Anal. Chem. 1995, 67, 2705-2713
Separation of Particles in Nonaqueous Suspensions by Thermal Field-Flow Fractionation Paul M. Shiundu,t Guangyue Liu,* and J. Calvin Giddings* Field-Flow Fractionation Research Center, Department of Chemistry, University of Utah, Salt Lake City, Utah 84 112
The ability of thermal field-flowfractionation (lXFFF') to retain and separate both submicrometer and micrometer size particles (latexes and silica) suspended in various organic carrier liquids is demonstrated. The dependence of particle retention on various factors is evaluated and discussed. These factors include solvent properties, amount of added electrolyte, particle size and composition, and cold-wall temperature. Evidence of retention perturbations due to electrostatic particle-wall interactions is provided, and the efficacy of modiijing the ionic strength of the organic carriers (by adding salt) to minimize the perturbations is demonstrated. General similarities and differences in retention behavior of particles suspended in nonaqueous as opposed to aqueous carriers are determined. One notable difference is the decrease in particle retention time in nonaqueous liquids with increasing cold-wall temperature; this trend is the same as that exhibited by polymers dissolved in organic carriers but is opposite to that shown by particles suspended in an aqueous medium. Thermal diffusion coefficients of several latex/solvent combinations are obtained from the ThFFF retention data. The present studies are limited primarily to polar organic solvents, except for some measurements involving cyclohexane and aqueous carriers. The investigation of nonpolar organic suspension media was hampered by either the solubility of the latexes or immiscibility between the original suspension media of the latexes and the nonpolar organic solvents. Field-flow fractionation (FFF) is a class of separation techniques whose operation is based on subjecting a sample to the combined effects of an external field applied perpendicular to the axis of a thin, open fractionation channel and to the axial flow of carrier liquid through the channel. Separation of sample components arises due to the differential interaction of the external field with the various components, which displaces them unequally toward the channel accumulation wall. As a result of the unequal displacement, different sample components end up in different stream laminae with unequal flow velocities. The result is differential component elution.' Field-flow fractionation methods have been used in the separation and characterization of macromolecules (both synthetic and natural) and of colloidal particles. Several kinds of "fields" (e.g., flow, electrical, thermal gradient, and sedimentation) have been ' Present address: Department of Chemism, University of Nairobi, P.O. Box 30197, Nairobi, Kenya. * Present address: DataChem Laboratories, 960 West LeVoy Dr., Salt Lake City, UT 84123. (1) Giddings. J. C. Science 1993,260,1456-1464. 0003-2700/95/0367-2705$9.00/0 0 1995 American Chemical Society
employed, giving rise to different FFF techniques with varying capabilities.'q2 Thermal field-flow fractionation (ThFFF) is a technique in which a temperature gradient is applied across the channel, thereby inducing displacement by thermal d ~ s i o n .Due to its intrinsically high selectivity, this technique is being increasingly used in the separation and characterization of polymers with molecular weights ranging from 104 to lo7and ThFFF has also provided basic thermal dfision data for polymer^.^-^ Various characteristics of ThFFF have been examined and discussed ThFFF has recently been found capable of separating particles as well as polymers. Thus, despite the fact that ThFFF has been applied almost exclusively to polymers in the past and that other FFF techniques, particularly flow FFF and sedimentation FFF, are highly effective in the separation and characterization of particulate matter, ThFFF promises to play a useful role in the fractionation of particles as well as polymers. This is because of the simplicity of the ThFFF operation and a high sensitivity to both particle size and composition. The ready compatibility of the apparatus with both aqueous and nonaqueous carrier solutions affords the technique greater flexibility than the other FFF techniques. ThFFF was recently shown to be applicable to aqueous particle separations over the submicrometer and larger particle size range.'O A strong compositional dependency of particle retention in the aqueous suspensions was observed. Thermal diffusion coefficients of a number of latexes and silica particles were reported. From those studies, it was established that the general range (1 - 3 x cm2s-' K-l) of thermal diffusion coefficients for particles in aqueous suspensions was lower than that reported for polystyrene polymers dissolved in organic solvents ((0.8 1.2) x cm2s-' KVi).il A difference in behavior between latex particles suspended in aqueous media and polymers dissolved in organic carriers was also observed with respect to their sensitivity to cold-wall temperatures. The particles showed an increase in retention time with increasing cold-wall temperatures at constant (2) Giddings, J. C. LTniFed Separation Science; Wiley and Sons: New York, 1991. (3) Gunderson, J. J.; Giddings, J. C. Anal. Chim. Acta 1986,189,1-15. (4)van Asten, A. C.; Venema, E.; Kok, W. Th.; Poppe, H.J.Chromatogr. 1993, 644,83-94. (5) van Asten, A. C.; Boelens, H. F.M.; Kok, W. Th.; Poppe, H.; Williams, P. S.; Giddings, J. C. Sep. Sci. Technol. 1994,29,513-533. (6) Kirkland, J. J.; Rementer, S. W. Anal. Chim. Acta 1992,64, 904-913. 405-421. (7) Schimpf, M. E./. Chromatogr. 1990,517, (8) Schimpf, M. E.; Giddings, J. C. /. Polym. Sci., Polym. Phys. Ed. 1989,27, 1317-1332. (9) Schimpf, M. E.; Giddings, J. C. /. Polym. Sci., Polym. Phys. Ed. 1990,28, 2673-2680. (10) Liu, G.; Giddings, J. C. Chromatographia 1992,34, 483-492. (11) Schimpf, M. E.; Giddings, J. C. Macromolecules 1987,20,1561-1563.
