Table of Contents
Shear-flocculation is a term used by Warren (1975) to describe a phenomenon observed when scheelite (CaWO4) was stirred in the presence of a surfactant. At high stirring speeds in the presence of 10 -4 M sodium oleate, the particle size of the scheelite slurry increased. However, in the absence of a collector and/or at low stirring speeds, aggregation did not occur. Warren interpreted this data to mean that the flocs formed were due to hydrophobic-hydrophobic interactions between oleate-coated particles. The high shear was necessary to overcome particle-particle repulsive forces due to diffuse layer overlap. Once this phenomenon, “shear-flocculation,” had occurred the particle-particle bonds were of sufficient strength to resist break-up during continued high shear.
The mechanisms of shear-flocculation are generally believed to be as follows: (1) a collector adsorbs onto a mineral surface, leaving an aliphatic chain pointing toward the bulk solution, which results in the particle having a hydrophobic surface; (2) if a collision of sufficient momentum occurs, the kinetic energy of the particles is adequate to overcome the surface repulsive force (Warren, 1975; Lu and Li, 1984, cited in Bilgen and Wills, 1991); (3) the hydrocarbon chains come together and form a hydrophobic bond (Pashley and Israelachvili, 1981), resulting in floc formation. The resulting floc is strong enough to withstand considerable turbulence, unlike flocs formed with high molecular weight polymers. However, the flocs are typically much smaller than polymer-based flocs.
Early methods of following the progress of shear-flocculation included turbidity measurements, Stokes’ settling tests, and indirect particle size analysis. Turbidity is an indirect measure of flocculation efficiency, and a useful descriptive tool (Raju, et al., 1991). For the purposes of modelling shear- flocculation, particle size determinations are essential (Koh, et al., 1987). Previous attempts to measure the particle size of shear-flocculated minerals have been limited to d50, the equivalent Stokes’ diameter corresponding to 50% by weight undersize. Because the floc densities are not known and must be assumed, the Stokes method tends to inaccurately characterize floc size (Warren, 1982). Indirect particle size measurements include removing or diverting a sample of shear-flocculated minerals to a batch-type particle size analyzer. Typically this method involves dilution of the slurry, which has indeterminate but likely deleterious effects on flocs.
The Bureau of Mines has recently investigated a new method of measuring shear-flocculation using an in-line particle size analyzer. The technique involves inserting a probe containing a laser source and a detector directly into the slurry. The method is fast (data collected every 9 s), flocs do not have to be removed from the slurry to be analyzed for size. This technique was used to study the hydrodynamics of shear-flocculation for silica particles.
Silica was sheared in the presence of a commercial flotation reagent for up to 20 min and the particle size was measured as a function of time. Stirring speed, solids content and reagent dosage were varied in studying the kinetics of shear-flocculation. The results were examined in light of hydrodynamic equations developed from a model for fine particle flotation.
Materials and Methods
Neosil A ground silica was obtained for use in the study. The particle size distribution is shown in figure 1. The mean particle size was 24 µm. Armac C, described as “97% cocoalkylamine acetate and 3% dicocoalkyl acetate,” was used as a collector.
To study the effect of surface chemistry on shear flocculation, zeta potential measurements were performed using a Zeta-Meter, Inc., Model 3.0 zeta meter microelectrophoresis cell. Slurries were diluted to 0.1% solids before measurement.
Figure 2 shows the experimental apparatus used to shear flocculate the silica. The appropriate amount of silica was added to 2 L of deionized water and placed in a 4-L polypropylene beaker, which was stirred with a 5-cm-diameter 4-blade impeller. The probe from the particle size analyzer was inserted near the impeller so as to measure the particle size in the most turbulent region of the slurry.
The particle size analyzer was a Partec 100 manufactured by Laser Sensor Technology, Inc. It consisted of a power supply and a 2.54 cm stainless steel probe with a sapphire window at the tip. The probe contained both the laser source and a detector for back-scattered light. Scan time was 1 s, and particle size measurements were saved to a disk every 9 s. The probe was inserted at a slight angle to prevent bubbles from collecting on the window. This also permitted continual washing of the probe by the slurry.
