Biosensors and Chemical Sensors - American Chemical Society

Chapter 23

Water and the Ion-Selective Electrode Membrane D. Jed Harrison, Xizhong Li, and Slobodan Petrovic

Downloaded by UNIV LAVAL on September 21, 2015 | http://pubs.acs.org Publication Date: April 23, 1992 | doi: 10.1021/bk-1992-0487.ch023

Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada

Water uptake in plasticized polyvinylchloride based ion selective membranes is found to be a two stage process. In the first stage water is dissolved in the polymer matrix and moves rapidly, with a diffusion coefficient of around 10 cm /s. During the second stage a phase transformation occurs that is probably water droplet formation. Transport at this stage shows an apparent diffusion coefficient of 2 x 10 cm /s at short times, but this value changes with time and membrane addititives in a complex fashion. The results show clear evidence of stress in the membranes due to water uptake, and that a water rich surface region develops whose thickness depends on the additives. Hydrophilic additives are found to increase the equilibrium water content, but decrease the rate at which uptake occurs. -6

-8

2

2

Ion selective membranes are the active, chemically selective component of many potentiometric ion sensors (7). They have been most successfully used with solution contacts on both sides of the membrane, and have been found to perform less satisfactorily when a solid state contact is made to one face. One approach that has been used to improve the lifetime of solid state devices coated with membranes has been to improve the adhesion of the film on the solid substrate (2-5). However, our results with this approach for plasticized polyvinylchloride (PVC) based membranes suggested it is important to understand the basic phenomena occurring inside these membranes in terms of solvent uptake, ion transport and membrane stress (4,6). We have previously reported on the design of an optical instrument that allows the concentration profiles inside P V C based ion sensitive membranes to be determined (7). In that study it was shown that water uptake occurs in two steps. A more detailed study of water transport has been undertaken since water is believed to play an important role in such membranes, but its exact function is poorly understood, and the quantitative data available on water in P V C membranes is not in good agreement (8-10). One key problem is to develop an understanding of the role of water uptake in polymer swelling and internal stress, since these factors appear to be related to the rapid failure of membranes on solid substrates. By adding water sensitive dyes to the P V C based membranes we are able to determine the distribution of water as a function of position inside the membrane, measured from the water interface, as a function of time (7). We present here a

0097-6156/92/0487-0292$06.00/0 © 1992 American Chemical Society

In Biosensors and Chemical Sensors; Edelman, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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HARRISON ET A L

Water and the Ion-Selective Electrode Membrane

293

detailed analysis of the transport of water which shows that water uptake occurs in two stages at very different rates, and that this is related to a phase transformation inside the membrane. Surprisingly, when the dye added is a hydrophilic salt the rate of water diffusion during the initial wetting process is an inverse function of the dyes concentration. Internal stress as a result of water uptake is demonstrated by the fact that there are two stages of water uptake, the diffusion constant for the second, slower process is a function of time, and the rate of evaporation of water is faster than the rate of uptake. The presence of a water rich surface region is also demonstrated.


Experimental A diagram of the instrument used to probe the internal concentration profile of a P V C based membrane is shown in Figure 1. A pulsed N2 pumped dye laser and two HeNe lasers (633 and 544 nm) are used as sources. The magnification power of the optics is 7 to 20 times. Further details of the instrument design are presented elsewhere (7). The cell dimensions in the figure are exaggerated for clarity. The optical path length is actually quite short, 0.15 mm, while the membrane thickness in the direction of transport (vertical direction in the figure) is in the range of 0.8 to 1.5 mm. The membranes studied were composed of 33% P V C , 66% dioctyladipate (DOA), and several other additives such as valinomycin, KB(C6H5)4 (KTPB), anhydrous C0CI2, and 2,6-diphenyl-4-(2,4,6-triphenyl-N-pyridinio)phenolate (Et30) in varying proportions up to 1%. Results and Discussion Figure 2 shows the uptake of water into a membrane containing 0.05% C0CI2 after a short period of time. The water enters from the right edge and evaporates into the atmosphere at the left edge. The decrease of absorbance observed in the bulk of the membrane arises from bleaching of the dye on reacting with water. At the surface the absorbance increase corresponds to the growth of light scattering centers in the membrane as the concentration of water increases, leading to an increase in absorbance. This effect indicates a phase transformation has occurred inside the polymer. It is most likely that this is a phase separation due to the formation of water droplets, as has been observed in other hydrophobic polymers in which salts or fixed charge sites are located (77). The light scattering centers penetrate much more slowly into the membrane than the water associated with bleaching of the dye. The phenomenon shown in Figure 2 may be understood in terms of there being at least two chemical states of water within the membrane. The first rapid ingress of water is due to the uptake of water that becomes dissolved in the membrane matrix. The second, slower step is associated with separation of the water phase into droplets. The processes can be modeled using Fick's laws of diffusion, and by separating the two stages in the analysis. A closed form solution is obtained if the diffusion coefficients are assumed to be described by a step function at the boundary between regions. Solutions are presented in a number of sources (12,13). If the data is analyzed at the early stages when semi-infinite boundary conditions are applicable then diffusion in the bulk region can be expressed by equation 1 (12,13). Ai=Ai

