EFFECTOF ELECTROLYTE AND UREAox MICELLE FORMATION
3585
Effect of Electrolyte and Urea on Micelle Formation'
by M. J. Schick Lever Brothers C o m p a n y , Research Center, Edgewater, N e w Jersey
(Received A p r i l 6, 1964)
The effect of counterion hydrstion on the critical micelle concentration (c.m.c.) of some n-dodecyl sulfates has been studied in aqueous salt solutions as a function of counterion concentration maintaining a constant sirnilion. I n order to supplement this study of hydration effects in the electrical double layer on the rnicellar periphery, the effect of urea on micelle formation and on hydrophobic bonding in the micellar core of some n-alkyl sulfates has been investigated as a function of urea concentration and temperature. The increments in c.m.c. values on urea addition increase in the following order: lithium ndodecyl sulfate, sodium n-decyl sulfate, sodium n-dodecyl sulfate, tetramethylammonium (TIMA) n-dodecyl sulfate. These results are interpreted in terrns of the structural concepts of water. For comparison with the data on ionic detergents, the effect of urea on the c.m.c. of n-dodecyl ether alcohol sulfates in aqueous solutions and of polyoxyethylene alkanols in a,queous and in electrolyte solutions has also been studied as a function of urea concentration and temperature. Increments in c.ni.c. values of the polyoxyethylerie alkanols on urea addition are attributed to increased hydration of the ethylene oxide chain caused by a reduction of the cooperative structure of water.
Introduction Recent investigations2-' have supported the contention that changes in water structure are an important factor in micelle formation. Therefore, an investigation has been carried out to increase our experimental knowledge of the effect of these changes in water structure upon micelle formation of ionic, nonionic, and the intermediate n-alkyl ether alcohol sulfate detergents I n addition, the effect of changing counterions haa been examined. Corrin and Ilarkins8 have shown in a study of the aggregation of long-chain electrolytes (1) that the c.rn.c. is related to only the concentration of the counterion and the nature of the sirnilion is without, effect, and (2) that the c.m.c. of a long-chain electrolyte is a linear function of the logarithm of the total concentration of the counterion. This holds only for univalent counterions and has been treated theoretically by Hobbs. The effect of changing counterions has been the subject of several investigations.'0-15 For example, the effect of Univalent cations on the c.m.c. of sodium n-dodecyl sulfate has been reported by Goddard, et a1.,I0 of univalent anions on the c.m.c. of dodecyl-
pyridinium chloride by Lange," and of univalent anions on the c.in..c. of dodecyl trimet hylainmoni uin bromide by Anack:er and Ghose.12 However, these (1) Paper presented in the Kendall Award Symposium on Behavior of Surfactants at Interfaces and in Solution at the 147th National Meeting of the American Chemical Society, Philadelphia, Pa., April 5-10, 1964. (2) E. D. Goddard, C. A. J. Hoeve, and G. C. Benson, J . P h y s . Chem., 61, 593 (1957). (3) E. D. Goddard and G. C. Benson, Can. J . Chem., 35, 986 (1957). (4) P. Mukerjee and A. Ray, J . P h y s . Chem., 67, 190 (1963). (5) W. Bruning and A. Holtzer, J . Am. Chem. Soc., 83, 4865 (1961). (6) K. W. Herrmann, J . P h y s . Chem., 66, 295 (1962). (7) M. J. Schick, ibid., 67, 1796 (1963). (8) M. L. Corrin and W. D. Harkins, J . Am. Chem. Soc., 69, 683 (1947). (9) M .E. Hobbs, J . Pht/s. Colloid Chem., 55, 675 (1951). (10) E. D. Goddsrd, 0. Harva, and T. G. Jones, Trans. Faraday Soc., 49, 980 (1953). (11) H. Lange, Kolloid-P:., 121, 66 (1951). (12) E. W. Anacker and H. M. Ghose, J . P h y s . Chem., 67, 1713 (1963). (13) J. M .Corkill and J F. Goodman, T r a n s . Faraday Soc., 58, 206 (1962). (14) K. J. Mysels and L. H. Princen, J . P h y s . Chem., 63, 1698 (1959). (15) E. W. Anacker, R. M . Rush, and J. S. Johnson, ibid., 68, 81 (1964).