After addition of the silica, the pH was adjusted with HCl(aq) to 6.5 and the solution was stirred for 63 s. At this point the surfactant was added and stirring was continued 10-20 min. However, the floe size typically reached equilibrium after 5 min.
Mathematical Model of Shear-Fl0cculation
It was mentioned earlier that carrier flotation and shear-flocculation were believed to be similar. Kelley has described flotation kinetics in terms of a probability of flotation, which is equal to a series of probabilities that several events will occur (Kelley, 1982). Adapting this concept to shear-flocculation, the rate expression becomes
Psf = PcPa(1 – Pd)……………………………………………………………………………(1)
where Psf is the probability of shear-flocculation occurring. Pc, the probability of collision, is the probability that particles in a slurry will come near enough to collide. Pa, the probability of attachment, represents the probability that particles which collide will aggregate. Pd, the probability of detachment, is simply 1 – Ps, where Ps represents the probability that flocs, once formed, flocs will remain stable throughout the experiment.
The probability of collision is proportional to the number of collisions Nc (Jordan and Spears, 1990). In modelling a collision between two particles p1 and p2, the particle p1 is held motionless while p2 moves past it in the slipstream. In flotation kinetics the stationary “particle” is the bubble. To maintain consistency, any following terms referring specifically to bubbles have been substituted with a similar term referring to p1.
Adapting Bischofberger and Schubert’s (1978) flotation equation to shear-flocculation, the number of collisions between two particles p1 and p2 becomes
where Np is the number of particles, rp is the particle radius, and Up is the particle velocity relative to the fluid.
The probability of attachment Pa in flotation is a function of bubble size (Sutherland and Wark, 1955). Adapting this probability to shear-flocculation, Pa becomes
where Up1p2 is the relative velocity of some particle to p2. The term rp2 3/2C1 is the induction time ti where Ci is a constant for each mineral and its surface chemistry (Jowett, 1980).
Woodburn (1971) has modeled the probability of stability for bubble-particle aggregates. This equation may be adapted to aggregates formed during shear-flocculation
where rmaxp is the maximum size of a particle that can remain attached at a given amount of turbulence. rmaxp decreases as the degree of turbulence increases.
Results and Discussion
Shear-flocculation is very similar to carrier flotation (Subrahmanyam and Forssberg, 1990). A series of flotation tests was made as screening experiments in order to determine the reaction conditions that would be suitable for shear- flocculation. Armac C floated silica well at pH 6.5. The effect of collector dosage was evaluated up to a maximum of 5 lb/st. Flotation screening tests revealed 3 lb/st Armac C was the minimum dosage necessary to float 100% of the silica. This data was interpreted to mean that an increase in the dosage of collector would not result in any additional surface hydrophobicity.
Figure 3 shows the results of shear- flocculation of silica as a function of time and stirring speed at 0.1% solids and 3 lb/st Armac C at pH 6.5. Figure 4 shows the same results for 0.5% solids, and figure 5 shows those for 1.0% solids.
Figure 5 shows that immediately after collector addition, a rapid increase in particle size occurred. For speeds below 1250 r/min, the increase in particle size was somewhat reduced, but at 1250 r/min, the particle size was tripled in just under 1 min. Curve fitting of the data showed that the particle size of the slurry increased logarithmically with time. A logarithmic function indicated that two different stages of shear-flocculation existed. One predominated during the first minute, and the other during the remaining 19 min of the test.
