XOV.,
1952 EFFECT O F SALT ON CRITICAL CONCENTRBTIONS OF
cyanole was first prepared, it was distinctly reddishviolet. On standing, the color became blue, and the change appeared complete in about 20 minutes. With a solution of the heptanoate of above the critical concentration, the change to blue seemed complete in about 10 minutes. Spectrophotometrically, it is found that at 490 mp the iiitensity of absorption decreases with time, whereas at 561 and 604 mp the optical densities increase with time. This time effect is veiy pronounced with the six and seven carbon soaps. The wide scattering of points a t 490 mp in Fig. 5 below the indicated C, which were determined a t from three to 15 minutes after the solutions were made, illustrates
POTASSIUM
~-ALKAKECARBOXYLATES 959
the importance of the time effect. The rate of change in optical density with time varies with soap conceiitration but i n general is significant only for ' / z hour. The readings a t 490 mp may decrease by nearly 50% during that period, and at 561 and 604 mp they may increase by as much as 10%. Ultimately, due to the instability of the dye, all three bands will fade. These observations are not in disagreement with the view that micelles continue to farm for a coiisiderable time after the longchain electrolyte has been dissolved. The discrepancies between the visual and spectrophotometric results reported herein may in part be egplained by this phenomenon.
EFFECT OF SALT ON T H E CKITICAL CONCENTRATIONS OF POTASSIU3.I r i - ~ ~ L I ~ d N E C A ~ ~ B O S ~AS L ADETEKPVIINED TES BY THE CH-INGE I N COLOR OF PINACYANOLE' BY SIMOK H. HERZFELD Depcwlrnent of Chemistry, Unicersilij of Chicago, Chicago 37, tllinois Receired September. 86, 1051
It has been shown that for the potassium Ii-alkaiiecalbosylates, the critical concentration and also the rate of change of the critical concentration cvith increase in t,he concentration of added potassium chloride, are decreasiiiy I'unctions of the concentration of the salt,. The data are found to fit equation ( 1 ) in which Cp is the critical molarity atid Ci the total volunie nornialit8y of count,erion. The slope of this line is found to be essentially indepentient of chain length. Hohbs2 interprets a as a function of the thermal energy and the work of inicellc formation. Corrin3 ~ ~ U R O I that I S b is the ratio of the number of couiiterions t o that of long-chain ions in the micelle at the critical concentration, and argues that the ratio is independent of the concentration of added salt. Both Mobbs and Corrin :irrive at equations of the same forin as the empirical equatiou shown above.
Introduction Numerous workers4-14 have reported that the presence of salt increases the tendency of lougchain electrolytes to micellize. The first systematic investigation of salt effect \vas carried out by Corrin and Harkiii~,~5 who studied the effects of various inorganic salts over wide ranges of c,oncentration on both anionic and cationic long-chain electrolytes. Corrin and Harkins, within the limits of their experiments, found that these effects were independent of the valence and composition of the ion possessing a charge of the same sign as that of the aggregating ion. Ions of opposite sign were
restricted to sodium, potassium, chloride and bromide. The rate of decrease of the critical molarity, C,, with iiicreasiiig iioimality of added salt was found to diminish rapidly with rise in salt concentration. In the cases studied, the data fitted the equation
J . A m . Chem. S o r . , 61, 644 (1939). (9) H. V. T a r t a r a n d R. D. Cadle, T H IJOURVAL. ~ 4 3 , 1173 (1939) ( I O ) H. F. Walton, J . Am. Chem. Soc., 68, 1180 (1946). (11) R. S. Steams. H. Oppenheiiner. E. Simon a n d W. D. Harkins. J . Chem. Phys.. 15, 406 (1947). ( 1 2 ) P. Debye. .4nn. iV. Y . .4cnd. S c i . , 51, 575 (1949). (13) P.Debye a n d E. W. Anacker, T H I SJ C I U R S A L55, , G4.L (1951). ( I 4 1 H. A. Scheraga and J. IC. Backus. J . A m . Chem. SOC..7 3 , 5133 (1951). ( 1 . 5 ) k1. 1., Cnrrin and M '. D. Harkiiis. ib;,/,, 69, 683 (lU47).
