*July, 1954
EFFECT OF PARAFFIN CHAINSALTSON CHARGE ON TEXTILE FIBERS
collected in Fig. 2. If two non-electrolyte molecules combine to form a single molecule, with no change other than the loss of translational entropy, the expected entropy change is -14 e.u. Consequently, if we can first evaluate the entropies of the pair of neutral molecules corresponding to the ions concerned, we need only add their entropies and subtract 14 units to have an estimate of the entropy of the activated complex. Let us consider, for example, the reaction
533
As examples, let us take the reactions OCIoc1-.
+ OCI+ ClOz-
= =
[-cloc1o-l* = CI[-ClOC102-]* = CI-
+ c102+ c103-
which were studied by Foerster and D01ch.l~ For the first reaction, the activated complex is of the type XOz--, which if it were a compact oxy-anion would have ail entropy of -23.5 e.u.; the estimated entropy of the complex is then -23.5 30 = +6.5 e.u., and the experimental value is found to be $8 e.u. For the second reaction, the activated complex NH4+ + OC1- = [NHIOCI]* = NHzCl 3- H20 is of the type XOs--, which if compact would have which has been investigated kinetically by Weil an entropy of - 10.5 e.u. ; the estimated entropy of 30 = +19.5 e.u., and Morris.18 We imagine the activated complex the complex is then -10.5 which may be compared with the experimental to be formed not from NH4+ and OC1-, but from NHP (26 e.u.) and HOC1 (31 e.u.). The estimated value of +21 e.u. It must in all fairness be pointed 31 - 14 out that this comparison of estimate with experientropy of the activated complex is 26 = 43 e.u. The experimental value for the entropy ment is not as good as it appears, because the experiments are of relatively low precision and the of the activated complex is 44 e.u. For those reactions in which the activated com- experimental values cited, though carefully calcuplex is still ionic, we need to estimate the entropy of lated from the original data, may be in error by as an ion which is not monatomic, or even relatively much as five to ten entropy units. To take another example in which the charges compact like the oxy-anions, but in which the charges are comparatively far separated. There are greater, there is the reaction are two methods of procedure here, which are, how- so3-- + s 1 0 3 - - = [--03ssSo~--]*= S*Oa-- + so3-ever, mathematically equivalent. The first, which which was studied by Ames and Willard by means was used successfully in ref. 11 to estimate the of radioactive exchange.20 The estimated entropy entropies of oxy-anions such as Crz07-- and Sz04-of an ion of the type XOe-4 is -64 e x . , so the estiis to estimate the .entropy of each charged end of mated entropy of the complex is -64 30 = the molecule as though it were separate, and then -34 e.n. From the average value of AS* (-33 t o subtract a suitable amount of entropy (say ,14 e.u.), and the entropy of SO3-- (-7 e.u.) and e.u.) for the loss on dimerization. The alternative S,O;l-- (+15 e.u.), the experimental value of the procedure is to estimate the entropy as though the entropy of the activated complex is -25 e.u. ion were a compact oxy-anion, and then to add Though the agreement is not very close, it must some 30 e.u. to make allowance for the greater en- be pointed out that the experiments were carried tropy of the extended ion (which presumably arises out a t ionic strength 2 rather than a t zero ionic from the additional vibration and internal rotation, strength where the estimate is applicable. as well as the increased moment of inertia of the (19) F. Foereter and P. Dolch, 2. Eleklrochem., '23, 137 (1917). molecule as a whole). (20) D. P. Ames and J. E. Willard, J . Am. Chem. Soc., 73, 164
+
+
+
+
(18) I. Weil and J. C. Morris, J . A m . Chem. SOC.,71, 1664 (1949).
(1951).
THE EFFECT OF PARAFFIN CHAIN SALTS ON THE CHARGE ON TEXTILE FIBERS BY J. S. STANLEY The University, Southampton, England Received June 16, 1966
The electrokinetic potential of cotton, wool and nylon in solutions of paraffin chain salts has been determined, using streaming potential measurements and the charge density a t the fiber surface calculated. On wool and nylon, paraffin chain anions are more strongly adsorbed from acid than from alkaline solutions; paraffin chain cations more strongly from :ilkdine than from acid solut,ions. The graph of the change of charge density, Aa, with concentration of paraffin chain salt, resembles an adsorption isotherm, but it has not proved possible to identify A c quantitatively with the amount of paraffin chain ion adsorbed.
