ELECTROKINETIC PROPEHTIES OF CELLULOSE
655
ELECTROKINETIC PROPERTIES .4SD SURFSCE CONDUCTIVITY OF CELLULOSE AND OXYCELLULOSE, WITH REFEREXCE TO THE CARBOXYL GROUP CONTENT GERTRUDE RABISOV
AND
E. HEYMAXS
Cheinistry Department, Cnztersity of illelbourne, Melbourne, Australia Received M a y 1 1 , 1943 1. INTRODUCTIOl’i
The presence of carboxyl groups in cellulose and oxycellulose has been confirmed by a number of investigators. Purified cotton moo1 contains about one carboxyl group per three hundred glucose residues, whereas oxycellulose may contain one carboxyl group per fifty glucose residues. In view of the relatively small number of these groups, \re cannot assume nith certainty that all, or even the majority, of the ions nhich constitute the diffuse double layer in a cellulosewater interface originate from the dissociation of carboxyl groups. However, these groups may be expected to be a contributing factor to the f-potential. .In investigation on the electrokinetic properties of cellulose of varying carboxyl content was therefore undertaken. The method employed n as the streamingpotential method due to Briggs (3), who \vas the first worker to take the surface conductivity into account. 11. EXPERIYEKTAL
The cell employed (diameter of the center cell 1.5 cm., distance between the electrodes 3.3 cm.) was narrower than that used by Briggs. It was found easier to pack the cellulose evenly in a cell of the above dimensions than in a wider cell. The electrodes were plates of pure platinum, 2 mm. thick, into which holes were drilled. The streaming potential was measured with a Cambridge valve potentiometer. This instrument measures the E.M.F. with an accuracy of about 0.2 millivolt, which is quite sufficient in view of the uncertainties introduced from other sources (see later). The resistance was determined by a Wheatstone net, using Tinsley inductionless shielded resistance boxes and a Sullivan triple range ratio arm and Wagner earth. The resistance was measured during streaming, Le., in the same condition as the streaming potential. The alternating current (-1000 cycles) was supplied by a valve oscillator. -4certain amount of polarization cannot be avoided during the streaniingpotential measurements by the Briggs method, as indicated by a residual potential between the electr&s, whose magnitude and sign varied without any evident rule. This residual potential \v&s determined before and after each streaming potential and resistance measurement and mas subtracted from the experimental potential. With the platinum electrodes employed in this investigation it was usually not greater than a few millivolts. To calculate the conductance of the cell filled with cellulose it was necessary to determine the “cell constant” for each filling. This mas done after each experiment, using potassium chloride solutions of various concentrations.
656
GERTRUDE RABINOV AND E. H E Y M A ”
Very little variation of the cell constant with concentration was found (usually below 1 to 2 per cent); hence extrapolation was generally not necessary (N/10 potassium chloride taken a~ standard). The samples under investigation were purified cottons, cotton wool, oxidized cellulose, mercerized cellulose, and a number of samples of regenerated ceIlulose (artificial silk). For methods of preparation and purification see reference 13. In most cases “hydrogen celluloses”, Le., celluloses containing free carboxyl groups were used; the metal ions were removed by the acid and distilled water treatment described previously (13). In a number of experiments, however, metal ions (e.g., calcium) were purposely introduced (e.g., by leaving the hydrogen cellulose in contact with concentrated calcium acetate solution and subsequently washing with distilled water). The carboxyl group content (acid value) was in all c a m determined by the calcium acetate method (13). The distilled water employed had a specific conductivity of about 1.5 X 10-’ mhos.’ In some experiments it was freed from carbon dioxide and the streaming potential measured with exclusion of carbon dioxide. No change in the experimental potential was found; it is, however, possible that cellulose contains adsorbed carbon dioxide which may be difficult to remove. 111. THE EVALUATION O F (-POTENTIAL AND SURFACE CONDUCTIVITY FROM THE EXPERIMENTAL DATA
The (-potential is calculated using the well-known Helmholtz-Smoluchowski equation, as modified by Briggs : ~ r = 4_D.~-rEK P
where 7 is the viscosity, D the dielectric constant (SO), P the pressure under which the liquid is forced through the diaphragm, E the streaming potential, and K the effective specific conductivity of the cellulose diaphragm soaked with liquid, which thus includes bulk conductivity and surface conductivity. This equation will not hold when the capillary diameter, or its equivalent in a diaphragm, falls below a certain limit and comes into the order of magnitude of the thickness of the double layer (4, 8, 17, 22). However, apart from this, there are indications that the Briggs equation may not be the correct function between and surface conductivity. I t appears that, in systems which possess a high conductivity, the Briggs equation yields too low values for (vide infra; cf. also reference 6). The values obtained for depend to a considerable extent on the evenness and tightness of packing of the cellulose. In extreme cases, the results may show variations of about kt10 per cent, even when equal amounts of cellulose are used.