The soap and pinacyanole were of the same lots used in the work described in the preceding report. The salt, pot,assium chloride of reagent grade, was dried at. 110". Measured volumes of soap and salt solutions of known normalit,ies, prepared a t 2 5 " , were mixed in varying rat,ios. Each mixture was brought by dilution at constant concentration of dye to t,he critical molarity in the manner employcd in the earlier work. The end-point,s usually became
log Cp = u
+ b log Ci
(1)
in which Ci is the total concentration in gram equivalents per liter of ions opposite in sign to the long-chain ion. The slope of this line was observed to vary moderately with the constitution of the micellixing ion and to remain less than one i n all cases. Merrill and GettyL6confirmed the findings (11 This investigation was carried o u t under the sponsorship of the of Corriii and Harkins for the case of sodium dodecReconstructioii Finanre Corporation, ORce of Rubber Reserve. i n anoate. coiiiiection with the Cover'ninent's synthetic rubber prograiii. S. H. The present investigation \vas undertaken to Herzfeid, Cheiniqtry Division, Office of Scientific Research, Air Research slid Develol~liientCoiiitiiaiid, Baltiiiiure 3, RId. study the salt effect i n a homologous series of long( 2 ) h.1. E. Hobbs, T H I SJ O U R X A L . ~675 ~ , (IUBI). chain electrolytes. The findings a t lower coiiFen( 3 ) A I . L. Corrin, J . Cu//osrl Sci., 3, 333 (lU.18). t,rations of salt have been demotiatrated to fit equa( 4 ) G . S. Hartley, Tmiix. F n r o d n y Jor., 30, 444 ( 1 0 3 4 ) . tion (l),and the slope has been shown to remain (j) R. C. Murray, i/Jid., 31, 1U9 (1935). ( G ) 0. S. Hartley a n d D. T;. Runiiirles, P m c . R O U .S q r . ( L . O I ( C / ~ I ~ ) constant within the limits of accuracy of the A168, 420 (1938). method. . (7) G . S.Hartley. J . C/iem. Soc., 19G8 (1938). Experimental Methods ( 8 ) K. A. Wright. .A. D. Abbott, V. Siverts and H. 1'. T a r t a r ,
-~
(115)
R. C. RIerrill and R. Getty, TIUSJOURNAL, 52, 774 (lY48).
SIMONH. HERZFELD
960 TABLE I
VARIATION OB' THE cR1'PICAL CONCENTRATIONS Ob' 1'OThSSIUM WALKANECARBOXYLATES WIT11 ADDEDPOTASSIUM
CHLORIDI AT 25' Salt
Concn., A l
AS DETERMINED B Y THE CHANGE IN OF PINACYANOLE CHLORIDE
Critical Concn., hf
IIeytanoate in 5 X 10-6 41 dye 0 0.780 0,146 ,728 .346 ,691 .633 ,633 I .08 ,542 1.72 ,430 2.38 ,340 2.80 ,280
Hcptmoate in 2.5 x 10-5 AI djrc 0 0 .670 0,066 .662 .I54 .GI7 ,289 .577 .407 ,543 .522 ,522 .728 .485 1.OB .430 1.50 .374 1.78 ,335 2.00 ,307 2.28 ,285 2.45 ,263 2.64 .247 2.76 .230 2.91 ,218
Salt Concn., A i
2.88 3.27
COLON
Critical Concn., A I
.033
Nonanoate in 5 X 10-5Af dyc 0. 0.200 0.037 187 .OB7 ,175 .150 ,159 .278 .139 ,461 ,115 .(577 ,097 ,855 ,086 1.29 ,064 1.63 .054 1.92 ,948 2.11 .042 2.30 ,040 2.64 038
Tridecanoate in 5 x 10-6 Af dye 0 0.0126 0.00134 ,0121 .0029 1 .0116 .00478 . .0112 ,00700 .OIOG .00929 .00929 .0134 .00892 .015G .00842 .0187 .00800 .0251 ,00717 ,0276 ,00690 ,0327 ,00654 ,0411 ,00587 ,0529 ,00529
I
Salt
Critical Concn., A f
Concn., A i
,0616 ,0913 .11G .144 ,169
,00492 ,00400 ,00357 ,00319 ,00294
,194 .2 I6 ,241 ,268 ,312
Critical Concn., M
,00277 .00262 ,00253 ,00238 ,00227 Computa-
Gip = V A M A / V T (2) wliioh ITA and Af.4 arc thc initial volunic arid niolarity of tlic soap solutiou, and VT t>hcsum of t.hc volumcs of all solut8ioiisatldctl u p l o the cntl point. Calculation of the concentration of salt, C"d, a t the end-point was made in a likc niaiiner.
