1345
J. Phys. Chem. 1986, 90, 1345-1349
Amphiphilic Coaggregation with Cetyltrimethylammonium Bromide Mohammad Abu-Hamdiyyah Chemistry Department, University of Kuwait, Kuwait (Received: June 7, 1985; In Final Form: October 21, 1985)
The coaggregation equation, K = -'/z(d In xf/dyf)yro + (d In cr/dyf)yro, relating the distribution coefficient of an amphiphilic additive ( K ) to its ability to strengthen hydrophobic bonding and its ability to increase the effective micellar degree of ionization ( a ) where xf and yr are the free monomer and additive mole fraction concentrations, respectively, is applied to coaggregation in cetyltrimethylammonium bromide (CTAB) aqueous solution. The effect of 18 additives, seven of which with a benzene ring in the nonpolar moiety, has been examined by electrical conductance measurements. As a first approximation an empirical relation between the last two terms of the coaggregation equation is found for all the additives which leads to the relation (-d In xf/dyf),,,.+ = OK with 0 = 0.78 i 0.07. However, it is possible to detect specificity among the additives depending on whether the nonpolar moiety contains a benzene ring (0 = 0.94 f 0.10) or not (6 = 0.63 k 0.07). The Kvalues obtained for alkanols by this method compare favorably with values in the literature obtained directly by chromatography or by solubility measurements. The free energies of coaggregation per mole of methylene groups in phenol homologues and in alkanols are -0.26 (k0.05) and -0.6 (10.06) kcal, respectively, indicating different environments of the methylene groups in these homologues in the micelle with the methylene groups in the phenol homologues exposed to water more than those in alkanols. The coaggregation equation and the specificity of amphiphilic action as well as the value of 0 are discussed.
Introduction It was recently found that it is possible to obtain reasonable values of the distribution coefficients of amphiphilic additives between sodium alkyl sulfates micelles and the surrounding aqueous solution ( K ) and hence the free energy of coaggregation using the equation
TABLE I: Coefficients of the Lines Cmc = a - bCadand S2 = a' + b%,, Obtained for the Variation of the Cmc of CTAB and of S2,the Slope of the Conductance-Concentration Line above the Cmc with Additive Concentration in the Low Concentration Range at 25 "C
no.
additive
1 propylene oxide 2 propylene
104a 9.27 9.28
103b 10w 0.29 f 0.04 25.4 1.08 f 0.03 25.2
10w 7 f l
43.4 f 0.3
carbonate
where xfand y f are the free monomer and additive mole fraction concentrations, respectively, and a the effective micellar degree of ionization.'S2 The method depends on finding an empirical relation between the two terms on the right side of the above equation thus yielding an equation of the form
with 8 a constant dependent on the surfactant chain length and related to the ability of the additive to increase the effective micellar degree of ionization. In the case of sodium lauryl sulfate (NaLS) at 25 OC it was noted' that the free energy of coaggregation per mole of methylene groups in phenol homologues to be -0.35 kcal compared to -0.60 kcal in alkanols, indicating some specificity of amphiphilic action. We, recently, presented preliminary experimental data which also show that an empirical relation between the ability to depress the cmc and the corresponding ability to increase a could be found for amphiphilic coaggregation in the aqueous solutions of cetyltrimethylammonium bromide, CTAB, a cationic ~ u r f a c t a n t . ~ The objective of this study is to extend the measurements to include alkanols whose K values have been reported in the literature and which were obtained by several independent methods and see whether the specificity detected in NaLS occurs also in CTAB. We shall also reexamine the derivation of the coaggregation equation and discuss the difference in the coaggregation tendency of amphiphilic additives in cationic (CTAB) and anionic (sodium hexadecyl sulfate) surfactant solutions. (1) Abu-Hamdiyyah, M.; El-Danab, C. J. Phys. Chem. 1983, 87, 5443. (2) Abu-Hamdiyyah, M.; Rahman, I. J . Phys. Chem. 1985, 89, 2377. (3) Abu-Hamdiyyah, M.; El-Danab C. A preliminary report presented to the 5th International Symposium on Surfactants in Solution, Bordeaux, France, July 1984, Abstract 30C1.
