10 - 6 M, an upper limit for spontaneous attachment, which corresponds to a surface coverage less than 2%. As for the electrochemical properties at the G / L interface, i.e., bubble surface, there seems to be no reliable experimental method by which the zeta potential of bubbles in aqueous solution is determined although some experimental studies have been reported so far (2528). It was reported (29) that, in the study of Dorn effect of bubbles, the adsorption densities of SHS (sodium hexadecyl sulfate) calculated from zeta potentials using the Gouy-Chapman theory increased with decreasing bubble size and were fairly close to those obtained by the Gibbs equation under equilibrium conditions when extrapolated to zero bubble di. . . . .~1

:>, "o

'

.......

I

........

I

70 ~ c

' ' ...... I

........

I

........

1

60

50

Ar/Sot

10~7

MC

10-6

10-5

10-4

10-3

5

........

-~ 4 o-

I

35

........

I

........

I

........

Ar I Solution O.01M NaCI04

i

......

/

>, 3

~2 ._~ <{ 010-7 '

10 6

10-5

10-4

10-3

~10-2

SD5 concentration, M

FiG. 8. Adsorption isotherm of SDS at Ar gas/solution

interface in the presenceof 0.01 M NaC104. ameter. This reflects that a smaller bubble whose rising velocity becomes smaller has a surface closer to a static air-water interface. Sotskova et al. (30) reported that the zeta potentials of bubbles were smaller than the Stern potentials obtained on the basis of adsorption data when the surfactant concentration is low, whereas the two potentials tended to be identical in the surfactant solution of high concentration. They considered the result as evidence for the absence of specific adsorption of counterions at the G / L interface having an equilibrium adsorption layer and they suggested that the G o u y - C h a p m a n model could be applied to calculate the Stern potential of an undeformed double layer induced by the adsorption of surfactant ions at the G / L interface. In the present study, as the mercury-bubble attachment experiment was performed under a static condition, the Stern potential of bubbles was determined by using the G o u y Chapman equation (Eq. [4]) from the adsorption data (Fig. 8), while assuming the DSions to be potential determining ions and no specific adsorption, i.e., - f f d = i f 0 ---- - - F F G / L . The variation of the Stern potentials thus obtained with SDS concentration is presented in Fig. 9. It can be seen that the Stern potential of bubbles increases its negative value as the SDS concentration becomes higher but shows an almost constant value near - 5 0 mV up to about 4 × 10 -6 M SDS.

10-2

SDS c o n c e n t r a t i o n . M

FIG. 7. Surfacetension versusconcentration curve for aqueoussolutionof SDS in the presenceof0.01 MNaCIO4.

DISCUSSION

In order to examine the behavior of mercury-bubble attachment quantitatively it is Journal of Colloid and Interface Science,

Vol.

139, No.

1, O c t o b e r

1, 1 9 9 0

36

DAI ET AL.

