Polymer Adsorption and Dispersion Stability - ACS Publications

7 Adsorption of Cellulose Ethers from Moderately Saline Aqueous Solutions J. E. GLASS, S. SHAH, D-L. LU, and S. D. SENEKER

Downloaded by LOUISIANA STATE UNIV on May 8, 2015 | http://pubs.acs.org Publication Date: February 10, 1984 | doi: 10.1021/bk-1984-0240.ch007

Polymers and Coatings Department, North Dakota State University, Fargo, ND 58105

Nonionic cellulose ethers, hydroxyethyl(HE) and hydroxypropyl (HP) cellulose, of variable molar substitution (M.S.) levels, were adsorbed on peptized sodium montmorillonite surfaces from fresh and saline (NaCl) aqueous solutions. The amounts adsorbed for 2 M.S. HEC and HPC and 4 M.S. HEC were insensitive to electrolyte concentration; the 4 M.S. HPC exhibited a notable increase in adsorption with increasing NaCl concentration. Entrapment in the interlayer of recovered sodium montmorillonite did not vary with salinity; the extent of entrapment was greater with the 4 M.S. HE and HP celluloses than either of the 2.0 M.S. polymers. Mixed ethers of HEC (2 M.S.) containing an anionic (carboxymethyl) or cationic (3-O-2-hydroxypropyltrimethylammonium chloride) group at 0.4 M.S. levels did not adsorb from fresh water. Adsorption of these polar mixed ethers increased with increasing electrolyte until electrostatic and solvation effects were negated in 0.54N NaCl solutions and the adsorbed amounts typical of a 2 M.S. HEC were observed. Interlayer entrapments comparable to the equivalent M.S. HEC were observed at lower (0.18N) electrolyte concentrations. I t has been shownQ) that carbohydrate polymers c o n t a i n i n g pendant ether linkages adsorb on peptized montmorillonite surfaces from f r e s h water solut ions i n amounts greater than u n d e r i v a t i z e d carbohydrate polymers having the same solut ion conformational characteristics. In t h i s study i t was a l s o demonstrated that ether linkages promote i n t e r l a y e r entrapment of the segmentally r i g i d macromolecules i n smectite c l a y s . This does not occur with u n d e r i v a t i z e d carbohydrate polymers. Several a n i o n i c carbohydrate polymers (e.g., carboxymethyl cellulose, xanthomonas campestris polysaccharide, c e l l u l o s e s u l f a t e ester, e t c . ) do not adsorb from f r e s h water s o l u t i o n s , but t h e i r adsorption i n s a l i n e s o l u t i o n s plays an 0097-6156/84/0240 0095506.00/0 © 1984 American Chemical Society In Polymer Adsorption and Dispersion Stability; Goddard, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