V o l u m e 68, Number 18 December, 196.4
3586
three investigations dealt with solutions containing mixtures of counterions,10-12 in which the possibility of forination of micelles with mixed counterion layers exists as shown by Corkill and Goodinan.13 In contrast, Mysels and Princen have compared the c.m.c. values of some n-dodecyl sulfates in solutions containing a single counterion. l4 In the present investigation, these data have been amplified with a study of the c.m.c. values of lithium, sodium, and tetraniethylaniinonium n-dodecyl sulfates in salt solutions containing a single counterion and a constant siniilion over the concentration range 0-0.4 M . Mukerjee and Ray4 and Bruning and Holtzer5 have used dissolved urea as a probe for investigating the water structure contribution to micelle forination and hydrophobic bonding. The choice of urea is based on its two outstanding properties in aqueous solutions, namely, its great ability to undergo hydrogen bonding with water because of the presence of three potential centers on each molecule and its sniall effect on the polarity of water.4 Urea actually increases the dielectric constant of water appreciably and the surface tension slightly. At high concentrations urea modifies t h e "iceberg" structure around solute niolecules as has been inferred from micellization and protein denaturation studies without unduly affecting interfacial effect~.~*~816,~~ In order to supplement our initial study of the hydration effects in the counterion layer of ionic micelles an attempt was made to elucidate the detailed nature of hydrophobic bonding in the niicellar core of ionic micelles. I n analogy with the two investigations cited a b o ~ e ,this ~ , ~was achieved by studying the effect of urea on the c.m.c. of n-alkyl sulfates. For comparison with the data on n-alkyl sulfates the effect of urea on the c.1n.c. of n-alkyl ether alcohol sulfates and of polyoxyethylene alkanols has also been studied.
Experimental Lithium n-dodecyl sulfate was prepared by treating Givaudan 1-dodecanol (99.6%) with so3 (Baker Sulfan). Dry nitrogen was passed over the liquid SO3 and the vapors introduced into the stirred 1-dodecanol at 40-50". The n-dodecyl sulfuric acid was neutralized by addition to an aqueous solution of LiOH until the paste was at pH 8. The paste was dried and recrystallized three times from 2-propanol. The molecular weight of lithium n-dodecyl sulfate was deterlnined by conversion to the free acid through ion exchange and subsequent titration. The observed molecular weight was 273.1 as compared to the theoretical one of 272.4. T h e Journal of Physical Chemistry
11.*J. SCHICK
Tetramethylammonium n-dodecyl sulfate was prepared by a similar procedure as lithium n-dodecyl sulfate. The n-dodecyl sulfuric acid was neutralized by addition to an aqueous solution of tetramethylaminoniuin hydroxide until the paste was at pH 8. The paste was dried and recrystallized four times from 2-propanol and five times from ethanol. The observed molecular weight was 339.9 as compared to the theoretical one, 339.6. Sodium n-dodecyl sulfate was prepared by the method of Dreger, et uZ.,~* recrystallized several times from ethanol, and extracted with petroleum ether. The synthesis of the sodiuni n-dodecyl ether alcohol sulfates has been referred to b e f ~ r e . ~The purity of these materials was readily checked from the shape of the surface tension us. logarithm of concentration plots near the c.m.c. The origin of the polyoxyethylene alkanols has been described previ~usly.~The water was redistilled from alkaline permanganate. Analytical reagent grade electrolytes and urea were used. For the procedure of the surface tension measurements see our previous comm~nication.~
Results Changes in the c.m.c., i.e., the maximum concentration of molecular dispersion, are a measure of the balance of forces causing the formation of micelles.1g Moreover, since interpretation of the experimentally measured behavior of micelles in terms of their structure requires extrapolation to conditions under which interaction between the micelles is negligible or, in other words, zero concentration, we were concerned with establishing this concentration, i e . , the c.m.c. Throughout this investigation, the c.1n.c. values were taken from the sharp breaks in the surface tension us. logarithm of detergent concentration plot^.^,'^ Ionic Detergents. The effect of the hydration of univalent counterions on the c.ni.c. values of some ndodecyl sulfates has been studied as a function of counterion concentration maintaining a constant similion at 25.0" and is illustrated in Fig. 1. In aqueous solutions, the c.1n.c. values decreased in the order of diminished hydration of counterions, uiz., Table I, from n-CIzH2&3O4Li(7.9 X M ) , n-ClzHz5SO4Na (7.2 X M ) to n-C12H26S04TblA(4.8 X (16) D. B. Wetlaufer, S. K. Malik, L. Stoller, and R. L. Coffin, J . Am. Chem. Soc., 86, 508 (1964). (17) W. Kauzman, A d a m . Protein Chem., 14, 1 (1959). (18) E. E. Dreger. G. I. Keim, G. D. Miles, L. Shedlovsky, and J. Ross, I n d . Eng. Chem., 3 6 , 610 (1944). (19) M. J. Schick, S. M. Atlas, and F. R. Eirich, J . P h y s . Chem., 66, 1326 (1962).