Dccanoate in 5 x 10-6 Ai dye 0 0.09'38. 0.0105 ,0949 ,0229 .0917 ,0379 ,0885 ,0559 ,0838 ,0787 ,0787 110 ,0731 ,151 . 0645 ,227 .0568 ,344 .0469 ,386 .0429
Octanoate iif 5 X 10-b d l d y e 0 0.400 0.075 ,376 .309 ,309 .530 ,265 .865 .216 1.22 ,175 1.52 ,152 2.22 ,111 2.73 ,091 2.97 ,085 ,080 3.20
Salt
Concn.,Ai
sliaryt~r\vit,Ii iiicrcasiiig concciilration of salt. tion of C'r again \vas iiiadc by iiieans of
.030
Undecanoate in 5 X 10-6 d i dye 0 0.0492 0.0048 ' ,0478 ,0144 ,0456 ,0187 ,0437 ,0273 .0409 ,0386 .0386 .OS34 ,0356 .OR4 .0327 .I03 .0294 .133 .0266 ,169 .0242 ,216 .0216 .287 ,0191 ,403 .ole1 ,506 ,0145 ,642 .0128 ,830 ,0111 1.01 ,0101 1.18 ,0094 1 35 ,0090 I .52 ,0087 1.70 .0085 2.01 ,008 1 2.22 .0074
Vol. 5G
ill
'
Results The variation of critical coiiceiitratioii ivith added potassium chloride has been determined for six potassium soaps: heptaiioate, octanoate, nonanoate, decaiioate, uiidecaiioate and tridecanoate. The measurements for the dodecanoate made by Corrin and HarkiiisI5 were not repeated. The new data are recorded in Table I. The values for the first four soaps are plotted in Fig. 1, which shows that the iiiitial effect of added salt increases with lengtheiiing of the carbon chain. Figures 2 and 3 illustrate in greater detail the very rapid initial drop of C, as a function of CS that is exhibited by the ele.ven aiid thirteen carbon soaps. In all cases studied, the absolute value of dC,/dCg was found to be a decreasing function of Cs. Most of the decrease in C, occurs before CS reaches 25 C,. It is significant17that with added alcohol, either in the absence of salt or in the presence of a constant concentration of salt, the value of dC,/dCa, where C, is the molarity of added alcohol, is a linear functiori of C, and is independent of C g . Plots of log C, as a function of log Ci are shown in Figs. 4 to 10. It is seen that the relationship is liiiear for all soaps at relatively low normalities of counterioii. Departure from linearity was demonstrated for five of the systems, which were studied to relatively high concentrations of added salt. The parameters of equation (1) were computed for the linear portions of the plots from the data in Table I by the method of least squares. The results are collected in Table 11, together with values for the dodecanoate obtained by Corrin and Harkiiis.'b The maximum ratio of equivalents of TABLE I1
c',
+'
VALUES OF PARAMETERS'IN LOG = n b POTASSIUM 71.-ALIGINECARBOXYLATgY Number Molarity of of carbon pinscyanole atoms x 105 a
Loa
~i FOR
b
-0.565 5 -0.160 - ,568 2.5 - ,270 - .621 8 5 - ,641 - .532 9 5 -1.008 - 562 10 5 -1,501 - ,521 11 5 -1,990 - ,570" 12 5 -2.617 - .538 13 5 -2.937 From data of Corrin and Harkins.I5 (17) 9. 11. IIorzIold,RI. L. Currinand W. D. Httrkins, ' l ' i I I f 3 J O U I I N A L
7 7
,
54, 271 (lQ50).
