Langmuir 1994,10, 1393-1398
1393
Electrosteric Stabilization of Polymer Colloids An-Min Sung and Irja Piirma' Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 Received May 10, 1993. In Final Form: January 20, 1994" Four electrostatic surfactants, C,Hz,+10(CHzCHzO),S03Na, with m averaging between 12 and 15 and n, the ethylene oxide chain length, being 3,7,9,or 15,were used to investigate the steric chain length effect in the stabilization of model latices and in emulsion polymerization of styrene. It was found that during polymerization, the number of particles stabilized and the rate of reaction decreased with increasing ethylene oxide chain length in the surfactant. The addition of an electrolyteto the polymerization system caused a decrease in the rate of polymerization. This electrolyteeffect decreased with increasing ethylene oxide chain length. Coagulation tests of latices stabilized by these four surfactants indicated a switch in stabilization mechanism from primarily electrostatic to steric stabilization between 9 and 15 ethylene oxide units.
Introduction With the van der Waals-London attractive forces acting continuously between colloidal particles, it is necessary, in order to maintain stability, to introduce a repulsive force to outweigh the attractive force. Two general modes are currently recognized for the stabilization of colloidal dispersions: electrostatic and polymeric stabilizations. Electrostatic repulsion is created between particles by electrical double layers that surround them when covering the particles with charged species,such as ionic surfactants. In polymeric stabilization the repulsion between particles is provided by steric repulsion of either adsorbed or attached amphipathic materials or is obtained by depletion of continuous phase soluble polymer chains between approaching particles.l-3 The DLVOPGtheory, developed in the 1940's, is regarded as describing electrostatic stabilization quite well both qualitatively and quantitatively. Although steric stabilization has not been as extensively investigated, the theoretical background and present day understanding of the mechanism are well covered in the literat~re.~J-'~ Enhancement of stabilization of colloids, especially during polymerization, has been attempted by combining electrostatic and steric stabilizations in order to make use of the unique qualities of each. Namely, electrostatically stabilized dispersions are kinetically stable but are very sensitive to the presence of electrolytes, while sterically stabilized dispersions are thermodynamically stable and are much less sensitive to the presence of electrolyte. In dispersions stabilized by polyelectrolytes, such as proteins, it has been difficult to make a distinction which mechanism is the dominating one. Whatever factors affect the contribution of one will quite often affect the contribution of the other. Abstract published in Aduance ACS Abstracts, April 1, 1994. (1)Ottewill, R. H. J. Colloid Interface Sci. 1977, 58, 357. (2) Overbeek, J. Th. G . J. Colloid Interface Sci. 1977,58,408. (3) Napper, D. H.Polymeric Stabilization of Colloidal Dispersions; Academic Press: New York, 1983. (4) Derjaguin, B. V.; Landau, L. Acta Physicochim. USSR 1941, 14, 633. (5) Verwey, E. J. W. Chem. Weekbl. 1942, 39, 563. (6) Verwey, E. J. W.; Overbeek, J. Th. G . Theory of Stabilization of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (7) Mackor, E. L. J. Collid Sci. 1951, 6, 492. (8) Heller, W.; Pugh, T. L. J. Chem. Phys. 1954,22, 1778. (9) Clayfield, E. J.; Lumb, E. C. J. Colloid Interface Sci. 1966,22,269. (IO) Fischer, E. W. Kolloid 2. 1958, 160, 120. (11) Doroszkowski, A.; Lambourne, R. J. Polym. Sci., C 1971,34,253. (12) Napper, D. H.J. Colloid Interface Sci. 1977,58, 390. @
0743-7463/94/2410-1393$04.50/0
Mathai and Ottewi1113J4 have observed that when dispersion particles are stabilized with mixtures of ionic and nonionic surfactants, the electrostatic repulsion decreases with increasing steric length of the nonionic surfactants and, finally, results in total domination of the steric stabilizations. The electrostatic contribution in potential energy decreases because of shielding. This is caused by the stabilizing moiety chains extending into the continuous phase beyond the particle surface and isolating the ions and counterions of the ionic surfactant. Napper and Netchey15 have reported similar observations with charged particles surrounded by nonionic polymeric surfactants: the contribution from the electrostatic repulsion was rendered negligible. The purpose of this research was to evaluate the stabilizing behavior of a series of surfactants called electrosteric, i.e. surfactants who have the ionic charge chemically bonded to the end of the hydrophilic nonionic moiety. A good example of such a molecule is the commercially available series of electrosteric surfactants, C,Hzm+lO(CHzCHzO),SO3Na, AvanelS (PPGIndustries). Because of the widespread applicability in the industry of this type of surfactant, and because of the expectation of enhanced stabilization capability, a study was undertaken to establish what type of stabilization mechanism predominates when these surfactants are used and whether there is enhancement of stabilization. The experimental studies were divided into two main parts. One part of the investigation concentrated on the steric chain length effect on stability during emulsion polymerization using the four surfactants. The other part measured coagulation kinetics and rheological behavior of model latices stabilized with these four surfactants.