EPPECl OF
ELECTROLYTE AND
3587
UREA ON AIICELLE I”OII3IATION
Table I : F2ffec.t of Urea on the C.m.c. of Some n-Alkyl Sulfate Solutions -
--C.m.c., M--
Urea, IM
0 1 2 3 4.5 6 0 1 2 3 4.5 6 0 1 2 3 4.5 6 0 1 2 3 4.5 6 0 3 6
100
7.4
x 10-3
1.06 X IOez 1.41 X 7 . 0 x 10-3
8.0
x
10-3
1.0 4.8
x x
10-2 10-3
1.0
x
3 . 1 x 10-2 3 . 2 x 10-2 3 . 5 x 10-2 3 . 7 x 10-2 4 . 3 x 10-2 5 . 0 x 10-2 7.2 x 10-3 7.4 x 10-3 8 . 0 x 10-3 ! L O x 10-3 1 . 0 2 x 10-2 1 20 x 10-2 7 . 9 x 10-3 8 . 0 x 10-3 8 . 2 x 10-3 8.8 X 9 . 4 x 10-3 1 . 0 x 10-2 4 . 8 x 10-3
5.0 x 10-3
6.6 X 10-2
7
450
250
5.8 X 6.9 x 7.9 x 1.0 x
low3 10-3 10-3 10-2
C.m.c./c.rn.c. (0)---
I-’
100
7.6
x
10-3
1.00
9.1
x
10-3
1.43
8 . 0 x 10-3
1.15 x
1.91 1 . 00
8 . 9 X 10-3
1.14
1.05 X 6 . 3 x 10-3
1.43 1 .00
x
10-3
1.37
1.08 X 6 . 0 x 10-4 6 . 4 x 10-4 7 . 9 x 10-4
2 08
7.9
250
1 .OO 1.03 1.13 1.19 1.39 1.61 1.OO 1.03 1.11 1.25 1.42 1.67 1.00 1.01 1.04 1.11 1.19 1.27 1.00 1.04 1.20 1.43 1.65 2.08
460
1.OO
1.20 1.52 1 .00
1.11 1.31 1. O O
1.25 1.72 1 .00 1.07 1 .g1
are more closely attached to the aggregates. These results conform with earlicr predictions of Goddard, et ~ l . , ~ O a nLar1ge.l’ d In addition to this study of hydration effects i n the counterion layer of n-dodecyl sulfates, the object of this investigatiori was also to understand hydrophobic bonding i n the niicellar core of ioriic detergents. Therefore, the effect of urea on the c m c . of n-dodecyl sulfate solutions has been determined as a function of urea concentration and the pertinent data are given in Table I and Fig. 2. Only a t high urea concentrations was a marked increase in c.1n.c. observed, which followed an almost li riear relation between c,m.c. / c.m.c. (0) us. urea concentration in the region of 2 to 6 log (c.1n.c.) = -3.55 - 0.68 log (Li+) (1) nioles/l. of urea. The ratio of the c.1ii.c. in the txesence log (c.ni.c.) = -3.60 - 0.66 log (Na+) of urea to the c.1ii.c. in the absence of urea, c.m.c./ (2) c.ni.c. ( O ) , has been adopted as a convenient way of log (c.m.c.) = -3.65 - 0.57 log (T;\IA+) (3) representing the data.4 The c.m.c. determinations of The intercepts vary with the degree of hydration of the the n-dodecyl sulfates given in Table I have been carried counterions,20 and the slopes decrease slightly in the out at 10.0, 25.0, arid 4,5.0”, whereas those of sodium order n-Cl2HZ5SO4Li,n-Cl2HZ5SO4Sa, to n-CI2H25SO4 n-decyl sulfate arid sodiuin n-hexadecyl sulfate have TMA. The latter may be attributed to a reduction in the effect of shielding of charges when the Counterions ( 2 0 ) E. I t . Nightingale, ,r, P h y s . Chem., 66,894 (1962). M).10314320 These results are in line with those reported by hIysels and P r i n ~ e n , who ’ ~ concluded that hydrated ions are less closely attached to the aggregates than unhydrated ones and, therefore, contribute less to their formation. I t is worth noting that these c.m.c. values determined by the surface tension method are somewhat lower coriipared to those of the lightscattering method, but follow the same order. The results of Fig. 1 follow the general relation that the logarithm of the c.m.c. of a colloidal electrolyte is a h e a r function of the logarithm of the total concentration of the counterion.s These results may also be expressed by the equations
Volume 68, Number 12
December, l9R4
2.0 I
3.5
1
J 2.5
2.0
1.0
1.5
0.5
0
-Loa ( A I + , rnoles/l.)
