J. Phys. Chem. 1990, 94,6445-6452
dehydration may be required to account for this. The present study, as shown above, provides us some evidence to indicate the structure of acid sites included in silica-alumina and silica-coated alumina. First, we can conclude that the silica on alumina does not possess such acid sites as found in the silica-alumina catalyst. In other words, silanol attached to AI does not show strong Bransted acidity. This is in agreement with the conclusion by Kawakami et al. on the basis of the quantumchemical cal~ulations.~They proposed a distorted structure consisting of silicon and aluminum for the strong Bransted acid sites. In the boundary layer between alumina and silica, the species as assumed or the substituted aluminum cation like in zeolite framework could be formed to play the role of the Bransted acid site. Because the electronegativity of the silicon cation (Si4+) is larger than that of the aluminum cation (AI3+),the silicon atom pulls an electron to become negatively charged; in place of that, the hydrogen of hydroxide becomes positively charged, Le., - A I U Si--OH+. Because of the induced effect, silanol attached to
6445
aluminum may behave as a weak Bransted acid site. It can be postulated that the distribution of the acid strength is fine due to the discrete structure of deposit silanol. Therefore, the silica monolayer on alumina could catalyze the appropriate reaction effectively. The sharp distribution of the acid site with a homogeneous strength of the acidity may be a characteristic of the material distinguishable from usual mixed oxides. Conclusions I . Chemical vapor deposition of silicon methoxide on alumina
formed the monolayer of silica fully covering the alumina surface. Further deposition of silica is possible; double-layer formation is suspected. 2. The silica monolayer does not possess the strong Bransted acidity for the cumene cracking but does possess the weak acidity to catalyze the isomerization of butene and the dehydration of tert-butyl alcohol. The acidity may be created by an induced effect of the assumed species - A I U S i - O H , and a fine distribution of the acidity strength is estimated.
Solubilization Tendency of I-Alkanols and Hydrophobic Interaction in Sodium Lauryl Sulfate in Ordinary Water, Heavy Water, and Urea Solutions Mohammed Abu-Hamdiyyah* and Kamlesh Kumari Chemistry Department, University of Kuwait, 13060 Safat, Kuwait (Received: January 16. 1990)
The effects of I-propanol, 1-butanol, I-hexanol, and 1-octanol on the micellization parameters of sodium lauryl sulfate (NaLS) in heavy water (D20) and in aqueous urea solutions, solvent systems with stronger and weaker hydrogen-bonded skeletal structures, respectively, than in ordinary water, have been determined at 25 OC. We have also determined the transfer free energy of an alcohol from H20to D20and also to urea solutions by head space gas chromatography. Results show the following: (1) The ability to depress the critical micelle concentration (cmc) increases in going from D20to H20to 5 M urea, the direction of decreasing structuredness of the solvent system. (2) The coaggregation equation ( J . Phys. Chem. 1983, 87, 5443), relating the ability to depress the cmc and the ability to increase the micellar degree of ionization to the distribution coefficient of the amphiphilic additive between the micelles and the surrounding solvent phase, is applicable in all these solvent systtems. (3) The transfer free energy from micelles in H20to micelles in urea solutions is positive and increases with urea concentrations for propanol and essentially butanol, while it is negative for hexanol and octanol and decreases with increasing urea concentration tending to reach a minimum at about 4 and 3 M urea, respectively. (4) The transfer free energy from micelles in H20to micelles in 3 M urea solutions tends to decrease with increasing alcohol chain length while it tends to increase for the transfer to micelles in D20.The results are interpreted in terms of the effect of the solvent "structuredness" on the extent of the hydrocarbon-solvent contact region in the micelles and the ability of an alcohol to reduce the extent of that region.
Introduction
It is now generally recognized that the phenomenon of hydrophobic bonding or interaction (HI), namely, the tendency of nonpolar moieties to associate together in aqueous solutions such as occurs on micelle formation, is related to the "structuredness" of the solvent water.' Studies on micelle formation in aqueous solutions whose structuredness was strengthened by substituting D20 for H 2 0 or weakened such as in urea solutions show a decrease and an increase, respectively, in the cmc of a given It is also well-established that nonpolar substances and nonionic amphiphiles at low concentrations strengthen the (1) Tanford, C. The Hydrophobic Effect, 2nd ed.; Wiley: New York, 1980. (2) Emerson, M. F.; Holtzer, A. J . Phys. Chem. 1967, 7/, 3320. (3) Mukerjee, P.; Kapauan, P.; Meyer, H. G. J . Phys. Chem. 1966, 70, 783. (4) Chang, N . J.; Kaler, E. W. J . Phys. Chem. 1985, 89, 2996. (5) Abu-Hamdiyyah, M. J . Phys. Chem. 1965, 69, 2720. (6) Abu-Hamdiyyah, M. In Solution Behaviour of Surfactants; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2.
