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Prevention of formation of dilatant sediments in suspension concentrates
Colloids and Surfaces, 18 (1986) Elsevier Science Publishers B.V.,
PREVENTION SUSPENSION
Th.F.
427-438 Amsterdam
OF FORMATION CONCENTRATES
427 -
Printed
in The Netherlands
OF DILATANT
SEDIMENTS
IN
TADROS
ICI, Plant Protection RG12 6EY (United (Received
2 July
Division, Kingdom)
1985;
Jealoff’s
accepted
Hill Research
in final form
Sfafion,
12 September
Bracknell,
Berkshire
1985)
ABSTRACT The prevention of formation of dilatant sediments in a practical suspension concentrate of ethirimol (a fungicide) was investigated using two methods. In the first method, the anionically stabilised suspension was flocculated using various electrolytes (NaCl, Na,SO,, CaCl, and AlCl,). The role of the cation was important with Ca’+ and Al”+ being most effective in reducing the formation of dilatant sediments at low concentrations. In the second method, a sterically stabilised suspension (produced by using a graft copolymer of the “comb” type) was flocculated using free, non-adsorbing polymers, namely polyethylene oxdie (PEO) with various molecular weights. Flocculation of the suspension occurred above a critical concentration of the free polymer (expressed in volume fraction o,‘), which decreased with increase of molecular weight of the polymer. Various flocculated structures could be produced depending on the concentration of the free polymer and its molecular weight. This provides an attractive system for practical application.
INTRODUCTION
The formation of solid-in-liquid suspension concentrates of drugs and pesticides requires, as a first step, preparation of the system in a colloidally stable state [l] . This is usually achieved by using surface-active agents and/ or macromolecules which are strongly adsorbed on the particle surface, providing a strong repulsive force either by electrostatic or steric interactions or a combination of both these forces [l]. However, since with most practical suspensions the particle size is outside the colloid range and the density of the particles is usually greater than that of the medium, then, on standing, the gravitational force exerted on the particles exceeds the weak agitation force produced by thermal fluctuations, and the particles settle to the bottom of the storage container. The repulsive forces between the particles, necessary to ensure stability in the colloid sense, enable the particles to move past each other to form a compact layer. This layer is dilatant and is usually referred to as hard “clay” or “cake”. As a consequence of the dense packing and the small spaces between the particles such dilatant sediments are difficult to redisperse.
0166-6622/86/$03.50
0 1986
Elsevier
Science
Publishers
B.V.
428
Several methods may be applied to prevent the formation of such dilatant sediments. The most common method is to add another disperse phase which forms a three-dimensional gel network in the continuous phase. This may be achieved, for example, by addition of a swelling clay such as sodium montmorillonite or a finely divided oxide such as silica [l].Alternatively, a high-molecular-weight material such as Xanthan gum may be used. An attactive method for preventing the formation of dilatant sediments is to induce weak flocculation by control of the conditions. For a chargestabilised suspension this can be produced by the addition of electrolyte, which may result in the formation of a relatively deep secondary minimum that is sufficient to produce a three-dimensional “gel network” of the particles (self-structured systems). Weak flocculation of sterically stabilised suspension can be produced by reducing the thickness of the adsorbed layer. Recently [2], we have shown that weak flocculation, produced by addition of free (non-adsorbing) polymer, can also be applied to prevention of the formation of dilatant sediments. In this paper, we will report some experimental results on prevention of the formation of dilatant sediments using suspensions of a fungicide, namely ethirimol, where the particles were stabilised either by electrostatic or steric repulsion. In the former case, the dilatant sediments were prevented by addition of electrolytes (NaCl, Na2S0,, CaCl, or AlCl,), whereas in the latter case, the phenomenon of depletion flucculation was applied by addition of polyethylene oxide (PEO). EXPERIMENTAL
Materials Ethirimol, a fungicide manufactured by ICI, is a’ white crystalline solid (density 1.21 g cme3) and was used as received. Two dispersing agents were used for the preparation of a colloidally stable suspension of ethirimol in water. The first was a condensate of phenol and formaldehyde that was sulphonated to produce an anionic-type agent. The material was a commercial substance of unknown molecular weight (probably in the region of l,OOO-2,000) and was simply used to produce a charge-stabilised suspension (SO:- groups). The second dispersing agent was a “comb’‘-type graft copolymer consisting of poly(methylmethacrylate methacrylic acid) backbone with methoxycapped PEO side chains (M, = 750). This was supplied by ICI, Paints Division (Slough, United Kingdom) and used as received. The exact molecular weight of the polymer is not known, but it is expected to be in the region of 20,000-30,000 (as indicated by Paints Division). Three PEO samples with nominal molecular weights of 20,000, 35,000 and 90,000 were used. These were supplied by Fluka, Hoechst and Union Carbide respectively, and used as received. All electrolytes were of Analar grade and used as received.
