1 Particle Size Analysis by Chromatography 1
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A. J. McHUGH , D. J. NAGY , and C. A. SILEBI
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Department of Chemical Engineering and Emulsion Polymers Institute, Lehigh University, Bethlehem, PA 18015
The use of column chromatography for fractionating polymer latex suspensions has been growing rapidly. Figure 1 shows a schematic breakdown of the several methods. One method, developed by Small (1,2), involves pumping the latex suspension through columns packed with nonporous beads. Separation by size results from the interaction between the electrostatically stabilized particles and eluant velocity gradients in the interstices between the packing. Thus the term Hydrodynamic Chromatography or HDC has been used to describe the process. Under conditions where van der Waals attraction between the particles and packing can predominate (such as at high eluant ionic strength), the particles may interact with or deposit onto the packing. The possibility exists for controlling the deposition -reintrainment behavior of the particles in this regime, based on either size or any of the physico-chemical parameters involved in the potential energy of interaction between the particles and packing. The term Potential Barrier Chromatography or PBC has been used to describe this process (3,4). A second chromatographic method, s i m i l a r i n o p e r a t i o n t o HDC, i n v o l v e s the use o f porous packing (as i n GPC) and has been r e f e r r e d t o as L i q u i d E x c l u s i o n Chromatography or LEC. Krebs and Wunderlich (5) were the f i r s t t o r e p o r t the use o f l a r g e pore s i l i c a g e l s f o r the f r a c t i o n a t i o n o f p o l y s t y r e n e and polymethylmethacrylate l a t e x e s . More r e c e n t l y , C o l l (6) and Singh and Hamielec (7.) have i n v e s t i g a t e d the s e p a r a t i o n o f p o l y s t y r e n e l a t e x e s up t o one micron i n diameter u s i n g c o n t r o l l e d - p o r e , s i l i c a g l a s s packing. T h e i r choice of packing s i z e and pore diameters c l e a r l y resembled those of t r a d i t i o n a l GPC systems. As a r e s u l t o f some o f the s t u d i e s t o be d i s c u s s e d i n t h i s paper, a separate regime i s p o s s i b l e when the packing pores are l a r g e Current address: department o f Chemical Engineering University of I l l i n o i s Urbana, I l l i n o i s 6l801 * A i r Products and Chemicals T r e x l e r t o w n , Pennsylvania 18105 0-8412-0586-8/80/47-138-001$06.25/0 © 1980 American Chemical Society
Provder; Size Exclusion Chromatography (GPC) ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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SIZE EXCLUSION CHROMATOGRAPHY
(GPC)
compared t o the p a r t i c l e s i z e . Such a process we r e f e r t o as Porous Hydrodynamic Chromatography. A t h i r d method i n v o l v e s f l o w o f the p a r t i c l e suspension through l o n g , s m a l l bore, open c a p i l l a r y tubes and has been r e f e r r e d t o as C a p i l l a r y Hydrodynamic Chromatography (8^9.). In t h i s process the f l o w s e p a r a t i o n mechanism appears t o be r e l a t e d t o the " t u b u l a r p i n c h e f f e c t " discussed i n the work of Segre and S i l b e r b e r g {10) and the name Tubular P i n c h Chromatography (TPC) has a l s o been a s s o c i a t e d w i t h i t . Since the phenomenon o n l y occurs above a c r i t i c a l Reynolds number ( l l ) , i t may be most a p p l i c a b l e t o p a r t i c l e s l a r g e r than a micron i n diameter. Another area o f r a p i d growth f o r p a r t i c l e s e p a r a t i o n has been t h a t