0022-3654/90/2094-6445$02.50/0
hydrophobic bonding tendency in ionic surfactant solutions.' This is manifested by a depression of the cmc and the coaggregation of the additive and the surfactant ions to form mixed micelles resulting in the distribution of the additive between the micelles and the surrounding aqueous phase. In the case of nonionic amphiphiles the coaggregation process is accompanied by the dilution of the micellar surface charge and the increase in the effective micellar degree of ionization ( a ) . We have developed an equation of state for the coaggregation process in nonionic amphiphile-ionic surfactant
involving the distribution coefficient K,the ability to depress the cmc, and the interaction of the additive with the surfactant ions in the micelles as measured by the ability to increase a. xf and (7) Abu-Hamdiyyah. M.; El-Danab, C. J . Phys. Chem. 1983,87, 5443. ( 8 ) Abu-Hamdiyyah, M. J . Phys. Chem. 1986, 90, 1345.
0 I990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 16. 1990
Abu-Hamdiyyah and Kumari
yf are the free monomer surfactant (cmc) and free additive
concentrations, in mole fractions, respectively. On introducing the emperical relationship between the two effects on the righthand side of eq 1 obtained from our conductance measurements (see Results section and Table 11), the following equation is obtained.
[ : Iyd
4 1
l-Octanol 1-Hexanol
= OK
- -
(2)
The proportionality constant obtained between the initial relation ability to depress the cmc and the distribution coefficient was shown7 to take a form indicated by the equation
-1
B = 2 - 2[ d In a dym
(3)
1-Bntanol I-Propanol -5 0
I
y,,,--o
where ym signifies the mole fraction of the additive in the micelle. It is of intrinsic interest to see how the coaggregation parameters, the ability to depress the cmc, and the corresponding ability to increase a as well as B would be affected when the solvent system is changed from H 2 0 to D,O or to aqueous urea solutions. This will throw light on the effect of strengthening or weakening the structuredness of the solvent on the partitoning of amphiphilic additives between the micelles and the surrounding solvent phase and also on the microenvironment of the amphiphilic additive in the micelle. Moreover, since the micellization process is often taken as a model for HI in biological systems, the results of such a study would also be of interest for the understanding of the effect of urea on the strengthening of the hydrophobic effect by amphiphiles in proteins which usually occurs during normal processes of life as in the formation of substrate-enzyme complexes, lipid-protein association, and neurotransmission or during medical treatment as in the action of anesthetic agent^.^.'^ Some studies recently appeared which dealt with some aspects Gonzales et al." investigated the of the above variation of the distribution coefficient of cyanonaphthalene between the micelles of cetyltrimethylammonium bromide (CTAB) and the surrounding aqueous phase in the presence of urea up to 3.3 M, who found K to decrease with increasing urea concentration. Candau et a1.I2studying the effect of isotopic composition of the solvent on the distribution coefficient of pentanol between the micelles of tetradecyltrimethylammonium bromide (TTAB) and the surrounding solvent (in 0.1 M KBr) using light scattering measurements found K to increase in going from H 2 0 to D 2 0 . However, Carlfors and StilbsI3 studying the solubilization of homologous aliphatic n-alcohols in sodium decanoate using Fourier transform N M R measurements concluded that no significant change occurs in the solubilization of the alcohols in going from HZO to DzO. The microenvironment in the micelles of sodium lauryl sulfates in D 2 0 and in aqueous urea solutions was also investigated by several authors using spectroscopic probes.14-17 In this study we will report on the coaggregation tendency of several alcohols in NaLS solutions as a function of urea concentration up to 5 M as well as in D 2 0 together with the corresponding changes in the micellization and coaggregation parameters. The transfer free energy of an alcohol from H 2 0 to D 2 0 and to aqueous urea solution is determined by head space gas chromatography (HSGC), and the corresponding transfer free (9) Abu-Hamdiyyah, M. Langmuir 1986, 2, 310.
