Langmuir 1992,8, 51-58
51
Kinetics and Properties of Colloidal Latex Aggregates Measured by Sedimentation Field-Flow Fractionation Bhajendra N. Barman+ and J. Calvin Giddings' Field-Flow Fractionation Research Center, Departmenf of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received July 6, 1991.I n Final Form: September 30, 1991 The unique capabilities of sedimentation field-flow fractionation (SdFFF) in the study of colloidal aggregation are demonstrated by the separation of latex clusters from one another, each composed of a different number of uniform poly(methy1methacrylate) (PMMA)or polystyrene (PSImicrospheres. Specifically, in SdFFF, the singlet, doublet, and higher order aggregated clusters elute on the basis of mass and thus appear as separate regularly spaced peaks in the elution profile or fractogram. The formation and breakup of these aggregates under different experimental conditions are readily recognized from the altered profile of the eluting peaks. Physical agitation such as ultrasonication is found to destroy some aggregates. As expected, the breakup of aggregates by such means depends on the sonication time. Some aggregated latex materialsare reduced totally to singletsafter sufficientsonication times but other aggregates cannot be completely disrupted by sonication. This variable stability of aggregates is attributed to different growth, aggregation, and aging cycles. From one PMMA latex sample, two distinct subpopulations of "doublet" clusters were isolatedby SdFFF. A retention time differenceof 10 % indicated a mass difference of 10%between the two. The lighter more compactly fused doublets were assumed to have first aggregated at an earlier stageof latex growth. Calculations based on the mass difference, augmented by the measurement of doublet dimensions from both subpopulations by electron microscopy, provided self-consistent values of all relevant singlet and doublet dimensions at both stages of growth, even though the first stage was not accessible to direct measurement. The controlled aggregation of both monodisperse PS and PMMA is induced by the addition of a cationic surfactant. It is found that the extent of aggregationdepends on the amount of cationic surfactant added up to a certain critical concentration, beyond which the latex population precipitates out. These aggregates are fairly stable and are easily separated and resolved by SdFFF. Scanning electron microscopy of collected fractions confirms that each peak contains clusters of unique aggregation number. N
N
Introduction There are numerous reports on the study of aggregation occurring in industrial, biological, and environmental particulate samples. The methods used for these studies include small-angle X-rays or neutron ~cattering,~-'Olight scattering,6J1-17 viscosity measure+ Present address: Texaco Inc., P.O.Box 1608,Port Arthur, T X 77641. (1)Kitano, H.; Iwai, S.; Ise, N.; Okubo, T. J.Am. Chem. SOC.1987,109, 66414644. - - .- - - ... (2)Cornell, R. M.; Goodwin,J. W.; Ottewill, R. H. J. Colloid Interface S C ~1979, . 71,254-266. (3)Zhang,H. X.;Sorensen, C. M.;Ramer,E. R.;Olivier, B. J.; Merklin, J. F. Langmuir 1988,4,867-871. (4)Samson.. R. J.:. Mulholland. G. W.: Gentrv. J. W. Lanpmuir 1987. 3,272-281. (5)Jeffrey, G. C.; Ottewill, R. H. Colloid Polym. Sci. 1990,268,179189. (6)Schaefer,D. W.;Martin,J.E.;Wiltzius,P.;Cannell,D. S.InKinetics of Aggregation and Gelation; Family, F., Landau, D. P., Eds.; Elsevier: New York, 1984;pp 71-74. (7)Weitz, D. A.;Huang, J. S.InKinetics of Aggregation and Gelation; Family, F., Landau, D. P., Eds.; Elsevier: New York, 1984; pp 19-28. (8)Sinha, S.K.; Freltoft, T.; Kjems, J. In Kinetics of Aggregation and Gelatton; Family, F., Landau, D. P., Eds.; Elsevier: New York, 1984;pp 87-90. (9)Ottewill, R. H. Langmuir 1989,5,4-9. (10) Wong, K.; Cabane, B.; Duplessix, R.; Somasundaran, P. Langm u v 1989,5, 1346-1350. (11)Cametti, C.; Codastefano, P.; Tartaglia, P. J. Colloid Interface Sci. 1989,131,409-422. (12)Cametti, C.; Codastefano, P.; Tartaglia, P. Phys. Reu. A : Gen. Phys. 1987,36,4916-4921. (13)Tang, P.; Colflesh, D. E.; Chu, B. J. Colloid Interface Sci. 1988, 126,304-313. (14)Pelton, R. H.; Pelton, H. M.; Morphesis, A.; Rowell, R. L. Langmuir 1989,5,816-818. (15)Klyubin, V. V.; Kruglova, L. A.; Sokolov, V. N. Kolloidn. Zh. 1988,50,864-872. _
I
menta at different shear rates,18 measurements of sedimentation rate^,^^^^^ optical density or turbidity meas~ r e m e n t a , ~and l - ~coulter ~ counter method.25 The unique capabilities of sedimentation field-flow fractionation (SdFFF) in studying aggregated colloids by separating and individually characterizing clusters of different mass were first demonstrated in the separation of aggregated viral rods.26 Recently, we have applied this method for the separation and determination of the polydispersity of poly(methy1methacrylate) (PMMA) latex aggregate^.^^^^^ We have also examined the aggregation of protein-coated polystyrene (PSI latex beads by SdFFFSz9 An advantage of SdFFFis that the separation and ready collection of fractions of different cluster sizes lends itself
