TECHNICAL REVIEW Effe d, of Surfactant on the Viscosity of Portla,ndCement-Water Dispersions Edward M. Petrie Colloids, Pofymrs, Surface Science Program, Carnegie-Mellon University, Pittsburgh, Pennsylvania 152 13
Edward M. Petrie is a Senior Research Engineer in the Polymers Department of Westinghouse Electric CorDoration’s Research a n d Deuelopment Center. Hereceiued his B.S. (1967) and M.S. (1976) deerees from Carnegie-Melloniiniuersity and M.B.A. degree (1972) from Duquesne Uniuersity. Since joining Westinghouse in 1967,Mr. Petrie has worked primarily in the areas of organic adhesiues and polymeric composites and has authored several papers in these fields. He is actiue in the Society of Plastics Engineers and is currently Educational Chairman of the Electrical a n d Electronic Diuision.
Introduction The effect of particle charge on the viscosity of colloidal dispersions is an area of commercial importance. The rheological properties of many common colloid systems such as cement mixes, paints, drilling fluids, and cosmetics can he significantly influenced by the addition of small amounts of surfactant. A crumbly mass of moist kaolin clay, for example, can he transformed into a free-flowing liquid by the addition of a fraction of a percent of an alkali polyphosphate and then resolidified by a trace of calcium chloride (20). The purpose of this work is to investigate the effects of surfactants on the rheology of portland cement dispersions and to determine the suitability of classical electroviscous theories in this case. This study centers primarily on Surfactants derived from polymerized sodium salts of alkyl naphthalenesulfonic acid. These have previously been found to he highly effective anionic dispersing agents for pigments in aqueous media, and their potential advantage as water reducers in concrete has been shown (18). The viscosities of various cement mixes a t different shear rates were determined in this investigation using a Brookfield viscometer. Attempts were also made to measure the electrophoretic mobility of cement particles with a commercial Zeta-Meter and to relate the data to rheological characteristics of the dispersion. Justification With many colloidal suspensions i t is desirable to achieve a reduction in viscosity without significantly adding substantial foreign or reactive material to the system (e.g., solvents, diluents, etc.). High filler loadings in castable epoxy resins, for example, act as reinforcement and lessen the cost of the resin system; however, high filler loadings are accompanied by an increase in viscosity and poorer processing characteristics. Control of viscosity through the electrical double layer would also apply to paint pigments, cosmetics, and a number of commercially important colloid suspensions. A special example of the attractiveness of possible viscosity reduction via surfactants is in the cement industry. As a result of the decrease in the viscosity of a cement paste, the water content of a concrete mix for a given consistency can he reduced. The water reduction results in concrete having superior strength while maintaining practical flow properties (18). Conventional field mixing of concrete calls for a cement-water ratio of 1:0.5 to ensure workability and proper flow. Stoichiometrically a cement-water ratio of approximately 4 1 is all that is required to complete the reaction. Cement strength decreases continually with increasing water concentration in the mix. Similar viscosity reduction requirements exist for mining slurries where it is desired to pump the most concentrated slurry with the least resistance due to viscosity. Theory The properties of colloidal dispersions can be changed in
242
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~~
Anionic surfactants based on certain commercial naphthalenesulfonic acid condensates appear to neutralize the attracting charge on portland cement particles by adsorption from solution onto active particle sites. The rheological properties of cement suspensions prepared in this manner behave similar to suspensions of nonattracting spherical particles of equal size and at high concentrations. The resultingviscosity reduction provided by the incorporation of surfactants allows the water content of a cement mix for a given consistency to be reduced. The various electroviscous theories are presented and related to these observations.