r
r
* Our cellulose easily picks up ions from water. If several liters of distilled water, showing no positive reaction for copper (diethyl dithiocarbamate test), are filtered through a small pad of cellulose, a positive test is obtained in the pad. Obviously, the water contained copper in an amount below the sensitivity of the test, and the metal was concentrated in the pad during filtration.
ELECTROKINETIC PROPERTIES OF CELLULOSE
657
The difficulties of reproducing results are greater with distilled water than with salt solutions, and appear to increase with increasing purity of the water. Apart from irregularities, observed by Briggs, when the cellulose is too loosely packed, we have observed further irregularities when the cellulose is very tightly packed. The (-potential found with very tightly packed diaphragms is lower than that found with diaphragms of medium tightness of packing, and decreases during the experiment. In one experiment the rate of flow fell from 20 ml. per minute ( p = 10 cm.) to 1.6 ml. per minute ( p = 10 cm.) after 2.5 liters of water were passed through the’diaphragm, and did not assume a steady value on further streaming. At the same time, E/P,which at constant resistance is proportional to (, fell from 10.2 to 5.1, whilst the resistance of the cell remained constant. The rate of flow remained low after the streaming was interrupted and the experiment started again. This showsthat the observed effects are mainly dueto an increasing amount of felting of the cellulose; a retardation of the flow rate due to a back E.M.F. may also occur, but is an effect of a smaller order. The low values for ( in the case of tightly packed diaphragms may in part be due to a breakdown of the Briggs equation when the “capillary diameter” is reduced. It is conceivable that in a tightly packed cotton wool diaphragm in distilled water, the average distance between the fibers may be of the order of magnitude of the thickness of the double layer. This may also account for the irregularity of the ( values. When, afterwards, an electrolyte solution is passed through the diaphragm, with concomitant reduction of the doubIe-layer thickness, the ( values become much more reproducible. Moreover, the rate of flow of an electrolyte solution streamed through a tightly packed diaphragm is somewhat larger than that of distilled water. The reduction of the double-layer thickness by the electrolyte apparently reduces the effect of a back E.M.F. on the rate of flow. Two important factors which may cause, in any diaphragm of fibrous material, the (-potential as calculated by the Briggs equation to be lower than the true (-potential have recently been pointed out by Bikerman (7) : (1) In the case of surfaces which are not smooth, the flow of liquid streaming between the microscopical “protuberances” of the surface is less rapid than that calculated from hydrodynamic equations postulating an ideally smooth surface. (8) In the case of solids possessing a fibrous structure and immobilizing water (swollen memavailable for ionic migration (and hence deterbranes), the cross section (8%) mining the conductivity of the system) is greater than the cross section (S1) available for the streaming liquid. Hence the right-hand side of the Briggs equation has to be multiplied by S2/S1, which is always greater than unity. Bikerman’s factor S Z / S may, ~ in part, account for some of the irregularities mentioned above. Slight variations of packing may produce an appreciable variation of S2/S1, and hence explain partly the difficulties in reproducing results. Moreover, with increasing tightness of packing, &/S1 will increase and hence ( aa calculated by the Briggs equation will be smaller than the true (. It is possible to estimate Sz/S1 roughly from the specific volume of cellulose (0.647, according to reference 12) and the volume inprease on swelling in water
658
GERTRUDE RABINOV AND E. HEYMANN
(cotton, 40-45 per cent; regenerated celluloses, 60-70 per cent, according to reference 11). With diaphragms of medium tightness, Le., 1.5 g. of cellulose in our 5.85 ~ mcell, . ~St/S1 is estimated as of the order of 1.1. If the packing is tighter, e.g., 2.5 g. in 5.85 ~ m .SS/& , ~ rises to about 1.2. Thus the factor &/SI does at first sight not seem to account for the large drop of f on streaming large quantities of water through a tightly packed diaphragm. However, “felting” which takes place during this process may immobilize quantities of water much in excess of those immobilized by actual swelling, and thus produce a considerable increase of Ss/SI. Iimemaru and coworkers (14) have observed a decrease of with increasing time of contact bet’ween cellulose and water which they regard as an indication for a progressive hydration of the cellulose (cf., however, Bikerman (7)). W e are inclined to interpret the drop of the apparent f-potential as being due to an increasing amount of felting, rather than to actual solvation. With diaphragms of medium tightness, v e find no variation exceeding =k6 per cent during the first’ 3 to 4 days. As our experiments were not usually extended over longer than about 4s hr., no such time effect need be taken into consideration, since the