thickness of the Stern layer was assumed to be 3.7 A (34) for both surfaces and/'el was calculated as a function of the distance between Stern planes of both surfaces. The calculation -100 was carried out by numerical integration using E -50 Simpson's method. tn The total force of interaction Ft was ob0_7 . . . . . . . . i ........ i ........ i ........ i ...... 10-6 10-5 10-4 10-3 10-2 tained as a sum of Fel and Fvw. As a criterion SD$ concentration,M for the attachment between mercury and bubFIG. 9. Stern potential of Ar gas bubble as a function of SDS concentration in the presence of 0.01 M NaC104. ble, the m a x i m u m value of Ft, i.e., the force barrier (designated as Ft,max), was investigated. Figures 10a and 10b illustrate the values of necessary to estimate the interactions between Ft,max as a function of electrode potential E the mercury and bubble. The mercury-bubble for various SDS concentrations. The chain attachment was analyzed in terms of the het- lines in Fig. 10a show the critical potentials erocoagulation theory which considers the between which attachments occur with no indouble layer force and van der Waals force. duction time. It can be seen from the figure In all the calculations the plane-parallel model that the force barrier extends from cathodic was used because dimensions of mercury drop to anodic branch of polarization and increases and bubbles were much larger than the dis- its magnitude with SDS concentration because tance over which surface forces were operating. of the increased negative charges on both In the previous paper ( 17 ) the van der Waals mercury and bubble surfaces, leading to the force was calculated by taking into account enhanced repulsive forces. Except for the case the effect of adsorbed layers on the basis of of SDS concentration below 1 X l 0 -6 M the the H a m a k e r (32) and Void (31 ) theories, in Ft,ma x shows high values in the order of magwhich the H a m a k e r constant of hydrocarbon nitude of 10 6 dyne cm 2 even under the conwas used as the constant of hydrocarbon layer ditions of spontaneous attachments. Figure 11 gives some examples of forces of of adsorbed film by assuming a closed packed monolayer adsorption on both mercury and interaction as a function of surface-to-surface bubble surfaces. In the present study the com- distance between mercury and bubble at E posite H a m a k e r constants composed of sol- = - 0 . 4 5 V for 5 X 10 -6 M SDS which is the vent (water) and adsorbate ( D S - ions) were highest critical concentration for spontaneous calculated using the surface coverage data in attachment. A critical force barrier of 5 X 105 accordance with the method of Vincent (33), dyne c m - 2 was selected in a previous study since the adsorption densities of SDS on both (16) as a criterion for spontaneous attachment mercury and bubble surfaces are fairly low. between the mercury and bubble considering However, the results showed that the correc- the fact that the mercury-bubble approach was tion for the effect of adsorption layers is of regulated by hand using a micrometer device. negligible significance if the surface coverage This value was obtained by analyzing total is less than 20%, and consequently the van der potential energy curves at critical conditions Waals force between a bare mercury drop and for coalescence of mercury droplets (14) and a bare bubble was calculated using the Ha- attachment between mercury and glass (15) maker constant for the mercury-water-bubble that were conducted using similar devices. It system being - 7 . 2 2 X 10 -13 erg (16). can be seen in Fig. 1 la that Ft,m~x reaches a The electrostatic force E e l w a s calculated magnitude of 3 x 106 dyne cm -2, six times according to the theory of heterocoagulation the critical force barrier, and still remains as proposed by Derjaguin (20) using Stern po- high as 2 X 10 6 dyne cm -2 even ifFvw is cortentials of mercury and bubble, in which the rected for postulated complete hydrocarbon ........

-200

. -~ -150

r

........

i

........

F

........

i

......

ArI $ o l u t i o l ~ 0.01M NaC/

Journal of Colloid and Interface Science,

Vol.

139, No.

I, O c t o b e r

1, 1 9 9 0

MERCURY-BUBBLE

37

ATTACHMENT

100 \

I

I

b

O.01M NaClOa

80

4C

5x10-6

2C ,

i

=

O.01M NaClO& [SDS], M

60! b

I

Ix10-5

40

,xto-. 2O

2C o i

i

,

1]

i

I ] . . . .

,

,

J

I

=

,

i

,

I

i

I

J

~c 40

I

2xt0_s b

~- 20 b x

[

~ ~°

=

~

ii

i

J

,

i

6°I

I

20

i

20

. . . .

I L

t

,

,J

7xl0-S

I

i

L

,

I ,

JI

. . . .

I

40

4O

20

20

6xllT s

J

[

L

,Li

0.0

i

i

-0.5

Polarization

i

L

i

i

0

-1.0 vs. SCE, V

potential

i

O0 Polorization

i

¢

I

J

i

-0.5 potential

vs.

i

,

-1.5 SEE, V

FIG. 10. Forcebarrieras a functionof the polarizationpotential(volt vs SCE)at variousSDS concentrations. NaC104:0.01 M (Ft = Fe~ + F,,,v). monolayers on the surfaces of mercury and of bubbles as indicated by 0 = 1 in Fig. 11 b. Strictly, the Hamaker constant is not truly constant but begins to decay rapidly at separation distances beyond about 50 A due to the retardation effect (12b), and precise computation of Fvw at all separations must be based

4C

c~ 3C

'

i

.uu / ~2C

i , .