,
Nov., 1962 EFFECT OF SALT ON CRITICAL CONCENTRATIONS OF POTASSIUM ?L-ALKANECARBOXTLATES 961 0.78
0.52 1.04 1.56 2.08 2.60 3.12 Concentration of potassium chloride (g. equiv./l.). Fig. 1.-Plots of the critical concentrations of four potassiuni n-alkanecarlioxylates showing the relative effect of added salt a t 25' in 5 X 10-6 molar pinacyanole.
5
-
.e
c
2$4 2 -0.21 - 0.33 e-
dEi
.2 c c. eI
u 0
-.\
-0.09 -
1
1
1
1
1
1
1
1
1
; -~
~
0
I X l O ' M PINALYAHOLF
-
-0.43 -
0
0 -
M
0
k
-0.57
-
I
l
l
I
I
I
I
I
1
1
0 0.4 0.8 1.2 1.6 2.0 Concentration of potassium chloride (9. equiv./l.). Fig. 2.-Variation of the critical concentration of potassium undecanoate with added salt a t 25" in 5 X 10+ inolar pinacyanole.
salt to equivaleiit weights of soap that falls on a straight line rises from about 4 for the heptanoate to more than 140 for the tridecanoate. The actual maximum salt coneelitration, however, drops from 1.7 to 0.3 normal. The cause for the departure from linearity is not kiiown to the writer. It is possible that linearity ceases wlien coalesceiicc of micelles into microcrystals heguis. I t is significant that the values of a slio\v a definite trend with increase in chain length, whereas b appears to be essentially independent of the carbon content in the homologous series investigated. The arithmetic mean of b for the seven soaps is -0.560 f 0.022. The variations in the slope of the straight line from the mean value show no trend and are compatible with the limits of accuracy
-
1
0
bo
0
* - O 0
-0.64
0
I 1 I I I I I I 1 1 -0.15 10.08 +0.21 $0.39 Loglo vation conrentration (g, equiv./l.). Fig. 6.-Linear logarithniic relation between critical roncentration of potassium heptanoat,e and total countcrion concentration a t 25" in 2.5 X 10'6 molar pinacyanole.
Discussion All measureinelits of equivalent conductivity have shown that ionic compounds containing large water-insoluble orgaiiic groups behave as typical
1
SIMONH. HERZFELD
962
Vol. 56 r
I
I
I
I
1
1
1
1
I
I
I
I
J
-I
-0.51
- 0.67 - 0.83 0
9 il
z
0
-0.99
t-
O O n
_i
-1.151 I I I I I I I I I I I 1 1 -0.40 -0.16 0.08 0.32 0.56 Loglo cation concentration (g. equiv./l.). Fig. 6.-Linear logarithmic relation between criticd concentration of potassium octanoatc and total counterion concentration a t 25" in 5 X niolar pinacyanole.