Briggs,I Bull and Gortner,2 Rabinov and HeyNeale and peter^,^ used streaming potentials tJostudy the electrokinetic or zeta potential between textNilefibers and aqueous solutions. Although in some cases the results are rather complicated, in ( I ) D. R. Brims, THISJOURNAL, 82, 641 (1928). (2) H. Bull and R. A. Gortner, ibid., 81, 309 (1931). (3) G.Rabinov and E. Heymann, ibid., 41, 655 (1943). (4) 8. M. Neale and R. H. Peters, Trans. Faraday SOC.,41, 478 (1946).
B.
general the potential tends t o decrease as the salt concentration increases. The zeta potential is the difference in potential between the part of the double layer k e d so tightly that it does not move when the solution streams along the surface, and the interior of the solution; its value depends on the charge density a t the surface and on the capacity of the double layer. The capacity increases as the thickness of the double layer decreases, and
J. S.STANLEY
534
since this thickness decreases with increasing concentration of electrolyte in the solution, the zeta potential should decrease, if the charge density remains constant, as the concentration of electrolyte increases. Assuming a Boltzmann distribution of ions in the diffuse double layer, the relation between the charge density u and the potential p is, for a univalent electrolyte5J u =
( 2NkT 7 "' sinh' ie rk ~)
e is dielectric constant, e electronic charge, N number of anions or cations per cc. of solution. Neale and Peters4 showed that the change in p with concentration of salt agreed fairly well with this equation, assuming u constant. Studies of the effect of surface-active solutes on p at fiber surfaces are few, but Neale and Peters4 showed that a basic dyestuff caused the potential of cotton, initially negative, t o become positive; and an acid dyestuff made p more negative. Experimental
The apparatus (Fig. 1) was modified from that described by Lauffer and Gortner.' The fiber was packed in the cell C, 15 mm. long, 6 mm. diameter, internally, between two perforated platinum plates El, Ez, on which was deposited silver and silver chloride (or bromide), to render the plates (electrodes) reversible to chloride (or bromide) ions. The leads, .LI, Lz, from the electrodes were insulated from the water in the thermostat by enclosing in thick rubber tubing. The solutions were forced through the cell by applying a
Vol. 58
measured suction to one of the reservoirs, capacity about 150 cc., the other being at atmospheric pressure. The streaming potential was measured by an electrometer valve circuit, and the resistance of the cell by an audio-frequency bridge. The zeta potential was calculated from the streaming potential H by the equation
P is the pressure driving the liquid through the cell,
q the viscosity coefficient, e the dielectric constant, taken as equal to that for water, K~ the specific conductivity of the solution in presence of the fibre. K~ was determined by measuring the resistance of the cell with the fiber plug in place, both with the solution and with KCl solution of known specific conductivity. The surface conductivity is thus included in K ~ . Measurements were a t first very erratic; this was traced to air bubbles forming on the surface of the fibers, and was cured by boiling the water used for the solution, and cooling under reduced pressure. The fiber plug was loosely packed; previous workers have found that 5 varies if the plug is too tightly packed, probably because some of the capillary channels are not much wider than the thickness of the double layer. The streaming potential was accurately proportional to the pressure ( L e . , to the velocity of streaming) between 0 and 50 cm. of mercury pressure. Cotton was first extracted with chloroform (Soxhlet, 6 hours), then with cold 2% acetic acid, and finally with water. Wool was extracted with light petroleum (40-60°), and with water, the temperature being kept always below 60". Nylon was extracted with chloroform and water. All the fibers were given a final extraction with light petroleum after packing in the cell, to remove traces of grease which might have contaminated the surface when handling. Cetylpyridinium chloride and cetylsodium sulfate were pure specimens provided by Professor Adam; dodecylsodium sulfate was prepared from carefully fractionated dodecyl alcohol (b.p. 102-103", at about 0.03 mm.; m.p. 23.5-23.8'), by action of chlorosulfonic acid in ethers; Na, found 7.94%, theory 7.97%. Dodecylpyridiniunl, bromide was prepared from dodecyl bromide by the method of Knight and S h a ~ . ~ To ensure that the electrodes were reversible, small concentration of chlorides (or bromides) were added to all solutions. These decrease' the critical' concentrations for micelle formation (c.m.c.); and the c.m.c. in the presence of salts was determined approximately by measuring the surface tension (using Sugden's methodlo), and by taking as the c.m.c. the concentration a t which the surface tension becomes practically constant. Table I shows the c.m.c.'s found.
