412
MELVINA.
COOKAND EUGENEL. TALBOT
In contrast, curve B (Fig. 6b), representing the homologous alkylsuccinic acids, shows very little increase in Bmelt with increasing chain length in going from octylsuccinic to octadecylsuccinic acid. This approximate constancy in angle, despite a ten carbon increase in the length of the aliphatic sidechain, ,indicates that the lateral forces between adjacent alkyl chains do not predominate in the packing of the molecules. The presence of the more bulky succinic acid portion of the molecule apparently limits the closeness of packing of the alkyl side-chains. This suggests an orientation in which the side-chains are oriented normal to the adsorbing surface so that their effective projected areas are less than those of the succinic acid groups. Acknowledgments.-The authors gratefully acknowledge the assistance of Mr. J. G. O’Rear and
VOl. 56
Mr. N. L. Smith of this Laboratory in the preparation and purification of a number of the compounds studied. The recent observations of Mr. E. M. Solomon, also of this Laboratory, that the advancing hydrophobic contact angle on films of stearic acid or octadecylamine adsorbed on platinum from cetane solution is 102” and the adoption of the resulting technique of increasing the size of the test drop of water to guarantee obtaining an advancing contact angle proved particularly valuable in this investigation. The decylsuccinic and octadecylsuccinic anhydrides were generously donated by Dr. George H. von Fuchs, Consulting Chemical Engineer. T h e phenylbutyric acid was kindly supplied by Dr. Abraham Schneider, Chemistry Department, Harvard University.
“7
SURFACE HYDROLYSIS IN SODIUM LAURYL SULFATE SOLUTIONS AND ITS EFFECT ON SURFACE TENSION AND ON ADSORPTION AT THE SOLIDAQUEOUS SOLUTION INTERFACE1” BY MELVINA. COOKAND EUGENE L. TALBOT'^ Department of hfetallurgy, University of Utah, Salt Lake City, Utah Received April 17, 1961
Hydrolytic adsorption a t the free surface of aqueous sodium lauryl sulfate solutions is shown (1) by pH measurements correlated with (foam) extract,ion of the soap, and (2) by means of surface tension vs. p H curves a t constant concentration and ionic strength. Values of the (apparent) surface h drolysis constant of 10-7 to 10-8 were obtained from the i u s . pH curves. However, the hydrolytic adsorption may perha s {e due merely to a fatty acid impurity in sodium lauryl sulfate. A series of surface tension-concentration curves were ogtained a t various ionic strengths and a thermodynamic analysis and interpretation given.
Arguments presented in the flotation theory developed in this Laboratory*13suggested that collectors always adsorb as whole molecules rather than as ions, usually, but not invariably, by hydrolytic adsorption, Le., as free acids or bases. The adsorption potential for hydrolytic adsorption is usually large enough in (chemisorbed) specific collectors that substantial surface coverage will occur on the mineral a t effective collector concentrations of to N (e.g., alkyl xanthic acids on metal sulfides). This collector theory is well supported by various independent experimental observations for the specific metal sulfide collectors.4-9 More(la) This work was sponsored by the Atomic Energy Commission, and was presented in the Symposium on Surface Tension of Solutions at the Boston Meeting of the American Chemical Society, April 1-5, 1951. ( l b ) Now at Minnesota Mining and Manufacturing Co., St. Paul. (2) M. A. Cook, 4 n g . Men. J . , 160, No. 2, 110 (1949): “Mechanism of Collector-Mineral Attachment in Flotation,” presented at El Paso meeting of A.I.M.M.E., Oct. (1948), San Francisco, Feb. (1949). (3) M. A. Cook and J. C. Nixon, Tars JOURNAL, LIP, 445 (1950). (4) C. M. Judson, A. G. Argyle, J. K . Dixon and D. J. Sally, J . Chem. Phye., 19, 378 (1951). (5) M. A. Cook and A. W. Last, University of Utah Experiment Station Bulletin No. 57, 40 (May 1950). ( 6 ) M. A. Cook and W . E. Wadsworth, “Free Acid and Base Adsorption on Solids From Aqueous Solutions of Strong Electrolytes.” Univeraity of Utah Expt. Sta. Bull. No. 51, Vol. 41, No. 9 (April 1951). (7) H. Mitsubishi Hagihari, Mining and Metallurgical Laboratory Research Report No. 1202 (1950). ( 8 ) G. A. Last and M. A. Cook, “Theory of Collector-Depressant Equilibria,” Accepted for publication, THISJOURNAL. (9) M. E. Wadsworth, R. G. Conrady and hf. A. Cook, TEIE JOURNAL, 66, 1219 (1952).