The Effect of Hydrodynamics on the Kinetics of Shear-Flocculation
In the first minute of shear-flocculation, rapid floc growth occurred. After 1 min, the mean particle size approached asymptotically a maximum mean particle size, dpmax the increase of the particle size of the slurry indicated that shear flocculation occurred. When a slurry is stirred, inter-particle collisions are possible. If the colliding particles are hydrophobic, aggregation can occur provided that the collision is energetic enough to overcome electrostatic energy barriers (Warren, 1975). Once aggregated, these “shear-flocs” are quite stable under high shear due to high bonding energies. Warren has calculated these energies to be on the order of 10³ kT or greater (Warren, 1975). As a comparison, flocs formed with high molecular weight polymers have binding energies on the order of 10 kT (Moudgil and Vasudevan, 1989) .
In the early stages of the shear-flocculation experiment (fig. 3-5), increases in dp occurred. This was because the term Pc x Pa predominates over Pd in equation 1. Pc is proportional to the number of particles present and their relative velocities. In the early stages of shear flocculation, relatively large numbers of particles are present in a turbulent environment and a large number of collisions occur which have sufficient energy to overcome the electrostatic barrier and cause aggregation. As aggregation occurs, the mean particle size increases (fig. 1). However, figures 3-5 shows that a given set of hydrodynamic conditions will permit floc growth only up to a certain maximum size.
Figure 6 demonstrates the importance of Pc in shear-flocculation. In figure 6, dpmax is shown as a function of stirring speed and particle concentration. At 0.1% solids (fig. 3), there is very little change in particle size up to 1500 r/min. However, at 1750 r/min, a significant increase in dpmax occurred. At 0.5% solids (fig. 4), dpmax changed very little up to 1250 r/min then increased sharply, showing at maximum at 1500 r/min. At 1.0% solids (fig. 5), the maximum is shifted to 1250 r/min.
Figure 6 confirms the existence of an energy barrier to shear-flocculation. Floe growth was negligible at low stirring speeds, but at a certain agitation rate, dpmax increased dramatically. This indicated that a large number of particles had sufficient momentum to overcome the energy barrier and aggregate. At higher stirring speeds, floc formation continued, but the rate of growth was less, indicating that the rate of floc breakage became significant. The rate of floc breakage increased with agitation rate, hence the maxima in figure 6.
The position of the maximum in figure 6 is a function of the solids content of the slurry. Pc, unlike Pa or Pd, is a function of Np (i.e., solids concentration). As particle concentration decreases (Equation (2)), the number of collisions also decreases, which lowers the probability of collision. When few collisions occur, very little shear-flocculation will occur even at high Pa. For shear-flocculation to transpire at low solids concentrations of a constant particle size, particle velocity must be increased by faster stirring. Conversely, at higher solids content where the Pc is higher, shear-flocculation occurs at low stirring speed. This accounts for the shift in the maximum in figure 6.
At stirring speeds below the maxima in figure 6, shear-flocculation does not occur to any great extent. What flocculation occurs is generally between larger particles, because of their greater Pa (Equation (3)). Figure 7 shows the particle size distribution at 0 and 10 min of silica shear-flocculated at 750 r/min (1% solids and 3 lb/st collector added). This stirring speed was below the energy barrier for shear-flocculation (fig. 6). At 10 min, the reaction was in the equilibrium region, the particle size distribution was essentially unchanged below 9.2 µm. However in the 9-40 µm region, there is a noticeable decrease in the distribution. The percentage of large floes (100-300 µm region) formed are relatively small, because of the low probability of collision.
When the slurry has sufficient energy for shear-flocculation to occur, a significant change in the particle size distribution is observed (Fig. 8). At 10 min, the particle size distribution is much broader due to the formation of significant quantities of floes in the 30-600 µm range. All but the smallest particles (<2 µm dp) have been flocculated.
The differences in particle size distribution at 10 min, as shown in Figures 7 and 8, can be accounted for by particle momentum. Particle momentum is a product of velocity and mass, and mass is a function of particle size. Pa is also a function of particle size and velocity. The particles in Figure 8 had higher momentum because of the increase in stirring speed. Consequently, even particles as small as 2 µm in diameter were able to overcome the energy barrier and aggregate. This implies that increased stirring speed raised the number of collisions and also increased the probability of attachment.
