Langmuir 1987, 3, 815-820 Plasma-TreatedPolypropylene (PPP) and Polyethylene (PPE). Polypropylene (PP; 'Il6-in. sheet, AIN Plastics Inc., Mount Vernon, NY) and unextracted PE-H were treated with an oxygen plasma in a Harrick Model PDC-23G plasma cleaner. The samples were treated for 10 and 90 min for PPE and PPP, respectively, at the medium setting, at an oxygen pressure of 200
815
Torr. The samples were rinsed in ethanol twice and allowed to dry in air. Untreated PP had Oa = 116' (pH 1);this value was reduced to 67' after plasma treatment. PECH had a contact angle of 103', which was reduced to 53'.
Registry No. PE, 9002-88-4.
Adsorption at Solid/Solution Interfaces. 3. Surface Charge and Potential of Colloidal Hematite+,$ Paul Hesleitner, Darko BabiC,I Nikola Kallay,l and Egon MatijeviE* Department of Chemistry and Institute of Colloid and Surface Science, Clarkson University, Potsdam, New York 13676 Received March 11, 1986. I n Final Form: December 26, 1986 The adsorption of potential-determining ions on monodispersed spherical hematite particles was measured by potentiometric titrations a t various concentrations of NaN03. Ultrasonic energy was applied during titrations to maintain an uncoagulated dispersion, and a detectable decrease in surface charge density was found when compared to values obtained with mechanically stirred samples. An analysis was developed which made it possible t o obtain the point of zero charge and the surface equilibrium constants directly from experimental data for individual hematite titrations.
Introduction Potentiometric titrations are most commonly used to characterize surface charge properties of dispersed solids. Although t h e procedure appears simple, neither the technique nor t h e interpretation of data is trivial. Experimentally, t h e inaccuracy increases considerably i n t h e range of high a n d low activity of potential-determining ions. I n order t o arrive at a correct evaluation of t h e surface charge per unit area, t h e dispersed matter should be well-defined in terms of particle size and shape (preferably spherical) and free of extraneous ions and t h e point of zero charge (pzc) should be known. In t h e course of t h e titration process t h e particles obtain low charges a n d aggregate, possibly causing a change in t h e available surface area. Thus, t h e effect of coagulation on t h e estimation of surface charge density should be taken into consideration. Commonly, t h e interpretation of ionic adsorption is based on surface complexation or site binding models.' These approaches require t h e knowledge of t h e specific surface equilibria a n d of t h e total number of surface sites. I n addition, the function relating surface potential to surface charge depends on t h e chosen double-layer model. One purpose of this work is t o address some of t h e problems by using a hematite dispersion of spherical particles of narrow size distribution, carefully purified t o remove any anions (e.g., C1-) t h a t affect t h e surface charge. Titrations were performed without a n d with t h e application of ultrasound. I n t h e latter case vigorous agitation kept t h e hematite particles dispersed over t h e entire pH range studied. To obtain t h e best value of the pzc, several methods were employed. In t h e interpretation of data two extreme cases were considered with respect to t h e electric double layer, i.e., t h e diffuse-layer a n d constant-capacitance models. Supported by NSF Grant No. CHE-83 18196. *Supported in part by a Hercules, Inc., Fellowship. Laboratory of Physical Chemistry, Faculty of Science, University of Zagreb, Zagreb, Yugoslavia.
T h e magnitudes of essential parameters were evaluated, including t h e surface potential dependence on the pH of t h e suspending solution.