I
0743-7463/92/2408-0051$03.00/0
(16)Rarity, J. G.; Seabrook, R. N.; Carr, R. J. G. Proc. R. SOC.London, A 1989,423,89-102. (17)Olivier, B. J.; Sorensen, C. M. J. Colloid Interface Sci. 1990,134. . . 139-146. (18)Utracki, L. A. J. Colloid Interface Sci. 1973,42,185-197. (19)Reynolds, P. A.;Goodwin,J. W. Colloids Surf. 1987,23,273-299. (20)Pires Costa, M. C.; Galembeck, F. Colloids Surf. 1988,33,175184. (21)Bensley, C. N.; Hunter, R. J. J. Colloid Interface Sci. 1982,88, 546-561. - - - - - -. (22)Ma, C. Colloids Surf. 1987,28, 1-7. (23)Jeffrey, G. C.; Ottewill, R. H. Colloid Polym. Sci. 1988,266,173-
-. ". 170
(24)Esumi, K.; Meguro, K. J. Colloid Interface Sci. 1989,129, 217221. (25)Pefferkorn, E.; Varoqui, R. J. Chem. Phys. 1989,91,5679-5686. (26)Caldwell, K.D.;Nguyen, T. T.; Giddings, J. C.; Mazzone, H. M. J. Virol. Methods 1980,1, 241-256. (27)Jones, H. K.; Barman, B. N.; Giddings, J. C. J. Chromatogr. 1988, 455.1-15. (28)Giddings, J. C.; Barman, B. N.; Li, H. J. Colloid Interface Sci. 1989,132,554-565. (29)Beckett, R.; Ho, J.; Jiang, Y.; Giddings, J. C. Langmuir 1991,7 , 2040-2047.
0 1992 American Chemical Society
52 Langmuir, Vol. 8, No. 1, 1992
Barman and Giddings
to combination with other methods that can be used to further characterize the isolated fractions. Electron microscopy (EM), for example, is a valuable adjunct of SdFFF, making it possible to identify structural features and dimensions of separated clusters. Without the fractionation provided by SdFFF, bound clusters cannot be readily distinguished from the transient clusters formed during sample preparation for EM. The size and cluster distribution obtained by EM (or other conventional methods) alone will thus provide an erroneous characterization of aggregated materials. Because of its relative simplicity, aggregation occurring in monodisperse latex suspensions is considered as a model process for understanding different aspects of aggregation p h e n ~ m e n a . ~ JA ~ ,controlled ~ ~ . ~ ~ low order aggregation process results in the formation of doublets, triplets, and higher order aggregated clusters in such samples. Knowledge of the conditions leading to the formation and breakup of such latex aggregates can be exploited to avoid, induce,30v31 or reverse5vZ4aggregation in other colloidal materials. This knowledge is useful to determine the range of conditions suitable for obtaining dispersions having acceptable levels of aggregation. In this work, sedimentation FFF (both with and without EM) has been applied to monitor aggregation phenomena in latex suspensions. SdFFF is a mass-based separation method that is highly effective in resolving particles having small mass differences in the colloidal size range. The elution of colloidal materials from a SdFFF flow channel is based on well-defined theoretical principles. The theoretical equations are advantageous for the optimization of experimental conditions as well as for the calculation of properties of colloidal particles based on their SdFFF fractogram (i.e., detector response versus time). Because of its high mass selectivity and resolving the method is uniquely positioned for the study of colloidal aggregation. Clusters of different aggregation number formed from a relatively monodisperse starting population can be readily resolved from one another by SdFFF.27s28 Because fractograms obtained by SdFFF are subject to relatively rigorous theoretical interpretation, they provide detailed information on the relative amounts, and changes in the amounts, of individual aggregated clusters in colloidal samples. Thus the formation and breakup of aggregates of different sizes can be readily tracked using this method. This study addresses the mechanisms of both the breakup and formation of latex aggregates. For breakup, we have used an ultrasonication device to agitate sample populations to convert aggregated species to singlets or other low-order clusters. Where possible, we have ultimately degraded the aggregates to a singlet population. The destruction of higher order clusters is apparent from the decrease in their peak areas and the corresponding increase in those of the singlet and lower order aggregates.28 The most common methods used to modify particleparticle interactions to induce aggregationare the addition of surfactants,l0,22,31t3 sa1ts,2,5,6,9,11,12,15,16,19,21,23,25 polym e r ~ , and ~ ~small * ~ organic ~ , ~ ~molecules such as pyridine" to the particle dispersion. Such dispersions are found to be stable in a certain range of surfactant concentration.
