luly 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
shift from the sulfur atom into the vacant &orbitals of the iron surface atoms. ACKNOWLEDGMENT
The authors wish to express their appreciation t o the Office of Saval Research for support of this work under Contract Nonr 375 (02). LITERATURE CITED
(1) Bdam, N. K., “Physics and Chemistry of Surfaces,” 3rd ed., p. 50, London, Oxford University Press, 1941. (2) Bowden, F. P., and Moore, A. C., Research (London), 2, 585 (1949). (3) Cook, E. L., and Hackerman, N., J . Phys. & Colloid Chem., 55, 549 (1951). (4) Dilke, M. H., Eley, D. D., and Maxted, E. B., Sature, 161, 804 (1948). (5) Greenhill,E. B., Trans. Faraday Soc., 45,625 (1949). (6) Hackerman, N., and Cook, E. L., J . Electrochem. Soc., 97, 1 (1950).
1485
(7) Hansen, R. S., Fu, Y . , and Bartell, F. E., J . Phys. & Colloid Chem., 53, 769 (1949).
( 8 ) Heilbron, I. M., and Bunbury, H. M., “Dictionary of Organic
Compounds,”London, Eyre and Spottiswodde,1943. (9) Hunter, E. C . E., and Partington, J. R., J . Chem. SOC.(London), 1931, 2062. (IO) Kumler, W. D., and Fohlen, G. M., J . Am. Chem. Soc., 64, 1944 (1942). (11) Makrides, A., and Hackerman, K.,unpublished information. (12) Meldrum, A. N., and Turner, W. E. S., J . Chem. SOC.(London), 97, 1605 (1910). (13) Ostwald, Wo., Kolloid-Z., 45, 56 (1928). (14) Pauling, L., and Sherman, J. J., J . Chem. Phys., 1, 606 (1933). (15) Ralston, A. W., “Fatty Acids and Their Derivatives,” New York, John Wiley & Sons, 1948. (16) Roebuck, A. H., Ph.D. dissertation, University of TexaE, June 1951.
(17) Sanders, J. V., Research (London),2, 586 (1949). (18) Trillat, J., and Brigonnet, J., Compt. rend., 228, 1587 (1949). (19) Zellhoefer, G. F., Copley, hl. J., and Marvel, C. S., J . Am. Chem. Soc., 60, 1337 (1938). RECEIVED for review December 21, 1953.
ACCEPTED March 4, 1954.
Aggregation of Suspensions by Polyelectrolvtes J
J
J
A. S. MICHAELS Massachusetts Institute of Technology, Cambridge, Mass.
T
HE ability of certain polyelectrolytes to cause the aggregation of soils and other finely divided solids in aqueous suspension has been well recognized during the past 2 years (1, 9, Q, IS), but comparatively little information (5, 6, 9, 11) has been published on the mechanism by which these reagents function. In their patent on fertilizing compositions, Mowry and Hedricli (8) list over 60 synthetic polyelectrolytes which were evaluated as “soil conditioners.” Of these, the most effective appeared to be copolymers of a t least one nonionic vinyl compound (acrylonitrile, acrylamide, and vinyl acetate or its hydrolysis product, vinyl alcohol) and a t least one ionic vinyl compound (sodium, ammonium, or calcium salt of maleic or acrylic acid). Polymers composed of purely ionic monomers-e.g., sodium polymethacrylate, sodium polyacrylate, sulfonated polystyrene-appear to be comparatively ineffective as aggregants (8,fd). The ability to function as an aggregant does not, however, seem to be exclusively confined to water-soluble polymers with electrolyte character. Certain starches, and other water-dispersible polysaccharides, have been used for this purpose for many years. On the other hand, water-soluble nonionic polymers such as polyvinyl alcohol or methylcellulose are a t best poor agents for this purpose. These qualitative observations suggest that certain aspects of molecular configuration and functionality determine whether a given compound will act as an aggregant. The fact, for example, that pure sodium polyacrylate (or polyacrylamide) is a poor aggregant, whereas partially caustic-hydrolyzed polyacrylonitrile or polyacrylamide is an effective one, suggests the necessity for a dual functionality (nonionic amide or nitrile groups, and ionic acrylate groups) to achieve high activity. To understand the role of these individual functional groups, in their reactions with solid surfaces and in altering the configuration of the polymer molecule, is to approach an understanding of the flocculation mechanism. The object of this study was to examine the aggregating capacity of a polymer whose degree of electrolytic character can be varied over a wide range. The compound selected for this purpose was the product obtained by the aqueous phase, redox-cata-
lyaed polymerization of acrylainide. This water-soluble polymer was hydrolyzed to controlled degrees with sodium hydroxide to produce compounds containing varying proportions of amide and carboxylate groups. The results of this study, it is believed, make it possible to formulate a tenable hypothesis for the mechanism of flocculation in the presence of water-soluble polymers which may be applicable not only to polyacrylates but also to other polyelectrolytes and to nonionic polymeric flocculants. EXPERIMENTAL WORK
This investigation can conveniently be divided into three phases: the synthesis of polyacrylamide, the controlled hydrolysis of polyacrylamide, and the evaluation of the aggregating capacity of the compounds produced.