Experimental Section Materials. Styrene(AldrichChemical Co.) was washed twice with 1 M NaOH solution and twice with distilled water, then
vacuum distilled, and stored in freezer under nitrogen atmosphere. Water was either double distilledor deionized by passing through a Corning Mega-Pure System. Avanel S surfactants (PPG Industries), a series of sulfonated polyoxyethylenated alcohols with EO chain length of 3 , 7 , 9 , and 15 called S30, S70,S90,and 5150,were used as received. Diisopropylbenzenehydroperoxide (DIBHP)(HerculesInc.),and tetraethylenepentamine(TEPA) (Aldrich)were used as received. (13) Ottewill,R.H.NonionicSurfactants;Schick,M.J.,Ed.;Surfactant Series, Marcel Dekker: New York, 1966 Vol. 1. (14) Mathai, K. G.; Ottewill, R. H. Trans. Faraday SOC.1966,62,759. (15) Napper, D. H.; Netachey, A. J. Colloid Interface Sci. 1971,237, 528.
0 1994 American Chemical Society
Sung and Piirma
1394 Langmuir, Vol. 10, No. 5, 1994 Table 1. Monodisperse Polystyrene Latices Stabilized with Avanel S Surfactant after First and Second Seeding 530
S70
s90
S150
114 115 1.01
113 114 1.008
180
1
160
-
150
-
I
After First Seeding D, nm Dw,nm Dw/&
114 115 1.016
111
112 1.009
DwlDn
238 246 1.03
224 227 1.01
4
s1
After Second Seeding D, nm Dw,nm
-0
224 226 1.008
229 232 1.01
Polymerizations. A total of three consecutive seeding emulsion polymerizations were carried out to prepare monodisperse model polystyrene latices stabilized solely by Avanel S surfactants. The polymerization recipe contained variable surfactant concentrations and a ratio of 100 g of styrene to 400 g of water. Redox initiator concentrations were 0.13 g of DIBHP and 0.26 g of TEPA. The particle diameters, obtained from electron micrographs, showed that monodisperse particles were obtained already after the first seeding (Table 1).The four model latices were subsequently saturation covered with the corresponding surfactants using the so-called "soap titration" technique.lBJ1 Adsorption isotherms at 25 "C of the four surfactants on polystyrene latex, illustrated in Figure 1for S30, were obtained using the Fisher Surface Tensiomat, Model 21. The point where the adsorption curve leveled off was considered the saturation coverage, thus allowing the calculation of the number of surfactant molecules per unit area at the interface. Model latices with diameters approximately 230 nm were then used in coagulation kinetics measurements, and those with 115 nm were used in a latex rheology study. In the emulsion polymerization studies with the four surfactants, the water-to-styrene ratio was again 400 g/100 g, with variableAvanel S surfactant and electrolyte, NaCl concentrations. In one set of experiments the amount of surfactant was varied from 0.002 to 0.009 mol while the ionic strength was adjusted to a constant value of 0.15 with the addition of sodium chloride. In another set of experiments the surfactant concentration was 0.007 mol with variable ionic strength of 0.2,0.4,0.6, and 1.0. Redox initiation was utilized to avoid establishment of residual charges from the decomposition of the initiator on the particle surface. The polymerization temperature was kept at 30 O C . The rates of polymerization were determined gravimetrically. The particle diameters were measured by transmission electron microscopy (TEM) and by quasi-elastic light scattering (DLS). The conductivity titrations were recorded with a YSI Model 31 conductivity bridge. Coagulation Kinetics Study. The electrolyte, MgS04 solution, at various concentrations, was mixed with the model latices in a quartz cuvette. The final solids content of the latex was controlled at 1.5 X 1W for the measurements. The high concentrations of electrolyte needed for the coagulation of the surfactant saturated latices forced a switch to 80% surfactant saturation for these experiments. The optical density change throughout the coagulation process was monitored by a UVvisible spectrophotometer a t a wavelength of 546 nm and was recorded and analyzed by a H P 8452A diode array. Latex Rheology. Viscosities of model latices at different electrolyte concentrations and rotational speeds were measured with the Brookfield coaxial viscometer, Model LTV.