96

POLYMER ADSORPTION AND DISPERSION STABILITY

i m p o r t a n t r o l e i n many p e t r o l e u m r e c o v e r y p r o c e s s e s . A d s o r p t i o n s t u d i e s from h i g h l y s a l i n e environments r e q u i r e a f o r m i d a b l e experimental e f f o r t . As a forerunner to these studies the a d s o r p t i o n o f n o n i o n i c c e l l u l o s e e t h e r s and the most h y d r o p h i l i c of these, h y d r o x y e t h y l c e l l u l o s e (HEC), c o n t a i n i n g both a n i o n i c and cationic groups a t e q u i v a l e n t degrees of substitution ( D . S . ) , were e x a m i n e d . The l o n g - t e r m g o a l o f t h i s i n v e s t i g a t i o n i s an u n d e r s t a n d i n g of the effect o f a p a r t i c u l a r mode o f a d s o r p t i o n on montmorillonite which ensures that u n d e r p r e s s u r e the c l a y p l a t e l e t s a l i g n p a r a l l e l t o a s o l i d s u r f a c e e v e n under s a l i n e conditions. This i s an important phenomenon i n petroleum recovery processes. EXPERIMENTAL The g e n e r a l e x p e r i m e n t a l methods o f a n a l y s i s and m a t e r i a l s u s e d i n t h i s s t u d y have b e e n p r e v i o u s l y d e s c r i b e d ( 1 , 2 ) w i t h t h e e x c e p t i o n of the i o n i c hydroxyethyl c e l l u l o s e polymers. C a r b o x y r a e t h y l h y d r o x y e t h y l c e l l u l o s e (CMHEC) was s u p p l i e d by Hercules Inc.; it is used in fracturing (petroleum) a p p l i c a t i o n s . The c a t i o n i c HEC was s u p p l i e d by U n i o n C a r b i d e Corp.; the cationic group is 3-0-2-hydroxypropyltrimethylamraonium c h l o r i d e . The p o l y m e r i s used p r i m a r i l y i n c o s m e t i c applications. Both cellulose polymers contain a molar s u b s t i t u t i o n (M. S.)(_3) o f h y d r o x y e t h y l g r o u p s o f a p p r o x i m a t e l y 2.0; t h e p o l a r g r o u p i s p r e s e n t a t an a p p r o x i m a t e M.S. (or D.S.) o f 0.4. A more q u a n t i t a t i v e d e s c r i p t i o n o f t h e p o l y m e r s i s p r o v i d e d i n T a b l e I I . The u n m o d i f i e d HEC p o l y m e r s , 2.0 and 4.3 M.S., were s u p p l i e d by U n i o n C a r b i d e ; t h e 2.5 M.S. by H e r c u l e s . H y d r o x y p r o p y l c e l l u l o s e s (HPC) o f 2.0 and 4.0 M.S. were s u p p l i e d by Hercules. These products have been quantitatively c h a r a c t e r i z e d d , 4 ) by M.S., percent unsubstituted v i c i n a l d i o l (%uVD) and glucopyranosyl (%uGP) a n a l y s e s and stochastic modeling. D i f f e r e n t i a l r e f r a c t i o n ( _ 5 ) and c o l o r i m e t r i c m e a s u r e m e n t s t 2 ) were u s e d t o d e t e r m i n e t h e amount o f c a r b o h y d r a t e polymer a d s o r b e d f r o m s o l u t i o n . A P h i l l i p s X - r a y d i f f r a c t o m e t e r was u s e d to q u a n t i f y t h e d e g r e e o f i n t e r l a y e r e x p a n s i o n ( _ 6 ) . The c a t i o n exchange capacity of the peptized sodium and potassium m o n t m o r i l l o n i t e was 105±5 meq./100g. In adsorption studies from saline environments i t is necessary to prepare the w a t e r - s o l u b l e p o l y m e r and p e p t i z e d m o n t m o r i l l o n i t e i n f r e s h w a t e r a t h i g h c o n c e n t r a t i o n s and t o add e a c h to a s a l i n e s o l u t i o n . P o l y e l e c t r o l y t e s w i l l f r e q u e n t l y n o t " y i e l d " t h e same v i s c o s i t y as when t h e y a r e d i s s o l v e d i n f r e s h water. M o n t m o r i l l o n i t e w i l l f l o c c u l a t e i n s a l i n e s o l u t i o n s . With f r e s h water mixing o f components, r e p r o d u c i b l e r e s u l t s are obtained in the saline studies. After component mixing, a g i t a t i o n o f the s l u r r y i s m a i n t a i n e d w i t h g e n t l e s t i r r i n g v i a

In Polymer Adsorption and Dispersion Stability; Goddard, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


7. GLASS ET AL.

Cellulose Ether Adsorption from Saline Solutions

micromagnet i c T e f l o n bars. Analyses of the continuous phase before and a f t e r c e n t r i f u g a t i o n of the polymer s o l u t i o n s were conducted to a s c e r t a i n that p r e c i p i t a t i o n of the polymer was not occurring. Variation in the montmorillonite concentration (.007g/cc to 0.014g/cc) d i d not i n f l u e n c e the adsorption of hydroxyethyl c e l l u l o s e (HEC) of v a r i a b l e molar s u b s t i t u t i o n s from f r e s h water or from 0.54N sodium c h l o r i d e s o l u t i o n s , nor the adsorption of the c a t i o n i c HEC from the s a l i n e s o l u t i o n . The data i n d i c a t e that the adsorptions noted i n t h i s study were not a s s o c i a t e d with a substrate induced precipitation of the water-soluble polymer. Adsorption on sodium montmorillonite as a f u n c t i o n of water-soluble polymer concentration i s i l l u s t r a t e d i n Figure 1 for HEC and the c a t i o n i c HEC from f r e s h and s a l i n e s o l u t i o n s . The c a t i o n i c HEC does not adsorb on the solid substrate from f r e s h water s o l u t i o n s . RESULTS AND