429
Preparation
of suspensions
For electrophoretic mobility measurements, a dilute suspension of ethirimol (1 g per 100 ml) was prepared using the sulphonated dispersing agent at various concentrations. For sediment height experiments, a 5% w/v suspension was prepared using 1% of the sulphonated dispersing agent and the sediment height was measured as a function of the electrolyte (NaCl, Na$O.,, CaCl, or A1C13) concentration. On the other hand, for the “comb” stabiliser, 50% w/v or 66.8% w/v suspensions were prepared using 2% w/v dispersing agent. The suspensions were prepared by bead milling using a Dyno Mill (Willey A. Bachofen, Basal, Switzerland). The particle size distribution (measured using an optical disc centrifuge) of the suspension was >50% below 2 pm, with an average particle diameter of - 1 pm. For rheological measurements (see below), samples of the 66.8 w/w suspension were mixed with PEO solutions, to give 55% w/w ethirimol, while varying the concentration of PEO. For sediment volume experiments, 5 g of the 50% w/v suspension was added to 5 ml PEO solution in stoppered cylinders and kept in constant temperature cabinets at 25 + 1°C. The sediment height was followed with time for several weeks until equilibrium was reached. At this point the tubes were mechanically inverted end-over-end and the number of revolutions required for redispersion was noted. Adsorption
iso therm
The adsorption of the “comb” stabiliser on ethirimol was determined as follows. About 1 g ethirimol powder [recrystallised ground material with a BET surface area of 0.29 m* g-’ (determined by Kr adsorption)] was weighed into 50-cm3 bottles and 25 cm3 of the dispersing agent solution, covering a concentration range of O-1000 ppm, were added. The powder was dispersed using a “Soniprobe” and then left overnight to reach equilibrium. A lo-cm3 aliquot of each suspension was then centrifuged for 30 min at 3000 rpm and then for another 30 min at 20,000 rpm. The concentration of the polymer in the supernatant liquid was determined using a calorimetric method based on complexation with IZ + KI [3]. This procedure has been described previously [4]. Desorption was investigated by redispersing the sedimented particles into distilled water, centrifuging and analysing for any adsorbed polymer. Electrophoresis
experiments
The electrophoretic mobility of ethimirol particles was determined as a function of the concentration of the anionic dispersing agent. For that purpose, a microelectrophoresis apparatus (Rank Bros, Bottisham, Cambridge, U.K.) was used.
430
Rheological
measurements
Two instruments were used to investigate the rheology of the suspensions flocculated by addition of PEO. The first was a Haake Rotovisko model RV2 (MSE Scientific Instruments, Crawley, Sussex, United Kingdom) fitted with a MK50 measuring head. This instrument was used to obtain steady-state shear stress (T)-shear rate (7 ) curves. From these curves, the Bingham yield of the linear portion of the + value, rp, can be obtained by extrapolation curveto? =O. The second instrument was a Deer rheometer (model PDR 881, Integrated Petronic Systems Ltd, London, United Kingdom) fitted with concentric cylinder platens. This instrument was used to measure the yield value by applying a series of stress values of equal increments and recording the response until flow occurred. RESULTS
Adsorption
iso therm
The adsorption of the “comb” stabiliser, which has been reported previously [5], was of the high affinity type, giving an apparent plateau value of -20 mg m-* which is much higher than the value obtained on polystyrene latex (-3 mg m-*) [2]. It should be mentioned, however, that the adsorption value was based on the BET surface area, which is probably an underestimate of the specific surface area of the suspension produced by dispersion of the powder in aqueous solution. The dispersion was aided by ultrasonics which may produce attrition resulting in an increase in the surface area. One can, therefore, only use the adsorption isotherm to give an indication of the minimum amount required for full coverage and reversibility of the adsorption. Both are important criteria for the investigation of depletion flocculation. The polymer concentration used for preparation of the suspensions (2%) was sufficient for full coverage based on the adsorption results. Moreover, no desorption was detected when the centrifuged sediment was redispersed into water. This implies strong “anchoring” of the chains to the surface and, therefore, the added “free” polymer (PEO) will not displace the adsorbed polymer.
Electrophoresis
measurements
The electrophoretic mobility of ethirimol particles showed a gradual increase with increasing concentration of the anionic dispersant, reaching a plateau at concentrations > 0.2%. Again, these experiments were used to locate the minimum dispersant concentration required for full coverage. All the suspensions used for flocculation or sediment volume results contained 1% w/v dispersant, i.e. well above the minimum required for saturation ad-
431
sorption. The particle mobility increased from its value of -2 X 10e4 cm2 V-’ s-l in the absence of dispersant to a plateau value of -5 X 10e4 cm’ V-’ s-l at dispersant concentration >0.2%. Using the Smoluchowski equation (which is valid when KU%l), the mobility data correspond to a decrease in the zeta potential from -26 mV in the absence of dispersant to -66 mV in the plateau region. The latter value is fairly high, ensuring sufficient electrostatic repulsion between the particles. Sediment
height and redispersion
Figure la shows plots of sediment height versus electrolyte concentration at 25°C for four different electrolyte types. The results show an increase in sediment height above a critical electrolyte concentration (critical flocculation concentration, CEC) which decreases in the order NaCl < Na2S04 < CaC1, < A1C13. Moreover, with NaCl and Na2S04, the sediment height increases gradually above the CFC, whereas with CaCl, and A1C13,the increase is fairly rapid. Similar results were obtained at 50°C. Figure lb shows the results of the redispersion experiments, in which the number of revolutions required to redisperse the suspension is plotted as a function of the electrolyte concentration. The results show a rapid decrease
-
10-6
IO5
Id4
10-3 lo?
Electrolyte corcentatlon
Fig. 1. Sediment CaCl,, and AICI,)
lo-'
I n-ddml-3
height and concentration.
redispersion
as a function
of electrolyte
(NaCl,
Na,SO,,
432
Fig. 2. Sediment fraction.
height
and
redispersion
as a function
of PEO
(M,
= 20,000)
volume
in the number of revolutions with increasing electrolyte concentration. The CFC obtained from sediment height and redispersion experiments agree reasonably well with each other. These are approximately lo-‘, 5 X 10m2, 5 X low3 and 10m3 mol dme3 for NaCl, NqSO,,, CaCl, and A1C13, respectively. Figure 2 shows the results for sediment height and redispersion for flocculation of the suspension obtained by the addition of a free polymer, namely PEO 20,000. The free polymer concentration is expressed as the volphase. Without added PEO, the suspenume fraction I$, in the continuous sion was very stable and turbid throughout. On standing, a dilatant sediment formed at the bottom of the tube leaving a turbid supernatant. The sediment was difficult to redisperse and required a large number of revolutions (>lOO). On addition of 0.5% PEO (4, = 0.007), the suspension separated into two layers, a lower concentrated layer and an upper dilute but turbid layer. However, the lower layer consisted of a fairly weak gel and the whole suspension could be redispersed with 5 revolutions of the tube. Increasing the PEO concentration resulted in phase separation, i.e. the supernatant liquid became clear. The sediment volume of the suspension reached a minimum between 1 and 2% PEO (op = 0.015-0.027). This sediment consisted of a stronger gel and required more revolutions for redispersion. A further increase in the PEO concentration resulted in phase separation with a clear supernatant liquid, but there was a continuous increase in sediment volume. Moreover, the gel strength of the sediment increased as indicated by the increase in the number of revolutions with the increase in PEO concentration. Rheological
measurements
Figure 3 shows the effect of addition of PEO (M, = 20,000, 75,000 and 90,000, respectively) on the extrapolated (Bingham) yield value. Figure 4 shows the corresponding results of the directly determined yield value (using the Deer rheometer). Although the trends obtained are the same, it is clear that the directly determined yield points are significantly lower than those obtained using the extrapolation procedure. Both sets of data show a rapid
433
Fig.