Results and Discussion A. Electrosteric Surfactants i n Model Latex Studies. The adsorption isotherms at 100% coverage of the monodisperse polystyrene latex particles were utilized to calculate the number of surfactant molecules per unit area at the particle-water interface. This number, in turn, was utilized to calculate the charge density on the particle surface by assuming that each surfactant molecule carried one charge. Finally, the surface area per surfactant (16)Maron, S.H.J. Colloid Interface Sci. 1954, 9, 89. (17) Ottewill, R.H.;Walker, T.Kolloid 2.Polym. 1966, 22, 108.
P
z:
v)
-M 5
!,,,,.I, , , , , I , , , ,
140
adsorption at 100% coverage s, x 109
surfactant
(mol/cm2)
S30 S70 s90 S150
1.72 1.54 1.22 0.41
I
, , , ,
I
,
,
,
,
charge density surface on particle area/moleculeat liq/solid interface surface Ami, X lo2 (nm2) u (rC/cm2) 166 149 118 40
9.7 10.8 13.6 40.6
0
0
5
IO
15
20
Ethylene oxide chain length
Figure 2. Effect of ethylene oxide chain length on the packing of electrosteric surfactants on liquid-solid interface.
molecule at the particle-water interface for the four surfactants was obtained and is listed in Table 2. It is obvious that the packing density of surfactant molecules decreases as the ethylene oxide (EO)chain length increased from 3 to 15 units. The drop in adsorption for EO 3 to mo1/cm2, then down 9 is from 1.72 X 10-9 to 1.22 X for EO 15. The tendency, illustrated in to 0.41 X Figure 2, suggests that, as the charge density is decreasing, t h e stabilization mechanism is changing. To investigate this possibility, coagulation kinetics studies and latex rheology measurements were carried out. Coagulation Kinetics Study. Prior to making spectrophotometric measurements, some static coagulation tests were conducted with the four latices using MgS04 as the coagulant. The latices, with particle sizes of 58,67, 64, and 78 nm stabilized with the S30, ,970, S90, and S150, respectively, were diluted to 1.5 X g/cm3solid content. Various concentrations of MgS04 solutions were mixed with the different latices in a volume ratio of 1part latex to 29 parts of coagulant solution. The mixtures were then left standing to observe the time needed for coagulation. The results are summarized in Table 3. The latices
Electrosteric Stabilization of Polymer Colloids
Langmuir, Vol. 10, No. 5, 1994 1395
Table 3. Static Coagulation with MgS0, of Latices Prepared with Different Surfactants Using Same Recipe MgSO4(aq), M-1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
S30 20 mina 15 min 9 min 4 min
RC RC
S70
21 days 9 days 19 h 4h lh
s90
100000 MS32
S150b S F F F
5
E M
-
30 days 11 days 30 h
‘r
1000
t
RC
lh Time neededto coagulatelatex: RC, rapid coagulation;S, stable; F, flocculation. b Flocculated latices could be redispersed either by heating or dilution.