I:
- 1.30
t
0
M
2
-1.50
O o o 0
00
3
-0.7 -0.4 -0.1 +0.2 +0.5 Loglo cation concentration (g. equiv./l.). Fig. 7.-Linear logarithmic relation I)etween critical concentration of potassium nonanoate and tot.al counterion concentration at 25" in 5 X 10-5 niolar pinacyanole.
g
-0.99
. c 3
E
5 . -1.11
I
zz
sg -e 2
$g - 1.23
.M 3
0
9
k
-1.35
-1.01 -0.81 -0.61 -0.41 Loglo cation concentration (g. equiv./l.). e Fig. 8.-Linear logarithniic relation between critical concentration of potassiuni decanoate and total counterion concentration at 25" in 5 X molar pinacyanole.
strong electrolytes at concentrations below their respective critical values. In dilute solutions of strong electrolytes, when the interionic potential energy is small in comparison to the average thermal energy, the repulsion between ions of like sign rises with increase in the magnitude of the charge in the manner prescribed by the principle of ionic strength and the more inclusive Debye-Huckel theory. With the onset of aggregation in solutions of longchaiii electrolytes, the charge on one ion undergoes a manifold increase; and the interionic potential
bo
2
-2.1 1
1
1
1
1
1
1
1
-1.32 -0.90 -0.48 -0.06 +0.36 Loglo cation concentration (g. equiv./l.). Fig. 9.-Linear logarithmic relation between critical concentration of potassium undecanoate and total counterion niolar pinacyanole. concentration a t 25" in 5 X
I
G
2
-2.7 -. .
t
1
I
I
4
I
I
I
-1.9 -1.0 -1.3 -1.0 -0.7 Loglo cation concentration (g. equiv./l.). Fig. 10.-Linear logari thmic relation between critical concentration of potassium tridecanoat>e and total counterion concentration at 25" in 5 X IO+ molar pinacyanole.
energy is no longer much less than the average thermal energy. Furthermore, the large potential thus established on the surface of a micelle repulses added salt ions of like sign so strongly that the concentrat'ion of t'he latter on the surface of the micelle becomes insignificant. Debyel8 has computed that this concentration is no more than 3 X lop4of that in the bulk of the solution. The magnitude of the charge on unaggregated ions of the same sign as the micelles acquires much less significance than before aggregation, therefore; and the influence of added salt is governed by the sum total of ionic charges rather than by the quantity of charge on individual ions bearing the same sign as the micelles. The composition or valence of salt ions of unlike sign of charge to that of the long-chain ion, however, may have a profound effect on the stability of the micelle. It has been demonstrated by Lottermoser and Puschel'g and by Samis and Hart1ey2O that the tendency of negative long-chain ions to micellize rises markedly with increase in the valence of the counterion. With positive long-chain ions, Samis and Hartley mere able to find only insignificant variations in the critical concentration with change in the valence and composition of the coun(18) P. Debye, THISJOURNAL, 53, 1 (1949). (19) d. Lottelmoser and F. Puschel, Kolloid 2.. 6 3 , 175 (1933). (20) C. S. Saliiis aiid G . 8. Hartley, Traris. F u m d a u Soc., 34, 1305 (1938).
Nov., 1952
FOAMING OF NON-IONIC SURFACE ACTIVEAGENTS
903
increase in the number of molecules per micelle. The slope of the line given by equation (1) has been interpreted by C0rrin3 to represent the ratio of the number of attached counterions to the number of long-chain ions in the micelle at the critical concentration. Corrin makes no assumption regarding the shape of the micelle, and treats the effect of added salt on C, by a direct application of the mass law to the equilibrium between the unassociated long-chain ions and counterions, and the aggregates. Corrin argues that the micelle content is unchanged by the addition of salt and shows that the value of b should be independent of the concentration of counterion, of the valences of the two kinds of ions composing the micelle, and, in the case of a homologous series of long-chain electrolytes, of the chain-length. The first and third of these statements are not in disagreement with the findings of the present report. The other remains to be tested. Acknowledgment.-The kindly guidance and inspiration given the author during this and the previous investigation by the late Professor William Draper Harkins are recognized gratefully. The pursuance of the work was facilitated greatly also by the constructive criticism freely offered by Dr. M. L. Corrin.