The Effect of Surface Chemistry on the Kinetics of Shear-Flocculation
The probability of attachment is also affected by particle surface chemistry and is accounted for in Equation (3) by the term Ci. In figure 9, the zeta potential of silica conditioned at pH 6.5 is shown as a function of collector dosage. Collector dosage is directly related to surface coverage. Sivamohan and Cases (1990) used dodecylamine hydrochloride (DDACl) as a collector for silica. They showed that zeta potential was related to dodecylamine concentration, and concluded that the collector served as a shear-flocculating agent by reducing the negative zeta potential. However, Warren (1975) used sodium oleate as a collector for scheelite. He showed that increasing the collector concentration did not reduce the negative zeta potential, and concluded that the role of oleate was to make the surface hydrophobic.
Figure 9 shows that collector dosage did not significantly increase the zeta potential. The data indicates that Armac C, called a “cocoalkylamine,” behaved more like sodium oleate than DDACl. This led us to the conclusion that Armac C (the collector used in this investigation) was an amide, probably of palmitic acid. Amides and carboxylic acids (such as sodium oleate) have similar interactions with mineral surfaces near pH 7 (Spears, et al., 1992). Thus, the role of the Armac C was to make the surface more hydrophobic.
Maximum mean particle size is a function of both agitation rate and collector dosage (fig. 10). For 0 lb/st collector dosage, dmax decreased slightly as the agitation rate increased. These results imply that the slurry
was being dispersed. No aggregation occurred because there were no hydrophobic interactions. The zeta potential at pH 6.5 was -29.8 mV, sufficiently negative to keep the particles dispersed and prevent loose aggregation due to charge effects. Therefore, interactions for hydrophilic silica particles were either van der Waals forces or hydrogen bonds through surface hydroxyls, both weak interactions. At high agitation rates, the shear forces on the particles would be adequate to overcome any such interactions and break any aggregates apart. The presence of collector changed slurry behavior considerably. At 1 lb/st dosage, dmax increased to a maximum at 1500 r/min, then decreased slightly at 1750 r/min agitation rate. At 3 lb/st dosage, the maximum was shifted to 1250 r/min, with dmax decreasing slightly up to 1750 r/min agitation rate.
Equation (3) shows that Pa is proportional to a constant C1 which is a function of the surface properties of the mineral, including hydrophobicity. The shift in the maximum in figure 10 occurred because particles with higher collector dosages had larger probabilities of attachment due to increased surface hydrophobicity, which translates to more sites being available on the surface for inter-particle attachment. Since collisions were more effective, fewer collisions were required to obtain flocculation. Therefore, a maximum in the dpmax versus stirring speed is observed at a lower agitation rate for 3 lb/st collector dosage than a 1 lb/st collector dosage. Further, since more sites were available at 3 lb/st for binding, floe stability increased,which resulted in the creation of larger flocs.
Figure 10 further confirmed the presence of an energy barrier. In the absence of collector, shear-flocculation did not occur. Floc growth was relatively small at low agitation rates, but at a certain agitation rate, dpmax increased dramatically when collector was present, which indicated that at this shear rate, the majority of the particles had sufficient momentum to overcome the energy barrier and aggregate. At stirring speeds above the energy barrier, floc formation continued but significant floc disruption also occurred. Additional collector lowered the energy barrier without significantly changing the zeta potential.
After the first 1-2 min of shear-flocculation, in which rapid floc growth occurs, an equilibrium was set up between floc growth and disruption. The rate of floc disruption was governed by Pd, which is 1-Ps. Equation (4) shows that Ps is governed by particle size and turbulence (rmaxp decreases with turbulence). This is evident from figure 11. In figure 11, particle size distribution is shown as a function of stirring speed after 10 min of shear-flocculation at 1.0% solids with 3 lb/st collector. At 10 min, the reaction was in
the equilibrium region. At 750 r/min, the particle size distribution is virtually unchanged from the unflocculated material (fig. 7). At 1250 r/min, the point of the maximum in figures 5 and 10, there was a substantial percentage of flocs 40-600 pm in diameter. However, at 1750 r/min, above the maximum in figures 5 and 10, there was a substantial decrease in the percentage of flocs larger than 100 µm in size.
