Langmuir 1996,11, 1347-1352
1347
Cetylpyridinium Chloride Sorption in an Ion-Exchange Resin-The Case of Sorption Kinetics Associated with Polymer Fracture Efigknia Amorimt and Fernando Galembeck” Institute of Chemistry, Unicamp, CP6154, CEP13083-970 Campinas SP, Brazil
Mauricio Urban Kleinke Znstitute of Physics “Gleb Wataghin”, Unicamp, CP6167, 13083-970 Campinas SP, Brazil Received October 12, 1994@ The adsorption of cetylpyridinium chloride (CPC)from aqueous solutions on a styrenic cation-exchange resin was determined, using dynamic (chromatographic)and static (batch)methods. Cation exchange site occupancy ( 8 ) is less than 1%for short (100 s) contact times of surfactant solution with resin, but 8 approaches unity after 20-30 h, at the highest concentrationsand temperatures used in this work. 8 us time curves are not smooth and show surgesof surfactant sorption, at some points. Microscopicexamination showed that resin particles swell and break down during surfactant sorption,evidencing that the mechanical tensions created in this process are sufficient to induce fracture in highly cross-linked,poorly plasticized particles.
Introduction Surfactant adsorption is a matter of great interest that has been studied by many authors. Many problems of colloid and surface science and technology are strongly dependent on surfactant adsorption, e g . particle formation and stabilization and wetting and spreading of liquids. Particle colloidalstability is in turn relevant to detergency and anti-redeposition mechanisms, the formulation of pigment and pharmaceutical dispersions, agricultural soil conditioning, emulsion polymerization, and flotation. Wetting is related to detergency, dispersability of powders, dyeing, flotation, the use of pesticides and herbicides, printing, and tertiary oil rec0very.l Most practical applications of surfactants are dynamic processes, in which the systems are not a t equilibrium. Finite times are required for surfactant adsorption, which depend upon the shape and size of the surfactant molecule and on the adsorption mechanisme2 The determination of surfactant adsorption kinetics has an intrinsic interest and also provides clues to the mechanism of adsorption as well as to the nature of the participating species; its understanding can help guide equilibrium adsorption ~tudies.~ A recent example of an adsorption study is the work of Kwok et al.,4 who studied static and dynamic adsorption of a nonionic surfactant (Triton X-100) on Berea sandstone. The adsorption mechanism can be described by the hemimicelle model. Maximum adsorption obtained from dynamic tests was consistently lower than that obtained from static tests, and this discrepancy increased as the flow rate was increased. The amount of surfactant adsorbed on sandstone decreased under alkaline conditions and increased as sodium chloride concentration
* Author to whom correspondence should be sent. Permanent address: Department of Chemistry, UFU, Campus Santa MBnica, C P 593, 38406-243Uberllndia MG, Brazil. Abstract published in Advance A C S Abstracts, February 15, 1995. (1)Hough, D. B.; Rendall, H. M. In Adsorption from Solution at the Solid /Liquid Interface; Parfitt, G. D., Rochester, C. H., Eds.; Academic Press: New York, 1983;Chapter 6. (2)Porter, M. Handbook ofSurfactants;Chapman & Hall: New York, 1991;Chapter 4. (3)Frisch, H. L.;Mysels, K. J. J . Phys. Chem. 1983,87,3988. (4)Kwok, W.; Nasr-El-Din, H. A., Hayes, R. E.; Sethi, D. Colloids Surf 1993,78, 193. +
@
0743-746319512411-1347$09.00/0