Aggregation tends to occur at surfactant concentrations falling outside the specified range.22 Particle size and potential measurements provide evidence for bilayer35or multiple layers of surfactant molecules on particle suror coagulation induced by salt is f a c e ~ .Aggregation ~~ caused by the decrease in electrical double layer thickness as supported by {potential measurement^.^^>^ The mixing of oppositely charged latex particles results in aggregation due to electrostatic attractions.lJ9 In this study, the formation of aggregates from uniform size PMMA and PS beads was induced by the addition of a cationic surfactant to a dispersion already exposed to an anionic surfactant. The appearance of multiple peaks instead of a single peak in the SdFFF fractogram of samples treated in this manner directly reveals the formation of aggregates from the monodisperse latex population. The relative populations are monitored by peak area measurements. The correspondence between the populations of different clusters and the amount of cationic surfactant added to the original dispersion is then established by SdFFF analysis.
where y is the dimensionless steric correction factor, of order unity. By combining eqs 1 and 2, or eqs 1, 2, and 4, one can estimate the particle mass and effective particle diameter of the singlet and of its aggregates in terms of their
(30)Moudgil, B. M.; Shah, B. D.; Soto, H. S.J . Colloid Interface Sci. 1987,119,466-473. (31)Thompson, L.J . Chem. Soc., Faraday Trans. 1 1984,80,16731688. (32)Myers, M. N.; Giddings, J. C. Anal. Chem. 1982,54,2284-2290. (33)Blackley, D.C. In Polymer Colloids; Buscall, R., Corner,T., Stageman, J. F., Eds.; Elsevier: New York, 1985;pp 247-288. (34)Russo, P. S.;Mustafa, M.; Cao, T.; Stephens, L. K. J. Colloid Interface Sci. 1988,122,120-137.
(35)Esumi, K.;Sakamoto, Y.; Yoshikawa, K.; Meguro, K. Colloids S u r f . 1989,36, 1-11. (36)Meguro, K.;Adachi, T.;Fukunishi, R., Esumi, K. Langmuir 1988, 4,1160-1162. (37)Watillon, A.;Joseph-Petit,A.-M. Discuss. Faraday SOC.1966,42, 143-153. Shaw, J. N. Discuss. Faraday SOC.1966,42,154(38)Ottewill, R.H.; 163. (39)Harding, H.J. Colloid Interface Sci. 1972,40,164-173.
Theory The theoretical details describing the fractionation and resolution of colloidalaggregates by SdFFF were provided e l s e ~ h e r e . However, ~ ~ , ~ ~ a few essential points are covered here for continuity of treatment. The retention ratio R of a given component or cluster is obtained experimentally as the ratio of the void volume Voof the SdFFF channel to the cluster retention volume V,. For normal FFF operation, R is related to the dimensionless retention parameter X by
(z)
V" 1 R =v r = 6X[ coth -2x1 The parameter A in sedimentation FFF is related to particle mass m or effective spherical diameter d by
where k is the Boltzmann constant, Tis the absolute temperature, G is the centrifugal acceleration, w is the channel thickness, ps is the particle density, and Ap is the difference in density between the particle and the carrier liquid. For highly retained particles with small R or A, R approaches the simple form
R = 6X (3) A modified form of eq 3 that provides a correction for steric (finite particle size) effects is R = 3 yWd + 6 X
(4)
Langmuir, Vol. 8, No. 1, 1992 53
Col loida 1 Latex Aggregates
measured retention volume Vr. Thus eqs 1,2, and 4 yield
where the final approximation, obtained by ignoring the steric term, shows that particle mass is roughly proportional to Vr. Thus aggregates of a monodisperse latex, each having a mass equal to some multiple of the underlying unit mass, are expected to produce a periodic array of emerging peaks, an expectation generally borne out by o b s e r ~ a t i o n .However, ~ ~ ~ ~ ~ the spacing between successive peaks is observed to decrease as the cluster size increases. This behavior is attributed to the steric exclusion mechanism and is consistent with the expanded center term of eq 5.