The loss of larger flocs at 1750 r/min coincided with a decrease in dpmax (fig. 6). As stirring speed increased, fluid and relative particle velocities increased (Jordan and Spears, 1990). This resulted in an increased probability of detachment, since rmaxp is lowered in turbulent conditions (Woodburn, 1971). Further, increased relative velocities decreased Pa (Equation (3)). This is because particles spend less time in contact at high velocities and so have less time to flocculate. The result is that particle and fluid velocities govern the position of the equilibrium between Pa and Pd.
A final point on floc disruption can be made from figure 6. At or above the stirring speed where shear-flocculation occurred, dpmax was larger at 0.5% solids than at 1.0% solids. At higher solids content, more collisions occurred (Equation (2)). It appears that collisions may be destructive as well as floc-forming, and the relative number of destructive collisions increases with particle concentration at fluid velocities where shear-flocculation occurs.
Shear-flocculation of silica occurs when silica is stirred in the presence of an aliphatic amine. The process of shear- flocculation is kinetically and mechanistically similar to the process of flotation. The kinetics of shear-flocculation can be described in terms of a probability of shear-flocculation, which is the product of a series of probabilities, the probability of collision, of attachment, and of detachment, describing the shear-flocculation process. The kinetics of shear-flocculation have two distinct stages: (1) in which rapid floc growth occurs; and (2) in which floc growth tapers off and an equilibrium is set up between the rate of floc growth and floe disruption. The position of the equilibrium is a function of hydrodynamic conditions and collector dosage.
A minimum stirring speed is required to achieve shear-flocculation. This is indicative of the amount of energy required to give particles sufficient momentum to overcome the electrostatic energy barrier. Shear-flocculation continues to occur above the minimum stirring speed, but the rate of floc disruption becomes significant and reduces the extent of aggregation. The amount of energy required to achieve shear-flocculation is reduced by increased solids content and collector dosage. Increased solids content raises the number of inter-particle collisions which occur, increasing the probabilities of collision and shear flocculation.
Collector dosage increases the number of surface sites available for attachment and inter-particle bonding. High collector dosages not only permit large floc growth, but increase floc stability. The extent of growth is also dependent on hydrodynamic conditions. Agitation causes a large number of collisions which can lead to aggregation despite unfavorable conditions such as poor collector coverage. However, floc stability is inversely proportional to shear, and too much shear can cause floc disruption.
Shear-flocculation is dependent on particle size. Larger particles floc first at a given agitation rate, and have greater momentum which allows them to overcome the energy barrier and attach. However, large flocs become increasingly unstable as stirring speed increases. This results in a narrowed particle size distribution at high stirring speeds.
Mechanism of Shear Flocculation of Silica with Fatty Acids
Minerals beneficiation often generates slurries of fine particles. Such slurries can contain valuable minerals which must be discarded as waste because of problems associated with recovering fines. Because of their size, fine particles are difficult or impossible to efficiently separate in gravity concentrators or classifiers. Further, the flotation kinetics of small particles are quite slow. In order to improve recoverability of fine slurries, it is desirable to increase their mean particle size by selective flocculation.
Warren (1975) developed a new process for particle aggregation which he termed “shear-flocculation.” In this process, a surfactant is added to a slurry where it interacts with particles and makes their surfaces hydrophobic. When placed in a field with a high enough rate of shear, the particles will attach, probably due to hydrophobic-hydrophobic interactions. Other types of interactions may also play a role. Flocs formed in this manner are much more stable in turbulence than are floes formed with high molecular weight polymers. Further, shear-flocculation can be used to selectively recover minerals. Flotation reagents can be used to shear-flocculate minerals and the adsorption of these reagents on mineral surfaces is often quite selective. Shear-flocculation research has been conducted rather sporadically over the last 15 years, but scant progress has been made in understanding the mechanism of the process.