max

[l-erf(x/V4Dït)]

0)

where A i is the decrease in absorbance due to bleaching of the dye. A i x is the apparent maximum change in absorbance (75) due to the equilibrium concentration of water, erf is the error function, χ is the distance from the edge of the membrane, D] is the diffusion coefficient for the dissolved water, and t is the time. m a


294

BIOSENSORS AND CHEMICAL SENSORS

Cell


Diode Array

toi'm

(633 nm)

(

7to20X

m

y

ϋ

·-

•oTn2

J86.7L I mm Γ

f

1

f

2

0.1-0.3 mm

f

3

• Interface

Figure 1. Optical arrangement to probe water distribution inside membranes. The magnification is 7 to 20 times.

Figure 2. Absorbance profile inside C0CI2 containing membrane during first 60 min. Water enters from the right and evaporates at the left edge. Smooth curves are fits to Eq 1 for dye bleaching.


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HARRISON ET A L

295

The second stage of uptake associated with the onset of light scattering can be described by equation 2. A +A =A 2

s

2 m a

x

-B erf(x/V3Dtf)

(2)

where A is the absorbance increase due to light scattering alone, A is the maximum value of A , and D is the diffusion coefficient associated with the apparent motion of the light scattering centers. It must be recognized that the scattering centers themselves are not moving, but H 0 molecules move from scattering site to site at this apparent rate. Referring to Figure 3, - A is the maximum decrease in absorbance that arises due to bleaching. It is smaller than A i since the total concentration of soluble water in the membrane exceeds the dye concentration, so that the dye is essentially saturated. The critical water concentration at which light scattering begins to occur, Ck, is different than this value, and the point at which this occurs in the membrane can be approximated as a moving boundary that occurs at position xk, which extends more deeply into the membrane with time. The parameter Β is a complex function of D i , D , Xk and t and has a value close to 1 when D i and D are very different. A detailed derivation and treatment of this function is given in ref. 13. The model curve in Figure 3 clearly resembles the data in Figure 2. To treat the experimental data the bleaching is first analyzed in the bulk region according to eq. 1. The absorbance due to light scattering alone is then obtained by correcting for the absorbance due to the dissolved water and for A . From this Xk is obtained, and the diffusion coefficient for light scattering, D , is estimated using eq. 2. Eq. 1 assumes no concentration perturbations occur near the surface, but this is not the case since the water concentration continues to change once light scattering centers begin to form. To correct for this the bleaching at xk is determined, Ai(xk), and this value is taken as unchanging in time. Then as the light scattering front moves deeper into the membrane A i is allowed to increase so that Ai(xk) is constant at the new value of Xk. A n equivalent correction is obtained by shifting the value of χ = 0 inwards and holding A i constant as xk shifts. Values of D i , D , and xk are then recalculated until the analysis is self-consistent, which usually requires one or two iterations. This method is clearly an approximation since the original boundary conditions did not allow for A x to change over time. 2

2 m a x

2

2

2

s


m a x

2

2

s

2

m a x

m a x

2

2 m a

Effect of Dye Concentration on Diffusion. Several curve fits resulting from these calculations are shown in Figure 2. For the C o C l dye the value D i decreases as the dye concentration increases, as is shown in Figure 4. The value in the absence of dye can be estimated by extrapolation to be 1 χ 10" cm /s, decreasing to 0.2 χ IO" cm /s at a concentration of 1.2 wt%. This result is surprising as adding a hydrophilic dye will increase the water uptake of a membrane, and it is unexpected that the rate of that process would decrease. It may result from the fact that the dye binds water to it and these hydrophilic sites must be saturated before water can pass deeper into the membrane. Whatever the cause, the results show that improving the rate of membrane wetting and water equilibration may not be achieved by what seems the most obvious approach. When the less hydrophilic dye Et30 is added to a membrane an absorbance profile similar to that in Figures 2 and 3 is obtained, since this dye also bleaches in the presence of water. For the initial uptake of dissolved water a diffusion coefficient of 1 to 2 χ IO" cm /s is obtained, indicating the validity of the extrapolation for the C o C l dye. The formation of light scattering centers is observed in membranes containing only P V C and DOA, and with the additives valinomycin, K T P B , C o C l , Et30 or 2

6

6

2

2

6

2

2

2




296

Figure 4. Diffusion coefficient vs concentration of C0Q2 (wt%) in membrane for the first rapid diffusion step (bulk region).