a
u,
Experimental Section Sedimentation FFF System. A description of the SdFFF apparatus was provided in recent p ~ b l i c a t i o n s . ~The ~ * ~ap~*~~ paratus is similar in most technical respects to the Model SlOl colloid/particlefractionator from FFFractionation, Inc. (SaltLake City, UT). A channel 0.0254 cm thick, 90.5 cm long (from inlet to outlet), and 2.0 cm in breadth was used. The channel is secured inside an aluminum rotor with a radius of 15.3 cm. The channel voild volume is 4.50 mL. Materials. The carrier liquids (used for SdFFF operation) were either doubly distilled water containing 0.05 % (w/v)sodium dodecyl sulfate (SDS) and 0.01 % (w/v) sodium azide, both from Sigma (St. Louis, MO), or water with a small amount (1drop/L) of the nonionic surfactant Alkawet N (Lonza Inc., Long Beach, CA). The carrier liquid with the nonionic surfactant was used to stabilize and analyze the aggregates formed after the addition of a cationic surfactant to a colloidal dispersion containing an anionic surfactant. The response from a UV detector (Model 153, Beckman Instruments, Fullerton, CA) working at 254 nm was monitored by a recorder from Houston Instruments (Austin, TX). The PMMA latex beads of 0.299 and 0.586 pm nominal diameters and the PS latex beads of 0.327 pm nominal diameter were obtained as a 10% (w/w) latex suspension from Seradyn Diagnostics(Indianapolis,IN). Each latex suspensionwas further diluted with the carrier liquid. Procedure for Aggregate Breakup. An ultrasonic device, Model SC lOlTH from Sonicor Instruments (Copiague, NY), was used to agitate latex samples suspended in water containing SDS. The sample was agitated for a specific period, then a small portion was injected for SdFFF analysis. The same sample was sonicated again for another specified period and subjected to a subsequent analysis. Procedure for Aggregate Formation. Tetrahexylammonium bromide (THAB) was the cationic surfactant used in these studies. The original 0.327 pm PS and 0.299 pm PMMA latex dispersions were prepared in SDS solution (0.05 % (w/v)). Different amounts of cationic surfactant were added to vials containing the original latex dispersion and mixed well. Each suspension was then allowed to stand for 15min or longer so that singlets and aggregates (if formed) could reach steady-state condition prior to injection into the SdFFF channel. (SdFFF results were invariant after the 15-min waiting period, thus justifying the steady-state assumption.) The resulting suspensions with THAB concentrations of 0.12,0.45, and 1.03 mM were found to be stable. The sample from each vial of stable suspension was analyzed by SdFFF. However, the latex populations with THAB concentrations of 1.18 or 1.74 mM were visibly altered, forming light or clumpy precipitates, respectively. We note that the concentration of SDS in the dispersion prior to the addition of cationic surfactant was 1.73 mM.
Results and Discussion In earlier studies we utilized SdFFF to fractionate and thus to discern the existence of numerous low and medium (40) Jones, H.K.; Phelan, K.; Myers, M. N.; Giddings, J. C. J. Colloid Interface Sei. 1987, 120,140-152.
I
I
0
50
I
100
I
I50
ELUTION VOLUME (mL) Figure 1. Fractogram for 0.586 pm PMMA latex aggregates. Inserted micrographs are for particles isolated in different fractions as indicated in the fradogram. Experimentalconditions: field, 9.9 g (240 rpm); carrier, 0.05% (w/v) SDS solution with 0.01% (w/v) sodium azide; volumetric flow rate, 0.38 mL/min.
order aggregates in various samples of “monodisperse” PMMA Changes in the population levels with sonication were tracked. However, not all aggregated PMMA samples displayed the simple pattern of resolved periodic peaks correspondingto singlets,doublets,triplets, etc. In particular, the 0.586 pm PMMA sample yields an extra peak (labeled b) sandwiched between the singlet peak a and the normal doublet peak c as shown in Figure 1. We elected to probe this anomaly further using a combination of SdFFF and EM. Separation and Characterization of Fused Doublets. The electron micrographs of several fractions collected from the SdFFF run of the 0.586 pm PMMA are shown in Figure 1. (Measurement of the singlets by EM suggests a diameter of 0.54 f 0.02 pm rather than 0.586 pm.) These micrographs show that peak b, like peak c, consists of doublets, but in peak b the doublets are clearly fused more tightly together than in peak c. Careful measurements of the images show that the doublets in b have diminished in length due to the fusion from the normal length (twice the diameter of the singlet) of 1.08 f 0.03 pm, to 0.88 f 0.04 pm. (The doublets in c are 1.09 f 0.08 pm from EM measurements, as expected for two spheres in contact.) However, it is not clear from the micrographs if the fused doublets of b have simply aged longer than those in c and thus “annealed” into a more compact binary particle or if some other mechanism is at work. SdFFF provides some important evidence to help resolve such questions. The observed retention volumes of the singlet, fused doublet, and normal doublet populations are 66.6, 96.4, and 107.1 mL, respectively. SdFFF was used first of all to verify the singlet diameter. A value of 0.539 pm was obtained from the measured retention volume Vr of the singlet from eqs 1,2, and 4 assuming y = 1and ps = 1.20 g/mL. This is in close agreement with the EM results. Similarly, effective spherical diameters for the fused and normal doublets are 0.625 and 0.655 pm, respectively. Turning now to the fused doublets, a simple annealing mechanism of fusion can be ruled out immediately on the basis of the different positions of peaks b and c. Since peak b precedes peak c by about 10% ,the fused doublets of peak b can be inferred to have -10% less mass than
54 Langmuir, Vol. 8, No. 1, 1992 1st stage latex
a.
