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The effect of added salt on the adsorbability of a synthetic polyelectrolyte
The Effect of Added Salt on the Adsorbability of a Synthetic Polyelectrolyte B. W. G R E E N E Physical Research Laboratory, The Dow Chemical Company, Midland, Michigan ~86~0 Received September 30, 1969; accepted April 12, 1971 The kinetics and statics of the adsorption of a synthetic polyelectrolyte, sodium poly(2-sulfoethylmethacrylate), at a solid-liquid interface have been examined as a function of the added salt concentration and polyelectrolyte molecular weight. It was found that at a given salt concentration, the maximum adsorption (expressed on a molar basis) was essentially independent of molecular weight; whereas at a given molecular weight, both the rate and extent of adsorption increased markedly with increasing salt concentration. Parallel viscosity experiments revealed that this latter was a consequence of the fact that the medium became a poorer solvent for the polyeleetrolyte as the salt concentration increased. Thus in general at a given polyelectrolyte molecular weight, adsorption was greater the lower the intrinsic viscosity. From this together with the finding that the adsorption could be completely reversed, it could be concluded that the polyelectrolyte was weakly adsorbed at the solid surface in the form of rather compact random coils. The dimensions of the latter were proportional to those of the free coils in solution. I. INTRODUCTION I t is generally well known t h a t depending primarily on the concentration and molecular weight, a given polyelectrolyte might be used to flocculate or stabilize a particular colloid. The behavior in either case is usually attributed to the adsorption of the polyeleetrolyte onto the colloid in question
(1-7). Until a few years ago, it was not known t h a t during the polymerization of m a n y commercial latexes--themselves colloids-polyelectrolytes were formed in situ in the aqueous phase (8-11). I n practice, it is found t h a t the concentration of polyelectrolyte so produced is sometimes so small as to be undetectable by most conventional techniques (9, 10). Yet such small quantities can have dramatic effects on final properties of the latex, e.g., thickener response and electrolyte tolerance (12). These effects are also commonly believed to be a consequence of polyelectrolyte adsorption. However, at present,
there are no published results to substantiate this view. Neither are there sufficient available data to establish t h a t polyelectrolyte adsorption would be favored from an aqueous environment (of moderate to relatively high ionic strength) similar to t h a t in a commercial latex. I t was the p r i m a r y goal of the present study to clarify this latter point; and thereby gain a more basic understanding of the manner by which in situ formed polyelectrolytes might affect latex properties. Therefore, in this study, the kinetics and statics of the adsorption of a synthetic polyelectro]yte onto a solid (similar in m a n y respects to a latex) have been determined as a function of added salt concentration and polyelectrolyte molecular weight. II. EXPERIMENTAL P R E P A R A T I O N AND CHARACTERIZATION THE P O L Y M E R U S E D
OF
The polymer used in this study was sodium poly(2-sulfoethylmethaerylate), N a P S E M .
Journal of Colloid and Interface ~gcience, VoL 37, No. 1, Sepl~ember 1971
144
A D S O R B A B I L I T Y OF A S Y N T H E T I C P O L Y E L E C T R O L Y T E
It is a strongly anionic polyelectrolyte having the monomer unit:
/
I
145
}00 ML SAMPLE WITH .475 ¥EQ POLY SEM ACID
0
C H 3 - - C - - C - - O - - C H 2 - - C H 2 - - S O ~ - N a+
l
CH~
l
TITRAINT -
The monomer itself has previously been described as an efficient comonomeric emulsifier for the polymerization of synthetic latexes (13, 14); and there is now recent evidence that during such polymerizations the polymer is formed i n s i t u in the latex serum phase (15). Several samples of N a P S E M were investigated. Each had been prepared by solution polymerization of the monomer to moderate conversion at 70°C using persulfatc as initiator; was subsequently purified by extraction with a nonsolvent; and then dialyzed several days to remove extraneous electrolyte. The viscosity average molecular weights of the samples used are listed in Table I. They were determined from the intrinsie viscosity In], measured in 0.8 M NaC1 using the M a r k Houwink relation, In] = K ' M ~ (the values used for the constants a and K ' were 0.470 and 3.72 X 10-4, respectively). The number average molecular weights of the samples which were determined by osmometry are also included in Table I. A typieal curve of the potentiometric titration of N a P S E M (Sample I which had been converted to the acid form by the addition of the stoiehiometric amount of HC1) with NaOH is shown in Fig. 1. It illustrates that during titration, the polymer behavior was characteristic of a strong electrolyte. Both the stoiehiometry at the titration end point, and the sharpness of the curve were indicative that the polymer was essentially free of impurities. TABLE I PROPERTIES OF THE SAMPLES OF N a P S E M Sample I II III
[7] (dl/gm) 3.7 4- 0.2 2.0 4- 0.1 0.5 4- 0.04
l~v. 10-~
M~-10-6
2.40 0.40 0.035
2.00 0.50 0.03
Measured in 0.8 M NaC1 solution.
: 4
[
I
.6 8 t.O Eq BASE / Eq. SO-3
FIG. 1. P o t e n t i o m e t r i c P S E M acid.
titration
I4
16
curve
of
THE ADSORBENT USED
A finely divided polyethylene (PE) powder manufactured by United States IndustI~als Chemical Company was used as the adsorbent. It constituted a nonporous, lowenergy hydrophobie surface which was either nonionie or very weakly anionic at the p H (6.5 4- 0.2) used in all adsorption experiments. The powder was dried at 60°C and stored for several days in a vacuum desiccator prior to usage. Some properties of the powder and the methods used to determine them are listed in Table II. THE SOLVENT
Deionized water (specific conductivity ca. 10-6 ohm -1 em -1) alone or containing various concentrations of analar grade NaCI was used as the solvent, p H adjustments on the solvent were made when necessary by addition of either NaOH or HC1. ADSORPTION ~'IEASUREMENTS
A given quantity of an aqueous solution containing known concentrations of NaC1 and N a P S E M was introduced into an adsorption tube which contained a weighed quantity of the adsorbent. The tube was stoppered and sealed; mounted on a shaker located in a room thermostated at 25 ± I°C ; Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
146
GREENE TABLE
II
PROPERTIES OF THE P O L Y E T H Y L E N E U S E D AS ADSOI~BENT
Manufacturer Grade Density Average particle diameter (listed by manufacturer) Surface area (N2 adsorption) Average particle diameter (based on N~ adsorption) Total oxygen content (Neutron activation analysis) Chemical form of the oxygen (I i~ spectral analy-
sis)
POWDER
U.