In order to better understand the mechanism of shear-flocculation and the underlying surfactant/surface interactions, the U.S. Bureau of Mines in conjunction with The University of Alabama is currently evaluating the application of electron paramagnetic resonance (EPR) spin labelling techniques.
Such studies have been carried out on other surfactant systems with considerable success (Baglioni, 1984; Waterman, 1986; Malbrel, 1990). In this paper preliminary results are presented of an EPR study of stearic acid/silica shear-flocculation using 16-doxyl-stearic acid.
EPR is a magnetic resonance technique sensitive to unpaired electrons, and to the environment of such unpaired electrons (Wertz and Bolton, 1986). A spin-label is a group such as the doxyl group (Figure 1a) which contains an unpaired electron delocalized between the ring nitrogen and the attached oxygen. As shown in figure 1b, such a spin-label can be attached to a molecule of interest, e.g. stearic acid, to study the environment and motion of that molecule. If the spin-label is tumbling freely in solution the EPR spectrum consists of three lines of equal height and width. The line-spacing (hyperfine coupling constant AN) is affected by the solvent polarity and can be used to distinguish between polar and non-polar environments. If the spin-label is not tumbling freely, the three lines become of unequal height and width. The heights and widths of the three peaks can be used to determine how fast the spin-label is tumbling around its molecular axes. Since the spin-label is attached to the molecule of interest the motion and environment experienced by the spin-label are similar to those of the molecule as a whole.
If the molecules exist in multiple environments, the spin-labels can also experience more than one environment. If a molecule is in a particular environment for a long period of time, the observed EPR spectrum will be the super-position of the individual EPR spectra from the spin-labelled molecules in each individual environment. If the molecule exists in each environment for only short periods of time an averaged EPR spectrum will be observed. If the time spent is intermediate between these two cases, the form of the EPR spectrum can be used to determine how long a molecule is in each environment. If the concentration of spin-labels is low (typically below 10 -5 M) , the EPR lines are narrow. As the concentration increases the EPR lines become steadily broader. Therefore, the width of the EPR lines can be used to determine the local spin label concentration. EPR shows considerable potential as a tool to study the mechanism and dynamics of shear-flocculation.
Currently, there is no direct experimental observation of the role of surfactants in shear-flocculation. This report contains the results of a study of the application of EPR spin-labelling techniques to study the role of stearic acid in shear-flocculation of silica.
5-, 12-, and 16-doxyl-stearic acids were obtained from Aldrich Chemicals.
These are stearic acids with a doxyl group (figure 1) at either the 5, 12, or 16 position on the alkyl chain. Neosil A (now called Tammsil 10) ground silica was obtained from Tammsco Silica (now Unimin Specialty Minerals, Inc.). The sample was listed by Tammsco as having a mean particle size of 2.1 µm, as determined by a Micromeritics Sedigraph 5100. This has been subsequently confirmed by examination under a scanning electron microscope.
The samples were prepared by making 0.1 M NaCl solutions from de-ionized water, adjusting the pH to 6.2 with NaOH, adding sufficient silica to make a 1 pct aqueous solution then adjusting the pH to 6.2 with HCl. The required amount of stearic acid or spin-labelled stearic acid was dissolved in the minimum amount of acetone required (typically 1 or 2 drops), then added to the silica solution while stirring.
Electron Paramagnetic Resonance (EPR) Spectroscopy
EPR spectra were recorded on a hybrid EPR spectrometer consisting of an X-band (approximately 9 GHz) Varian E-109 bridge with an IBM ER 073 10-in magnet. The samples were contained in a 0.3 mm flat cell in a Varian 4531 rectangular cavity. All spectra were scanned at 1.0 Gauss modulation; checks at lower modulation did not show any improvement in resolution.