SYNTHESIB OF POLYACRYLAMIDE. Ten grams of pure acrylamide (American Cyanamid Co.) were dissolved in distilled water a t room temperature to prepare a solution containing 10% by weight of the monomer. I n this solution was dissolved a redox catalyst consisting of an equimolal mixture of sodium thiosulfate and ammonium persulfate. Two molecular weight ranges of polyacrylamide were prepared by varying the catalyst concentration; in one case, 0.5 mole of catalyst mixture per 100 moles of acrylamide was employed, and in the other, 1 mole of catalyst per 100 moles of acrylamide. Polymerization was allowed to proceed for 24 hours a t room temperature, after which the perfectly clear, viscous solutions were diluted to exactly 2 liters. Samples of these two solutions were further diluted to concentrations of 0.2 and 0.05 gram of polymer per liter of solution, and subjected to viscosity measurement a t 23.8” C. with an Ostwald viscometer. The intrinsic viscosities thus determined are shown in Table I.
TABLE I. INTRINSIC VISCOSITY OF AQCEOCSPOLYACRYLAMIDE Polymer
A B
(Temperature, 23.8’ C.) Catalyst, Mole $6 0.6 1.0
[v I
.
3.8 1.9
INDUSTRIAL AND ENGINEERING CHEMISTRY
1486
mixtures were allowed to react a t room tem erature for 24 hours. The polyacrylamides of both high a n f low molecular weight were treated with quantities of caustic equivalent t o 30, 50, SO, and 95% of the amounts stoichiometrically required for complete conversion to sodium polyacrylate (1.41 gram-equivalents of sodium hydroxide per 100 grams of acrylamide). The actual degree of hydrolysis of the samples wa9 determined in the following manner. An aliquot of the polyacrylamide-caustic solution after aging was diluted with water, and titrated with standard hydrochloric acid solution (0.01N) to a methyl orange end point. At t'he color change of methyl orange (approximately pH 3.5), all residual sodium hydroxide and free ammonia are neutralized, and almost all polyacrylate salts present' are converted to the free acid.
-.
50
40
1.
k
2 b
30
h
20
$
IO
5
Vol. 46, No. 7
k
8 20
10
1
I
0 0
IO
10
30
40
MOLE
SO
60
70
80
90
I00
HYDROLYSIS
Figure 1. Effect of Degree of Hydrolysis on Soil Sediment Density
At the catalyst concentration levels employed, the average degree of polymerization of the product would be expected to vary approximately inversely xit,h the catalyst concentration. If (as may be the case with aqueous solutions of polyac the polymer chains are flexible and free draining, the S equation should be applicable, and the molecular v,eight should be proportional to the intrinsic viscosity. These two observations appear to be consistent with the data of Table I, so that the assumption that the two polymer samples have an average molecular weight ratio of 2 to 1 seems justifiable. It has been suggested (IO)that cross linkage of polya chains via imide formation may have taken place during polymerization, with the result' that subsequent caustic hydyolysis may have caused polymer degradation. Inasmuch its polymerizations were carried out in relatively dilute solutions at room temperature, the polyacrylamide Eolutions were clear, precipitated no gel, and were essentially a t t'he same pH as the monomer Bolutions, and the caustic-hydrolyzed polymer solutions were iii all cases more viscous than the unhydrolyzed polymer solutions, it would appear that imide cross linking occurred to a negligible extent. The above observations regarding the relation between intrinsic viscosity and catalyst concent'ration also suggest little to no cross linking. COSTROLLED HYDROLYSIS O F POLYACRYLAMIDE. TO measured volumes of the polyacrylamide solution concentrate were added measured volumes of 0.5.V sodium hydroxide solution, and the
The equivalents of amide hydrolyzed were t'herefore stoichiometrically equal to the difference between the equivalents of camtic added initially and acid required for neutralization. While amides hydrolyzed in acid solution, there was no evidence of significant acid hydrolysis under the conditions employed in these analyses; no fading of the methyl orange end point was observed even after prolonged aging of the titrated solutions. The rate of hydrolysis of aqueous polyacrylamide on addition of sodium hydroxide a t room temperature appears to become extremely slow after only a few hours; no significant change in extent of hydrolysis occurred after 24 hours' aging. Nevertheless, to avoid the possibility of further reaction, hydrolyzed samples of the polymer mere neutralized t o pH 7 (using concentrated hydrochloric acid and Hydrion paper) before being bottled for use in the flocculation experiments.