DISCUSSION

Fresh Water Solutions The d i f f e r e n c e between h y d r o p h o b i c i t i e s of the hydroxyethyl (HE) grouping and the hydroxypropyl (HP) u n i t i s evident i n the r e l a t i v e aqueous s o l u t i o n surface tens ions(7,8) of the two c e l l u l o s i c polymers; i n these comparative references the M.S. of the products i s not equal. The dramatic i n f l u e n c e of the more hydrophobic HP groupings on surface pressures i s i l l u s t r a t e d i n Figure 2. The amounts adsorbed on montmorillonite ( 2500 ppm polymer, 0.8 to 1.0 wt.% peptized montmorillonite) and i n t e r l a y e r expansion ( d ^ ^ ) i n the c l a y s (Table I) recovered from f r e s h water s o l u t i o n s do not r e f l e c t the d i f f e r e n c e s i n h y d r o p h o b i c i t i e s noted i n Figure 2. Equivalent adsorption and 0*001 values are observed among polymers of equal M.S.. There i s an i n s e n s i t i v i t y to the monovalent c a t i o n ( i . e . , potassium or sodium vs. ammonium used i n previous s t u d i e s Q ) ) . Hydrophobic!ty i s not important i n adsorption on montmorillonite surfaces; the cation-ether i o n - d i p o l e i n t e r a c t i o n Q ) i s the c r i t i c a l f a c t o r . This i s true i n a d i f f e r e n t sense i n the hydroxyethyl c e l l u l o s e polymers containing i o n i c groups (CMHEC and HECN Me^Cl ; these shorthand chemical formulas w i l l be used f o r the polymers i n the remainder of t h i s a r t i c l e ) . Neither i o n i c group a f f e c t s the surface tension of water compared to the equivalent M.S. HEC (Figure 3), but both inhibit significant adsorption on montmorillonite from f r e s h water, despite the presence of hydroxyethyl (HE) u n i t s (at a 2.0 M.S. l e v e l ) that through a cation-ether ion-dipole interaction promote adsorption and i n t e r l a y e r entrapment. Montmorillonite i s e l e c t r o n e g a t i v e l y charged (105 ± 5 meq./ 100g) i n water and the carboxylate anions i n CMHEC are r e p e l l e d


97


98


2000

4000

6000

POLYMER C O N G , ppm.

Figure 1:

Adsorption (g/g) dependence on water-soluble polymer (W-SP) c o n c e n t r a t i o n (ppm). Substrate: peptized sodium montmorillonite. Open symbols adsorbed from fresh water s o l u t i o n s , c l o s e d symbols from 0.54N NaCl s o l u t i o n s . W-SP:Ο, · , hydroxyethyl c e l l u l o s e (HEC) M.S.= 2.0; A ,HEC (M.S.=2.0) containing 3-0-2 hydroxypropyltrimethy1ammonium c h l o r i d e (M.S.=0.4) ( H E C N M e C l ~ ) , polymer was extracted and gave pH=6.7 i n clay s l u r r y . +

3

TIME (Hre.) Figure 2:

Surface pressure (raN/m) dependence (air-water i n t e r f a c e ) on time (hrs.) f o r hydroxyethyl (HE) hydroxypropyl (HP) c e l l u l o s e (1000 ppm). Molar s u b s t i t u t ions of adduct: Ο ,M.S. = 2.40 ( H E ) , 0.13 (HP) ,M.S. = 1.52 ( H E ) , 0.62 (HP) ,M.S. = 1.19 ( H E ) , 1.03 (HP)

S


GLASS ET AL.


