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Flocculation Studied with the ESA Technique
Journal of Colloid and Interface Science 217, 49 –59 (1999) Article ID jcis.1999.6119, available online at http://www.idealibrary.com on
Flocculation Studied with the ESA Technique Charlotte Walldal Department of Physical Chemistry, Go¨teborg University, S-412 96, Go¨teborg, Sweden Received September 8, 1998; accepted January 28, 1999
to titrate a particle dispersion with a polymer solution. A disadvantage is that at high polymer concentrations the measuring probe has to be cleaned during the experiment to remove adsorbed flocs. This paper is about flocculation between colloidal silicic acid and cationic polyacrylamide or cationic polyamine. The silica dispersion was also flocculated with a positively charged surfactant, CTAB (cetyltrimethylammonium bromide). The ESA technique has earlier been used to study adsorption of surfactants and polymers onto colloidal particles. In his thesis, Miller (4) compared zeta potentials, obtained with the more recent electroacoustic technique and traditional microelectrophoresis, for titania and titania–polyvinyl alcohol. It was found that the zeta potentials measured with the two techniques were directly comparable provided that the inertial correction factor was calculated independently for the electroacoustic measurement. Carasso (5) studied the effects of adsorbed surfactants, polymers, and proteins on the electroacoustic behavior of colloidal silica particles. The Acoustosizer was used to measure the zeta potential, size, and dynamic mobility of the particles during the addition of polymers. Flocculation was avoided. The ESA 8000 has previously been used to study the adsorption of cetylpyridinium chloride onto kaolinite. It was found that the isoelectric point coincided with the cation exchange capacity of the clay. The surfactant appeared to adsorb in two steps, a monolayer and a bilayer (6). Bijsterbosch (7) studied adsorption of CTAB onto hydrophilic silica with adsorption, depletion, and electrophoresis. The CTA 1 isotherm was single step and Br 2 started to adsorb once a certain minimum concentration was exceeded. The adsorption of Br 2 increased the adsorption of CTA 1 compared to if the adsorption of Br 2 was prevented.
The aim of this paper is to show that the ESA (electrokinetic sonic amplitude) technique can be used to study the flocculation between cationic polyelectrolytes and negatively charged silica particles. The ESA signal was measured when cationic polyacrylamide (CPAM) or cationic polyamine (POL) was mixed with silica particles. The dynamic mobility of the formed flocs was calculated from the corrected ESA signal. The amount of nonadsorbed polyelectrolyte was determined by viscosity. The floc sediment volume was also measured. It was found that the polyacrylamide molecule was more efficient than the polyamine molecule in neutralizing the charges on the silica particles due to the longer CPAM molecules and the larger distance between the CPAM charges which was in better accordance with the distance between the silica charges compared to POL. © 1999 Academic Press Key Words: dynamic mobility; electroacoustics; flocculation; polyelectrolytes; silica particles.
1. INTRODUCTION
The ESA (electrokinetic sonic amplitude) technique has mainly been used to measure the dynamic mobility of different colloidal particles. Recently the same technique has been applied to polyelectrolytes (1, 2, 3). In this investigation it is shown that the technique can be used to study “real time” changes in the ESA signal when a flocculation process takes place. Flocculation is of great importance since it occurs in many industrial processes such as water purification, powder technology, and paper making. When a polymer adsorbs onto a solid substrate, dispersion forces, dipolar forces, hydrophobic forces, and hydrogen bonding are all possible types of interactions. In polyelectrolyte adsorption, electrostatic interactions play a very important role. These interactions mainly depend on charge densities (both polymer and substrate) and salt concentration. Since the ESA signal depends on the charge of the particles the ESA technique should be suitable to study flocculation. The main advantages with using the ESA technique are that it is not limited to dilute systems or sensitive to contamination. Along with the ESA measurements the pH and conductivity can also be measured. The shear rate of the flocculating suspension can be changed by changing the stirring rate. The measurements can be performed over a short or long period of time (from 1 s up to 167 h). It is also possible
2. THEORY
There is no theory for calculation of the dynamic mobility of particles flocculated by polymers. The closest approach is a paper called “The dynamic mobility of a porous particle” by O’Brien (8). The aim of that paper is to derive a formula for the dynamic mobility of a porous spherical floc. The mobility formulas are obtained for the case where the double layer thickness is much smaller than the pore radius and for the case 49
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CHARLOTTE WALLDAL
where the zeta potential is small (for any double layer thickness). With electroacoustic measurements over a range of frequencies it would according to this theory be possible to determine both size and zeta of the flocs. Since the ESA-8000 is limited to 0.8 –1.2 MHz it is not possible to obtain the size of the flocs with this technique. In the porous sphere theory the floc has a radius R and consists of primary particles with radius a (which also is the pore size). It is assumed that ( v a 2 )/v ! 1, where v is the angular frequency and v is the kinematic viscosity of the suspending liquid. At 25°C, 1 MHz, and a particle radius of 50 nm, ( v a 2 )/v 5 0.015; thus, the theory would apply in this study. The formula for the dynamic mobility of a rigid floc, i.e., the particles are so strongly bound together that they move with the same velocity, is
md 5
Ds 1 i vr Vs~1 2 3 f / 2! ez p Ds 1 i v Ms h
[1]
which relates the dynamic mobility to the Smoluchowski mobility. i is the imaginary number, r is the liquid density, h is the liquid viscosity, e is the permittivity, and z is the zeta potential. Vs is the volume of the floc, Vs 5 (4/3) p R 3 , and Ms is the mass of the floc, Ms 5 (r(1 2 f) 1 r pf)V s. f is the volume fraction of particles in the floc and Ds is the drag coefficient,
S
Ds 5 6 p R h 1 1 ~1 1 i!
Î
D
a a 1i p , 2 9
[2]
where a 5 ( v R 2 )/v.
FIG. 1. The chemical structures of (a) cationic polyacrylamide and (b) polyamine.
TABLE 1 The Molecular Weight, Amount of Charges/Gram and Distance between the Charges Tabled for CPAM and POL
Polymers
MW (g/mole)
Titratable charge (meq/gram)
Kinetic charge (meq/gram)
Charge separation (Å)
CPAM POL
5 3 10 6 50–200 3 10 3
1.2 6
0.31 2.4
20 7
Equations [1] and [2] also apply for a flexible floc at low frequencies if ( v aR)/v ! 1. According to these equations the dynamic mobility equals the Smoluchowski value for a solid particle of the same size and density at low frequencies or at small floc sizes. At high frequencies or large floc sizes the dynamic mobility approaches a limiting finite value in contrast to a solid particle for which the mobility tends to zero. 3. EXPERIMENTAL
3.1. Materials a. Colloidal silicic acid. The colloidal silica used was supplied by Eka Chemicals AB with a hydrodynamic diameter of 122 nm determined by a Brookhaven particle sizer. Photos taken with an electron microscope showed monodisperse spherical particles and the size was estimated to be between 100 and 120 nm. The specific surface area according to the BET method was 31.3 m 2/g (which corresponds to a particle diameter of 87 nm) and with Sear’s method (9) 25.4 m 2/g (which corresponds to a diameter of 108 nm). At pH 7 and 10 mM NaCl the silica particles had a dynamic mobility of about 22.5p10 28 m 2/Vs which was fitted using Mangelsdorf and White’s (10) computer program to be a zeta potential of 239 mV. The charge density at the slip plane was calculated to be 21.04 mC/cm 2. From the charge density the amount of charges at the slip plane was calculated (with specific surface from Sear’s method) to be 0.0027 meq/g for the particles. This will later be used in the presentation of the experimental results. Potentiometric titrations were also performed on the particles. The silica dispersion was diluted so the final weight% concentration was 0.25% when the dispersion was mixed with the polymers. The electrolyte concentration was 10 mM NaCl and pH about 7. The amount of silica used in the flocculation experiments was constant through the investigation. Milli-Q water was used in this study as a dispersion medium. b. Surfactant. CTAB was obtained from BDH Chemicals Ltd. and used as received. c. Polyelectrolytes. Two types of polyelectrolytes with different molecular weight were supplied by Eka Chemicals AB. The high weight polymer was a linear cationic polyacrylamide (CPAM) with a molecular weight of 5 3 10 6 g/mole with 10% of the acrylamide monomer exchanged for trimeth-
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
51
FIG. 2. Amount of adsorbed CPAM (■) and POL (}) charge plotted against amount of added polymers charge/silica charge.