terion. Recent work by Kraus and co-workers,21,22 however, has demonstrated for several quaternary ammonium and pyridiiiium salts a very marked effect of the coiwtitution of the counterion upon the critical concentration. In a group of five n-octadecyltrimethylammonium salts, four of which were of the.uni-univalent type, the critical concentration varied more than fourfold, with the lowest value being exhibited by the oxalate. On the basis of available data, therefore, no correlation can be drawn between the critical concentration of a cationic long-chain electrolyte and the valence of the anion associated with it. Possibly the influence of valence exists but is masked by a stronger force. The parameter a in equation (1) has been given a meaning recently by Hobbs,2 who adopts the view that the micelle is a double layer of long-chain molecules of approximately cylindrical symmetry, with the polar groups oriented in the flat surfaces of the disk-shaped structure. Hobbs deduces that a is a,function of both the energy of micelle formation and the thermal energy of the solute. He supports the view of Debyel*that the effect of salt on the critical concentration is associated with an (31) E. C. Evers and C. A. Kraus, J . A m . Chem. Soc., 70, 3049 (1948). (22) P. F. Grieger a n d C. A. Icraus, ibid., 7 0 , 3803 (1948).
FOAMING OF NON-IONIC SURFACE ACTIVE AGENTS BY MANUELN. FINEMAN, GEORGEL. BROWNAND ROBERT J. MYERS Research Laboratories, Rohm and Haas Go., Philadelphia, Pa. Received September 87, 1961
The foaming properties of noli-ionic surface active agents vary markedly with the temperature of the test aiid with the solubility of the surfactants as characterized by their cloud points. The principal rffect of temperature is that of altering the solubilities. An attempt has been made to deduce inforination concerning the shapes of their ethylene oxide distribution curves from analysis of thcir foaming behavior.
Non-ionic surface active agents have become widely available only within the past decade. Because of their comparatively recent origin, the literature contains relatively few accounts of their properties, in contrast to the voluminous investigations reported for soap and synthetic ionic surface active compounds. The reaction of ethylene oxide with an alcohol or phenol is capable of producing a spectrum of non-ionic surface active materials. For a given alcohol or phenol, the hydrophilic nature is increased as the polyoxyethylene chain is lengthened ; and for a specific polyoxyethylene chain the hydrophobic nature increases with the length of the hydrocarbon base. In view of a lack of published reports on the subject, the foaming of aqueous solutions of several materials of this type has been examined. The compounds studied in greatest detail were those obtained from reaction of p-t-octylphenol with ethylene oxide. The prototype molecule may be represented by the symbol OPE,, where z represents the average number of moles of ethylene oxide. The foam of homologous compounds con-
taining longer aliphatic chain substituents on the aromatic nucleus was also examined briefly. In view of the nature of the reaction of ethylene oxide with the various organic groups, the molecular weight must be regarded only as a number average. Presumably the molecular weight distribution could be calculated by the method of Flory. As opposed to colloidal electrolytes, for which the solubility in water increases with increasing temperature, these ethylene oxide polymers are insolubilized as the temperature is raised, presumably as the result of loss of water associated with the hydrophilic ether linkages in the chain. Upon heating, a dilute aqueous solution becomes turbid at a definite temperature known as the cloud point. The cloud point is substantially constant for solutions of 0.5 to 10% concentration12and measurements are customarily conducted on 1% solutions. Compounds of the OPE, series in which 2 is Ci or less are very hydrophobic, and since they are in'
(1) . 'I J. Flury, J . A m . Chem. SOC.,62, 15til (1940). ( 2 ) J. AI. Cross, Official Proceedings, 36th M i d - y e a r Meeting. Cheiiiical Specialties Manufacturing Association.