E=-O45V FI=FeI*F~O'O1M NaCiO4*SxlO-GM SDS F~

Ft

F

g -10

-2C

50

100

150

50

i 100

(

150

Seporotion distance,

FIG. 1I. Electrostatic force (Fel), van der Waals force (F~) and total force (Ft) of interaction between mercury and bubble as a function of separation distance. (a) F~ contains no hydrocarbonmonolayer(0 = 0). (b) F,~ contains complete hydrocarbon monolayer (0 = 1). (E = -0.45

V; 5

X 10-6 MSDS).

on Lifshitz's theory (35). At shorter separations, e.g., around 30 A, where force barriers appear in all cases of this study, Fvw is approximately nonretarded and the distance dependence of the Hamaker constant may be ignored. Furthermore, it should be noticed that Derjaguin's method used in calculating Fe~ is based on the assumption of constant surface potential model, which corresponds to constant Stern potential in the present study. The Stern potential calculated as a function of separation distance under the assumption of constant surface potential showed that the dependence of the Stern potential on distance is less distinct when Kd >~ 1 (K is the Debye reciprocal length parameter, d is half the distance of separation), particularly in the presence of specific adsorption (36). It is difficult to analyze the double layer interaction on the basis of the surface regulation model (37), because no data are available on the charging behavior at the mercury and bubble surfaces with respect to the separation distance. However, it should be mentioned that the constant surface potential model gives the lowest value Journal of Colloid and Interface Science, Vol. 139, No. 1, O c t o b e r 1, 1990

38

DAI

ET

of F~ as compared to other models such as the surface charge constant model and mixed model, i.e., constant surface potential-constant surface charge model. In other words, if F~I were calculated using the other double layer interaction model /'el, Ft,max would give a higher force barrier. Thus, the high value of Ft,max shown in Figs. 10 and 11 cannot be diminished only by making modifications on either F¢~ or Fvw. An additional attractive force must be incorporated to account for the attachment results. A strong attractive force, i.e., hydrophobic force between two hydrophobized mica surfaces has been demonstrated by Israelachvili and Pashley (6) and some other investigators (7-11 ). The existence of such a force in mercury-SDS aqueous solution-bubble system was also proposed in the previous paper (17). The hydrophobicity or hydrophilicity of mercury surfaces depends on the adsorption behaviors of surfactant ions which can be known from double layer capacity measurements. Figure 12 illustrates a typical differential capacity-polarization potential curve obtained in a concentrated SDS aqueous solution in the presence of concentrated Na2SO4 =

- - - - - No2SO4 --

>, L 13

No2SO4* SD S SDS anion

~)

cation

>,

p, \,~

Desorption

,Tt5

(a)

010

-01.5

-1!0

1 2

(b)

(+)

3

6

-115 E,V (vs. SCE)

solution

(-)

Hg

FIG. 12. (a) Differentialcapacity as a function of polarization potential E (volt vs SCE). (b) Schematic representation of SDS ions adsorbedat the mercury/aqueous solution interfaces. Journal of Colloid and Interface Science, Vol. 139, No. 1, October 1, 1990

AL.

30

'

'

i

.

'

,

,

i

. . . .

t

. . . .

I

O.01M NaCIO4

p,o.

~\x ~t X.

,/',\ _

/

\NaCI04

~ 6x10-6 • 3xt0 -6

20

7_0 ~_ 10 0.0

-0.5

-1.0

-1.5

Polorization potential vs. SCE,V

FIG. 13. Differentialcapacities measured on SME for 0.01 M NaCIO4aqueous solution in the presenceof SDS. Arrow representsthe PZC of mercury in base solution.

together with the schematic adsorption models. According to Eda (38), at negative E D S ions adsorb on mercury with negatively charged head groups toward the solution but begin to desorb when mercury is more negatively polarized as indicated by peak c (desorption peak) in Fig. 12a. While at positive E the orientations of D S - ions are reversed with hydrocarbon chains turned to the solution, which is reflected by peak b, the reorientation peak, thus making mercury hydrophobic. Figure 13 gives the capacity vs E curves obtained in the present study. The arrow indicates the PZC and the vertical dashed lines show the potential range within which the attachment takes place spontaneously. Though the capacity peaks become less distinct because of the low SDS concentration, it may be considered that mercury is made hydrophilic when E is more negative than - 0 . 5 V and begins to be hydrophobized when E becomes less negative than about - 0 . 5 V. In view of the above, the hydrophobic force may be responsible for the additional attractive force although the surface coverage of the surfactant is far below the monolayer which is the case of Israelachvili and other researchers (6, 7). Thus Ft becomes Ft = Eel + Fvw + Fh,

[5]

where Fh is the hydrophobic force. Fh can be calculated as a function of monolayer-tomonolayer distance D by the equation

MERCURY-BUBBLE ATTACHMENT Fh = - - ( 1 × 108 C J D o )

× e x p ( - D / D o ) dyne cm -2.