ylaminoethyl–acrylatechloride. As a low weight polymer a branched cationic polyamine (polydimethyldiallylammoniumchloride) (POL) with a molecular weight of 50 3 10 3–200 3 10 3 g/mole was used. All molecular weights are number averages and according to the manufacturers. The structural formulas of the polyelectrolytes are shown in Fig. 1. The total charge of the polyelectrolytes was determined by titration with potassium polyvinylsulfate (11) at pH 7 and low ionic strength (,300 mS/cm). The polyamine had a surface charge of 6 meq/g and CPAM 1.2 meq/g. The dynamic mobilities of the polyelectrolytes have been determined with the ESA 8000 in a high sensitivity cell in a previous investigation (1). (The ESA signal from polyelectrolytes is too weak to be measured in the standard cell of the ESA 8000 instrument.) The mobilities thus were found to be 0.56p10 28 m 2/Vs for CPAM and 2.4p10 28 m 2/Vs for POL at 10 mM NaCl. In the same investigation a kinetic charge density is calculated by modeling the polyelectrolytes as cylinders. The kinetic charge density divided by the charge density of the cylinder gives a measure (the charge fraction) of how large a fraction of counterions is located outside the slip plane (12). To obtain the amount of charges at the slip plane, the meq/g at the surface is multiplied by the charge fraction. This gives a charge at the slip plane of 2.4 meq/g for POL and 0.31 meq/g for CPAM (1). The properties of the polymers are summarized in Table 1.
Solutions of cationic polymers were prepared with concentrations ranging from 0.0005 up to 0.01% and with 10 mM NaCl. The pH was adjusted to 7 just before the experiments. 3.2. Methods a. ESA. The ESA measurements were performed on an ESA 8000 instrument (Matec Applied Sciences). With this device it is possible to obtain the dynamic mobility and the conductivity as pH is changed. The measuring cell holds 220 ml of sample and the stirring speed can be varied between about 0 and 2500 rpm. To determine the charge on the particles, potentiometric titrations were performed at two different particle concentrations (5 and 8 weight%) using an automatic titrator. From pH 9.5 the particles were titrated down to pH 2.5 with 80 mM HCl and then up to pH 10 with 80 mM NaOH. Blank titrations were also performed so the amount of NaOH–HCl consumed by the electrolyte solution at a given pH could be subtracted. The ESA 8000 is operating at a field strength of about 100 V/cm and a field frequency of 0.8 –1.2 MHz. When the alternating field is applied the particles start to move backward and forward. As the particles move they displace an equal volume of liquid. If the density of the particles is different to the density of the liquid this will give rise to a sound wave. The ESA signal depends on the particle velocity and from that the dynamic mobility, m d, can be determined. The dynamic mo-
52
CHARLOTTE WALLDAL
FIG. 3. Sediment volume plotted against added amount of POL charge/silica charge.
bility of the particles depends on their size, zeta potential, and the frequency of the applied field. It depends on size because the particle motion at high frequencies is affected by inertia forces. For the 100 nm silica particles used in this investigation the inertia force is negligible and therefore the dynamic mobility is approximately equal in magnitude to the electrophoretic mobility for the solid silica particle. As mentioned earlier the ESA effect can also be measured for polymers but due to the low density difference between polymers and solvents a special cell with high sensitivity is required. With the standard measuring cell used in this investigation the ESA signal of the polymers cannot be distinguished from the background noise. The contribution to the measured ESA signal from free polymers can therefore be neglected. For dilute colloidal suspensions (up to approximately 5% volume fraction) the relation between the ESA effect and the dynamic mobility, m d, of the charged particles is given by (13) ESA~ v ! 5 A~ v ! f ~D r / r o! md,
[3]
where ESA is measured in Pa/(Vcm 21), A(v) is an instrument factor, f is the volume fraction of the particles, and Dr is the density difference between the particle density (r p) and that of the solvent (r o). The ESA measures the maximum in the pressure of the sound wave (Pa) per unit applied electric field strength (Vcm21).
The obtained ESA signals in the flocculation experiments were corrected by subtracting the ESA signal from the background electrolyte solution. b. Flocculation procedure. First, 110 ml of the silica suspension was mixed with 110 ml of the polyelectrolyte or the surfactant solution. Equal volumes of added particle dispersion and polyelectrolyte solution were chosen in order to obtain good mixing and to avoid high local concentrations of polymers. Two methods of mixing were used. Either the particles and the polyelectrolytes were poured together through a funnel into the ESA beaker or alternatively the polyelectrolytes were poured into the particle dispersion which was already in the beaker. Each mixing procedure took about 20 s. The stirring speed was about 1700 rpm. The ESA signal was measured over 10 min with 30 measurements/min. The results obtained were independent of the mixing method which suggested that good mixing was achieved by both methods. When high concentrations of polyelectrolytes (mainly CPAM) were used the measurement had to be stopped and the measuring probe cleaned (wiped off) to remove adsorbed material. If the probe was not cleaned, irreproducible results occurred. The pH of the solutions was maintained during the experiments by adding NaOH or HCl if needed. The charge of the silica particle is sensitive to the concentration of OH 2-ions around pH 7.
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
53
FIG. 4. Sediment volume plotted against added amount of CPAM charge/silica charge.