1 Q
VIRGINIA SANDY
I
I
PO
30
01
0
IO
40
MOLE /o '
Figure 2. TABLE
I
CLAY
+ POLYMER
I
I
I
I
I 60
70
50
B
1
I 80
90
I00
HYDROLYS/S
Effect of Degree of Hydrolysis on Soil Sediment Density
O F HYDROLYSIS O F POLYACRYLA~IIDE 11. DEGREE
Theoretical NaOH Added, %
Polymer
.4
0 20 30 80 93 0 30 50
B
80
.
95 Large excess
Amide Hydrolyzed, 9l 0
11 5 26 5
39.8 6 1 , s (after boiling) 0 14.0 29.0 39.1 6 2 , O (after bojljng)
100.0 (after boiling)
T o achieve high ext'ents of hydrolysis, it was found necessary to add a large excess of caustic to the polyacrylamide solution, and t o boil and evaporate the solution. When this procedure was employed, both the distillate and tmheresidue were titrated with hydrochloric acid, and the degree of hydrolysis u-as calculated as outlined above. The extent of hydrolysis achieved with the two polymers studied, as a function of the quantity of sodium hydroxide added, is shown in Table 11. FLOCCULATION TESTS. When a finely divided solid suspended in liquid is caused to flocculat,e, the rate of sedimentation is in-
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1954
I
I
I
I
1487 I
I
OF SOILSTUDIED TABLE 111. PROPERTIES
Characteristios Coarser t h a n 0.06 mm., % Between 0.06 and 0.002 mm..
%
Finer than 0.002 mm., % Liquid limit. % Plastic limit, % Composition, %
Virginia Sandy Clay (VSC) 40
New Hampshire Silt ("6) 18
32 28 40
75 7
22
Quartz, 40 Kaolinite 25 Hydrous oxides of iron and aluminum
27 27 uarts 55 eldgpar and mica
8
creased and the density of the sediment formed is reduced. Previous research ( 7 ) has indicated that the reduction in sediment density brought about by the addition of flocculating agents to suspensions is a convenient quantitative measure of flocculating ability. This measurement was the one employed in this investigation. Samples of two natural soils of widely differing composition (properties and characteristics as shown in Table 111) were prepared in the following manner. The soils were air-dried and pulverized to disintegrate large lumps, and were passed through a 65-mesh screen to remove pebbles and large aggregates. Small samples were weighed, and then dried to constant weight in an oven a t 150' C. to determine the air-dry moisture content. A 50-gram (based on bone-dry weight) soil sample was placed in a 250-ml. graduate and water was added to bring the total volume to exactly 250 ml. The contents of the graduate were then agitated gently until all the soil was in suspension, and the graduate was returned to a vertical position, and allowed to remain undisturbed for 24 hours. A t the end of this period, the volume occupied by the sediment was measured, and the sediment density calculated. Three duplicate measurements were made of the sediment density of the untreated soil samples, and the average of these figures was used as the blank. To determine the effect of the polyelectrolytes on these soils, a 50-gram soil sample was placed in a 250-ml. graduate, approximately 100 ml. of water added, and a measured quantity of polymer solution added from a buret. The total volume was then brought to about 240 ml., and the system mildly agitated as described above. Particles adhering to the side of the graduate were rinsed into the solution with a wash bottle, and the volume brought to exactly 250 ml. The sus ension was allowed to stand undisturbed for 24 hours, after whic! period the volume of sediment was measured. Four polymer concentrations (based on the dry soil weight) were employed in this study: 0.01, 0.04,0.07, and 0.10% polymer calculated as pure sodium polyacrylate, irrespective of the degree of hydrolysis of the individual polymer samples. In other words, the soils were treated with the same number of milliequivalents of acrylate constituent (0.105, 0.42, 0.74, and 1.05 meq. per 100 grams of dry soil, respectively) for each compound examined. The relative aggregating effectiveness of the polymers employed was determined by calculating the percentage reduction in sediment density over that of the untreated soil, due to polyelectrolyte addition. RESULTS
The results of this investigation, showing the reduction in soil sediment density on polymer addition as a function of soil type, degree of hydrolysis, concentration of polymer, and molecular weight of polymer, are shown in Tables IV and V and Figures 1to3. DISCUSSION OF RESULTS
Several important trends are evident from the data presented. The most significant of these are: In nearly every case studied, the aggregating capacity (as exhibited by a given soil, a t a given polymer concentration, and a t a given degree of hydrolysis) of the polymers employed is higher for the higher molecular weight species.