[6]

Equation [6] was derived from the force law measured by Israelachvili and Pashley for partially hydrophobic CTAB monolayers with an advancing contact angle a o f 65 ° (6). Here the force constant Ch = 22.3 dyne cm -~ (corresponding to 0.14 N m - l / 2 ~ - in Ref. (6)), and decay length Do = 10 A were assumed (6). Pashley et al. (7) obtained the force law for fully hydrophobic monolayers ( a = 95 °) of dihexadecyldimethylammonium acetate ( D H D A A ) , in which they adopted Ch = 56 erg c m - 2 and Do = 14 A. Also, Claesson et al. (8) performed the same measurement for dimethyldioctadecylammonium ions ( D D O A + ) monolayer (o~ = 94 °) and obtained a force law composed of two terms, each having the same form as Eq. [6], with Ch and Do of the first term being 57 erg cm -2 and 12 A, respectively. Their results indicate that the strength of the hydrophobic force, reflected by the force constant Ch, is strongly related to the degree of surface hydrophobicity, while Do seems to depend on the nature of the medium. To estimate the contributions of Fh to Ft under various conditions of surface coverages of D S - ions, Eq. [6] was also used. It leaves room for argument whether Eq. [6] derived in a homointeraction system could be applied in heterointeraction system. As there are little studies focused on the dependence of Fh on the surface hydrophobicity, it is worth trying to find a little more quantitative information about how Fh is influenced by the hydrophobicity. In using Eq. [6] Ch was regarded as a variable and Do is set to be 14 ~. Hence the equation is expressed as

× exp[-(D-

surfaces were assumed to be covered by a layer of vertically oriented hydrocarbon chains. The effect of Fh on Ft at the critical conditions for spontaneous attachment, i.e., E = - 0 . 4 5 V and 5 × 10 -6 M SDS, is shown in Fig. 14. The force constant Ch was determined in such a way that Ft . . . . in Fig. 14a was lowered to the critical force barrier (5 × l0 s dyne cm -2, Fig. 14b) and was found to be 3.0 dyne cm -J Such calculations of Ch were carried out for other SDS concentrations at E = - 0 . 4 5 V and are plotted against the surface coverage of D S ion, 0, at the mercury surface as shown in Fig. 15. Here 20 A2, instead of 18.5 A2, was used in calculating 0. The value of 20 A2 is well known, from experiments of insoluble monolayers on the G / L interface, to be an effective area occupied by one straight chained hydrocarbon when packed, while 18.5 ~2 is a theoretical one representing the absolute crosssectional area of hydrocarbon chains, which is adequate for the calculation of volume fraction of hydrocarbon in determining the composite H a m a k e r constant of adsorption layer. The reasons why only 0 of the M / L interface was considered are, ( 1 ) 0 of the G / L interface is very low as compared with that of the M / L interface and (2) hydrocarbon chains adsorbing on the bubble are inside the gas 40

.....

11.6)/14]

[7]

in which 11.6 is the length of hydrocarbon chain of D S - ion (39) and D is now the surface-to-surface separation distance, in which

~=:0'dv'

,20

QO1MNoCIO4*SxlO-61~ SOS

30t F

t=Fel*F~w Fet

!o co~

50

Fh = --(1 × 108Ch/14)

39

J I

X~ 150 50 SeporQtion distonce,

/

(b) 100

150

FIG. 14. Electrostatic force (Fe~), van der Waals Force (F,~) and total force (Ft) of interaction between mercury and bubble as a function of separation distance, in which force of hydrophobic interaction (Fh) is not (a) and is (b) incorporated into the calculation of Ft (E = -0.45 V; 5 × 10-6 M SDS). Journal of Colloid and Interface Science, Vol. 139, No. I, October 1, 1990

40

DAI ET AL 12 10 8

~s (.)2 4

2

. . . . I 0.01M NoCIO4 E=-0.45V

'

,

'

6 • 5 4 • //

// Z , '

. . . .