S. Industrial Chemicals Co., Tuscola, Ill. Microethene 0.915 gm/ml <20 t~ 0.55 m~/gm 10.9 0.1394- 0.005% by wt C--OH and
~C=O
and agitated slowly for a desired period of time. I t had been established prior to the adsorption experiments that N a P S E ~ was not precipitated from aqueous solutions containing 0.1-2.0 M l~aC1. The difference in the concentration of polymer solutions before and after being in contact with the adsorbent was determined as described below. I t was used together with the weight of the adsorbent to calculate the amount and rate of adsorption in meq/100 gm I (or m e q / c m ~) and meq/100 gm/hr, respectively. Two different techniques were used to anMyze for residual polyelectrolyte in each solution. T h e y were the turbidimetric titration method introduced by Fuoss and Sadek (16) which takes advantage of the antagonistic reaction between po]yanions and polycations; and conductometric titration using Hyamine 1622, a cationic surface-active agent as titrant. A Brice-Phoenix light scattering photometer was used for the former measurements; and an Industrial Instruments conductivity bridge and cell operative at 1000 cps was used for the latter. The agreement between the two techniques was found to be satisfactory (4.2%) except for 1 One equivMent of polymer is equal to the molecular weight of a monomer unit.
solutions of very low ionic strengths for which the turbidity was found to change erratically with time. Results obtained by conductometric titration on duplicate samples of a given polymer solution always agreed within 4.5 %. RESORPTION
MEASUREMENTS
Desorption experiments were carried out as follows: At the completion of adsorption equilibrium experiments (which usually lasted for a period of about a week) polymer solutions were carefully withdrawn from the adsorbent in the adsorption tubes and replaced by equivalent volumes of deionized water adjusted to pH 6.5. The tubes were then stoppered, sealed, and subjected to gentle agitation at 25°C for a period of at least a week. Thereafter, the supernatant solutions were withdrawn from the adsorbent and analyzed for polymer by the eonduetometrie titration technique described above.
VISCOSITY
MEASUREMENTS
Viscosity measurements on aqueous solutions of iUaPSEIV[ in the presence of various concentrations of l~aCl were carried out at the same pH and temperature as the adsorption experiments. This was done in order to determine if changes in the conformation of the polymer in solution could be correlated with polymer adsorption characteristics. The measurements were carried out in a Ubbelohde-type viscosimeter which was mounted in a water bath thermostated at 25.0 40.02°C. The flow time of 25 ml of water through the viscosimeter was 257.7 4- 0.2 sec and sufficiently great to neglect kinetic energy corrections. Stock polymer solutions used in viscosity as well as adsorption equi]ibrium experiments were diluted isoionically, i.e., by systematically increasing the NaCI content of the diluent as the I~raPSEM concentration was decreased so as to maintain the total ionic strength constant. At a given NaC1 concentration, plots of reduced viscosity, 7~p/C, against polymer concentration were made and extrapolated to zero concentration to obtain the intrinsic viscosity, [7]. The root-mean-square end to end length ((~))1/~ of N a P S E M in solution was calculated from [7] using the F l o r y - F o x
Journal of Colloid and Interface Ncience, Vol. 37, No. 1, September 1971
ADSORBABILITY
OF A SYNTHETIC
equation for random polymer coils (17, 18),
POLYELECTROLYTE
l&7
16 14
=
where M is the polymer molecular weight; and ¢ (assumed here to have its usual value of 2.1 X 102.) is the universal Flory constant. The root-mean-square radius of gyration, R~, of the polymer in solution was obtained from the relation (18),
E2 tO
8
4 2
0
I 0.4
I I I r 0.8 1.2 h6 2.0 CONC. NaCl, Moles/liter
2.4
and the intramoleeular expansion factor, ~, which increases from a value of unity to FzG. 3. The dependence of the intrinsic vishigher values as the solvent power (i.e., cosity of NaPSE1VI on the concentration of NaC1 affinity) of the medium for the solute in- in solution. creases was obtained from the relation (18), =
•
Here [~] is the intrinsic viscosity of the polymer at any given salt concentration; and [~]0 is the limiting intrinsic viscosity at the salt concentration just corresponding to the minimum in the [v] vs lEaC1 concentration plot. III. RESULTS AND DISCUSSION VISCOSITY
STUDY
The curves shown in Fig. 2 illustrate how the reduced viscosity of N a P S E M (Sample I) depended on the concentrations of polymer and lgaC1 in solution. A plot of the limiting reduced viscosities, [v] values, obtained from Fig. 2 against pertinent NaC1 16 [
/ . ~ 0 4
[
N~O CONC IN MOLES/LITER: 1
..I
I
.02
!
I
'
.04 06 08 I0 CONC NoPSEM,g/lO0 m]
.12
34
FIG. 2. Reduced viscosity--concentration curves of NaPSEM.
concentrations is given in Fig. 3. It shows that with increasing NaC1 concentration [v] decreased to a limiting value and thereafter remained constant. If it is assumed that [v] is a measure of the effective volume of the polymer molecule in solution, then this result signified that the hydrodynamic dimensions of NaPSEIV[ decreased with salt concentration up to a concentration of about 0.8 M NaC1. Alpha and (<~})1/2 values derived from [v] at various salt concentrations are given in Table III. The data listed in column 3 of Table I I I show that a also decreased to its limiting value with increasing NaC1 concentration. This indicated that as the salt concentration was increased, the medium (water) became a poorer solvent for NaPSEM. Since the water solubility of charged polymers generally decreases with decreasing net charge density (19), this in turn indicated that the primary effect of the added salt was to screen the polymer charge (i.e., to reduce the net charge of the polymer). That there was a contraction of the polymer coils as a consequence of this is clear from the values of ((S})lm listed in column 4 of this table. The limiting value of ((~})1/2 which occurred at concentrations >=0.8 M NaC1 indicated that the minimum hydrodynamic dimensions of NaPSEL~/I in salt solution had been reached. Thus as the salt concentration increased (or a decreased), NaPSESI assumed a more compactly coiled conformation in solution; consequently, [s] decreased. It will
Journal of Colloid and Interface Science, ¥ o l . 37, No. 1, S e p t e m b e r 1971
148
GREENE
:F
TABLE IH THE INTRINSIC VISCOSITY AND HYDRODYNAMIC DIMENSIONS OF N a P S E M (SAMPLE I) AS A FUNCTION OF THE CONCENTRATION OF ADDED SALT CNaOI (moles/Iiter)
[~] (dl/gm)
0.04 0.12 0.40 0.80 1.20 1.60
13.05 7.20 4.40 3.70 3.75 3.70
a
1.53 1.25 1.06 1.00 ~1.00 1.00
INITIAL CORCRATIO NoPSEM ADSORBENT :
IAMOUNT ADSORBED, Meq/IOOg I 5
-:
4
2.46 2.02 1.73 1.62 1.63 1.62
The adsorption-time curves obtained when N a P S E M (Sample I) was adsorbed from aqueous solutions having an initial concentration ratio of N a P S E M to adsorbent of 0.2 meq/liter to 4 gin/liter ( - 5 meq/100 gin) and containing various concentrations of NaC1 are shown in Fig. 4. It is seen that at a given NaCI concentration, the adsorption was initially time dependent, i.e., the amount of N a P S E M adsorbed at first increased with increasing time of contact between the polymer solution and the adsorbent. However, when deionized water which contained no added salt was used, the adsorption which was practically immeasurable appeared to be instantaneous and exhibited no further changes with time. It can also be seen from the curves in Fig. 4 that the maximum amount adsorbed in the plateau (or apparent equilibrium adsorption) region was greater the higher the concentration of NaC1 in solution. Similarly, the rate of adsorption as measured by the initial slopes of the adsorption-time curves was greater the higher the NaC1 concentration. These results, therefore, showed that both the amount and rate of adsorption increased with increasing salt concentration. On the other hand, those in the preceding section showed that, in general, [n] as well as a decreased as the NaC1 concentration increased. Thus, it eould be concluded that the adsorption of N a P S E M increased as [n] decreased, i.e., as the polymer assumed a more
:
~C
S
s
t
-
-
.