two ways: by (a) effecting a change in particle size distribution or (b) changing the electrical double layer surrounding the particles by adding electrolyte or surface-active agents which can adsorb onto the suspended particles. This investigation will examine the viscosity component resulting from the electrical double layer (the electroviscous effect). The well-kown Einstein (9) expression relating the relative viscosity of a sol and the amount of dispersed solids '7 = 70 (1
+ 2.54)
'*
f
(1)
(where 7 is the viscosity of the suspension, 70 is the viscosity of the dispersion medium, and 4 is the volume fraction occupied by spherical particles) is valid only for spherical particles, dilute suspensions where the spheres are far enough apart not to influence each other, and no interparticle attraction or repulsion. However, in most common suspensions and in cement dispersions the particles are nonspherical, relatively concentrated, and probably characterized by interactive forces. When particle interaction cannot be neglected, Einstein's formula is no longer valid; the viscosity of the suspension will be greater than predicted due to an electroviscous effect. If the suspension is concentrated, the particles may link to form a gell-like substance having a certain yield stress. The magnitude of the yield stress is related to the force required to break the interactive bonds between particles. On addition of the proper surfactant which can adsorb on the particles, the links may weaken and if sufficient repulsion exists between the particles the yield stress disappears and the viscosity is reduced. If the suspended particles are electrically charged as is generally true, the viscosity of the suspension is greater than that predicted by the Einstein equation because of the effect of the electrostatic field. By reducing the charge effect on these particles through proper choice of surfactant, it should be possible to reduce suspension viscosity. At least three electroviscous effects have been described in the literature. The First Electroviscous Effect. When a colloid particle carries electrical charges there is a distribution of gegenions about the particle. When the particle is in motion, the distortion of the ionic atmosphere gives rise to an electrostatic contribution to the viscosity of the dispersion. Smoluchowski ( 2 1 ) in 1916 pointed out that for a charged particle in an electrolyte, the electric double layer around the particle might be expected to increase the effective viscosity, and he gave the following relation
where 70 is the viscosity of the solvent, e is its dielectric constant, u is the specific conductivity of the electrolyte, a is the radius of the solid particle, and {is the electrokinetic potential. Smoluchowski gave no derivation for relation 2, but in 1936 Krasny-Ergen (15)derived an almost identical expression (3) The Smoluchowski expression when compared with the Einstein equation indicates an increase in viscosity on account of electrical forces by
r-l
3 0.6. a 0.4
0.2 1
.
2
._.______ -
3
ka Figure 1. Comparison of A[q] vs. b = ment.
4
b-
KU
for theory and experi-
(4)
This indicates that on addition of added salts, ionic strength and u increase, and the {potential diminishes causing a reduction in viscosity. It can also be seen from expression 4 that measurable primary electroviscous effects would not be expected unless the particles were very small. In the Smoluchowski-Krasny-Ergen derivation, the restricting assumption was made that the double layer is thin compared to the size of the particle and no overlapping or interaction of double layers could occur. These assumptions lead to generally higher predictions of the increase in viscosity than what is measured experimentally. Booth ( I ) in 1948 recalculated the magnitude of the electroviscous effect without Smoluchowski's restriction that the double layer is thin compared to particle size. Booth's expression can be written 17 = 70 11
+ 2.54 [ 1 + q *
(5)
z(b)(l
+ b)?]]
where e is the elementary charge, k is the Boltzmann constant, T is the absolute temperature, z ( b ) is a function which can be obtained from Booth's data, 1 / K is the Debye thickness of the double layer, a is the hydrodynamic radius of the particle, mi is the number concentration of ions of type i, zi is the valency of type i ions, and wi is the mobility of the type i ions. If the mobilities of the small ions are assumed to be equal, eq 5 may be reworked to give
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 4, 1976
243
Equation 6 differs from Smoluchowski’s formula by the addition of the multiplying factor r b 2 (1 b ) * z ( b ) .The viscosity increments calculated according to Booth’s and Smolrichowski’s expressions are compared with experimental results in Figure 1 where 1 [ 7 ] , the electroviscous contribution tu intrinsic viscosity is plotted against KU (7). Obviously, neither theory fits the experimental data exactly, although Booth’s theory is largely used to date. Comparisons with experimental data are often hindered by limited accuracy of viscosity measurements a t high dilutions and the simultaneously occurring but difficult-to-separate secondary and tertiary effects. The primary electroviscous effect can offer some practical insight into the viscosity of suspensions. In general the viscosity can be expressed in the form
+
17 = 1 + 2.5@(1+ e )
(7)
770