over, it has been shown that the inorganic depressants generally employed in these systems, namely cyanide and sulfide, adsorb as the free acids, HCN and HzS, respectively. There appeared very early in this development, however, a serious problem in regard to the non-selective strongly surface active long chain paraffin collectors. Even the paraffin chain salts of strong acids and bases showed good collector properties over wide ranges of pH, Le., under conditions where acid soap concentrations in the bulk solution were quite negligible. Furthermore, the long chain paraffin salts in general behave very much alike as collector agents, irrespective of their hydrolysis constants, except when chemisorption is i n v o l ~ e d . ~These long chain collectors are generally non-selective and non-specific, i.e., they cause flotation of the metallic and/or non-metallic minerals and gangue alike. Plantel0 and Rogers, et aZ.,ll studied the collector properties of many synthetic and natural soaps, and mapped the pH and concentration ranges where these substances are applicable as hydrophobic film forming reagents. The non-specific collector properties of these compounds apparently preclude them as adsorbing i0nically.3.~ Also, the low potential associated with non-selective adsorption requires (10) E. Plante, Am. Insl. Mining Met. Eng., Tech. Pubs., 2163, July (1947). (11) J. Rogers, K. L. Sutherland. E. E. Wark and I. W. Wark, Trona. A m . Insl. Minino Met. Eng., 169, 287 (1946).
c
March, 1952
’
SURFACE HYDROLYBIS IN SODIUM LAURYL SULFATE
that the bulk solution concentrations of the effective collector should be much larger to obtain a given surface coverage than when the specific (high potential) adsorbates are used. I n spite of this, one frequently obtains high contact angles on solids even when treated in very dilute solutions of the para& chain salts of strong acids. For example, sodium lauryl sulfate will cause flotation of minerals like-fluorite or calcite a t a total soap concentration N even in basic solutions of pH 8 to as low as 10. What then is the source of the effective collector in such synthetic soap solutions? The present study is the result of one attempt to answer this question. However, the work of Brady12 and the earlier work of Miles and Shedlovskyl* which came to our attention during the course of this study, showed clearly that a relatively small amount of a non-ionic impurity (evidently an alkyl alcohol) in sodium lauryl sulfate influenced the surface properties of the soap solution tremendously. They demonstrated, moreover, that the anomalous (type 111) surface tension (7)-concentration (C) curve of sodium lauryl sulfate was due (almost) entirely to this impurity. The word “almost” is added here because the results of the present study show another “trace impurity” effect, namely the influence of hydrolytic adsorption even in this strong acid salt! There now appears to be little or no doubt that one or anothers, or both of these factors are responsible for the collector properties of many of these synthetic soaps. Our flotation studies also led us to suppose that some of the features of the anomalous (type 111) y vs. C curves .as well as the normal (type I) ones could be explained by taking into account hydrolytic adsorption even in such strong acid salts as sodium lauryl sulfate and, in addition, the conditions associated with surface saturation and the effects of high surface pressures in the adsorbed films. Included in this paper, therefore, are the results ,of measurements of several y-C curves a t different ionic strengths and pH’s and an interpretation of these curves. Materials and Equipment.-The sodium lauryl sulfate used in this study was a high quality (inorganic free) sample obtained from the du Pont Company. Fresh solutions were used in each series of tests and were made with twice distilled and reboiled (carbonate free) water. Surface tension measurements were made with a “Cenco” du Nouy Precision Tensiometer. The y-pH curves were obtained by measurements of y and pH in the same solutions (under a nitrogen atmosphere) by inserting the electrodes from a Beckman Model G meter (glass-calomel electrodes) into a specially constructed test cell in which y could also be measured directly. All solutions were stored in a thermostat at 29” until used, and the nitrogen atmosphere was maintained at 29O, all measurements being made a t this temperature. The nitrogen both for the surface tension and the foaming experiments was purified by passing it successively through soda lime, pyrogallic acid in potassium hydroxide, sulfuric acid and silica gel. It was then saturated with distilled water at 29” before introducing it into the test system. The use of this conditioning atmosphere aided materially in obtaining reproducible results and in reducing the time for ‘ equilibrium In the surface tension measurements. The foaming apparatus was a 30-mm. Pyrex column, one meter long equipped at the bottom with a sintered disk for dispersing the nitrogen stream, and with a stopcock for (12) A. P. Brady, Tma JOURNAL, 63, 5G (1940). (13) G. D. Milcs and L. Shedlovsky, ibid., 48, 5.7 (1944).