:Qt
0
ag-
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d0
Barman and Giddings
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fused doublet
-mfd = 1 - 3 - + 2 -h2 mnd d12
growth
h3 d13
(9)
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Figure 2. Formation of fused and normal doublets. Fused doubletsare formed by further polymerization on two aggregated first stage latex particles.
the normal doublets of peak c (see eq 5). Somewhat more precisely, the application of eq 5 (in its expanded form with yd = 0.625 pm for fused doublets and yd = 0.655 pm for normal doublets) to the retention data suggests that the mass ratio mfd/mndof fused to normal doublets is 0.87; the simplified form of eq 5 yields 0.90. An alternate mechanism of doublet formation consistent with the observation of a reduced mass of the fused doublet relative to that of the normal doublet entails the formation of the two doublets at different stages of latex growth (see Figure 2). If a two-stage growth process was used for these beads, then two discrete populations of doublets could arise according to whether aggregation occurred after the first stage (Figure 2a) or after the second and final stage (Figure 2b). (Unfortunately the records detailing the preparation of this sample are not available, but a twostage process was frequently used for larger latexes such as this at the time of preparati~n~l.) Aggregationoccurring continuously in a single stage of growth can be ruled out by the discretenessof the doublet populations as exhibited by the two SdFFF peaks (b and c) of Figure 1. The proposed mechanism of aggregation and the dimensions of the particles involved are shown in Figure 2. We assume that the fused doublets first aggregated after the spheres reached diameter do. In the second stage both spheres and existing doublets were coated with an additional layer of thickness h. The final sphere diameter dl would become d, = do + 2h
(6)
and the length Lfd of the fused doublets would be (7) By contrast, the length of the “normal” doublets formed after the completion of the second growth stage would equal
The consistency of the two-stage mechanism can be checked by comparing results based on relative SdFFF peak positions and those obtained from EM measurements made on the fractionated doublets. Since EM observations yield 2dl = 1.08 f 0.03 pm and Lfd = 0.88 f 0.04 pm, eqs 6-8 can be used to approximate h and thus do: h = dl Lfd2 = 0.10 f 0.03 pm and do = Lfd - dl = 0.34 f 0.05 pm. The SdFFF calculation is based on the reduction of mass of the fused relative to the normal doublet caused by the exclusion of second stage growth from the contact region of the initial doublets. It can be shown that the relative mass of fused and normal doublets is given by (41) Bangs, L. Personal communication, March 9, 1988.
As indicated previously, eq 5 shows this mass ratio to lie in the approximate range 0.87-0.90; we can reasonably represent this ratio by 0.88 f 0.02. Equation 9 then yields (h/&1)= 0.22 f 0.01. If either the SdFFF value, dl = 0.539 f 0.006 pm, or the EM value, dl = 0.54 f 0.02 pm, is used, we get h = 0.12 f 0.01 pm, which is consistent with but perhaps somewhat more reliable than the EM value reported above, h = 0.10 f 0.03 pm. Similarly, SdFFF yields do = dl - 2h = 0.30 f 0.02 pm. The above study shows that a combination of SdFFF and EM is capable of providing a wealth of information on aggregated colloidal species not otherwise available. The consistencyof the mass differencesof the two doublets obtained from SdFFF and the dimensional differences from EM, assuming a two-stage aggregation process, is strongly suggestive that the dual growth mechanism is correct, whereas an aging (annealing) process can be unequivocally ruled out. While such detailed information would be more difficult to acquire in polydisperse samples, it is expected that these samplestoo would provide a fertile area of investigation because, through mass-based SdFFF fractionation and the subsequent use of dimensionalsensitiveEM, mass could be correlatedwith morphological and dimensional features over a wide particle size range. Kinetics of Cluster Breakup. The fused doublets in Figure 1 cannot be converted into singlet beads by sonication. On the other hand, most latex aggregates can be broken up providing conditions are such that disrupted singlets or lower order aggregates are stabilized in the suspending medium. Some clusters, however, are found to be quite stable although they do not appear to be fused. We previously encountered such clusters in a 0.230 pm PMMA sample where only a portion of multiplets could be destroyed by sonication to obtain singlets and lower order aggregates.28 A sonication for more than 2 h failed to convert all the multiplets of the 0.230 pm PMMA sample to a singlet population. The high stability of these aggregates is likely caused by polymer molecules (or adsorbed polymeric surfactant molecules used for latex ~tabilization~~) that penetrate from one bead into another, thus building bridges of polymer chains across the beads. This is expected to act as a binding mechanism for latex beads sterically stabilized by a grafted layer of polymer chain^.^,^^ The resulting clusterscould not be easily broken apart. This hypothesis is consistent with the observation of aggregates in “old” latex suspension^^^^^^ where more bridges could form with time. Such aggregation phenomena and stability against breakup can be characterized using SdFFF since changes in the individual cluster populations can be observed and measured. Below we describe a case in which the time course of cluster breakup is tracked by SdFFF and the kinetics of the dissociation process is established. We used sonication to disrupt the aggregated species found in the 0.299 pm PMMA sample and then examined the resulting distribution of clusters displayed by the fractograms as a function of the time of sonication. Figure 3 shows the effects of sonication time on this sample. The evolving elution pattern shows that a gradual destruction (42) Lafuma, F.; Wong, K.; Cabane, B. J . Colloid Interface Sci. 1991, 143, 9-21. (43) Barman, B. N.; Giddings, J. C. In Particle Size Distribution II: Assessment and Characterization;Provder, T., Ed.; ACS Symposium Series 472; American Chemical Society: Washington, DC, 1991; pp 217228.