where e depends on parameters of the electric double layer. From both Booth and Smoluchowski’s theories, the electroviscous effect can be minimized by decreasing double layer thickness, 1 / (increasing ~ b = Ka). This would be expected on physical grounds, for the electroviscous effect arises from the distortion of the symmetrical double layer field around the particle. T h e Secondary Electroviscous Effect. The primary electroviscous effect is generally small for most commercial colloid systems because particles are generally large causing Ka to become large, and both Booth and Smoluchowski’s derivations assumed no interaction between particles (relatively dilute suspensions). The secondary electroviscous effect may provide the greatest influence on rheological parameters on common, relatively concentrated colloid systems. The secondary electroviscous effect arises from repulsion effects where particles approach each other in a flowing fluid. In order for two particles to pass one another they must be displaced in a direction perpendicular to the direction of flow. When the particles are charged an extra displacement is necessary to account for the radius of the ionic atmosphere. An enhancement of viscosity due to the electrical double layer was first examined by Harmsen et al. in 1953 ( 1 4 ) . No quantitative theory yet exists for the second electroviscous effect, but various investigators have undertaken attempts to correlate rheological parameters of synthetic latices and the parameters of the electrical double layer (3,13,19).As might be expected, the secondary electroviscous effect increases in proportion to the square of the particle concentration and increases at constant concentration with a decrease of ionic strength (increase in effective double layer radius). Since the second electroviscous effect is very significant when the double layer thickness is equal to or greater than the particle radius, the effect is especially strong with small particles and low ionic strength. Schaller (19)has also shown that secondary electroviscous effects can be of importance a t low particle concentrations as well as in concentrated suspensions. The only quantitative expression for apparent viscosity increaqe due to the electroviscous effect has been presented by Elton ( 1 1 ) in a study of the sedimentation of charged spherical particle of radius a which at small values of {can be expressed as
where m is the number of particles per cubic centimeter. This relationship has, however, yet to be verified experimentally. Again in the case of the secondary electroviscous effect, it appears that the rheological properties of a suspension are 244
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
influenced by the thickness of the electrical double layer. However, a quantitative estimate of the nature of the total electroviscous effect is clouded by the unavailability of an appropriate secondary electroviscous theory and the inseparable influence of both primary and secondary effects. The T h i r d Electroviscous Effect. The third electroviscous effect is present in colloids owing to particle-shape changes associated with modification of their charge by ionization in neutral salts. This effect is generally encountered with nonelectrolytic polymers in solutions. An excellent review of the third electroviscous effect as it applies mainly to polymeric colloids is presented in ref 10. If a polymeric molecule can undergo ionization by reaction with a base or by reaction with some other ion-producing substances, electrostatic repulsion between like charges introduced on the polymer chain modifies the average radius of the polymer coil and the partial molar free energy of the polymer in solution. This contributes to an increase in the viscosity of the system through a large coil volume and resulting effects on the primary and secondary electroviscous forces. Since the development of a third electroviscous theory is difficult and not applicable to most colloid suspensions, further analysis will not be presented here. General Expression. The three electroviscous effects cannot be quantitatively separated by experimental methods. Generally, their overall effect is considered in a viscosity expression similar to 7 = 70[1 +
(hi
+ h2 + h,&I
(9)
where k l , h2, h3 represent the corresponding contributions of each electroviscous effect. In many investigations reported in the literature, the expression for the viscosity of disperse systems is Mooney’s emperical relationship (17) written in the form
where cy and
are emperically determined.
History Few published investigations have examined the viscosity reducing characteristics of surfactants in colloidal suspensions (3,13,18,20,22)and none has completely explained the results of the experimental work. This is most probably due to inseparable influences of the electroviscous effects. Street (22) has shown that the decrease in yield stress of sodium clay suspensions is attributed to neutralization of the positive edge charges by polyanion adsorption. Sennet and Olivier (20) use mass transport electrophoresis to show a relationship between {potential and viscosity of Kaolinite suspensions with addition of surfactant. In construction work surface-active additives to concrete have been used to render the mixture more fluid or “workable” a t a given water content. These agents have primarily fallen into two classes: (a) technical dispersing agents such as salts of the lignosulfonic acids and formaldehyde-naphthalenesulfonic acid condensates, and (b) hydroxycarboxylic acids or their salts; e.g. citric, tartaric, and tetrahydroxy adipic acids ( 5 ) .Laboratory studies of calcium lignosulfonate in cement pastes have shown that the lignosulfonate anions are adsorbed on cement particles making them negatively charged (12,24). This process is claimed to cause the particles to become mutually repulsive when they approach one another closely. In addition, it is believed that the adsorbed anions attract sheaths of water molecules around cement particles which prevent their approaching one another closely. Both of these effects are believed to decrease the viscosity of cement pastes. Although the electrophoretic mobility of cement particles has been observed (12),no known data have been presented as to the parameters of the electrical double layer.