3. Variation
molecular
Fig. 4. Variation molecular
of extrapolated
(Bingham)
yield
value
with
@Q, for PEO
with
various
with
various
weights. of
directly
determined
yield
value
with
@p for
PEO
weights.
increase in 7p above a critical PEO concentration range. This concentration corresponds to the CFC of the free polymer (o,‘). Assuming the latter to be the concentration above which a detectable yield value can be measured, f 0.001 and 0.005 + 0.001 for PEO with M, = then $+, = 0.02 f 0.002,O.Ol 20,000, 35,000 and 90,000, respectively. These values are similar to those obtained using a model suspension of polystyrene latex. It should also be mentioned that the polymer concentration at which 7p begins to rise corresponds to the minimum observed in the sediment volume. DISCUSSION
The results obtained in the present investigation clearly demonstrate that dilatant sediments produced in suspension concentrates, which are stable in the colloid sense, can be prevented by a process of controlled flocculation. For charge-stabilised suspensions (as in the case where the suspension is stabilised with an anionic dispersing agent), such controlled flocculation can be achieved by the addition of electrolytes. On the other hand, for sterically stabilised suspensions (as for example the case where graft copolymers of the “comb” type are used), the flocculation can be controlled by the addition of free non-adsorbing polymer. The results obtained from the two types are discussed separately below.
434
Controlled
flocculation
of charge-stabilised
suspensions
The dispersing agent used in this study is a commercial material whose composition is not exactly known. However, previous studies showed that such sulphonated condensates of phenol (or naphthalene) and formaldehyde consists of a number of oligomeric chains with 2-9 units [6]. Thus, although the stabilizing mechanism is essentially electrostatic, some steric contribution may be obtained from the relatively long chains which may not be flat on the surface [7]. Indeed calculations using the DLVO theory [ 81 showed that the CFC is smaller than that obtained in the present system. The addition of electrolytes produces flocculation of the suspension as a result of compression of the double layers and collapse of the chains towards the surface. However, with the suspensions used in the present study, secondary minimum flocculation will occur since the particle size is relatively large (average radius 0.5 pm). This is illustrated in Figs 5 and 6 which show the potential energy-distance curves at various NaCl concentrations. In Fig. 5, the calculations were based on the DLVO theory (VT = VR + V, where V, is the electrostatic repulsion and V, the van der Waals attraction) using the following parameters: A (Hamaker constant) = 5 X 10e2’ J, a (par-
3c
2c F Y 2 10
Ti
-lC IO
m
Fig. 5. Energy-distance curves using the DLVO (mol dm-‘): (I) 10m3; (II) 10e2; (III) 5 X 15m*.
theory
at three
NaCl
concentrations
435
Fig. 6. Energy-distance NaCl concentrations
(mol
curves dme3):
using a three-force (I) lo-‘; (II) 10.‘;
equation (V, (III) 5 X lo-‘.
+ VA + V,) at three
title radius) = 0.5 pm, { = -42, -40 and -39 mV in lo-“, 10e2 and 5 X 10m2 mol dm-” NaCl. The results of the calculation show a deep secondary minimum (-25 hT) in lo-‘mol drne3 and rapid coagulation in 5 X 10e2 mol dmm3. However, experimental results showed only weak flocculation which started at 5 X 10e2 mol dme3. This must be due to the contribution of steric interactions resulting from the presence of adsorbed layers. Calculations using a three-force equation, VT = V, + VA + VS, where Vs is the steric repulsive force are shown in Fig. 6. In this calculation V, was taken to be equal to the mixing contribution, using an adsorbed layer of 5 nm and a chain-solvent parameter of 0.499. The results of the calculation show an energy minimum of -20 hT at 10m2mol dmT3and -50 hT at 5 X lob2 mol dme3 NaCl. As a result of the steric repulsion, flocculation only occurs in the secondary minimum. This secondary minimum flocculation is reflected in the weak nature of the gel formed which could be redispersed just by end-over-end rotation of the suspension. The electrolyte concentration at which flocculation starts and the nature of the flocculated structure produced depend to a large extent on the nature of the electrolyte used. Figure la clearly shows the im-
portance of the nature of the cation. With Na+ ions (NaCl and Na.$O,), flocculation occurs gradually over a concentration range and the floes formed are relatively compact. This is reflected in the relatively smaller sediment volumes obtained with these two electrolytes. In contrast with CaCl, and Al&, the flocculation is sharper over a narrow concentration range and the floes formed are relatively more open, giving higher sediment volumes. Indeed with A1C13,the flocculated structure nearly fills the whole suspension. This difference in the structure of the floes is also reflected in the ease with which they can be redispersed (Fig. lb). Floes using NaCl and Na$04 are more difficult to redisperse when compared with those produced using CaCl, and A1C13.Thus, for producing loose and open floes with anionically stabilised suspension, it is preferable to use polyvalent cations. Low concentrations are sufficient to induce flocculation and the floes formed are relatively open, causing minimum separation. Controlled
flocculation
by addition
of free non-adsorbing
polymer