TABLE I ADSORPTION AND INTERLAYER EXPANSION OF CELLULOSE ETHERS FROM FRESH WATER ON PEPTIZED SODIUM MONTMORILLONITE CELLULOSE DERIVATIVE


Hydroxyethyl(HEC)

Hydroxypropyl(HPC)

Hydroxyethyl carboxyraethyl (CMHEC) Hydroxyethyl -3-0-2-hydroxypropyltrimethyl ammonium c h l o r i d e (HECN Me Cl)

3

ADS(g/g)

M.S.

a

ά

Λ°λ

2.0 2.5 4.3 2.0 4.0

.68(.68) .70(.70) .98(.92) .66(.67) .97(.96)

2.1 2.2 2.4 2.1 2.6

2.0 0.4

.14(.12)

1.2

.16(.07)

1.2

b

2.0 ^ 0.4

3

a. Values i n parentheses were determined c o l o r i m e t r i c a l l y , s i m i l a r data were obtained with potassium peptized c l a y . b. d ^ . of Na montmorillonite =1.2nm at 0% r e l a t i v e humidity.

L-J

Ο

ι-ΛΛ 2

1

I TIME

Figure 3:

1

1

3

4

1

1

5

6

^— 7

1

(hrs.)

Surface pressure (mN/m) dependence (air-water i n t e r f a c e ) on time (hrs.) f o r HEC ( Ο ), M.S.= 2.0 and i o n i c HEC (M.S. = 2.0) mixed ethers. Ionic groups : • , c^rboxymethyl , M.S. = 0.4; Δ , Ν Me C l , note Figure 1. Polymer concentrations : 1000 ppm.




100

despite the presence of HE groupings i n a f i v e f o l d excess on a M.S. comparison ( a two fold excess on a degree of s u b s t i t u t i o n , D.S.(4), b a s i s ) . With the frequency and chain extension(_l) of HE groups i t i s s u r p r i s i n g that a s i g n i f i c a n t adsorption i s not observed from f r e s h water, f o r the s u b s t i t u e n t d i s t r i b u t i o n data i n Table I I i n d i c a t e that the CM groups are attached to the glucopyranosyl r i n g rather than to the hydroxyls of the pendant HE groupings. An equivalent M.S. HEC c o n t a i n i n g a 3-0-2-hydroxypropyltrimethy1ammonium c h l o r i d e group (at an equivalent M.S. to the CM grouping) a l s o does not e x h i b i t s i g n i f i c a n t adsorption from f r e s h water s o l u t i o n s . The lack of adsorption and the i n a b i l i t y of the HE groupings to promote i n t e r l a y e r entrapment i n the mixed ether polymers i s r e f l e c t e d i n d ^ spacings equal to those observed i n untreated montmorillonite. I t i s w e l l known that amine containing compounds adsorb s t r o n g l y on the surface, particularly i n the i n t e r l a y e r where the d r i v i n g force i s protonation bv_ the highly a c i d i c surface. The quaternary amine of HECN Me^Cl i s charged and s t e r i c a l l y r e s t r i c t e d , and i t s ability to inhibit the more prevalent HE groupings from promoting adsorption and i n t e r l a y e r entrapment i s s u r p r i s i n g . Based on the r e l a t i v e transport of water i n t h i n films (90 containing both quaternary amine and carboxylate groups the lack of adsorption of HECN Me^Cl can be related to the high s o l v a t i o n of the quaternary ammonium group. S a l i n e Solutions With i n c r e a s i n g e l e c t r o l y t e (sodium c h l o r i d e ) c o n c e n t r a t i o n there appears to be a s l i g h t increase ( w i t h i n experimental e r r o r ) i n adsorption among the lower M.S. nonionic c e l l u l o s e ethers (Figure 4 ) . With i n c r e a s i n g M.S. the amount adsorbed i s greater and the amount adsorbed with increasing salinity increases for HPC, outside the l i m i t s of experimental e r r o r . This can be understood i n terms of s o l v a t i o n e f f e c t s . C e l l u l o s e i s s u b s t i t u t e d i n order to d i s r u p t hydrogen bonding among the glucopyranosyl hydroxy1 groups ; adducts such as ethylene or propylene oxide do not increase the h y d r o p h i l i c i t y of the cellulose chain. The solubility of the high M.S. HEC is dependent on the ether-water i n t e r a c t i o n ; the 2 M.S. HEC and HPC are less dependent. The dependence i s greatest i n the 4 M.S. HPC, where e s s e n t i a l l y a l l of the glucopyranosyl hydroxyls have been s u b s t i t u t e d by propylene oxide which imparts a s i g n i f i c a n t hydrophobicity to the macromolecule. These conclusions are supported by the observation that the 4 M.S. HPC p r e c i p i t a t e s i n f r e s h water at 45 C and by i t s greater i n t r i n s i c v i s c o s i t y sens i t i v i t y to s a l i n i t y (Figure 5 ) . P r e c i p i t a t i o n of the 4 M.S. HPC occurs at sodium c h l o r i d e concentrations i n excess of I N , which i s greater than the s a l i n i t i e s used i n t h i s study.