Table I
Surfactant
Supplier
Type and description
Darvan #1
R. T. Vanderbilt co. Rohm & Haas
Sodium salt of condensed naphthalenesulfonic acid
Tamol N
co.
Orzan S.
Sodium salt of condensed naphthalenesulfonic acid Sodium lignosulfonate
Crown Zellerbach Sodium silicate Fisher Scientific co. Sodium Fisher Scientific hexametaCo. phosphate
Torque
F-.
1
Figure 3. “Up and down” curves obtained on a thixotropic material in a rotational viscometer and forming a hysteresis loop. E represents the equilibrium relationship.
100
i
50
m 42
20
2
10
r:
VI
20
40
Torque S c a l e R e a d i n g
60 ( x 6.737
-
dyne-cm)
VI
,”
3000 -
i
..
.
..
0
YI V
2
~.
2000
V
i P)
4 i
: 1000
&
20 -
5u 100 Shear Rate, RPM
Figure 2. Rheological properties of Type I portland cement suspensions (water:cementratio of‘O.5 by wt): (a) shear rate vs. shear stress; (b) viscosity vs. shear rate.
Experimental Section P r e p a r a t i o n of Cement Mixes. All cement mixes were prepared from Universal Atlas Type I portland cement and deionized water a t 23 “C. The cement powder was added to the water and mechanically mixed with an air-powdered propeller type laboratory stirrer for 1 min. When used, surfactants were first added to the water and dissolved before addition of the cement powder. Table I lists the commercially available surfactants which were used throughout this investigation. All experimental mixes were prepared on a gravimetric basis. T o convert to a volumetric basis, an approximate specific gravity of cement powder of 3.1 was employed. Viscosity Measurements. Viscosity measurements were made on a Brookfield Synchro-Lectric Viscometer Model RVF-100. The device is commonly used in the paint and
plastic industries and is capable of measuring viscosity at four shear rates, In construction and method of operation, it is similar to the classical Couette viscometer. The force required to rotate a spindle at constant rpm in the test fluid is directly read on the viscometer. The relationship between torque ai:d dial reading is absolutely linear, as is the rate of shear to spindle rpm. Couette and capillary viscometers have proven unsatisfactory because of their tendency to push water from the cement paste causing false viscosity and even premature setting of the cement. Approximately 900 g of sample was prepared in a quart container which also served as the container for the mix during viscosity measurements. l o avoid effects due to cement setting, the time between mixing and viscosity determinations (1 min) and equilibrium time a t constant spindle speed (30 s) were standardized. Operation of the Brookfield Synchro-Lectric Viscometer is described in ref 4 . Electrophoretic Mobility. Electrophoretic mobility measurements of cement particles in water and surfactant solution were attempted using a Zeta-Meter with Riddick Type I1 UVA electrophoresis cell. In general, the results obtained with this instrument were unsatisfactory because of settling-out of the larger cement particles. However, a qualitative estimate of electrophoretic direction of migration could be made. Procedures in operation of the Zeta-Meter are described in ref 23. Normal Cement Dispersions I t is possible to describe the rheology of normal cement dispersions in terms of yield stress and plastic viscosity, the yield stress being the force which must be applied to a material before any movement is produced, and viscosity being the measure of fluid friction or ratio of shear stress to rate of shea7 Figure 2a shows a shear plot of cement dispersion a t the commonly used 0.5 water:cement ratio using a Brookfield viscometer. The relation between viscosity and rate of shear stress is illustrated in Figure 2b. The shapes of these curves are typical of non-Newtonian. thixotropic materials with a measurable yield stress. The yield stress is the intercept of the stress axis with the extension of the curve plotting rate of shear against shear stress. Although the slope of curve 2a 15 increasingly less positive as shear rate is increased, the material shows thixotropic properties (viscosity decreasing with increasing shear rate) since the terms of shear stresshate of shear are measured from the origin. These observations are consistent with general theories regarding the rheology of suspended particles. If the suspensions are concentrated and attractive forces are present, the Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 4, 1976
245
I -$
7000
100,
t
0 3
n
6000
e.