Sediment volume, redispersion and rheological results all indicate that the addition of free non-adsorbing polymer to a sterically stabilised suspension can be applied to control the flocculation of the suspension concentrate. It is perhaps essential at this stage to define the free polymer concentration at which flocculation can first be detected. We have arbitrarily chosen this to be the concentration above which the suspension shows a measurable yield value. Clearly flocculation starts at lower polymer concentrations, as is clear from the sediment volume results. At 0.5% PEO (M, = 20,000), i.e. @n = 0.007 which is well below the point at which a measurable yield value is obtained,the suspension shows a phase change and it is almost certain that some flocculation occurs since the sediment formed is weakly gelled. However, the upper layer in this case is turbid and contains stable particles. It seems, therefore, that at this point there is some equilibrium between flocculated and stable suspensions. At 1% PEO, i.e. @JP= 0.015, the upper layer now becomes completely clear, although the yield value is unmeasurable. Only at Gp = 0.02 is one able to measure the yield value and this was arbitrary taken to be the polymer concentration above which depletion flocculation occurs. Thus, with concentrated suspensions one should clearly define the criteria used for identifying flocculation. It should be mentioned that the results of sediment volume were obtained using a polydisperse system which may complicate the interpretation as a result of the difference in flocculation behaviour between smaIl and large particles. However, recent investigations using a model monodisperse polystyrene latex suspension [ 91 showed similar trends in sedimentation behaviour. This shows that the results of the variation of sediment volume with $p are not due to the polydispersity of the systems but are the results of changes in the flocculated structure as described below. Whether dealing with dilute or concentrated suspensions, it is clear that
431
the flocculation produced and its nature depend on the concentration of free polymer and its molecular weight. Various flocculated structures may be produced and this is reflected in the sediment volume results shown in Fig. 2. In a recent study, Sperry [lo] investigated the floe morphology of the acrylic latices flocculated by addition of hydroxyethyl cellulose (HEC) polymers with various molecular weights. Using low magnification microscopy, he demonstrated that the floe morphology ranges from liquid-like to rigid and the latter from relatively amorphous to pseudocrystalline. The final structure was found to depend on three main parameters, the polymer concentration, its molecular weight and the particle size of the suspension. The results obtained in our study are for relatively coarse suspensions (average radius 0.5 pm) compared with those studied by Sperry (maximim radius 0.3 pm). The first addition of PEO at a concentration of 0.5% (&-, = 0.007), which is below +p, preduces some flocculation and phase separation into a suspension rich layer and an upper turbid layer with low particle number concentration. Any floes formed are probably non-rigid and the sediment can be somewhat compressed, giving a soft gel with relatively low sediment volume. On further increase of PEO concentration to 1% (&, = O.O15), the floe structure seems to change, becoming less rigid, and the sediment volume decreases further. Note in this case that the supematant liquid is clear, i.e. all the particles are now within the flocculated structure. At @p = 0.027, i.e. above qp, the sediment volume decreases further. With a further increase in &,, more rigid and irregular floes are produced and this is reflected in an increase in sediment volume and the flocculated structure becomes increasingly more difficult to redisperse. Our results seem to be in good agreement with the speculative structure suggested by Sperry [lo]. Indeed, the trend in sediment volume versus HEC concentration for the 0.3 pm particles is identical to that observed in the present system. The trends in the variation of rp with @p are similar to these obtained using a model polystyrene latex suspension [ 11. As mentioned above, the @p values obtained in the present practical system are also similar to those obtained with the model suspension. This demonstrates the generality of the phenomenon of depletion flocculation which can be applied to practical systems to prevent the formation of dilatant sediment. At the onset of flocculation, the suspension shows marked viscoelasticity, particularly when the volume fraction of the suspension is high ($ = 0.6 in the present case). With concentrated suspensions, depletion of the free polymer from the particle interstices may result in the formation of a “gel” network in the continuous phase as a result of polymer entanglement and/or interaction with “pockets” of flocculated suspension. This provides the system with sufficient elasticity to prevent the collapse of the network and the formation of hard sediments. Depending on the application, the magnitude of the interaction needed may be controlled by controlling the concentration of free polymer and its molecular weight.
438 CONCLUSIONS
Dilatant sediments that form in colloidally stable suspension concentrates (when the particle size is outside the colloid range and their density is greater than that of the medium) may be prevented by control of the flocculation of the suspension. For electrostatically stabilised suspensions this may be achieved by the addition of electrolytes which produce flocculation in the secondary minimum. When anionic dispersing agents are used, polyvalent cations (e.g., Ca2+ or Al”+) are preferable since flocculation can be controlled using a relatively low electrolyte concentration. For sterically stabilised suspensions, the flocculation may be controlled by the addition of free nonadsorbing polymer. The higher the molecular weight of the polymer, the lower the concentration of the free polymer needed for flocculation. Various flocculation states may be reached which can be controlled by controlling the concentration and molecualr weight, depending on the application. ACKNOWLEDGEMENTS I am indebted to Mr R. Rajaram for carrying out the rheological experiments during a six month industrial training period from Brunel University. I am also indebted to Mr S. Douglas for carrying out the adsorption experiments and to Mr D. Heath for carrying out the sediment volume experiments.
REFERENCES 1 2 3 4 5 6 7 8 9 10
Th.F. Tadros, Adv. Colloid Interface Sci., 12 (1980) 141. D. Heath and Th.F. Tadros, Faraday Discuss. Chem. Sot., 76 (1983) 203. B. Baleux, C.R. Acad. Sci. Ser. C, 274 (1980) 1617. Th.F. Tadros and B. Vincent, J. Phys. Chem., 80 (1980) 1575. D. Heath, R.D. Knott, D.A. Knowles and Th.F. Tadros, ACS Symp. Ser., 254 (1984) 11. M.J. Garvey and Th.F. Tadros, Kolloid Z. Z. Polym., 250 (1972) 967. M.J. Garvey and Th.F. Tadros, Proceedings of the Fourth International Congress Surface Activity, Zurich, Vol. IV, 1972, p. 715. E.J.W. Verwey and J.Th.G. Overbeek, Theory of Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. C. Prestidge and Th.F. Tadros, to be published. P.R. Sperry, J. Colloid Interface Sci., 91 (1984) 97.