7. GLASS ET AL.


101

TABLE I I ADDUCT DISTRIBUTIONS IN SELECTED CELLULOSE ETHERS


CELLULOSE DERIV.

b

%uGP

HEC

2.0

24

68

12.6

CMHEC

2.0 0.4

13

29

10.1

2.0 0.4

13

44

11.7

HECN Me Cl 3

%uV.D.

b

M.S.

Int.Vise. dl/g

a. D e t a i l e d c h a r a c t e r i z a t i o n of nonionic c e l l u l o s e ethers are given i n reference 1. b. See reference 4 f o r d e s c r i p t i o n of a n a l y s i s of unsubstituted v i c i n a l d i o l (%uVD) and glucopyranosyl (%uGP) and f o r other p e r t i n e n t references. c. Determined i n IN NaCl.

0.13 025 CONCENTRATION OF

Figure 4:

038 NaCl (N)

0.50

Adsorption (g/g) dependence of nonionic c e l l u l o s e ethers (2500 ppm) on s a l i n i t y (N, NaCl) of aqueous solution. Substrate : peptized sodium montmorillonite. Open symbols, HEC: O , M.S. = 2.0; Φ , 2 . 5 ; C M . 3 Closed symbols, Hydroxypropyl c e l l u l o s e (HPC): · , M.S. =2.0; φ ,4.0.




102

The hydration of polyoxyethylene (POE) i s d r a m a t i c a l l y a f f e c t e d by the anion present(10) i n the aqueous phase. The adsorption of HEC (both 2.0 and 4.3 M.S.) was therefore studied i n Na^SO^ and Na^PO, at equivalent n o r m a l i t i e s . The m u l t i v a l e n t anions are more e f f e c t i v e i n p r e c i p i t a t i n g POE than i s the c h l o r i d e ion. The amounts adsorbed and the i n t e r l a y e r expansions at n o r m a l i t i e s below p r e c i p i t a t i o n conditions are given i n Table I I I . The i n f l u e n c e of m u l t i v a l e n t anions on the i n t r i n s i c v i s c o s i t y of v a r i a b l e M.S. HECs i s i l l u s t r a t e d i n Figure 6. The increased amounts adsorbed are w i t h i n experimental e r r o r , but the decrease i n d^ with the 4.3 M.S. HEC i s notable. The d^ changes i n the absence of increased adsorption are not e x p l a i n a b l e i n terms of s o l v a t i o n e f f e c t s . The observed changes i n i n t e r l a y e r expansion (Table IV) i n the montmorillonite clays a l s o do not p a r a l l e l the adsorption and i n t r i n s i c v i s c o s i t y changes with s a l i n i t y f o r the 4.0 M.S. HPC recovered from NaCl s o l u t i o n s . The l a t t e r polymer as noted above p r e c i p i t a t e s i n NaCl s o l u t i o n s above IN and notable, gradual increases i n adsorption occur with i n c r e a s i n g NaCl c o n c e n t r a t i o n . This trend i s not observed i n d^ . The greater i n t e r l a y e r entrapment of higher M.S. c e l l u l o s e ethers has been related(4) to a combination of fewer unsubstituted glucopyranosyl u n i t s and to a higher frequency of extended ether chains from the c e l l u l o s e backbone, which r e s u l t s i n a higher density of cation-ether i n t e r a c t i o n s . A d d i t i o n a l studies are required to c l a r i f y the apparent c o n f l i c t between adsorption and α"θ01 changes with s a l i n i t y i n a l l of the s a l t s o l u t i o n s . With increasing electrolyte concentration the c e l l u l o s e mixed ethers e x h i b i t a marked increase i n a d s o r p t i o n (Figure 7 ) . This i s expected as a r e s u l t of decreased electrostatic r e p u l s i o n and h y d r a t i o n of the polar groups. The adsorption of CMHEC and HECN Me Cl~ increases u n t i l at 0.54N NaCl s o l u t i o n s both approach that of an equivalent M.S. HEC. The decrease i n electrostatic repulsions i s accompanied by the decrease i n intrinsic v i s c o s i t i e s with salinity (Figure 8 ) . The CMHEC contains approximately 20wt.% salt which d i d not a f f e c t adsorption from f r e s h water s o l u t i o n s . The c a t i o n i c HEC contained only 3.2wt.% s a l t , which i s minimal i n 2500ppm W-SP s o l u t i o n concentrations. The s a l t i n part a r i s e s from the n e u t r a l i z a t i o n a c i d ( i . e . , a c e t i c , n i t r i c , phosphoric, etc.,) used i n the f i n a l step of c e l l u l o s e ether syntheses. The pH o f the c a t i o n i c polymer (2500 ppm) s o l u t i o n was 5.0, with the peptized montmorillonite, 5.7. E x t r a c t i o n with a tertiary butanol(75%)-water(25%) s o l u t i o n lowered the ash content from 3.2 to 2.3 wt.% and r a i s e d the s o l u t i o n pH to 9.4 (at 2500 ppm). With peptized montmorillonite s l u r r i e s the pH was 6.7, approxi mate with the 6.9 - 7.0 values observed with the other c e l l u l o s e ethers. E x t r a c t i o n did not change the adsorption behavior of the c a t i o n i c HEC. Adsorption on peptized montmorillonite from 0.54N NaCl s o l u t i o n s p a r a l l e l e d that observed with HEC (Figure 1 and +

