2413
J . Phys. Chem. 1986, 90, 2413-2418 reaction with hydroxide ion more than with cyanide ion, although the influence of the nucleophile upon the reaction of these substrates is not only concerned with kinetic parameters such as kM but also with the equilibrium parameters Ks and K P . The assumption that /3 is constant for CTABr and CTACl implies that the micelle is effectively saturated with Br- and C1hydrophobic counterions. This assumption is supported experi~ ~ ~ ~ ~ ~ ~recent ~ ~ measurements mentally and t h e ~ r e t i c a l l y , 'although of micellar charge by quasi-elastic light scattering suggest that /3 increases on addition of counterions to the s o l ~ t i o n .Even ~~~~~ for ions that bind very strongly to the micelle, to consider p as (38) Stieter. D. Prop. Colloid Polvm. Sci. 1978. 65. 45. (39j R$he,'A.; Sacukmam, E. J. Colloid 1nte;face Sci. 1979, 70,494; J . Phys. Chem. 1980,84, 1598. (40) Kratohvil, J. P. J . Colloid Interface Sci. 1980, 75, 271.
a constant is only an approximation. According to this evidence a new model was developed36which assumes independent equilibrium constants between the aqueous and micellar pseudophases for each of the ions in solution and permits calculation of the fraction of micellar head groups neutralized, 0. We have tested this kinetic model with the results found in the reactions of both substrates la and l b with hydroxide and cyanide ions. The kinetic results with hydroxide ion cannot be explained with this model, but it explains well the kinetic results with cyanide ion. Acknowledgment. F.O. gratefully acknowledges financial support from the "Plan de Formacion de Personal Investigador" of the Spanish Government. Registry No. la, 21412-03-3; l b , 17378-70-0; CTABr, 57-09-0; CTAC1, 112-02-7; CTAOH, 505-86-2; CTACN, 74784-26-2; CN-, 5712-5; OH-, 14280-30-9.
Relationship of Structure to Properties of Surfactants. 13. Surface and Thermodynamic Properties of Some Oxyethylenated Sulfates and Sulfonates Manila1 Dahanayake,+ Anna W. Cohen,f and Milton J. Rosen* Department of Chemistry, Brooklyn College, City University of New York, Brooklyn, New York 1I 2 1 0 (Received: October 12, 1985; In Final Form: January 8, 1986)
Surface and thermodynamic properties of the oxyethylenated anionic surfactants C10H210C2H4S03Na, C12H2,0C2H4S03Na, C,,HZ5OC2H4SO,Na,and C12HzS(OC2H4)2S0,Na were investigated at 10,25, and 40 OC in aqueous solution and in NaCl solution of 0.1 and 0.5 M total ionic strength. At all ionic strengths and temperatures studied, compounds with an oxyethylene group (EO) in the molecule are more surface-activethan the corresponding compounds without an EO in the molecule. Sulfated compounds, with or without an EO, are somewhat more surface-active than the corresponding sulfonates. The introduction of the first EO does not increase the area per molecule at the aqueous solution/air interface significantly,but the introduction of the second does. In pure water, surface pressure at the cmc (rcmc) is not increased by the introduction of one EO into the molecule, but in 0.1 or 0.5 M NaCl it is. The introduction of a second EO reduces rcmc at all ionic strengths. The first EO increases the negative value of AGOmicby 3 kJ mol-' and AGOad by 3.6 kJ mol-'; the second EO increases the values by 0.5 and 1.4 kJ mol-', respectively. The unusual properties of the EO-containing compounds may be due to complex formation between the ether oxygens and Na'.
Introduction As part of a study of the relationships between the chemical structures of well-purified surfactants and their surface and thermodynamic a number of anionic surfactants with oxyethylene units adjacent to the hydrophilic head group were synthesized and their properties investigated. Interest in anionic surfactants of this type stems in part from the industrial importance of these materials4 and in part from the poorly understood, unusual effects of the oxyethylene units on the properties of the compounds. Early workSy7had shown that although sulfated polyoxyethylenated alcohols containing a few oxyethylene units were much more water-soluble than the corresponding (nonoxyethylenated) alkyl sulfates, the former appeared to be more surface-active than the latter, as indicated by their smaller cmc values, which decreased with increase in the number oxyethylene units. This has been confirmed in recent work.4 In nonionic polyoxyethylenated alcohols with 2-8 oxyethylene units, increase in the number of units caused AGOad to become more negative and AGOmic to become less negative, the latter effect being attributed to steric inhibition of micellization.' To clarify the role of the oxyethylene groups in anionic surfactants, in the present investigation both oxyethylenated sulfates and oxyethylenated sulfonates were studied and the properties of the compounds compared with the corresponding non-oxyethylenated materials. Present address: Betz Paper Chemicals, Inc., Jacksonville, FL 32216. *Present address: Pall Corp., Glen Cove, NY 11542.