TABLE I Paraffin chain salt
C.m.c., M
Temp.,
OC.
Conon. of added salts
5 X 10-4 55 0.004M NaCl and Cetylsodium NaOH a t pH 11" sulfate 5.8 X 10-8 25 0.004M NaCl and Dodecylsodium NaOH a t p H 11" sulfate 6.8 X 10-4 25 0.001 M NaCl a t CetylpyridipH 6' nium chloride 9 . 3 X 10-8 25 0.001 M KBr at Dodecylpyridip H 65 nium bromide The pH was measured with a glass electrode (Marconi pH meter).
Results
p is negative for cotton, and is decreased continu-
Fig. 1,-Streaming (5)
potential apparatus.
H.A. Abramson, "Electrokinetic Phenomena," Chem. Catalog
GO.. NewYork, N. Y., 1984, p. 110. (6) 8. M. Neale, Trans. Faraday Soc., 42, 473 (1946). (7) M.A. Lauffer and R.A. Gortner, THISJOUBNAL, 48,641 (1938).
ously by increasing concentrations of NaCl (Fig. 2a) ; u is negative and is increased by NaCl until a steady maximum is reached a t about 0.002 M . This increase is probably due to preferential ad(8) A. GrUn and T. Wirth, Bey., 66, 2206 (1922). (9) G. A. Knight and B. D. Shaw, J . Chem. Soc., 682 (1938). (IO) 8. Sugden, ibid,, 27 (1924).
*
EFFECT OF PARAFFIN CHAINSALTSON CHARGE ON TEXTILE FIBERS
July, 1954
535
Figures 4a and 4b show that, at pH 11, wool adsorbs many more dodecyl sulfate ions than does cotton; the maximum adsorption on cotton appears to be reached about 0.001 M , while on wool the adsorption is still increasing a t 0.0025 M . Figure 4c shows that { becomes constant a t con-3.0 .2icentrations slightly below the c.m.c., with cetylg x sodiumsulfate; with both fibers { changes by a -"O;-. nearly the same amount, 7 mv. Figure 4d 80 -20 e shows that cetylpyridinium, below the c.m.c., -1.0 -40 'Q-'3Z requires a very much smaller concentration to 0 b 0 a produce a given change in f than does dodecyl2.0 4.0 6.0 8,010.0 0 2.0 4.0 6.0 8.0 10.0 pyridinium; the ratio of concentrations required concn. X 10-8, llf concn. X 10-8, M . to reduce {, -44 mv. initially, to zero, is about (a) 0) 80. This agrees with Traube's rule, since there Fig. 2.--$ and u on cotton; NrtCl solutions, pH 6.4, 25'. are four additional CH2 groups in the cetylpywhere u is constant must be due to the thickness of ridinium ion, so that the ratio of concentrations the double layer decreasing, so that its capacity in- should be 34 = 81. { again becomes nearly concreases. stant for some distance above the c.m.c.; a t still Figures 3a and 3b show the effect of pH and of do- higher concentrations { diminishes slightly, perhaps decylpyridinium bromide on wool. All concentra- bec,ause the salt concentration is increasing and the tions were well below the c.m.c. (0.0093 M for this thickness of the double layer is decreasing, Au was concentration of added salt). I n the absence of not calculated for the cetyl salts, since above the paraffin chain salt, { is constant between pH 7 and c.m.c. the concentration of ions, N , required for 11.5, indicating that the number of dissociated basic equation 1, is unknown. groups on the wool surface becomes negligible a t pH -5.0 7. Between pH 6 and 3, falls almost linearly with decreasing pH, the iso-electric point being 3.4. -4.0
sorption of chloride ions. Figure 2b closely resembles? quantitatively, the curve of Q against concentration found by Abramson" for graphite in NaCl solutions. The decrease in { a t concentrations
.r(
*n
I : l v i2. ]
-10.0
-30
5 2 e3
-40
4 .B
'2 0
Ir,
-40
gX 22 -2.0
-
.* *n - 5 0 b y , 8.0
aa
6.0
'8
4.0
3
2.0
e *
.e2 -3.0
2
a -60
"*
-40
$
0 1.0 2.0 conon. X 10-8, M
-1.0
3.0
0 1.0 2.0 concn. X 10-8,M
(b)
(a)
- 50
b
-40
4
8
1210
PH
(a)
6
4
2
0 1.0 2.0 3.0 4.0 concn. X 10-8, M .