3 butyramide 4 1-butanol 5 caprolactam
6 butylurea 7 phenol 8 benzyl alcohol 9 2-phen ylethanol 10 3-phenylpropanol 11 1-hexanol 12 benzocaine 13 4-ethylphenol 14 ethylparaben 15 1-octanol 16 1-decanol
9.27 9.25
9.27 9.28 9.12 9.26 9.21 9.22 9.27 9.29 9.24 9.26 9.26 9.27
1.0 f 0.2 2.0 f 0.5 1.8 f 0.3 3.9 f 0.1 11.0 f 0.7 14 f 2 19 f 8 45 f 8 21 f 2 110 f 4
160 f 10 180 f 10 188 f 10 800 f 20
25.1
27 f 1
25.8 25.0
60 f 90 66.8 f 0.3 56.0 f 0.6 170 f 30 300 f 40 360 f 10 400 f 10 629 & 15 1410 f 50 1298 f 8 5600 f 700 6200 f 400 27860 f 500
24.1
25.1 25.1 25.1
24.9 25.2 24.7 24.9 24.9 25.1 25.3
Experimental Section Chemicals. 1-Butanol, 1-hexanol, 1-octanol, and 1-decanol were all 99% from Sigma; propylene oxide (puriss), benzyl alcohol (puriss), 2-phenylethanol (purum), 3-phenylpropanol (purum), benzocaine (purum), ethylparaben (purum), and 6-caprolactam (purum) were from Fluka; phenol was laboratory reagent from British Drug Houses; ethylene glycol and ethylene glycol monomethyl ether were both proanalysis from Merck and propylene carbonate (puriss) was from Koch and Light. All these compounds were used as received without further purification. Butyramide (pure) from Koch and Light and n-butylurea (purum) from Fluka were both recrystallized from benzene; 4ethylphenol (purum) was recrystallized from chloroform; CTAB (purum) from Fluka was recrystallized from acetone. All were dried under vacuum before use. No minimum was observed in the surface tension-CTAB concentration plot. Apparatus and Procedure. The electrical conductance measurements of the various solutions, which were made up volumetrically, have been carried out at 25 f 0.02 OC with the same apparatus and procedure described previously.' Results The cmc values in the presence and absence of the additives were determined from the intersections of pairs of straight lines
0022-3654/86/2090-1345%01.50/0 0 1986 American Chemical Society
Abu-Hamdiyyah
1346 The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 A This study 0
-I
Miyashita B Hayano
0 .9 0
E
t
0
0 0
r
0
u
0
0. B
0 0
u '0 \
0
I
0 .?
0.0005
0.0010
0.0015
Additive Concentrotion ( M I Figure 1. Variation of the cmc of CTAB as a function of additive concentration at low concentrations. Refer to Table I for meaning of numbers.
-5
0 -0 Y
1
c
2
-6
-11
1
I
1
5
4
6
I
1
I
1
7
8
9
IO
n in CnH2n+IOH
Figure 3. Ability of an alkanol to depress the cmc as a function of the number of carbon atoms in the alkanol.
t
O 0 0
U
I
0 024
0.05
0.01
0.015
Additive Concentrotion ( M Figure 2. Variation of the slope of the conductanceconcentration line above the cmc as a function of additive concentration, at low concentrations. Refer to Table I for meaning of numbers.
obtained by plotting solution specific conductances against surfactant concentrations. In the absence of the additive the cmc of CTAB falls in the range 0.93 f 0.01 m M in agreement with reported values in the literature.6 The slopes of the specific conductance-concentration lines in the absence of the additive below (Slo) and above (Sz0)the cmc cm-' mol-', respectively. were 0.093 f 0.001 and 0.025 f 0.001 P1 The values of the cmc and of S2 as a function of additive concentration in the low concentration range are summarized in Table I, in the form of the coefficients of the lines cmc = a - bCad and S2 = a' b'Cad. Figures 1 and 2 illustrate these effects. All the additives except ethylene glycol and ethylene glycol monomethyl ether depress the cmc and simultaneously increase S2. Ethylene glycol and the monomethyl ether derivative did not significantly affect the cmc or S , at very low concentrations; however, at higher concentrations they increased the cmc. In Figure 3 the natural logarithm of the negative of the rate of change of cmc with alkanol concentration as a function of the number of carbon atoms in the alkanol molecule obtained in this study is compared with the corresponding values obtained by using Miyashita and Hayano's r e ~ u l t s . ~Both sets of measurements appear to be consistent. No corresponding data on S2appear to exist in the literature.