0022-3654/86/2090-2413$01.50/0
Surface tension, y, vs. bulk-phase surfactant concentration, C, data were used to calculate surface excess concentrations, rmax, at surface saturation. For solutions in pure water, the equation2 rmax
= [dr/d(log C + logf*)Imax/4.6RT
(1)
was used. r is the surface pressure (= yo - y, where yo is the surface tension of the solvent), and f* is the mean activity coefficient, evaluated* from the Debye-Huckel equation
+
log fz = - B l z + ~ - l I ' / ~1/ ( 0.33(~1'/*)
(2)
I is the ionic strength based on the free ions in the solution, (Y is the mean distance approach of the ions (in A) (taken as 0.6 for (1) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. Y . J . Phys. Chem. 1982, 86, 541. (2) Rosen, M. J.; Dahanayake, M.; Cohen, A. W. Colloids Surf. 1982,5, 159. (3) Danayake, M.; Rosen, M. J. In Relationship between Structure and Performance in Surfactants; Rosen, M. J., Ed.; American Chemical Society: Washington, DC, 1984; ACS Symp. Ser. No. 253, p 49. (4) Schwuger, M. J. In Relationship between Structure and Performance in Surfactants; Rosen, M. J., Ed.; American Chemical Society: Washington, DC, 1984; ACS Symp. Ser. No. 253, p 1. ( 5 ) Bistline, R. G.; Stirton, A. J.; Weil, J. K.; Maurer, E. W. J . Am. Oil Chem. SOC.1957, 34, 516. (6) Weil, J. K.; Bistline, R. G.; Stirton, A. J. J . Am. Oil Chem. SOC.1958, 62, 1083. (7) Weil, J. K.; Bistline, R. G. Stirton, A. J. J . Phys. Chem. 1958,62, 1796. (8) Boucher, E. S.; Grinchuk, T. M.; Zettlemoyer, A. C. J . Am. Oi/ Chem. SOC.1968, 49, 45.
0 1986 American Chemical Society
2414
Dahanayake et al.
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
the surfactant ion and 0.3 for the counterions), and B is 0.497 at 10 "C, 0.509 at 25 OC, and 0.524 at 40 OC. log f* in eq 1 is assumed to equal (log f + + log f-)/2. For a dilute solution of the 1:l anionic surfactant R-X', in the presence of a swamping amount of electrolyte containing a common nonsurfactant ion, the equation is2 rR-,max = [dr/d(log
CR-+ log f+)]/2.3RT
(3)
The minimum area per molecule at the air/aqueous solution interface, A,,, (in nm2), is then given by the relationship Amin
= 10'4/mmax
(4)
Standard free energies of micellization were calculated by the use of the relationship
+
AGO,,, = 2.3RT[log aR-,cmc K(1og a x + ) ]
= 2.3RT[log cmc
34' 3 d
(5)
where K is the effective electrical coefficient of micellization, obtained from the slope of the linear plot of (log cmc log f - ) vs. (log c,++ log f+). Standard entropies and enthalpies of micellization, ASo,,, and AH',,,, were calculated from the relationships
+
d(AGo)/dT = -ASo
(6)
+ TASO
(7)
Standard free energies of adsorption, AGOad, were calculated by use of the relationship9 AGo,d = 2.303RT IOg aR-x+ - r A m I n
(8)
Here, the standard state for the surface phase is defined as a surface filled with a monolayer of surface-active agent, i.e., F = rmax, but at a surface pressure of zero. In the present case, this becomes
+ log Cx+ + log f + ) 6.023 X IO-'xA,,,
c
2 2
l a
44
0
Figure 1. Surface tension vs. log of the concentration of C,oH21S03Na in aqueous solution at 10 ( A ) , 25 (0),and 40 OC (0);in NaCl solution of 0.1 M total ionic strength at 10 ( A ) ,25 (D),and 40 "C and in NaCl solution of 0.5 M total ionic strength at 10 (4), 25 ($), and 40
(a,
OC
(6).