413
sampling. At the top was a side delivery tube for removing foam, and a glass stoppered neck for adding solution. Foam Extraction Tests.-A solution of known concentration and pH was placed in the foaming column, and foam extracted with purified and water resaturated nitrogen gas forced through the sintered disk a t a controlled rate such as to allow time for most of the entrained bulk solution to drain back as t.he foam ascended in the column. Measurements of pH were made by withdrawing and testing samples of the extracted solution a t the end of the foaming test or, in some cases, at intervals during extraction. Foaming was usually continued until most of the soap was extracted to allow a maximum pH change. The soap concentration in the extracted solution was estimated only roughly from the approximate volume of foam extracted and subsequently from the surface tension of the solution which, a t the end of the tests, was usually above 70 ergs./cm.a. Table I contains the initial and final results of the pH measurements of the foam extracted solutions.
TABLE I EFFECT OF FOAMING: ON THE pH OF SODIUM LAURYL SULFATE SOLUTIONS Initial concentration (molal)
1X 1X 2 X 2 X 1X 1X 5 X 5 X 5 X
(no added salt) lo-‘ (no added salt) lo-‘ (no added salt) lod4(no, added salt) (0.1 N NaCl) lo-‘ (0.1 N NaCl) 10-6 (0.1 N NaCl) 10-6 ( 0 . 1 N NaCl) 10-6 ( 0 . 1 N NaCI)
(OH)-
Initial p H Final
6.55 5.80 6.50 6.30 6.65 5.60 5.50 6.24 5.80
7.65 7.12 7.94 7.75 6.50 6.65 6.65 7.63 6.50
Generated (mols./l.) X 10-8
0.67 1.6 1.2 1.0 1.9 2.3
3.0 1.0 1.6
While no appreciable hydrolysis of sodium lauryl. sulfate will occur in the bulk solution, i t is not a priori certain that hydrolysis will not occur in the free surface or a t a solidaqueous solution interface. Actually the soap exists in the surface a t concentrations as much as 106 times greater than in the bulk solution; also it may be shown that K. > > Kh for long chain soaps, where K. is the (a parent) surface hydrolysis constant and Kh is the bulk lydrolysis constant. If the ratio HX/X- (HX-free acid, X--soap anion) were the same in the surface as in the solution, foam extraction would not affect the pH of the sol~tion.’~One requires, therefore, that (HX/X-), > > (HX/X-) to account for the appreciable increase in pH observed in these tests. This results from a shift toward the right (due to extraction of (HX),) of the equilibrium (HX)a (OH-). (X-1 HzO Here the subscript s refers to the surface, and no subscript is used when solution concentrations are designated. Experimentally HX/X- is quite negligible; we could detect no pH increase u on solution of dry sodium lauryl sulfate in pure water. &nce the foaming resulted in a substantial increase in pH, this can mean only that the ratio (HX/X-). was appreciable as a result of which the continuous removal and reforming of the surface phase caused an accumulation of OH- in tjhe bulk solution as shown in the last column of Table I. From the (integral) values of (OH)- gcncratctl one obtains average (integral) values of (HX/X-). in tho range 0.01 to 0.04. This gives for K. (as defined in ref. 14) the value 10-8 to if it is assumed that the free acid soap results from surface hydrolysis of the sodium lauryl sulfat,e. However, one has no assurance that this “apparent” 1 4 % hydrolysis may not be due to a fatty acid soap impurity present to about this extent in the sodium lauryl sulfate. A long chain fatty acid salt would be ionized completely but hydrolyaed to only a very small extent in the bulk solution at pH 7. However, it would be nearly completely hydrolyzed in the surface, at this pH as shown, for exam le, by the data of Long, Nutting, and Harkins,’6J6 and g y the theoretical analysis of the 6 (degree of hydrolysis) uersus C
+
+
(14) M. A. Cook, ibid., 61, 383 (1951). (15) F. A. Long and G. C. Nutting, J . Am. Chem. 800.. 63,84 (1941). (16) F. A. Long, a. C. Nutting and W. D. Harkins, ibid., 19, 2197
(1937).