+
~~
~~
(4)Miyashita, Y.; Hayano, S. J . Colloid Inferfoce Sci. 1982, 86, 344. ( 5 ) Gettins, J.; Hall, D.; Jobling, P.L.; Rassing, J. E.; Wyne-Jones E. J . Chem. Soc., Faraday Trans. 2 1978, 74, 1957. (6) Mukerjee, P.;Mysels, K. J. Natl. Stand. ReJ Data Ser. (US., Natl. Bur. Stand) 1971, No. 36.
-' 0
--
-1
-2
/
1
-'I -4
-5t -6
1
-5
-4
-3
-2
-1
In ( dS,/ dC o d
0
-
1
2
3
0
Figure 4. Ability of an additive to strengthen hydrophobic bonding vs. its ability to increase the slope of the conductance-concentration line above the cmc. Refer to Table I for meaning of numbers.
Strengthening of Hydrophobic Bonding and the Increase in SZ(a). The ability of an additive to strengthen hydrophobic bonding in aqueous CTAB has been plotted against its ability to increase S2,Figure 4. A linear relation is obtained between In (4cmc/dCa&& and In (dS,/dCad)cd--o with a slope, intercept, and correlation coefficient of 1.OO f 0.06, -3.05 0.15, and 0.974, respectively. The numbers in the figure correspond to the serial numbers in Table I. Considering amphiphilic additives containing a benzene ring as one family, the rest of the additives with paraffinic hydrocarbon moieties as another, we obtain for the slope, intercept, and correlation coefficient, 0.94 & 0.17, -2.12 & 0.20, and 0.925, and 0.96 f 0.05, -3.41 f 0.16, and 0.990 respectively. The additives with a benzene ring show a large scatter. We shall assume for the present that all these additives fall on the same line whose
*
The Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1347
Coaggregation with Cetyltrimethylammonium Bromide TABLE 11: Micelle-Water Distribution Coefficients in Mole Fraction Units As Obtained by Our Method, K , and Also according to Treiner’s q , in CTAB Solutions at 25 OC additive ethylene glycol ethylene glycol monomethyl ether phenol benzyl alcohol 2-phen ylethanol 3-phenylpropanol 4-ethylphenol butyramide caprolactam butylurea propylene carbonate propylene oxide benzocaine ethylparaben I-butanol 1-hexanol 1-0ctanol 1-decanol
K
4
0 0
A
Klq 32 41
830 f 148 1041 f 186 1420 f 254 3374 f 603 12172 f 2174 7 4 f 13 134 f 24 289 f 521 80 f 14 22 f 4 8149 f 1456 13378 f 2390 99 f 2 1558 f 148 13905 f 740 59328 f 1483
1383 1730 2347 5498 19659 169 274 521 169 66 13190 21607 205 2567 22448 95600
/
-
I!
0.60 0.60 0.61 0.61 0.62 0.44 0.49 0.56 0.47 0.33 0.62 0.62 0.48 0.61 0.62 0.62
/J
10-
H ;5Rn
M 9a
il
8-
7-
slope is unity and intercept -3.05. Converting to mole fractions using xp = cmc0/55.4;
This S t u d y Ref. ( 7 1 Ref. ( 8 ) p-Substituted Phenols
5
m 1
0
y f = Cad/55.4
3
2
, R,-
n in C6H5(CH,),0H
5
4
6
C,H50H
Figure 5. Variation of the natural logarithm of the distribution coefficient of phenol homologues C6H5(CH2),0H and of para-substituted phenols H N 6 H 5 R ,as a function of the number of carbon atoms in the substituent.