CORC IN MOLES/LITER:~
~06M
~
I /H20 o
~
t
3 50
ADSORPTION ~4~INETICS
:
\1.2 M
10 5 ( < ~ 2 > ) 1 / s (cm)
be shown later that this had a marked effect on the polymer adsorption characteristics.
5 Meq IOOg
i IO0
~
L 150 200 CONTACTTrME, H O U R S
I
250
300
350
FIG. 4. Adsorption of NaPSEM from NaC1 solutions of various concentrations onto polyethylene powder. compactly coiled conformation in solution owing to its poorer solubility in the medium. The empirical relation found here between [v] and the adsorption characteristics of N a P S E M is shown in the graph given in Fig. 5. Several previous studies have shown that the adsorption of uncharged polymers varied inversely with the solvent power of the medium (20-23). However, in those cases, the solvent power was altered by varying the type rather than the concentration of the solvent as was done here and in a similar study of polyelectrolytes (24). The adsorption-time curves of Samples ( I - I I I ) of N a P S E M of different molecular weights are compared in the graph shown in Fig. 6. The initial concentration ratio of polyelectrolyte to adsorbent and the concentration of NaC1 in solution were in all cases 5 meq/100 gm and 0.6 M, respectively. It is seen from this graph that the initial rate of adsorption decreased with increasing polymer molecular weight; whereas the maximum amount adsorbed (on a molar basis) in the apparent equilibrium region was practically independent of polymer molecular weight. As will be shown later in the section on equilibrium adsorption, this latter result indicated that the conformation of adsorbed N a P S E M was similar to that of free N a P S E M in NaC1 solutions. The molecular weight dependence found here suggested that the rate of N a P S E M adsorption was initially diffusion-controlled. If this were the
Journal o] Colloid and Interface Science, Vol. 37, No. 1, S e p t e m b e r 1971
ADSOI~BABILITY OF A SYNTHETIC POLYELECTROLYTE i01
149
[2 NaCI CONC,MOLES/LITER] I0
i0 o m
//°
/ °o: 2
Moximum Amount dsorbed meq / I 0 0
F"
,
61
//
i0-I Initiol Rote of . \ Adsorption (meq/100 g/hr)~
L
0
IO-2tP ; I ,I
i
i0 o
I
I I I[lll
I
i0 t
ADsoeBEo, MEQ/ OOG I
- 0~~ I
MY: 35 x I04
0
~
112
•
6 M y24×[0 :
o
50
I 150 200 250 CONTACT TIME, HOURS
I00
35 xEO4 I 300
i 350
400
Fro. 6. The adsorption of NaPSEM from 0.6 M NaCl solution onto polyethylene powder.
case then the early stages of the adsorption would be expected to conform to the Longmuir-Schaefer relation (25), r
=
6
i O-~.TI/z,SEC~12
8
I0
12
through the origin. T h a t this was approximately the ease for the adsorption of N a P S E M (Sample I) is clear from the lineality of the plots shown in Fig. 7. However, the failure of the lines to pass through the origin indicated that Eq. [1] was not strictly valid for large molecules as it is for small ones. Plots similar to those in Fig. 7 were also obtained using the adsorption data of Samples II and III. ADSORPTION STATICS
V 40 x I05
0
4
FzG. 7. The dependence of the polymer adsorption on the square root of the adsorption time.
FIG. 5. Empirical relation between iv] and the adsorption characteristics of NaPSE1Vi.
61
2
[q
Here £ is the amount adsorbed per unit area of adsorbent; C, the bulk concentration of the adsorbate; D, the diffusion coefficient of the adsorbate; and t, the adsorption time. According to this relation at a given adsorbate concentration and molecular weight, a plot of the amount adsorbed against the square root of the adsorption time should yield straight lines passing
All equilibrium adsorption studies were carried out at 25°C in 0.8 M NaC1 solution. This latter NaCl concentration was essentially equivalent to the one at which the limiting [~] of N a P S E M was reached, i.e., where maximum coiling of the polymer chains was indicated. All adsorption measurements were made after the adsorbent had been in contact with the polymer solution 172 ± 0.5 hr. ~ On the basis of adsorption kinetics results, this was more than sufficient time to establish equilibrium. The equilibrium adsorption isotherms of the three polymer samples are shown in Fig. 8. The desorption isotherm shown in Fig. 9 illustrates that the adsorption was essentially completely reversible when the experiments were carried out by replacing the 2 The quantity of adsorbent used (4 gm/liter) was the same as in the adsorption kinetics experiments.
Journal of Colloid and Interface Science, Vol. 37, No. 1, S e p t e m b e r 1971
150
GREENE
SAMPLE E : o
~
10
"rr" : .
o-
10 2
~ 6 g iO I
2
0
--
L
0'.o Ce,
/
I
I
,.0
1.4
i0 °
meq / l i t e r
f
10 4
FIG. 8. Isotherms for the adsorption NaPSEM on polyethylene powder at 25°C.
I
I
IIIIII
I
I
I
IIIIII
10 5
I
I
I
10 6
of Fro. 10. The dependence of maximum adsorption on NaPSEM molecular weight.
lZ o
8 t~ m
g G m o 4 ~7- A d s o r p t i o n
~ 2
o
T
0 0
0.2
n
-
I
~
I
I
I
I
0.4
0.6
0.8
i.e
1.2
1.4
Ce,
1.6
Meq/liler
FIG. 9. Equilibrium adsorption--desorption isotherm of N~PSEM (Sample II).
residual polymer solution by deionized water as described earlier. On the other hand, desorption of the polymer from the PE powder into aqueous NaC1 solutions was found to be negligible. From the adsorption isotherms given in Fig. 8, it is seen that the affinity of the adsorbent for the polymer (~s measured by the initial slopes of the curves) increased somewhat with increasing polymer molecular weight. However, the maximum amount adsorbed at saturation (on a molar basis) was essentially independent of polymer molecular weight. This latter w~s also found in the adsorption kinetics study. Now when the maximum adsorption (N8.2 meq/100 gm) was expressed as gr~m of polymer chains adsorbed, it was found to increase with polymer molecular weight as shown in Journal of Colloid and Interface Science,
V o l . 37, N o .