variations with different fillings of the cell exceed this amount. After longer periods we generally find a decrease in It is evident from the foregoing considerations that absolute values for f are unlikely to be obtained by the streaming-potential method. However, for the system cellulose-water this method (or the equivalent elect,roosmosis) is the only method that is experimentally feasible. Results of comparative significance can be obtained only by careful standardization of the experimental conditions,2 and the results obtained have to be discussed in conjunction with the modifying factors mentioned above. In this investigation, the amount of cellulose in t’hecell was always kept near 1.5 g., in order to have as little as possible variation of tightness of packing. Great pains were taken to pack the cellulose evenly in the cell. The constancy of E / P R ( R = resistance) with varying pressure ( P ) ,which is postulated by the Briggs equation, provided we are dealing with Newtonian flow, proved t o be a good criterion for even and unvaried packing. Experiments which showed a drift in E / P R mere discarded. The most reasonable way of expressing the surface conductivity would be as
r
r.
K
- Kb
__ , where
K = C/R, Kb is the specific conductivity of the liquid in bulk, a is the a effective surface area per gram of cellulose, C is the cell constant, and R is the observed resistance. However, since a is unknown, and cannot easily be estimated, the density of packing ( d ) was substituted for it, Le., the weight of cellulose per milliliter of diaphragm space: KI
=
K
- Kb d
I _
is thus the surface conductivity per unit weight. It is a measure for the surface conductivity per unit effective surface only if the specific effective surface K*
2
This was early appreciated by Briggs and Gortner and coworkers.
ELECTROKINETIC PROPERTIES OF CELLULOSE
659
is the same in all cases. The procedure is quite permissible when the surface conductivity of the same diaphragm in relation to the electrolyte concentration is investigated. On comparison of various celluloses, however, a possible variation of the specific effective surface must be considered. K~ varies very little with varying tightness of packing. IV. RESULTS AND DISCUSSION
A . Systems involving distilled water The results are summarized in table 1. A is the acid value in milliequivalents per gram of cellulose; for hydrogen celluloses A is also the carboxyl group content; in all the other cases it is the free carboxyl hydrogen. W is the weight of the cellulose diaphragm, dried at 100°C. All the other symbols, defined previously, refer to 20°C. The samples listed under (a) in table 1 are celluloses containing free carboxyl groups (“hydrogen celluloses”). The reproducibility of f in all systems involving distilled water is poor. Nevertheless, it may be seen that there is a general trend towards lower f with increasing A value, Le., increasing carboxyl group content. On the other hand, there is a marked increase in surface conductivity with increasing A value. It appears reasonable that even if the f-potential is determined only in part by the dissociation of the carboxyl groups of the cellulose, the f-potential should increase with increasing surface density of the carboxyl groups. This effect will be modified by a simultaneous decrease of the double-layer thickness (owing to the increase of charge density); nevertheless, it is surprising that a decrease of should result. It may be argued that the f-potential may not be determined at all by the carboxyl groups; in this case, however, the increase of surface conductivity with increaeing carboxyl-group content would be difficult to understand except on assumptions different from those made usually (el. section B, end). The decrease of f is very unlikely to be explained by Bikerman’s factor &/&. From the scanty knowledge about the swelling of cellulose and oxycellulose (19), there is little indication that &/SIshould differ appreciably in the two cases; nor is it likely that inhomogeneities of the surfaces should falsify results to a different extent in the two cases, as the oxycelluloses are prepared from the same cotton wool. There is, however, a possibility that f is in error, in the systems with high acid value, because of an inadequacy of the Briggs equation mentioned above. It has been found with systems in which the surface conductivity is high, e.g., parchment paper of high acid value (2), that .( calculated by the Briggs equation has extraordinarily low values. According to G. M. Willis of this Department, parchment papers have A values of the order of 35 to 45 X These are higher than the A values of cotton wool, but lower than those of most oxycelluloses. The surface conductivity of the parchment papers has not been determined. It appears reasonable to assume that the surface conductivity of a parchment paper is higher than that of cellulose, even if the acid value were the same, since the acidity of the
660
GERTRUDE RABINOV AND E. HEYMANN
E -
P
IN mLLICELLWLOSES
?;pZN.x x 10'
A X'O1
Kb
x
10'
XI
MHOS
B O S
x 106 MHOS
TIMETER OF MERCURY
~ ~ _ _ _ _ _ _ _ _ ( a ) Hydrogen celluloses: Cotton D . . . . . . . . . . . . . . . . . . . . . . . .