Langmuir, Vol. 8, No. 1, 1992 55
Colloidal Latex Aggregates 1.0
0. Unsonicated sample
singlet
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Figure 3. Effect of sonication on 0.299 pm PMMA latex aggregates. Experimental conditions: field, 54.7 g (565 rpm); carrier,0.05% (w/v)SDS solutionwith0.01% (w/v) sodium azide; volumetric flow rate, 1.10 mL/min. Sample volumes injected were (a) 25, (b) 20, (c) 20, (d) 15, (e) 10, (0 8, and (9) 8 pL.
of higher order aggregates occurs as the sonication period is increased. Ultimately, after the sonication extends to a period of 20 min, the only observable peak remaining is that for the singlet population. We note that the injected sample volumes used for sedimentation FFF analysis in Figure 3 were different for different sonication time intervals in order to maintain similar peak heights. However, if all disruption steps follow first-order kinetics (as expected), and second and higher order reaggregation processes during sonication are unimportant (as suggested by the final and apparently irreversible reduction of all clusters to singlets), the fractograms in Figure 3 would provide a direct comparison of changes in the relative populations of the singlets and multiplets for different sonication periods independent of sample size. Second and higher order kinetic processes, if they exist, would yield different peak area (or height) ratios for different sample amounts, other conditions being constant. A fully detailed analysis of the kinetics of cluster breakup would require the precise monitoring of the concentrations of clusters of different sizes as a function of sonication time. Unfortunately, the detector used in this study provides a response based largely on light scattering, which, while proportional to concentration, is also a complex function of cluster size and shape. Although compensated response after light scattering corrections can be obtained for spherical p a r t i c l e ~ , 4the ~?~ formulation ~ of a similarly corrected response for odd-shaped clusters of spheres is not available. Accordingly, peak area measurements will accurately provide only the relative population changes for the different cluster sizes. (44) Kirkland. J. J.: R e m e n t e r , S. W.: Yau, W. W. Anal. Chem. 1981, 53,1730-1736. (45) Yang, F.-S.; Caldwell, K. D.; Giddings, J. C. J. Colloid Interface Sci. 1983, 92,81-91.
Figure 4. Plots of the logarithm of relative peak area vs sonication time for different clusters. Despite the difficulty in measuring absolute concentration levels, changes in the relative peak area (normalized by the total area of all peaks) provide a good measure of relative concentration changes and can be used for the analysis of first-order kinetics. Normalizing by the total (summed) peak area is justified by the observation that the total area deviates by no more than 10% from its mean value found for fixed sample amounts but for widely different sonication times. Figure 4 provides a plot of the logarithm of the relative peak area of different clusters versus the sonication time. The data points in this figure are derived from the fractograms in Figure 3 and two additional fractograms obtained for sonication times of 1and 12 min. The relative peak area of the “quadruplet” includes the area for the unresolved higher order aggregates as well. The logarithmic plots of relative peak areas shown in Figure 4 approach linearity for doublets, triplets, and quadruplets as their concentrations become small compared to that of singlets but large compared to that of the higher multiplets whose own decomposition might augment the concentration level of the cluster in question. The asymptotic negative slopes of these plots can therefore be identified with the first-order rate constants for aggregate disruption. The rate constants are found to be 6.8 X and 8.5 X for doublets,triplets, 4.5 X and quadrupleWhigher order aggregates, respectively. These values are consistent with the expectation that quadruplets or higher order aggregateswill break up faster than triplets, and triplets will dissociate more readily than doublets. We note that the ratio of rate constants is 2.0: 3.0:3.8, approximately equal to the ratio of aggregation numbers for the respective clusters. We are not aware of any previous studies in which it has been possible to measure the absolute and relative dissociation rates of aggregated clusters of different sizes. Surfactant-Induced Cluster Formation. Figure 5 shows a relative increase in the population of doublets and higher order aggregates as increasing amounts of the cationic surfactant, THAB, are added to the 0.299 pm PMMA singlet population in the presence of 0.05 % (w/v) SDS. We note that the starting dispersion containing the singlet population was obtained for the sample by sonicating the original 0.299 p m PMMA latex for 20 min or longer,producing the total breakup of higher order clusters, as confirmed by Figure 3g. We observe that a small doublet peak emerges with a THAB concentration as low as 0.12 mM. Peaks for doublets and triplets are clearly visible with a THAB concentration of 0.45 mM. The populations of doublets and higher order aggregates are found to be more pronounced with a concentration of 1.03 mM THAB.