• ?

6. IX10 "5 7. 3x10"5 8. 5xl0"5

~ d "~" I 0(~ ' '

I

8

[SDS]. M 1. 2x10"6 2. 3x10"6 3. 5x10-6 4. (i~10-6 5. 7x10"6

9. f "

'

Ch= :~ .0 i/

3 I 10

. . . .

Surfoce

I 20

covero0e

. . . .

I 30

'

of S D S e . %

FIG. 15. Variation o f critical force constant Ch with the

surface coverageof SDS at Hg/solution interface.

phase and hence are thought to be unable to affect the structure of water molecules to cause a structural force. It should be noted that it is only the critical condition for spontaneous attachment (point No. 3 indicated by the arrow in Fig. 15 ) which allows Ch. Points other than No. 3 give apparent values of Ch with which the force (Fh) calculated using Eq. [7] lowers Ft,max to the critical force barrier. It was assumed that at low adsorption densities, say below 20%, the force constant Ch is proportional to the surface coverage O as represented by the dashed line drawn through the origin and the point No. 3. Then, it is seen that the plotted points to the left of point No. 3 lie near or slightly below the line whereas those to the right of point No. 3 lie above the line. This means that below the SDS concentration of 5 × 10 -6 M the hydrophobic force exceeds the force that is needed to lower the Ft, maxto the critical value (5 × l0 s dyne cm -2) and hence spontaneous attachment occurred. On the other hand, above 5 × 10 -6 M the plotted points tend to deviate progressively from the dashed line as 8 increases. This means that some forces in excess of Fh are needed in order to lower Ft, m~xto the critical value, which makes the spontaneous attachment difficult. Ft, m~x was again plotted as a function of E for various SDS concentrations in Fig. 16, in which Fh was incorporated using Ch = (3/1 1 )8 as shown in Fig. 15. The broken lines show Journal of Colloid and Interface Science. Vol. 139, No. 1, October 1. 1990

the critical potentials for the spontaneous attachment. The horizontal axes locate the critical barrier, 5 × l0 5 dyne cm -2, an Ft,max higher than that indicating no spontaneous attachment. It can be seen that, except for the case of low SDS concentration, within the polarization range o f - 0 . 2 to - 0 . 6 V, the values o f Ft,max c a n be compared with the attachment behavior. Fh at SDS concentration higher than 1 × 10-5 M has not been calculated because no reliable Ch -- 0 relation can be obtained. Extrapolation to 100% 8 yielded a Ch at 27.3, about half of the 56 obtained by Pashley, which seems to suggest that the Ch -- 0 relation i s n o t linear over the whole range of 0 but concave. Also, in the above analyses, 5 × 105 dyne cm-2 was adopted for a critical barrier and vertical orientation of adsorbed D S - ions was assumed. The evaluation Of Ch was made by using different values of critical force barrier and different thicknesses of adsorption layer separately, and it was found that both factors do not affect the conclusion stated above except that the points of Fig. 15 shift upward or downward depending upon the factors selected

0.01M N o e l 0 4 [SDS], M lx10-5 ,

,

,

I

. A :'::+

,

1°I 5110-6 .

~ .

,

t

~10-6

i

uS" 10 I •

5

J

r ,

; ,

~ iI

,

~

,

i I

. 0.0 Potarization

,i

,

,

I

,

I

2x10-6 , lx10-6

.i .... -0.5 potential

-l.O vs. SCE, V

FIG. 16. Force barrier as a function o f the polarization

potential (volt vs SCE) at various SDS concentrations. NaCIO4:0.01 M (Ft = Fel + F,~ + Fh).

