The following is a slide show of the presentation I gave at the 4th Research Forum on Recycling in Quebec, QC, Canada in Oct,1997:

The central tower to the Chateau Frontenac in the old city of Quebec where the Forum was held! Pretty nice!

Click on title or image for full page scale slides.

Quebec Slide 1: Fluid Interfacial Separations for Secondary Fiber Recovery as Probed with Atomic Force Microscopy

I'm Eric Aston from the Department of Chemical Engineering at the University of Washington working for Professor John Berg. The behavior of ink particles at water-fluid interfaces is fundamental to all separation processes for secondary fiber recovery. We've begun research in this field by studying a system that we are already familiar with, that is, oil-assisted agglomeration for de-inking toner-printed papers. Specifically, we've focused on a direct, single-particle analysis using atomic force microscopy (a.k.a. AFM) set up for colloidal probing. This colloidal force microscopy is not a conventional AFM mode and has only come into practice for fluid/fluid interfacial studies in the last two years. Our very first results were published in Langmuir this last February.

Quebec Slide 2: Toner-Ink Oil-Assisted Agglomeration

Industry has recently moved away from oil-assisted agglomeration for recycling toner-printed office waste in favor of densification processes. This change was spurred on by the much larger volume of toner usage and by ever increasing restrictions on volatile organic carbon (VOC) emissions. However, oil-assisted agglomeration is still useful in other respects. The critical separation step in toner de-inking by oil-assisted agglomeration is the coalescence of dispersed toner and oil into large aggregates that can be easily removed from the repulped paper slurries leaving clean cellulose fibers. Furthermore, the key is to create aggregates of adequate strength to remain stable against the high shear forces experienced in the repulpers. It has been shown that cationic polymers commonly present in most papers inhibit oil-toner coalescence by forming electrosteric adlayers. Agglomerating agents of simple oils with surfactants are able to overcome this steric stabilization [to wetting the toner]. The addition of a surfactant into the oil phase allows coalescence to occur even with starch-covered particles.

Quebec Slide 3: Cationic Starch Adlayers

In recent tests, we used a commercial brand, modified potato starch (STA-LOK 400--derivatized with quaternary ammonium groups) as an adsorbing polymer similar to cationic starches present in many papers known to inhibit toner aggregation (separation from paper slurries by oil agglomeration processes). The cationic groups on the starch chain (electrosterically) associate with the negative surface charges on the toner particle and the oil-aqueous interface, forming adlayers found to be 49 nm and 33 nm in thickness from dynamic light scattering. Significant amounts of starch were also adsorbed by the oil/surfactant blend, but with no detectable change in droplet size. Along with trends in zeta potential (electrokinetic), this reveals the quite remarkable situation of hydrophilic starch engulfment into the oil drop leaving no adlayer on the aqueous side. Our oil/surfactant blend was a commercial agglomerating agent from Betz Paperchem, Inc. It consists of roughly 60% simple aliphatic oils and 40% largely hydrophobic (HLB--low hydrophile-lipophile balance) surfactants which reside almost completely in the oil phase. The presence of the oil surfactants is enough to allow starch penetration into the oil. Even though the Betz product requires elevated temperatures for maximum performance, the AFM is only suited for making room temperature comparisons.

Quebec Slide 4: Objectives

Our main objective was to use the atomic force microscope (AFM) to examine forces of interaction between an oil droplet under water and a hydrophobic toner particle. The three parameters we've chosen to focus on with AFM are the attractive "snap-in" force pulling the toner and oil together, the applied force required to induce particle attachment or film rupture, and the induction time, which we estimate from the time the toner first encounters the interface until it enters the oil (initial repulsion until final snap-in). The resulting force-distance curves are compared with advancing oil contact angles on toner flats under water. This single-particle AFM analysis is also the first step toward a direct comparison to bulk agglomeration studies that gauge the final degree of toner dispersion (aggregation).