PREVENTION SUSPENSION
Th.F.
427-438 Amsterdam
OF FORMATION CONCENTRATES
427 -
Printed
in The Netherlands
OF DILATANT
SEDIMENTS
IN
TADROS
ICI, Plant Protection RG12 6EY (United (Received
2 July
Division, Kingdom)
1985;
Jealoff’s
accepted
Hill Research
in final form
Sfafion,
12 September
Bracknell,
Berkshire
1985)
ABSTRACT The prevention of formation of dilatant sediments in a practical suspension concentrate of ethirimol (a fungicide) was investigated using two methods. In the first method, the anionically stabilised suspension was flocculated using various electrolytes (NaCl, Na,SO,, CaCl, and AlCl,). The role of the cation was important with Ca’+ and Al”+ being most effective in reducing the formation of dilatant sediments at low concentrations. In the second method, a sterically stabilised suspension (produced by using a graft copolymer of the “comb” type) was flocculated using free, non-adsorbing polymers, namely polyethylene oxdie (PEO) with various molecular weights. Flocculation of the suspension occurred above a critical concentration of the free polymer (expressed in volume fraction o,‘), which decreased with increase of molecular weight of the polymer. Various flocculated structures could be produced depending on the concentration of the free polymer and its molecular weight. This provides an attractive system for practical application.
INTRODUCTION
The formation of solid-in-liquid suspension concentrates of drugs and pesticides requires, as a first step, preparation of the system in a colloidally stable state [l] . This is usually achieved by using surface-active agents and/ or macromolecules which are strongly adsorbed on the particle surface, providing a strong repulsive force either by electrostatic or steric interactions or a combination of both these forces [l]. However, since with most practical suspensions the particle size is outside the colloid range and the density of the particles is usually greater than that of the medium, then, on standing, the gravitational force exerted on the particles exceeds the weak agitation force produced by thermal fluctuations, and the particles settle to the bottom of the storage container. The repulsive forces between the particles, necessary to ensure stability in the colloid sense, enable the particles to move past each other to form a compact layer. This layer is dilatant and is usually referred to as hard “clay” or “cake”. As a consequence of the dense packing and the small spaces between the particles such dilatant sediments are difficult to redisperse.
0166-6622/86/$03.50
0 1986
Elsevier
Science
Publishers
B.V.
428
Several methods may be applied to prevent the formation of such dilatant sediments. The most common method is to add another disperse phase which forms a three-dimensional gel network in the continuous phase. This may be achieved, for example, by addition of a swelling clay such as sodium montmorillonite or a finely divided oxide such as silica [l].Alternatively, a high-molecular-weight material such as Xanthan gum may be used. An attactive method for preventing the formation of dilatant sediments is to induce weak flocculation by control of the conditions. For a chargestabilised suspension this can be produced by the addition of electrolyte, which may result in the formation of a relatively deep secondary minimum that is sufficient to produce a three-dimensional “gel network” of the particles (self-structured systems). Weak flocculation of sterically stabilised suspension can be produced by reducing the thickness of the adsorbed layer. Recently [2], we have shown that weak flocculation, produced by addition of free (non-adsorbing) polymer, can also be applied to prevention of the formation of dilatant sediments. In this paper, we will report some experimental results on prevention of the formation of dilatant sediments using suspensions of a fungicide, namely ethirimol, where the particles were stabilised either by electrostatic or steric repulsion. In the former case, the dilatant sediments were prevented by addition of electrolytes (NaCl, Na2S0,, CaCl, or AlCl,), whereas in the latter case, the phenomenon of depletion flucculation was applied by addition of polyethylene oxide (PEO). EXPERIMENTAL
Materials Ethirimol, a fungicide manufactured by ICI, is a’ white crystalline solid (density 1.21 g cme3) and was used as received. Two dispersing agents were used for the preparation of a colloidally stable suspension of ethirimol in water. The first was a condensate of phenol and formaldehyde that was sulphonated to produce an anionic-type agent. The material was a commercial substance of unknown molecular weight (probably in the region of l,OOO-2,000) and was simply used to produce a charge-stabilised suspension (SO:- groups). The second dispersing agent was a “comb’‘-type graft copolymer consisting of poly(methylmethacrylate methacrylic acid) backbone with methoxycapped PEO side chains (M, = 750). This was supplied by ICI, Paints Division (Slough, United Kingdom) and used as received. The exact molecular weight of the polymer is not known, but it is expected to be in the region of 20,000-30,000 (as indicated by Paints Division). Three PEO samples with nominal molecular weights of 20,000, 35,000 and 90,000 were used. These were supplied by Fluka, Hoechst and Union Carbide respectively, and used as received. All electrolytes were of Analar grade and used as received.
429
Preparation
of suspensions
For electrophoretic mobility measurements, a dilute suspension of ethirimol (1 g per 100 ml) was prepared using the sulphonated dispersing agent at various concentrations. For sediment height experiments, a 5% w/v suspension was prepared using 1% of the sulphonated dispersing agent and the sediment height was measured as a function of the electrolyte (NaCl, Na$O.,, CaCl, or A1C13) concentration. On the other hand, for the “comb” stabiliser, 50% w/v or 66.8% w/v suspensions were prepared using 2% w/v dispersing agent. The suspensions were prepared by bead milling using a Dyno Mill (Willey A. Bachofen, Basal, Switzerland). The particle size distribution (measured using an optical disc centrifuge) of the suspension was >50% below 2 pm, with an average particle diameter of - 1 pm. For rheological measurements (see below), samples of the 66.8 w/w suspension were mixed with PEO solutions, to give 55% w/w ethirimol, while varying the concentration of PEO. For sediment volume experiments, 5 g of the 50% w/v suspension was added to 5 ml PEO solution in stoppered cylinders and kept in constant temperature cabinets at 25 + 1°C. The sediment height was followed with time for several weeks until equilibrium was reached. At this point the tubes were mechanically inverted end-over-end and the number of revolutions required for redispersion was noted. Adsorption
iso therm
The adsorption of the “comb” stabiliser on ethirimol was determined as follows. About 1 g ethirimol powder [recrystallised ground material with a BET surface area of 0.29 m* g-’ (determined by Kr adsorption)] was weighed into 50-cm3 bottles and 25 cm3 of the dispersing agent solution, covering a concentration range of O-1000 ppm, were added. The powder was dispersed using a “Soniprobe” and then left overnight to reach equilibrium. A lo-cm3 aliquot of each suspension was then centrifuged for 30 min at 3000 rpm and then for another 30 min at 20,000 rpm. The concentration of the polymer in the supernatant liquid was determined using a calorimetric method based on complexation with IZ + KI [3]. This procedure has been described previously [4]. Desorption was investigated by redispersing the sedimented particles into distilled water, centrifuging and analysing for any adsorbed polymer. Electrophoresis
experiments
The electrophoretic mobility of ethimirol particles was determined as a function of the concentration of the anionic dispersing agent. For that purpose, a microelectrophoresis apparatus (Rank Bros, Bottisham, Cambridge, U.K.) was used.