stabilized with S30 coagulated rapidly when the MgS04 concentration in the system was higher than 1.6 M (the critical coagulation concentration (CCC) of the system). Lower electrolyte concentrations resulted in longer times for the onset of coagulum. This time dependency demonstrates a typical behavior of a charge stabilized system. The latices stabilized with S70 and S90 exhibited similar behavior, except that rapid coagulation occurred at much higher concentration of MgS04,l.g M for S70 and higher for S90. None of the above coagulated latices were redispersible. The data in Table 3 also show that the coagulation behavior of the S150 stabilized latex was totally different from the other three surfactants. Flocculation occurred at 1.3 M MgS04 and there was almost a total lack of time dependency, indicating a behavior associated with steric stabilization. This concentration of electrolyte is approximately the same as is needed to generate B-conditions for poly(oxyethy1) chains in water.3 The 5150 stabilized latex, when flocculated, was repeptizable by dilution and by warming it to 62 7 2 “C. This behavior indicates that an entropic steric stabilization is involved rather than the enthalpic steric stabilization usually observed in aqueous system with poly(oxyethy1ene)as the stabilizing m0iety.~J8Jg In order to obtain a complete understanding of the stabilization mechanism, the kinetics of coagulation of monodisperse latices was monitored using spectrophotometry. The optical density change, dE, at 546 nm wavelength with the addition of electrolyte, MgS04, was recorded as a function of time. The initial slope of the line, dE/dt, was then related to the stability ratio W a s dEldt = k’l W. According to von Smoluchowski,20 the kinetics of coagulation of primary particles -dN/dt = kiVo2,where NO is the number of particles per unit volume initially present and k is a rate constant. Rapid coagulation is considered a diffusion-controlled process, and k = ko = &Da, with D being the diffusion constant and a the radius of the particle. When energy barrier is present, the coagulation is slowed. A special term for slow coagulation, defined by Fuchs,21is the stability ratio, W = ko/k, where ko is the rate constant for fast coagulation and k the rate constant for slow coagulation. The initial rate of disappearance of particles -dN/dt = kdVo2/ W, can be measured experimentally by particle counting or light scattering. For rapid coagulation W = 1 and leads to a value for ko, which can be used to obtain k and Wfor slow coagulation. The double logarithmic plot of W (dt/dE was used) against electrolyte (18) Napper, D. H. J. Colloid Interface Sci. 1970,32, 106. (19) Napper, D. H. J. Colloid Interface Sci. 1977,58, 390. (20) von Smoluchaowski,M. 2.Phys. Chem. 1917, 92, 129. (21) Fuchs, N. 2.Phys. 1934, 89, 736.
T=25 O C 1= 546 nm
MS72
i
E
p -
0.11 mom
A= 546 nm
MS152
10000 ., 1.72 mow
M
1000
1.2 mow
01
!p‘
1
Log
10
s
Figure 4. Double logarithmic plot of dt/dE versus concentration of MgSO4 for polystyrene latices stabilized by S90 and S150. Table 4. Coagulation Studies on Monodisperse Polystyrene Latices S30 S70 S90 5150 224 224 229 Dn, nm 238 227 226 232 D,, nm 246 1.01 1.008 1.01 DdDn 1.03 CCC (mol/L) 0.11 0.32 1.72 1.2 1.8 X 1.7 X 10-19 1.6 X 1O-l8 V, (J/mol)
concentrations, C,, is shown in Figures 3 and 4, where the CCC and/or critical flocculation concentration (CFC) can be obtained from the intersection of the two lines. The stability ratio, W, is related to the total potential energy, which, according to Reerink and Overbeek,22 is mainly determined by the height, V, of the energy barrier. The approximate equation that relates W to Vm is W = k”exp( Vm/kBT),where k g is the Boltzmann constant and T the absolute temperature. By use of Figures 3 and 4 the critical coagulation concentrations were obtained, and the extrapolation of the lines to 1V M electrolyte concentration yielded the stability ratio. The maximum energy barrier, Vm, was calculated using the approximate equation shown above. The data, tabulated in Table 4,show an increase in the critical coagulation concentration from 0.1 M MgSO4 for S30 to 1.7 M for S90. However, the value decreases for S150 surfactant stabilized particles to 1.2 M MgS04. This value is close to the 1.3 M found in the static coagulation tests. The results thus far indicate that the electrostatic repulsion does not dissappear gradually as the ethylene oxide number in the surfactant increases from 3 to 9; instead there is enhancement of stability. This is indicated by the exceptionally high CCC values compared to either 74.