Figure 1. Log-log plot of the effecats of salts on the c.rn.c. of sorrie n-dodecyl sulfate solutions st 25.0'.
dodecyl sulfate within experiinental accuracy. I n contrast, a much lower increase in c.ii1.c. on urea addition was observed with sodiurn n-hexadecyl sulfate than with the shorter chain homolog a t 45.0" The increments in c.iii.c. values of n-dodecyl sulfates on urca addition dccrease on raising the tcriiperature froiii 10.0 to 45.0". Thesc results are a t variance with those of JIukerjec and Ray,4 who obscrved 110 teiiipcrature deperiderice in the c.1n.c. of dodecylpyridiniuiii iodide solutions on urea addition. Thc effect of urca on the c.m.c. of n-dodccyl sulfatcs as a function of the hydration of the counterions is shown jii Fig. 2. The increments in c.m.c. values on urea addition, iri the concentration region from 0 to 6 JI, increase in the order of diminishcd hydration of the c o ~ r i t c r i o r i: ' ~ ~ ~ ~ lithium n-dodecyl sulfatc, sodium n-dodecyl sulfate to T h I A n-dodecyl sulfate. Nonionzc Detergents. For comparison with the data on ionic detergents, the effect of urea on the c.11i.c. of sodium n-dodecyl ether alcohol sulfates arid polyoxyethylene alkanols in aqueous solutions has also been studied as a function of urea coriccritration arid the pertinent data are given in Tables I1 and 111. With the sodium ether alcohol sulfates, the incrernents in c.11i.c. valucs on urea addition increase with increasing lcngth of the polyethcr chain. Thus, the transition from the moderate effects of ionic detcrgents to the larger effects of nonionic detergents with comparable hydrophobic groups, viz., Tables I and 111, is reflected in these results. With the polyoxycthylcne alkanols, the increments in c.ni.c. values on urea addition increase in the order of thcir c.1ii.c. values in the absence of urea: 1-hexadecanol 30 EO, l-dodecano1 7 EO, and 1-dodecanol 30 130. The effect of 6 M urea on the c.11i.c. values of polyoxyethylerie alkanol solutions as a function of temperaturc is shown in Fig. 3. For comparison, data of sodium ndodecyl sulfate and of sodium n-dodecyl ether alcohol sulfates have been included and are represented by the dotted lines. The values of c.ni.c./c.in.c. (0) in the c.m.c./c.m.c. (0) us. teriipcrature plots of the polyoxyethylene alkanols decrease with increasing tcmperature and follow the order: 1-dodecanol 30 EO, 1-dodecanol 7 EO, and 1-hexadecanol 30 EO. In analogy the c.in.c./c.in.c. (0) values in the c.m.c./ c.m.c. (0) us. temperature plots of sodiuiii n-dodccyl sulfate arid the intcrincdiatc sodium n-dodecyl ether alcohol sulfates decrease iri the tcinpcraturc rangc from 10.0 to 45.0'. An interprctation of these data of polyoxyethylene conipounds is givcri in the discussion in terms of changes iri the hydration of the cthylcnc oxide chains. Finally, the effect of urea on the c.1n.c. of l-dodeca-
+
0
1
2
3
5
IJreu. inules/l.
Figure 2. Effect of urea on the c.rn.c. of w m e n-alkylsulfate solutions a t 25.0".