Experimental Section Materials. Stock solutions of sodium hydroxide and nitric acid were prepared from Baker Dilut-it reagents. The base was standardized by potentiometric titration with potassium hydrogen phthalate and stored in a glovebox containing an overpressure of argon. This solution was then used to determine the nitric acid concentration. The ionic strength was adjusted with sodium nitrate. Doubly distilled water, used in preparation of all solutions, was kept free of COz by storage under argon. The hematite dispersion was produced by forced hydrolysis mol dm-, HCl) solutions of iron(II1) chloride of acidified (1X (1.5 X lo-, mol dm-3 FeCl,) and aged at 100 "C for 24 h.233 The modal diameter of hematite particles prepared as such was 0.12 pm with a standard deviation of 0.02 pm. The specific surface area, as determined by BET, was 13.2 m2 g-l, which agrees with the geometric area calculated by using the above-mentioned diameter of spheres and a density of 3.8 g cm-,. This density is somewhat less than that for crystalline hematite, which is not surprising in view of the hydrated state of the suspended particles. In calculations to follow the BET area was used. Electrophoretic measurements were carried out with a Rank Brothers Mark I1 apparatus. In calculating {potentials Henry's equation was applied. In view of qualitative comparisons of these potentials, no more sophisticated corrections seemed necessary. It is well-known that iron oxides prepared in the presence of chloride ions incorporate varying amounts of this anion, which affect the particle surface charge properties. For this reason it is necessary to carefully purify the dispersed solids before any characterization is carried out. It was shown that electrophoretic measurements can be used for detecting surface contamination by the chloride ion. Figure 1illustrates the shift in the mobility as a function of pH with repeated washing of a hematite dispersion. The solid curve, typical of purified particles, shows an isoelectric point (iep) of 7.1 and is symmetrical around it. With (1) Davis, J. A.; James, R. 0.; Leckie, J. 0. J. Colloid Interface Sci. 1978, 63, 480.
( 2 ) MatijeviE, E.; Scheiner, P. J . Colloid Interface Sci. 1978, 63, 509. (3) MatijeviE, E. Annu. Reu. Materials Sci. 1985,15, 483.
0743-746318712403-0815$01.50/0 0 1987 American Chemical Society
Hesleitner et al.
816 Langmuir, Vol. 3, No. 5, 1987
---
PH Figure 1. Electrophoretic mobility of spherical hematite (aFe20 ) particles (0.12 wm in diameter) as a function of pH a t 1 X 10-\ mol dm-3 NaN03: completely cleaned (35 washing cycles) (-), after 20 washing cycles (- - -), and freshly prepared (-e).
extended storage of the sol, particle mobility changes again, indicating the diffusion of chloride ions from the solid bulk into the interface. On further washing, the anions could be again removed and the mobility restored to that of pure hematite. The leaching procedure consisted of adding a 4 mol dmT3NaOH solution until the p H was >12 while the system was being treated with an ultrasound probe (80 W) for 5 min. Afterward, the coagulated dispersion was permitted to settle, the clear supernatant solution was removed, and the solids were redispersed with the same basic solution. The procedure was repeated as many times as necessary until the electrophoretic measurements indicated a pure system; up to 35 washings may be required. The final stock dispersion of pH 8, contained in polyethylene bottles, was stored in a glovebox under argon to avoid contamination with carbon dioxide. Hematite particles used in the titrations were never dried or calcined. Titrations. The titration cell was jacketed to maintain constant temperature by passing water from a bath and was covered with an airtight lid. The latter was fitted with ports for two Fisher universal glass electrodes and an electrode bridge containing a Fisher silver-silver chloride reference electrode, a gas inlet/outlet, a thermometer, a Manostat micropipet, and a Branson ultrasound probe (Model 165). The reference electrode bridge had a pinhole junction to minimize the suspension effect, and it was filled with a sodium nitrate solution of the same