o f F i e l d - F l o w F r a c t i o n a t i o n (FFF) o r i g i n a l l y developed by Giddings (12^, 13,1^,15.) (see a l s o papers i n t h i s symposium s e r i e s ) . L i k e HDC, the s e p a r a t i o n i n f i e l d - f l o w f r a c t i o n a t i o n (FFF) r e s u l t s from the combination of f o r c e f i e l d i n t e r a c t i o n s and the convected motion o f the p a r t i c l e s , r a t h e r than a p a r t i t i o n i n g between phases. In FFF the f o r c e f i e l d i s a p p l i e d e x t e r n a l l y w h i l e i n HDC i t r e s u l t s from i n t e r n a l i n t e r a c t i o n s . This paper w i l l be l i m i t e d t o a d i s c u s s i o n of our packed column s t u d i e s i n which we have addressed a t t e n t i o n t o questions r e g a r d i n g , (a) the r o l e o f i o n i c s t r e n g t h and s u r f a c t a n t e f f e c t s on both HDC and porous packed column b e h a v i o r , (b) the e f f e c t s of pore s i z e and pore s i z e d i s t r i b u t i o n on r e s o l u t i o n , and (c) the e f f e c t s of the l i g h t s c a t t e r i n g c h a r a c t e r i s t i c s o f p o l y s t y r e n e on s i g n a l r e s o l u t i o n and p a r t i c l e s i z e d i s t r i b u t i o n d e t e r m i n a t i o n . The d i s c u s s i o n s i n c l u d e r e f e r e n c e s t o previous p u b l i c a t i o n s which c o n t a i n d e t a i l e d development o f some o f the m a t e r i a l presented here. Nonporous Packing:
HDC
i ) Background. D e t a i l s o f the experimental aspects o f HDC column design and o p e r a t i o n are given i n s e v e r a l references ( l , l 6 , 1/7,18). The b a s i c technique i n v o l v e s pumping a d i l u t e suspension o f l a t e x p a r t i c l e s through beds packed w i t h styrene-divinylbenzene copolymer beads. P a r t i c l e s are detected by monitoring the t u r b i d i t y o f the eluant stream i n a flow-through c e l l at 25U nm. Over a range o f i o n i c s t r e n g t h s , p a r t i c l e s e l u t e from the columns ahead o f a d i s s o l v e d marker species (dichromate ion) w i t h p a r t i c l e residence time decreasing w i t h i n c r e a s i n g diameter. P a r t i c l e s e p a r a t i o n can be c h a r a c t e r i z e d by the s e p a r a t i o n f a c t o r , R , which i s the r a t i o o f eluant t o p a r t i c l e e l u t i o n volumes, o r , by the d i f f e r e n c e i n e l u t i o n volume, AV, between p a r t i c l e and eluant marker t u r b i d i t y peaks. For p o l y s t y r e n e monodisperse standards, a l i n e a r r e l a t i o n s h i p occurs between the l o g o f the p a r t i c l e diameter and AV, w i t h a s e r i e s of p a r a l l e l l i n e s r e s u l t i n g f o r d i f f e r e n t c o n c e n t r a t i o n o f e i t h e r s a l t or s u r f a c t a n t below i t s c r i t i c a l m i c e l l e c o n c e n t r a t i o n (17,18,19). The s e p a r a t i o n f a c t o r has a l s o been shown t o be independent o f eluant F
Provder; Size Exclusion Chromatography (GPC) ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
1.
MCHUGH E T A L .
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Particle Size Analysis
flow rate (l8,1£). To q u a n t i f y Rp i n terms o f a fundamental model f o r t h e p a r t i c l e r e s i d e n c e time t h e d e f i n i t i o n i n terms o f average v e l o c i t e s i s used,
F
(1)
where v and vg r e f e r t o t h e p a r t i c l e and eluant a x i a l v e l o c i t y , and t h e b r a c k e t s r e f e r t o t h e a p p r o p r i a t e averaging taken over the cross s e c t i o n o f t h e assumed e q u i v a l e n t bed i n t e r s t i t i a l geometry. For t h e c a p i l l a r y bed model ( l 8 ) p
J
= —
R -R o p
x -