For both soils the aggregating capacity of polyacrylamide reaches a maximum a t a degree of hydrolysis in the neighborhood of 28 to 35%. At the lower polymer concentration levels (0.01 and 0.04%), there also appears t o be a minimum in aggregating ability a t a low (about 10%) degree of hydrolysis. In nearly every case with these two soils, the unhydrolyzed polyacrylamide appears to be more effective than the completely hydrolyzed compound. For the two soils, the degree of flocculation increases rapidly with increasing polymer concentration up to a value of about 0.03 gram per 100 grams of dry soil. Above this concentration, the extent of flocculation is increased slightly, if a t all, and in some cases is actually decreased. The effect of polymer concentration on flocculation changes in no consistent fashion ~ i t h degree of hydrolysis. The dependence of extent of flocculation on polymer concentration is, however, greatest for the most active compounds.
.
These observations can, it is believed, be put t o good use in formulating a hypothesis for the mechanism by which waterRoluble polymers can cause aggregation, and thus in predicting what types of polymers are best suited for use as aggregants. A t the outset, it seems reasonable to postulate that, for a polymer to contribute directly to flocculation of a suspension, the polymer molecules must become adsorbed on the solid surfaces. Flocculation can then occur by two basically different phenomena: (1) The electrokinetic potential of the particle surfaces may be reduced sufficiently to permit particle aggregation and cohesion via residual valence forces; or (2) the polymer molecules may (if of adequate size) adsorb on more than one particlc, and thus be directly responsible for interparticle bonding. For a polymer to cause flocculation by reducing the electrokinetic potential of solid surfaces, it must of necessity be of ionic rharacter, and, furthermore, must carry a charge opposite in sign to that of the solid surface. Inasmuch as most soils and common finely divided solids carry a negative charge in an essentially neutral aqueous environment, cationic polyelectrolytes would be expected to be active flocculants for such systeme. This expectation has been borne out by experiment (3,7 , 11). Naturally, a cationic polyelectrolyte molecule may not only cause flocculation by reducing electrokinetic potential, but may
INDUSTRIAL AND ENGINEERING CHEMISTRY
1488
TABLE Iv. FLOCCL-LATIOS DATA
OX
NEW AMPS SHIRE SILT
Average sediment volume occupied by S O grams of untreated dry soil in water, 43.9 cc Average sediment density of untreated soil in water, 1.14 grams per cc. Hydrolysis of Polymer,
0.01% 0.04% 0.07% 0 10% ______ D V D % V D % 5‘ D COXEXTRATIOX O F POLYMER A (0.5% CATALYST) I N SOIL^ 56.0 0.893 21.6 5 5 . 0 0 910 0 56.0 0.893 21.6 5 5 . 5 0 . 9 0 2 20.8 5 9 . 5 0.841 2 6 . 2 60.0 0.834 1 1 . 5 5 0 . 5 0 , 9 9 0 1 3 . 1 5 6 . 5 0.886 2 2 . 3 0.695 0.725 36.4 7 2 . 0 3 9 . 1 7 4 . 0 7 0 . 5 0.710 26.0 69.0 0.676 40.7 6 6 . 5 0 . 7 5 3 3 3 . 9 71 0 0.705 3 8 . 1 39.8 64.0 0 , 7 8 2 3 1 . 4 6 7 . 0 0 747 61.8 61.0 0.820 2 8 . 0 6 0 . 0 0.834 2 6 . 8 5 9 . 5 0.841 2 6 . 2 6 0 . 0 0 . 8 3 4 C O N C E N T R ~ TOI O F K POLYXER B (1.0% CATALYST) I N SOIL* 5 1 . 0 0.981 1 4 . 0 5 4 . 5 0 , 9 1 7 1 8 . 7 5 7 . 5 0.869 2 3 . 8 5 6 . 5 0 885 0 5 5 . 0 0.909 2 0 . 2 6 9 . 5 0.840 1 4 . 0 5 1 . 0 0.981 14.0 60.0 0,834 26.8 6 5 . 5 0.763 3 3 . 1 6 3 . 5 0.787 3 1 . 0 5 9 . 5 0.840 5 9 . 5 0,840 2 6 . 3 29.0 6 1 . 5 0.813 2 8 . 7 63.0 0.794 62.0 0,807 29.2 3 9 . 1 5 6 . 5 0.885 2 2 . 4 5 3 . 5 0 , 9 3 4 1 8 . 1 58.0 0 , 8 6 2 2 4 . 4 5 0 . 0 1.000 62.0 5 1 . 5 0,970 14.9 100.0 48.0 1 , 0 4 1 8 . 8 5 3 . 0 0.944 1 7 . 2 56 0 0 . 8 9 3 21 7 58.0 0.862 Weight yo based on sodium acrylate on dry soil. Volume occupied by 50 grams of sedimented soil. 40 decrease in bulk density after treatment.