crystalline product (23% of the theoretical yield). The purity was shown to be 99.5% by two-phase titration with Hyamine 1622, using mixed indicator. Preparation of Sodium Salts of Sulfated Polyoxyethylenated I -Dodecanol, C1 H2 OC,H4S04Na (C l E OS 0 ) and Cl H2 (0C2H4)2S04Na (C12E02SO).These were prepared by reacting the respective ether alcohols, C , 2 H 2 5 0 C 2 H 4 0 H and C12H25(OC2H,),OH (Nikko Chemical Co., Tokyo, Japan; >98% purity), in chloroform with 1 mol% excess freshly distilled chlorosulfonic acid. The temperature of the reaction mixture was maintained below 10 OC. The reaction mixture was neutralized with sodium hydroxide dissolved in ethanol. The chloroform was removed by distillation, the residue was taken up in absolute ethanol, and the mixture was filtered hot to remove insoluble matter. Crystallization of the residue four times from 2-propanol gave a white, crystalline product. The purity of C,2H,50C,H4S0,Na and C12H2j(OC2H4)2S0 4 N a was 99.2% and 99.0%, respectively, as determined by two-phase titration with Hyamine 1622, using mixed indicator. Sodium decanesulfonate (ClOS),sodium dodecanesulfonate (C12S),and sodium dodecyl sulfate (C12SO)of >98% purity were purchased from Research Plus, Bayonne, NJ. Before being used for surface tension measurements, aqueous solutions of the surfactants (in water that had been first deionized and then distilled twice, the last time from alkaline permanganate solution through a 3-ft-high Vigreaux column with quartz condenser and receiver) were further purified by repeated passage" through minicolumns of octadecylsilanized silica gel to remove any traces of impurities more surface-active than the parent compound. The concentration of surfactant in the effluent from these columns was determined by two-phase titration with Hyamine 1622. Sodium chloride used to increase the ionic strength of solutions was analytical grade material, which was then baked for several hours in a porcelain casserole at red heat to remove traces of organic compounds. Surface Tension Measurements. Measurements were made by two different investigators, using the Wilhelmy plate technique, with a sandblasted platinum plate of ca. 5-cm perimeter.'s2 Instruments were calibrated against quartz-condensed water (specific conductivity 1.1 X mho cm-I at 25 "C) each day that measurement was made. Sets of measurements were taken at 15-min intervals until no significant change occurred.
,
and
AGo,d = 2.303RT(log CR- + logf-
26 IbQ
+ log f - + K(1og Cx++ log f + ) ]
AHo = AGO
30
(9)
when A,,, is in nm2, x in m N m-l, and AGOad in kJ mol-'. Standard entropies of adsorption, ASoad, and enthalpies of adsorption, AHoad, were calculated from eq 6 and 7 .
Experimental Section Preparation of Sodium 2-Decoxyethanesulfonate, C,oH210C,H4S03Na (C,&OS). Sodium methoxide (0.1 1 mol) was prepared by dissolving freshly cut sodium metal in 50 mL of methanol cooled in an ice bath. After all the sodium metal had reacted, the methanol was removed and the product was dissolved in 250 mL of dimethyl sulfoxide (Me2SO) at 80-90 OC. The sodium salt of isethionic acid (GAF) (17.7 g, 0.12 mol), dissolved in 100 mL of MezSO at 90 OC, was added to sodium methoxide in M e 2 S 0 over a period of half an hour. After the methanol formed during the reaction was removed, bromodecane (26.2 g, 0.12 mol) was added over a period of 4-5 h and the reaction was continued overnight at 80 OC. The solution was cooled and filtered, and the residue was washed with 1-butanol. The yellow solid obtained was crystallized from 1-butanol, 2-propano1, and finally quartz-distilled water, to obtain 5.5 g of a white crystalline product (17.5% of the theoretical yield). The purity was shown to be 99.0% by two-phase titration of the product with Hyamine 1622, using mixed indicator.1° Preparation of 2-Dodecoxyethanesulfonate,CI 2HzsOC2H4S0 , N a (CI2EOS).The procedure was basically the same as for the previous compound. The crude product, sodium 2-dodecoxyethylenesulfonate, was recrystallized twice from 1 -butanol and twice from quartz-distilled water to obtain 7.2 g of a white (9) Rosen, M. J.; Aronson, S. Colloids Surf. 1981, 3, 201. (10) Reid, V. W.; Longman, G. F.; Heinerth, E. Tenside 1967, 4. 292.