(22) Reerink,H.; Overbeek, J. Th. G. Discrcss. Faraday SOC.1954,18,
Sung and Piirma
1396 Langmuir, Vol. 10, No. 5, 1994 20
Latices Stabilized b y Surtacianr 530
Latices Stabilized b y Surfactant S70
15
15
--
T - 2 5 'C A
0.05M
2-
10
1.
--
T-25
*
NO added NaCl
A
0.05 M
1
0.1 M
..
.*
f
03M
O3 Ol0
0005
0 -0005
~
0 000
-000s
-
15 No added N l C l
5
0010
Latices Stabilized by Surtacranr 5150
--
T - 2 5 'C
lo
0 005
20 Latices Stabilized by Surfactanl S90
70
.
. I /
0.000
20
IS
O C
--
A
0.05 M
*
0.1 M
*
0.3 M
.... I
No Bddcd NsCI
2 10
A
--
A
* 5
--
0.05 M 0.1 M
?I
:I
. t
T - 2 5 'C
!
b
0.3 M
--
0,
O T
electrostatically or sterically stabilized systems and by an increasing CCC with increasing EO chain. Normally, only M of any electrolyte would be sufficient to coagulate an electrostatically stabilized system and 1M of MgS04 to coagulate at room temperature a sterically stabilized dispersion. However, an increase of the EO chain to 15 units did not enhance the stability any further; for the S150 stabilizedlatex, the critical coagulation concentration of MgS04 actually decreased, and as seen in Figure 4,the linearly decreasing behavior was disrupted. Furthermore, the numerical value of the maximum energy barrier, V , (Table 41, for the ,990stabilized system is 1 order of magnitude higher than for S30 and S70 stabilized latex, also indicating enhancement of stability. The V , value could not be obtained for the S150 stabilized latex, since in this case the linearly decreasing behavior was totally disrupted, indicating a change in stabilization mechanism from electrostatic to steric. The stability ratio actually increased with increasing electrolyte concentration beyond the CFC. Obviously the S150 surfactant, with 15 unit ethylene oxide chains on the particle form a brushlike layer thick enough for water molecules to be trapped, creating a time-delay for coagulation by electrolyte. It has been shown that the time needed to collapse the structure of an electrical double layer with the addition of an electrolyte solution is several orders of magnitude shorter than that for the adjustment of the conformation of an adsorbed chain23 From the data presented thus far, it can be rationalized that if the particles covered with the four Avanel S surfactants are viewed as macroions, the dissociation of counterions increases as the packing density decreases since the surface charge density decreases (Table 2). With the increase in dissociation of counterions, the double layer ~~
(23) Overbeek, J. Th.G. J. Colloid Interface Sci. 1977, 58, 408.
thickness increases with increasing ethylene oxide chain length. This contributes to the synergistic effect of increased colloidal stability due to the increse in both the electrostatic and steric repulsion. When the number of ethylene oxide units is lower than nine, electrostatic stabilization is still the predominant mechanism. With 15 EO units the charge density on the particle surface becomes low, 0.41 mol/cm2,which renders the double layer ineffective, and the only repulsive force is provided by the steric layer created by the EO chains. Latex Rheology Studies. In order to provide additional evidence as to the nature of interparticle interactions, the rheological behavior of the four latices was measured. The variables involved were the electrolyte concentration and the shear rate. It is well-known that interactions of electrical double layers between particles cause electroviscous effects.24 These effects and the microstructure formation between particles could increase the viscosity and change the rheological behavior of colloidal dispersions from Newtonian to non-Newtonian shear thinning.25 Latices with particles covered to saturation with the four surfactants were concentrated by centrifugation to 39 wt % solids. The viscosity in centipoise was measured at different spindle rotation speeds and NaCl concentrations utilizing a Brookfield coaxial viscometer. Dividing by the viscosity of the medium, the measured data were converted to relative viscosities. The dimensionless shear T, no is the rate was calculated through ( n o y a 3 ) / k ~ where viscosity of the medium, y is the shear rate which is equal to 3.2 times the spindle rotation speed, a is the particle radius, kg the Boltzmann constant, and T the absolute (24) Conway, B. E.; Dobry-Duclanx, A. Rheology, Theory and A p plication; Eirich, F., Ed.; Academic Press: New York, 1960; Vol. 3. (25) Russel, W. B.; Seville, D. A.; Schowalter, W. R. Colloidal Dispersion; Cambridge University Press: New York, 1989.