been liniitcd to one teniperaturc, i . e . , 25.0 arid 45.0", respectively. Sodium n-hexadecyl sulfate is insoluble bclow 45.0". No sigriificaiit difference in susceptibility to changes in c.ni.c. on urea addition was observed at 2,5.0° between sodiuni n-decyl sulfate arid sodium n-
The Jozirnal of Physical Chemistry
+
+
+
+ +
EFFECT O F
ELECTROLYTE AKD UREA ON MICELLE
FORMATION
3589
-
.--
-
Table I1 : Effect of Urea on the C.m.c. of Sodium n-Dodecyl Ether Alcohol Sulfate Solutions Urea,
~ - C I ~ H ~OC2H&-S"34Na ~--(
n-ClzHz6-(OCzH)l?.5-SOaNa
-
C.m.e.,
I----
M
100
0 3 6 0 3 6
1 . 1 x 10-4 1.55 X 10-4 2.85 x 10-4 6 . 5 x 10-5 1 . 3 x 10-4 2.48 X
450
25O
1.0 x 1.5x 2.5 x 6.0 x 1.2 x 2.2 x
10-4 10-4 10-4 10-5 10-4 10-4
,--10"
1 . 2 x 10-4 1 . 9 3 x: 10-4 2.70 x 10-4 4 . 5 x 10-5 7.02 x 10-6 1 . 4 1 x 10-4
1.00 1.41 2.60 1.oo 2.00 3.80
C.m.c./c.rn.c.(O)-
7
25O
450
1.oo 1.50 2.50 1.00 2.00 3.70
1.00 1.61 2.25 1.oo 1,56 3.13
Table 111: Effect of Urea on the C.m.c. of Polyoxyethylene Alkanols in Aqueous Solutions
__
Urea, Hydrophokie
n-Dodecanol n-Dodecanol n-Dodecanol n-Dodecanol n-Dodecanol n-Dodecanol n-Hexadecanol n-Hexadecanol n-Hexadecanol
nEO
7
30 30
M
100
C. ni,e., M2.50
450
100
25O
450
0 3 6 0 3 6 0 3 6
8 . 0 x 10-5 1 . 2 x 10-4 2.08 x 10-4 9 . 0 x 10-5 3 . 6 x 10-4 7 . 2 x 10-4 2 . 0 x 10-6 3 . 2 x 10-6 4 . 0 X 10-6
5 . 0 x 10-6 6 . 2 5 x 10-5 1.25 x 10-4 8 . 0 x 10-5 1 . 6 x 10-4 2 . 5 x 10-4 1 . 1 x 10-5 1 . 6 x 10-5 2 . 0 x 10-5
2 . 8 x 10-5 3 . 4 x 10-5 5.6 X 4 . 8 x 10-5 6 . 7 x 10-5 9 . 4 5 x 10-5 5.0 X 113-8 6 . 3 5 X 10-0 7 . 9 x 10-0
1.00 1.50 2.60 1.00 4.00 8.00 1.00 1.60 2.00
1.00 1.25 2.50 1.00 2.00 3.13 1.00 1.45 1.82
1.00 1.22 2.00 1.00 1.40 1.97 1.00 1.27 1.58
------C.m.o./c.rn.c.(O)-----
7
+
no1 30 EO in electrolyte solutions has been determined a t 25.0", and the results are listed in Table IV. In previous communications we have shown that a t a specific temperature the lowering of the c.1n.c. of a nonionic Table IV: Effect of Urea on the C.m.c. of n-Dodecanol EO in Electrolyte Solutions at 25.0"
Solvent
Hz0 0.43 M 0.43M 0.43 M 0.86 M 0.86 M 0.86 M 0.86 M 0.86 M 0.86 M 0.86 M 0,86M 0.86 M 0.86 M 0.86 M 0.86 M 0.86 M 0.86 M 0.86 M a
NaCl NaCl NaCl NaCl
NaCl NaCl NaCNS NaCKIS NaCKIS l/zNa~SO4 1/2Na2S04 '/zNazSO4 LiCl LiCl LiCl
T1CIAC1" TMACl" TMAC1"
Urea, M
0 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6 0 3 6
Tetramethylammonium chloride
C.rn.c., M
8 . 0 x 10-5 3 . 0 x 10-5 4 . 5 x 10-5 1 . 0 x 10-4 2 . 0 x 10-5 5 . 0 x 10-5 1 . 2 x 10-4 5 . 5 x 10-5 8 . 7 x 10-5 1 . 1 x 10-4 1 . 2 x 10-5 2 . 9 x 10-5 3 . 6 x 10-5 4 . 0 x 10-5 5 . 0 x 10-5 5 . 0 x 10-5 3.0 x 5 . 7 x 10-6 1.0 x 10-4
+ 30
C.m.0. ____ C.m.c. ( 0 )
1.00 1.50 3.34 1.00 2.50 6.00 1.oo 1.58 2.00 1.00 2.40 3.00 1.00 1.25 2.00 1 .oo 1.90 3.33
\ 0
10
20
30
40
50
Temp., OC.