Results and Discussion Plots of the surface tension, y, of aqueous solutions of CloS, CI2S,C,,EOS, C,,EOS, C,,EOSO, and C12E02S0 vs. log of their ( 1 1)
Rosen, M. J. J . Colloid Interface Sei. 1981, 79, 587.
The Journal of Physical Chemistry, Vol. 90, No. J l , 1986 2415
Structure vs. Properties of Surfactants TABLE I: Surface Properties of Cl&Iz1S03Na(Cl&3) medium
H20 0.1 M NaCl 0.5 M NaCI
cmc,
T, OC
mol dm-3 X lo3
mol cm-2 X 1O'O
Amin, nm2 X 100
pC20
mN m-'
cmc/Czo
10 25 40 10 25 40 10 25 40
47.9 42.7 39.8 25.7 21.1 18.2 7.94 7.33 6.53
3.37 3.32 3.05 4.06 3.85 3.67 4.46 4.24 4.04
49.2 51.8 54.4 40.9 43.1 45.2 37.2 40.6 41
1.70 1.69 1.66 2.29 2.2, 2.27 2.89 2.87 2.84
33.0 31.0 29.2 34.4 32.6 31.1 38.1 37.1 35.7
2.4 2.1 1.8 5.0 4.1 3.4 6.2 5.4 4.5
pC2, 2.38 2.36 2.33 3.4, 3.38 3.30 4.1, 4.06 3.9,
mN m-'
cmc/C2,
33.0 31.0
2.8 2.4
36.4 34.8
5.9 4.8
39.00
6.8
rmsv
.,
*cmw
TABLE 11: Surface Properties of C12H#03Na (C&) medium
H20 0.1 M NaCl 0.5 M NaCl
cmc, mol dm-' X lo3
T, OC 10 25 40 10 25 40 10 25 40
rmX,
Amin,
mol cm-2 X l O l o
nm2 X 100
3.02 2.93 2.78 3.92 3.76 3.55 3.98 3.85 3.60
54.9 56.7 59.7 42.4 44.2 46.8 41.7 41.7 43.8
12.4 11.4 2.47 2.42 0.79
*cmcr
TAB E III: Surface Properties of Cl&lZlOCH2CI -S03Na (CIoEOS) medium H,O 0.1 M NaCl 0.5 M NaCl
cmc,
rIll.aX9
Amin,
*cmc.
T, OC
mol dm-3 X lo3
mol omb2X 1O1O
nm2 X 100
pC2,
mN m-'
cmc/C2,
10 25 40 10 25 40 10 25 40
20.50 15.90 15.40 6.49 5.46 5.01
3.3,
494 51.5 53.3 39.4 43.1 45.2 37.5 38., 41.5
2.0a 2.10 2.0~ 2.93 2.92 2.87 3.60 3.54 3.4,
32.9 30.8 28.8 36.5 34.7 33.0
2.2 2.0 1.7 5.5 4.5 3.7
39.0 37.7
7.1 5.6
2.04 2.00
TABLE I V Surface Prowrties of CI~H2aOCH,S0.rNa (Cl,EOS) r m X 9
medium
H,O 0.1 M N a C l
0.5 M NaCl
T , OC 10 25 40 10 25 40 10 25 40
Amin,
mol cm-2 X loLo nm2 X 100 3.07 2.92 2.74 3.93 3.73 3.57 4.22 3.97 3.81
54.0 56.; 60.5 42.2 44.3 46.5 39.4 41.8 43.6
pC20 2.7s 2.7; 2.67 4.14 4.07 3.98 4.76 4.68 4.60
bulk concentration in mol dm-' (log C) in quartz-condensed distilled water and in NaCl solutions of 0.1 or 0.5 M total ionic strength a t 10,25, and 40 OC are shown in Figures 1-6. From these, plots of ?r vs. (log C logj) were obtained, and rmnwas calculated from the maximum slope in each case. A& values were and Ami, calculated by use of eq 4. Tables I-VI list cmc, rmax, values, together with rCmc (the effectiveness of surface tension reduction), pC2, (the efficiency of surface tension reduction), and cmc/C2, ratios. At all ionic strengths and temperatures investigated, the oxyethylenated materials are more surface-active than the corresponding non-oxyethylenated surfactants (their pC2, values are greater and their cmc values are smaller), with surface activity increasing with increase in the number of oxyethylene units in the molecule. This is consistent with previous data4-' showing that the cmc values of sulfated polyoxyethylenated alcohols, R(OC2H4)$04Na, where x = 1-4, are smaller than those of the corresponding alkyl sulfates and decrease as x increases from 1 to 4. The sulfated materials, both oxyethylenated and non-oxyethylenated, appear to be somewhat more surface-active than the corresponding sulfonates, as reflected in the pC2, values for C12EOS0 vs. ClzEOS and pC2, and cmc values for CI2S0vs.