4.2
1.37 1.52
86 73
4.05 4.4
1.54 1.60
11 37 11 ' 34
Cotton B . . . . . . . . . . . . . . . . . . . 5.6
1.47 1.26
78 97
4.15 3.65
1.50 1.51
11 10
34.5 37.5
Cotton 8 . .. . . . . . . . . . . . . . . . . . . .
8.3
1.30
99
3.80
1.50
10
38
Cotton wool . . . . . . . . . . . . . . . . . . . 20.5
1.60 1.59
25 27.2
9.9 : 1.56 10.2 1.52
31 33
1.25 1.33
1 19.4 18.3
10.2 11.1
1.45 1.50
;;
1.52 1.50
78 66
Oxycellulose E . . . . . . . . . . . . . . . . 52.2 Oxycellulose -4. . . . . . . . . . . . . . . . 79.0 Oxycellulose D . . . . . . . . . . . . . . .104.8
Cotton
wool,
hydrogen-satu-
1
1
1.36 15.5 1.30 114.9 1.61 9.0 1.55 10.2
19.2 15.7
'
24.1 18.4
~
.
1.50 1.48
~
1 ,
84 66
.
1 ~
26.5 29
21 21.5 31 25 23 20
ELECTROKINETIC PROPERTIES OF CELLULOSE
661
former is partly due to SOsH groups which may be assumed to be more strongly ionized than COOH groups. Partial substitutionSof hydrogen by calcium (in the calcium celluloses, table 1 (b)) or by sodium (in the mercerized cellulose, table 1 (c)) causes a slight decrease of [. Regenerated celluloses give small values for [. This may in part be due to a larger factor &/SI, as the regenerated celluloses show stronger swelling than native celluloses (cf. reference 11). Most investigators of surface conductivity in the past have been concerned with systems containing electrolytes, and often relatively inert surfaces, such aa glass. The effects are generally interpreted as being due to an ionic atmosphere which derives its constituents mainly from the solution. In the system cellulose-water, the surface contains dissociating carboxyl groups. A rough estimate, based on our previous computation of an acid dissociation constant of cellulose (13), shows that the dissociation of the carboxyl groups may yield sufficient hydrogen ions to account for the experimental surface conductiirity. The surface conductivities are better reproducible than the r-potentials, except for some of the oxycelluloses, where duplication may be more difficult because of the inhomogeneity of the samples. There is a marked increase of K* with increasing carboxyl-group content ( A value) among the hydrogen celluloses. Although the increase of K~ with increasing A is very plausible, caution must be exercised. K~ is defined per unit weight; on the other hand, the specific effective surface may vary for various samples. Nevertheless, the oxidized celluloses have all been prepared from cotton wool. Although during oxidation the fibers may break and become shorter, this need not involve an appreciable increase in the effective surface.4 Table 1 shows an effect which points very clearly to the carboxyl hydrogen as a factor contributing strongly to the surface conductivity in distilled water, Partial substitution of carboxyl hydrogen by calcium (with a corresponding decrease of the acid value) leads in all cases to a reduction of the surface conductivity. In the case of cotton wool K* X IOe drops from 32 to 20; with oxidized cellulose C drops from 72 to 41. This may be qualitatively explained by the fact that the mobility of the calcium ion is smaller than that of the hydrogen ion. A similar effect is shown with mercerized cellulose where the product containing free carboxyl has a surface conductivity ( K J of about 30, whereas the ordinary mercerized cellulose, which has most of its carboxyl hydrogen substituted by sodium (low acid value), has a surface conductivity ( K ~ of ) 21, probably due to the mobility of Na+ being smaller than that of H+.6 These observations The hydrogen is only partially substituted, a s A is.not zero For the method of substitution, see section 11. A comparison of K~ of the regenerated celluloses with t h a t of the native cellulases is not feasible, because of the extreme difference of texture and specific effective surface, 6 In addition, the structure of the surface layer has t o be considered. Since salts of carboxylic acids are more strongly ionized than the acids themselves, there may be more ions in the diffuse part of the double layer in the case of the calcium and sodium celluloses than in the case of the hydrogen cellulose. This would counteract the mobility effect; although the “free” mobility of H’ is greater than t h a t of +Caw or Na+, there will be less H+ present
662
GERTRUDE RARINOV AND E. HEYMANN