Electrosteric Stabilization of Polymer Colloids
Langmuir, Vol. 10, No. 5, 1994 1397
I5
--
1-26 *E
4
-?. 10 q.
--
Mu1 MI71
0.05 M NmCl
I 1
Mu1
MI161
A
t
.
c
A. 1
* A
e
.
e
5 --
5
--
s
a
.
- .
" r
-000.'
--
0 00s
OOOO
0010
(%Pb')/kT
4.00s
O.Oo0
0 003
0010
(%*')lkT
Figure 6. Relative viscosity, versus dimensionlees shear rate of polystyrene latices at different concentration levels. Latices MS31, MS71, MS91,and MS151 stabilized by 530,S70,S90,and S150,respectively.
temperature. Figures 5 and 6, where the relative viscosity is plotted against the dimensionless shear rate, show the effect of electrolyte on the latices stabilized with the four electrosteric stabilizers. Latices saturated with the 530, S70,and S90 stabilizers show fairly similar results: both the relative viscosity and the shear thinning decrease as the concentration of the NaCl is increased. At low electrolyte concentrations, the viscosity increased considerably as the shear rate increased, which indicates high shear thinning behavior. The relative viscosity changed less significantly with shear rate a t higher electrolyte concentrations, indicating insignificant shear thinning behavior. The latex stabilized with the S150 surfactant demonstrated different behavior: there was little change in relative viscosity and the rheological behavior with increasing electrolyte concentration. Obviously, these experiments again point out the difference in the stabilization mechanism with the shorter EO chains and the 15 EO unit surfactant. In the former, the electrostatic repulsion is the predominant force between approaching particles, and electroviscous effects play a predominant role in the rheological behavior. The 5150 stabilized latex, on the other hand, shows very little effect in the presence of increasing NaCl concentration as would be expected for a sterically stabilized system. Conclusions. The results obtained thus far indicate that the electrostatic stabilization provided by the Avanel S surfactants differ from results reported for mixed ionicnonionic surfactants.l4J5 In the mixed surfactant system the surface charge density on particles does not change with increasing EO chain length of the nonionic surfactant, only the surface potential drops significantly due to the increase of steric length. This means that the contribution
from the electrostatic repulsion decreases monotonically while the contribution from steric repulsion increases linearly. On the other hand, with the Avanel S surfactants, the electric charge is at the end of the ethylene oxide chain, and therefore, the surface charge density decreases with increasingEO chain length (Table 2). Experiments showed that there was enhancement of stabilization with EO chain length from 3 to 9 indicating contribution from both electrostatic and steric repulsion. While this steric repulsion kept increasing with the ethylene oxide chain length increase to 15, the charged double layer became too diffuse; the surface charge density on particles dropped from u = 118 pC/cm2 to u = 40 pC/cm2. B. Electrostatic Stabilizers in Emulsion Polymerization. The Avanel S surfactants were used to investigate the steric length as well as the added electrolyte effect on the emulsion polymerization of styrene. Generally it has been observed that the polymerizations using ionic surfactants have faster rates of polymerization and stabilize a larger number of particles at the smaller size level than nonionic surfactants. One reason is that ionic surfactants stabilize colloidal particles very effectively since the electrostatic repulsive force is a long distance interaction, which the steric repulsion is not. At the same time, ionically stabilized systems are more sensitive to environmental changes, such as ionic strength of the medium, and any change in pH could cause coagulation. Sterically stabilized systems would not be affected by these changes, unless 0 conditions are reached. Ethylene Oxide Chain Length Effect. In emulsion polymerizations of styrene with the Avanel S surfactants, each surfactant was used at five molar concentration levels, while other recipe ingredients were kept constant. Again
Sung and Piirma
1398 Langmuir, Vol. 10, No. 5, 1994 20
25
1
I
I
S30
$ 7
_.