E
i
x
rl 42
n
0
0
i
n
* .a
n
e
V 3
0
m
.a
ux
% 0 b
d Y)
0 A 0
k0
$ V 3
20
a
c i Y. 0
b zu
30
WJ
50
Volume Percent o f Cement
Figure 5. Viscosity of various suspensions: A, Einstein’s relationship, = 1 t 2.54, where 70 = 70 for cement = 10 cps; B, Non-attracting spheres of very disperse size, q/qo = (1 - 4)-* where 70 = 10 cps; C, nonattracting spheres of equal size at high concentrations 7/70 = (1 - 1 . 3 5 ~ $ ) -where ~ ~ 70 = 10 cps; D, experimental data for Type I portland cement. 7/70
26--
-- -30
-94
38
k2
b6
Volume Percent o f C e m e n t
It
Figure 4. Viscosity of Universal Atlas type I portland cement suspensions as a function of cement volume. particles link to build rigid networks, and thus a certain yield stress is developed. The magnitude of the yield stress is related to the force required to break a link between two particles and to the number of such links per unit volume. When a thixotropic suspension is sheared, the structure is progressively destroyed and in general, the higher the rate of shear, the higher the rate of destruction and the lower the viscosity. At a constant rate of shear a kinetic equilibrium state is attained in which the rate of structural breakdown by shear is equal to the natural rate of formation of structure. Thixotropic materials also show a hysteresis loop similar to that shown in Figure 3. The reason for the loop is that as speed is reduced, the structure begins to re-link but usually at a slow rate so that a t a particular speed and time sequence, the amount of structure present and therefore the torque are less for the down-curve than for the corresponding speed on the up-curve. Cement particles, however, show an unusual backward loop. I t is probable that in the case of cement suspensions the high rates of shear break up the structure into a more dispersed condition and a t lower rates of shear the particles can then come into more favorable positions and orientations for strong structure formation. This implies that when cement mixes are sheared a t very high rates, they may cure to form stronger strength because of improved dispersion but workability will be impaired. Such observations have been observed in the laboratory (18). I t may also be useful to examine the effect of cement concentration on viscosity. These relationships are presented in Figure 4 for a range of concentrations common to the construction industry and in Figure 5 for a broader concentration range. At high cement concentrations viscosity is strongly dependent on concentration. In Figure 5 the viscosity relationship is compared to theoretical models based on nonattracting spheres of equal size ( 2 ) ,nonattracting spheres of very diverse size (2), and Einstein’s relation (9).Cement particles 246
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
1’ 0
.a
1 Figure 6. Simultaneous action of forces of attraction (A) and of repulsion (R) gives rise to a net force of either (a) repulsions at separations greater than the critical value ro or (b) attraction at all separations.
somewhat approximate spheres ranging in size from 0.5 to 45 in diameter. The significant difference in viscosity between cement suspensions and the theoretical models for nonattracting spheres indicates the presence of significant forces of attraction. A charge on Atlas Type I cement particles can be seen by observing the electrophoretic migration of suspended cement fi
-- ft
of repulsion is reduced in magnitude or range, net attraction results for greater range of separations causing thixotropic structure in more cpncentrated suspensions.