+
Figure 2. Surface tension vs. log of the concentration of C12H25S03Na in aqueous solution at 10 (A),25 (0),and 40 OC (0);in NaCI solution of 0.1 M total ionic strength at 10 (A),25 and 40 "C and in NaCl solution of 0.5 M total ionic strength at 10 (&, 25 and 40 OC (0).
(a,
(a; (o),
CI2S. This, again, is consistent with previous datal2 on these last two compounds. On the other hand, the introduction of one oxyethylene group into the alkyl sulfates or alkanesulfonates has little effect on the area per molecule, as seen by the comparable A- values for C I S and C,,EOS, C12Sand C12EOS,and CI2SOand C12EOS0,respectively. The introduction of a second oxyethylene group (CI2EO2SO),however, increases Amin. The effectiveness of surface tension reduction, given by acme, is almost the same in pure water for the monooxyethylenated (12) Bujake, J. E.; Goddard, E. D. Trans. Faraday SOC.1965, 61, 90.
2416
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
Dahanayake et a].
TABLE V Surface Properties of CI2Hz5SO4Na(C1zSO) and Cl2HZ50CH2CH2SO4Na (C,,EOSO) compd CI._ ,so CIZEOSO
medium
T , OC
cmc, mol dm-j X lo3
mol cm-' X 10"
nm2 X 100
pC,,
mN m'l
H7O 0.i M NaCl H20
25 22 IO 25 40 10 25 40 10 25 40
7.94 1.29 4.73 3.91 4.14 0.48 0.43 0.49 0.14 0.13 0.15
3.ln 4.0; 3.07 2.9, 2.80 4.0, 3.8, 3.60 4.6, 4.4, 4.1,
52.5 41.2 54., 56.8 59., 41., 43., 46., 36., 37., 40.,
2.5, 3.67 2.8, 2.8, 2.72 4.29 4.2, 4.09 4.9, 4.8, 4.67
32.5 38.0 35.6 32.8 30.4 40.6 38.6 36.4 44.4 42.4 40.8
0.1 M NaCl 0.3 M NaCl
rmax,
Amin,
Pcmc
~
~~
cmc/czo 2.6 6.0 3.1 2.6 2.2 7.8 7.3 6.0 11.6 8.3 7.1
TABLE VI: Surface Properties of ClzHz5(OCHzCHz)2S04Na(ClZEO2SO)
H,O 0.1 M NaCl 0.5 M NaCl
I1
cmc, mol dm-3 X IO3
T, OC 10 25 40 10 25 40
medium
3.09 2.88 2.78 0.32 0.29 0.28 0.1 1 0.10 0.10
10 25 40
Amin,
rmam
mol cm-2 x
nm2 x 100
IOIO
2.76
60.0 63.2 66.4 45.5 47., 50.3 42.0 43+ 46.5
C,,
C,oH,,0CH,CH,S03Na
H,,OCH,H,SO,
acme,
pC2, 2.9, 2.9; 2.86 4.4, 4.3, 4.2, 5.04 4.98 4.g9
mN m-l
cmc/C2,
32.6 30.6 28.6 38.0 36.5 34.8 41.5 40.2 38.6
2.8 2.5 2.0 7.9 6.7 4.8 12.1 10.0 1.9
No
62
4
58i-
54-
-,
E 50-
-x E
46-
42 -
38
-
34 -
i
I
35
1
1
25
3.0
42 I
2.0
-lag C
Figure 3. Surface tension vs. log of the concentration of CIOHIIOCH2CH2SOpNain aqueous solution at 10 (A), 25 (0),and 40 OC ( 0 ) ;in
(a,
NaCl solution of 0.1 M total ionic strength a t 10 (A),25 and 40 OC (p?; and in NaCl solution of 0.5 M total ionic strength at 10 ( h ) , 25 (+), and 40 OC (0).