concentration as the solution being titrated. The initial pH of all systems was adjusted by the addition of a known amount of nitric acid. Before titration was started, the acidified hematite suspension (8.9 g dm-3) was agitated by means of an ultrasound probe (80 W) for 5 min to peptize any aggregates. Readings of the electromotive force were taken 2 min after each addition of a small amount of sodium hydroxide. Electrodes were calibrated by titrations of sodium nitrate-nitric acid solutions of desired concentrations with and without the application of ultrasound. Each run consisted of titration of the blank, then of the hematite dispersion, again of the blank and of hematite, and finally of the blank solution. Tests have shown that the electrodes were not damaged by the ultrasound, even a t the highest intensity used. Figure 2 gives plots of electromotive force against the relative volume of the titrant (NaOH) in the absence and in the presence of ultrasound (66 W) for a 1 X mol dm-3 NaNO, solution and for a hematite dispersion containing the same concentration of NaN03. From this graphical plot it would seem that the energy input had no discernible effect on the titrations; however, closer examination of titration data showed a distinct decrease in the emf. Calibration of the Electrode System. Data collected from blank titrations may be described by the Nernst equation E = E" + cy In aH+ (1) where a is the slope (in the ideal case equal to R T / n and E" is the standard electromotive force of the electrode system. An expression for aH+in terms of the added titrant volume needs to
I
I
I
I
1 I I -2 00; 05 I 1.5 2 RELATIVE VOLUME OF TITRANT (Na OH) vlV,
Figure 2. Electromotive force as a function of the titrant volume normalized at the equivalent volume of NaOH with and without 0)and of the hematite ultrasound of the blank solution (0, dispersion (v, A) both a t 1 X low2mol dm-3 NaN03. be derived. If only NaN03, HN03, and NaOH are present in the system, the condition of electroneutrality requires CHC + CNa+ - COH- - CNOB- = 0 (2) The thermodynamic equilibrium constant of water, K,", is given by K," = aH+aOH- = y2CH+COH(3) where the activity coefficient for monovalent ions, y, is expressed as log
= 0.11- A
W / (+~ z14
(4)
A is the Debye-Huckel constant and I the ionic strength (expressed in mol dm-3):
(5) V, is the initial volume of the solution, u the volume of the titrant added, co the initial concentration of NaN03, [NaOH] the concentration of the titrant, and V, the equivalent volume of the titrant:
V,[NaOH] = V0[HNO3],
(6)
The concentrations of Na+ and NO; ions in solution are given by cNa+= (cOVO + v[NaOH])/(V, CNO;
= (cove
+ U)
+ V,[NaOHI)/(Vo + u )
(7) (8)
The combination of eq 2-8 yields
4Kw"]li2) (9) Substituting eq 9 into 1 makes it possible to obtain the electromotive force as a function of the volume of the added titrant. The values of E", a , and K," that best fit the experimental data characterize the electrode system and may be used as constants for the interpretation of the corresponding titration of hematite dispersions. Titration of Hematite Dispersions. The activity of protons, uH+,in the presence of hematite particles dispersed in a NaN03 solution can be established in a manner very similar to that already described for the blanks. In treating the titration data the adsorption of ions by the charged particles must be taken into consideration.
Adsorption at SobidlSolution Interfaces
Langmuir, Vol. 3, No. 5, 1987 817
The mass balance of ions in solution is expressed as = CN,+(tOtal) - CNa+(adS)=
CN,+
coVo + u[NaOHl
vn + u
and combining with eq 20-22 yield
- CN,+(adS) (10)
where CNa+(adS)indicates the concentration of Na+ ions located in the diffuse double layer or in the Stern layer (when the constant-capacitance model was used) and CNOC
=
Furthermore [CNa+(adS)- C~o,-(adS)]/ y s = - U o / F
(12)
where y is the mass concentration of hematite (g dm-3), S is the specific surface area of hematite (m2 g-l), and uo is the surface charge density on the particle. Equation 12 implies that only NO, and Na+, respectively, are compensating for the surface charge. This assumption is reasonable because at low pH the OHcounterion concentration in the double layer is negligible as compared to the NOc concentration. The same is true for cations He and Na+ in the double layer in systems at high pH. In addition, no specific adsorption was assumed. A combination of eq 2-5 and 10-12 yields