c. Floc size. After some of the experiments a floc sample was taken from the ESA measuring cell with a syringe partly filled with the appropriate background solution. The sample was then instantly injected in a laser diffraction instrument (Malvern 2600 particle sizer) and the floc size distribution was measured. From the volume size distribution it was possible to transform the results into number distribution using numerical transformations. The observation cell was equipped with a magnetic stirrer which prevented the flocs from sedimenting. No further flocculation was observed for samples left for several hours in the observation cell with the stirrer on. d. Adsorption isotherm. The flocculated suspension was left to sediment and the sedimented floc volume was measured. The remaining suspension was centrifuged for 30 min at 7000 rpm. Viscosity measurements were performed on the supernatant with a Cannon–Fenske capillary flow viscometer to determine the concentration of the free polymer. All samples were filtered with a millipore filter with a poresize of 2 mm before the measurements. 4. RESULTS
a. Potentiometric Titration The surface charge density was calculated using
s O 5 2F~G H1 2 G OH2! 5 F~@H 1 #!~X 2 v/mA!,
[4]
where F is the Faraday constant, G H1 and G OH2 are the adsorption densities of H 1 and OH 2, and [H 1] is the concentration of acid. X is the total amount of HCl (cm 3) added and v is the volume of acid required to change the pH of the silica dispersion by a certain amount. A is the surface area per unit mass of solids and m is the mass of the silica sample. The two different particle concentrations used (5 and 8%) gave the same result. At pH 7 and 10 mM NaCl the surface charge density was about 21.9 mC/cm 2 which corresponds to 0.005 meq/g. It is mainly the surface charge of the polyelectrolytes and particles which will be used in the presentation of the results. b. Adsorption Isotherm In Fig. 2 the amount of surface charge (in meq) of adsorbed polyelectrolytes is plotted against the ratios of the added amount of polymer surface charges to the amount of surface charges on the silica particles. The arrows indicates the points of charge reversal (from Figs. 6 and 7). The maximum adsorbed amount of polyelectrolyte charges was larger for polyamine than CPAM. The adsorption curve for polyamine was more rounded and it is uncertain if the curve has reached its plateau. The adsorption curve for polyamine starts to flatten out at an almost three times larger amount of added polyamine charges than added CPAM charges. If instead the weight amount of adsorbed polymer is compared, about three times
54
CHARLOTTE WALLDAL
FIG. 5. The corrected ESA signal is plotted against added amount of CTAB.
more CPAM was adsorbed at maximum adsorption. The arrows indicate the points of charge reversal. The sedimented floc volumes are plotted in Figs. 3 and 4 as a function of amount of added polyelectrolyte charges/silica charges. The sediment volume curve for polyamine starts at zero sediment volume at low addition of polyamine followed by a sudden increase in sediment volume which then increases slightly until it drops to a smaller volume at a large addition of polyamine. At zero and small sediment volumes the supernatants were turbid; otherwise the supernatants were clear. When a clear supernatant is observed it is assumed that all silica particles are flocculated. The sediment volume curve of CPAM and silica particles has a different shape which resembles the adsorption isotherm. The supernatant is turbid up to about a ratio of 2 and then it is alternating clear and turbid until about a ratio of 3.7 when the supernatant becomes clear. At a ratio of about 10 the supernatant becomes turbid again. Between 2– 4.5 CPAM charges/ silica charge (point A, Fig. 4) the sediment volume does not increase as more CPAM is adsorbed. c. Surfactant Adsorption In Fig. 5 the ESA signal is plotted as a function of CTAB concentration. As the CTAB concentration increases the ESA
signal decreases almost linearly. At about 0.065 mM added CTAB there is a change in the slope of the curve. d. Polyelectrolyte Flocculation In Fig. 6 the corrected ESA signal is plotted against the amount of CPAM charges/silica charge. The ESA signal becomes less negative as more CPAM is added and changes sign at 3 added polymer charges per silica charge. The total system then becomes positively charged. The ESA signal reaches a plateau value of about 0.017 Pa/Vcm 21 at 4.5 added CPAM charges/ silica charge. At low addition of CPAM the ESA signal decreases linearly in the same manner as when CTAB is adsorbed. The sediment volume increases as more CPAM is added up to about 2–3 polymer charges/silica charge (Fig. 4) where the increase is less. The ESA signal on the other hand continues to change. It seems therefore that the change in ESA signal is not affected by the formation of flocs and any inertia effect originating from the aggregation of particles to larger units can then be neglected. If a comparison is made with the adsorption isotherm (Fig. 2) it is obvious that the change in ESA signal reflects the increased adsorption of polymer. This conclusion is supported by comparing the adsorption isotherm for POL and the change in ESA signal when POL is mixed with the silica dispersion (Fig. 7). The change in ESA signal
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
55
FIG. 6. The corrected ESA signal is plotted against added amount of CPAM charge/silica charge.
when POL is used is different compared to when CPAM is used. At low addition of POL the ESA signal does not change linearly probably because the polyamine has such a high charge density and low molecular weight so the charges are less effectively used to neutralize the particle charges at low concentrations. When larger polyamine concentrations are used the ESA signal continues to decrease as more polyamine is added in agreement with the adsorption isotherm (Fig. 2). More remarkable is that the magnitude of the largest obtained positive ESA signal (0.09 Pa/Vcm 21) exceeds the negative magnitude of the pure silica particles (20.066 Pa/Vcm 21). Obviously such a large amount of polyamine is adsorbed that strongly positively charged areas with high polymer densities are formed which contribute to the positive ESA signal (14). e. Time Aspects It took about 20 s to mix the polyelectrolytes and surfactant with the silica dispersion. The measured ESA signals were constant immediately for the surfactant adsorption. For the large CPAM flocs the ESA signal was also constant at low concentrations of added CPAM. At higher concentrations of CPAM the measuring probe had to be cleaned and therefore different ESA signals are observed before and after the cleaning. In the case of the smaller POL molecules the ESA signal became constant after about 3 min for zero POL concentrations
up to the amount required for charge reversal. For higher POL concentrations a constant ESA signal was obtained immediately. DISCUSSION
The ESA measurements of CTAB (Fig. 5) showed a change in the slope of the curve when CTAB corresponding to a surface area of 35 Å 2 was added. The headgroup area of ionic surfactants is usually reported to range from 40 to 60 Å 2 per molecule (15, 16). This implies that the adsorbed amounts of CTA 1 are packed closely enough onto the silica surface for the hydrocarbon tails to interact. Hydrophobic attractions probably start to be important at the intersection and a bilayer can be formed. It is also possible that the adsorption of counterions starts due to the build-up of a bilayer at the intersection which facilitates the adsorption of CTA 1 (7). At first the CTAB molecules adsorb due to electrostatic attraction head down to the surface groups. In the first part the adsorption of CTAB onto the silica particles can be regarded as charge neutralization. In the next step the adsorption of CTAB molecules involves hydrophobic attraction between the surfactant tails. Since we are mainly interested in observing the electrostatic effects the flocculation experiments are limited to a few results when the particles are charge reversed.
56
CHARLOTTE WALLDAL
FIG. 7. The corrected ESA signal is plotted against added amount of POL charge/silica charge.
By comparing Figs. 3 and 4 it can be seen that the growing of CPAM flocs starts at a smaller amount of added CPAM charges compared to POL charges even though the adsorption isotherm (Fig. 2) clearly shows that there is an adsorption already at small amounts of added POL. In fact Fig. 2 shows that the adsorption of polyelectrolyte charges is independent of molecular weight at low concentrations. The change in ESA signal (Figs. 6 and 7) at low polymer concentrations seems therefore to be related to the onset of flocculation. By comparing Figs. 4 and 7 it is seen that an increased change of the ESA signal as larger amounts of POL are added corresponds to the formation of flocs. This critical amount of POL molecules needed to flocculate the silica dispersion is not observed for CPAM. Above 10 POL charges/silica charge, the dispersion becomes positively stabilized which is shown by the small sediment volume and the observed turbidity in Fig. 3, the change of ESA signal is again reduced. Obviously the formation of flocs gives a more effective use of the POL charges as one molecule can adsorb to more than one particle. The greater ability of CPAM to flocculate is due to the higher molecular weight and the linear structure of the molecule which gives a more extended conformation in the solution which enables CPAM to more easily bridge between particles. The maximum sediment volume obtained is larger for the POL flocs than the CPAM flocs which at first sight seems strange when the floc sizes are much larger for flocs made of
CPAM than of POL (Table 2). But there was a large density difference of the sediments. The CPAM flocs were much more closely packed than the POL flocs which seemed to float on top of each other. The large CPAM flocs are more sensitive to the stirring rate and are more easily ruptured than the smaller POL flocs which partly explains the higher density of the CPAM flocs. In general polyelectrolytes tend to adsorb in rather flat conformations. The formation of a thick polymer layer is prevented due to the electrostatic repulsion between the segments. An exception is polymers with very low charge density which form loops and tails at the surface, while polymers with higher charge density adsorb in a flat configuration. It is not possible to obtain the polymer conformation in the flocs with TABLE 2 The Floc Diameter for Different Amounts of Added CPAM and POL Measured with a Malvern 2600 Particle Sizer CPAM
POL
CPAM (meq)/ silica (meq)
Diameter (mm)
POL (meq)/ silica (meq)
Diameter (mm)
1 2.7 3.4
25 52 60
0.67 2.5 2.7
2.5 4.6 8.6
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
57
FIG. 8. The dynamic mobility divided by the Smoluchowski mobility is plotted against floc size for POL with f 5 0.026 (■), POL with f 5 0.033 (}), CPAM with f 5 0.036 (Œ), and CPAM with f 5 0.05 (✖).