increased. Couzis and Gulari5did in situ observations on the interaction dynamics and the structure of sodium laurate adsorbed from its aqueous solution onto an alumina surface, using infrared-attenuated total internal reflection spectroscopy. The results of this study show that the adsorption process is not elementary and that the complexity of the mechanism is unique to the system studied. They have also shown that the structure of the adsorbed film and the reversibility of the adsorption process are strongly linked. Dynamic aspects of surfactant adsorption are important, because they determine the time scale of the related phenomena: wetting, spreading, detergency, and others. Beyond that, sorbents are often used in surfactant analysis as well as in their removal from liquid e M ~ e n t s . ~In, ~these processes, it is very important to know which are the relevant time scales, to achieve complete surfactant adsorption. Most surfactant adsorption studies reported in the literature involve the determinations of equilibrium amounts adsorbed8-I3 and the structure and properties of the adsorbing layer a t equilibri~m.l~-~O However, the observation of surfactant sorption associated with polymer swelling and fracture is rare. (Indeed, we did not find any case analogous to the present one, in the literature.) (5)Couzis, A.;Gulari, E. Langmuir 1993,9, 3414. (6)Cheremisinoff, N. P. Applied Fluid Flow Measurement; Marcel Dekker: New York, 1979;p 1. (7)James, A. In Pollution, Causes, Effects & Control, 2nd ed.; Harrison, R. M., Ed.; Royal Society of Chemistry: Cambridge, 1992; Chapter 4. (8)Hanna, H. S.;Somasundaran, P. J . Colloid Interface Sci. 1979, 70,181. (9)Rathman, J. F.;Scamehorn, J . F. J . Phys. Chem. 1984,88,5807. (10)Scamehorn, J. F.; Schecter, R. S.; Wade, W. H. J . J . Colloid Interface Sci. 1982.85. 463. (li) Scamehorn,J. F:; Schecter,R. S.;Wade, W. H. J . Colloid Interface Sei. 1982,85, 479. (12)Scamehom,J.F.;Schecter,R. S.;Wade, W. H. J . Colloid Interface Sci. 1982,85, 493. (13)Timmons, C. 0.; Zisman, W. A. J . Phys. Chem. 1965,69,984. (14)Tompkins, H. G.; Allara, D. L. J . Colloid Interface Sci. 1974,49, 410. (15)Patzk6, A.; DBkiny, I. Colloids S u r f . 1993,71,299;812. (16)Golden, W. G.; Snyder, C. D.; Smith, B. J . Phys. Chem. 1982, 86,4675. (17)Allara, D. L.;Nuzzo, R. G. Langmuir 1986,1,52. (18)Malbrel, C. A.; Somasundaran, P.;Turro, N. J.J . Colloid Interface Sei. 1990,137,600. (19)Kung, K. S.;Hayes, K. F. Langmuir 1993,9, 263. (20)Trompette, J.L.;Zajac, J.;Keh, E.; Partyka, S. Langmuir 1994, 10,812.
0 1995 American Chemical Society
1348 Langmuir, Vol. 11, No. 4,1995
Amorim et al. Time 1000
0
2000
(5)
4000
3000
2ooo
.
stop
1500
Figure 1. Schematic view of the apparatus used in the adsorption measurements by using the chromatographic technique: (a) pump; (b) valve injection; (c) column; (d) UV photometric detector; (e) capillary; (0balance; (g) Plexiglas cover; (h) microcomputer.
In this work, we describe the time-dependent sorption of the cationic surfactant cetylpyridinium chloride (CPC), in a styrenic cation exchange resin, and the associated sorption-dependent polymer fracture. Two types of adsorption experiments were done: chromatographic runs and static experiments. For chromatographic runs,21in which a known amount of surfactant is injected in an aqueous stream and passes through a bed of the exchange resin, the effluent concentration is determined by using both tensiometric and photometric detectors. The former is a balance, adapted to on-line determination of surface tension by the dropweight method. These experiments are suitable for the determination of the adsorbed amounts, at short surfactant-substrate exposure times. In static the surfactant aqueous solution is added to the resin and the supernatant concentration is determined, after given times. By using both methods, we can cover surfactant-resin contact times ranging from ca. 1min to many days, thus covering a broad time span.