430
Rheological
measurements
Two instruments were used to investigate the rheology of the suspensions flocculated by addition of PEO. The first was a Haake Rotovisko model RV2 (MSE Scientific Instruments, Crawley, Sussex, United Kingdom) fitted with a MK50 measuring head. This instrument was used to obtain steady-state shear stress (T)-shear rate (7 ) curves. From these curves, the Bingham yield of the linear portion of the + value, rp, can be obtained by extrapolation curveto? =O. The second instrument was a Deer rheometer (model PDR 881, Integrated Petronic Systems Ltd, London, United Kingdom) fitted with concentric cylinder platens. This instrument was used to measure the yield value by applying a series of stress values of equal increments and recording the response until flow occurred. RESULTS
Adsorption
iso therm
The adsorption of the “comb” stabiliser, which has been reported previously [5], was of the high affinity type, giving an apparent plateau value of -20 mg m-* which is much higher than the value obtained on polystyrene latex (-3 mg m-*) [2]. It should be mentioned, however, that the adsorption value was based on the BET surface area, which is probably an underestimate of the specific surface area of the suspension produced by dispersion of the powder in aqueous solution. The dispersion was aided by ultrasonics which may produce attrition resulting in an increase in the surface area. One can, therefore, only use the adsorption isotherm to give an indication of the minimum amount required for full coverage and reversibility of the adsorption. Both are important criteria for the investigation of depletion flocculation. The polymer concentration used for preparation of the suspensions (2%) was sufficient for full coverage based on the adsorption results. Moreover, no desorption was detected when the centrifuged sediment was redispersed into water. This implies strong “anchoring” of the chains to the surface and, therefore, the added “free” polymer (PEO) will not displace the adsorbed polymer.
Electrophoresis
measurements
The electrophoretic mobility of ethirimol particles showed a gradual increase with increasing concentration of the anionic dispersant, reaching a plateau at concentrations > 0.2%. Again, these experiments were used to locate the minimum dispersant concentration required for full coverage. All the suspensions used for flocculation or sediment volume results contained 1% w/v dispersant, i.e. well above the minimum required for saturation ad-
431
sorption. The particle mobility increased from its value of -2 X 10e4 cm2 V-’ s-l in the absence of dispersant to a plateau value of -5 X 10e4 cm’ V-’ s-l at dispersant concentration >0.2%. Using the Smoluchowski equation (which is valid when KU%l), the mobility data correspond to a decrease in the zeta potential from -26 mV in the absence of dispersant to -66 mV in the plateau region. The latter value is fairly high, ensuring sufficient electrostatic repulsion between the particles. Sediment
height and redispersion
Figure la shows plots of sediment height versus electrolyte concentration at 25°C for four different electrolyte types. The results show an increase in sediment height above a critical electrolyte concentration (critical flocculation concentration, CEC) which decreases in the order NaCl < Na2S04 < CaC1, < A1C13. Moreover, with NaCl and Na2S04, the sediment height increases gradually above the CFC, whereas with CaCl, and A1C13,the increase is fairly rapid. Similar results were obtained at 50°C. Figure lb shows the results of the redispersion experiments, in which the number of revolutions required to redisperse the suspension is plotted as a function of the electrolyte concentration. The results show a rapid decrease
-
10-6
IO5
Id4
10-3 lo?
Electrolyte corcentatlon
Fig. 1. Sediment CaCl,, and AICI,)
lo-'
I n-ddml-3
height and concentration.
redispersion
as a function
of electrolyte
(NaCl,
Na,SO,,
432
Fig. 2. Sediment fraction.
height
and
redispersion
as a function
of PEO
(M,
= 20,000)
volume
in the number of revolutions with increasing electrolyte concentration. The CFC obtained from sediment height and redispersion experiments agree reasonably well with each other. These are approximately lo-‘, 5 X 10m2, 5 X low3 and 10m3 mol dme3 for NaCl, NqSO,,, CaCl, and A1C13, respectively. Figure 2 shows the results for sediment height and redispersion for flocculation of the suspension obtained by the addition of a free polymer, namely PEO 20,000. The free polymer concentration is expressed as the volphase. Without added PEO, the suspenume fraction I$, in the continuous sion was very stable and turbid throughout. On standing, a dilatant sediment formed at the bottom of the tube leaving a turbid supernatant. The sediment was difficult to redisperse and required a large number of revolutions (>lOO). On addition of 0.5% PEO (4, = 0.007), the suspension separated into two layers, a lower concentrated layer and an upper dilute but turbid layer. However, the lower layer consisted of a fairly weak gel and the whole suspension could be redispersed with 5 revolutions of the tube. Increasing the PEO concentration resulted in phase separation, i.e. the supernatant liquid became clear. The sediment volume of the suspension reached a minimum between 1 and 2% PEO (op = 0.015-0.027). This sediment consisted of a stronger gel and required more revolutions for redispersion. A further increase in the PEO concentration resulted in phase separation with a clear supernatant liquid, but there was a continuous increase in sediment volume. Moreover, the gel strength of the sediment increased as indicated by the increase in the number of revolutions with the increase in PEO concentration. Rheological
measurements
Figure 3 shows the effect of addition of PEO (M, = 20,000, 75,000 and 90,000, respectively) on the extrapolated (Bingham) yield value. Figure 4 shows the corresponding results of the directly determined yield value (using the Deer rheometer). Although the trends obtained are the same, it is clear that the directly determined yield points are significantly lower than those obtained using the extrapolation procedure. Both sets of data show a rapid
433
Fig.