the ESA technique. The measured sediment volumes could, however, give an indication of the polymer conformation. Lower floc densities are obtained for the POL flocs compared to the CPAM flocs, even though the POL flocs are much smaller. This is an indication that the POL molecules have a more extended conformation in the flocs than the CPAM molecules. An explanation could be that the collision rate between the particles is faster than the rate of attainment of adsorption equilibrium. Since the CPAM molecule is more flexible than the POL molecule due to a lower charge density and a linear structure, the CPAM molecule attains a more flat conformation in the flocs. The observed turbidity of the supernatant at high CPAM concentrations (Fig. 4) could be explained by the flocs reaching a limiting size which depends on the stirring rate. Even if more CPAM is added the flocs will not obtain a larger size due to the fact that small fractions of the flocs are torn off. Those small fractions would give a turbid supernatant. The measured ESA signal does not increase either. The observation that between 2– 4.5 CPAM charges/silica charge the sediment volume does not increase as more CPAM is added could be interpreted as the flocs becoming more compact. The ESA measurements show that about 50% more POL than CPAM charges are needed to obtain a charge reversal (Figs. 6 and 7). Together with the structural difference between
the polymers discussed earlier this can also be understood in terms of the different charge densities on the polyelectrolytes. The mean charge distance between the POL charges is about 7 Å and a rough estimation of the length between the charges at the silica surface gives 30 Å. For CPAM the distances are in better accord (about 20 Å between the CPAM charges). CPAM therefore better balances the surface charges with fewer extra charges. Dynamic Mobility There is a practical interest in calculating a dynamic mobility from the ESA signal. The dynamic mobility obtained by using O’Briens formula (Eq. [1]) is divided by the Smoluchowski mobility (h/ez) and the result is shown in Fig. 8. The volume fractions used are the ones obtained when the solution above the sediment has been nonturbid. As can be seen in Fig. 8 the change of the dimensionless mobility is larger at higher particle volume fractions, even though the difference is at maximum 9.5% in this case. For floc sizes larger than 10 mm a limiting value is reached. In Table 2 some floc diameters obtained with the Malvern 2600 are given. The floc sizes for polyamine are smaller than 10 mm which means that there is a size dependence of the mobility for these flocs whereas for the larger CPAM flocs, the mobility is affected by the floc size to
58
CHARLOTTE WALLDAL
FIG. 9. The dynamic mobility is plotted against added amounts of POL (■) and CPAM (}) charge/silica charge.
the same extent for all flocs. O’Brien’s theory only accounts for flocs with the same charge sign and then preferably before a charge reversal of the primary particles is obtained. The observation that when the total flocculating system becomes positively charged and the magnitude of the ESA signal increases at the same time as the floc size increase or remains constant has no correspondence in the porous sphere theory. Truly more theoretical studies are needed in this field. Equation [3] shows the relation between dynamic mobility and ESA signal for a particle dispersion. The results are shown in Fig. 9 where Eq. [3] is used to calculate the dynamic mobility of the silica particles flocculated with the polyelectrolytes. The results will not be discussed here since they are proportional to those in Figs. 6 and 7. The use of Eq. [3], which relates the dynamic mobility to the ESA signal for particles, to calculate the dynamic mobility of the flocs can be justified with the following discussion. The contribution to the measured ESA signal from nonadsorbed polymers can be neglected as discussed previously. The ESA signal must originate from silica particles with or without adsorbed polymers which neutralize the particle charges. The question then arises whether the bridging of polymers between the particles affects the ESA signal or not. Does the polymer bridging prevent the particles from moving in the applied field more than what is caused by them having their charges reduced?
The amplitude of the particle motion in the applied field is quite small. It is calculated to be about 2.5 Å, which can be compared to the flexibility of the polymers. It is likely that particles with the same charge reduction move the same distances in the field. If the polymers are unevenly distributed in a floc the movement of the particles with less adsorbed polymers would be hindered. The flocs probably move forward and backward as one unit in the applied field with a speed proportional to the average charge density of the flocs. The inertia forces acting on a solid particle with the same size as the flocs would reduce the ESA signal toward zero for the large CPAM flocs. On the contrary we observe an increase in the ESA signal as the flocs grow larger after the charge reversal. The change of ESA signal shows the same behavior as the adsorption isotherm. There are probably no artefacts arising from the particles being connected by polymers and Eq. [3] should be a valid way to obtain the dynamic mobility for a floc. Also according to the porous sphere theory at most only 10% of the mobility for these flocs depends on their size the rest is due to charge neutralization, which further justifies the use of Eq. [3]. All results in this study have been presented with charge ratios based on the surface charges. In the flocculation– coagulation process the attraction felt by the aggregating species is more likely controlled by the potential at the slip plane (zpotential). If instead the charge densities (for both polyelectro-
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
lyte and particles) at the slip plane are used to calculate the point of charge reversal it occurs at 1.6 CPAM charges/silica charge and 3.5 POL charges/silica charge. With this approach less polyelectrolyte is needed to neutralize the silica charges and CPAM becomes even more effective as a flocculant compared to POL. Finally it can be concluded that it is possible to use the ESA 8000 to obtain information about the dynamic mobility “in real time” for a dispersion flocculated by polyelectrolytes. However, additional information about floc sizes and polymer conformation obtained with the same technique would be of interest. This could be obtained if an electroacoustic technique measuring over a wide range of frequencies is used, although the theoretical treatment has to be improved. ACKNOWLEDGMENT Prof. Bob Hunter is acknowledged for reading and having valuable comments on this manuscript. All the materials in this investigation were supplied by Eka Chemicals AB.