1, S e p t e m b e r
Fig. 10. It was also found to be considerably more than could be accommodated in a monolayer (approximately 40 times more if it is assumed that each polymer segment having a molecular area of ca. 40 As is in contact with the adsorbent surface). Both these latter results gave some clues as to the structure of adsorbed NaPSEM on the adsorbent surface. For it is usually found when nonporous adsorbents are used that the molecular weight dependence of the maximum adsorption is in accord with the empirical equation (20-23, 26, 27) A mox = K M ~,
[2]
where M is the molecular weight of the polymer; and K and a are constants. If the polymer in the extended configuration were to lie parallel to the adsorbent surface until adsorption was complete, then the number of polymer chains adsorbed would decrease as M increased. However, the amount adsorbed on a gram basis would be independent of M (i.e., a = 0 in Eq. [2]). On the other hand, if the polymer in a coiled or looped configuration were to be attached at only one point on the adsorbent surface, the number of polymer chains adsorbed would be independent of M. However, the amount adsorbed on a gram basis would increase with increasing M (a = 1). It would, no doubt, be fortuitous if either of these extreme eases represented the actual state of 1971
ADSORBABILITY OF A S Y N T H ETI C POLYELECTROLYTE
mum adsorption in mg/gm. Values of Ra calculated in this manner as well as values of R~ calculated from viscosity results of the polymer samples in 0.8 M NaC1 solution are listed in Table IV. Comparison of these values reveals that R~ was always much smaller than Ra. This suggested that there was considerable compression of the polyelectrolyte coils upon adsorption. It is conceivable that this could have occurred because the concentration of Na + ions per polymer chain was considerably higher iD the adsorbed state than in solution. However, this is not clearly understood.
the adsorbed polyelectrolyte here. However, it is obviously the latter case which is more compatible with the adsorption and viscosity results reported here for NaPSEM. Therefore, it can be said that NaPSEM in a more or less compact randomly coiled conformation was attached through only a few of its segments to the surface of the PE powder. On the basis of the above analysis, it is reasonable that a monolayer consisting of randomly coiled polymer chains was formed at adsorption saturation. If it is assumed that this was approximately the case then a comparison can be made between the radius of the adsorbed polymer coils, RA , and the radius of gyration of the free polymer coils, RG (determined from viscosity measurements). RA in angstr6ms can be calculated from the equation (27), R~ = (SM/6.023 7rA)~2,
TABLE IV COMPARISON OF THE DIMENSIONS OF ADSORBED AND
FREE
NaPSEM
Sample
10-6 My
RA (A)
RG (A)
I I2 III
2.40 0.40 0.035
62.8 25.6 7.6
662.0 296.0 82.6
SAMPLE 1.0
CONFORMANCE OF THE ADSORPTION STATICS TO LANGMUIR ADSORPTION THEORY
[31
where S is the specific surface area of the adsorbent in m2/gm; M, the molecular weight of the polymer; and A, the maxi-
151
The adsorption isotherms of the three NaPSEM samples (Fig. 8) were of the Langmuir type. An attempt was, therefore, made to fit the data using the Langmuir adsorption equation, abCe A - 1 + bC~"
[4]
Here A is the amount adsorbed per gram; C~ is the bulk concentration of the adsorbate; and a and b are constants. The Langmuir plots of the data of the three polymer samples are shown in Fig. 11. The lines in these plots are seen to be linear except in the concentration region where saturation is approached. Thus the adsorption of NaPSEM like that of many uncharged polymers conformed reasonably well to the
[ : o ]1: A ~D. •
<" .6
'o .4
.2
0
I 5
110
1T5
2PO
1/C e , l i t e r / m e q
FIG. 11. Langmuir plots for the adsorption of NaPSEM on polyethylene powder. Journal of Colloid and Interface Science.
¥ol. 37, No. 1, September 1971
152
GREENE TABLE V
THE DEPENDENCE OF THE CONSTANTS a AND b IN TIIE LANGMUIR ADSORPTION EQUATION ON N a P S E M MOLECULAR WEIGHT Sample
My"10-6
a (meq/gm
a (liter/meq)
I II III
2.40 0.40 0.035
.29 ± .29 ± .27 ±
.73 ± .53 ± .42 ±
.02 .02 .01
.05 .04 .02
Langmuir equation. Theoretically, the Langmuir type isotherm is expected for polymers (28) when only a few of the chain segments are attached to the adsorbent surface and the remainder are looped up away from the surface toward the continuous phase. Still the validity of the Langmuir isotherm implies monolayer adsorption. According to Silberberg (29) this holds true even for polymeric adsorbates. However, it must be assumed, as was done here, that the occurrence of the Langmuir type isotherm for polymers means that at saturation a monolayer of polymer coils is formed. With this interpretation in mind, values of a and b in the Langmuir equation have been calculated for each of the polymer samples. If these constants were to have the same meaning here as they have when small molecules are the adsorbates, then a should be proportional to the area of an adsorbed segment at the completion of the monolayer; and b should be a measure of the energy of adsorption. The data listed in Table V show that the value of the constant a was essentially the same for all three polymer samples as was expected on the basis of the adsorption results in Fig. 8. However, the value of the constant b was found to increase with polymer molecular weight. This indicated, as was mentioned before, that the affinity of the polymer for the adsorbent increased with polymer molecular weight.
main impetus for the enhanced polymer adsorption observed here as it led to a decrease in (i.) the electrostatic repulsion between similarly charged adsorbing and already adsorbed polymer molecules and (i.i.) the density of adsorbed polymer segments on the P.E. powder. V. SUMMARY The kinetics and statics of the adsorption of a synthetic polyelectrolyte, NaPSEM, from aqueous salt solutions onto a PE powder have been examined. It was found that at a given salt concentration, the adsorption on a molar basis was essentially independent of polyelectrolyte molecular weight; and could be essentially completely reversed. On the other hand, at a given molecular weight, both the rate and extent of adsorption were found to increase with increasing salt concentration. Parallel viscosity measurements showed that this latter result could be correlated with changes in the conformation of the polyelectrolyte in solution (i.e., changes in iv]). In general, adsorption was found to be greater the lower iv]. It could, therefore, be deduced that NaPSEM in a more or less compressed coiled conformation was weakly attached to the adsorbent surface. The adsorption of the polyelectrolyte from aqueous salt solutions was shown here to be similar to that of many uncharged polymers with regard to its (i) dependence on molecular weight; (ii) dependence on the solvent power of the medium; and (iii) conformance to the Langmuir adsorption theory. VI. A C K N O W L E D G M E N T S Acknowledgments are due to Mr. D. A. Kangas from whom the polymer samples were obtained; to Dr. James P e t e r s for the sample of the polyethylene powder; and to Dr. Wilfried Heller for a m o s t informative discussion. REFERENCES
IV. M E C H A N I S M OF NAPSEM A D S O R P T I O N
As stated earlier, the primary effect of the added salt was to reduce the net charge of the polymer. Thus as' the salt concentration was increased, the polymer approached the isoelectric state (minimum solubility) and assumed a more compact randomly coiled conformation. This seemingly provided the
1. RUEI-IRWEIN, R. A., AND WARD, D. W., Soil Sci. 73,485 (1952). 2. MICHAEbS, A. S., Ind. Eng. Chem. 46, 1485 (1954). 3. MIGH2J-ELS, A. S., AND MORELOS~ 0., Ind. Eng. Chem. 47, 1801 (1955). 4. PUGH, T. L., AND HELLER, W . , J . Polym. Sci. 47,219 (1960).