(IO) Abu-Hamdiyyah, M.; Kumari, K. Langmuir 1989,5, 808.
2 3 Urea Concentration .M
5
4
F g v e 1. Ability of an alkanol to depress the cmc of NaLS as a function of urea concentration.
energy of an alcohol from micelles in H 2 0 to micelles in DzO or to micelles in aqueous urea is thus obtained. The results will be discussed in terms of the effect of increasing or decreasing "structuredness" of the solvent system on the size of the hydrocarbon-solvent contact region in the micelle18 and the ability to accommodate the nonpolar moiety of the alcohol therein.
Experimental Section Chemicals. Ethanol and 1-propanol were both proanalysis from E. Merck. 1-Butanol and 1-hexanol were from Fluka, and 1octanol was from Sigma. Urea was Aristar quality from British Drug Houses. Heavy water was 99.7%from Koch-Light. They were all used without further purification. Sodium lauryl sulfate, especially pure from British Drug Houses, was recrystallized twice from ethanol and dried under vacuum to remove the small minimum in the surface tension-concentration plot. Apparatus and Procedure. All solutions were made up volumetrically. The conductance measurements were carried out using the apparatus described p r e v i o ~ s l y . ~Gas ~ chromatography experiments for the determination of the transfer free energy of an alcohol from H 2 0 to D 2 0 or to urea solutions and for the determinations of the distribution coefficient of some alcohols for verification purposes were initially carried out using a PerkinElmer Sigma 1 B instrument. Five-milliliter samples were injected in the column manually with a pressure lock syringe from the vapor space above the solutions, which were in sealed bottles equilibrated at 25 "C. Later on, however, all the experiments were repeated using a Shimadzu GC- 14A gas chromatograph.with a capillary column coupled with an automatic head space analyzer, HSS-2B. The samples were programmed for a conditioning time of 50 min at 40 "C with each sample analyzed three times. The volume of each vapor sample was IO mL. Results For the evaluation of K in eq 1 the two terms on the right-hand side of the equation, the initial relative ability to depress the cmc, i.e., (-d In xf/dyf)yro and the initial relative ability to increase a, i.e., (d In a/dyf)yd, are both evaluated basically from conductance measurements. The former term is given by d In xf
d(cmc)
nt
and the latter term by
( I I ) Gonzales, M.; Vera, J.; Abuin, E. B.; Lissi, E. A. J . Colloid Interface Sci. 1984, 98, 152. (12) Candau, S.; Hirsch, E.; Zana, R. J. Colloid Interface Sci. 1982, 88, 428. (13) Carlfors, J.; Stilbs, P . J. Colloid Inferfoce Sci. 1985, 104, 489. (14) Miyagishi. S.;Asakawa, T.; Nishida, M. J. Colloid Interface Sci. 1987, 115, 119. ( 1 5 ) Grieser, F.; Lay, M.; Thistlewaite, P. J. J. Phys. Chem. 1985, 89, 2065. (16) Plonka, A.; Kevan, L. J . Phys. Chem. 1984, 88, 6348. (17) Plonka, A.; Kevan, L. J . Phys. Chem. 1985, 89, 2087.
where n, is the total number of moles in 1 L of solution, cmc' and a" are the values of the cmc and of a in the absence of ( I 8) Abu-Hamdiyyah, M.; Rahman, 1. R. J . Phys. Chem. 1985,89, 2377. (19) Abu-Hamdiyyah, M.; Mansur, E. L. J . Phys. Chem. 1979,83, 2236.