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Walldal, C., and Åkerman, B., submitted. Rasmusson, M., and Åkerman, B., Langmuir 14, 3512 (1998). Larsson, A., and Rasmusson, M., Carbohydr. Res. 304, 315 (1997). Miller, N. P., “Electroacoustic and Electrophoretic Determination of Zeta Potential: Effect of a Polymer Adlayer and Particle Porosity.” University of Washington, thesis, 1992. Carasso, M. L., “The Effect of Polymer Adsorption on the Electroacoustic Signals of Colloidal Silica.” University of Sydney, thesis, 1996. Rowlands, W. N., and Hunter, R. J., Clays Miner. 40, 287 (1992). Bijsterbosch, B. H., J. Colloid Interface Sci. 47, 186 (1974). O’Brien, R. W. J., J. Colloid Interface Sci. 171, 495 (1995). Sears, G. W., J. Anal. Chem. 28, 1981 (1956). Mangelsdorf, C. S., and White, L. R., J. Chem. Soc. Faraday Trans. 88, 3576 (1992). Horn, D., Progr. Colloid Polym. Sci. 65, 251 (1978). Schellman, J. A., and Stigter, D., Biopolymers 16, 1415 (1977). O’Brien, R. W., J. Fluid Mech. 212, 81 (1990). O’Brien, R. W., Personal communication. Tanford, C., J. Phys. Chem. 76, 3020 (1972). Weiss, J., Zografi, G., and Simonelli, A.P., J. Pharm. Sci. 63, 381 (1974).
Flocculation Studied with the ESA Technique Charlotte Walldal Department of Physical Chemistry, Go¨teborg University, S-412 96, Go¨teborg, Sweden Received September 8, 1998; accepted January 28, 1999
to titrate a particle dispersion with a polymer solution. A disadvantage is that at high polymer concentrations the measuring probe has to be cleaned during the experiment to remove adsorbed flocs. This paper is about flocculation between colloidal silicic acid and cationic polyacrylamide or cationic polyamine. The silica dispersion was also flocculated with a positively charged surfactant, CTAB (cetyltrimethylammonium bromide). The ESA technique has earlier been used to study adsorption of surfactants and polymers onto colloidal particles. In his thesis, Miller (4) compared zeta potentials, obtained with the more recent electroacoustic technique and traditional microelectrophoresis, for titania and titania–polyvinyl alcohol. It was found that the zeta potentials measured with the two techniques were directly comparable provided that the inertial correction factor was calculated independently for the electroacoustic measurement. Carasso (5) studied the effects of adsorbed surfactants, polymers, and proteins on the electroacoustic behavior of colloidal silica particles. The Acoustosizer was used to measure the zeta potential, size, and dynamic mobility of the particles during the addition of polymers. Flocculation was avoided. The ESA 8000 has previously been used to study the adsorption of cetylpyridinium chloride onto kaolinite. It was found that the isoelectric point coincided with the cation exchange capacity of the clay. The surfactant appeared to adsorb in two steps, a monolayer and a bilayer (6). Bijsterbosch (7) studied adsorption of CTAB onto hydrophilic silica with adsorption, depletion, and electrophoresis. The CTA 1 isotherm was single step and Br 2 started to adsorb once a certain minimum concentration was exceeded. The adsorption of Br 2 increased the adsorption of CTA 1 compared to if the adsorption of Br 2 was prevented.
The aim of this paper is to show that the ESA (electrokinetic sonic amplitude) technique can be used to study the flocculation between cationic polyelectrolytes and negatively charged silica particles. The ESA signal was measured when cationic polyacrylamide (CPAM) or cationic polyamine (POL) was mixed with silica particles. The dynamic mobility of the formed flocs was calculated from the corrected ESA signal. The amount of nonadsorbed polyelectrolyte was determined by viscosity. The floc sediment volume was also measured. It was found that the polyacrylamide molecule was more efficient than the polyamine molecule in neutralizing the charges on the silica particles due to the longer CPAM molecules and the larger distance between the CPAM charges which was in better accordance with the distance between the silica charges compared to POL. © 1999 Academic Press Key Words: dynamic mobility; electroacoustics; flocculation; polyelectrolytes; silica particles.
1. INTRODUCTION
The ESA (electrokinetic sonic amplitude) technique has mainly been used to measure the dynamic mobility of different colloidal particles. Recently the same technique has been applied to polyelectrolytes (1, 2, 3). In this investigation it is shown that the technique can be used to study “real time” changes in the ESA signal when a flocculation process takes place. Flocculation is of great importance since it occurs in many industrial processes such as water purification, powder technology, and paper making. When a polymer adsorbs onto a solid substrate, dispersion forces, dipolar forces, hydrophobic forces, and hydrogen bonding are all possible types of interactions. In polyelectrolyte adsorption, electrostatic interactions play a very important role. These interactions mainly depend on charge densities (both polymer and substrate) and salt concentration. Since the ESA signal depends on the charge of the particles the ESA technique should be suitable to study flocculation. The main advantages with using the ESA technique are that it is not limited to dilute systems or sensitive to contamination. Along with the ESA measurements the pH and conductivity can also be measured. The shear rate of the flocculating suspension can be changed by changing the stirring rate. The measurements can be performed over a short or long period of time (from 1 s up to 167 h). It is also possible
2. THEORY
There is no theory for calculation of the dynamic mobility of particles flocculated by polymers. The closest approach is a paper called “The dynamic mobility of a porous particle” by O’Brien (8). The aim of that paper is to derive a formula for the dynamic mobility of a porous spherical floc. The mobility formulas are obtained for the case where the double layer thickness is much smaller than the pore radius and for the case 49
0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
50
CHARLOTTE WALLDAL
where the zeta potential is small (for any double layer thickness). With electroacoustic measurements over a range of frequencies it would according to this theory be possible to determine both size and zeta of the flocs. Since the ESA-8000 is limited to 0.8 –1.2 MHz it is not possible to obtain the size of the flocs with this technique. In the porous sphere theory the floc has a radius R and consists of primary particles with radius a (which also is the pore size). It is assumed that ( v a 2 )/v ! 1, where v is the angular frequency and v is the kinematic viscosity of the suspending liquid. At 25°C, 1 MHz, and a particle radius of 50 nm, ( v a 2 )/v 5 0.015; thus, the theory would apply in this study. The formula for the dynamic mobility of a rigid floc, i.e., the particles are so strongly bound together that they move with the same velocity, is
md 5
Ds 1 i vr Vs~1 2 3 f / 2! ez p Ds 1 i v Ms h
[1]
which relates the dynamic mobility to the Smoluchowski mobility. i is the imaginary number, r is the liquid density, h is the liquid viscosity, e is the permittivity, and z is the zeta potential. Vs is the volume of the floc, Vs 5 (4/3) p R 3 , and Ms is the mass of the floc, Ms 5 (r(1 2 f) 1 r pf)V s. f is the volume fraction of particles in the floc and Ds is the drag coefficient,
S
Ds 5 6 p R h 1 1 ~1 1 i!
Î
D
a a 1i p , 2 9
[2]
where a 5 ( v R 2 )/v.
FIG. 1. The chemical structures of (a) cationic polyacrylamide and (b) polyamine.