and the relation2
This study
we get for the linear relation in Figure 4,
l4
The Distribution Coefficient ( K ) . Using the above empirical relation in eq 3 we obtain
(T)
= 0.78(*0.07)K
A Gettins e1 PI
t
Miyoshita 8 Hayano
(4)
Yf-0
The distribution coefficients calculated from the cmc data by using (4)are listed in Table 11. The general trend observed is that the value of K tends to increase with the hydrophobicity of the additive. For example, K increases in value in going from phenol to 3-phenylpropanol and 4-ethylphenol or in going from butanol to decanol. However, the trend and absolute values for the distribution coefficients of the phenol homologues obtained in this study are at variance with those obtained by Lissi et al.’ who determined the partitioning of these alcohols between water and CTAB micelles in 0.5 M ZnS04 by the fluorescence method using NiZ+as a quencher. This difference is illustrated in Figure 5 which shows how the K values obtained by Lissi et al. decrease in going from phenol to benzyl alcohol reaching a minimum at 2-phenylethanol then increasing sharply in going to 3-phenylpropanol. The K values have been obtained by multiplying the binding constant by 55.4 M. Figure 5 also shows the results of Bunton and Sepulvedas (converted to mole fraction scale) for para-substituted phenols HOC6H5R,. The value for 4-ethylphenol obtained in this study shown in the figure falls below that reported in ref 8. It is clear that psubstituted phenols have a stronger coaggregation tendency than C6H5(CH2),0Hhomologues; however, the pattern observed for the p-substituted phenols looks similar to that exhibited by the results of this study for C6H5(CH2),,0Hseries. In contrast to the discrepancies noted above for the phenol homologues, Figure 6 shows the distribution coefficients of alkanols as obtained in this study together with distribution coefficients (7) Lissi, E.; Abuin, E.; Rocha, A. M. J . Phys. Chem. 1980, 84, 2406. (8) Bunton, C. A,; Sepulveda, L. J . Phys. Chem. 1979, 83, 680.
a
3
4
5
I
I
6
7
8
I
I
9
1
0
n in C,H,-,OH
Figure 6. Variation of the natural logarithm of the distribution coefficient of an alkanol as a function of the number of carbon atoms in the alkanol chain.
reported in the literature obtained by chromatography4 and also by solubility5 measurements. In K is plotted against the number of carbon atoms in 1-alkanols. The K values of Gettins et aL5 have been converted from the molarity to the mole fraction scale assuming a micellar density of 0.86 g m-’. It is obvious that although there are variations in the absolute values of the distribution coefficients, the three sets of values are consistent. Gettins et aL5 also reported the binding constants of the four alkanols determined by relaxation measurements which when converted to distribution constants on the mole fraction scale also follow a similar pattern as exhibited in Figure 6. The absolute values, however, are consistently lower than those obtained by the solubility method by about one unit on the scale. The distribution coefficient q according to Treiner: see eq 12, is included also in Table 11. The values of q are consistently higher (9) Treiner, C. J . Colloid Interface Sci. 1982, 90, 444.
1348
The Journal of Physical Chemistry, Vol. 90, No. 7 , 1986
than the corresponding K values by a factor approaching 1.6 for strongly hydrophobic molecules. Standard Free Energy of Coaggregation of Methylene Groups in Alkanols and Phenyl Homologues. We have plotted AGo,oa,,(-RT In K) for alkanols against the number of carbon atoms in the alkanol molecule and obtained a straight line with a slope, intercept, and correlation coefficient of -0.63 (f0.06), 0.37 (f0.45),and 0.990, respectively. Likewise when AGOcoagg for phenol homologues C,H,(CH,),-OH is plotted against n, the number methylene groups in the molecule, a straight line is obtained with a slope, intercept, and a correlation coefficient of -0.26 (fO.OS), -3.89 (f0.09), and 0.966 respectively. The value of the slope in each case corresponds to the contribution of one mole of methylene groups to the standard free energy of coaggregation while that of the intercept represents the contribution of the rest of the molecule (per mole). The free energy of coaggregation of methylene groups in alkanols with CTAB (-0.63 kcal mol-') is comparable to the value found','0 in NaLS system (-0.60 kcal mol-'). The corresponding quantity in phenol homologues, ACo(CH,) = -0.26 (jzO.05) kcal mol-', is smaller than that in the alkanol case but is of comparable magnitude to the value -0.30 kcal mol-' obtained by Bunton and Sepulveda8 for the transfer of methylene groups from water to CTAB micelles in p-substituted phenols (Figure 5) and also in p-substituted phenoxides. It was previously1found using eq 2 that the free energy of coaggregation per mole of methylene groups in phenol homologues in NaLS is -0.35 kcal which is in agreement with the value -0.36 kcal obtained by Hirose and Sepulveda" using more direct methods for determining K of p-substituted phenols in NaLS. The difference in values of the free energy of coaggregation of methylene groups in alkanols and in phenol homologues found in CTAB seems to confirm the previously observed difference found in NaLS indicating a difference in the corresponding environments on coaggregation with the ionic surfactant.
