2 Size Exclusion Chromatography of Model Latices
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A Feasibility Study J. E. JOHNSTON, C. L . COWHERD, and T. B. MacRURY Union Carbide Corporation, South Charleston, WV 25303 The r a p i d , a c c u r a t e , and p r e c i s e measurement of p a r t i c l e s i z e d i s t r i b u t i o n s i s required i n both l a b o r a t o r y synthesis and commer c i a l production. It i s e s s e n t i a l f o r the design of new l a t e x pro ducts and processes and f o r the monitoring of e x i s t i n g commercial processes. Two o f the chromatographic technqiues which o f f e r pro mise i n the area o f p a r t i c l e s i z e d i s t r i b u t i o n determinations are hydrodynamic chromatography (HDC) and l i q u i d or s i z e e x c l u s i o n chromatography (SEC). The former has been i n v e s t i g a t e d by Small 2^ 3), McHugh and co-workers (4, 5, 6j, and S t o i s i t s (7_) and the l a t t e r by Krebs and Wunderlich X"8), Gaylor and James (9J, C o l l , Fague and R o b i l l a r d (10) and Singh and Hamielec —
ΟΙ)·
The two techniques d i f f e r in that HDC employs a nonporous s t a t i o n a r y phase. Separation i s a f f e c t e d as a r e s u l t of p a r t i c l e s of d i f f e r e n t s i z e sampling d i f f e r e n t v e l o c i t i e s i n the i n t e r s t i t i a l spaces. S i z e e x c l u s i o n chromatography i s accomplished by superimposing a s t e r i c s e l e c t i o n mechanism which r e s u l t s from the use o f a porous bed. The pore s i z e s may vary over a wide range and the separation occurs as a r e s u l t o f e s s e n t i a l l y the same processes present i n the gel permeation chromatography of macromolecules. I f a s i n g l e pore s i z e i s employed i n the s t a t i o n a r y phase, which i s l a r g e r than the l a r g e s t p a r t i c l e to be a n a l y z e d , the technique has been termed porous HDC. A model f o r the separation i n t h i s type o f system has been described by DiMarzio and Guttman (12, 11). The nature of the s t a t i o n a r y phase i n SEC precludes the e l u t i o n of sample p r i o r to a volume equivalent to t h a t contained i n the i n t e r s t i t i a l spaces, V j . T h i s i s a l s o known as the excluded volume, V^. The remainder of the t o t a l system void volume, V-r, i s made up of the pore volume, Vp. For an e l u t i n g species the volume increment at which i t appears, V5 , w i l l be r e l a t e d to the i n t e r s t i t i a l and pore volumes a s : V
S = I V
+
k
d P V
0-8412-05 86-8/ 80/47-13 8-027$05.00/0 © 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)
where k 1 . An analogous s i t u a t i o n e x i s t s in SEC. Because of tne presence o f pores i n the s t a t i o n a r y phase, p a r t i c l e s of a given s i z e w i l l only sample a p o r t i o n of the a v a i l able pore volume, i . e . , Lj < 1 . For t h i s reason, SEC w i l l tend to e x h i b i t higher values o f the separation f a c t o r , Rf, than HDC. It has been observed that improvement i n the values o f Rf may be obtained in HDC by decreasing the packing s i z e (1_). This i s done at the cost of p a r t i c l e s i z e range however, since decreasing the packing s i z e decreases the i n t e r s t i t i a l channel diameter. In the case of SEC, Rf may be increased by i n c r e a s i n g the range and d e n s i t y of pores without changing the packing diameters and hence without s i g n i f i c a n t l y a f f e c t i n g the p a r t i c l e s i z e range. These observations are i l l u s t r a t e d i n Figure 1 where the e f f e c t of changing packing s i z e on Rf in HDC i s shown i n comparison to the e f f e c t of changing pore s i z e on Rf i n SEC. The other component of the r e s o l u t i o n i n chromatographic systems i s the tendency f o r p a r t i c l e s of the same s i z e to e l u t e at d i f f e r e n t r e t e n t i o n volumes because of a x i a l d i s p e r s i o n or i n strument spreading. The r e s u l t o f t h i s spreading phenomenon i s that the e f f e c t i v e r e s o l u t i o n of the chromatograph i s decreased to the extent that instrument spreading i n c r e a s e s . This e f f e c t i s p a r t i c u l a r l y pronounced i n SEC of p a r t i c l e s because o f d i f f u s i o n i n the porous matrix. In the case of gel permeation chromatogra phy, the magnitude of the c o n t r i b u t i o n of instrument spreading to the observed chromatogram i s c o n s i d e r a b l y l e s s than in SEC of l a t i c e s because of the l a r g e r values f o r the d i f f u s i o n constants o f molecules as compared with c o l l o i d a l p a r t i c l e s . In a d d i t i o n to instrument s p r e a d i n g , which i s g e n e r a l l y t r e a t e d as being Gaussian i n nature (15, 1 6 ) , skewing can a l s o be observed i n SEC of l a t i c e s because of entrapment of p a r t i c l e s w i t h i n the porous m a t r i x . This e f f e c t g e n e r a l l y increases with increasing p a r t i c l e s i z e . The d e t e c t o r most often employed in SEC of p a r t i c l e s i s the u l t r a v i o l e t photometer. Detection takes place at e s s e n t i a l l y 180 m
Provder; Size Exclusion Chromatography (GPC) ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
2.
JOHNSTON ET AL.
SEC of Model Lattices
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degrees r e l a t i v e to the i n c i d e n t beam. Hence, i t i s the t u r b i d i t y , τ , which i s measured by the decrease i n transmitted l i g h t : τ = 1 In (100/T) I = optical
(1)
path length
Τ = % transmittance In g e n e r a l , the number o f p a r t i c l e s present i n the c e l l a t a r e t e n t i o n volume (v) i s r e l a t e d to the t u r b i d i t y a s :
Ν(ν)ατ ( ν ΐ Κ ί ν Γ ' Ό ί ν ) "
2
(2)
where K(v) = the s c a t t e r i n g c o e f f i c i e n t s from Mie theory (18, 19) D(v) = the p a r t i c l e s i z e at r e t e n t i o n volume ν based on a c a l i b r a t i o n curve The assumptions made at t h i s point are that there i s no m u l t i p l e s c a t t e r i n g , i . e . , concentrations