compounds and for their corresponding non-oxyethylenated increases with increase in the cmc/C,, compounds. Since rcmc ratio and with decrease in the Ah value,13 this appears to be linked to the similar cmc/C,, and surface area values for these compounds. In contrast to what is observed in pure water, the monooxyethylenated compounds in 0.1 and 0.5 M NaCl show larger ,r values than their corresponding non-oxyethylenated analogues. This greater effectiveness in surface tension reduction appears to be a consequence of the larger cmc/C,, ratios of the oxy(13) Rosen, M. J. J . Colloid Interface Sci. 1976, 56, 320
I
I
,
8
5.0
4.0 -log
3.0
c
Figure 4. Surface tension vs. log of the concentration of C12H250CH2CH2S0,Na in aqueous solution at 10 (A), 25 (0),and 40 "C (0);in NaCl solution of 0.1 M total ionic strength at I O (A),25 (a), and 40 'C and in NaCl solution of 0.5 M total ionic strength at 10 25 (Q),and.40 OC ($).
(o),
(m;
TABLE VII: Effective Electrical Coefficients of Micellization K
compd CIOS CI2S CIoEOS CI2EOSO C,,EO,SO
10 o
c
0.94 0.84 0.89 0.78
25 OC
40 OC
0.89 0.88 0.74 0.83 0.77
0.89 0.85 0.73 0.82 0.76
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2417
Structure vs. Properties of Surfactants 64
TABLE VIII: Standard Thermodynamic Parameters of Micellization
58
ClOS 52
5
-
46
E
h
CloEOS 40
c12so Cl2EOSO
34
28 55
40
50
c
-100
20
30
I5
Figure 5. Surface tension vs. log of the concentration of C12H250CH2CH2S04Nain aqueous solution at 10 (A), 25 (0),and 40 OC (0);in NaCl solution of 0.1 M total ionic strength at 10 (b), 25 (a),and 40 "C and in NaCl solution of 0.5 M total ionic strength at 10 (4),
(m;
25
(e),and 40 OC (0). 64
C12E02SO
Cl2S CloEOS CIZEOS
c12so
CIiEOSO C12E02SO
32 60
5 0 -100
c
40
20
30
Figure 6. Surface tension vs. log of the concentration of C12H25(OCH2CH2)S04Nain aqueous solution at 10 (A),25 (0),and 40 OC (0); in NaCl solution of 0.1 M total ionic strength at 10 @), 25 and 40 OC 0 ;and in NaCl solution of 0.5 M total ionic strength at 10 (b),
-14.77 -15.69 -16.60 -21.00 -22.15 -17.37 -18.35 -19.36 -21.97 -24.06 -25.26 -26.17 -24.26 -25.74 -27.06
2.56 2.40
0.061 0.061
2.06
0.077
1.10 0.925
0.065 0.065
-0.05 -1.4 -7,2
2.64 o,48
0.073 0.080 0.061 0.095 0.088
TABLE I X Standard Thermodvnamic Parameters of Adsomtion
compd CIOS
t
10 25 40 10 25 40 10 25 40 25 10 25 40 10 25 40
T, OC 10 25 40 10 25 40 10 25 40 10 25 40 25 10 25 40 10 25 40
AGoad,
kJ mol-' -23.64 -25.02 -26.16 ~.~. -30.20 -31.61 -32.70 -27.11 -28.50 -29.81 -34.48 -35.48 -36.78 -32.25 -34.35 -35.90 -37.00 -35.69 -37.32 -38.91
AHoad, kJ mol-l
kJ mol-' K-'
2.4 -1,8
0.092 0.078
-3,74 -5.7
soad.