3




7. GLASS ET AL.

1

1

«

u-J

05 1.0 1.5 CONCENTRATION OF NaCl (N)

Figure 5:

103

2.0

I n t r i n s i c v i s c o s i t y (dl/g) dependence on s a l i n i t y (N, NaCl) of aqueous s o l u t i o n s . W-SP: symbols given i n Figure 4.

TABLE I I I HYDROXYETHYL CELLULOSE ADSORPTION BEHAVIOR IN MULTIVALENT ANION SOLUTIONS SALINITY HEC M.S. 2.0

4.3

0.18N g/g d

Q 0 1

0.36N g/g d

Q 0 1

0.54N g/g d

Q 0 1

SALT Na SO Na^PO, 3 4 Na SO NafpO,

.74 .73

2.1 2.1

1.04 1.9 1.04 2.2

.76 .76

2.0 2.1

1.06 1.9 1.06 2.2

.78 .78

2.1 2.1

1.08 1.8 1.08 2.1

10 , Δ , 10 • , 10 ) a t 2500 ppm on s a l i n i t y (N, N a C l ) o f aqueous s o l u t i o n . S u b s t r a t e : p e p t i z e d sodium m o n t m o r i l l o n i t e .




112

Literature Cited 1. Glass, J . E.; Ahmed, H.; Lu, D-L; Seneker, S. D.; McCarthy, G. J. J.Colloid Interf. Sci., in press. 2. Malone, T. R.; Raines, R. H.; Weintritt, D. J., Society of Petroleum Engineers Publication No.8741, 6200 N. Central Expressway, 64706, Dallas, Texas 75206. 3. Savage, Α. Β., in "Encyclopedia of Polymer Science and Technology"; Mark, H. F.; Gaylord, N. G.; Bikales, Ν. M. Eds.; Interscience, New York, 1965; Vol. 3, p. 512. 4. Glass, J. E.; Buettner, A. M.; Lowther, R. G.; Young, C. S.; Cosby, C. A. Carbohydr. Res, 1980,84,245. 5. Brice, B. A.; Halzer, H. J. Opt. Soc. Amer., 1951,41,1033. 6. "Crystal Structures of Clay Minerals and Their X-Ray Identi fication, " Brindley, G. W.; Brown, G., Eds., Mineralogical Society Monograph No. 5, London,1980; Chapter 5. 7. Glass, J. E.J. Polym. Sci.,Part C,1971,34,141. 8. Chang, S.A.; Gray, D. G. J.Colloid Interf. Sci.,1978,67,255. 9. Hill, L. W.; Lu,D-L J . Water Borne Coatings, 1981,3,3. 10. Bailey, F. E.; Callard, R. W. J. Appl. Polym.Sci.,1959,1,56. RECEIVED October

7, 1983


Polymer Adsorption and Dispersion Stability - ACS Publications

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