The Journal of Physical Chemistry. Vol. 94, No. 16, 1990 6447
Solubilization Tendency of I-Alkanols additive, and -d(CmC)/dC,d and da/dCad terms are the initial slopes of the cmc-Cad and .-cad plots, respectively. The CmC'S of NaLS in aqueous urea solutions (0-5 M) and in D 2 0 were determined from the intersection of two straight lines obtained on plotting the specific conductance against surfactant concentration, one below the cmc with slope SIand the other above the cmc with slope S2. The initial slopes of cmc-Cad plots have been determined by the least-squares method, and their values are given in Table I. Figure 1 illustrates the variation of the ability to depress the cmc given by In (-d(cmc)/dCad)c,d,o as a function of urea concentration. The ability to depress the cmc increases linearly with urea concentration for each of the alcohols examined. The ability of a given alcohol to depress the cmc in D 2 0 is less (by 9%) than that in H 2 0 . The least-squares slopes for the ability to depress the cmc, in Figure I , together with the correlation coefficients are 0.092 f 0.007, r = 0.988; 0.141 f 0.017, r = 0.971; 0.22 f 0.02, r = 0.983; and 0.167 f 0.142, r = 0.986 for propanol, butanol, hexanol, and octanol, respectively. In aqueous urea solutions (0-5 M) as well as in D 2 0 the ability to depress the cmc increases linearly with alcohol chain length. The slopes of these lines with their correlation coefficientsare 1.09 f 0.06, r = 0.997; 1.00 f 0.06, r = 0.996; 1.02 f 0.08, r = 0.994; 1.06 f 0.12, r = 0.998; 1.06 f 0.10, r = 0.991; 1.07 f 0.11, r = 0.989; and 1.07 f 0.1 1, r = 0.988 for D 2 0 , H 2 0 , and 1, 2, 3, 4, and 5 M urea solutions, respectively. I t is also worth noting that the cmc values in aqueous urea solutions in the absence of the additives which are shown in Table I 1 are higher than those reported in the literature20 by 3% at 5 M and up to 16% at 1 M urea. The latter values were obtained by surface tension measurements which usually tend to give lower cmc values than those obtained by specific conductance. The cmc value for NaLS in D 2 0 agrees quite well with that reported by Mukerjee et a1.j but higher than the value reported by Chang and Kaler4 by about 4%. The values of the cmc of NaLS as a function of urea concentration (up to 5 M) at 25 OC is given by In (cmco/M)) = -4.7925 + O.O589[u], r = 0.996, where M signifies molarity. The effective micellar degree of inonization has been related to S2,the slope of the specific conductance-concentration plot above the cmc, by the equation S2 = dK/dC = cro(A+
+Fp)m3
9
7
9
A t
9
9
Q
9
9
-4
(4) 9
where A+, F, and p are the counterion conductivity at the cmc, Faraday constant, and micellar electrophoretic mobility, respectively. In our previous studies'* where we were concerned with one solvent system it was pointed out that at very low concentration of the additive and near the cmc the effect on A+ and p are negligible, and (d In ct/dyf),fl was equated to (d In S2/dy[),+ which is given by (dS2/dCad!cfl-,yz,/Sso, where S20 signifies the value of S2in absence of additive. The values of (dS,/dCd)c,d4 are reported in Table I . In the present study we are dealing with solvent systems where the intrinsic values of A+ and of p may be different. Thus, in order to safely compare the results in H20, D20, and aqueous urea, corrections have been applied to eq 4 in accordance with Mukerjee's method.21,22The corrected equation is given by
where Ao,cyo, and 9' signify respectively the values of counterion equivalent conductivity at the cmc, the effective micellar degree of ionization, and the viscosity coefficient in water in the absence (20)Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; NRDS NBS 36; US. Printing Office: Washington, DC, 1971. (21) Sugihara, G.; Mukerjee, P. J . Phys. Chem. 1981, 85, 1612. (22) Mukerjee, P.; Korematsu, K.; Okawauchi, M.; Sugihara, G. J . Phys. Chem. 1985,89, 5308.
9
9
3
Abu-Hamdiyyah and Kumari
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990
6448
TABLE 11: Coefficients of the Lines In -(d In x/ldy,)vro
= s In (d In a/dy,),,.+
and Aaueaus Urea Solutionsa
+ I Together with the Values of SZ0,cmco, and 8 in D20,HzO,
~~~~~~~
a
I I ooos,o , "a mM 8 a0
n1
D2O
0
I
1.02 f 0.12 0.124 & 0.62 17.50 7.93 0.80 0.23 55.25
0.99 f 0.06 0.160 f 0.37 23.15 8.32 0.72 0.25 55.4
1.00 f 0.08 0.730 f 0.47 27.10 8.77 0.90 0.28 49.7
urea concentration, M 2 3 1.10 f 0.12 1.09 f 0.13 0.820 f 0.71 1.080 f 0.64 29.75 31.0 9.48 9.85 1.08 1.27 0.3 1 0.33 49.5 49.4
4 1.03 i 0.04 1.730 f 0.18 33.3 10.7 1.45 0.36 49.3
Units of S2are in f2-l cm-I L mol-I. n, is the total number of moles of H,O and of urea in 1 L of solution. ao values in solutions were obtained from eq 5 . a
E
5 1.06 f 0.03 1.870 i 0.10 33.6 11.1 I .62 0.38 49.3
D20and in aqueous urea
-3 -4
!0
I
2
3
Urea Concentration
4 ~
5
0 4
Y
Figure 2. Ability of an alkanol to increase the micellar degree of ionization as a function of urea concentration.