% ’
0
b c
Vb
70 20 2 26.8 37 7 34.4 26 8
22 4 26 3 26.3 30 4 12.3 24.4
TABLE lr. FLOCCULATION Da.r.4 o s T‘IRGIVIL S ~ S D Y C ~ i r Average volume occupied b y 50 grams of dry soil in water, 58.7 cc. Average density of untreated soil in water, 50/58.7 = 0.852 grain per cc Hydrolysis of Polymer,
70
b c
0 01% D
%c
V
0 04% I)
%
V
0 0790 D
%
20.1 71 0 0 705 17 3 1 6 . 6 7 4 . 0 0 G76 1 8 . 8 23.0 82 5 0 006 2 7 . 0 20.9 74.0 0.676 18.8 14.2 72 0 0 . 0 9 5 1 6 . 6 C O N C E N T R A T I O N Oi‘ P O L Y Y E R % (1.0% C.4TALYST) IX S O I L a 5.4 6 9 . 5 0 . 7 2 0 1 5 . 5 7 1 . 0 0 705 1 7 . 8 0 6 2 . 0 0.806 68 5 0 . 7 3 0 1 4 . 3 6 8 . 0 0.736 1 3 . 6 6 5 . 5 0 , 7 6 4 10 3 14.0 67.5 0 , 7 4 2 1 3 . 2 72 2 0 , 6 9 0 1 9 . 0 7 3 . 0 0.685 1 9 . 6 29.0 1 6 . 1 7 2 . 0 0 . 6 9 4 18 5 72.5 0.690 19.0 0.715 7 0 . 0 39.1 7 . 3 6 7 . 5 0.741 1 3 . 0 61; 5 0 752 1 1 . 7 62.0 63.5 0.788 100.0 6 2 . 5 0.800 6 . 1 6 5 . 0 0.770 9.6 ii6.0 0.758 1 1 . 0 Weight % based on sodium acrylate on dry soil. Volume occupied b y 50 grams of sedimented soil % decrease in bulk density after treatment.
0 11.5 26.0 39.8 61.8
(1
Vb 70.5 68.0 77.5 76.0 69.0
0,710 0,736 0.646 0.658 0,725
16.7 11.7 22.3 20 9 13.0
73.5 72.0 77.5 76.0 70.0
0.681 0 695 0.6413 0.658 0,715
(if large enough) take act’ive part in the flocculating process by adsorbing on more than one particle. The observation that cationic polyelectrolytes cause flocculation of markedly different character on negatively charged suspensions than t,hat produced by simple polyvalent ions-e.g., thorium-leads one to suspect bhat interparticle bridging may be an important factor in t>heir action. On the other hand, anionic polyelectrolytes (such as those studied here, as well as the majority of commercial products now being considered for use as soil conditioners and industrial flocculants) cannot be responsible for flocculat’ion by mere alteration of electrokinetic potential, as they carry charges of the same sign as that of electronegative colloids. Furthermore, the anionic groups on these polymers cannot easily be visualized as undergoing strong adsorption on negatively charged surfaces. Thus, if adsorption does indeed t,ake place, it must occur by virtue of other active groups present in the polymer molecule. In the case of caustic-hydrolyzed polyacrylamide, t’he only other “active group” present is the amide group. Were this analysis correct, it would be expected that pure polyacrylamide, which contains no anionic carboxylate groups, would exhibit a higher aggregating capacity than partially hydrolyzed polyacrylamide. This, hon-ever, is not in accord n-ith the facts, as the present data indicate. That a certain degree of hydrolysis (roughly, one carboxylate per two amides) iL q necessary to achieve maximum aggregation, indicates clearly that ionic groups in the polymer play an important role in the flocculating process. It is probable, however, that these groups serve predominantly to alter the properties of the polymer molecules, without themselves playing a direct role in the flocculating mechanism, The analysis which leads to t,his conclusion is as follo~vs.