aH+= i ( y (
V,-V v, + 0 [NaOH] - ySuo/F
>' ]"3 + 4K,"
(13)
The above equation is also used to calculate the surface charge if the equivalent volume of the titrant, Ve, is known. Furthermore, the proton activity can be evaluated from surface equilibria. For hematite, the following surface reactions are considered:
+ H+; K, + EFeO- + H+; Kb
=FeOH2+ + =FeOH =FeOH
(14) (15)
where
$, is the surface potential, I' is the surface concentration, and R,
T, and F have their usual meanings. The total surface concentration of active sites is rtot
=
rFeOH
+ rFeOHz+ + rFeO-
(18)
and the surface concentrations of individual kinds of sites are given by eq 19-21: rFeOH
= rtat
1
(19)
If the relationship between uoand gois known, uomay be evaluated for each point of the titration by setting eq 13 to equal eq 23. Several approa~hes'~~-'~~J~ have been used to relate uo to $, Since all of them involve certain assumptions and approximations, two have been considered in this work: the diffuse-layer and the constant-capacitance models. In the former case the original Gouy-Chapman relationship holds (in SI units):
RT In F
q0 = 2-
[(
(8eR 20 ' 'I2
) + (*
8cRTI
+ 1)1'2]
(24)
where e is the permitivity of water. In the constant-capacitance model *o =
c UO
where C is the double-layer capacitance, which is treated as an additional adjustable parameter. Fleming14 derived an approximate analytical surface charge/potential expression for spheres that is identical to eq 24 if one substitutes his constant C as follows:
C = 2 ~ z e a / & T ( 1+ U K )
(26)
where the other symbols have the usual meaning. Computations. Nonlinear regression analysis was carried out by using the Marquardt procedure contained in a statistics package written by the SAS Institute on the IBM Model 4341 (Group 2) mainframe computer. In the evaluation of titration data, two different procedures were applied, which utilized the same equations but in different manners. In the first method, the pH, and associated quantities V, and K& were not fitted. Instead, the initial concentration of HN03, [HN03],, was assumed to be known. In the second method the product K&, which is directly related to the pHpzc and Ve, was adjusted. Method I: Blank titrations were interpreted by fitting the experimental values of electromotive force (emf) against the titrant volume (eq 1 , 4 , and 9). The value of K," was fixed at 1.0 X Equal statistical weight was given to each point. This analysis yielded best values for E o , a,and V,, which characterized the properties of the electrode system. An analogous procedure was adopted in the evaluation of titrations of hematite dispersions. The experimental values of the emf were first converted to the corresponding uH+ (with constants obtained from the blank titrations) and then used in eq 13 to obtain values of uo based on the knowledge of V,. The pH,,, for the titrations was found, and the best values of K , and Kb were computed by stepwise variation of parameters in eq 23, together
+ [ a ~ exp(F$o/Rr)/K,I + + [Kb e x p ( - F + ~ / R r ) / a ~ + ]
rFeOHo+
=
(4) Lyklema, J. Croat. Chem. Acta 1971,43, 249. (5) Parks, G. A.; DeBruyn, P. L. J. Phys. Chem. 1962, 66,967.
(6) Parks, G. A. "A Study of the Surface of Ferric Oxide in Aqueous Systems"; Ph.D. Thesis, Department of Metallurgy, MIT, 1960. (7) Onoda, G. Y.; DeBruyn, P. L. Surf. Sci. 1966, 4, 48. (8) Hazel, F.; Ayres, G. H. J. Phys. Chem. 1931, 35, 2930. (9) Breeuwsma, A.; Lyklema, J. J. Colloid Interface Sci. 1973,43,437. (10) Atkinson, R. J.; Posner, A. M.; Quirk, J. P. J. Phys. Chem. 1967, 71, 550. (11) Albrethson, A. E. "An Electrochemical Study of the Ferric Oxide-Solution Interface";Ph.D. Thesis, Department of Metallurgy, MIT, 1963. (12) Smith, G. W.; Salman, T. Can. Metall. Q. 1966,5,93. (13) Yoon, R. H.;.Salman, T.; Donnay, G. J. Colloid Interface Sci. 1979, 70, 483. (14) Fleming, B. A. J. Colloid Interface Sci. 1984, 101, 577.
Hesleitner et al.