MERCURY-BUBBLE ATTACHMENT with the same trend with respect to the abscissa. In the cathodic branch (more negative than - 0 . 6 V), there may be an additional hydrophilic repulsion, rather than the attractive hydrophobic force, between mercury and bubble resulting from the adsorption of D S - ion with the polar group out-oriented on the mercury surface. This hydrophilic force together with the repulsive double-layer force and van der Waais force may hinder the mercury-bubble attachment. It is apparent from Fig. 16 that there is a contradiction between the results obtained experimentally and theoretically when E is more positive than - 0 . 3 ~ - 0 . 4 V, where Eel turned to be attractive and, at the same time, the mercury surface is expected to be more hydrophobized (Figs. 12, 13). Nevertheless no attachment was observed in this region. Similar abnormal behavior at anodic branch was reported in KF solutions and it was attributed to the surface viscous effect resulting from the accumulation of F - ions near a positively polarized mercury surface (16). Perhaps the same cause may be considered in the present case because SDS content is fairly dilute. Huhnerfuss (40, 41) measured the surface viscosity in the presence of monomolecular surface films and showed that hydrophobic interactions between the film-forming molecules (long chain alcohols) and the adjacent water layer gave rise to a reduction in mobility of the water molecules within the vicinal water layer and thus to an increase in surface viscosity. Nakamura et al. (42) pointed out that the activation energy of thinning of the dimple formed between an air bubble and a glass plate increased with increasing concentration of SDS and they regarded the increment as the activation energy of the surface flow. In this way, attachment can be hindered by obstructing the drainage of the intervening thin film. Besides, it may be considered that the bilayer adsorption of DS- ions takes place, whereby the mercury surface is made hydrophilic. Harwell et al. (43) pointed out a possibility of the forming of the second layer on top of the first

41

almost simultaneously as the monolayer formed patchwise on the surface. It is probable for DS- ions to have such a tendency to aggregate each other because of the strong hydrophobic interaction between long alkyl chains even at sparse surface coverage. Especially when the Stern potential becomes negative at the anodic branch caused by the specific adsorption of D S - ions as shown in Fig. 6 at higher SDS contents, it is energetically favorable that D S - ions adsorb on top of the ions previously adsorbed by hydrocarbon chain association rather than adsorb directly on the mercury surface although the surface potential E is positive. The true reasons of the abnormal behaviors of mercury-bubble attachment at the anodic branch need to be further investigated. CONCLUSIONS The range of the polarization potential E (volt vs SCE) of the mercury electrode at which attachment between a mercury droplet and an argon gas bubble takes place with no induction time was about - 0 . 3 ~ - 0 . 6 V, when SDS concentration is less than 5 X 10-6 M in 0.01 M NaCIO4 aqueous solutions. At SDS concentrations higher than 5 × 10-6 M no spontaneous attachment was observed. Under the conditions of spontaneous attachment, the Stern potentials of both mercury and bubble were negative and the surface coverages of D S - ions on their surfaces were less than 10% and 2%, respectively, assuming a cross-sectional area ofalkyl chain of 18.5 ~2. Analyses in terms of the heterocoagulation theory suggest that at E ~ - 0 . 3 ~ - 0 . 6 V an additional attractive hydrophobic force exists between the mercury and bubble, and the attachment behavior depends on the balance between the attractive hydrophobic force and the repulsive double-layer force and van der Waals force. The hydrophobic force becomes stronger with increasing surface coverage of the surfactant ion. No spontaneous attachment at E more negative than ~ - 0 . 6 V was explained by Journal of Colloid and Interface Science, Vol. 139, No, 1, October I, 1990

42

DAI ET AL.

considering the repulsive hydrophilic force due to the adsorption of the DS- ion with the polar group oriented outward, in addition to the double layer force and van der Waals force.