Journal of Colloidand Interface Science, Col. 37, No. 1, September 1971
ADSORBABILITY OF A SYNTHETIC POLYELECTROLYTE 5. NEMETIt, R., AND MATIJEVIC, E., Kolloid-Z. Z. Polym. 225, 155 (1968). 6. SO~I=~ERAUEa,et al., Kolloid-Z. Z. Polym. 225, 147 (1968). 7. TEO% A. S., AND DA~IELS, S. L., Environ. Sci. Technol. 3,825 (1969). 8. MuRoI, S., J. Appl. Polym. Sci. 10,713 (1966). 9. M~TROI, S., AND HosoI, K., Kobunshi Kagaku 26, 416 (1969). 10. lV[uRoi, S., AND t-Iosor, K., J. Appl. Polym. Sci. 11, 2331 (1967). 11. GREENE, B. W., FILER, T. D., .~ND SHEETZ, D. P., J. Colloid Interface Sci. 32, 90 (1970). 12. GREENE, B. W., unpublished results. 13. MILLs, T. L., AND YOCU~, R. H., J. Paint Technol. 39, 532 (1967). 14. HEI~ENZ, P., J. Polym. Sci. Part C 27, 253
(1969). 15. VAN DELL, i~., unpublished results. 16. Fuoss, R. M., AND SADEK, N., Science 110,
552 (1949). 17. Fox, T. G., AND FLORY, P. J., ar. Amer. Chem. Soe. 73, 1909 (1951). 18. FLOR¥, P. J., " P r i n c i p l e s of P o l y m e r C h e m -
153
istry," pp. 295, 401, 408, 611, 620. Cornell University Press, Ithaca, New York, 1953. 19. MORAWETZ, H., "Maeromolecules in Solution," p. 83. Wiley (Interseience) New York, 1965. 20. KOR~L, J., ULL~AN, R., AND EIRICH, F. R., Y. Phys. Chem. 62,541 (1958). 21. LUCE, J. E., ~ND ROBERTSON, A. A., J. Polym. Sci. 51,317 (1961). 22. PERXEL, K., AND ULLMAN, R., J. Polym. Sci. 54, 127 (1961). 23. MIZU•ARA, K., HAR.~, K., AND IMOTO, T., Kolloid-Z. Z. Polym. 229, 17 (1969). 24. SHYLUK, W. P., J. Appl. Polym. Sci. 8, 1063 (1964); J. Polym. Sei. 6, 2009 (1968). 25. LANGMUIR,I., AND SCI-IAEFER,Y. J., J. Amer. Chem. Soc. 59, 2400 (1937). 26. HELLER, W., Pure Appl. Chem. 12,258 (1966). 27. BURNS, H., AND CARPENTER, D. K., Macromolecules 1,384 (1968). 28. FRiscI~, H. L., AND SIMMS, R., J. Phys. Chem. 58,507 (1954). 29. SILBERBEaG, A., J. Phys. Chem. 66, 1872 (1962); Ibid p. 1884.
Journal of Colloid ancl Interface Science,
Vol. 37, No. 1, September 1971
(1-7). Until a few years ago, it was not known t h a t during the polymerization of m a n y commercial latexes--themselves colloids-polyelectrolytes were formed in situ in the aqueous phase (8-11). I n practice, it is found t h a t the concentration of polyelectrolyte so produced is sometimes so small as to be undetectable by most conventional techniques (9, 10). Yet such small quantities can have dramatic effects on final properties of the latex, e.g., thickener response and electrolyte tolerance (12). These effects are also commonly believed to be a consequence of polyelectrolyte adsorption. However, at present,
there are no published results to substantiate this view. Neither are there sufficient available data to establish t h a t polyelectrolyte adsorption would be favored from an aqueous environment (of moderate to relatively high ionic strength) similar to t h a t in a commercial latex. I t was the p r i m a r y goal of the present study to clarify this latter point; and thereby gain a more basic understanding of the manner by which in situ formed polyelectrolytes might affect latex properties. Therefore, in this study, the kinetics and statics of the adsorption of a synthetic polyelectro]yte onto a solid (similar in m a n y respects to a latex) have been determined as a function of added salt concentration and polyelectrolyte molecular weight. II. EXPERIMENTAL P R E P A R A T I O N AND CHARACTERIZATION THE P O L Y M E R U S E D
OF
The polymer used in this study was sodium poly(2-sulfoethylmethaerylate), N a P S E M .
Journal of Colloid and Interface ~gcience, VoL 37, No. 1, Sepl~ember 1971
144
A D S O R B A B I L I T Y OF A S Y N T H E T I C P O L Y E L E C T R O L Y T E
It is a strongly anionic polyelectrolyte having the monomer unit:
/
I
145
}00 ML SAMPLE WITH .475 ¥EQ POLY SEM ACID
0
C H 3 - - C - - C - - O - - C H 2 - - C H 2 - - S O ~ - N a+
l
CH~
l
TITRAINT -
The monomer itself has previously been described as an efficient comonomeric emulsifier for the polymerization of synthetic latexes (13, 14); and there is now recent evidence that during such polymerizations the polymer is formed i n s i t u in the latex serum phase (15). Several samples of N a P S E M were investigated. Each had been prepared by solution polymerization of the monomer to moderate conversion at 70°C using persulfatc as initiator; was subsequently purified by extraction with a nonsolvent; and then dialyzed several days to remove extraneous electrolyte. The viscosity average molecular weights of the samples used are listed in Table I. They were determined from the intrinsie viscosity In], measured in 0.8 M NaC1 using the M a r k Houwink relation, In] = K ' M ~ (the values used for the constants a and K ' were 0.470 and 3.72 X 10-4, respectively). The number average molecular weights of the samples which were determined by osmometry are also included in Table I. A typieal curve of the potentiometric titration of N a P S E M (Sample I which had been converted to the acid form by the addition of the stoiehiometric amount of HC1) with NaOH is shown in Fig. 1. It illustrates that during titration, the polymer behavior was characteristic of a strong electrolyte. Both the stoiehiometry at the titration end point, and the sharpness of the curve were indicative that the polymer was essentially free of impurities. TABLE I PROPERTIES OF THE SAMPLES OF N a P S E M Sample I II III
[7] (dl/gm) 3.7 4- 0.2 2.0 4- 0.1 0.5 4- 0.04
l~v. 10-~
M~-10-6
2.40 0.40 0.035
2.00 0.50 0.03
Measured in 0.8 M NaC1 solution.
: 4
[
I
.6 8 t.O Eq BASE / Eq. SO-3
FIG. 1. P o t e n t i o m e t r i c P S E M acid.
titration
I4
16
curve
of
THE ADSORBENT USED
A finely divided polyethylene (PE) powder manufactured by United States IndustI~als Chemical Company was used as the adsorbent. It constituted a nonporous, lowenergy hydrophobie surface which was either nonionie or very weakly anionic at the p H (6.5 4- 0.2) used in all adsorption experiments. The powder was dried at 60°C and stored for several days in a vacuum desiccator prior to usage. Some properties of the powder and the methods used to determine them are listed in Table II. THE SOLVENT
Deionized water (specific conductivity ca. 10-6 ohm -1 em -1) alone or containing various concentrations of analar grade NaCI was used as the solvent, p H adjustments on the solvent were made when necessary by addition of either NaOH or HC1. ADSORPTION ~'IEASUREMENTS
A given quantity of an aqueous solution containing known concentrations of NaC1 and N a P S E M was introduced into an adsorption tube which contained a weighed quantity of the adsorbent. The tube was stoppered and sealed; mounted on a shaker located in a room thermostated at 25 ± I°C ; Journal of Colloid and Interface Science, Vol. 37, No. 1, September 1971
146
GREENE TABLE
II
PROPERTIES OF THE P O L Y E T H Y L E N E U S E D AS ADSOI~BENT
Manufacturer Grade Density Average particle diameter (listed by manufacturer) Surface area (N2 adsorption) Average particle diameter (based on N~ adsorption) Total oxygen content (Neutron activation analysis) Chemical form of the oxygen (I i~ spectral analy-
sis)
POWDER
U.