23. HARRISON ET A L


297

some combination of these. Although the effects are not identical the differences are a matter of degree, as suggested in Figure 5, showing a dependence of D on the additive and its concentration. The behaviour of membranes with C0CI2 added has been examined in the most detail, and is presented here. The transport behavior associated with the light scattering centers depends on C 0 C I 2 concentration and time. The diffusion process is psuedo-Fickian. The maximum concentration expressed as A2max increases with time, while the value of D2 decreases with time at low dye concentrations, and increases with time at higher concentrations. In Figure 5 the water is penetrating from the left edge and the magnification is increased so that only a portion of the membrane is visible. It can be seen that the water has penetrated a much greater depth into the membrane after 24 h for the membrane with a higher C0CI2 concentration. Figure 6 shows that as the concentration of dye increases the apparent rate of diffusion of the light scattering centers decreases, when measured 10 minutes after exposure to water. The same is true after 20 min. or 1 h. However, if the extent of penetration of this water rich region is determined after about 24 h the trend is reversed, as seen in Figure 6. At low dye concentration the light scattering centers remain fixed in a region near the surface, and D 2 decreases to less than 10" cm /s. At concentrations of 0.4% or more the water rich region continues to penetrate the membrane and D2 increases with time. It should be noted that eq. 2 is derived using assumptions that are clearly poor approximations for this complex system. In fact the surface concentration represented by A2max increases with time, and D 2 is likely to be a function of concentration, time or position in the membrane. These factors mean the magnitudes of D2 calculated here may not be accurate, but they should reflect the real trends in D2 with C0CI2 concentration. The behavior of water in the membrane once the phase transformation is induced is complex, but can be understood in the following general terms. When the concentration of water reaches a critical value droplet formation will occur. At this point the polymer begins to be stressed due to volume changes and the diffusion coefficient for water transport decreases. {The dependence of diffusion rates on stress in polymers is a well recognized phenomenon (13-15).} This would predict that at short times both D i and D2 should follow the same trend with dye concentration, and this is the case as shown in Figures 4 and 6. At low dye concentration the increasing stress with water uptake causes a further decrease in D2 until the penetration of water is essentially halted. At high concentrations of the salt C0CI2 the increased hydrophilicity of the membrane overcomes the effect of stress and so it continues to absorb large amounts of water in the bulk region. The net result is that adding a hydrophilic salt to the membrane does increase the uptake of water as would be expected, but at the same time the initial rate of uptake is decreased.


2

9

2

Effect of Indicator Dye Selected. The surface region that can be seen in Figure 5 has a width that depends on the dye concentration and the additive itself. For 0.05% C0CI2 this region is about 90 μπι thick, while for the zwitterion Et30 or the hydrophobic NO2" carrier bromo(pyridine){5, 10,15, 20 tetraphenylporphyrinato}cobaltate it is about 50 μπι thick. For a standard K+ sensitive membrane containing valinomycin and K T P B this water rich region is about 20 μπι thick. The trend observed within these compounds is a decreasing thickness with decreasing hydrophilicity of the additive. Figure 7 shows the light scattering in a valiomycin membrane with KTPB after 40 h of exposure to water. Weak light scattering is observed in the central region, while a large change is seen at the edges. Developing this distribution requires about 24 h, and it is stabilized within 40 h, indicating the transport process is as slow as with the water sensitive dye additives. The most interesting feature is shown by the



298


60.0

120.

180.

240.

300.

360.

DISTANCED) Figure 5. Absorbance profile in membrane after 24 h exposure to water with a) 0.5% C o C l , b) 0.05% C0CI2 , c) no additives. 2

Weight % of C o C I

2

in membrane

Figure 6. Diffusion coefficient vs concentration of C0CI2 (wt%) in membrane for the second step associated with light scattering. The values depend on time, and are shown for the same membranes after 10 min. and 24 h.