0)
Fig. 3.-Wool, dodecylpyridinium bromide solutions, 25': (a) variation of with pH; concentration of dodecylpyridinium: ( a )0; (a) 6 X.10-6; ( 7 ) 2 X lov4; (6) (E) 3.6 X 10-8 M ; (b) variation of A u wlth c: 0 , pH 9; pH 7; X, pH 5 ; A, pH 3.
+,
The dodecylpyridinium ion renders the surface more positive there is a definite change in the slope about PH 7, when dodecYlPYridinium ions are present, { is not constant but slowly rises as the alkalinity increases. It seems possible that ion-pairs are formed between dodecylpyridinium and carboxyl ions on the wool surface, and that these tend to be dissociated in the presence of large hydroxyl ion concentrations. The iso-electric point is displaced to more alkaline pH by the dodecylpyridinium ions, no doubt because these ions neutralize many of the COO- ions on the surface. The increase in Au as the acidity decreasks (Fig. 3b) may be due t o the number of dissociated COO- groups increasing between pH 3 and 7 so that the adsorptive capacity of the surface for dodecylpyridinium ions increases in this range of pH. The curves in Fig. 3b resemble adsorption isotherms. (11) H. A. Abramson, Trans. Faraday Soc., 86, 15 (1940).
3.0
i
i
E -40
a
0
*I
+40 +80 0
.-a
.r
-30
0 1.0 2.0 concn. XlO-8, M (c)
3.0
2.0 4.0 6.0 8.010.0
concn.
x
10-8,Y
(4
Fig. 4.-(a) and (b), effect of dodecylsodium sulfate solutions, pH 11, 25', on and AU for wool ( 0 )and cotton (+); (c) effect of cetylsodium sulfate solutions on for wool ( 0 ) and cotton (+), H 11 55'; (d) effect of dodecylpyridinium bromide ( +I a n d cetyjpyridinium chloride ( o ) solutions on cotton, pH 6.4, 25'.
r
r
f for nylon was found to vary with pH in much the Same way as found by Ne& and Peters, except that the isoelectric point was found at pH 2.6 instead of 3.1 and the negative f-potentials were larger than found by Neale and Peters, in alkaline solutions. Figures 5%to 5d show that the adsorption of dodecyl sulfate ion is greater a t pH 3 than a t pH 11; while adsorption of dodecylpyridinium is greater a t pH 11 than a t pH 3. This difference is no doubt due to the sign and magnitude of the charge on the fiber. Adsorption does not reach saturation even at 0.005 M ,
J. S. STANLEY
536
- 100 - 80
Vol. 58
tronic units, except at very low concentrations. This difference may be due in part to the paraffin chain ions displacing some of the ions which im.$f .-02 -8.0 parted the charge to the fiber surface before paraf-60 EX fin chain ions were present; perhaps also to ad** -40 sorption of gegenions on top of the adsorbed r, o -4.0 b paraffin chain ions, and to differences between 220 a the structure of the interfacial films between water, and liquid and solid paraffin. 2*o 3.0 A few experiments were made on solutions of ooncn. X IO-*, M concn. X 10-3,M the commercial non-ionic detergent Lissapol N, (a) (b) which is a condensation product of polyethylene -80 16.0 oxide with an alkylated phenol. At pH 11, 1 is > decreased on wool from -70 mv. to -23 mv., a t jf 120 - 40 quite low concentrations of Lissapol N, about 0 half that (c.m.c. in Fig. 7) a t which the surface $ 8.0 tension of solutions of this detergent become 2r, 0 i s constant. At pH 3 Lissapol N has almost no eftu 40 feet on I . A specimen of nearly pure p-octyl-I40 a phenyloctaglycol ether, supplied by I.C.I. (Dyestuffs) Ltd. gave very similar results at pH 11, 0 1.0 2.0 3.0 4.0 5.0 concn. X 10-8, M concn. X 10-8, M the minimum value of 1 being reached a t about (C) (4 0.0006 M . Fig. 5.