Gas Chromatography (GC)
A 250 mL aqueous solution of 1 pct silica and stearic acid was made up as described above, and allowed to stir for 30 minutes. Tests showed that within this time the sample reached equilibrium. A 10 mL aliquot was filtered to remove the silica. The filtrate was extracted with petroleum ether. To the ether extract 3 mL of 10% BCl3/methanol solution was added. The solution was heated for 3 minutes to form the methyl ester of stearic acid. To destroy any excess BCl3, 3 mL of water was added. After this, 5 mL petroleum ether was added and the solution was allowed to stand for 15 minutes; the ether layer was separated and dried with CaCl2. Ten mL of 10-³ M anthracene and naphthalene were added as internal standards. A 2 mL sample of the petroleum ether solution was injected into a gas chromatograph, and the amount of the methyl ester determined by comparison with the internal standards. Tests on solutions of known stearic acid concentration were used to determine the overall efficiency of the extraction/esterification procedure (found to be 65 + 4% overall) and the data were adjusted accordingly.
Shear-Flocculation of Silica
Figure 2 shows the experimental apparatus
used to shear flocculate the silica 20 g of silica and 2 L of deionized water were added to a 4 L polypropylene beaker, which was stirred with a 5-cm-diameter 4-blade impeller. The probe from the particle size analyzer was inserted near the impeller so as to measure the particle size in the most turbulent region of the slurry.
The particle size analyzer was a Partec 100. It consisted of a power supply and a 2.54 cm stainless steel probe with a sapphire window at the tip. The probe contained both the laser source and a detector for back-scattered light. The scan time was 1 second, and particle size measurements were saved to a disk every 9 seconds. The probe was inserted at a slight angle to prevent bubbles from collecting on the window. This also permitted continual washing of the probe by the slurry.
After addition of the silica, pH was adjusted with HCl(aq) to 6.2 and the solution was stirred for 63 seconds. At this point the surfactant was added. Stirring was continued 10-20 minutes. However, the floc size typically reached equilibrium after 5 minutes.
Results and Discussion
When a 1 pct aqueous slurry of silica was mixed at 1750 rpm with 10 -4 M (5 lb/st) stearic acid in 0.1 M NaCl, a 63 pct increase in particle size was observed after 10 minutes. No significant particle size increase was found at 4 x 10 -5 M (2 lb/st) stearic acidconcentrations, indicating that there is a critical or threshold stearic acid concentration below which significant flocculation doesn’t occur. Without NaCl, no increase in particle size was observed even at 4.5 x 10 -4 M (22.5 lb/st). Experiments run with 5- and 16-doxylstearic acids showed similar increases in particle size, indicating that the addition of a doxyl group does not significantly affect the flocculation properties.
Interestingly, no flocculation occurred at pH 12 with either stearic or doxylstearic acid. A qualitative EPR determination showed that very little doxylstearic acid was bound to silica at this pH. Therefore is not surprising that no flocculation occurred at pH 12.
In order for flocculation to occur, it is necessary for the stearic acid to interact with the surface of the silica particles. To understand the interaction between the stearic acid and the silica, an adsorption isotherm was determined by gas chromatography as shown in figure 3. At concentrations above approximately 3 x 10 -5 M (1.5 lb/st) stearic acid the majority of the stearic acid was associated with the surface. Although the concentration of stearic acid in solution increased with total stearic acid concentration, the amount of stearic acid associated with the silica rose more rapidly.
To investigate the type of interaction between the stearic acid and the surface, EPR spectra were run on 5-, 12-, and 16-doxylstearic acid at different doxylstearic acid concentrations using the same conditions used for the flocculation studies and the adsorption isotherm determination. Figure 4 shows the spectra for 16-doxylstearic acid. At low doxylstearic acid concentrations, the spectra were dominated by three narrow lines indicative of isolated doxylstearic acid molecules. As the doxylstearic acid concentration increased, a broad peak began to grow. When the concentration of 6 x 10 -4 M (30 lb/st) was reached, this broad peak became the predominant feature of the spectrum. This broad peak was indicative of the formation of aggregates of doxylstearic acid (Waterman et al., 1986; Baglioni, et al., 1984). In solution these aggregates would be micelles; on the surface they could be hemi-micelles, micelles, monolayers, bilayers or some other type of aggregate.