Discussion The Coaggregation Equation. At the cmc where the concentration of the micelles is negligible, the monomer-micelle equilibrium may be represented R T In xf = Ap",
(5)
where R is the gas constant, T the absolute temperature, and Ap', is the difference in the standard chemical potential of the surfactant ion in the micelle and in bulk solution (water). As a first approximation A g o m may be split into two components, ApoHp, the hydrophobic bonding component, and the electrostatic one. Amphiphilic coaggregation affects both these terms. On coaggregation with the ionic surfactant the additive is positioned between the ionic heads, reducing the electrostatic repulsion between them and displacing counterions and water and thus increasing the effective degree of micellar ionization.2 The effect of the additive on the electrostatic component is d ( m ) / d y f and that on the hydrophobic bonding component is d(ApoHp)/dyP The additive's effect on is equivalent to a lengthening of the surfactant This effect may be evaluated from the change of the cmc with increasing surfactant chain length in a homologous series. For a homologous series the cmc (xf) is relatedi5to the number of carbon atoms ( n ) in a hydrocarbon chain by R T In x, = A - Bn, where A and B are constants, with essentially the only factor with significant contribution to the change in the cmc as n is increased being the hydrophobic bonding component,I6 ApoHP. Thus, if the effective increase in the sur-
Abu-Hamdiyya h factant chain length is dn in presence of dCd/55.4 (-dyf) at very low concentration of the additive so that the surfactant chain length becomes effectively n dn, then the effect of the additive on ApoHp is given by
+
d In xf
-z-
(apo d In xf
=
(dyi)jpo
with d In xf/dn being the rate of change of the (natural logarithm of) the cmc with surfactant chain length and (dn/dyf)ypo the effective rate of increase of the surfactant chain length with additive concentration in the limit of infinite dilution. The additive's effect on the electrostatic component has been derived previously.'
Thus the total effect of the additive on the monomer-micelle equilibrium at very low concentration of the additive is obtained by differentiating eq 5 to give
This is eq 1 which was previously obtained1 by equating d(FS/RT)/dyf to zero. Equation 8 can be integrated to give In xf/xp = 2 In x,
+ 2 In a / a o
(9)
or Xf
= xp(a/aO)2(xm)2
(10)
where x, is the mole fraction of surfactant ion in the micelle and the superscript denotes the quantity in absence of additive. The values calculated for the distribution coefficients of amphiphilic additives in NaLS and in CTAB using eq 8 compare favorably with values obtained more directly by chromatography4 or by ~olubility,~ lending some support for the validity of the above relation. It is expected that the coaggregation equation would be useful for calculating distribution coefficients as long as the concentration of the additive is sufficiently small so that the change in the micellar aggregation number is insignificant.2 Miyagishi" attempted to estimate the dependence of the cmc on the additive concentration from the degree of dissociation of the micelle. Using the Stern equation for the adsorption of ions in the double layer and the electrostatic energy as given by Gouy-Chapman theory he obtained In x f / x f " =
y2 In x, - i/z In D / D O + y2 Inf, - In f / P
(1 I )
where D is the dielectric constant a n d f , and f are the activity coefficients of the surfactant ion in the micelle and in bulk solution, respectively. However, the activity coefficient terms were undetermined and were taken as proportional to the concentration of the additive in bulk solution, Le., lnfs - 2 In f/j" = Klyf with K , taken to fit the experimental data. Treiner9 derived eq 12 relating the distribution q = (2.3 X 1000)/18(2K, - Ks")
(10) Manabe, M.; Koda, M. Bull. Chem. SOC.Jpn. 1979, 51, 1599. (11) Hirose, C.; Sepulveda, L. J . Phys. Chem. 1981, 85, 3689. (12) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J . Chem. SOC. Faraday Trans. 2 1916, 72, 1525. (13) Evans, D. F.; Ninham, B. W. J . Phys. Chem. 1983, 87, 5 0 2 5 . (14) Lin, 1. J.; Metzer, M. J . Phys. Cbem. 1971, 75, 3000. (15) Shinoda In "Colloidal Surfactants"; Hutchinson, E., Von Rysselberghe, P., Eds.; Academic Press: New York, 1963, p 42. (16) Tanford, C. "The Hydrophobic Effect", 2nd ed.: Wiley: New York. 1980; p 71.