818 Langmuir, Vol. 3, No. 5, 1987 0 2,"
0.2 0 A
I
I
0 0
HEMATITE, 2 5 T NONO, I x i o 2 mol dm'3
?E
0
E 5.1
0
% O
\
X
'
LW
0
A
n
5! v)-
19
I
NO ULTRASOUND
w
0
33
A I
66W
-
9.5
v)
2
"
W
W
0
(3
%
w
a
A%
~
4
80
W
L8
I --95
0
~
h
-O"
a
v)
-0 21
4
I
5
6
8
7
9
1-1 9 10
bo
k
v)
s
4
>:
2: I
0.c
0
V
2
-0.2
5
v)
; i
6
9
PH
PH
Figure 3. Surface excess concentration and surface charge density of spherical a-Fe O3particles (0.12 wm in diameter) as a function of pH at 1 X 10-2 mol dm-3 NaN03 in the absence of ultrasound (0) and at ultrasound intensity of 33 W (u).The surface excess and surface charge density were calculated by concentration, rex, method I.
Figure 4. The same system as in Figure 3 in the absence of ultrasound (0) and at ultrasound intensities of 33 (0) and 66 W (A). The parameters were calculated by method 11.
, HEMATITE.
26%
Table I. Capacitance Computed by Method I for Spherical Hematite Particles (0.12-wm Diameter) at 25 OC" capacitance/ (F m-? I / (mol dm-3) ow 33 w 10-3 1.04 0.72 10-2 1.73 1.35 10-1 4.04 2.81 "Fixed parameters: K,Kb = 2.88 mol m-2 (6 sites/100 A2).
X
and
rtot= 1.0 X
with the expression for the relationship between and U~ When the diffuse-layer model was used, eq 24 applied. The constantcapacitance model used eq 25 and contained an adjustable parameter, i.e., the capacitance of the double layer, C. In both cases the values of rtotand K&b were fiied. The mean pH, was used to set the value of K,Kb since pH,,, = l/z log (K&b). Method 11: Blank titrations were evaluated just as in method I. Hematite titration data (emf vs titrant volume) were fitted in one step using eq 1, 4,13, and 23 combined. The same expressions (eq 24 and 25) were used for the two different double-layer models. In contrast to method I, K,, Kb, Fbt, and V , were taken as adjustable parameters. The Marquardt curve fitting procedure included an iterative loop to evaluate the ionic strength for each experimental point (eq 5 ) .
Results Figure 3 gives the surface charge as a function of pH for the hematite dispersion of ionic strength 1 X loV2mol dm-3 in the absence and in the presence of ultrasonic power of 33 W, computed by method I. This result shows that the input of energy had a detectable effect on the adsorption equilibria. An almost identical shift of the curve is found
."4
5
6
7
8
9
10
PH
Figure 5. Surface charge density of a-Fe O3particles as a function of p H a t three ionic strengths: 1 X lo-?(-), 1 X lo-' (---), and 1 X lo-' mol dm-3 NaN03 (-). Calculated by method I1 and the constant-capacitance model.
mol dm-3. in the cases of 1 X 10-1 and 1 X Figure 4 is an analogous plot based on computational method 11, with ultrasonic power of 0, 33, and 66 W. The influence of the ionic strength (NaN03)on the same system at 33 W, as calculated by method I1 assuming the constant-capacitance model, is illustrated in Figure 5. All three curves cross at the same pH, yielding a point of zero charge of 7.2 f 0.2. The same trend is seen when method I is applied, except that the curves cross at pH 6.8 f 0.2. It should be noted that the volume fraction of hematite (>0.2%) was sufficiently low not to influence the ionic
Table 11. Parameters Computed by Method I1 for Adsorption of Potential-Determining Ions on Spherical Hematite Particles (0.12-pm Diameter)" capacitance/ I/(mol d m 3 model Ka Kb rtot/(mol m-2) (F m-? PH,z,b PHpzcC PH,E,d 10-3 C 1.3 X lo-' 1.4 X lo-" 1.5 X 10-6 1.5 7.9 4.6 x 10-9 1.0 x 10-5 7.3 5.5 x 10-7 D 10-2 C 1.5 X 10-5 3.5 x 10-10 1.5 X 10-5 1.8 7.1e 7.2 & 0.2 3.7 x 10-9 1.0 x 10-5 7.3 6.2 x 10-7 D 1.6 X lo-' 1.7 x 10-5 1.3 7.4 1.1 x 10-7 10-1 C } 7.2 3.3 x 10-9 1.0 x 10-5 7.0 D 3.0 X lo4
j::
I
In aqueous dispersion treated by ultrasound with intensity of 33 W at 25 O C . C, constant-capacitance model; D, diffuse-layer model. bpHp,, = 1 / 2 log (K,K,). 'As determined from uo vs. pH curves. d A s determined by the intersection of go vs. pH curve at different ionic strengths ,Figure 5). eNote that pH,,, = 7.1 (Figure 1).