Experimental Section Materials. CPC (Hopkin &Williams) solutions were prepared using deionized water. The ion exchanger was a cross-linked sulfonated polystyrene (Lewatit S-100 from QEEL, Si50Paulo, CA); prior to use, it was activated with 6% hydrochloric acid (Merck). The capacity of cation exchanger was determined volumetrically via an ion-exchange reaction.23 The result thus obtained is 4.70 mequiv HVg of dry resin. The apparatus used in the chromatographic experiments is presented in Figure 1; it is built around a FPLC Pharmacia chromatograph, containing a LCC-500 Plus control unit, two high-precision P-500 pumps, a UV-1 photometric detector ( A = 280 nm), an automatic MV-7 injection valve, and a homemade PTFE column (15 mm long, 2 mm i.d.), for the resin. The tensiometric detector is a capillary which delivers drops to a polyethylene container on a Mettler AE 163 analytical balance connected to a PC 386 microcomputer through an ME338750 interface. The balance stands on a vibration-free table under a tight Plexiglas cover, to eliminate air vibrations. The weight of the container is measured in real time during the experiment, with a 1Hz frequency of acquisition. Approximately 23 mg (on a dry resin basis, weighted to 0.01 mg) of activated sorbent was manually packed within (21) Sharma, S. C.; Fort, T. JR. J.Colloid Interface Sei. 1973,43,36. (22)Attwood, D. and Florence A. T. Surfactant Systems, 2nd ed.; Chapman and Hall: New York, 1985; pp 20-21. (23) Fisher, S.; Kunin, R. Anal. Chem. 1955,27, 1191.
1
1050
.
.
.
.
.
6
1100
.
.
.
1640 1150
T i m e (s)
Figure2. Atypical output of the drop-weightdetector. Weights of the drops are obtained from the step heights, as shown in the magnifiedview (b). Curve a givesthe balanceoutput before, during, and after the passage of a surfactant zone.
the PTFE column, in each run. The whole system was connected via 0.5 mm i.d. Teflon tubing. Methods. (i) ChromatographicRuns. The sorbent was equilibrated with deionized water by pumping the latter through the column for 30 min. The flow rate was 0.1, 0.2, or 0.3 mumin. Room temperature was kept at 26 f 2 "C. Areas under the chromatographic bands were obtained by numeric integration. Runs were performed a t least in duplicates. We attempted to calculate surfactant concentrations from the surface tension data, using calibration curves obtained by pumping surfactant solutions of known concentration through the detector and using the applicable corrections. These corrections are needed because the drop weight in a flowing system depends not only on (dynamicor time-dependent)y but also on hydrodynamic effects. McGee2* found an empirical correlation (eq 1) between drop weight and drop formation time (eq 1)
M(t) = Me
+
(1)
where M(t)is the weight of a drop, for which the formation time is t , and Me and S are the linear and angular coefficients. Drop weights were thus measured for CPC solutions of uniform concentration, at various concentrations and flows, and used to determine the coefficients in eq 1. However, this did not give correct values for the surfactant concentrations within the surfactant zones, in the chromatographic experiments. For this reason, we further calibrated both the tensiometric and the photometric detectors with pulses of surfactant solution, of known concentration and volume, injected in the stream of water. By using this procedure, excellent agreement was obtained for the amounts of surfactant in the zones, as determined by both methods. (In a forthcoming paper, we will give a detailed report on this topic.) Real-time mass recording of the container in which the liquid drops (from the capillary tip) gives a series of steps (see Figure 2); the height of each step is the mass of a (24) Miller, C.; Neogi, P. Interfacial Phenomena: Equilibrium and Dynamic Effects; Surfactant Sci. Series; Marcel Dekker: New York, 1985; Vol. 17, p 172.
Cetylpyridinium Chloride Sorption
Langmuir, Vol. 21, No. 4, 1995 1349 1 . 2 [ . ' . " " ' ' ~ ' " ' ~ ' ' " ~ " ' ' ~ 1.0
0.020
0.8 0.015
0.6
m 0.4
0.010
0.2
0.005
0.0 20
10
0
30
40
50
0.000
1
0
2
3
Volume
4
/
5
0
/ h
Time
ml
1.2
: b 1.0
rl
I
/'
fi
2
fi
/ '
/'
._........1 .................................. ......
....... L,.,"" I._._ ................ .... 7 ........ &.-.e
0.8
70
I.'
/'
-8
:
e...
0.6 0
0.4
-.-.-
......
.........