0
1200
c
&:
f
Cement Dispersions with Surfactant The effect of the addition of various surfactants on the viscosity of cement paste a t a 0.5 watercement ratio is shown in Figure 7 . Also investigated were Tam01 N, a material similar to Darvan # 1which exhibited essentially equivalent viscosity reducing properties and Orzan S, a sodium lignosulfonate, which produced viscosity reduction initially followed by significant stiffening of the cement paste during shear. Because of its superior viscosity reducing characteristics and economic attractiveness as a concrete admixture, Darvan # 1,an anionic sodium salt of condensed napthalene sulfonic acid, was examined in depth. Different surfactants are not equally efficient in their ability to lower the viscosity of a given suspension due to their different dissociation products and preferential adsorption on cement particles. All of the surfactants examined were similar in that as their concentration was increased the viscosity remained relatively stable to a point where viscosity changed considerably with the addition of further surfactant. Then at a sufficiently high concentration, viscosity again showed little change with addition of surfactant. This point at which the viscosity curve finally levels out must represent the optimum concentration where surfactant has been absorbed on all of the possible active cement sites. Further addition of surfactant is wasted with regard to viscosity reduction. Figure 8 illustrates the effect of progressive addition of Darvan #1 on the shape of the shear rate vs. shear stress consistency curve. As the concentration is increased, the yield stress and thixotropic characteristics of the cement suspension are gradually reduced until at surfactant concentrations greater than 1%of cement weight (roughly the range at which adsorption has occurred on all possible particle sites) the suspension behaves as a nearly Newtonian fluid. Such Newtonian behavior indicated by the lack of yield stress and a linear shear rate-stress curve is generally associated with suspensions of nonattracting particles.
-L . A
I 0
.2
.4
.6
.B
1. 2
1 0
1.Y
1.0
1.
Weight P e r c e n t o f S u i f a c t a n t B a s o d o n Cement
F i g u r e 7. Viscosity reduction of Type I portland cement with (A) Darvan # 1; (B) sodium hexametaphosphate; and (C) sodium silicate.
particles at very low concentrations in a Zeta-Meter. The particles migrate to the cathode indicating a positive charge. Quantitative measurements of mobility could not be made because of rapid settling of the dense cement particles. These findings are different from those of Ernsberger and France (12) who claim no electrophoretic mobility for cement particles in water and migration toward the anode for particles dispersed in calcium lignosulfonate solutions. Their method of measurement was not disclosed. The result of a charge on the cement particle is a force of repulsion the magnitude of which depends on the charge of the particles, on the nature of the associated ions, and on the total concentration of electrolyte present. Superimposed on this force of repulsion there is a force of attraction generally arising from temporary dipoles set up by the electrons and nuclei of atoms near the surface. These forces and the effect of their combined total are shown in Figure 6. If the forces of repulsion are of longer range than the force of attraction, net repulsion exists a t normal particle separations ( r > T O ) minimizing the development of thixotropic structure. If the force
100
t
t
f o
io
fo--
30
io
Torque S c a l e Reading
-
0
10 20 T r -bo Torque S c a l e R e a d i n g
( 0 )
50
20
0
10 20 30 UO Torque S c a l e Reading
Torque S c a l e Reading
Figure 8. Rheological properties of portland cement Type I suspensions with different weight percentages of Darvan 1 surfactant based on cement: (a) no surfactant; (b) 0.6%; (c) 0.8%; (d) 1.0%; (e) 1.2%. Water:cement ratio of 0.5 by weight. Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
247
.
..
2m" g
50
ff t
Y .d n 0
2
-
60
V
20 10
0
c .3 Y 0
Torque S c a l e Reading
i
Figure 9. Viscosity of 0.5 water:cement suspension with 1.2%Darvan # 1based on cement weight. Mixing was repeated after 5-min intervals.
It is interesting to note that when excess Darvan # 1is used in cement suspensions, the viscosity of the mix decreases on repeated mixing (Figure 9). This seems to imply that the excess surfactant is adsorbed more efficiently because of the greater dispersion o i particles due to mixing, but more likely the excess surfactant is being adsorbed on the reaction products of the cement hydration. This theory agrees with the experimental work of Bruere (5), who found significant dependence on mixing sequence for viscosity reducing agents such as calcium lignosulfonate in cement mortar. The plot of viscosity as a function of cement fraction in surfactant solution, Figure 10, may illustrate the possible mechanism for viscosity reduction by Darvan # 1. When 1% Darvan # 1is added to cement suspensions, the suspensions behave remarkably like the theoretical model developed by Boscoe ( 2 )
for nonattracting spheres of equal size at high concentrations. i3oscoe's analysis also shows that the fact that aggregates may not be strictly spherical should not appreciably affect the validity of eq 11. A possible reason for these observations is that the addition of Darvan # 1 to cement suspension neutralizes the charge on the cement particles through adsorption of surfactant. This yields suspensions with little or no interparticle attraction or repulsion. Cement particles suspended in Darvan # 1solution showed no noticeable electrophoretic migration when examined by a Zeta-Meter. The larger particles were also noticed to settle more rapidly in the surfactant solution than in pure water. The drag of counterions on a sedimenting particle inhibits the steady rate of fall, aiid uncharged particles show greater sedimentation velocities (8). These observations also suggest that the electroviscous effect may be absent or significantly reduced in cement suspension in solutions of surfactants such as Darvan # 1. Expressions for the electroviscous component of viscosity such as eq 4 cannot be negative due to the presence of the term. It is therefore very unlikely that the viscosity of portland cement suspension can be decreased below the theoretical relation for nonattracting spheres presented in Figure 10. Search for viscosity reducers superior to Darvan #1 type surfactants would likely be unfruitful. Future Work It was initially the intent of this investigation to derive a relationship between the electrical double layer parameters of cement particles and rheological properties. This was found 248
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
1
10
20
30
OW
-
Percent Cement by Volume
Figure 10. Viscosity of Type I portland cement suspensions as a function of cement volume: -e-, no surfactant; -0-, 1.0% by weight Darvan # 1 based on cement; curve (a) is the theoreticzl model for nonattracting spheres of equal size a t high concentrations where 70 = 10 cps.