I
...e
'
0
*. 0
S30
A
S90
S70
....
...... 0. -
't
I
S150
04
0
0002
0004
0006
0008
001
0
02
04
Concentration of Surfactant (Mol)
08
0.6
12
1
Ionic Strength
Figure 7. Effect of ethylene oxide chain length on the rate of polymerization in the emulsion polymerization of styrene at 30
"C.
Figure 9. Effect of electrolyteon the rate of polymerization in the emulsion polymerization of styrene at 30 "C. IO 1
IO 8
/
S30
S30
66 -
a
S90
S150
I
E
$ $ w
A S70
8
S150
-/.!-a-
v/
r
s 2
/ -
4 -
M
'
____--
_ - --*
, '8
04
0 0
0002
0004
0006
0008
001
Concentration of Surfactant (Mol)
Figure 8. Effect of ethylene oxide chain length on the number of particles in the emulsion polymerization of styrene at 30 "C (from TEM measurements).
a redox initiation system (DIBHP/TEPA) was used to avoid residual initiator charges on particles. The results are shown in Figures 7 and 8. An overlap in results with the S70 and 5390, especially on the number of particles formed, was observed, and therefore the ,970 points were omitted from the graph. A comparison of S30 results with those obtained with S90 and S150 does provide some conclusive results, however. As expected, the ,930 surfactant gave the smallest particle size and the fastest rate of polymerization, exhibited good electrostatic particle stabilization. There is, as expected, an increase in particle number, N , and rate of polymerization, R,, with increasing surfactant concentration. There is also a gradual decrease in N and R, with increasing ethylene oxide chain length. The difference in the behavior of the four surfactants becomes more noticeable in the presence of electrolyte, NaC1. In Figures 9 and 10 the number of particles and rate of polymerization, respectively, are plotted against the ionic strength, which was varied from 0.2 to 1.0 with the help of sodium chloride. The addition of this electrolyte to the S30 stabilized latex created quite a noticeable effect, a considerable decrease in the number of particles stabilized. This is an obvious sign of decreasing stabilization efficiency created by the addition of charged ions to the continuous phase. The effect of electrolyte was much less noticeable for the polymerizations stabilized with the other three surfactants, and the effect, especially on the rate of polymerization was decreasing with increasing ethylene oxide chain length. No electrolyte effect was observed with the 54150 surfactant. Most importantly, the change was gradual from shortest EO chain to the longest, 15 EO, studied, meaning that the change from electrostatic to steric stabilization was gradual contrary
02
04
06
08
1
12
Ionic Strength
Figure 10. Effect of ionic strength of continuous phase on the number of particles stabilized in the emulsion polymerization of styrene at 30 "C.
to observations made in model latex studies. Obviously, the S30, S70, and S90 surfactants are losing their ionic character, but the electrostatic stabilization is still the predominant mechanism of stabilization of particles. In polymerization with S150 surfactant the steric stabilization is the predominant mechanism, making the monovalent electrolyte addition ineffective. The model latex study pointed out one of the reasons for the change, the dramatic drop in charge density of particle surface from Q = 118 for S90 to u = 40 for ,9150. The surface area per surfactant molecule on the particle surface at saturation adsorption to 1.22 X 10-9 mol/cm2 decreased from Sm= 1.72 X when EO chain length increases from 3 to 9 and then dropped to 0.41 X mol/cm2 for surfactant with 15 EO units. These results suggest that the area taken up by a surfactant molecule increases with EO chain length. The value at liquid-solid interface increases from Amin = 9.7 X to 40.6 X nm2 with increasing EO units from 3 to 15, which is the reason for the charge density drop at particle surface.
Conclusions. The results with the Avanel S surfactants as stabilizers in styrene emulsion polymerizations showed that an increase in the number of ethylene oxide units in these electrostatic surfactants caused a decrease in the number of particles, N , formed and in the rate of polymerization, R,. An increase in the EO chain length resulted in a decrease in the surfactants effectiveness as ionic stabilizer, since it decreased the surface charge density. This made them less sensitive to added electrolyte. The experimental data actually showed that when the EO chain length reached 15, the stabilization mechanism had switched from electrostatic to steric repulsion.