S. Industrial Chemicals Co., Tuscola, Ill. Microethene 0.915 gm/ml <20 t~ 0.55 m~/gm 10.9 0.1394- 0.005% by wt C--OH and
~C=O
and agitated slowly for a desired period of time. I t had been established prior to the adsorption experiments that N a P S E ~ was not precipitated from aqueous solutions containing 0.1-2.0 M l~aC1. The difference in the concentration of polymer solutions before and after being in contact with the adsorbent was determined as described below. I t was used together with the weight of the adsorbent to calculate the amount and rate of adsorption in meq/100 gm I (or m e q / c m ~) and meq/100 gm/hr, respectively. Two different techniques were used to anMyze for residual polyelectrolyte in each solution. T h e y were the turbidimetric titration method introduced by Fuoss and Sadek (16) which takes advantage of the antagonistic reaction between po]yanions and polycations; and conductometric titration using Hyamine 1622, a cationic surface-active agent as titrant. A Brice-Phoenix light scattering photometer was used for the former measurements; and an Industrial Instruments conductivity bridge and cell operative at 1000 cps was used for the latter. The agreement between the two techniques was found to be satisfactory (4.2%) except for 1 One equivMent of polymer is equal to the molecular weight of a monomer unit.
solutions of very low ionic strengths for which the turbidity was found to change erratically with time. Results obtained by conductometric titration on duplicate samples of a given polymer solution always agreed within 4.5 %. RESORPTION
MEASUREMENTS
Desorption experiments were carried out as follows: At the completion of adsorption equilibrium experiments (which usually lasted for a period of about a week) polymer solutions were carefully withdrawn from the adsorbent in the adsorption tubes and replaced by equivalent volumes of deionized water adjusted to pH 6.5. The tubes were then stoppered, sealed, and subjected to gentle agitation at 25°C for a period of at least a week. Thereafter, the supernatant solutions were withdrawn from the adsorbent and analyzed for polymer by the eonduetometrie titration technique described above.
VISCOSITY
MEASUREMENTS
Viscosity measurements on aqueous solutions of iUaPSEIV[ in the presence of various concentrations of l~aCl were carried out at the same pH and temperature as the adsorption experiments. This was done in order to determine if changes in the conformation of the polymer in solution could be correlated with polymer adsorption characteristics. The measurements were carried out in a Ubbelohde-type viscosimeter which was mounted in a water bath thermostated at 25.0 40.02°C. The flow time of 25 ml of water through the viscosimeter was 257.7 4- 0.2 sec and sufficiently great to neglect kinetic energy corrections. Stock polymer solutions used in viscosity as well as adsorption equi]ibrium experiments were diluted isoionically, i.e., by systematically increasing the NaCI content of the diluent as the I~raPSEM concentration was decreased so as to maintain the total ionic strength constant. At a given NaC1 concentration, plots of reduced viscosity, 7~p/C, against polymer concentration were made and extrapolated to zero concentration to obtain the intrinsic viscosity, [7]. The root-mean-square end to end length ((~))1/~ of N a P S E M in solution was calculated from [7] using the F l o r y - F o x
Journal of Colloid and Interface Ncience, Vol. 37, No. 1, September 1971
ADSORBABILITY
OF A SYNTHETIC
equation for random polymer coils (17, 18),
POLYELECTROLYTE
l&7
16 14
=
where M is the polymer molecular weight; and ¢ (assumed here to have its usual value of 2.1 X 102.) is the universal Flory constant. The root-mean-square radius of gyration, R~, of the polymer in solution was obtained from the relation (18),
E2 tO
8
4 2
0
I 0.4
I I I r 0.8 1.2 h6 2.0 CONC. NaCl, Moles/liter
2.4
and the intramoleeular expansion factor, ~, which increases from a value of unity to FzG. 3. The dependence of the intrinsic vishigher values as the solvent power (i.e., cosity of NaPSE1VI on the concentration of NaC1 affinity) of the medium for the solute in- in solution. creases was obtained from the relation (18), =
•
Here [~] is the intrinsic viscosity of the polymer at any given salt concentration; and [~]0 is the limiting intrinsic viscosity at the salt concentration just corresponding to the minimum in the [v] vs lEaC1 concentration plot. III. RESULTS AND DISCUSSION VISCOSITY
STUDY
The curves shown in Fig. 2 illustrate how the reduced viscosity of N a P S E M (Sample I) depended on the concentrations of polymer and lgaC1 in solution. A plot of the limiting reduced viscosities, [v] values, obtained from Fig. 2 against pertinent NaC1 16 [
/ . ~ 0 4
[
N~O CONC IN MOLES/LITER: 1
..I
I
.02
!
I
'
.04 06 08 I0 CONC NoPSEM,g/lO0 m]
.12
34
FIG. 2. Reduced viscosity--concentration curves of NaPSEM.
concentrations is given in Fig. 3. It shows that with increasing NaC1 concentration [v] decreased to a limiting value and thereafter remained constant. If it is assumed that [v] is a measure of the effective volume of the polymer molecule in solution, then this result signified that the hydrodynamic dimensions of NaPSEIV[ decreased with salt concentration up to a concentration of about 0.8 M NaC1. Alpha and (<~})1/2 values derived from [v] at various salt concentrations are given in Table III. The data listed in column 3 of Table I I I show that a also decreased to its limiting value with increasing NaC1 concentration. This indicated that as the salt concentration was increased, the medium (water) became a poorer solvent for NaPSEM. Since the water solubility of charged polymers generally decreases with decreasing net charge density (19), this in turn indicated that the primary effect of the added salt was to screen the polymer charge (i.e., to reduce the net charge of the polymer). That there was a contraction of the polymer coils as a consequence of this is clear from the values of ((S})lm listed in column 4 of this table. The limiting value of ((~})1/2 which occurred at concentrations >=0.8 M NaC1 indicated that the minimum hydrodynamic dimensions of NaPSEL~/I in salt solution had been reached. Thus as the salt concentration increased (or a decreased), NaPSESI assumed a more compactly coiled conformation in solution; consequently, [s] decreased. It will
Journal of Colloid and Interface Science, ¥ o l . 37, No. 1, S e p t e m b e r 1971
148
GREENE
:F
TABLE IH THE INTRINSIC VISCOSITY AND HYDRODYNAMIC DIMENSIONS OF N a P S E M (SAMPLE I) AS A FUNCTION OF THE CONCENTRATION OF ADDED SALT CNaOI (moles/Iiter)
[~] (dl/gm)
0.04 0.12 0.40 0.80 1.20 1.60
13.05 7.20 4.40 3.70 3.75 3.70
a
1.53 1.25 1.06 1.00 ~1.00 1.00
INITIAL CORCRATIO NoPSEM ADSORBENT :
IAMOUNT ADSORBED, Meq/IOOg I 5
-:
4
2.46 2.02 1.73 1.62 1.63 1.62
The adsorption-time curves obtained when N a P S E M (Sample I) was adsorbed from aqueous solutions having an initial concentration ratio of N a P S E M to adsorbent of 0.2 meq/liter to 4 gin/liter ( - 5 meq/100 gin) and containing various concentrations of NaC1 are shown in Fig. 4. It is seen that at a given NaCI concentration, the adsorption was initially time dependent, i.e., the amount of N a P S E M adsorbed at first increased with increasing time of contact between the polymer solution and the adsorbent. However, when deionized water which contained no added salt was used, the adsorption which was practically immeasurable appeared to be instantaneous and exhibited no further changes with time. It can also be seen from the curves in Fig. 4 that the maximum amount adsorbed in the plateau (or apparent equilibrium adsorption) region was greater the higher the concentration of NaC1 in solution. Similarly, the rate of adsorption as measured by the initial slopes of the adsorption-time curves was greater the higher the NaC1 concentration. These results, therefore, showed that both the amount and rate of adsorption increased with increasing salt concentration. On the other hand, those in the preceding section showed that, in general, [n] as well as a decreased as the NaC1 concentration increased. Thus, it eould be concluded that the adsorption of N a P S E M increased as [n] decreased, i.e., as the polymer assumed a more
:
~C
S
s
t
-
-
.