3. Variation
molecular
Fig. 4. Variation molecular
of extrapolated
(Bingham)
yield
value
with
@Q, for PEO
with
various
with
various
weights. of
directly
determined
yield
value
with
@p for
PEO
weights.
increase in 7p above a critical PEO concentration range. This concentration corresponds to the CFC of the free polymer (o,‘). Assuming the latter to be the concentration above which a detectable yield value can be measured, f 0.001 and 0.005 + 0.001 for PEO with M, = then $+, = 0.02 f 0.002,O.Ol 20,000, 35,000 and 90,000, respectively. These values are similar to those obtained using a model suspension of polystyrene latex. It should also be mentioned that the polymer concentration at which 7p begins to rise corresponds to the minimum observed in the sediment volume. DISCUSSION
The results obtained in the present investigation clearly demonstrate that dilatant sediments produced in suspension concentrates, which are stable in the colloid sense, can be prevented by a process of controlled flocculation. For charge-stabilised suspensions (as in the case where the suspension is stabilised with an anionic dispersing agent), such controlled flocculation can be achieved by the addition of electrolytes. On the other hand, for sterically stabilised suspensions (as for example the case where graft copolymers of the “comb” type are used), the flocculation can be controlled by the addition of free non-adsorbing polymer. The results obtained from the two types are discussed separately below.
434
Controlled
flocculation
of charge-stabilised
suspensions
The dispersing agent used in this study is a commercial material whose composition is not exactly known. However, previous studies showed that such sulphonated condensates of phenol (or naphthalene) and formaldehyde consists of a number of oligomeric chains with 2-9 units [6]. Thus, although the stabilizing mechanism is essentially electrostatic, some steric contribution may be obtained from the relatively long chains which may not be flat on the surface [7]. Indeed calculations using the DLVO theory [ 81 showed that the CFC is smaller than that obtained in the present system. The addition of electrolytes produces flocculation of the suspension as a result of compression of the double layers and collapse of the chains towards the surface. However, with the suspensions used in the present study, secondary minimum flocculation will occur since the particle size is relatively large (average radius 0.5 pm). This is illustrated in Figs 5 and 6 which show the potential energy-distance curves at various NaCl concentrations. In Fig. 5, the calculations were based on the DLVO theory (VT = VR + V, where V, is the electrostatic repulsion and V, the van der Waals attraction) using the following parameters: A (Hamaker constant) = 5 X 10e2’ J, a (par-
3c
2c F Y 2 10
Ti
-lC IO
m
Fig. 5. Energy-distance curves using the DLVO (mol dm-‘): (I) 10m3; (II) 10e2; (III) 5 X 15m*.
theory
at three
NaCl
concentrations
435
Fig. 6. Energy-distance NaCl concentrations
(mol
curves dme3):
using a three-force (I) lo-‘; (II) 10.‘;
equation (V, (III) 5 X lo-‘.
+ VA + V,) at three
title radius) = 0.5 pm, { = -42, -40 and -39 mV in lo-“, 10e2 and 5 X 10m2 mol dm-” NaCl. The results of the calculation show a deep secondary minimum (-25 hT) in lo-‘mol drne3 and rapid coagulation in 5 X 10e2 mol dmm3. However, experimental results showed only weak flocculation which started at 5 X 10e2 mol dme3. This must be due to the contribution of steric interactions resulting from the presence of adsorbed layers. Calculations using a three-force equation, VT = V, + VA + VS, where Vs is the steric repulsive force are shown in Fig. 6. In this calculation V, was taken to be equal to the mixing contribution, using an adsorbed layer of 5 nm and a chain-solvent parameter of 0.499. The results of the calculation show an energy minimum of -20 hT at 10m2mol dmT3and -50 hT at 5 X lob2 mol dme3 NaCl. As a result of the steric repulsion, flocculation only occurs in the secondary minimum. This secondary minimum flocculation is reflected in the weak nature of the gel formed which could be redispersed just by end-over-end rotation of the suspension. The electrolyte concentration at which flocculation starts and the nature of the flocculated structure produced depend to a large extent on the nature of the electrolyte used. Figure la clearly shows the im-
portance of the nature of the cation. With Na+ ions (NaCl and Na.$O,), flocculation occurs gradually over a concentration range and the floes formed are relatively compact. This is reflected in the relatively smaller sediment volumes obtained with these two electrolytes. In contrast with CaCl, and Al&, the flocculation is sharper over a narrow concentration range and the floes formed are relatively more open, giving higher sediment volumes. Indeed with A1C13,the flocculated structure nearly fills the whole suspension. This difference in the structure of the floes is also reflected in the ease with which they can be redispersed (Fig. lb). Floes using NaCl and Na$04 are more difficult to redisperse when compared with those produced using CaCl, and A1C13.Thus, for producing loose and open floes with anionically stabilised suspension, it is preferable to use polyvalent cations. Low concentrations are sufficient to induce flocculation and the floes formed are relatively open, causing minimum separation. Controlled
flocculation
by addition
of free non-adsorbing
polymer