MELVINA. COOKAND EUGENE L. TALBOT
414
curve of Pawney and Jordan given in reference 14. (On the other hand perhaps 1 5 5 0 % hydrolysis of the fat,ty acid salt would have occurred in the solutions of minimum initial p H shown in Table I, but this would still allow for considerable increase in pH during foaming as the free acid soap is gradually extracted.) Surface Tension os. pH Curves.-One should also be able t o demonstrate surface hydrolysis by means of surface tension vs. p H curves at constant soap concentration and total ionic strength. For this purpose a special flask was constructed so that either pH or y could be measured in the same solution, as described previously. The p H was adjusted by additions of constant boiling HC1 on the acid side and concentrated ( COe-free) sodium hydroxide on the basic side. Measurements were made in 0.01 and 0.1 N NaCl solutions, thus the p H adjustments did not affect the total ionic strength appreciably. Figure 1 shows the results of two separate series of measurements. In both instances an increase in p H resulted in an increase in y occurring primarily over a range of about two pH units, which theoretically would include about 90% of the change if one were dealing with surface hydrolysis. The point of maximum change of y with pH should correspond to the condition p H = pK,, from which one obtains K,, Ei 10-8 in one case and 10-7 in the other. Here K,. is the (apparent) acid dissociation constant in the surface phase. This gives for K , the values 10-8 and lo-', corresponding to the different values obtained from the separate curves. The fact that this is around 10 to 100 times larger than the value estimated from the foam extraction test gives credence to the free acid impurity possibility. In any event it is clear from these experiments that there is adequate surface. hydrolysis to account for the observed collector properties of sodium lauryl sulfate solutions by hydrolytic adsorption. I n other words there are a t least two sources of whole molecule adsorption in these solutions when the Brady impurity is also considered. Since sodium lauryl sulfate has no apparent special properties as a collector which cannot be obtained with a.fatty acid soap, it did not seem worthwhile to go further into the source of the acid soap. Instead, attention was directed to the y us. C curves on the assumption that pH and ionic strength control might eliminate the anomalous minima in the Type I11 curves.
,.-..--.--
60
i
Vol. 56
Figure 2 shows the results obtained in solutions of 0.1 and 0.01 N NaCl a t two pH values (5.5 and 10). One will observe that while the pH and ionic strength control did not completely eliminate the minima in the y-C curves, it reduced the sharpness of them materially. The experimental points in Fig. 3 are the results obtained at different NaCl concentrations. Since previous tests showed that there was no hydrolysis in the bulk solutions, and only a very small p H effect anyway, no attempt was made to maintain pH control in obtaining the results shown in Fig. 3. While again the minima were observed, they were not nearly as pronounced as in the work of Brady (Fig. 4).
J
I
I
.
- 01 N N a C l
---
001 N NoCl
.-. h 'D
4
3
-6
No CI
70
I. N O N E
-
.