(6)
(12)
coefficient (termed q ) to the ability of the additive to depress the cmc and to a salting-out effect. K , is the slope of log cmcO/cmc vs. Cadline, KsN the salting-out constant, and cmc' is the cmc in absence of the additive. It was assumed that the additive mixes ideally with the surfactant ions in the micelle; i.e., the effect of (17) Miyagishi, S.Bull. Chem. SOC.Jpn. 1974, 47, 2972.
'he Journal of Physical Chemistry, Vol. 90, No. 7, 1986 1349
Coaggregation with Cetyltrimethylammonium Bromide the amphiphilic additive on the counterions condensed between the ionic heads and at the micellar surface was not taken into account. Ignoring the salting-out effect, which is negligible for hydrophobic solutes, we find q and K to be related by q = 28K. In terms of integral quantities, eq 2 becomes In xf/x? =
y2 In x,
(13)
Treiner noted that the values of q obtained through eq 12 for alkanols in NaLS are consistently higher than the values obtained experimentally by chromatography'* (Kexptl).This was attributed to q being an ideal partition coefficient and the ratio Kexptl/qwas considered as an activity coefficient which includes all non-ideal interactions between the solute and the micelle. The value of this ratio approaches for strongly hydrophobic solutes the value (28)-'. The Constant 8. 8 is the constant linking the effect of the additive on the cmc to the distribution coefficient of the additive between the micelles and the surrounding aqueous solution. It is given by eq 14 8 = 2 - 2(d In a/dy,),m,o
(14)
where y , is the mole fraction of the additive in the micelle. According to eq 14, 8 is related to the relative increase of the micellar degree of ionization with additive concentration in the micelle. As a first approximation 8 is constant for a given surfactant independent of the amphiphilic additive. For example, it was recently, found that 8 is equal to 1.33, 1.00, 0.71, and 0.59 for hexadecyl, tetradecyl, dodecyl, and decyl sodium sulfate, respectively, at 44 "C. It was concluded that 8 is a function of the extent of the hydrocarbon-water contact region in the micelle. It was also found that the amphiphilic additive's ability to coaggregate with an ionic surfactant increases in the direction of increasing hydrocarbon-water contact region which is the direction of decreasing surfactant chain length or decreasing 8. The value of 8 obtained for CTAB in this study using all the additives is 0.78 with a standard deviation of 0.07. This value of 8 is significantly smaller than that of sodium hexadecyl sulfate (NaCS), suggesting that the hydrocarbon-water contact region is greater in CTAB than in NaCS. The extent of this region is dependent on the repulsion between the ionic heads which must be different in these two types of surfactants. This is in line with and 6.1 the cmc values of these surfactants in water, 9.4 X X lo4 M, respectively, for CTAB and NaCS. This is an example of ionic head specificity. 0 may be looked upon as a measure of the penetrability of the micelle by the amphiphilic additive. The smaller the value of 8 the larger the extent of the hydrocarbonwater contact region and the greater the amphiphilic coaggregation tendency. Locus of Aromatic and Aliphatic Moieties. It has already been noted in a previous study' using eq 2 that the free energy of coaggregation per mole of methylene groups, AG"(CH,),,, with the ionic surfactant NaLS is less negative when the CH, is part of a moiety containing a benzene ring than when it is only part of a paraffinic moiety. The same trend has now been found for such additives in CTAB. Furthermore the absolute value of AGO (CH,)coaggis approximately the same (-0.3 kcal mol-') for the phenol homologues C6H5(CH2),0H in both NaLS and CTAB. The corresponding quantity for alkanols is also the same, about -0.60 kcal mol-', in both of these systems. The similarity in the values of AGo(CH2)coagg in both of these surfactant systems suggests a similarity in the extent of penetrability of CTAB and NaLS micelles. The difference in the values of AGo(CH2)coagg (18) Hayase, M.; Hayano, S. Bull. Chem. SOC.Jpn. 1977, 50, 83.