Sediment volume, redispersion and rheological results all indicate that the addition of free non-adsorbing polymer to a sterically stabilised suspension can be applied to control the flocculation of the suspension concentrate. It is perhaps essential at this stage to define the free polymer concentration at which flocculation can first be detected. We have arbitrarily chosen this to be the concentration above which the suspension shows a measurable yield value. Clearly flocculation starts at lower polymer concentrations, as is clear from the sediment volume results. At 0.5% PEO (M, = 20,000), i.e. @n = 0.007 which is well below the point at which a measurable yield value is obtained,the suspension shows a phase change and it is almost certain that some flocculation occurs since the sediment formed is weakly gelled. However, the upper layer in this case is turbid and contains stable particles. It seems, therefore, that at this point there is some equilibrium between flocculated and stable suspensions. At 1% PEO, i.e. @JP= 0.015, the upper layer now becomes completely clear, although the yield value is unmeasurable. Only at Gp = 0.02 is one able to measure the yield value and this was arbitrary taken to be the polymer concentration above which depletion flocculation occurs. Thus, with concentrated suspensions one should clearly define the criteria used for identifying flocculation. It should be mentioned that the results of sediment volume were obtained using a polydisperse system which may complicate the interpretation as a result of the difference in flocculation behaviour between smaIl and large particles. However, recent investigations using a model monodisperse polystyrene latex suspension [ 91 showed similar trends in sedimentation behaviour. This shows that the results of the variation of sediment volume with $p are not due to the polydispersity of the systems but are the results of changes in the flocculated structure as described below. Whether dealing with dilute or concentrated suspensions, it is clear that
431
the flocculation produced and its nature depend on the concentration of free polymer and its molecular weight. Various flocculated structures may be produced and this is reflected in the sediment volume results shown in Fig. 2. In a recent study, Sperry [lo] investigated the floe morphology of the acrylic latices flocculated by addition of hydroxyethyl cellulose (HEC) polymers with various molecular weights. Using low magnification microscopy, he demonstrated that the floe morphology ranges from liquid-like to rigid and the latter from relatively amorphous to pseudocrystalline. The final structure was found to depend on three main parameters, the polymer concentration, its molecular weight and the particle size of the suspension. The results obtained in our study are for relatively coarse suspensions (average radius 0.5 pm) compared with those studied by Sperry (maximim radius 0.3 pm). The first addition of PEO at a concentration of 0.5% (&-, = 0.007), which is below +p, preduces some flocculation and phase separation into a suspension rich layer and an upper turbid layer with low particle number concentration. Any floes formed are probably non-rigid and the sediment can be somewhat compressed, giving a soft gel with relatively low sediment volume. On further increase of PEO concentration to 1% (&, = O.O15), the floe structure seems to change, becoming less rigid, and the sediment volume decreases further. Note in this case that the supematant liquid is clear, i.e. all the particles are now within the flocculated structure. At @p = 0.027, i.e. above qp, the sediment volume decreases further. With a further increase in &,, more rigid and irregular floes are produced and this is reflected in an increase in sediment volume and the flocculated structure becomes increasingly more difficult to redisperse. Our results seem to be in good agreement with the speculative structure suggested by Sperry [lo]. Indeed, the trend in sediment volume versus HEC concentration for the 0.3 pm particles is identical to that observed in the present system. The trends in the variation of rp with @p are similar to these obtained using a model polystyrene latex suspension [ 11. As mentioned above, the @p values obtained in the present practical system are also similar to those obtained with the model suspension. This demonstrates the generality of the phenomenon of depletion flocculation which can be applied to practical systems to prevent the formation of dilatant sediment. At the onset of flocculation, the suspension shows marked viscoelasticity, particularly when the volume fraction of the suspension is high ($ = 0.6 in the present case). With concentrated suspensions, depletion of the free polymer from the particle interstices may result in the formation of a “gel” network in the continuous phase as a result of polymer entanglement and/or interaction with “pockets” of flocculated suspension. This provides the system with sufficient elasticity to prevent the collapse of the network and the formation of hard sediments. Depending on the application, the magnitude of the interaction needed may be controlled by controlling the concentration of free polymer and its molecular weight.
438 CONCLUSIONS
Dilatant sediments that form in colloidally stable suspension concentrates (when the particle size is outside the colloid range and their density is greater than that of the medium) may be prevented by control of the flocculation of the suspension. For electrostatically stabilised suspensions this may be achieved by the addition of electrolytes which produce flocculation in the secondary minimum. When anionic dispersing agents are used, polyvalent cations (e.g., Ca2+ or Al”+) are preferable since flocculation can be controlled using a relatively low electrolyte concentration. For sterically stabilised suspensions, the flocculation may be controlled by the addition of free nonadsorbing polymer. The higher the molecular weight of the polymer, the lower the concentration of the free polymer needed for flocculation. Various flocculation states may be reached which can be controlled by controlling the concentration and molecualr weight, depending on the application. ACKNOWLEDGEMENTS I am indebted to Mr R. Rajaram for carrying out the rheological experiments during a six month industrial training period from Brunel University. I am also indebted to Mr S. Douglas for carrying out the adsorption experiments and to Mr D. Heath for carrying out the sediment volume experiments.
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Th.F. Tadros, Adv. Colloid Interface Sci., 12 (1980) 141. D. Heath and Th.F. Tadros, Faraday Discuss. Chem. Sot., 76 (1983) 203. B. Baleux, C.R. Acad. Sci. Ser. C, 274 (1980) 1617. Th.F. Tadros and B. Vincent, J. Phys. Chem., 80 (1980) 1575. D. Heath, R.D. Knott, D.A. Knowles and Th.F. Tadros, ACS Symp. Ser., 254 (1984) 11. M.J. Garvey and Th.F. Tadros, Kolloid Z. Z. Polym., 250 (1972) 967. M.J. Garvey and Th.F. Tadros, Proceedings of the Fourth International Congress Surface Activity, Zurich, Vol. IV, 1972, p. 715. E.J.W. Verwey and J.Th.G. Overbeek, Theory of Stability of Lyophobic Colloids, Elsevier, Amsterdam, 1948. C. Prestidge and Th.F. Tadros, to be published. P.R. Sperry, J. Colloid Interface Sci., 91 (1984) 97.