"E
60-
0
rn D
50h
40-
30 I
o
I
I
I
10'5
10'~
10-2
c. Fig. %--Effect of ionic strength on surface tension. 60
PH
Fig. 1.-Surface
tension vs. p H in sodium lauryl sulfate.
Surface Tension us. Concentration Curves of Sodium Lauryl Sulfate.-Surface tension-concentration measurements were made with a platinum ring of R / r value 40.1, and in a platinum dish. Both ring and dish were flamed before each use, and measurements were carried out under a bell-jar. A small hole was bored in the top of the bell-jar to allow a quartz fiber, connecting the ring and tensiometer, to pass through. The solution and bell-jar could be raised and lowered by means of a threaded screw adjustment to keep the balance arm level. Pure (water saturated) nitrogen under a slight positive pressure and a t 29" was maintained above the solutions during measurements. A weighed quantity of soap was dissolved in a known amount of water to make a stock solution from which other solutions were made up by dilution to volume after additions of a weighed amount of NaC1. Concentrated HCl or NaOH was added a t the tip of a flamed platinum wire for p H adjustment. The samples were then stored a t 29' until used. Measurements on each sample were continued every few minutes until the surface tension remained substantially constant over a period of 15 minutes. Harkins and Jordan17 corrections were applied in all measurements. (17) W. D. Harkins and H. F. Jordan, J . A m . Cliem. Soc., 62, 1731 (1930).
i
-!
I Original Material
-, 5 0 -
e Collapsed Foam
E
.t!
-E
-
o.Residue From Foaming 0
Miles 8 Shedlovsky
40-
n
-
0
-30
02
01
03
C(rnoles/lit )
Fig. 4.-Surface
tension of foam fractionated sodium lauryl sulfate (BradyI2).
Discussion It%isof interest to evaluate the experimental surface tension-concentration curves thermodynamically in order to obtain approximate contributions of various factors to the total adsorption potential. For this purpose we shall assume that since the film is gaseous, the surface phase of aqueous sodium
March, 1952
SURFACE
HYDROLYSIS IN S O D I U M LAURYL SULFATE
lauryl sulfate solutions obeys the two-dimensiona,l van der Waals equation of state (r
+ :)(u
- b)
=
kT
where P is the surface pressure, u the area/molecule, and a and b are constants. Also we shall assume that a t constant pH we may treat the soap solution as a single component system so that
d.lr - RT d In (yl
r(l)
C)
(2)
where y*B is the mean activity coefficient of the soap. The thermodynamic equilibrium constant of the reaction soap (solution) )r soap (surface) is
- e-X(~,T)/RT
K = - ut y*S
(3)
CE
where y A s is the activity coefficient in the adsorbed phase, X is the adsorption potential [Le., -AF (adsorption)] which is a function of the density of solute in the surface phase (or u), and the temperature. It is assumed that X may be broken down as X = Xo
+ 625 n + + ( u , T )
(4)
where ho is the contribution to X of the head groupcounter ion-solvent interaction. This component of X might be considered to be dependent on the ionic strength (p) ; however, we shall include this dependence in y/.tBand y*. The second term is the Traube potential with n the number of carbon atoms in the chain. The 9 term accounts for intractions between adsorbed molecules and will be taken as zero at some particular (large) u1, or (low) ?rl. From the equations (2), (3) and (4) one obtains
where has been set equal to CS7 with T the average thickness of the adsorbed film which we shall assume is 6A and does not vary with C,. It will also be assumed that y*E = y si (p) and we shall attempt to determine y 18 (p) from the experimental data. At constant p one obtains the relation
The left side of Equation (6) may be set equal to -b) a RT eul R T
(Ul
-Q
euRT
(Gb>,
and integrated graphically. It is assumed that equation (6) will apply to the y-C curve for all soap concentrations below the effective solution saturation concentration C2, ie., below the normal micelle region of the y-C curves. At concentrations above C2,y should become constant (at constant pH and ionic strength) in a pure soap. The minimum prevailing in the y us. C curve after pH and ionic strength control may possibly be simply a type of nucleation limitation of the impurity. In other words, at C2 the impurity would not have reached solution saturation and y would thus continue to drop above C2 as the concentration of
