Chapter 3
Particle Size Determination Using Turbidimetry Capabilities, Limitations, and Evaluation for O n - L i n e Applications
Downloaded by CORNELL UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: September 24, 1991 | doi: 10.1021/bk-1991-0472.ch003
Theodora Kourti and John F. MacGregor McMaster Institute for Polymer Production Technology, Department of Chemical Engineering, McMaster University, Hamilton, Ontario L8S 4L7, Canada
A critical discussion on the use of turbidimetric methods for particle size determination in polydisperse suspensions, with emphasis on latexes, is presented, and the potential of these methods for on-line particle size determination during latex production is evaluated. The advantages, disadvantages and limitations of several turbidimetric techniques that have been used for the estimation of an average size or the full particle size distribution are discussed. Some conflicts and inconsistencies reported in the literature and an existing controversy are resolved. Also, some observations reported concerning the type of average of the PSD that can be estimated from one specific turbidity measurement are shown to be coincidental and therefore should not be generalized. Specific problems arising when turbidimetry is used for latex particle size determination are discussed, and the use of the method for on-line particle size measurements during latex production is evaluated. F o r a l m o s t four decades t u r b i d i m e t r i c techniques have been used w i d e l y for the determ i n a t i o n o f p a r t i c l e size i n suspensions (1-49). T u r b i d i m e t r y i s e x p e r i m e n t a l l y s i m p l e , c a n be used for a wide range o f p a r t i c l e sizes (from l o w e r s u b m i c r o m e t e r to m a n y micrometers) a n d does not d i s t u r b the s y s t e m u n d e r i n v e s t i g a t i o n It is also fast, reprod u c i b l e a n d inexpensive. These advantages m a k e i t a v e r y a t t r a c t i v e m e t h o d for t h e d e t e r m i n a t i o n o f p a r t i c l e size i n suspensions. It i s the fastest a n d e x p e r i m e n t a l l y t h e s i m p l e s t of the l i g h t s c a t t e r i n g methods a n d is p r o m i s i n g for o n - l i n e applications. T h e r e are n u m e r o u s reports on the use o f t u r b i d i m e t r i c techniques for the determ i n a t i o n o f e i t h e r a n average size o r the p a r t i c l e size d i s t r i b u t i o n (PSD) i n polydisperse suspensions, i n a v a r i e t y o f systems, s u c h as s i l v e r b r o m i d e sols i n w a t e r (3.4.19). poly(styrene) latexes (10.29.34). p o l y ( v i n y l acetate) latexes (14-17.21.24.26.27). h o m o genized m i l k , o i l i n w a t e r e m u l s i o n s (11.18.42). coupler dispersions (22), m i n e r a l s (39). T h e r e a r e even a t t e m p t s to use t u r b i d i m e t r y for the d e t e r m i n a t i o n o f p a r t i c l e size i n p o l y m e r blends (25.49). A wide range o f wavelengths have been u t i l i z e d for the t u r b i d i t y m e a s u r e m e n t s ; p a r t i c l e size d e t e r m i n a t i o n from t u r b i d i t y measurements a t w a v e l e n g t h s i n the i n f r a r e d r e g i m e has been w i d e l y reported (11. 18. 19. 22). T h e d e t e r m i n a t i o n o f p a r t i c l e size u s i n g o n - l i n e t u r b i d i m e t r y has also a t t r a c t e d a lot of a t t e n t i o n ( 1 4 - 1 6 . 1 9 . 24. 2 7 . 3 1 . 4 5 ) . Some of the w o r k reported i n the l i t e r a t u r e o n the a p p l i c a t i o n of the m e t h o d to different m a t e r i a l s has been s u m m a r i z e d (in order of d e c r e a s i n g v a l u e of m) i n T a b l e I.
0097-6156/91/0472-0034$08.50A) © 1991 American Chemical Society
In Particle Size Distribution II; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
Downloaded by CORNELL UNIV on August 2, 2012 | http://pubs.acs.org Publication Date: September 24, 1991 | doi: 10.1021/bk-1991-0472.ch003
3.