’i’
0 10% D
%
70.0 76.0 87.0 74.0 69.0
0 715 16 1 0.658 20 9 0 . 5 7 3 30 6 0 . 8 7 6 18 8 0 723 13 0
70 0 69 0 72.0 69 0 67.5 64.0
0 713 0 721 0.694 0 721 0 741 0 782
16.1 15 4 18 ti 15.4 13.0 8.2
Vol. 46, No. 7
There are (as noted earlier) a numbrr of water-soluble polymers without electrolyte character which act as powerful flocculants. Practically all of these compounds are polysaccharides (polyglucosides, po 1y g a l a c t o m a n n a n s! etc. i. These compounds appear to function satisfactorily as flocculants in aqueoue solution over wide ranges of pH, and in the presence of appreciable concentrations of miscellaneous electrolytes. [In this respect, they differ markedly from most anionic polyelectrolytes, whose aggregating capacity becomes very poor a t relatively low (3.5) or high (9 t’o 9.5) pH’s, and in the presence of polyvaleiit metal salts.] These compounds are characterized by being predominantly linear in structure, and containing a rather high molecular concent,ration of hg-droxyl groups. Most of these polysaccharides can be precipitated from aqueous solut’ion by the addition of small amounts of boric acid, the reaction occurring being a hydrogen-bonding process or a borate-ester formation, with consequent cross linking and gelation of the polymer. Because of t,he similarity in functionality and in spatial distribution of atoms in silicic acid, aluminum hydroxide, and the hydroxides of other weakly basic polyvalent, metals to t,hose of boric acid, it seems not unlikely that the same type of reaction may take place between polysaccharides and the hydrated surfaces of minerals containing these elements. As these polymers are of fairly large size, it is possible that the Process of flocadsorption via hydrogen bonding and:or and “bridging” of particles via polysaccharide
culation involves “ester” formation, chains. If this picture is correct, then there is no reason to suspect that other polyhydroxy compounds, such ae polyvinyl alcohol, or compounds of comparable functionality-e.g., polyamide or unionized carboxyl compounds-should not act’ similarly. (Polyamino compounds should certainly fall into this category, hut are not discussed here, because they function as polycations in aqueous solution; it thus becomes difficult to differentiate between their ionic and nonionic functions in flocculation processes.) However, polyvinyl alcohol and polyacrylamide are relatively poor aggregants. The explanation of t,his anomaly may possibly lie in differences in molecular configurat,ion of polysaccharides and polyvinyl alcohol or polyacrylamide. The linear polysaccharides, because of the rigidity of t,he inter-ring linkages, exist in solution as fairly stiff, extended rods. I n this extended condition, the polyhydroxy groups are exposed and made available either for adsorption on solid surfaces, or for association with hydroxy groups on neighboring molecules, the latter process leading to the formation of molecular aggregates of considerably greater size than the individual molecules. On the other hand, the vinyl polymers are flexible chains which can fold, kink, or coil in solution. If, as appears likely, there is a tendency for associat,ion via hydrogen bonding between adjacent hydroxyl or amide groups, this tendency can be satisfied by molecular coiling with the carbon-carbon chain polymers but not with the polysaccharide. Since intramolecular bonding of this type can inactivate groups which have a latent’ adsorptive affinity for solid surfaces, and more importantly greatly reduce the effective length of the molecule in solution, it is thus possible to account for the
July 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
high aggregating ability of the rigid-chain polysaccharides, and the low aggregating ability of flexible-chain, coiled polymers such as polyvinyl alcohol and polyacrylamide. If, however, it were possible to force polyvinyl alcohol or polyacrylamide chains to become “stretched out” in aqueous solution, this above analysis would lead one to expect a marked enhancement in aggregating ability. This change can be brought about by introducing ionized groups a t intervals into the polymer chain; under these conditions, the electrostatic repulsion between adjacent charged groups is responsible for the stretch-out. (One welldocumented effect attributable to this cause is the marked increase in viscosity of polyacrylic acid solutions on treatment with caustic.) Clearly, the number of ionizable groups introduced into the chain must be high enough to achieve appreciable chain extension; on the other hand, if anionic groups are introduced, and a negatively charged solid is to be flocculated, too high a charge density on the polymer molecule may prevent close approach of the molecule to the solid surface, and thus prevent adsorption. Consequently, one would expect the aggregating ability (as measured on anionic colloids) of polyvinyl alcohol or polyacrylamide to increase as the extent of anionic substitution is increased up to a point a t which maximum chain extension is achieved without marked electrostatic interference with polymer adsorption, followed by a gradual decrease in this capacity as the polymer is rendered more electronegative. This picture adequately explains most of the observed trends in aggregating capacity of polyacrylamide subjected to varying degreee of hydrolysis, but does not account for the noted reduction in aggregating ability (compared with unhydrolyzed polyacrylamide) a t low degrees of hydrolysis. This latter phenomenon can, however, be reasonably explained when it is realized that unhydrolyzed polyacrylamide is not completely nonionic, but undoubtedly contains a small percentage of cationic “amidinium” groups (-CONHI+). This slight cationic character thus favors adsorption on negative surfaces, as well as slight degree of chain extension in solution. A slight amount of hydrolysis, with formation of isolated anionic carboxylate groups, repders the polymer electrostatically neutral-Le., isoelectric--a condition conducive to maximum coiling and hence minimum aggregating ability. These changes in character of polyacrylamide on hydrolysis are perhaps shown more clearly by the diagram in Figure 4. I t is clear that, if the sole function of anionic groups on polyhydroxy or polyamide polymers is to cause chain extension, the maximum concentration of ions on the polymer chains which can be tolerated without interference with hydroxy or amide adsorption on solid surfaces will vary with the electrokinetic potential of the solid surface-that is, a low zeta-potential solid can adsorb a highly anionic polymer, and vice versa. Evidently there is a maximum negative zeta potential above which flocculation with anionic polyelectrolytes is impossible; this is the potential a t which the polyelectrolyte with the lowest charge density conducive to chain extension will be unable to adsorb on the solid. This picture can thus explain why aluminosilicate soils and clays, when suspended in water a t relatively high pH in the presence of alkali ions (under which conditions the zeta potential is large) do not respond to flocculation by usual anionic polyelectrolyte aggregants. Similarly, this hypothesis satisfactorily explains Ruehrwein’s and Ward’s observations (11) that anionic polyelectrolytes are active as aggregants only in soils and clays suspended in the presence of free electrolyte; for under these conditions the electrokinetic potentials are low. The use of carboxylate or other weakly acidic groups as “chain extenders” for polymeric aggregants appears to have some advantage over strongly acidic-e.g., sulfonate or sulfategroups, as the degree of ionization (and hence the polymer charge density) can be relatively easily controlled by adjustment of pH, or addition of polyvalent metal ions-e.g., calcium-which can form chelates with these groups. Furthermore, it is probable
1489 UNHYDROL YZED : WEAULY CAT/ON/C SLlGHTLY EXTENDED CHAIN MEDIOCRE FLOCCULANT SL IGHTL Y HYDROLYZED : NON-IONIC (JSOEL ECTRIC) TIGHTLY COILED CHAIN POOR FLO~CULANT
33 Yo HYDROLYZED:
GOOD FLOCCULANT
-@
-
Figure 4.