Barman and Giddings
56 Langmuir, Vol. 8, No. 1, 1992 a. Original sample -singlet
A
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b. THAB cow: 0.12mM
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[-single+
ELUTION VOLUME (mL)
Figure 5. Formation of 0.299 pm PMMA latex aggregates by using different amounts of cationic surfactant, THAB. Experimental conditions: field, 46.3 g (520 rpm); carrier, water containing Alkawet N; volumetric flow rate, 1.08mL/min. Sample volumes injected were (a) 5, and (b, c, and d) 6 pL.
sinalets
r
0
I
25
I
doublets tridets
20
I
75
bo
auadrudets
I
125
ELUTION VOLUME (mL) Figure 6. Fractogram showing where fractions of 0.299 pm PMMA latex aggregates were collected. The aggregates were formed when 1.03 mM THAB was added to the singlet suspension in the presence of 0.05% (w/v) SDS solution. Sample volume: 17 pL. Experimental conditions are the same as in Figure 5. Inserted micrographs are for fractions collected in the positions indicated by dashed lines. (Note that the “quadruplets” fraction contains two quintuplet clusters and one double quadruplet, the latter undoubtedly formed upon preparation for microscopy.)
The formation of 0.299 pm PMMA aggregates is verified by the electron microscopic examination of collected fractions from different peaks of the fractogram as shown in Figure 6. The fractogram in Figure 6 was obtained for the sample with a THAB concentration of 1.03 mM. The inserted micrographs in the figure show that the first four peaks correspond to singlets, doublets, triplets, and quadruplets, respectively. The fifth peak, eluted after turning off the centrifugal field,corresponds to aggregates larger than quadruplets. It is interesting to note that the relative amounts of different aggregated clusters in the original sample (as shown in Figure 3a) and those in the
ELUTION VOLUME (mL)
Figure 7. Formation of 0.327 pm PS latex aggregates with different cationic surfactant (THAB) concentrations. Sample volumes injected were (a, b, and c) 3 and (d) 2.5 pL. Experimental conditions: field, 61.6 g (600 rmp); carrier, water containing Alkawet N; volumetric flow rate, 0.424 mL/min.
sample where aggregation was induced by a cationic surfactant (as shown in Figures 5d and 6) are quite different. A larger population of higher order aggregates in the commercial sample is probably due to aging and/or a difference in the suspending medium used for latex stabilization. The presence of polystyrene aggregates, particularly in aged samples, has been reported by other workers.20 However, in our work with numerous polystyrene samples, extending over many years, we have rarely observed aggregates. Recently, we detected trace amounts of doublets of the major size component of a blended fourcomponent PS sample.43 We have also observed, as noted earlier, the aggregation of protein-coated PS beads.29It is of interest to further examine conditions that will induce low-order aggregation in this relatively stable latex. For this purpose we have applied SdFFF to study induced aggregation in a sample of 0.327 pm nominal diameter PS latex beads. Fractograms of the original PS latex sample and of three of the PS samples to which different amounts of THAB were added are shown in Figure 7. The originalpolystyrene suspension,yielding only a singlet peak, was prepared with SDS as described earlier. A doublet peak for the sample exposed to 0.12 mM THAB can be found upon close examination. Both doublet and higher order aggregate peaks emerge with 0.45 and 1.03 mM THAB concentrations. A careful examination reveals four peaks in fractogram d which was obtained for the sample containing 1.03 mM THAB. Six PS aggregate peaks are observed clearly in the fractogram of Figure 8, which was obtained by using a larger volume of the sample containing 1.03 mM THAB. Six fractions cut from the six resolved peaks were collected for microscopic examination. The micrographs obtained from these cuts are shown in Figure 9. As might be expected, singletsthrough sextupletsare observed in these cuts. The large final peak, appearing after the field was turned off, indicatesthe presence of clusters larger than sextuplets. Their low individual concentrations combined with the secondary effects of both polydispersity and shape effects may prevent the observation of many additional resolved peaks.28
Langmuir, Vol. 8, No. 1, 1992 57
Colloidal Latex Aggregates
surfactants on the particle surface.51 Evidence for the buildup of such multiple adsorption layers was presented in a recent The buildup was achieved by adding both anionic and cationic surfactants alternately to positively charged particle surfaces. In the framework of the above studies, it can be concluded that the formation of latex aggregates is due to the partial or complete neutralization of surface charges by the cationic surfactant followed by the coalescence of particles having neutral (or even patches of oppositely charged) surfaces. The particle cluster is then stabilized by the adsorption of excess anionic surfactant molecules present in the carrier and/or by adsorbed anionic surfactant molecules which were not neutralized.