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299


1.10

DISTANCE

Figure 7. Light scattering profile in valinomycin membrane after 40 h equilibration with H 2 0 on both sides. A decrease in absorbance with time is then initiated when water is removed from the right side. changes that occur at one side of the membrane after the water is removed at that interface. The water rich region at the surface disappears very rapidly. Further, the decrease in light scattering in the bulk of the membrane appears as a moving front, rather than as a Fickian diffusion process distributed across the entire bulk region. Only an hour or so is required to reverse the effects of 24 hours of water uptake. This effect has not yet been analyzed in detail. However, such differences in ingress and loss of solvent from a polymer matrix are well known and arise from changes in the diffusion coefficient (13-15). In this case the data indicate the diffusion coefficient increases as the water content decreases. This is consistent with the measurements presented above, and also with the hypothesis that there is stress in the membrane following significant uptake of water. Summary Based on our analysis of the water uptake phenomena in P V C based membranes we can make several statements about the behavior of water. (1) There must exist at least two chemical states of water inside the membrane. This is inferred from the fact that two stages of water uptake occur, and that there is a phase transformation leading to the formation of light scattering centers. Although not proven, we take these centers to be water droplets. There is a vast difference in the rates of penetration of these two types of water into the membrane. The first, dissolved form enters rapidly with a diffusion coefficient of about 10~ cm /s, while the second form associated with the phase transformation shows a complex dependence on time and additive concentration, but is in the range of 10 cm /s initially, and this decreases with time leaving a water rich surface region. (2) Stress develops within the membrane with the uptake of water. This is demonstrated by the fact that the diffusion coefficient for the second stage of water uptake changes in time, and by the differences in the rate of loss and entry of water into the membrane (Figure 7). The stress field inside may explain the development of a water rich surface region over time, instead of what would be the expected uniform distribution. (3) The nature and concentration of membrane additives strongly affect the behavior of water. Even for the first stage of water uptake that involves dissolved 6

2

-8


2

300



water, increasing the concentration of the hydrophilic salt C0CI2 has an effect. The rate of diffusion is seen to decrease as more dye is added. The formation and distribution of a water rich surface region in which light scattering centers form depends on the additive and its concentration. It appears that the more hydrophilic the additive the greater the depth this surface layer penetrates into the membrane bulk. Also, at least for C0CI2, the initial rate of water uptake decreases as the dye concentration increases. This effect is inverted after long equilibration times, and essentially means that the water rich region penetrates throughout the bulk of the membrane. Consequently, adding hydrophilic dye has the expected effect of increasing the water content, but its effect on the rate of uptake may be undesirable for fast wet-up times. Acknowledgments. We thank the Natural Sciences and Engineering Research Council of Canada, and Ciba-Corning Diagnostics for financial support of this research.

Literature Cited 1. Janata, J. In Solid State Chemical Sensors; Janata, J., Huber, R. J., Ed.; Academic Press: London, 1985; Chapter 2. 2. Blackburn, G.; Janata, J. J. Electrochem. Soc. 1982, 129, 2580-2584. 3. Satchwell, T.; Harrison, D. J. J. Electroanal. Chem. 1986, 202, 75-81. 4. Harrison, D. J.; Cunningham, L. L.; Li, X.; Teclemariam, Α.; Permann, D. J. Electrochem. Soc. 1988, 135, 2473-2478. 5. Moody, G. J.; Thomas, J. D. R.; Slater, J. M . Analyst 1988, 113, 1703-1707. 6. Harrison, D. J.; Teclemariam, Α.; Cunningham, L. L. Anal. Chem. 1989, 61, 246-251. 7. Li, X.; Petrovic, S.; Harrison, D. J. Sens. Actuat. 1990, B1, 275-280. 8. Thoma, A. P.; Viviani-Nauer, Α.; Arvantis, S.; Morf, W. E.; Simon, W. Anal. Chem. 1977, 49, 1567-1572. 9. Morf, W. E.; Simon, W. Helv. Chim. Acta 1986, 69, 1120-1131. 10. Marian, S.; Jagur-Grozinski, J.; Kedem, O.; Vodsi, D. Biophys. J. 1970, 10, 901-910. 11. Southern, E.; Thomas, A. G. ACS Symp. Ser. 1980, 127, 375-386. 12. Buck, R. P.; Berube, T. R. J. Electroanal. Chem. 1988, 256, 239-253. 13. Crank, J. The Mathematics of Diffusion; Oxford Press: Oxford, 1956. 14. Petropoulos, J. H.; Rousis, P. P. J. Membrane Sci. 1978, 3, 343-356. 15. Rudolph, F.; Peschel, G. Ber. Bunsenges. Phys. Chem. 1990, 94, 456-461. RECEIVED October 22, 1991


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