0.2
: : 0.5 mM : 4mM
2 mM
0.0
0 0
1
2
3
4
5
e
Volume / ml Figure 3. CPC zone detection: a, photometric; b, drop-weight; Open circles, injected zone (0.35pmol); closed circles, the same but a h r passing through the ion-exchange resin bed (5-100 Lewatit, 23 mg).
10
Time / h Figure 4. Time dependence of CPC adsorption on S-100 Lewatit: (a) effect of temperature (initial amount of CPC, 50 pmol within 100 mL solution, added to ca. 11 mg resin); (b) effect of concentration (2' = 25 "C).
(K-l)
1/T
drop. To improve the precision of data collection, step heights were obtained in the following way: the linear equations for the plateau lines seen in Figure 2 were calculated, and the vertical distances between two consecutive horizontal lines were thus evaluated. The transient in the response of the balance between consecutive plateaus lasts 1-3 s, and the distance between plateaus was taken in the middle of this period. Typical mass us time curves are shown in Figure 2, in which we can see the result of injecting a zone of CPC solution in a water current. When the injected CPC solution attains the detector, the step height decreases. This is observed in the middle of the upper curve in Figure 2. In the bottom of Figure 2 is shown a zoomed step in the mass us time curve. The surface tension of the flowing solution is calculated from the drop weights (eq 2):
0.003
,
0.3
#.*
c
,'/
0.0
.
,
.
.
0.004 ,-Q
,
I
I
*
0
where m is the mass of the drop, g is the local gravity, r is the radius of the capillary, and f is an empirical correction function which depends on the ratio of r to the cube root of the volume of the drop. (ii) Static Adsorption Runs. In these experiments, resin and CPC solutions were placed within capped glass test tubes, each containing a known amount (ca. 11mg) of the activated resin and 100 mL of aqueous CPC solution (0.5,
.
,
, 45OC
I
2mf
.
.
11
0.4 0.5
0.1
y==-
50
40
30
20
2000
Time
4000
/
6000
s
Figure 5. (a) Initial CPC adsorption rates (from 100 mL, 0.5 mM aqueous solution) at 5 , 25, and 45 "C and (b) Arrhenius plot from data in part a. 2, and 4 mM). The tubes were mounted on water baths, were they could be tumbled, at a low rate (25 rpm) for the
1350 Langmuir, Vol. 11, No. 4, 1995
Amorim et al.
a d
Figure 6. Optical micrographs of S-100Lewatit resin particles. (a)original particles, activated and dried; (b)particles as observed after 24 h immersion within a 2.0 mM CPC solution at 25 "C, in a tumbler; (c) the same as b, but using a 10 mM NaCl solution.
required time intervals, at 5, 25, and 45 "C. The experiments were terminated by decantation of the resin, which takes ca. 1 min. CPC concentration in the. supernatant was determined spectrophotometrically in a W-Hitachi instrument, at A = 258 nm. All samples and measurements were done in duplicates. Surface site coverage 8 was calculated as follows:
e=
adsorbed mmol CPC resin mequiv H+
(3)
Results . Chromatographic Experiments. Figure 3 shows chromatograms of CPC injected in the presence and absence of sorbent and detected photometrically as well as tensiometrically. These curves show that CPC adsorption is essentially irreversible (this means that desorption is negligible in this time scale) in this resin, since no
desorption is observed aRer the passage of the zone of surfactant. Other chromatograms were obtained, a t three and 0.30mumin) and four different flow rates (0.10,0.20, and different amounts of injected CPC (0.064,0.14,0.34, 0.75 pmol). From the detector records we calculated the amount of adsorbed surfactant and the degree of adsorption site coverage, 8, as given in Table 1. On the other hand, times of contact of the surfactant zone with the resin bed can be estimated from the zone volume and the current flow. Results for 8 obtained by using both techniques are in good agreement, and show the following trends: (i) 8 values obtained under these conditions are always very low, amountingto a few tenths of 1%, in many cases; (ii) 8 values obtained for the lowest amounts of injected CPC are nearly independent of flow; in this case, adsorption is almost complete; for the larger amounts of injected surfactant, 8 decreases with flow, indicating that
Cetylpyridinium Chloride Sorption
time of contact of the zone with the surfactant bed is on
the order of magnitude of loz s. 62
I
0
10
5
15
Time
n
m u
.
s a 5 -
u 3
.