in phenol homologues and in alkanols suggests that the two types of methylene groups have different locations in the micelle with the (CH,) groups in C6H5(CH2),0Hexposed to water more than the (CH,) groups in an alkanol moiety. This could be due either to the relative bulkiness of the phenyl group compared to that of the methylene groups, with the benzene ring (rotating) forming the thick end of a wedge pointing toward the hydrocarbon core of the micelle so that there is more water accessibility at the thin end of the wedge than in the case of a uniform wedge in an alkanol moiety, or it could be due to the intrinsic difference in hydrophobicities of the benzene ring and of a saturated hydrocarbon (moiety). The benzene ring being more h y d r o p h i l i ~than ' ~ ~ ~the ~ (CH,) groups and with the hydroxyl group assumed to be located in water, the (CH,) groups in phenol homologues would be exposed to water more than (CH,) groups in alkanol moiety, i.e., nearer the micellar surface. If it is the latter situation that prevails, then it means there is some specificity regarding the action of amphiphilic additives depending on whether the nonpolar moiety of the additive contains a benzene ring or not. This difference should be reflected in 8 and thus in the intercepts of In (-dcmc/dCad) vs. In (dS2/dCad)plots. Although the data on the additives (phenol, benzyl alcohol, 2-phenylethanol, 3-phenylpropanol, 4-ethylphenol, benzocaine, and ethylparaben) show scatter there appears to be indications of specificity. The best straight line through the data (In -dcmc/dCad vs. In dS2/dCad)gives an intercept of -2.72 f 0.20 giving 8 = 0.94 f 0.10. For additives containing no benzene ring but with paraffinic moieties only, an intercept of -3.41 f 0.16 is obtained giving 8 = 0.62 f 0.07. These results indicate the micelle to be less penetrable by an additive containing a benzene ring than an additive with paraffin chain only. These values of 8 would increase the values of K shown in the table for the additives containing no benzene ring and would decrease it slightly for the additives containing a benzene ring. Summary and Conclusion. 1. The coaggregation equation relating the distribution coefficient of the additive to its effect on the monomer-micelle equilibrium appears to be valid for amphiphilic coaggregation with anionic and cationic surfactants at very low concentrations of the additive. 2. The value of 8 for CTAB is smaller than that of NaCS, an anionic surfactant with equal surfactant chain length, indicating that the cationic surfactant is more penetrable than the anionic surfactant (ionic head specificity). 3. The free energy of coaggregation per mole of methylene groups in a nonpolar moiety containing a benzene ring is less negative than that in a completely paraffinic moiety in both anionic and cationic surfactants. 4. Finally, the pattern of K values for phenol homologues obtained in this study is different from that reported in the literature, a point which needs further investigation. Acknowledgment. I thank the Research Council of the University of Kuwait for their support of this work. Registry No. CTAB, 57-09-0; ethylene glycol, 107-21-1; ethylene glycol monomethyl ether, 109-86-4; phenol, 108-95-2; benzyl alcohol, 100-51-6; 2-phenylethanol, 60-12-8; 3-phenylethanol, 122-97-4; 4ethylphenol, 123-07-9; butyramide, 541-35-5; caprolactam, 105-60-2; butylurea, 592-31-4; propylene carbonate, 108-32-7; propylene oxide, 75-56-9; benzocaine, 94-09-7; ethylparaben, 120-47-8; I-butanol, 71-36-3; I-hexanol, 11 1-27-3; I-octanol, 11 1-87-5; I-decanol, 112-30-1. (19) Mukerjee, P.; Cardinal, J. R. J . Phys. Chem. 1978, 82, 1620. (20) Nagarajan, R.; Chaiko, M. A,; Ruckenstein, E. J . Phys. Chem. 1984, 88, 2916.