TABLE 1 The Molecular Weight, Amount of Charges/Gram and Distance between the Charges Tabled for CPAM and POL
Polymers
MW (g/mole)
Titratable charge (meq/gram)
Kinetic charge (meq/gram)
Charge separation (Å)
CPAM POL
5 3 10 6 50–200 3 10 3
1.2 6
0.31 2.4
20 7
Equations [1] and [2] also apply for a flexible floc at low frequencies if ( v aR)/v ! 1. According to these equations the dynamic mobility equals the Smoluchowski value for a solid particle of the same size and density at low frequencies or at small floc sizes. At high frequencies or large floc sizes the dynamic mobility approaches a limiting finite value in contrast to a solid particle for which the mobility tends to zero. 3. EXPERIMENTAL
3.1. Materials a. Colloidal silicic acid. The colloidal silica used was supplied by Eka Chemicals AB with a hydrodynamic diameter of 122 nm determined by a Brookhaven particle sizer. Photos taken with an electron microscope showed monodisperse spherical particles and the size was estimated to be between 100 and 120 nm. The specific surface area according to the BET method was 31.3 m 2/g (which corresponds to a particle diameter of 87 nm) and with Sear’s method (9) 25.4 m 2/g (which corresponds to a diameter of 108 nm). At pH 7 and 10 mM NaCl the silica particles had a dynamic mobility of about 22.5p10 28 m 2/Vs which was fitted using Mangelsdorf and White’s (10) computer program to be a zeta potential of 239 mV. The charge density at the slip plane was calculated to be 21.04 mC/cm 2. From the charge density the amount of charges at the slip plane was calculated (with specific surface from Sear’s method) to be 0.0027 meq/g for the particles. This will later be used in the presentation of the experimental results. Potentiometric titrations were also performed on the particles. The silica dispersion was diluted so the final weight% concentration was 0.25% when the dispersion was mixed with the polymers. The electrolyte concentration was 10 mM NaCl and pH about 7. The amount of silica used in the flocculation experiments was constant through the investigation. Milli-Q water was used in this study as a dispersion medium. b. Surfactant. CTAB was obtained from BDH Chemicals Ltd. and used as received. c. Polyelectrolytes. Two types of polyelectrolytes with different molecular weight were supplied by Eka Chemicals AB. The high weight polymer was a linear cationic polyacrylamide (CPAM) with a molecular weight of 5 3 10 6 g/mole with 10% of the acrylamide monomer exchanged for trimeth-
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
51
FIG. 2. Amount of adsorbed CPAM (■) and POL (}) charge plotted against amount of added polymers charge/silica charge.
ylaminoethyl–acrylatechloride. As a low weight polymer a branched cationic polyamine (polydimethyldiallylammoniumchloride) (POL) with a molecular weight of 50 3 10 3–200 3 10 3 g/mole was used. All molecular weights are number averages and according to the manufacturers. The structural formulas of the polyelectrolytes are shown in Fig. 1. The total charge of the polyelectrolytes was determined by titration with potassium polyvinylsulfate (11) at pH 7 and low ionic strength (,300 mS/cm). The polyamine had a surface charge of 6 meq/g and CPAM 1.2 meq/g. The dynamic mobilities of the polyelectrolytes have been determined with the ESA 8000 in a high sensitivity cell in a previous investigation (1). (The ESA signal from polyelectrolytes is too weak to be measured in the standard cell of the ESA 8000 instrument.) The mobilities thus were found to be 0.56p10 28 m 2/Vs for CPAM and 2.4p10 28 m 2/Vs for POL at 10 mM NaCl. In the same investigation a kinetic charge density is calculated by modeling the polyelectrolytes as cylinders. The kinetic charge density divided by the charge density of the cylinder gives a measure (the charge fraction) of how large a fraction of counterions is located outside the slip plane (12). To obtain the amount of charges at the slip plane, the meq/g at the surface is multiplied by the charge fraction. This gives a charge at the slip plane of 2.4 meq/g for POL and 0.31 meq/g for CPAM (1). The properties of the polymers are summarized in Table 1.
Solutions of cationic polymers were prepared with concentrations ranging from 0.0005 up to 0.01% and with 10 mM NaCl. The pH was adjusted to 7 just before the experiments. 3.2. Methods a. ESA. The ESA measurements were performed on an ESA 8000 instrument (Matec Applied Sciences). With this device it is possible to obtain the dynamic mobility and the conductivity as pH is changed. The measuring cell holds 220 ml of sample and the stirring speed can be varied between about 0 and 2500 rpm. To determine the charge on the particles, potentiometric titrations were performed at two different particle concentrations (5 and 8 weight%) using an automatic titrator. From pH 9.5 the particles were titrated down to pH 2.5 with 80 mM HCl and then up to pH 10 with 80 mM NaOH. Blank titrations were also performed so the amount of NaOH–HCl consumed by the electrolyte solution at a given pH could be subtracted. The ESA 8000 is operating at a field strength of about 100 V/cm and a field frequency of 0.8 –1.2 MHz. When the alternating field is applied the particles start to move backward and forward. As the particles move they displace an equal volume of liquid. If the density of the particles is different to the density of the liquid this will give rise to a sound wave. The ESA signal depends on the particle velocity and from that the dynamic mobility, m d, can be determined. The dynamic mo-
52
CHARLOTTE WALLDAL
FIG. 3. Sediment volume plotted against added amount of POL charge/silica charge.
bility of the particles depends on their size, zeta potential, and the frequency of the applied field. It depends on size because the particle motion at high frequencies is affected by inertia forces. For the 100 nm silica particles used in this investigation the inertia force is negligible and therefore the dynamic mobility is approximately equal in magnitude to the electrophoretic mobility for the solid silica particle. As mentioned earlier the ESA effect can also be measured for polymers but due to the low density difference between polymers and solvents a special cell with high sensitivity is required. With the standard measuring cell used in this investigation the ESA signal of the polymers cannot be distinguished from the background noise. The contribution to the measured ESA signal from free polymers can therefore be neglected. For dilute colloidal suspensions (up to approximately 5% volume fraction) the relation between the ESA effect and the dynamic mobility, m d, of the charged particles is given by (13) ESA~ v ! 5 A~ v ! f ~D r / r o! md,
[3]
where ESA is measured in Pa/(Vcm 21), A(v) is an instrument factor, f is the volume fraction of the particles, and Dr is the density difference between the particle density (r p) and that of the solvent (r o). The ESA measures the maximum in the pressure of the sound wave (Pa) per unit applied electric field strength (Vcm21).
The obtained ESA signals in the flocculation experiments were corrected by subtracting the ESA signal from the background electrolyte solution. b. Flocculation procedure. First, 110 ml of the silica suspension was mixed with 110 ml of the polyelectrolyte or the surfactant solution. Equal volumes of added particle dispersion and polyelectrolyte solution were chosen in order to obtain good mixing and to avoid high local concentrations of polymers. Two methods of mixing were used. Either the particles and the polyelectrolytes were poured together through a funnel into the ESA beaker or alternatively the polyelectrolytes were poured into the particle dispersion which was already in the beaker. Each mixing procedure took about 20 s. The stirring speed was about 1700 rpm. The ESA signal was measured over 10 min with 30 measurements/min. The results obtained were independent of the mixing method which suggested that good mixing was achieved by both methods. When high concentrations of polyelectrolytes (mainly CPAM) were used the measurement had to be stopped and the measuring probe cleaned (wiped off) to remove adsorbed material. If the probe was not cleaned, irreproducible results occurred. The pH of the solutions was maintained during the experiments by adding NaOH or HCl if needed. The charge of the silica particle is sensitive to the concentration of OH 2-ions around pH 7.