of the additive. The corresponding unsuperscripted symbols represent the values in the solvent system ( D 2 0 or aqueous urea) in question. The values of A/Ao have been obtained experimentally while those of q o / q have been taken from the literat ~ r e . ~ ~ - ~ ~ Using our experimental value of S20 (0.023 75 0-Icm-' L mol-') in water with2' X + O = 50.1 cm2 V-I S-' mol-', po = 4.57 X lo4, and F = 96485 C mol-', one obtains ao(H20)= 0.25. Thus, taking ordinary water as the reference point, we have calculated a values in D 2 0 and in urea solutions in the absence and presence of the additives using our experimental values of S2.The values of a obtained in D 2 0 and in urea solutions in the absence of the additives are shown in Table 11. The values of a in the presence of additives have been plotted as a function of the additive concentration, and the initial slopes were determined by the leastsquares method. Figure 2 illustrates the values of the logarithm of the initial slopes, In (da/dC,d)c,d,o, for the various alcohols as a function of urea concentration. It shows that the ability of an alcohol to increase the effective micellar degree of ionization tends to decrease with increasing urea concentration. The values of the initial slopes of .-cad plots in D20for propanol, butanol, and hexanol are respectively 0.12,0.55, and 3.47 compared to 0.1 5,0.58, and 5.79 in ordinary water. It indicates that the ability of an alcohol to increase the effective micellar degree of ionization is less in D 2 0 than in ordinary water. B Values. Our results show that the logarithm of the initial relative ability to depress the cmc, In (4In xf/dyf)yfl: is linearly related to the logarithm of initial relative ability to increase a , In (d In a/dyf),+,, in all the solvent systems examined. The values of the coefficients of these lines are given in Table 11. We shall assume that in each case the slope is equal to unity. Using these relations between these two terms on the right-hand side of eq 1 in each solvent system gives eq 2, with the values of 6 obtained shown in Table 11. The value of B in D 2 0 is 0.80 compared to (23) Warren, J. R.; Gorden, J. A. J . Phys. Chem. 1966, 70, 297. (24) MacDonald. J. C.;Guervera. J. J. J . Chem. Eng. Dum 1970, IS, 546. (25) Lewis, G. N.; McDonald, R. T. J . Am. Chem. Soc. 1933,55,4730. (26) Baker, W. N.; Lamer, V. K . J . Chem. Phys. 1935, 3, 406. (27) Stigter, D.;Mysels, K. J. J. Phys. Chem. 1955, 59, 45.
I s 03
0
I
1
t
I
1
1
2
3
4
5
0 2
Urea Concentration, M
Figure 3. Variation of ao and of (da/dy,&+
with urea concentration.
0.72 in ordinary water. In aqueous urea 0 increase from 0.72 in 0 M to 1.62 in 5 M urea. The variation of 8 with urea concentration is given as In 8 = -0.28 0.16[u], r = 0.99. It is worth mentioning that In (d In S2/dyl)yfl for alcohols in H 2 0 when plotted against In (d In a/dyf)yl,o based on a values obtained by eq 5 gives a straight line passing through the origin with slope equal to unity within the experimental error. Micellar Degree of Ionization and the Rate of Change of Micellar Degree of Ionization with Mole Fraction of Alcohol in the Micelle. The values of the effective micellar degree of ionization of NaLS micelles in D20, H 2 0 , and urea solutions in the absence of the additive (a') have been evaluated and are given in Table I1 and plotted in Figure 3. ao decreases on going from H 2 0 (0.25) to D 2 0 (0.23), and it increases in going from H 2 0 to urea solutions. a' increases linearly with urea concentration. The rate of change of a with mole fraction of additive in the micelles (daldy,), +) is obtained from eq 3. Figure 3 depicts the rate of change ora with mole fraction of alcohol in the micelle in the limit of infinite dilution. It is clear that cyo and (da/dy,),+-, have opposite tendencies, the former increasing while the latter decreasing with increasing urea concentrations. It is to be noted also that the value of (da/dy,),m_.o in D 2 0 is 0.14 compared to 0.16 in ordinary water. The Coaggregation Tendency. The values of the distribution coefficients of the alcohols have been determined from eq 2. The coaggregation tendency (-AGO,,/RT) value are shown in Table Ill and illustrated for aqueous urea solutions in Figure 4 showing the spread in the individual values. Also shown in Figure 4 and Table I11 are the values of the distribution coefficients for butanol,
+
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6449
Solubilization Tendency of 1-Alkanols lor
9
p
=
=
b
2
l-Octanol
60
1
Y 0
cl
3
tt--t--t--h, 9
I
I
I
I
I
2
3
4
5
1-Prop.nol 1 6
Urea C o n c e n t r a t i o n , Y
Figure 4. Coaggregation tendency (-AGo,/RT) of various I-alkanols as a function of urea concentration ( 0 )by eq 2 at 25 OC and (0)by head space gas chromatography at 40 OC.