16. Usui, S., Sasaki, H., and Hasegawa, F., Colloids Surf. 18, 53 (1986). 17. Kaisheva, M., Usui, S., and Dai, Q., Colloids Surf 29, 147 (1988). 18. Broadhead, D. E., Hansen, R. S., and Potter, G. W., J. Colloid Interface Sci. 31, 61 (1969). 19. Smolders, C. A., and Duyvis, E. M., Rec. Tray. Chim. ACKNOWLEDGMENT 80, 635 (1961). The authors thank Dr. Katsuo Takahashi, at the Institute 20. Derjaguin, B. V., Discussion Faraday Soc. 18, 85 (1954). of Physical and Chemical Research, Japan, for his valuable 21. Devereux, O. F., and de Bruyn, P. L., "Interaction of discussion in double layer capacity measurements. Plane-Parallel Double Layers," MIT Press, Cambridge, MA, 1963. REFERENCES 22. Hogg, R., Healy, T. W., and Fuerstenau, D. W., Trans. Faraday Soc. 62, 1638 (1966). 1. Derjaguin, B. V., and Dukhin, S. S., Trans. Inst. Min. 23. Leja, J., "Surface Chemistry of Froth Flotation," p. Metall. 70, 221 (1969). 421, Plenum Press, New York, 1982. 2. Derjaguin, B. V., and Shukakidse, N. D., Trans. Inst. 24. Grahame, D. C., Chem. Rev. 41,441 (1947). Min. Metall. 70, 569 ( 1961 ). 25. Collins, G. L., Motarjemi, M., and Jameson, G. J., J. 3. Somasundaran, P., Chandar, P., and Chaff, K., ColColloid Interface Sci. 63, 69 (1978). loids Surf. 8, 121 (1983). 26. Usui, S., and Sasaki, H., J. Colloid Interface Sci. 65, 4. Sonntag, H., and Strenge, K., "Coagulation and Sta36 (1978). bility of Disperse Systems," p. 20, Israel Program 27. Sharovarnikov, A. F., Kolloidn. Zh. 46, 97 (1984). for Scientific Translations, Jerusalem, 1972. 28. Sharovarnikov, A. F., KoUoidn. Zh. 46, 191 (1984). 5. Blake, T. D., and Kitchener, J. A., Trans. Faraday 29. Usui, S., Sasaki, H., and Matsukawa, H., J. Colloid Soc. 1 68, 1435 (1972). Interface Sci. 81, 80 ( 1981 ). 6. Israelachvili, J. N., and Pashley, R. M., Nature (Lon- 30. Sotskova, T. Z., Poberezhnyi, V. Ya., Bazhenov, don) 300, 341 (1982); J. Colloid Interface Sci. 98, Yu. F., and Kul'skii, L. A., Kolloidn. Zh. 45, 108 500 (1984). (1983). 7. Pashley, R. M., McGuiggan, P. M., and Ninham, 31. Vold, M. J.,J. ColloidSci. 16, 1 (1961). B. W., Science 229, 1088 (1985). 32. Hamaker, H. C., Physica 4, 1058 (1937). 8. Claesson, P. M., Blom, C. E., Herder, P. C., and Nin- 33. Vincent, B., J. Colloidlnterface Sci. 42, 270 (1973). ham, B. W., J. Colloid Interface Sci. 114, 234 34. Devanathan, M. A. V., and Tilak, B. V. L. S. R. A., (1986). Chem. Rev. 65, 635 (1965). 9. Claesson, P. M., Herder, P. C., Blom, C. E., and Nin- 35. Dzyaloshinskii, I. E., Lifshitz, E. M., and Pitaevskii, ham, B. W., J. Colloid Interface Sci. 118, 68 L. P., Adv. Phys. 10, 165 (1961). (1987). 36. Usui, S., J. Colloid Interface Sci. 97, 247 (1984). 10. Rabinovich, Y. I., and Derjaguin, B. V., Kolloidn. 37. Chan, D., Perram, J. W., White, L. R., and Healy, Zh. 49, 682 (1987). T. W., J. Chem. Soc. Faraday Trans. 1, 1046 11. Claesson, P. M., and Christenson, H. K., J. Phys. (1975). Chem. 92, 1650 (1988). 38. Eda, K., Nippon Kagaku Zasshi 80, 349 (1959). 12. (a) Israelachvili, J. N., "Intermolecular and Surface 39. Tanford, C., J. Phys. Chem. 78, 2469 (1974). Forces," p. 105, Academic Press, London, 1985; 40. Huhnerfuss, H., J. Colloid Interface Sci. 107, 84 (b) p. 152. ( 1985 ). 13. Watanabe, A., and Gotoh, R., Kolloid Z. 191, 36 41. Huhnerfuss, H., J. Colloid Interface Sci. 120, 281 (1963). (1987). 14. Usui, S., Yamasaki, T., and Simoiizaka, J., J. Phys. 42. Nakamura, M., Hara, M., and Uchida, K., Z Colloid Chem. 71, 3195 (1967). Interface Sci. 123, 317 (1988). 15. Usui, S., Yamasaki, T., J. Colloid Interface Sci. 29, 43. Harwell, J. H., Hoskinns, J. C., Schechter, R. S., and 629 (1969). Wade, W. H., Langmuir 1,251 (1985).

Journal of Colloid and Interface Science. Vol. 139, No, 1, October 1, 1990