Langmuir, Vol. 3, No. 5, 1987 819
Adsorption at SolidlSolution Interfaces
>
100
E
\
L))
b
? A
0
9
s W
I-
x
-100
-200
4
I
1
I 5
6
7
I 8
I 9
1
0
PH
Figure 6. Electrokinetic({, -) and surface ($, 0,0) potentials of a-Fe203particles as a function of pH at 1 X mol dm-3 NaN03. Dashed line corresponds to a slope of 59 mV/pH unit. The surface potentials were calculated by method I1 and either the diffuse-layer model (0, D.L.)or the constant-capacitance
model (0,C.C.).
activity coefficients. The diffuse-layer model gave analogous trends. Table I lists the capacitances for the hematite dispersion at three ionic strengths computed by method I, while Table I1 summarizes all parameters for the same systems calculated by method I1 using the two models. Although the experimental data could be fitted, it was quite obvious that the calculated individual values of equilibrium constants of surface reactions (K,and Kb) differed considerably. In contrast, reasonably consistent results were obtained for other parameters. It is noteworthy that the total surface site concentration agrees quite well for all ionic strengths, as calculated from both models (Table 11). The capacitance of 1.5 F m-* is also approximately constant. The pzc, determined by three different methods, appears rather reliable, and it does not differ from the isoelectric,point (iep) at pH 7.1 established by electrokinetic measurements. Finally, Figure 6 shows the surface potential as calculated from both models as a function of pH. The dotted line gives the Nernst plot with the slope of 59 mV drawn through the pzc. For comparison, the corresponding { potentials are shown as the solid curve. The Nernst plot is located between the surface potential curves calculated by using the two models, while the electrokineticpotentials are lower at all pH values except around the iep.
Discussion One of the aims of this work was to establish if coagulation of dispersed particles in the course of the titration experiment would have an effect on the adsorption capacity. All experimental techniques employed heretofore used only mechanical stirring for the titrated dispersions. Since the pH range always includes the pzc, coagula formed by electrically neutral particles cannot be fully dispersed by such relatively mild agitation. In order to test any double-layer theory on such systems, it is necessary to know the surface charge density on an absolute scale. It was readily observed that the application of ultrasonic energy prevented the coagulation of hematite sols used in this study. In order to properly evaluate the titration data, it was necessary to test if the ultrasound probe had any effect on the performance of the electrode system. Since consistent values of Eo and a were able to adequately
fit the experimental data, it was justified to carry out titrations of hematite dispersions in a cell provided with the ultrasound probe. Somewhat surprisingly, the surface charge on the particles at any given pH was reduced with the application of ultrasound. I t has been reported that ultrasonic waves, introduced into a colloidal suspension, produce alternating potentials of the same frequency between points separated by onehalf wavelength.l“ls This phenomenon is associated with the distortion of the diffuse layer of ions surrounding each colloidal particle. The effect has been measured with pulse-modulated ultrasonic waves at carrier frequencies of 2-1000 kHz for several aqueous colloidal systems. Acoustic waves cause a periodic displacement of the colloidal particles and of ions in the diffuse double layer. The effect in the latter case is much stronger due to the difference in respective masses and frictional coefficients. As a result, the distribution of ions of the diffuse layer about the central colloidal particles is no longer symmetrical. Since the diffuse layer has a charge opposite to that of the particle, the system acts as a dipole of periodic moment. The application of ultrasound would seemingly affect the measured capacitance of the double layer. The close-packed structure near the particle surface would certainly be disturbed since the ions can vibrate more easily than the particle, resulting in the net displacement