- -
-
-
- -
6 7 % HYDROLYZED: STRONGLY ANIONIC EXTENDED CHA/N POOR FL OCCULAN T
Effectof Hydrolysis on Polyacrylamide Character
that un-ionized carboxylic acid groups function also as nonionic hydrogen-bonding groups, and can contribute to polymer adsorption and bridging. There are attendant disadvantages, however, since chain coiling or precipitation of polymer may occur with such compounds in fairly acidic or even solutions of relatively low polyvalent cation content. This appears to be a plausible explanation for the flocculation inactivity of hydrolyzed polyacrylonitrile and polyvinyl acetate-maleate copolymers in mildly acidic media, or in polyvalent metal salt solutions. Previous work ( 1 2 ) has shown that, for an active aggregant, there is an optimum polymer concentration for maximum flocculation; this phenomenon is to an extent borne out by this investigation. These observations are consistent with the postulation that flocculation occurs (or is a t least enhanced) by polymer adsorption and interparticle bridging. For if excess polymer is available in solution, all available adsorptive sites on the suspended particle surfaces can be satisfied by adsorption of individual molecules, and “bridging” will be minimized. Inasmuch as the adsorbed polymer is strongly hydrophilic and (for polyelectrolytes) is charged as well, this will result in stabilization of the dispersion by protective colloid action. The effect of polymer concentration on flocculation may be represented schematically as shown in Figure 5. An apparent exception to this hypothesis is evidenced by the observation that the very poor activity of slightly hydrolyzed (10%) polyacrylamide is exhibited only a t low concentration levels, while a t higher concentrations this material is fairly effective. This discrepancy may be reconciled by granting that a sample of slightly hydrolyzed polyacrylamide contains some small percentage of molecules whose degree of hydrolysis corresponds to that required for maximum flocculating effectivenesj. At low concentration levels, there is an insufficient quantity of thie active material present to cause significant aggregation, whereas a t the higher concentration levels there is enough of this material present for this purpose. I n this regard, it should be kept in mind that “degree of hydrolysis’’ as employed in this discussion is a statistical quantity, and does not truly describe the configuration of those molecules which specifically take part in the flocculation process. SUMMARY
In summary, the hypothesis presented postulates that the aggregating or flocculating action of nonionic or anionic watersoluble polymers is caused by adsorption (via ester formation or
INDUSTRIAL AND ENGINEERING CHEMISTRY
1490
hydrogen bonding) of hydroxyl or amide groups on the solid surfaces, each polymer chain adsorbing on, and bridging between, more than one solid particle. The presence of ionic groups in these compounds is necessary only for flexible-chain compounds, and serves only to extend the polymer chains, to prevent intramolecular polar group association and permit interparticle bridging. While this hypothesis has been developed on a basis of data POLYMER
SUSPENDED SOL ID
(FLOCCULATION)
LOW
(DlSPERS/ON STAEILJZAT/ON)
:. .
HIGH
Figure 5 .
course, be more satisfactory flocculants for infrequently encountered positively charged dispersions, such as strongly acidic heavy metal hydroxide systems. It appears probable that the ultimate clarification of the mcchanism of flocculation by hydrophilic polymers must come from a detailed study of the adsorption of these compounds from solution by various solids. The hypothesis developed herein is, it is believed, in agreement with fundamental chemical and colloidal principles, and consist’ent wit’h a reasonably large amount of circumstantial information on both synthetic and natural aggregants; at,t,emptsto test it by means of precise deductive research are under m y .