on the change of K~ with substitution of carboxyl hydrogen by calcium or sodium are not affected by any uncertainty due to the fact that K* is defined per unit mass. As the substitution is carried out on the same sample of cellulose, we are concerned with the same effective surface. It is interesting that there i s apparently no relation between surface conductivity and [-potential in distilled water. All the mathematical relations between [ and gs (4,5, 6, 10) postulate that these two quantities vary together. There is, however, a possibility that the parts of the double layer which determine K~ and those which determine [ are not identical. This point will be discussed further in the next section. N x ‘01
2
I
3
14
5
Fro. 1. Plot of {-potential against concentration (in N X lo4)for several electrolytes. a, sulfuric acid; b , hydrochloric acid; c , potassium nitrate; d , sodium chloride; e, calcium chloride and barium chloride; f , thorium nitrate; g, lanthanum nitrate.
B. Systems involving electrolyte solutions Figure 1 shows the p-potential of hydrogen cellulose (cotton wool) in a number of electrolyte solutions plotted against the concentration. The curves are similar to those obtained by Briggs and by Bull and Gortner (9) with purified wood celluloses and filter papers. Our values are higher than those of the other investigators; moreover, we find a stronger sign reversal in the case of thorium nitrate. It is unlikely that these curves should be strongly in error by neglect of Bikerman’s factor SJS1 or of surface roughness, as these are little influenced
r
in the diffuse part of the double layer. On the other hand, there are indications t h a t the calcium- and nodium-substituted celluloses behave as “insoluble salts,” as i t is very difficult to hydrolyze the metal ions away completely by treatment with water (13). In this case, the metal ions would be expected to be mainly in the fixed part of the double layer; hence a very great reduction of x I will result when hydrogen is replaced by calcium or sodium.
663
ELECTROKINETIC PROPERTIES O F CELLULOSE
at the very low electrolyte concentrations under investigation. Our curves for alkali chlorides, like those of many other investigators of various systems, show a maximum at low concentration. Many authors, in particular Bikerman (5, 7) and A. J. Rutgers (18),suggest that this maximum may be spurious and may not be found if the true ( is measured. Bikerman ( 5 ) originally suggested that the experimental maximum may be due to the fact that, when the surface conductivity is determined with high frequency A.C. (in our case 1000 cycles), only part of it may be measured, and as a consequence, too low values for ( would be obtained in systems in which the surface conductivity is a large fraction of the total conductivity, i.e., at low electrolyte concentration and in distilled water. In Bikerman’s quantitative theory of surface conductivity, it is assumed that the surface conductivity is composed of a term depending on electroosmosis (first investigated by Smoluchowski),and a term depending on the ionic mobilities; at 1000 cycles we may be above the dispersion region with regard to the electroosmotic term. It may, however, be shown that in order to make the maximum disappear in our potassium chloride curve, the total surface conductivity would have to be at least twice as high as that determined experimentally at 1000 cycles. It is therefore possible to test this point in a semiquantitative manner by measuring the resistance of our system containing distilled water or very dilute electrolyte solution first with 1000-cycle A.C. and afterwards with ordinary 50-cycle A . C . and vibration galvanometer. The latter arrangement is not very sensitive in our system of high resistance, but preliminary experiments have shown beyond doubt that there is no difference in resistance exceeding a few per cent between the experiment at 1000 cycles and that at 50 cycles. Hence, the electroosmoticterm is either very small, or its dispersion region is above 1000 cycles. Urban, Feldman, and White (20) came to a similar conclusion with regard to the electroosmotic term of the surface conductivity in Pyrex slits. Rosenhead and Miller ( 5 ) , in a mathematical analysis, found that the dispersion region is at frequencies higher than lo6cycles. At any rate, there is little doubt that the total surface conductivity has been measured in our arrangement as well as in those of other investigators. Since the maximum in the (-C curve for potassium chloride can thus not be explained by a dispersion of the surface conductivity, and since a possible slight variation of &/St (see above) is unlikely to account for it,6we are faced with the alternative of either regarding the maximum as real, or assuming that the experimental ( is too low at lorn concentrations and in distilled mater, owing to a possible inadequacy of the Briggs equation referred to previously; this mould make itself felt to an increasing extent at low concentrations, when the contribution of the surface conductivity to the total conductivity is high. The latter possibility can be substantiated only after further theoretical research regarding the Briggs equation. The lowering of the {-potential of cellulose by lanthanum and thorium salts (with sign reversal in the case of the latter) is, in accordance with the assump-
-
6 We also satisfied ourselves t h a t the maximum is not due t o an exchange reaction between hydrogen cellulose and the salt (cf. 13). Curves obtained with hydrogen cellulose and with sodium-substituted cellulose are almost identical
664
GERTRUDE E A ~ I S O V . \imE. HEYMAXN
tions of other Xvorkers, due to preferential adsorption of cations? The position is, however, not so clear in the case of salts of uni- and bi-valent cations, even if it be assumed that the maximum in the r-C curve is spurious. In figure 2 the surface conductivity is plotted against concentration. An expression
-
Ke Ku ___
u+v
where K, = ( K - K& ( c j . section 111, end) in distilled water and u and u are the mobilities of cation and anion of the salt, may be called the reduced surface conductivity. This notation renders possible a comparison of the contribution of various electrolytes to the surface conductivity, almost independent of their individual ionic mobilities. It is seen that the curves (figure 3) of the reduced surface conductivity of various electrolytes of the same valency type almost coin-
200
KSx Id 100
J
0
I
3
2 Nx
4
5
104
FIG.2. Plot. of K~ X IOa against concentration. a , sulfuric acid; b, hydrochloric acid; c, potassium nitrate; d , sodium chloride; e, calcium chloride and barium chloride; f , thorium nitrate; g, lanthanum nitrate.
cide. If the surface Conductivity is regarded as being due to the ions in the mobile part of the Stern double layer, the above expression may be regarded as a measure of the inequality of adsorption of the ions of opposite sign. In first approximation it may also be regarded as an estimate of the charge density in the mobile part of the double layer (cf. also the calculations of Cole (10)). The salts furnishing tervalent and quadrivalent cations cause a lowering of the surface conductivity (K*) at low concentration (figure 2). This is understandWith lanthanum nitrate, and to a lesser degree with thorium nitrate, rapid changes of Possibly in these solutions we are not merely concerned with ion adsorption and a change in charge density b u t , in part, with an actual deposition of hydrolysis products. A suggestion that the hydrolysis products and not the ions may be partly operative in the surface discharge by adsorption may also be seen in the fact t h a t the quadrivalent and hexavalent ions of complex cobalt salts lower the {-potential of glass less than the tervalent and quadrivalent lanthanum and thorium ions (15). 7
r occurred in very dilute solutions, which impaired the accuracy.
665
ELECTROKINETIC PROPER7iES OF CELLULOSE
able, as the p-potential is simultaneously decreased, although the shape of the p-C curve is not easy to interpret. More experiments will be necessary with high-valent ions. In the case of uni-univalent and bi-univalent electrolytes K~ as well as ‘ 3 u+u show a marked increase with the bulk concentration (C) of electrolyte (in equivalents per liter). This increase is slightly less with the bi- than with uni-univalent electrolytes, which presumably means that the inequality of adsorption is less with bi- than with uni-univalent electrolytes, or, in other words, there may be more ion-pair adsorption in the first case than in the second. It is not easy to reconcile the increase of surface conductivity with the simultaneous decrease of r-potential. If the decrease of p is attributed to a preferential adsorption of cations on the negatively charged surface, a simultaneous decrease, and not an
U‘ v
0
I
2
3
4
5
Nx IO4
FIG.3. Plot of ‘C against w concentration for several electrolytes. a, sulfuric acid; b, u+fJ hydrochloric acid; c , potassium nitrate; d , sodium chloride; e , calcium chloride and barium chloride; f , thorium nitrate; g , lanthanum nitrate.