of the ions away from the surface and, thereby, effectively decreasing the capacitance. Numerical procedures were employed in determining the surface charge, which differ from the usually adopted graphical method. To obtain the absolute value of the surface charge, it was necessary to know the equivalent volume, V,, or the initial amount of the acid (which equals V,[NaOH]), in addition to the experimentally determined parameters u and aH+(emf)(eq 13). If V , is not given with certainly or if the point of zero charge is not available, only relative values of uo can be evaluated. By comparison of the results obtained with different computational methods, it becomes apparent that great care must be taken in how the experimental data are analyzed. Method I gave a pH,,, somewhat different from that by Method 11, which is not surprising since the former assumed a value of V, whereas the latter fitted this quantity independently. In the absence of specific adsorption of extraneous ions, the isoelectric point (iep) and the point of zero charge (pzc) must be identical and can be readily obtained from electrokinetic measurements. Titration experiments independently yield the pzc from the crosspoint of the charge vs. pH curves for different ionic strengths. In this study, the pzc was also calculated from a fitted value of the electromotive force (method 11), which represents a novel approach to establishing this important quantity. The procedure yielded values in good agreement with other (15)Rutgers, A. J. Physica (Amsterdam) 1938,5, 46. (16)Hermans, J. J. Philos. Mag. 1938,26, 674. London, Sect. A 1952, (17)Booth, F.;Enderby, J. A. R o c . Phys. SOC., 65A,321. (18)Dietrick, H.;Yeager, E.; Bugosh, J.; Hovorka, F. J. Acoust. SOC. Am. 1953,25,461. (19)Miaw, H. L.“The Effect of Iron Oxide Slime Coatings on Flotation of Quartz and Other Minerals”; M.S. Thesis, Department of Metallurgy, MIT, 1957. (20) Yopps, J. A.;Fuerstenau, D. W. J.Colloid Interface Sci. 1964,19, 61. (21) Korpi, G. K. “Measurement of Streaming Potentials”; M.S. Thesis, Department of Metallurgy, MIT, 1960. (22)Penners, N. H.G. “The Preparation and Stability of Homodis-
perse Colloidal Haematite”;Ph.D. Thesis, Agricultural University Wageningen, The Netherlands, 1985.
820 Langmuir, Vol. 3, No. 5, 1987 determinations (Table 11). The points of zero charge ascertained from equilibrium constants show some scatter, but the average does not deviate much from the values established by other methods. The agreement with the iep clearly indicates the absence of specific adsorption of either sodium or nitrate ions, and it confirms that the cleaning procedure to remove chloride ions was efficient. The discrepancies in values of equilibrium constants (Table 11) are due to the computational procedure in solving eq 23 and either eq 24 or 25. In the case of the constant-capacitance model, four parameters were fitted simultaneously (Ka,Kb, C, and rtot). Thus, the K values are directly related to the corresponding rtot and C and consequently must be considered together. The diffuselayer model uses one less parameter, and the deviation in does not much affect the values of K is smaller. Since rtot product K,Kb, the calculated pzc’s are more consistent than the corresponding individual K values. Both models yield a reasonable total surface site conwhich varies between 1.0 X loW5 and 1.7 centration, rtot, X mol m-2 (corresponding to 6-10 sites/100 A2). This quantity is essential in the charge characterization of the dispersed particles. The two models used in calculations of the critical parameters represent extreme physical situations, yet both fit the experimental data. Consequently, the latter cannot be used to decide on an a priori given model. It is reasonable to expect that the real case should lie between these two extremes. In view of the complexity of interfacial structure it makes little sense to introduce more elaborate schemes (Stern model, triple-layer model, etc.) in an attempt to refine the interpretation of data; there are simply too many adjustable parameters that cannot be separately evaluated. Figure 6 corroborates the above conclusions; the linear relationship corresponding to the Nernst slope of 