0.093 0.087
-0.7 -2.7
0.093 0.086
-6.9 -9,7
0.096 0.086
-5.1
-'2'5 -4.9 -5.8
0.103 0.078 0.109 0.106
(Table VIII) by about 3 kJ mol-', which is equivalent to adding one X H 2 - group to the hydrophobic group, while the introduction of the second oxyethylene unit (ClzEO2SO)increases it only by about 0.5 kJ mol-'. The smaller increase in the negative value 25 (o), and 40 OC (0). of AGominupon introducing the second oxyethylene group may ethylenated compounds in solutions of high ionic strengths. On again be due to steric inhibition of micellization since, as seen the other hand, with introduction of a second oxyethylene group above, the surface area of the molecule appears to increase only is reduced at into the molecule (C12EOzSO),the value of rcmc upon the introduction of the second oxyethylene group. The standard free energies of adsorption (Table IX) also become all ionic strengths. This appears to be linked mainly to the increase in the area molecule a t the surface (Amin). more negative upon the introduction of oxyethylene units into the Standard free energies of micellization are listed in Table VIII. molecule. Here, the value becomes more negative by about 3.6 Values of K,the effective electrical coefficient of micellization, kJ mol-', equivalent to adding a little more than one -CHz- group and Somic are listed in Table VU. Table VI11 also lists Wmic to the hydrophobic group, upon introduction of the first oxyvalues. ethylene units into the molecule (C,,EOS, C12EOS,ClzEOSO) Standard free energies of adsorption at the aqueous solution/air and by 1.4 kJ mol-' more upon introduction of the second oxyinterface were calculated by eq 9. For compounds C,& C,,EOS, ethylene unit (C12E02SO). Again, the smaller change upon ClzEOSO,and Cl2EO2SO,the maximum surface pressure, A,,, introduction of the second oxyethylene unit may be attributed to was used together with the activities a t the critical micelle consteric inhibition of the process, with the steric effect expected to centration for the calculation of AGOadvalues. For compounds be less significant at the planar liquid/air interface than at the CI2Sand C12EOS,where the cmc could not be reached due to convex micellar surface. The AHomicand AHoad values for ClzE02S0are consistent with this explanation. Whereas both their poor solubility, the activity and Aminvalues at A = 20 were W f iand c W dvalues become more negative upon introduction used. The AGOad values, together with the M 0 a d and h S o a d values, are listed in Table IX. of the first oxyethylene unit into the molecule, the introduction The standard free energies of micellization for the oxyof the second oxyethylene group in C12EOzS0causes these values ethylenated sulfates and sulfonates are more negative than those to become more positive than those for C12EOS0under the same conditions. This indicates greater dehydration of the ClzE02S0 for the corresponding non-oxyethylenated compounds. This is in molecule in the micellization and adsorption processes, necessitated contrast to the effect in nonionic polyoxyethylenated alcohols,' by the greater bulkiness of the hydrophilic head with two oxywhere AGOdc was found to become less negative with the increase ethylene units. in oxyethylene content of the molecule, an effect attributed to steric inhibition of micellization. The unusual surface and thermodynamic properties of the Introduction of the first oxyethylene group into the molecule polyoxyethylenated anionics may be the result of complex formation between the ether oxygen atoms of the oxyethylene groups (CloEOSand CI2EOSO)increases the negative value of AGOmic
(a,
2418
J . Phys. Chem. 1986, 90, 2418-2421
and the Na+ counterion. Alkali-metal cations have been shown to be capable of complexing with polyoxyethylenated nonionics in the presence of anionic surfactants and other large anions,14 and these complexes have been used as a basis for the analysis of this type of nonionic.15J6 In polyoxyethylenated anionics, the negative charge of the surfactant ion should increase the tendency of the ether oxygens to interact with Na+. Interaction with Na+ imparts partial zwitterionic character to the surfactant. Such zwitterionic character should result in a decrease in the cmc and (14) Toei, K.; Motomizu, S.; Umano, T. Talanra 1982, 29, 103. (15) Anderson, N. H.; Girling, J. Analyst (London) 1982, 107, 836. (16) Tsubouchi, M.; Yamasaki, N.; Yanigasawa, K. Anal. Chem. 1985, 57, 783.
C,, values and an increase in the cmc/C2, ratio3,when compared to those of the corresponding non-oxyethylenated materials. All these changes are observed in the oxyethylenated compounds studied. Moreover, as would as expected from an interaction involving Na', these changes increase with the Na+ content of the solution, providing support for the complex formation hypothesis. Acknowledgment. This material is based upon work supported in part by the National Science Foundation under Grant No. ENG-7825930. Registry NO. ClOEOS, 101225-35-8; C12EOS, 20829-85-0; CIZEOSO, 15826- 16-1; Cj2EOZS0, 3088-31-1.
Size of Sodium Dodecyl Sulfate Micelle in Concentrated Salt Solutlons Jin-Ming Chen, Tzu-Min Su, and Chung Yuan MOU* Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China 107 (Received: October 28, 1985; In Final Form: January 8. 1986)
We investigate the effect of high salt concentration on the average aggregation number of sodium dodecyl sulfate (SDS) over the temperature range 30-70 O C . The method is based on the increase of self-quenching of the fluorescence of micelle-solubilized pyrene through excimer formation. Transient fluorescence decay is measured and analyzed. It is shown that an exponent weighted averaged aggregation number ( n ) eis obtained by this technique; it is smaller than weight average aggregation number for a polydisperse micelle system. For SDS in NaCl solution, we observe an increase of ( n ) , as the temperature is lowered and ionic strength is increased. We use the thermodynamic model developed by Missel et al. to calculate ( n ) e . Agreements between theory and our experimental results are quantitative.