to be difficult with equipment such as the Zeta-Meter due to settling of the cement particles and large dilutions required. However, a mass-transport electrophoresis method has been developed and patented which allows for measurement in suspensions of high concentration (16). Cement and concrete are lo^ cost construction materials, and the addition of admixture can contribute to a significant portion of the final material cost. Therefore, surfactants which are low cost and can completely adsorb on cement particles at very low concentrations would be of continued interest to the cement industry.
Conclusions It is assumed that anionic surfactants based on certain naphthalenesulfonic acid condensates such as Darvan # 1 neutralize the attracting charge on portland cement particles by adsorption from solution onto active particle sites. Cement suspensions produced in this manner behave similar to suspensions of nonattracting particles of equal size and at high concentration. Their relative viscosity conforms roughly to the following model: 7/70 = (1- 1.354)-2.5. The following observations tend to confirm this theory. (1) Normal cement suspensions behave as non-Newtoaian, thixotropic fluids; suspensions in sufficient surfactant solution behave as Newtonian fluids. ('2) Experimental viscosity measurements of cement suspensions in sufficient surfactant solution conform well with the theoretical viscosity of suspensions of nonattracting spheres of equal size at high concentrations. (3) Cement particles in pure water migrate to the cathode, and particles suspended in surfactant solution show no electrophoretic mobility. (4) Cement particles suspended in surfactant solution show greater sedimentation velocity indicating the reduction or absence of charge caused by surfactant adsorption. Acknowledgment This work is the result of the author's research project for the M.S. program in Colloids, Polymers, and Surface Sciences
at Carnegie-Mellon University. The author is grateful to both Carnegie-Mellon University and Westinghouse R & D Center for supplying the analytical instruments used in this work.
Literature Cited (1) Booth, F.. Proc. Roy. SOC.London, Ser. A, 203, 533 (1950). (2) Boscoe, R., Brit. J. Appl. Phys., 3, 267 (1952). (3) Brodngan. J. G.. Kelley, E. L.. J. Colloid Sci., 20, 7 (1965). (4) Brookfield Synchro-Lectric Instruction Manual. Brookfield Eng. Labs., Stoughton, Mass. (5) Bruere, G. M., Constr. Rev., 16-21, 164 (Feb 1964). (6) Bruere, G. M.. "Effects of Mixing Sequence on Morter Consistencies When Using WaterReducing Agents," Symposium on Structure of Portland Cement Paste and Concrete Highway Research Board, 1966. (7) Chan, F. S., Gering, D. A. T.. J. Colloid Sci., 22, 371 (1966). (8) Davis, J. T., Rideal, E. R., "Interfacial Phenomena", p 139, Academic Press, New York, N.Y., 1961. (9) Einstein, A., Ann. Phys., 4, 19 (1906). (10) Eirich, F. R., "Rheology", Vol. 3, pp 9-1 19, Academic Press, New York, N.Y., 1960.