Aexanol Concentration, Y
Figure 5. Henry's law plots for hexanol in 5 M urea (A), HzO (B), and DzO (C) at 40 OC.
TABLE 111: Values of the Coaggregation Tendency (-ACo,/RT) of Alcohols in D20, H20, and Aqueous Urea Solutions at 25 "C As Obtained by the Coaggregation Equation with a Calculated According to Mukerjee's Method21q22(Eq 5) and by Head Space Cas Chromatography in Parentheses at 40 OC DroDanol butanol hexanol octanol 4.00 (4.42) 5.30 (5.67) 7.35 (7.93) 9.00 (9.4) HZO 5.23 (5.75) 7.38 (7.75) 8.85 (9.4) 1 M urea 3.75 2 M urea 3.80 5.35 (5.89) 7.50 (7.81) 8.97 (9.2) 3 M urea 3.61 5.09 (5.72) 7.45 (7.72) 8.93 4 M urea 3.49 5.02 (5.50) 7.39 (7.64) 8.80 (9.04) 5 M urea 3.33 4.86 (5.45) 7.27 (7.16) 8.69 (8.8) DZO 3.87 (4.32) 5.19 (5.57) 7.11 (7.75)
hexanol, and octanol determined by HSGC at 40 "C. The values obtained by HSGC28+29 tend to be higher than those obtained by the coaggregation equation by about 10% in the worst case although sometimes the two values tend to fall in the same range. In urea solutions in the presence of NaLS the variation in peak areas is about 7%. As can be discerned from Table 111, the coaggregation tendency in D 2 0 is less than that in water by 2.5%. Transfer Free Energy
The transfer of an alcohol from micelles in H 2 0 to micelles in DzOor to micelles in urea solution, Apom(HzO+X) = pom(X) - p0,(H20), is given by Ap",( HZO-X) = -RT In [Km(X)/Km(H,O)I + AM"(H,O+X) (6)
Km(X) and K,(H,O) are the distribution coefficients of the alcohol between the micelles and the surrounding solvent X (DzOor urea solution) and that between the micelles and the surrounding water ( H 2 0 ) , respectively. Apo(H20-X), the transfer free energy of an alcohol from water solution to D 2 0 or to urea solutions in the absence of NaLS, has been estimated by gas chromatography using the ratio of peak areas (PA) as a measure of the vapor pressure ratios for a given alcohol concentration in the solution, Apo(H20-X) = RT In [PA(X)/PA(H20)], and are shown in Table IV. Figure 5 illustrates Henry's law plots for hexanol in H 2 0 , D20, and 5 M urea solutions at 40 "C. The error in the PA readings is about 1% or less in most cases; however, in the worst case it was about 4%. The resulting Apo(HzO+u) values are plotted in Figure 6. For ethanol and propanol these values are positive, which increase with increasing urea concentration. A reversal in this trend occurs for butanol, hexanol, and octanol. These alcohols show a negative (28) Hayase, K.;Hayano, S. Bull. Chem. Soc. Jpn. 1977, 50, 83. (29) Spink. C. H.; Colgan, S. J . Colloid Inferface Sci. 1984, 97, 40.