It was possible to determine the relative amounts of doxylstearic acid which are in an isolated form or in an aggregate by doubly integrating the derivative spectra shown in figure 4 (Wertz and Bolton, 1986). To correct for baseline drift and other artifacts a non-linear least squares fitting procedure was used.
Wavemetrics, Inc., Program Igor was used to fit a Lorentzian curve to the broad peak and Gaussian curves to the three narrow peaks. Analytical expressions (Poole, 1983) were then used to calculate the integral.
Figure 5 shows the fraction of 16-doxylstearic present as monomers as well as the amount of aggregate formed. Figure 5 shows that although the measured monomer concentration in solution showed considerable scatter, the concentration of the aggregated form of stearic acid had very little scatter. This was because essentially all the stearic acid was present as aggregates, even at the lowest concentration(10 -5 M (0.5 lb/st)). Another source of error comes from the method of determining solution spin probe concentrations. The calculation is sensitive to any background signal. This problem is, however, significantly reduced for the broad- line signal. Within the accuracy of measurement, the monomer concentration was constant.
Significant aggregation was present even at 10 -5 M stearic acid, as determined by the non-linear curve fitting procedure, even if this is not apparent from figure 4. This aggregation may have resulted from the presence of micelles. The critical micelle concentration (CMC) of stearic acid in water in the absence of salt is approximately 6 x 10 -4 M (Klevens, 1938) . The addition of salt is known to lower the CMC significantly (Harrold, 1959), so it is not clear if the presence of the silica was enhancing aggregation, as was observed for adsorption of doxylstearic acid on alumina (Waterman et al., 1986).
No difference was found between the coupling constant, line width and relative line heights for the isolated doxyl-stearic acid molecules with or without the presence of silica. Since the doxyl group formed part of the carbon-chain (figure 1), it acted as a very sensitive reporter of the environment and motion of the aliphatic tail of the surfactant. This strongly suggested that individual stearic acid molecules were not strongly bound to the surface. However, aggregates of stearic acid molecules did interact strongly with the surface.
Neosil A silica had a reported surface area of 2 m²/g. At 10 -4 M, total stearic acid concentration, where significant flocculation occurred, the amount of stearic acid associated with the surface was 7.5 x 10 -5 M for 1% silica. This corresponded to an average distance of approximately 20A between stearic acid molecules. However, the broad-line signal suggests the nitroxides are much closer, possibly no more than 10A apart. This meant that the average stearic acid concentration available at the surface was well below monolayer coverage. Given the hydrophobic nature of the alkyl tail of the stearic acid, it is reasonable to assume that the stearic acid associated with the surface does so as either a micelle or as a bilayer consisting of polar carboxylate groups attached to the surface and facing out into the solution. It is reasonable to picture flocculation occurring by a mechanism in which silica particles continually collide due to the shear forces, and when a micelle/bilayer is present at the point of impact, the stearic acid in the micelle/bilayer acts as hydrophobic “glue” to bind the particles together.
In conclusion, shear-flocculation of silica occurred at stearic acid concentrations of >10 -4 M at pH 6 in the presence of 0.1 M NaCl. Where stearic acid was not strongly bound because of pH or the absence of NaCl, no shear- flocculation occurred. The presence of a pendant doxyl group did not alter the behavior of the surfactant. At >10 -4 M at pH 6, the stearic acid was predominantly adsorbed on the surface in an aggregated form; however, the amount of stearic acid adsorbed was less than monolayer surface coverage. When stearic acid was not present in an aggregated form, no shear-flocculation occurred. This demonstrated that aggregates of surfactant molecules at the surface were responsible for the inter-particle bonding which results in flocculation.