Figure 3. Effect of 6 M urea on the c.m.c. of polyoxyethylene alkanol solutions as a function of temperature.
detergent on electrolyte addition is proportional to the electrolyte concent ration, but inversely proportional Volume 68,Number 12 December, 1.96'4
M. J. SCHICK
3590
to the lyotropic number of the anion^.^^'^^^^ The effect of variations in the lyotropic number of the anions is more pronounced than that of the cations. This also follows from the results of Table IV, which demonstrate both the effects of electrolyte concentration and lyotropic number. In contrast, the c.m.c. of l-dodecanol 30 EO is raised on urea addition (see Table 111). From this it follows that two opposing effects must take place in this simultaneous addition of electrolyte and urea to 1-dodecanol 30 EO solutions. Let us now compare the results in Table IV a t the highest urea concentration level, i.e., 6 M . In general, the magnitude of the c.m.c./c.m.c. (0) values in these solutions correlates with the lowering of the c.m.c. in electrolyte solutions containing no urea. Thus, the increase in c.m.c./c.m.c. (0) values is proportional to the electrolyte concentration, 3.34 for 0.43 M NaC1 and 6.00 for 0.86 M NaCl but is inversely proportional to the lyotropic number of the anions, 2.00 for 0.86 M NaCKS and 3.00 for 0.86 M 1/zNa2S04,or cations 2.00 for 0.86 M LiCl and 3.33 for 0.86 M TMAC1. No marked difference of the effect of variations in the lyotropic number of the anions compared to that of the cations in c.m.c./c.m.c. (0) in the presence of 6 M urea was observed in contrast to the large difference in the absence of urea.
+
+
Discussion
surrounded by the water structure represents a comparatively low energy state but the concomitant restriction of motion provides a driving force to aggregation, which is an entropy effect at lower temperature^.^ Furthermore, Frank and Wen26and Nightingale20have shown a gradation of ions in their effect of altering the structure of water. Thus, alkali metal ions are structure breakers whereas the tetramethylammonium ion is a structure promoter in aqueous solutions. According to Frank and Wen,26structure breaking ions orient the neighboring solvent molecules, restricting their participation in hydrogen-bonded water clusters and leading to a region of disorder around the solvated solute molecules. The role of urea may be defined similarly. In order to explain the observed anomalously low viscosities of urea solutions, Rupleya4 suggests that urea disrupts the water structure. The proposed mechanism of the disruption of water structure by urea follows the same line as that given before for the structure breaking by ions.a4 Thus, we are led to hypothesize that ions and urea may modify the “iceberg” structure around the hydrocarbon chain of the single ions and consequently affect micelle formation of ionic detergents. The effects in the electrical double layer of ionic micelles are considered first. The decrease in c.m.c. values of the n-dodecyl sulfates, see Table I, can be simply related to diminished hydration of the univalent cations,29 i.e., from Lif, Yaf to TMAf, for which Mysels and PrincenL4have given a plausible explanation in terms of differences in the closeness of attachment of the counterions to the aggregates. However, in order to explain the drastic decrease in c.m.c. by the symmetrical tetramethylammonium ion despite
Water has always provided an interesting scientific challenge. In order to explain the properties of water and to elucidate its structure, many qualitative and quantitative theories have been proposed. Several of the more recent theories of water structure and the influence of various ions on it are given in the papers referred to below.22-a2 In particular, interest has (21) M . 3. Schick, J . Colloid Sci., 17, 810 (1962). been focused on the relation between hydrophobic bonding in proteins and water stru~ture.‘6,17,30-~~ (22) H. S. Frank and M. W. Evans, J . Chem. Phys., 13, 507 (1945). From the results of our previous c o m m u n i ~ a t i o n s ~ ~(23) ~ ~ F. S. Feates and D. J. G. Ives, J . Chem. Soc., 2798 (1956). (24) H. S.Frank, Proc. Roy. SOC.(London), A247, 481 (1958). and related investigation^,^-^^^^ it was inferred that (25) H. S.Frank and W. Y . Wen, Discussions Faraday Soc., 24, 133 changes in the water medium are an important factor (1967). in stabilizing an equilibrium size distribution in micelle (26) H. S.Frank, “Desalineation Research Conference,” Publication 942 of National Academy of Science, National Research Conference, formation. Therefore, it seems appropriate to inWashington, D.C., 1962, p. 141. terpret the results of this investigation in terms of the (27) H. S. Frank, “The Effects of Solutes on the Structure of Water,” structural concepts of water. Paper presented at the 144th National Meeting of the American Chemical Society, New York, N. Y., September 9-13, 1963. Let us first define the “iceberg” picture and the role (28) R. P. Marchi and H. Eyring, J . P h y s . C h e n . , 68,221 (1964). of ions and urea in aqueous solutions more accurately. 