415
this impurity increases. However, when micelles become sufficiently abundant adsorption of the impurity on the micelles may be initiated and the potential for it increases with the average size of the micelle at sufficiently high concentrations as a result of which y returns toward the C2 value at sufficiently high concentrations. This implies simply that the impurity saturation concentration is greater in a solution containing only this impurity than in a solution containing a sufficiently large concentration and average size of sodium-lauryl sulfate micelles. Such an assumption can easily be justified theoretically. Since Cbvaries only slowly over the greater part of the y-C between surface pressures above about 5 dynes/cm. and the micelle region, one might expect y+s to be relatively constant over this region. Since y/.t should vary exponentially with p'lz, one would obtain log yr,l y.t = k'p'/z where k' is a constant. Experimentally this is not t,he case. Instead, the relation u;:y*
=
(7)
gr'/2
seems to fit the condition quite well, where g is a constant. We have no theoretical explanation for this situation as yet. Assuming that Equation (7) is valid, one may write Equation (6) as l a e - 4 / R T c h = 4 X 106eAot/RTP'/2C
where =
A0
+ RII' In g
TABLE I1 ' RESULTSO F GRAPHICAL INTEGRATION a
0 1.0, 2.5 5.0 7.5 10 12.5 15 17.5 20 22.5 25 27.5 30 32.5 35.0
(8) (9)
OF EQUATION
J;;~I,-@/RT~~
,-WR Tyl&.(* c
0 1.2 7.5 70 158 295 455 700 950 1285 1615 2040 2470 2900 2400 4250
0 3 x 10-6 1 . 9 x 10" 1.75 x 10-4 4 . 0 x 10-4 7 . 3 x 10-4 1 . 1 x 10-3 1.75 x 10-3 2 . 4 x 10-3 3 . 2 X 10-8 4.1 X 5 . 1 x 10-3 G.2 X 7 . 3 x 10-3 8 . 5 x 10-3 1.06 x
(6)
Table I1 presents the results of the graphical integration of the left side of equation (6), using the valxes a = 4 x 104 ergs. A.4/cm.2~mo1ecules2,b = 50 A.2. Figure 5 presents a comparison of experimental and computed y-C data for the values A; = 3.25 kcal./mole. The use of the empirical equation (7) seems to be well justified from the results shown in Fig. 5. Moreover the empirical values of XA, a and b seem entirely reasonable ones, although the shape of the 7~ vs. log C curve is not very sensitive to small changes in either a or b. A vnriatioii of y i s with ionic strength in bulk solu-
MELVINA. COOKAND EUGENE L. TALBOT
U G I
I
70t 'IL h
40
3
-6
lag
c.
Fig. 5.-Comparison of experimental and theoretical surface tension-concentration curves at constant pH and ionic strength.
tion may possibly be explained qualitatively by considering the average ionic atmosphere radius in the various solutions. A t ,C = lo4 and no NaCl this radius is about 300 A. Adsorption is then hindered by simple anion repulsion and the potential required to absorb counter-ions to overcome it. At the same soap concentration and 0.1 Naql, the average atmosphere radius is about 10 A.
Vol.
SG
Hence the anion repulsion is largely eliminated at this high ionic strength because the counter-ions (Na+) are then more easily brought into the surface film with the anions to produce charge stabilization. Actually the absolute values of X depend to some extent on the choice of a in equation (1). Since this choice cannot be made very accurately in the present study, because a influences the 7 vs. C curve appreciably only at low surface pressures, ie., at high y, the absolute values of Xo may be in error by perhaps as much as 0.5 kcal. Furthermore the inclusion of g in the Xa term obscures the significance of the numerical values of Xo. It is interesting to note that the minima were shifted to lower concentrations by increasing L.I to almost the same extent as the displacement of C, (ie., the concentration a t constant surface pressure). The latter displacement in turn was almost independent of the surface pressure. The displacement of the minimum shows that the potential for micelle formation varies with ionic strength in about the same manner as the product y7By+ which is consistent with the theory discussed in reference 14.
f
4