:
25
30
35
/ h
0 . 5 mM tumbled 2 m M , under rest
0
'
.
I-
..-
p:
0
10
20
30
40
50
Time / h Figure 7. Time dependence of resin particle breakdown. The abscissa is the ratio between broken (particlefragments)and whole particles, as measured from optical micrographs as those in Figure 6: (a) effect of temperature (initial amount of CPC, 50 pmol within 100 mL solution, added to ca. 11mg of resin); (b) effect of concentration (T= 25 "C). Table 1. Determination of the Degree of Coverage (0) in the Chromatographic Experiments,by Using the T w o Different Detectors flow rate (mumin)
injected CPC hmol)
0.10 0.10 0.10 0.10 0.20 0.20 0.20 0.20 0.30 0.30 0.30 0.30
0.064 0.14 0.34 0.75 0.64 0.14 0.34 0.75 0.064 0.14 0.34 0.75
e x 103 photometric tensiometric detection detection 0.61 1.4 3.5 6.3 0.60 1.3 3.2 6.0 0.60 1.2 2.9 5.8
Static Sorption Experiments. Data for 8 as a function oftime, obtained a t three different temperatures and feed concentrations, are given in Figure 4a,b. A remarkable feature of these curves is that they are not smooth at all. Although there is always a growing trend, the curves present inflexions and steps that are not expected in adsorption or sorption kinetics measurements. As shown in the next section, this is associated with resin particle breakdown. As expected, 8 grows faster at higher temperatures and a t higher surfactant concentrations. Using the 8 growth rate and assuming that sorption sites are uniformly distributed throughout the spherical ion-exchanger particles, we can estimate the rate of surfactant diffusion within the resin. This is done as follows: during sorption, cetylpyridinium ions penetrate a radial distancex, from the surfaces of the particles. This distance is the difference between external particle radius and the innermost radius reached by CP ions, within a given time, t. Surfactant ion diffusion coefficient within the resin particle may be approximately calculated by using Einstein's equation:
-
..
h
b q 4 . 2O :h
20
4 mM tumbled 2 mM tumbled
b
2% : a
Langmuir, Vol. 11, No. 4,1995 1351
0.60 1.4 3.6 6.4 0.60 1.3 3.3 6.1 0.62 1.1 3.0 5.9
surfactant uptake is incomplete if the contact time of resin with the surfactant solution is too short. Note that the
The results thus obtained are in the 10-locm2s-l range, which is of course much less than diffusion coefficients for small ions in aqueous mediaz5(Na+at 25 "C, D = 2 x cmz s-l) or even for the diffusion coefficientz5of Na+ ion a t 25 "C, in a sulfonated styrenic resin (8%DVB, D = 7.2 x lo-' cmz s-l). Initial adsorption rates of CP ions were obtained at three different temperatures (Figure 5). In these experiments, the CP concentration was obtained from an Arrhenius plot, using the initial sorption rates (see the insert in Figure 5). The value thus obtained is 27.5 kJ.mo1-'. Microscopic Observation of Resin Particle Breakdown. The steps and inflexions observed in the 8 us time curves are rather unexpected, for which reason we decided to examine the resin particles in an optical microscope, before and after exposure to surfactant. The new, unused particles are spherical and rather uniform in size, as shown in the micrographs in Figure 6a. On the other hand, many broken particles appear in the resin after surfactant sorption. Some control runs were also performed, using other electrolytes and different experimental conditions. Particle breakdown, as observed in these cases, is shown in Figure 6b. To have a measure of particle breakdown frequency, particles observed in each picture were classified into two groups, whole spheres and broken particles, and the particles within each group were counted. A minimum of 50 random counts were made on each sample, and the ratio ( R )among broken particles and spheres was calculated. R grows with time, as shown in Figure 7. We can observe that it also depends on surfactant concentration and temperature, as follows: at 45 "C, the number of particle fragments increases much faster and reaches much larger values than at either 25 or 5 "C. On the other hand, larger CPC concentrations contribute to increase the rate of particle breakdown. Consequently, there is a qualitative parallel between the growth of 0 and the growth of particle fragments. (25) Helfferich,F. IonEzchange;McGraw-Hill Book New York, 1962; pp 308, 112.