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
53
FIG. 4. Sediment volume plotted against added amount of CPAM charge/silica charge.
c. Floc size. After some of the experiments a floc sample was taken from the ESA measuring cell with a syringe partly filled with the appropriate background solution. The sample was then instantly injected in a laser diffraction instrument (Malvern 2600 particle sizer) and the floc size distribution was measured. From the volume size distribution it was possible to transform the results into number distribution using numerical transformations. The observation cell was equipped with a magnetic stirrer which prevented the flocs from sedimenting. No further flocculation was observed for samples left for several hours in the observation cell with the stirrer on. d. Adsorption isotherm. The flocculated suspension was left to sediment and the sedimented floc volume was measured. The remaining suspension was centrifuged for 30 min at 7000 rpm. Viscosity measurements were performed on the supernatant with a Cannon–Fenske capillary flow viscometer to determine the concentration of the free polymer. All samples were filtered with a millipore filter with a poresize of 2 mm before the measurements. 4. RESULTS
a. Potentiometric Titration The surface charge density was calculated using
s O 5 2F~G H1 2 G OH2! 5 F~@H 1 #!~X 2 v/mA!,
[4]
where F is the Faraday constant, G H1 and G OH2 are the adsorption densities of H 1 and OH 2, and [H 1] is the concentration of acid. X is the total amount of HCl (cm 3) added and v is the volume of acid required to change the pH of the silica dispersion by a certain amount. A is the surface area per unit mass of solids and m is the mass of the silica sample. The two different particle concentrations used (5 and 8%) gave the same result. At pH 7 and 10 mM NaCl the surface charge density was about 21.9 mC/cm 2 which corresponds to 0.005 meq/g. It is mainly the surface charge of the polyelectrolytes and particles which will be used in the presentation of the results. b. Adsorption Isotherm In Fig. 2 the amount of surface charge (in meq) of adsorbed polyelectrolytes is plotted against the ratios of the added amount of polymer surface charges to the amount of surface charges on the silica particles. The arrows indicates the points of charge reversal (from Figs. 6 and 7). The maximum adsorbed amount of polyelectrolyte charges was larger for polyamine than CPAM. The adsorption curve for polyamine was more rounded and it is uncertain if the curve has reached its plateau. The adsorption curve for polyamine starts to flatten out at an almost three times larger amount of added polyamine charges than added CPAM charges. If instead the weight amount of adsorbed polymer is compared, about three times
54
CHARLOTTE WALLDAL
FIG. 5. The corrected ESA signal is plotted against added amount of CTAB.
more CPAM was adsorbed at maximum adsorption. The arrows indicate the points of charge reversal. The sedimented floc volumes are plotted in Figs. 3 and 4 as a function of amount of added polyelectrolyte charges/silica charges. The sediment volume curve for polyamine starts at zero sediment volume at low addition of polyamine followed by a sudden increase in sediment volume which then increases slightly until it drops to a smaller volume at a large addition of polyamine. At zero and small sediment volumes the supernatants were turbid; otherwise the supernatants were clear. When a clear supernatant is observed it is assumed that all silica particles are flocculated. The sediment volume curve of CPAM and silica particles has a different shape which resembles the adsorption isotherm. The supernatant is turbid up to about a ratio of 2 and then it is alternating clear and turbid until about a ratio of 3.7 when the supernatant becomes clear. At a ratio of about 10 the supernatant becomes turbid again. Between 2– 4.5 CPAM charges/ silica charge (point A, Fig. 4) the sediment volume does not increase as more CPAM is adsorbed. c. Surfactant Adsorption In Fig. 5 the ESA signal is plotted as a function of CTAB concentration. As the CTAB concentration increases the ESA
signal decreases almost linearly. At about 0.065 mM added CTAB there is a change in the slope of the curve. d. Polyelectrolyte Flocculation In Fig. 6 the corrected ESA signal is plotted against the amount of CPAM charges/silica charge. The ESA signal becomes less negative as more CPAM is added and changes sign at 3 added polymer charges per silica charge. The total system then becomes positively charged. The ESA signal reaches a plateau value of about 0.017 Pa/Vcm 21 at 4.5 added CPAM charges/ silica charge. At low addition of CPAM the ESA signal decreases linearly in the same manner as when CTAB is adsorbed. The sediment volume increases as more CPAM is added up to about 2–3 polymer charges/silica charge (Fig. 4) where the increase is less. The ESA signal on the other hand continues to change. It seems therefore that the change in ESA signal is not affected by the formation of flocs and any inertia effect originating from the aggregation of particles to larger units can then be neglected. If a comparison is made with the adsorption isotherm (Fig. 2) it is obvious that the change in ESA signal reflects the increased adsorption of polymer. This conclusion is supported by comparing the adsorption isotherm for POL and the change in ESA signal when POL is mixed with the silica dispersion (Fig. 7). The change in ESA signal
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
55
FIG. 6. The corrected ESA signal is plotted against added amount of CPAM charge/silica charge.
when POL is used is different compared to when CPAM is used. At low addition of POL the ESA signal does not change linearly probably because the polyamine has such a high charge density and low molecular weight so the charges are less effectively used to neutralize the particle charges at low concentrations. When larger polyamine concentrations are used the ESA signal continues to decrease as more polyamine is added in agreement with the adsorption isotherm (Fig. 2). More remarkable is that the magnitude of the largest obtained positive ESA signal (0.09 Pa/Vcm 21) exceeds the negative magnitude of the pure silica particles (20.066 Pa/Vcm 21). Obviously such a large amount of polyamine is adsorbed that strongly positively charged areas with high polymer densities are formed which contribute to the positive ESA signal (14). e. Time Aspects It took about 20 s to mix the polyelectrolytes and surfactant with the silica dispersion. The measured ESA signals were constant immediately for the surfactant adsorption. For the large CPAM flocs the ESA signal was also constant at low concentrations of added CPAM. At higher concentrations of CPAM the measuring probe had to be cleaned and therefore different ESA signals are observed before and after the cleaning. In the case of the smaller POL molecules the ESA signal became constant after about 3 min for zero POL concentrations
up to the amount required for charge reversal. For higher POL concentrations a constant ESA signal was obtained immediately. DISCUSSION
The ESA measurements of CTAB (Fig. 5) showed a change in the slope of the curve when CTAB corresponding to a surface area of 35 Å 2 was added. The headgroup area of ionic surfactants is usually reported to range from 40 to 60 Å 2 per molecule (15, 16). This implies that the adsorbed amounts of CTA 1 are packed closely enough onto the silica surface for the hydrocarbon tails to interact. Hydrophobic attractions probably start to be important at the intersection and a bilayer can be formed. It is also possible that the adsorption of counterions starts due to the build-up of a bilayer at the intersection which facilitates the adsorption of CTA 1 (7). At first the CTAB molecules adsorb due to electrostatic attraction head down to the surface groups. In the first part the adsorption of CTAB onto the silica particles can be regarded as charge neutralization. In the next step the adsorption of CTAB molecules involves hydrophobic attraction between the surfactant tails. Since we are mainly interested in observing the electrostatic effects the flocculation experiments are limited to a few results when the particles are charge reversed.
56
CHARLOTTE WALLDAL
FIG. 7. The corrected ESA signal is plotted against added amount of POL charge/silica charge.