I Cut No:
r'
Conclusions
6
;5
io
715
I60
ELUTION VOLUME (mL) Figure 8. Fractogram showing where fractions were collected for 0.327 PS pm aggregates formed when 1.03 mM THAB was added to an original PS latex suspension in the presence of 0.05 76 (w/v) SDS solution. Sample volume was 1 2 pL. Experimental conditions are the same as in Figure 7.
Singlets (cut 1)
ri Quads. (cut 4)
Doublets (cut 2)
Triplets (cut 3)
Sexts. (cut 6) U
I
1Pm Figure 9. Electron micrographs for particles isolated in the different cuts shown in Figure 8.
The effects of surfactant on the stability of colloidal suspensionshave received major attention.22,35,36p46-51 The adsorbed anionic or cationic surfactant molecules, with their associated electrical double layer, control colloidal stabilization by an electrostatic m e c h a n i ~ m . ~On ~ *the ~~ other hand, nonionic surfactants lead to the steric stabilization of colloidal particle~.~~950 Oppositely charged colloidal particles tend to form aggregates due to electrostatic a t t r a ~ t i o n . ~The , ~ ~interaction between cationic surfactant and adsorbed anionic surfactant is expected to induce the formation of stable aggregates through the latter mechanism. It is likely that cationic surfactant molecules adsorb on the particle surfaces and neutralize the opposite surface charges. If a sufficient amount of cationic surfactant is present, it may reverse the charge carried by the particles and finally restabilize the d i ~ p e r s i o n .This ~ ~ is equivalent to the bilayer adsorption of anionic surfactants on positively charged particles50or the formation of ion-pairs due to the adsorption of cationic and anionic (46) Piirma, I.; Chen, S.-R. J. Colloid Interface Sei. 1980, 74,90-102. (47) Paxton, T. R. J. Colloid Interface Sei. 1969, 31, 19-30. (48) Varennes, S.; Van de Ven, T. G. M. Colloids Surf. 1988,33,63-74. (49) Kayes, J. B. J. Colloid Interface Sei. 1976,56, 426-442. (50) Meguro, K.; Tomioka, S.; Kawashima, N.; Esumi, K. Prog. Colloid Polym. Sei. 1983, 68, 97-100. (51) Huang, Z.; Yan, Z.; Gu, T. Colloids Surf. 1989, 36, 353-358.
Sedimentation FFF has been shown to provide detailed information on the physical state of aggregation by resolving different cluster sizes and masses. The relative amount of each aggregate formed under specific experimental conditions is readily monitored by this method. The information obtained is far more complete than that provided by the measurements of average particle diameter,13-15~22,24,34-36,50 which is commonly the only information available on the state of aggregation. The combination of SdFFF with complementarytools like EM is particularly adept at probing fundamental aggregation processes and structures. This capability is utilized above to examine the influence of a number of factors (sonication time, cationic surfactant concentration,etc.) on cluster populations and on the dynamics of population shifts. It would be possible to elucidate aggregation/breakup kinetics in still greater detail if specific cluster subpopulations were isolated by SdFFF and their kinetics examined individually. In this way one could measure various cluster-to-cluster transition rates. One could determine, for example, if (and under what circumstances) cluster breakup occurs predominantly by the shedding of singlets or by more symmetrical cluster fission. The main difficulty in pursuing such a study would be the small quantities (in the milligram range) of pure clusters that could be readily isolated by SdFFF, which would require special sample handling techniques and highly sensitive measurement. While the monodisperse latexes used above are useful models for the study of aggregation, they obviously do not replicate all the properties of polydisperse colloids,whose aggregation is a widespread problem of great practical significance. The study of the aggregation of polydisperse colloids by any means would clearly be more difficult than the corresponding study involving monodisperse microspheres. SdFFF applied to such complexaggregateswould not be able to resolve clusters of different aggregation numbers as reported above for latexes. However, SdFFF is an unusually sensitive method for detecting the small (or sometimes large) mass shifts that inevitably accompany aggregate formation or breakup. This method used in conjunction with electron microscopy would provide information relating the mass and morphology of different aggregated species. Useful information might again be gained by isolating narrow mass fractions by SdFFF and then following changes in the isolated population with time or with changes in the environment. These studies are not expected to be simple but, in comparison to studies using other methods that might be applied to polydisperse colloids, approaches based on SdFFF promise a wealth of useful data not previously available.
Barman and Giddings
58 Langmuir, Vol. 8, No. 1, 1992
Acknowledgment. This work was supported by Grant CHE-8800675from the National Science Foundation.
mass of normal doublet component retention ratio absolute temperature channel void volume component retention volume channel thickness
Glossary d do di G k h Lfd Lnd
m mfd
effective spherical diameter of particle diameter of first stage singlet latex final diameter of singlet latex centrifugal acceleration Boltzmann constant thickness of added layer on particle length of fused doublet length of normal doublet particle mass mass of fused doublet
Greek Y
AP
x Ps
steric correction factor density difference between particle and carrier liquid dimensionless retention parameter particle density
Registry No. PMMA, 9011-14-7; PS,9003-53-6; THAB,432813-6.