1. Introduction
The formation of micellar aggregates in amphiphile solutions is a well-known phenomenon clearly established in early lightscattering studies.' Usually, ionic amphiphiles form small nearly spherical micelles at low ionic strength. When one increases the concentration of counterion, above some ionic concentration the aggregation number can increase up to 20-fold and the micelle is interpreted to undergo a sphere-to-rod shape transition. The most studied system is sodium dodecyl sulfate (SDS) in NaCl solution by either statiS4 or dynamic light-scattering techniquesw For aqueous solutions of SDS plus NaCl, in the range C(NaC1) = 0-0.8 M at 30 OC, it is found that aggregation number ranges from 60 to 1000; it drastically decreases at higher temperature. In fact, a t 80 OC, the micelles are again small with an average aggregation number around 100 at C(NaC1) = 0.8 M. Around the same time, fluorescence quenching of the micelle-solubilized fluorophore was investigated to obtain an aggregation number of SDS in NaCl solution.1w12 It was found (1) Mysels, K.J.; Princen, L. H. J . Phys. Chem. 1959, 63, 1699. (2) Emerson, M. F.; Holtzer, A. J . Phys. Chem. 1967, 71, 1898. (3) Anacker, E. W. In Solution Chemistry of Surfactants. Vol. 1, Mittel, K. L., Ed.; Plenum: New York, 1979. (4) Ikeda, S.; Hayashi, S.; h a c , T. J . Phys. Chem. 1981, 85, 106. ( 5 ) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976,80, 1075. ( 6 ) Missel, P. J..; Mazer, N. A.; Benedek, G. B.; Young, C. Y.; Carey, M. C. J . Phys. Chem. 1980, 84, 1044. (7) Missel, P. J.; Mazer, N. A,; Benedek, G. B.; Carey, M. C. J . Phys. Chem. 1983,87, 1264. (8) Corti, M.; Degiorgia, V. J. Phys. Chem. 1981, 85, 711. (9) Flamberg, A.; Pecora, R. J. Phys. Chem. 1984, 88, 3026. (IO) Turro, N. J.; Yekta, A. J . Am. Chem. SOC.1978, ZOO, 5951.
0022-3654/86/2090-2418$01 SO10
that the apparent average size measured is much less than the light-scattering results when a large rodlike micelle is detected at high salt concentrations. For example, at 0.8 M NaCl and 35 OC fluorescence quenching experiment gives an average aggregation around 240 while light scattering gives an average size about 1000. Many workers"-13a'4 suspected that the fluorescence technique gives too low a value for the micelle size. Lianos and Zana" believe that fluorescence quenching technique is limited to a size of n less than 200 because of incomplete excimer formation within its own lifetime (-400 ns), and the method simply fails at [NaCl] = 0.8 M. Because of this, the fluorescence quenching technique has not been applied to large rodlike micelles since then. However, there is another perfectly reasonable explanation: the large rodlike micelle is very polydisperse and the difference in the measured average aggregation size reflects different ways of averaging. In fact, this is indicated by Missel et al.;6,7they have a simple thermodynamical model that gives the size distribution function P ( n ) . They applied the model to their QLS study and explained their measured average aggregation of SDS very well. We therefore think it is necessary to reopen this question again using the available distribution to examine the average size determined by the fluorescence quenching technique. In this paper, we use solubilized pyrene as a fluorescene probe. We monitor the time-resolved fluorescence quenching due to excimer formation. The relative importance of monomer fluorescence and excimer formation is determined by the partition of pyrene among the micelles. By assuming random partitioning, ( 1 1 ) Lianos, P.; Zana, R. J . Phys. Chem. 1980, 84, 3339. (12) Atik, S. S.; Nam, M.; Singer, L. A. Chem. Phys. Let?. 1979, 67, 75. (13) Kfatohvil, J. P. J . Colloid Interface Sci. 1980, 75, 271. (14) Lindman, B.; Wennerstrom, H. Top. Current Chem. 1980, 87, 1.
0 1986 American Chemical Society