KOURTI & MACGREGOR
Particle Size Determination Using Turbidimetry
35
V e r y f r e q u e n t l y , w o r k e r s t r i e d to compare t u r b i d i m e t r y w i t h other p a r t i c l e s i z i n g methods. T h e r e are n u m e r o u s reports where the average d i a m e t e r s or the p a r t i c l e size d i s t r i b u t i o n s e s t i m a t e d by t u r b i d i m e t r y a n d by other methods, were i n good agreement. T h e r e a r e some reports (12.22) however, on " f a i l u r e " of some t u r b i d i m e t r i c techniques to provide the f u l l P S D . T h e r e i s a controversy i n the l i t e r a t u r e (12.16.17) on w h e t h e r o r not t u r b i d i m e t r y c a n provide the t r u e P S D . T h e controversies a n d c o n t r a d i c t o r y reports i n the l i t e r a t u r e m o t i v a t e d a d e t a i l e d i n v e s t i g a t i o n o n the c a p a b i l i t y of the m e t h o d to provide the f u l l P S D , a n d t h i s i n v e s t i g a t i o n is p r e s e n t e d i n a n o t h e r c h a p t e r i n t h i s v o l u m e (28). T h e m a i n conclusion f r o m t h i s i n v e s t i g a t i o n was t h a t i s i m p o s s i b l e to generalize t u r b i d i m e t r i c methods across refractive indices a n d p a r t i c l e size ranges. T h e c a p a b i l i t y of a t u r b i d i m e t r i c method to provide a n average p a r t i c l e size or the p a r t i c l e size d i s t r i b u t i o n o f the s u s p e n s i o n a n d i t s s e n s i t i v i t y to e x p e r i m e n t a l e r r o r c a n n o t be discussed w i t h o u t r e l a t i n g i t to the m a n d a v a l u e s of the suspension, a n d conclusions cannot be e x t r a p o l a t e d f r o m one (m,a) r e g i m e to a n o t h e r a n d f r o m one method to another. A c r i t i c a l r e v i e w (27) of the w o r k reported on t u r b i d i m e t r y i n d i c a t e d t h a t c o n t r a r y to the above c o n c l u s i o n , there i s a tendency i n the l i t e r a t u r e to generalize or extrapolate c o n c l u sions a n d observations across methods a n d (m,a) r e g i m e s , a n d t h a t most of the conflicts a n d the e x i s t i n g controversy a r i s e f r o m t h i s tendency. T h e observations reported by each i n v e s t i g a t o r were correct a n d c o u l d be j u s t i f i e d for t h e i r s y s t e m , by t a k i n g i n t o account o u r a n a l y s i s i n (28). B y c h e c k i n g the (m,a) v a l u e s of these systems, t u r b i d i m e t r y was expected to succeed i n p r o v i d i n g the P S D i n the cases t h a t was reported as a successful m e t h o d , a n d was expected to f a i l i n the cases t h a t i t f a i l e d . H o w e v e r , a l t h o u g h t h e i r observations were correct, conflicts arose w h e n these i n v e s t i g a t o r s e x t r a p o l a t e d t h e i r observations to other systems (other m , a regimes) a n d other methods. T h e objective of t h i s chapter i s to give a n i n s i g h t into t u r b i d i m e t r y . T h e methods a v a i l a b l e for p a r t i c l e size d e t e r m i n a t i o n u s i n g t u r b i d i m e t r y are discussed. A f t e r a b r i e f t h e o r e t i c a l b a c k g r o u n d the advantages a n d l i m i t a t i o n s of each method are g i v e n a n d the (m, a) regimes where the e s t i m a t i o n of the f u l l P S D is possible a r e i d e n t i f i e d . T h e s e n s i t i v i t y of the r e s u l t s to e x p e r i m e n t a l e r r o r is investigated. S e v e r a l p u b l i c a t i o n s on t u r b i d i m e t r y are c r i t i c a l l y d i s c u s s e d i n o r d e r to h i g h l i g h t the l i m i t a t i o n s o f some approaches t h a t are not w i d e l y used, a n d to point out some c o i n c i d e n t a l observations t h a t m a y r e s u l t to m i s l e a d i n g conclusions. F i n a l l y , the p o t e n t i a l of the method for o n - l i n e a p p l i c a t i o n s is i n v e s t i g a t e d a n d problems r e l a t e d to a p p l i c a t i o n s for latex systems a r e discussed. General Definitions T u r b i d i t y gives a measure of the a t t e n u a t i o n of a b e a m of l i g h t t r a v e r s i n g a suspension of particles: !
I
i = ± € t a I I
(1)
w h e r e I a n d I represent i n t e n s i t i e s of the incident a n d t r a n s m i t t e d beams, r e s p e c t i v e l y , a n d € i s the l e n g t h of the o p t i c a l p a t h (cell l e n g t h , cm). T h e t u r b i d i t y of a suspension c a n be m e a s u r e d w i t h a n y spectrophotometer, after c e r t a i n m o d i f i c a t i o n have been made (22). F o r a suspension of s p h e r i c a l , non a b s o r b i n g , isotropic p a r t i c l e s , i n the absence of multiple scattering, 0
In Particle Size Distribution II; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
In Particle Size Distribution II; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991. 350-1000 400-800
1.17, 1.15
1.166, 1.21
poly(butadienestyrene) latexes
Kodacolor dispersions
coupler dispersions
Latexes: Hevea, cis-polyisoprene
Styrene divinylbenzene copolymer latexes
Gledhill (7)«
Haseler (22)a
Wales (6)a
BagchiandVold(13)
3
Maron etal(9)&
3
poly(styrene) latexes d
2
2
Wallach and Heller (10)a
2
1.136
1.15
1.2
1.9 1.33 1.86 1.163
400-600
650,800,950, 1100
400,700 400 nm, 2.22 pm
440-610 basis: 546
436,546, 650,800
infrared (2.22 pm)
Minerals: Ti0 A1 0 A1(0H) Si0
1.7
Yang and Hong (39)
450,800
2.
silver bromide grains
436,546
1.
Wavelengths (nm)
Haseler and P a r k i n (19)»
1.7
m
stable silver bromide sols i n water
System
Meehan and Beattie (4)
AUTHORS
N
12