Quebec Slide 5: AFM Cell For Liquid-Liquid Interfaces

This schematic shows an unfused toner particle roughly 10 microns in diameter glued at the end of a flexible, (commercial) AFM cantilever. This assembly is then lowered into the liquid cell for probing. A Teflon tube (ID = 2.25 mm) glued to the bottom of the cell anchors the n-hexadecane, or oil/surfactant, under water. The cell sits on the AFM scanner that uses piezoelectric transducers to move, specifically, the oil-water interface toward and away from the toner. When the toner is near the interface, it will experience attractive or repulsive forces deflecting the cantilever down or up, respectively. The interface is also free to deform as a result of long-range interactions and physical contact with the toner. A laser reflecting from the back of the cantilever is directed to a photodetector (PSPD) measuring the shifts in position. The stiffness of the cantilever is known in terms of a spring constant, so the deflection corresponds to a quantifiable force typically in the nanoNewton range (from pico- to micro-Newtons).

Quebec Slide 6: AFM Force vs. Distance

We've observed three common interactions in our oil-aqueous-toner systems. Force profile "A" illustrates the main features for contacting oil and toner in pure water, and curve "B" shows the decidedly different behavior when cationic starch is added to the water. The horizontal line is the undeflected cantilever signal, or zero interaction force, as the oil approaches the toner. There is a (1) rapid increase in force at or near contact with the oil interface. Hydrodynamic forces initially repel the two hydrophobic materials until the long-range attraction precipitates snap-in and contact. An intervening aqueous film may also resist the attraction until it drains and ruptures. Further advancing continues with the cantilever deflecting in constant compliance with the oil interface. Reversing the direction, (2) the toner drags the interface along and must be pulled back more than 50 microns to detach it from the oil. This large pull-off force can only correspond to a capillary force resulting from a three-phase contact line with radius approaching the toner size. When cationic starch is present to form steric adlayers, (3) the approach curve is monotonically repulsive. No attractive snap-in is observed, and little or no pull-off force is required to detach the toner. In this case, the oil and toner cannot coalesce and agglomeration will fail.

Quebec Slide 7: Toner Particle Approach to Oil-in-Water

These AFM experiments used a toner particle approaching the interface of interest at approximately 30 um/s. This simulates the high-shear environment experienced in repulpers. This approach curve for the pure water case shows the toner being engulfed by the oil after some initial viscous repulsion. Optical images of a larger toner particle (~100 um) interacting with the oil under water were acquired to confirm the events as interpreted from the AFM data. While watching the live video, the snap-in was quite obvious, though it is difficult to appreciate here. The interface was rapidly pulled up and formed a neck with the toner particle.

Quebec Slide 8: Toner and Oil with Cationic Starch Adlayers

The difference with the 0.1 wt% aqueous starch dispersion was anticipated from bulk agglomeration studies but is still quite remarkable. The approach curve, given on the same scale as the previous plot, is monotonically repulsive; that is, no attractive snap-in was detected. Again, optical images show the extreme to which a toner particle can deform the oil drop without wetting, and then be pulled back cleanly, when both are covered with starch adlayers.

Quebec Slide 8B: Toner and Oil/Surfactant Blend

This slide show images and force profiles never used in the talk although the outcome of these data are summed up in the tables of comparative results following. The Betz oil/surfactant always wet the toner surfaces whether in the presence of cationic starch or not. The plots show not great differences in character for either case.

Quebec Slide 9: Review of Results

Now, we can make some gross, quantitative comparisons with the snap-in forces, or distances, the induction times, and the repulsive loads before wetting contact. For oil in pure water, the induction time for the toner was 52 ms at an average applied force of 5.6 nN before the very aggressive snap-in which was off scale and greater than 16.6 nN. The snap-in distance of >332 nm was calculated directly from the force and includes a significant degree of toner entry into the oil; with further analysis we'll get a better idea for the true interfacial separation just prior to snap-in which will be much smaller than this value. The addition of cationic starch at least delayed interfacial rupture beyond 191 ms, we suspect indefinitely, and above 77 nN applied load. These pure oil results are in agreement with the starch-induced stability against coalescence previously observed.
When the pure oil is replaced with the Betz oil/surfactant blend, we once again record a measurable induction time, about 133 ms ( 26), at loads around 0.7 nN ( 0). The snap-in is much weaker than recorded with pure oil (3.5 0.57 nN). This time the addition of starch has a much less dramatic effect on coalescence (151 47 ms, 1.4 0.16 nN, 2.5 0.18 nN) agreeing with the past observations of the oil/surfactant's ability to imbibe the cationic starch adsorbed to both toner and oil. The induction time and load are slightly elevated with a smaller snap-in, suggesting that the starch may still be working against complete wetting of the toner.