increase of the surface conductivity would be expected, if we assume that the surface conductivity is determined by the diffuse part of the double layer. On the basis of the latter assumption, the increase of surface conductivity and charge density of a negatively charged surface on addition of electrolyte may be understood, if the primary process is assumed to be preferential union adsorption. The simultaneous decrease of f (with or without initial maximum) would have to be interpreted as being due to a reduction of the thickness of the double layer on addition of electrolytes. That this occurs at lower concentration with bithan with uni-univalent salts would be understandable on the basis of the ionic strength principle. The fact that hydrochloric acid lowers the f-potential more strongly than sodium chloride may be explained on the assumption that a common-ion effect of hydrochloric acid on the ‘Ldissociation”of the hydrogen cellulose is superimposed on the other effects.
666
GERTRUDE RABINOV AND E. HEYhUNN
If the hypothesis of preferential anion adsorption be accepted as a working hypothesis, the surface conductivity is then mainly determined by the cations in the diffuse part of the Stern double layer. Neglecting the electroosmotic term of the surface conductivity, and also the interionic forces in the diffuse double layer, we may write K,
- K,
= const. X C, X u
where C, is the average excess cation concentration in the diffuse double layer and u the mobility of the cation. Hence
K
- Kui , which is proportional to C.,
-!--U
may be regarded as a truer measure for the surface charge density than =.
U S V
0
I
2
3
4
5
Nx IO‘
FIQ.4. Plot of
against concentration. a , sulfuric acid; b, hydrochloric acid; c,
potassium nitrate; d , sodium chloride; e, calcium chloride and barium chloride.
In figure 4, ‘S is w plotted against the bulk concentration of th2 electrolyte U
(in equivalents per liter). The general shape of the curves thus obtained is not unlike the charge density curves of Abramson, calculated from I-potential data. Like Abramson’s curves (1) they may be regarded as adsorption curves, if the above reasoning is permissible. The curves for acids are below those for salts. This is compatible with the assumption made previously that the large influence of acids on the {-potential is partly a result of a repression of ionization of carboxyl groups. Preferential anion adsorption on negatively charged surfaces in order to explain f-C curves has been suggested previously. Nevertheless, a weak point in this hypothesis is the primary process. It is feasible that an inert surface may be negatively charged in an electrolyte solution, because anions, owing to their greater polarizability, may be more strongly adsorbed than cations. However,
ELECTROKINETIC PROPERTIES OF CELLUMBE
667
after a certain amount of anion adsorption has occurred, further selective adsorption will be resisted by the electrostatic repulsion between them and the negatively charged surface. Preferential anion adsorption will occur on the negative cellulose surface only as long as the charge density is low. Because of this difficulty the mechanism outliped above may have to be abandoned. On the other hand, if, in accordance with many workers in the field, the assumption is made that the lowering of the r-potential on addition of alkali and alkaline-earth salts, as well as of acids, is mainly due to a preferential adsorption of cations with actual surface discharge, this assumption would be very difficult to reconcile with the simultaneous strong increase of the surface conductivity. The dilemma may be resolved by the rather unorthodox assumption that the parts of the double layer which determine the {-potential, and those which determine the surface conductivity, are not identical. If, for instance, we m u m e that adsorbed ions present in the “inner Stern layer”, which cannot be sheared off in a streaming-potential experiment, still possess mobility in an electric field, we could understand why, on addition of electrolyte, the {-potential, determined only by the ions in the outer (diffuse) Stern layer, decreases, whereas the surface conductivity determined by the ions in the inner and outer layers simultaneously increases? It is needless to say that if this assumption were correct, all mathematical theories of surface conductivity would be very incomplete. Nevertheless, a similar assumption has been envisaged by Urban, White, and coworkers (21,23) in their discussions on surface conductivity. Moreover, Monaghan, White, and Urban (16) discuss the possibility that the streaming potential may be determined only by the outer layer, whereas electro&mosis may be determined by both layers. Whatever the final solution of these problems, any theory explaining the processes that occur on an initially negative surface when electrolytes are added will have to explain the simultaneous decrease of {-potential and increase of surface conductivity. SUMMARY
1. Experimental conditions and theoretical considerations affecting the streaming-potential method in the case of cellulose are investigated and discussed. 2. In distilled water, the {-potential decreases and the surface conductivity increases with increasing carboxyl-group content of cellulose. Substitution of carboxyl hydrogen by calcium and sodium lowers the surface conductivity, but affects the r-potential little. A theoretical interpretation of these results is given. 3. The maximum in the I C curve for alkali chlorides is not due to a dispersion of the surface conductivity. 8 Bikerman (J. Chem. Phys. 0,88 (1941)) has recently suggested that the immobile layer in electrokinetics may simply be due to the roughness of all solid surfaces. The independent variation of surface conductivity and I-potential would be compatible with Bikerman’s aasumption, 88 the immobile layer may contribute t o the ionic mobility term of the surface conductivity.