Analytical Chemistry, Vol. 67, No. 75,August 7, 7995 2705
temperature drop AT across the channel thickness,'O a trend opposite to that exhibited by polymers dissolved in organic s o l v e n t ~ . ~However, ~J~ because of the limited scope of experiments performed, it could not be ascertained whether the observed differences were due to the contrast in the type of suspending media used (aqueous versus nonaqueous) or to an intrinsic difference in behavior between particles and polymers when subjected to a thermal gradient. Signiiicant evidence in regard to this question is provided by this study. In this paper, various parameters controlling the retention of particles in nonaqueous media are evaluated, and the results are compared with those obtained using aqueous suspensions. Speciiic studies reported in this paper include the influence of particle size, composition, carrier liquid, and cold-wall temperature on particle retention. In addition, we provide experimental evidence that electrolytes added to either aqueous or nonaqueous camer liquids substantially increase particle retention times. This behavior is presumably a result of particle-wall electrostatic interactions (of a repulsive nature) which decrease with increasing ionic strength of the carrier. Thus, for low ionic strength camers, particles in the ThFFF channel are repelled away from the accumulation wall (due to the relatively large double-layer thickness) into the faster streamlines of the parabolic flow profile, thereby causing the particles to elute earlier. The predicted trend of increased particle retention for higher electrolyte concentrations was consistently observed. This apparent particle-wall electrostatic phenomenon is not, however, unique to ThFFF. Hansen et al. attributed retention perturbations observed in the sedmentation FFF of aqueous latex suspensions to particle-wall interactions having a strong electrostatic c ~ m p o n e n t . ' They ~ , ~ ~ observed a variability in the apparent electrostatic interactions with different surface materials.
for this departure from the parabolic flow m ~ d e l . ~All J ~numerical values reported in this paper have been corrected accordingly to account for these perturbations. The parameter ,Iis a measure of the level of the component's interaction with the applied field. For ThFFF, the parameter ,I can be expressed as18J9
A = D/D,BAT
(3)
where D is the component's ordinary diffusion coefficient, DT is its thermal diffusion coefficient, and AT is the temperature drop applied across the channel. The parameter 0 provides a correction term that converts AT into a temperature gradient as needed to describe the operation of thermal diffusion; in most cases 8 is close to unity.Ig By combining eqs 1-3, we show, as an approximation, how the experimentally measurable quantity tr depends on D and &: t,/to
= DTAT/6D
(4)
Of these two parameters, only D is well characterized, being related to the hydrodynamic diameter d of the particle under investigation by the Stokes-Einstein expression,"
D = kT/3nvd
(5)
where k is Boltzmann's constant, T is the temperature in the region occupied by the particles (usually approximated by the temperature T, of the accumulation wall or cold-wall near which the particles accumulate), and q is the viscosity of the suspending medium at temperature T. The substitution of eq 5 into eq 4 gives
THEORY
t,/to
nqdDTAT/2kT
(6)
The standard model for retention in normal mode FFF yields the expression16
R = tO/t, = 6A[coth(1/22)
- 221
(1)
where R is the retention ratio, to is the channel void time, tr is the retention time, and /I is the retention parameter defined by d = l/w, where 1 is the mean thickness of the component zone compressed against the accumulation wall and w is the channel thickness. For highly retained sample components (i.e., R THF > MeOH > HzO > CYH. This order, however, contrasts with that observed for the latex particles in which the level of retention increases with solvent polarity. Silica particles show no obvious trend in the present study. For example, CYH and HzO both yield the weakest retention (i.e., the largest dAT values) for silica particles. There are, however, some consistent differences in the retention behavior of particles suspended in aqueous versus nonaqueous carrier liquids. One such difference is found in the effect of cold-wall temperature T, on particle retention. Previous studies with particles in aqueous suspensionsshowed a linear dependency between tr/F and T,, with the slope of the plots increasing with particle diameter.1° However, our studies involving PS latexes in several nonaqueous suspensions show that the retention times decrease with increasing T,. This behavior is the same as that of polymers in nonaqueous solvents. The observed trends are illustrated in Figure 12, which shows linear plots of log(t,/tO) versus log T, for different sizes of PS latex particles suspended in ACN and MeOH, respectively. All plots have a negative slope. These experiments utilized a concentration of TBAP of 1.0 mM, a flow rate of 0.38 mL/min, and a AT of 55 K As was demonstrated in an earlier publication,IO ThFFF can be used to determine the physicochemical properties of particles in both aqueous and nonaqueous suspensions. According to eqs 3, 4, and 6, thermal diffusion data are easily obtainable (taking care that temperature and viscosity effects are corrected). For example, Figure 13 shows plots of 1versus l/AT for three sizes of PS particles in ACN containing 1.0 mM TBAP. The slopes of the plots, according to eqs 3 and 5, can be used to calculate thermal diffusion coefficients. For the three sizes of PS particles (0.091, 0.198, and 0.426 pm), & values were determined to be 1.67 x 1.56 x and 1.02 x cmz ssl K-I, respectively. As with aqueous media,1° 4 .values vary not only with particle composition but (unlike polymers) with size. This is demonstrated by Figure 14, which shows plots of & versus particle diameter d for various particles in both aqueous and ACN suspensions. An interesting feature of these plots is that the slopes are positive for aqueous carriers and negative for ACN. The reason for this contrasting dependence of & on particle size between aqueous and nonaqueous carrier liquids is not understood, but it does explain the reported disparity (above) in the magnitudes of the Sd values between the two carrier types. Analytical Chemistry, Vol. 67, No. 15, August 1, 1995
2711
1.5
I
(a) ACN 0.300pm PS
. -
7 + 1
.