Quebec Slide 10: Final Comparisons

These single-particle measurements in oil-aqueous-toner systems further illustrate the roles of cationic starch and oil surfactants in the coalescence of toner and agree with bulk agglomeration studies. Steric stability against toner attachment to oil due to the adsorbed starch is overcome by the addition of a largely hydrophobic surfactant to the oil phase. A qualitative comparison can be made between the snap-in forces as from the AFM and the advancing oil contact angles. Only the case of pure oil in the presence of cationic starch showed interfacial stability (no interfacial rupture) and a nonwetting contact angle (95).
Direct, quantitative comparisons between snap-in force and contact angle for the oil and the Betz product are not easily made because of the effects that interfacial stiffness, viscosity and particle entry have on the AFM analysis. But wetting vs. nonwetting comparisons are still applicable. Both pure water cases show that toner is wet by the oil phase, but toner covered with starch more easily attaches to the oil/surfactant than the pure oil. Looking the Betz experiments with contact angles suggests that the starch has little effect on toner wettability with the oil/surfactant additive.

Quebec Slide 11: Snap-in Interpretation

Our most recent work with recording interactions at liquid interfaces shows great promise for finally generating a true force-separation curve which can be directly compared to theoretical plots of strictly intermolecular forces. Previously, we've been restricted to snap-in values from the AFM with the knowledge that they included not only the actual long-range jump into contact but also a significant degree of particle entry into the oil. A new technique uses an oscilloscope to collect AFM data much faster than is normally considered. Through a simple dynamic force analysis, we are able to subtract most, if not all, of the external, competing forces, like mechanical, capillary, and hydrodynamic effects, to reveal the intermolecular forces.
The toner and oil interface seem to interact in a three-step sequence during snap-in. A long-range attraction first pulls the interface toward the toner particle. When the force field gradient exceeds the stiffness of either the interface or the cantilever to which the particle is glued, the dynamic snap-in begins. The separation rapidly decreases until the intervening water phase forms a thin film that resists the motion via viscous film drainage. The toner particle is finally wet by the oil only after the thin film drains and ruptures against the applied pressure.
The snap-in distance is commonly used to compare the range of attractive interactions between systems. But it is experimentally underestimated because the AFM measurement does NOT include the film thickness. Ideally, the snap-in distance would be the maximum separation at which a free particle is attracted into direct, physical contact with the interface. Because our particle is not free but is attached to a cantilever, the AFM again underestimates this value.

Quebec Slide 12: Dynamic Force Analysis

After the AFM data are modified by this dynamic force analysis, the already significant differences in the behavior of oil and oil/surfactant (with toner) are magnified. The new force curve shown in red emphasizes the incredibly strong attraction for hydrophobic toner particles to the pure n-hexadecane. The toner particle jumps about 70-100 nm closer to the oil droplet where it meets with what we've interpreted as viscous film drainage. After film rupture, the oil wets the particle and pulls it in (by capillary forces). The oil/surfactant blend provides a very much weaker attraction relying mostly on its enhanced wetting ability for coalescence. The amphiphilic surfactant molecules at the oil interface reduce the hydrophobicity and no long-range attraction is evident from the dynamic analysis. A snap-in of several nanometers or less would not have been detected with this design. Increased magnification of the force profile shows slow wetting taking place, and optical images confirm engulfment.
We're really one step away from a complete analysis that can extract the intermolecular force profile from this type of AFM experiment. Hopefully, the last phase will be to model the viscous film drainage to confirm our thin film hypothesis and then subtract its effects from the existing curve. The result will be an even greater well depth in the force profile. In addition to generating new fundamental information, we appear to be on our way to developing a practical tool for testing the affect of surface and solution chemistry on such processes as air-flotation and oil-assisted agglomeration.