59 mV drawn through the pzc is located in the region of true potentials. The deviation between the Nernst potentials and those calculated from the experimental data becomes larger as the pH is further removed from the iep. This effect is to be expected, if one considers the increasing surface coverage by potential-determining ions. It should be noted that the diffuse-layer model yields a slope around the pzc that is considerably larger than 59 mV/pH, which would indicate this treatment to be inadequate. The electrokinetic potentials as a function of pH show consistently lower absolute values than the surface potential, as they should. It is obvious that equating l potentials with surface potentials in any interpretation of interactions involving charged solids (coagulation, heterocoagulation, peptization, adhesion) may at best represent crude approximations and at worst lead to erroneous conclusions. It should be noted that the discrepancies between f and surface potentials increase with rising electrolyte concentration, which is particularly important in studies of electrolytic coagulation. Table I11 summarizes the literature data on the points of zero charge and isoelectric points of hematite particles. It is noteworthy that the majority of samples prepared by precipitation exhibit a similar pzc or iep (8.0-8.8) while natural samples are characterized by showing considerable scatter (2.2-6.7). This difference is to be expected in view
Hesleitner et al. Table 111. Summary of Data on the Point of Zero Charge (pzc) and the Isoelectric Point (iep) of Colloidal Hematite Samples either Prepared by Precipitation or Obtained from Natural Sources PHPZC
8.5
PHiep ref PH,,, PHiep Precipitated Hematite 5 8.68 8.0, 8.7 5 8.68
8.4, 9.03 8.3 8.4
6 7
9.0 8.0 8.3 9.04 9.5 6.0-6.6 8.3
7
7.2, 8.3 8.6
8.5 8.6, 8.9, 9.3 8.7, 8.8 8.7
7 8 9 10 11 11
8.5 8.0
ref 12 12
13 19 20 21 22 2
23 24 24
Natural Hematite
4.2 6.5, 6.7 5.3-6.0 5.4. 5.7
25 26 27
4.5-5.0 6.7 6.9
28 29
2.2
6.9
17
of anionic contaminants present in the soil, such as phosphates, which differ from place to place. I t has been pointed out2, that hematite particles, equilibrated in a solution of pH > pHpzc,will lose protons due to leaching and thereby cause a change in the particle surface charge. The lower pzc value of the solids used in this work could not be caused by such an effect; long equilibration of hematite samples had no effect on the pzc. Most hematite dispersions used by other authors were prepared from nitrate solutions. This anion is much less strongly complexed with the ferric ion and consequently will neither be incorporated in the particles nor affect their potential. The estimate of capacitances (Table I) shows a decrease by the application of ultrasound and an increase in its value with higher ionic strength. If all countrions were in the Helmholtz layer, C should be independent of the ionic strength. In view of data in Table I, one concludes that the counterions are distributed between the fixed and diffuse double layers and that the latter is more compressed with increasing electrolyte content. The GouyChapman theory permits the calculation of the differential capacitance, whereas the average capacitance can only be estimated by a straight-line approximation in the range of surface charges of interest. The capacitances calculated as such range between 0.4 and 2.6 F m-2 as the NaNO, concentrations increase from to 10-1 mol dm-,. Obviously, these values are lower than those obtained by methods I or I1 (Tables I and 11). (23) Troelstra, S. A.; Kruyt, H. R. Kolloid 2.1942, 101, 182. (24) Johansen, P. G.; Buchanan, A. S. Aust. J. Chem. 1957, 10, 392. (25) Johansen, P. G.; Buchanan, A. S.Aust. J . Chem. 1957,10, 398. (26) Ahmed, S. M.; Maksimov, D. Can. J . Chem. 1968, 46, 3841. (27) Joy, A. S.; Watson, D. Bull. Inst. Min. Metall. 1964, 687, 323. (28) Chwastiak, S. In “Adaptation of New Research Techniques to
Mineral Engineering Problems”; progress report to USAEC NYO-10, October, 1963, No. 31, 330 (MITS-51). (29) Iwasaki, I.; Cooke, S. R. B.; Choi, H. S. Trans. Am. Inst. Min., Metall. Pet. Eng. 1960, 217, 237. (30) Schuylenborgh,J.; Sanger, A. M. H. R e d . Trau. Chim. Pays-Bas 1948, 68, 999