(11) Elton, G. A. H., Proc. Roy. SOC.London, Ser. A, 197, 568 (1949). (12) Ernsberger, F. M., France, W. G., lnd. Eng. Chem., 37, 598 (1945). (13) Fryling, C. F., J. ColloidSci., 18, 7B(1963). (14) Harmsen, G. J., Schoater, J. V., Overbeek, J. Th. G., J. ColloidSci., 8, 64, 72 (1953). (15) Krasny-Ergen, W., Kolloid-Z., 74, 172 (1936). (16) Micromeritics Corp., Norcross, Georgia. (17) Mooney, M., J. ColloidSci., 6, 162 (1951). (18) Petrie, E. M., Westinghouse Research Center, unpublished work, 1974. (19) Schaller, E. J., Humphrey, A. E., J. Colloid lnterface Sci., 22, ST3 (1966). (20) Sennett, P., Olivier, J. P., lnd. Eng. Chem., 57, 33 (1965). (21) Smoluchowski, N., Kolloid-Z., 18, 190 (1916). (22) Street, N., J. ColloidSci., 12, l(1957). (23) Zeta-Meter Manual, Zeta-Meter Inc., New York, N.Y. (24) Zhuravlev, V. F., Tikhonor, V. A,, J. Appl. Chem. USSR, 25, 1317 (1952).
Received for review April 15, 1976 Accepted July 30, 1976
Catalytic Characteristics of a Rhodium Complex Attached to an Aromatic Polyamide Tae H. Kim and Howard F. Rase* Department of Chemical Engineering, The University of Texas at Austin, A'ustin, Texas 787 12
The characteristics of the versatile Wilkinson homogeneous catalyst, tris(triphenylphosphine)chlororhodium(l) bound to a rugged, high-melting polymer, poly( mphenyleneisophthalamide), have been studied in hydrogenation of olefins. Steam treatment of the polymer produced a high surface area which in turn gave the most active polymer-bound catalyst. The bound catalyst exhibited similar selectivities and poison resistance as the homogeneous form, and was less active but more resistant to deactivation at higher temperatures.
In comparison to heterogeneous catalysts, homogeneous catalysts often exhibit improved selectivity and higher activity at modest temperatures. This latter characteristic has caused an even greater interest in homogeneous catalysts as a means for reducing energy consumption in commercial processes. Homogeneous catalysis is employed in manufacturing approximately 15%of the total value of products based on catalytic processes (Heinemann, 1971). One of the reasons preventing further growth in commercial use is the difficulty in recovering the catalyst for reuse in the large majority of cases where the catalyst is too valuable to discard. In highly successful processes such as the Oxo process, the difficulty and cost of recovering the catalyst (Lemke, 1966) are apparently overshadowed by its unique character and efficiency. One major means under investigation for overcoming the separation problem is to attach the homogeneous catalyst, usually a transition metal complex, to an insoluble polymer or silica. Such techniques for preparing so-called heterogenized homogeneous catalysts have been thoroughly reviewed by Burwell (1973), Heinemann (1971), Manassen (1969), Michalska and Webster (1975), and Pittman and Evans (1973). See also Delmon and Jannes (1975). Many problems remain to be solved before widespread commercial use will be
possible. These include in many cases a markedly lower activity of the bound catalyst compared to that of the free catalyst in solution and, in the case of polymer-bound complexes, a low maximum use temperature dictated by the melting point of the polymers generally employed, polystyrenes and polymethacrylates. The purpose of the present study was to develop and evaluate a polymer-bound catalyst using a rugged, high-melting polymer. For this purpose we selected a commercial poly( m -phenyleneisophthalamide), Nomex, which has a melting point of 427 "C and is soluble only in concentrated sulfuric acid at room temperature and in boiling dimethylacetamide with 3% CaC12. Such a support could increase the utility of any bound catalyst by expanding the range of reactants and solvents that might be used with it and increasing the range of operable temperature so that either reactions could be conducted a t higher temperatures when necessary or temperature runaways would not destroy the integrity of the support. The versatile Wilkinson hydrogenation catalyst was selected, tris(tripheny1phosphne)chlororhodium(I), because it has been thoroughly studied in the homogeneous form, and it has also been attached to a chloro-methylated styrene-divinylbenzene with a 1.8% cross-linking by Grubbs and coworkers (1971, 1972). It has Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 4, 1976
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