By comparing Figs. 3 and 4 it can be seen that the growing of CPAM flocs starts at a smaller amount of added CPAM charges compared to POL charges even though the adsorption isotherm (Fig. 2) clearly shows that there is an adsorption already at small amounts of added POL. In fact Fig. 2 shows that the adsorption of polyelectrolyte charges is independent of molecular weight at low concentrations. The change in ESA signal (Figs. 6 and 7) at low polymer concentrations seems therefore to be related to the onset of flocculation. By comparing Figs. 4 and 7 it is seen that an increased change of the ESA signal as larger amounts of POL are added corresponds to the formation of flocs. This critical amount of POL molecules needed to flocculate the silica dispersion is not observed for CPAM. Above 10 POL charges/silica charge, the dispersion becomes positively stabilized which is shown by the small sediment volume and the observed turbidity in Fig. 3, the change of ESA signal is again reduced. Obviously the formation of flocs gives a more effective use of the POL charges as one molecule can adsorb to more than one particle. The greater ability of CPAM to flocculate is due to the higher molecular weight and the linear structure of the molecule which gives a more extended conformation in the solution which enables CPAM to more easily bridge between particles. The maximum sediment volume obtained is larger for the POL flocs than the CPAM flocs which at first sight seems strange when the floc sizes are much larger for flocs made of
CPAM than of POL (Table 2). But there was a large density difference of the sediments. The CPAM flocs were much more closely packed than the POL flocs which seemed to float on top of each other. The large CPAM flocs are more sensitive to the stirring rate and are more easily ruptured than the smaller POL flocs which partly explains the higher density of the CPAM flocs. In general polyelectrolytes tend to adsorb in rather flat conformations. The formation of a thick polymer layer is prevented due to the electrostatic repulsion between the segments. An exception is polymers with very low charge density which form loops and tails at the surface, while polymers with higher charge density adsorb in a flat configuration. It is not possible to obtain the polymer conformation in the flocs with TABLE 2 The Floc Diameter for Different Amounts of Added CPAM and POL Measured with a Malvern 2600 Particle Sizer CPAM
POL
CPAM (meq)/ silica (meq)
Diameter (mm)
POL (meq)/ silica (meq)
Diameter (mm)
1 2.7 3.4
25 52 60
0.67 2.5 2.7
2.5 4.6 8.6
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
57
FIG. 8. The dynamic mobility divided by the Smoluchowski mobility is plotted against floc size for POL with f 5 0.026 (■), POL with f 5 0.033 (}), CPAM with f 5 0.036 (Œ), and CPAM with f 5 0.05 (✖).
the ESA technique. The measured sediment volumes could, however, give an indication of the polymer conformation. Lower floc densities are obtained for the POL flocs compared to the CPAM flocs, even though the POL flocs are much smaller. This is an indication that the POL molecules have a more extended conformation in the flocs than the CPAM molecules. An explanation could be that the collision rate between the particles is faster than the rate of attainment of adsorption equilibrium. Since the CPAM molecule is more flexible than the POL molecule due to a lower charge density and a linear structure, the CPAM molecule attains a more flat conformation in the flocs. The observed turbidity of the supernatant at high CPAM concentrations (Fig. 4) could be explained by the flocs reaching a limiting size which depends on the stirring rate. Even if more CPAM is added the flocs will not obtain a larger size due to the fact that small fractions of the flocs are torn off. Those small fractions would give a turbid supernatant. The measured ESA signal does not increase either. The observation that between 2– 4.5 CPAM charges/silica charge the sediment volume does not increase as more CPAM is added could be interpreted as the flocs becoming more compact. The ESA measurements show that about 50% more POL than CPAM charges are needed to obtain a charge reversal (Figs. 6 and 7). Together with the structural difference between
the polymers discussed earlier this can also be understood in terms of the different charge densities on the polyelectrolytes. The mean charge distance between the POL charges is about 7 Å and a rough estimation of the length between the charges at the silica surface gives 30 Å. For CPAM the distances are in better accord (about 20 Å between the CPAM charges). CPAM therefore better balances the surface charges with fewer extra charges. Dynamic Mobility There is a practical interest in calculating a dynamic mobility from the ESA signal. The dynamic mobility obtained by using O’Briens formula (Eq. [1]) is divided by the Smoluchowski mobility (h/ez) and the result is shown in Fig. 8. The volume fractions used are the ones obtained when the solution above the sediment has been nonturbid. As can be seen in Fig. 8 the change of the dimensionless mobility is larger at higher particle volume fractions, even though the difference is at maximum 9.5% in this case. For floc sizes larger than 10 mm a limiting value is reached. In Table 2 some floc diameters obtained with the Malvern 2600 are given. The floc sizes for polyamine are smaller than 10 mm which means that there is a size dependence of the mobility for these flocs whereas for the larger CPAM flocs, the mobility is affected by the floc size to
58
CHARLOTTE WALLDAL
FIG. 9. The dynamic mobility is plotted against added amounts of POL (■) and CPAM (}) charge/silica charge.
the same extent for all flocs. O’Brien’s theory only accounts for flocs with the same charge sign and then preferably before a charge reversal of the primary particles is obtained. The observation that when the total flocculating system becomes positively charged and the magnitude of the ESA signal increases at the same time as the floc size increase or remains constant has no correspondence in the porous sphere theory. Truly more theoretical studies are needed in this field. Equation [3] shows the relation between dynamic mobility and ESA signal for a particle dispersion. The results are shown in Fig. 9 where Eq. [3] is used to calculate the dynamic mobility of the silica particles flocculated with the polyelectrolytes. The results will not be discussed here since they are proportional to those in Figs. 6 and 7. The use of Eq. [3], which relates the dynamic mobility to the ESA signal for particles, to calculate the dynamic mobility of the flocs can be justified with the following discussion. The contribution to the measured ESA signal from nonadsorbed polymers can be neglected as discussed previously. The ESA signal must originate from silica particles with or without adsorbed polymers which neutralize the particle charges. The question then arises whether the bridging of polymers between the particles affects the ESA signal or not. Does the polymer bridging prevent the particles from moving in the applied field more than what is caused by them having their charges reduced?
The amplitude of the particle motion in the applied field is quite small. It is calculated to be about 2.5 Å, which can be compared to the flexibility of the polymers. It is likely that particles with the same charge reduction move the same distances in the field. If the polymers are unevenly distributed in a floc the movement of the particles with less adsorbed polymers would be hindered. The flocs probably move forward and backward as one unit in the applied field with a speed proportional to the average charge density of the flocs. The inertia forces acting on a solid particle with the same size as the flocs would reduce the ESA signal toward zero for the large CPAM flocs. On the contrary we observe an increase in the ESA signal as the flocs grow larger after the charge reversal. The change of ESA signal shows the same behavior as the adsorption isotherm. There are probably no artefacts arising from the particles being connected by polymers and Eq. [3] should be a valid way to obtain the dynamic mobility for a floc. Also according to the porous sphere theory at most only 10% of the mobility for these flocs depends on their size the rest is due to charge neutralization, which further justifies the use of Eq. [3]. All results in this study have been presented with charge ratios based on the surface charges. In the flocculation– coagulation process the attraction felt by the aggregating species is more likely controlled by the potential at the slip plane (zpotential). If instead the charge densities (for both polyelectro-
FLOCCULATION STUDIED WITH THE ESA TECHNIQUE
lyte and particles) at the slip plane are used to calculate the point of charge reversal it occurs at 1.6 CPAM charges/silica charge and 3.5 POL charges/silica charge. With this approach less polyelectrolyte is needed to neutralize the silica charges and CPAM becomes even more effective as a flocculant compared to POL. Finally it can be concluded that it is possible to use the ESA 8000 to obtain information about the dynamic mobility “in real time” for a dispersion flocculated by polyelectrolytes. However, additional information about floc sizes and polymer conformation obtained with the same technique would be of interest. This could be obtained if an electroacoustic technique measuring over a wide range of frequencies is used, although the theoretical treatment has to be improved. ACKNOWLEDGMENT Prof. Bob Hunter is acknowledged for reading and having valuable comments on this manuscript. All the materials in this investigation were supplied by Eka Chemicals AB.
59
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