129) E. E. Nightingale, ibid., 63, 1381 (1C J ) . The currently accepted hypothesis for micelle forma(30) G. NBmethy and H. A. Scheraga, J . Chem. Phys., 36, 3382 tion of ionic detergents invokes the ‘%eberg” structure (1962). of Goddard, Hoeve, and Benson have (31) G. NBmethy and H. A. Scheraga, ibid., 36, 3401 (1962). (32) G. NQmethy and H. A. Scheraga, J . P h y s . Chem.. 66, 1773 given an explanation for the observed positive values for AR, at lower temperatures in terms of a water (so- (1962). (33) J. M. Corkill, f . F. Goodman, and S. P. Harrold, T r a n s . Faracalled “iceberg”) structure around the hydrocarbon dag Soc., 60,202 (1964). chain of the single ions.2q22 The hydrocarbon chain (34) J. A. Rupley, J . P h y s . C h e n . , 68, 2002 (1964). T h e Journal of Phyeical Chemistry
EFFECTOF ELECTROLYTE AND UREAON N~ICELLEFOXMATION
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a reduction in electrostatic interaction, Myselsd5 suggests that other nonelectrostatic interactions between hydrophobic mrfaces of aggregates and ions must also be a t work. Contact between these hydraphobic surfaces of aggregates and ions can reduce the hydrocarbon-water interface in the solution and, consequently, stabilize an equilibrium size distribution at a lower concentration level. From this it follows that other functions besides the electrostatic interaction may be assigned to counterions in micelle formation. In line with the “iceberg” picture, it is anticipated that a structure-promoting ion should enhance micelle formation, whereas a structure-breaking ion should reduce it. This is confirmed to a large extent by our results of the n-dodecyl sulfates. The structure-promoting tetramethylammonium ion exhibits a drastic decrease in c.m.c. from the high values in the presence of the structure breaking alkali metal ions, as shown in Table I. In contrast, the discrepancy between the c.m.c. values of the lithium and sodium n-dodecyl sulfates is small and follows the order of hydration rather than the efficacy of structure breaking. The wide discrepancy between the effect of structurepromoting and structure-breaking ions on micelle formation is shLown also in the results of urea addition to n-dodecyl sulfates, as illustrated in Fig. 2. The highly ordered “icebergs” around the solute molecules in the presence of structure promoting tetYmethy.1ammonium ion are more susceptible to modification by urea than the disordered “icebergs” around the solute molecules in the presence of structure-breaking alkali metal ions. The c.ri.c./c.m.c. (0) values follow the same order as 1he c.m.c. values, These results suggest, a multiplicity olf functions for the counterions in micelle formation; however, further work is required l,o explain the relative contributions from electrostatic interaction, ion hydration, water structure changes by ions, and other factors. Likewise, the argument of the “iceberg” picture is ueleful to explain the results of the temperature diependence of the C.M.C.of n-dodecyl sulfate solutions in the presence of urea, as given in Table I. On raising the temperature, a gradual decrease of ordered regions in water occurs above 25.0°.28 Consequently, it is anticipated that the modification of the “iceberg” structure around solute molecules on urea addition gradually decreases at temperatures above 25.0’. This is borne out by our results of Table I, which show a decrease in the increments in c.m.c. values of the n-dodecyl sulfates on urea addition, on raising the temperature from 10.0 to 45.0’. Secandly, the effects exhibited by nonionic micelles are considered. It is generally recognized that the
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over-all solubility of nonionic detergents depends on the extent of hyldration of the hydrophilic group through forma tion of hydrogen bonds between ether oxygens and water molecules.*l The c.m.c. values of nonionic detergents depend on the balance of forces between the van dler Waals interactions in the hydrophobic groups and the opposing hydration of the ethylene oxide chains. Therefore, it is expected, if we accept the validity of Rupley’sa4postulate of the disruption of water structure by urea, that addition of urea to polyoxyethylene surfactants increases the hydration of the ethylene oxide chains and consequentlly raises the c.m.c. values of these solutions. Thus, the marked increments in c.m.c. values of polyoxyethylene alkanol solutions on urea,addition, as shown iin Table 111, may be attributed to increased hydration caused by a decrease of ordered regions in water. As may be deduced from the inverse solubility relation, the hydration of the ethylene oxide chains decreases with increasing temperature.21 This is attributed to the fact that at higher temperatures the tendency of water to hydrogen bond to the ether oxygens is reduced. Consequently, the c.m.c. values of nonionic detergents decrease with increasing temperature.’ It has been stressed already that on raising the temperature, a gradual decrease of ordered regions in water occurs above 25.0°.28 Therefore, it is anticipated that the effect of urea on the hydration of the ethylene oxide chain gradually decreases a t temperaitures above 25.0’. This follows also from our results given in Fig. 3, which show a decrease in the increments in c.m.c. values of the polyoxyethylene alkanols in the presence of 6 M urea on raising the temperatture from 10.0 to 45.0’. After having given an explanation for the temperature dependence of c.m.o. values in urea solutions of n-dodecyl sulfates and polyoxyethylene 1-dodecanols, it is clear that both these explanations apply to the intermediate n-dodecyl ether alcohol sulfates. Finally, the results of Table IV on the addition of electrolyte and urea to 1-dodecanol 3. 30 EO solutions are briefly discusseld. The effects of electrolytes on nonionic detergents have been attributed to a saltingout m e ~ h a n i s r n . ’ ~The ~ ~ ~magnitude of this salting out effect depends on the electrolyte concentration and lyotropic number of the “counterion,” see Table IV. I t is concluded from examination of the two opposing effects that nonionic detergents under con-
(35) K. J. Mysels, Final Report Project NR 356-254, Office of Naval Research Contract NONR-274 (004).
Volume 68, Number 18 December, 1964
E.H. CROOK,G. F. TRBBBI, AND D. B. FORDYCE
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ditions of maximum dehydration of the ethylene oxide chain on electrolyte addition are prone to maximum hydration of the ethylene oxide chain on urea addition. Acknowledgment. The author wishes to express
his gratitude to Lever Brothers Co. for permission to publish this paper, to X r . D. Manning and Mr. E. Beyer for carrying out the experimental work, and to Mr. W. Pease for synthesizing the ionic detergents.
Thermodynamic Properties of Solutions of Homogeneous p,t-Octylphenoxyethoxyethanols(OPE,- lo)
by E. H. Crook, G. F. Trebbi, and D. B. Fordyce Research Laboratories, Rohm and Haas Co., Philadelphia, Pennsylvania
(Received A p r i l 8 , 2.964)
Surface tension as a function of concentration and temperature has been determined for aqueous solutions of single species p,t-octylphenoxyethoxyethanols(OPE1From these data surface tension a t the critical micelle concentration (yc.m.c.), area per molecule, and c.m.c. have been obtained and are presented as a function of temperature and ethylene oxide chain length. y o decreases with temperature, the dependence being greatest for compounds with ethylene oxide chain lengths greater than five EO units. This behavior may be attributed to the dehydration of the surfactant molecules which form a lower surface energy film at the air-water interface. Area per molecule at the air-water interface increases as a function of temperature due to the greater surface area swept out by the molecules with increased thermal-kinetic motion. Applying the pseudo-phase model of micellization, changes in enthalpy (AHmT)and entropy (ALLT) of micellization are calculated for OPEl-lo. AHmTand ASmT increase with increasing EO chain length and decrease with temperature. With increasing EO chain length, this corresponds to a greater positive energetic contribution to the micellization process due to the breaking of an increasing number of hydrogen bonds, and with increasing temperature at constant EO chain length, there is a reduced contribution to the energetics of the micellization process because of a lesser degree of initial hydrogen bonding.
Introduction In a previous investigation,l the surface tension of single species OPE1-loas a function of concentration was determined a t 25’. At this temperature, several conclusions were drawn as to the effect of ethylene oxide chain length on the surface physicochemical properties of these compounds. Only a few recent investigations2-10 have dealt with the effect of temperature on the surface tension properties of nonionic surfactants. It is commonly known that temperature has a T h e Journal of Physical Chemistry
drastic effect upon the bulk properties of certain nonionic compounds. This is demonstrated by the SOcalled “cloud point” phenomenon.11112 In this investigation a systematic study of the effect of temperature (1) E. H. Crook, D. B. Fordyce, and G. F. Trebbi, J . P h y s . Chern., lg8’ (1963). (2) M.J. Schick, ibid., 67, 1796 (1983). (3) M. J. Schick, J . Colloid Sci., 17, 801 (1962). (4) J. M. Corkill, J. F. Goodman, and R. H. Ottewill, Trans. Faraday sot., 57, 1627 (1961).
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