CORC IN MOLES/LITER:~
~06M
~
I /H20 o
~
t
3 50
ADSORPTION ~4~INETICS
:
\1.2 M
10 5 ( < ~ 2 > ) 1 / s (cm)
be shown later that this had a marked effect on the polymer adsorption characteristics.
5 Meq IOOg
i IO0
~
L 150 200 CONTACTTrME, H O U R S
I
250
300
350
FIG. 4. Adsorption of NaPSEM from NaC1 solutions of various concentrations onto polyethylene powder. compactly coiled conformation in solution owing to its poorer solubility in the medium. The empirical relation found here between [v] and the adsorption characteristics of N a P S E M is shown in the graph given in Fig. 5. Several previous studies have shown that the adsorption of uncharged polymers varied inversely with the solvent power of the medium (20-23). However, in those cases, the solvent power was altered by varying the type rather than the concentration of the solvent as was done here and in a similar study of polyelectrolytes (24). The adsorption-time curves of Samples ( I - I I I ) of N a P S E M of different molecular weights are compared in the graph shown in Fig. 6. The initial concentration ratio of polyelectrolyte to adsorbent and the concentration of NaC1 in solution were in all cases 5 meq/100 gm and 0.6 M, respectively. It is seen from this graph that the initial rate of adsorption decreased with increasing polymer molecular weight; whereas the maximum amount adsorbed (on a molar basis) in the apparent equilibrium region was practically independent of polymer molecular weight. As will be shown later in the section on equilibrium adsorption, this latter result indicated that the conformation of adsorbed N a P S E M was similar to that of free N a P S E M in NaC1 solutions. The molecular weight dependence found here suggested that the rate of N a P S E M adsorption was initially diffusion-controlled. If this were the
Journal o] Colloid and Interface Science, Vol. 37, No. 1, S e p t e m b e r 1971
ADSOI~BABILITY OF A SYNTHETIC POLYELECTROLYTE i01
149
[2 NaCI CONC,MOLES/LITER] I0
i0 o m
//°
/ °o: 2
Moximum Amount dsorbed meq / I 0 0
F"
,
61
//
i0-I Initiol Rote of . \ Adsorption (meq/100 g/hr)~
L
0
IO-2tP ; I ,I
i
i0 o
I
I I I[lll
I
i0 t
ADsoeBEo, MEQ/ OOG I
- 0~~ I
MY: 35 x I04
0
~
112
•
6 M y24×[0 :
o
50
I 150 200 250 CONTACT TIME, HOURS
I00
35 xEO4 I 300
i 350
400
Fro. 6. The adsorption of NaPSEM from 0.6 M NaCl solution onto polyethylene powder.
case then the early stages of the adsorption would be expected to conform to the Longmuir-Schaefer relation (25), r
=
6
i O-~.TI/z,SEC~12
8
I0
12
through the origin. T h a t this was approximately the ease for the adsorption of N a P S E M (Sample I) is clear from the lineality of the plots shown in Fig. 7. However, the failure of the lines to pass through the origin indicated that Eq. [1] was not strictly valid for large molecules as it is for small ones. Plots similar to those in Fig. 7 were also obtained using the adsorption data of Samples II and III. ADSORPTION STATICS
V 40 x I05
0
4
FzG. 7. The dependence of the polymer adsorption on the square root of the adsorption time.
FIG. 5. Empirical relation between iv] and the adsorption characteristics of NaPSE1Vi.
61
2
[q
Here £ is the amount adsorbed per unit area of adsorbent; C, the bulk concentration of the adsorbate; D, the diffusion coefficient of the adsorbate; and t, the adsorption time. According to this relation at a given adsorbate concentration and molecular weight, a plot of the amount adsorbed against the square root of the adsorption time should yield straight lines passing
All equilibrium adsorption studies were carried out at 25°C in 0.8 M NaC1 solution. This latter NaCl concentration was essentially equivalent to the one at which the limiting [~] of N a P S E M was reached, i.e., where maximum coiling of the polymer chains was indicated. All adsorption measurements were made after the adsorbent had been in contact with the polymer solution 172 ± 0.5 hr. ~ On the basis of adsorption kinetics results, this was more than sufficient time to establish equilibrium. The equilibrium adsorption isotherms of the three polymer samples are shown in Fig. 8. The desorption isotherm shown in Fig. 9 illustrates that the adsorption was essentially completely reversible when the experiments were carried out by replacing the 2 The quantity of adsorbent used (4 gm/liter) was the same as in the adsorption kinetics experiments.
Journal of Colloid and Interface Science, Vol. 37, No. 1, S e p t e m b e r 1971
150
GREENE
SAMPLE E : o
~
10
"rr" : .
o-
10 2
~ 6 g iO I
2
0
--
L
0'.o Ce,
/
I
I
,.0
1.4
i0 °
meq / l i t e r
f
10 4
FIG. 8. Isotherms for the adsorption NaPSEM on polyethylene powder at 25°C.
I
I
IIIIII
I
I
I
IIIIII
10 5
I
I
I
10 6
of Fro. 10. The dependence of maximum adsorption on NaPSEM molecular weight.
lZ o
8 t~ m
g G m o 4 ~7- A d s o r p t i o n
~ 2
o
T
0 0
0.2
n
-
I
~
I
I
I
I
0.4
0.6
0.8
i.e
1.2
1.4
Ce,
1.6
Meq/liler
FIG. 9. Equilibrium adsorption--desorption isotherm of N~PSEM (Sample II).
residual polymer solution by deionized water as described earlier. On the other hand, desorption of the polymer from the PE powder into aqueous NaC1 solutions was found to be negligible. From the adsorption isotherms given in Fig. 8, it is seen that the affinity of the adsorbent for the polymer (~s measured by the initial slopes of the curves) increased somewhat with increasing polymer molecular weight. However, the maximum amount adsorbed at saturation (on a molar basis) was essentially independent of polymer molecular weight. This latter w~s also found in the adsorption kinetics study. Now when the maximum adsorption (N8.2 meq/100 gm) was expressed as gr~m of polymer chains adsorbed, it was found to increase with polymer molecular weight as shown in Journal of Colloid and Interface Science,
V o l . 37, N o .