POLYMER CONCENTRATION
1
POLYMER CONCENTRATION
Effect of Polymer Concentration on Flocculation
on hydrolyzed polyacrylamide, it appears equally suitable for explaining the aggregating action of hydrolyzed polyacrylonitrile, of salts of maleic acid->Tiny1 acetate copolymers (which probably exist in aqueous solution as salts of polyvinyl alcohol-maleate), of the majority of the anionic copolymers described by Mowry a n d Hedrick (8), and of sodium carboxymethylcellulose of high molecular weight, and low carboxylate content (lb). This hypothesis also leads to the prediction that most linear polyhydroxy or polyamide polymers-e g., polyvinyl alcohol, polyvinyl cyclohexane-triol, polyallyl alcohol, polymers of furoamide, etc -can be converted into flocculants by introductlon of, or copolymeri7ation with, controlled concentrations of ionic constituents---e.g., carboxylate, sulfate, sulfonate, phosphate, xanthate; primary, secondary, and tertiary alkyl or aryl amino groups: quaternary ammonium salts, etc. It is also likely that the use of cations as ionic groups in these compounds will produce flocculants which will be much more effective with negatively chaiged dispersions, and will be less sensitive to the presence of estrancous ions in the suspending medium. Anionic polyelectrolytes will, of
Vol. 46, No, 7
ACKNOWLEDGAIENT
This article is a contribution of the N.I.T. Soil Stabilization Laboratory, and is part of a broad program of fundamental research sponsored by industrial contributions. The experimental data herein described were taken from an undergraduate thesis (zj carried out under the author’s supervision. LITERATURE CITED
Allison, L. E., Soil Sci., 73, 443 (1952). Gardner, W. M., and Montemayor, L. X., “Factors Influencing the Flocculating Capacity of Polyacrylates,” Undergraduate thesis in chemical engineering, Massachusetts Institute of Technology, May 1953. (3) Hedrick, R. M., and Alowry, D. T., Soil Sci., 7 3 , 4 2 7 (1952). (4) Martin, W. P., Ibid., 73, 456 (1952). (5) Michaels, A. S., “Altering Soil-Water Relationships by Cheiniea1 Means,” Proceedings of Conference on Soil St,abilization, Massachusetts Institute of Technology, June 18 to 20, 1952, (1) (2)
p. 59.
(6) Michaels, A. S., “Some Colloidal and Physico-Chemical Aspects
of Soil-Chemical Interactions,” address before Sortheastern Section, AH. CHEY.SOC., April 10, 1952. (7) Nichaels, 9. S., and Lambe, T. W., J . Ag. Food Chem., 1, 835 (1953).
(8) Mowry, D. T., and Hedrick, R. 31. (to Aionsanto Chemical Co.), U. 5. Patent 2,625,471 (Jan. 13, 1953). (9) Quastel, J. H., SoilSci., 7 3 , 4 1 9 (1952). (IO) Ruehrwein, R. A., private communication. (11) Ruehrwein, R. A,, and Ward, D. W., Soil Sci., 73, 485 (1952). (12) Soil Stabilization Laboratory, M.I.T., “Soil Stabilization Research Sponsored by Industrial Funds,” October 1952. (13) Weeks, L. E., and Colter, W. G., Soil Sci., 73, 473 (1952). RECEIVED for review Xovember 16, 1953.
.4CCEPrED
March 13, 1954.
Alkylation of Cellulose with Esters of p-Toluenesulfonic Acid J. W. WEAVER
C. A. MACKEXZIE AND D. A. SHIRLEY]
Southern Regional Research Laboratory, :Yew Orleans, La.
Tulane University, iVew Orleans, La.
H
IGHLY substituted cellulose ethers are difficult to obtain by commercial methods of preparation ’Involving alkyl halides. The use of dimethyl sulfate as the alkylating agent introduces the hazard of toxicity and the element of close control necessary with a highly exothermic reaction. Esters of p-toluenesulfonic acid have been used as alkylating agents for alcohols (S), amines (6, 10, l a ) , mercaptans ( 1 1 ) , phenols (4, 5 ) , and thiophenols ( 1 1 ) . A survey of the literature re-qealed only one reference ( 7 ) to the alkylation of cellulose with this type of agent; no experimental details or other useful data were given. 1 Present address, Department of Chemistry, the University of Tennessee, Knoxville. Tenn.
The work described in the present paper has been concerned with the establishment of optimum conditions for the alkylation of cellulose with esters of p-toluenesulfonic acid. Certain other experiments were conducted with a view t o gaining a better understanding of the reaction. MATERIALS
Commercially purified cotton l i n t m were ground to pass a 20mesh sieve and allowed to come to moisture equilibrium with the surrounding atmosphere. The moisture was then determined and the cellulose handltd in this moist state with calculations based on the anhydrous weight. The methyl, propyl, n-butyl, and phenyl p-toluenesulfonates were reagent grade; the ethyl ester was technical grade.