Amorim et al.
1352 Langmuir, Vol. 11, No. 4, 1995
As seen in Figure 7b, particle fracture is more important in the tumbled particles than when particles were immersed in surfactant but at rest. However, there is fracture even in the absence of any external mechanical action on the system. Indeed, particle fracture may be so violent that we have observed “projectile”particles ejected from a bed of resin, right after a change ofthe surrounding liquid medium. This is observed not only with CPC but also when water and aqueous solutions of NaC1, LiCl, and NaOH are placed in contact with dry resin, at near-boiling temperature. This may be related to the breakdown of a few particles, in which larger tensions are frozen. However, a gradual increase in the number of broken particles was only observed in the case of CPC solutions.
Discussion CPC sorption in the resin is slow; the estimated diffusion coeficients are very low, as compared to data in the literature at 25 “C,for liquid or gel media.24 Slow sorption may be understood as follows: the overall ion-exchange process consists in the following steps: (a) ion diffusion within the solution, to the particle surface; (b) cation adsorption at the surface; (c) cation diffusion within the particle; (d) cation binding to anionic sites, releasing H+ ions; (e) H+ diffusion within the particle, toward the particle-solution interface; (0 H+ desorption from the particle; and (g)outward H+diffusionwithin the solution. The apparent activation energy for the sorption process may be estimated by taking initial sorption rates from solutions at the same concentrations and different temperatures. The value thus obtained was 27.5 kJ mol-’. The activation energyz5 of diffusion in standard ionexchange resins is about 25.14-41.9 k J mol-’. The rate of CPC sorption follows a complex pattern, because of particle breakdown. This may be wellunderstood, considering the following: CPC sorption is a diffusion-limited process; as the surfactant penetrates the resin particles, the volumes of the outward shells increase (as seen in the micrographs) and introduce tension in the particles which break down, as discussed in the next paragraph. Consequently, new and clean surface is now offered to the surfactant solution, which allows a surge of surfactant adsorption. For this reason, the adsorption
rate curves are not smooth; their many inflexions are the result of an associated “catastrophic”phenomenon, which is particle breakdown. The shapes of many resin fragments resemble spherical shells, for which the radius of curvature is significantly larger than that of the original resin particles. This suggests that particle breakdown follows these steps: CPC sorption causes a volume increase, in the resin. Dilation of spherical particle shells creates mechanical tension a t the interfaces between swollen and unswollen neighboring regions, within each particle. Tension leads to fracture: shell fragments come out leaving smaller spherical fragments behind. On the other hand, fragments having other shapes are also observed, which (for instance, halfmoon fragments) may arise due to sorption nonuniformity within each particle, creating tangential tensions at the sphere surfaces. It is well-known that sorption may cause large volume changes within a polymer; swellingz5pressures may reach values as large as 1000 atm. However, the observation of polymer fracture associated with swelling or sorption phenomena is rare (indeed, so far we did not find any case analogous to the present one, in the literature). The uniqueness of the findings reported in this article is probably due to one factor: in most cases of sorption in polymers, the swelling or sorbed agent is also a polymer plasticizer; this means that the swelling pressures generated within the system are countered by the elongation of flexible chains. Moreover, at least part ofthe mechanical energy built up within the system due to sorption may be dissipated by plastic deformation and polymer chain flow, within most polymer solids. In the present case, the sorbed CPC does not plasticize the resin sufficiently, which is easily understood: CP ions bind to the anionic sites within the resin and the resulting salts should contribute to the formation of aqueous domains. Electrostatic binding should predominate over hydrophobic binding to the polymer backbone chains. Moreover, CPC does not have the ability to break down the polymer chain covalent crosslinks. Consequently, the resin swells without plasticizing the network, thus leading to particle brittle fracture. IA940795L