1, S e p t e m b e r
Fig. 10. It was also found to be considerably more than could be accommodated in a monolayer (approximately 40 times more if it is assumed that each polymer segment having a molecular area of ca. 40 As is in contact with the adsorbent surface). Both these latter results gave some clues as to the structure of adsorbed NaPSEM on the adsorbent surface. For it is usually found when nonporous adsorbents are used that the molecular weight dependence of the maximum adsorption is in accord with the empirical equation (20-23, 26, 27) A mox = K M ~,
[2]
where M is the molecular weight of the polymer; and K and a are constants. If the polymer in the extended configuration were to lie parallel to the adsorbent surface until adsorption was complete, then the number of polymer chains adsorbed would decrease as M increased. However, the amount adsorbed on a gram basis would be independent of M (i.e., a = 0 in Eq. [2]). On the other hand, if the polymer in a coiled or looped configuration were to be attached at only one point on the adsorbent surface, the number of polymer chains adsorbed would be independent of M. However, the amount adsorbed on a gram basis would increase with increasing M (a = 1). It would, no doubt, be fortuitous if either of these extreme eases represented the actual state of 1971
ADSORBABILITY OF A S Y N T H ETI C POLYELECTROLYTE
mum adsorption in mg/gm. Values of Ra calculated in this manner as well as values of R~ calculated from viscosity results of the polymer samples in 0.8 M NaC1 solution are listed in Table IV. Comparison of these values reveals that R~ was always much smaller than Ra. This suggested that there was considerable compression of the polyelectrolyte coils upon adsorption. It is conceivable that this could have occurred because the concentration of Na + ions per polymer chain was considerably higher iD the adsorbed state than in solution. However, this is not clearly understood.
the adsorbed polyelectrolyte here. However, it is obviously the latter case which is more compatible with the adsorption and viscosity results reported here for NaPSEM. Therefore, it can be said that NaPSEM in a more or less compact randomly coiled conformation was attached through only a few of its segments to the surface of the PE powder. On the basis of the above analysis, it is reasonable that a monolayer consisting of randomly coiled polymer chains was formed at adsorption saturation. If it is assumed that this was approximately the case then a comparison can be made between the radius of the adsorbed polymer coils, RA , and the radius of gyration of the free polymer coils, RG (determined from viscosity measurements). RA in angstr6ms can be calculated from the equation (27), R~ = (SM/6.023 7rA)~2,
TABLE IV COMPARISON OF THE DIMENSIONS OF ADSORBED AND
FREE
NaPSEM
Sample
10-6 My
RA (A)
RG (A)
I I2 III
2.40 0.40 0.035
62.8 25.6 7.6
662.0 296.0 82.6
SAMPLE 1.0
CONFORMANCE OF THE ADSORPTION STATICS TO LANGMUIR ADSORPTION THEORY
[31
where S is the specific surface area of the adsorbent in m2/gm; M, the molecular weight of the polymer; and A, the maxi-
151
The adsorption isotherms of the three NaPSEM samples (Fig. 8) were of the Langmuir type. An attempt was, therefore, made to fit the data using the Langmuir adsorption equation, abCe A - 1 + bC~"
[4]
Here A is the amount adsorbed per gram; C~ is the bulk concentration of the adsorbate; and a and b are constants. The Langmuir plots of the data of the three polymer samples are shown in Fig. 11. The lines in these plots are seen to be linear except in the concentration region where saturation is approached. Thus the adsorption of NaPSEM like that of many uncharged polymers conformed reasonably well to the
[ : o ]1: A ~D. •
<" .6
'o .4
.2
0
I 5
110
1T5
2PO
1/C e , l i t e r / m e q
FIG. 11. Langmuir plots for the adsorption of NaPSEM on polyethylene powder. Journal of Colloid and Interface Science.
¥ol. 37, No. 1, September 1971
152
GREENE TABLE V
THE DEPENDENCE OF THE CONSTANTS a AND b IN TIIE LANGMUIR ADSORPTION EQUATION ON N a P S E M MOLECULAR WEIGHT Sample
My"10-6
a (meq/gm
a (liter/meq)
I II III
2.40 0.40 0.035
.29 ± .29 ± .27 ±
.73 ± .53 ± .42 ±
.02 .02 .01
.05 .04 .02
Langmuir equation. Theoretically, the Langmuir type isotherm is expected for polymers (28) when only a few of the chain segments are attached to the adsorbent surface and the remainder are looped up away from the surface toward the continuous phase. Still the validity of the Langmuir isotherm implies monolayer adsorption. According to Silberberg (29) this holds true even for polymeric adsorbates. However, it must be assumed, as was done here, that the occurrence of the Langmuir type isotherm for polymers means that at saturation a monolayer of polymer coils is formed. With this interpretation in mind, values of a and b in the Langmuir equation have been calculated for each of the polymer samples. If these constants were to have the same meaning here as they have when small molecules are the adsorbates, then a should be proportional to the area of an adsorbed segment at the completion of the monolayer; and b should be a measure of the energy of adsorption. The data listed in Table V show that the value of the constant a was essentially the same for all three polymer samples as was expected on the basis of the adsorption results in Fig. 8. However, the value of the constant b was found to increase with polymer molecular weight. This indicated, as was mentioned before, that the affinity of the polymer for the adsorbent increased with polymer molecular weight.
main impetus for the enhanced polymer adsorption observed here as it led to a decrease in (i.) the electrostatic repulsion between similarly charged adsorbing and already adsorbed polymer molecules and (i.i.) the density of adsorbed polymer segments on the P.E. powder. V. SUMMARY The kinetics and statics of the adsorption of a synthetic polyelectrolyte, NaPSEM, from aqueous salt solutions onto a PE powder have been examined. It was found that at a given salt concentration, the adsorption on a molar basis was essentially independent of polyelectrolyte molecular weight; and could be essentially completely reversed. On the other hand, at a given molecular weight, both the rate and extent of adsorption were found to increase with increasing salt concentration. Parallel viscosity measurements showed that this latter result could be correlated with changes in the conformation of the polyelectrolyte in solution (i.e., changes in iv]). In general, adsorption was found to be greater the lower iv]. It could, therefore, be deduced that NaPSEM in a more or less compressed coiled conformation was weakly attached to the adsorbent surface. The adsorption of the polyelectrolyte from aqueous salt solutions was shown here to be similar to that of many uncharged polymers with regard to its (i) dependence on molecular weight; (ii) dependence on the solvent power of the medium; and (iii) conformance to the Langmuir adsorption theory. VI. A C K N O W L E D G M E N T S Acknowledgments are due to Mr. D. A. Kangas from whom the polymer samples were obtained; to Dr. James P e t e r s for the sample of the polyethylene powder; and to Dr. Wilfried Heller for a m o s t informative discussion. REFERENCES
IV. M E C H A N I S M OF NAPSEM A D S O R P T I O N
As stated earlier, the primary effect of the added salt was to reduce the net charge of the polymer. Thus as' the salt concentration was increased, the polymer approached the isoelectric state (minimum solubility) and assumed a more compact randomly coiled conformation. This seemingly provided the
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Journal of Colloid ancl Interface Science,
Vol. 37, No. 1, September 1971