-Effect of (a) and (b), dodecylsodium sulfate solutions My thanks are due to the Council of the a t T" 11 ( 4-1and PH 3 ( 0); (C) and (d), d:decylr.wdln!um British Launderers' Research Association for a bromide solutions on and AU for nylon, 25 , a t pH 11 \ a ) maintenance grant, and to Mr. E. W. Balson and and at pH 3 (+). Professor N. K. Adam for valuable suggestions. An attempt was made to estimate whether the change in charge density, ACT,caused by the paraffin chain salts, was quantitatively equal to the number of ions adsorbed. It is not possible to determine .s the number of ions adsorbed on a fiber surface with r , o t +-++--+-+ any accuracy; but it was hoped that, by comparing the values of Au on paraffin wax, and of I?, the sur$40 face excess at the interface between paraffin oil 0 0.05 0.10 0.15 0.20 concn., %. and the aqueous solutions, some information on this point could be obtained. The paraffin wax was cut Fig. 7.--Effectof Lissapol N solutionon r f o r wool, 25": 0 , pH 11; +, pH 3. into lumps 1 to 2 mm. across before packing in the Streaming potential cell. Was calculated from NOTEby Professor N. K. Adam.-The value 3.4 found measurements of interfacial tension, made by the here for the isoelectric p H of wool is equal to the lowest re-
3
s
-
-12.0
0
"3
ported previously; Neale and Peters4 found 3.4 on woolJibers in salt solutions with little buffering 3 power, as used in this work. Harris12 obtained 3.4 % . I: n. on ground wool in buffer solutions containing 12,0'$,. 0 1200 ~20.0 phthalate ions, but later Sookne and Harris's found 12.0 .$g 2 X 8" 4.2 to 4.5 on ground wool or wool cortical cells in 8 0 $ x acetate buffers, ascribing the lower value to adsorp80.0 8.0 f % im. tion of phthalate ions on the ground wool partia83 40.0 cles. Adsorbed ions would displace the isoelectric 40.0 .; .Oa 40.0 40 ! point to a lower pH. Neale and Peters cleaned Q h L a their wool with an anionic detergent; but in this work there seems no obvious source of adsorbed an0 1.0 2.0 3 0 0 1.0 2.0 30 ions since the wool was cleaned with light petroleum. concn. X 10-8, M . concii. X 10-8 M . All these determinations were made by streaming (4 (b) potential or electrophoretic measurements, and reon paraffin wax: fer to the surface, either of the intact fiber or of the pig. 6.-Comparison of r on paraffin with (a) dodecylpyridinium bromide 0 , pH 3 and 11 r on oil; + p H ground WOO1 Particles. Determinations of the 11, A U on wax; X, pH 3, A U on wax. (b) dodecylsodium sujfate z n t ~ n a l soe electric Point by combination with solutions ( 0 )pH 3, A, pH 11, r on oil (both); X, pH 3, A,, on acids, bases, or other ions give results usually at wax; pH 11, A U on wax. least 4.5 and sometimes considerably higher." Whether the low value obtained here and by Neale drop-volume method with a micrometer syringe, and Peters4 is the correct value for the surface of the intact wool fiber, or is due to some adsorbed substance using Gibbs' adsorption equation which lowers isoelectric p H , is difficult to decide with 1 37 certainty; but it seems not unlikely that the keratin r = -(3) molecules are oriented at the surface of the wool fiber so 2RT 3 In c that the surface isoelectric pH is very different from that in 2 being introduced in the denominator because the the interior. 160.0
-.
160.0
4
.z
OI
::
I
.i
+,
-
salt has two ions. Figures 6a and 6b show that the curves of r and Aaigainst concentration are of shape, but that r, measured in ions per 'q* em., is about twenty times larger than A u in elec-
(12) M. Harris, Am. D w W E Reporter. 91, 399 (1932). (13) A. M.Sookne and M. Harris, ibid., 28, 593 (1939). (14) J. B. Speakman and E. Scott, Trans. Faraday SOC.,80, 539 (1934).
.