668
GERTRUDE RABINOV AWD E. HEYMANN
4. For uni-univalent and bi-univalent electrolytes, the surface conductivity shows a marked increase with concentration. The 1-C curves, in these cases, are found to be similar to those obtained by Briggs and by Bull and Gortner with different celluloses. The difficulty of reconciling the increase of surface conductivity with a simultaneous decrease of p-potential is pointed out, and trvo tentative explanations suggested. 5 . The investigation is concerned with purified cotton, cotton wool, oxycellulose, mercerized cellulose, and regenerated cellulose. REFEREKCES ic Phenomena, p. 186. The Chemical Catalog Com-
( 2 ) RABOROVSKY, J . , AND BURGL,R . : Collection Czechoslov. Chem. Commun. 3, 563
(1931). D. R . : J. Phys. Chem. 32, 641 (1928). (3) BRIGGS, (4) BIKERMAS, J. J.: 2 . physili. Chem. A163, 378 (1933); 171, 209 (1934); Z.Elektrochem. 39, 526 (1933). (5) BIKERMAN, J. J . : Kolloid-Z. 72, 100 (1935). R O S E K H E ~L., D , AND ~ I I L L E R J., C. P. (with a note by J. J. Bikerman): Proc. Roy. Soc. (London) A163, 298 (1937). (6) B I K E R X ~ N J . , J . : Trans. Faraday SOC.36, 154 (1940). (7) BIKERXAK, J. J . : J. Phys. Chem. 46, 721 (1912). (8) BULL,IT.E., AKD GORTNER, R . A . : J. Phys. Chem. 36, 111 (1932). BULL,H . B., A N D MOYER,I.. S.: J. Phys. Chern. 40, 9 (1936). (9) BULL,1%. B.,A N D G O R T K E R , R . A , : J. I'hys. Chem. 36, 309 (1931). (IO) COLE,K . S.: Physics 3, 114 (1932). (11) COLLINS, C . E . : J.Textile Inst. 21T,311 (1930). For further literature vide K . HESS: Die Chemie der Cellulose, Lpipzig (1928). (12) D.~VIDSOK, C . F . : ,J. Textile I n s t . 18T,175 (1925). K , E.. AND R.~SINOV, GERTRTDE: J. Phys. Chem. 46, 1152, 1167 (1941). Roiloid-Z. 77, 351 (1936). (14) I C s s h a r a ~ uIC.: , Kaxvau.4~v.K . , T A K A D A T., , 451) AK.AIIV.L, K . : Kolloid-Z. 83, 28s (19381, P. C . : Kolloid-Z. 46, 307 (1928). (15) KRCYT,€I. R., ANI) VAS DERWILLIGEZ, (16) A~o.~-.AGH,As, BETTY,WHITE,€I. I,., A N D IJRBAN, F.: J. Gen. Physiol. 18, 523 (1935). (17) REICII.~RDT, 15.: Z. pliysik. Chem. A166,433 (1933). A. J.: Trans. Faraday SOC.36, 69 (1940). (18) RUTGERS, (19) SHEPPARD, S. E., A N D XEFSOME,P. T.: J. Phys. Chem. 33,1817 (1929). (20) CRBAS, F., FELDMAK, S., AND WHITE,13. L.: J. Phys. Chem. 39, 605 (1935). F . , WHITE,IT. L., ASD S m a a s N m , E. A , : J. Phys. Chem. 39,311 (1935). (21) URBAN, F., A N D KRICK,E. T . : J. Phys. Chem. 36, 120 (1932). (22) WHITE:H. L., URBAN, (23) WHITE,H . L., URBAS,F., AND VAN ATTA,E , A , : J. Phys. Chem. 36,3152 (1939).