v L
0.220 pm 3 PS
R
*PS PB
-
an
-.t- Silica
"1n .., s L.46
/
A..... "..& 2.41
2.48
2.49
:::::::.... ....:::::::..
-
..a.....a...
1
1
"-..n.....D... ps
lhcd
-...m... pB
2.50
*...s' .
....
log (T,IK)
/%L'*
(b) MeOH 0.300 pn PS
-
Aqueous
PMMA
0.1
0.2
0.3
0.4
g'.oh d (um)
1.2
Figure 14. Plots of thermal diffusion coefficient & versus particle diameter dfor PS, PB, PMMA, and silica particles. The concentration of TBAP in ACN was 1.O mM; the aqueous carrier solution contained 0.1% FL70 (as surfactant) plus 0.01% sodium azide (as bactericide).
0.105 pm PS
0.6 0.8
~ . 4 6 2.41
2.48
2.49
2.50
2.51
log (T,IK)
Figure 12. Logarithmic plots of t,/P versus cold-wall temperature Tcfor PS particles suspended in (a) ACN and (b) MeOH. Experimental conditions: V = 0.38 mumin, A T = 55 K, [TBAP] = 1.0 mM. U.UJ
0.04
-
0.091 pn PS
0.03 -
x
..
0.00
0.01
0.02
0.03
0.04
0.05
0.06
1 / A T(K-')
Figure 13. Plots of retention parameter A versus l / A T f o r three diameters of polystyrene latex particles in ACN. Experimental conditions: V = 0.30 mumin, [TBAP] = 1 .O mM.
CONCLUSIONS The study reported in this paper demonstrates that ThFFFcan be used to fractionate both latex and inorganic particles suspended in nonaqueous carrier liquids according to size and composition. 2712 Analytical Chemistry, Vol. 67, No. 15, August 1, 1995
The ability of ThFFF to retain and fractionate both polymeric and particulate species, and the possibility of tuning the separation by variations in electrolyte concentration,make ThFFF a particularly promising technique for the analysis of complex composite materials. ThFFF can also be used effectively to determine thermal diffusion coefficient data for particles suspended in both aqueous and nonaqueous carriers. Presently, such data are scarce. Our studies demonstrate that there are both similarities and differences in retention behavior between particles suspended in aqueous and nonaqueous carriers. All the particles studied show retention dependence on particle composition in both types of suspensions. The level of retention of latex particles appears to depend on the polarity of the carrier medium, whereas for silica particles, retention appears uncorrelated with carrier liquid polarity, and the silica particles are retained to different extents in the different carrier liquids. Another contrast in observed trends is that particles suspended in aqueous carriers show an increase in retention time with increasing cold-wall temperatures, whereas particles suspended in nonaqueous carriers act oppositely. This latter behavior is similar to that observed with polymers in organic carriers. Despite advances in developing ThFFF as an alternative technique for particle size analysis,further studies are still needed for ThFFF to become a viable technique for broadly based particle characterization and analysis. A better understanding of the dependence of the thermal driving force on factors such as particle composition, surface properties, and solvent type is necessary for ongoing progress.
ACKNOWLEWMENT This work was supported by Grant No. CHE-9322472from the National Science Foundation. We acknowledge Dr. Grant Von Wald of Dow Chemical for supplying us with polybutadiene latex samples as well as Dr. J. N. Kinkel of E. Merck for providing us with silica particles.
GLOSSARY molar concentration of electrolyte particle diameter ordinary diffusion coefficient thermal diffusion coefficient Boltzmann’s constant mean thickness of sample zone Avogadro’s number retention ratio universal gas constant diameter-based selectivity retention time channel void time temperature cold-wall temperature
channel flow rate channel thickness charge on ionic species solvent dielectric constant vacuum permittivity constant Debye length (or electrical doublelayer thickness) temperature drop applied across channel viscosity thermal conductivity correction factor, (dT/dd/ (AT/w) Received for review October 18, 1994. Accepted May 15, 1995.e AC941015G @
Abstract published in Advance ACS Abstracts, June 15, 1995.
Analytical Chemistry, Vol. 67, No. 15, August 1, 1995
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