Ethanol
-2oooL 0
I I
1
2
I 3
I
I
s
4
Urea Concentration
1
J
6
7
,Y
Figure 6. Transfer free energy of ethanol, propanol, butanol, hexanol, and octanol from water to aqueous urea solutions at 40 OC. TABLE IV: Transfer Free Energy Values of Alcohols Ar0(H2O+X) for H 2 0 to D20or to Urea Solutions at 40 OC
urea concentration, M alcohol ethanol propanol butanol hexanol octanol
1 108 -10 -129 -215 -665
2 266 0 -171 -474 -1390
3 266 38 -167 -731
4
216 28 -188 -726 -1778
5 466 83 -218 -1028 -1740
D,O 0 57 137 269
transfer free energy which becomes more negative with increasing urea concentration. For a given urea concentration the transfer free energy decreases (become less positive or more negative) with increasing alcohol chain length. The transfer free energy of an alcohol from micelles in H 2 0 to micelles in urea solutions has been obtained by using eq 6 and distribution coefficients by eq 1 utilizing the values of Ap0(H2O-u) shown in Table IV and illustrated in Figure 6. Apo,(H2Ct.u) values obtained accordingly are shown in Figure 7. The transfer free energy from micelles in H 2 0 to micelles in urea solution for propanol is positive and increases with urea concentration. For butanol Apom(H20-u) is nearly zero or slightly negative up to 2 M, and then it becomes positive and
6450
Abu-Hamdiyyah and Kumari
The Journal of Physical Chemistry, Vol. 94, No. 16, I990
ALL~,,,(H,O---~ M urea) displays an opposite tendency, i.e., decreasing (becoming less positive) with increasing alcohol chain length. The values of Apom(H20-D20) obtained utilizing K values by eq 1 and by HSGC are in agreement. However, although the corresponding values of Apom(H20-.u) give the same general trend as a function of urea concentration as indicated in Figure 7 , the absolute values are not the same due to the difference in temperature in the first term on the right-hand side of eq 6.
2001
I001
-
Discussion The first point to note is the applicability of eq 1 to the coaggregation of nonionic amphiphiles with lauryl sulfate ions in urea or D,O solutions. This is clear from Table 11, which shows that the slope of In -(d In xf/dyf) vs In (d In a/dyf) for each of these solvent systems is equal to unity within the experimental error. Secondly, although the absolute values of In K obtained by eq 1 and by HSGC are not in very good agreement (they differ by as much as 10% in some cases), both results show essentially the same general trends as can be seen in Table 111 and in Figure 4. We shall henceforth be dealing with the results obtained by the coaggregation equation. Aqueous Urea. The effect of urea on coaggregation tendency of an alcohol in NaLS solutions is the sum of the effects on the various factors in eq 2 as shown in eq 7 . The last term containing n,, the total number of moles of H2O and of urea in 1 L of solution, is negligible as seen from Table 11.
-
0
E
-
7
3
!' 0,
I .a E
I
-loo(
-ZOO(
Figure 7. Transfer free energy of I-alkanols from N a L S micelles in water to micelles in aqueous urea solutions at 25 O C using Apo(H20+X)
-200oL--0
1
I
2
4
,
6 n an C , H z n . , O H
1
1
1
e
10
12
Figure 8. Transfer free energy of I-alkanols from H 2 0 to D20at 40 'C and from N a L S micelles in H 2 0 to micelles in D20and to micelles in 3 M urea as a function of alcohol chain length at 25 O C .
increases with increasing urea concentration. The transfer free energy is negative for hexanol and octanol and becomes more negative with increasing urea concentration, tending to reach a minimum between 4 and 3 M, respectively, with an apparent tendency for bom(H2O-.u) to increase (becoming less negative) at higher urea concentrations. For butanol the minimum appears to be around 2 M. The transfer free energy values of alcohols from H 2 0 to D 2 0 are shown in Figure 8. Ago(H20-.D20) increases linearly with alcohol chain length. Figure 8 also compares the transfer free energy of an alcohol from micelles in H 2 0 to micelles in D20and to micelles in 3 M urea. It shows that as the alcohol chain length increases A~o,(H20-.D20) tends to become more positive while
d In K - d[uI =[
d In (-d(cmc)/dC,d) d[uI
]-[%I-
The effect on the cmc' term reflects the effect of urea on the standard free energy of micellization in the absence of the additive. This factor works against coaggregation, Le., tends to decrease the value of the distribution coefficient as urea concentration is increased. The rate of increase of the standard free energy of micellization per 1 M urea is 0.06RT,which is relatively small. Effect on 0. The value of 0 depends on the value of the effective micellar degree of ionization in the absence of the alcohol and on the ability of the alcohol to increase the micellar degree of ionization (eq 3). These two factors are functions of the surfactant chain length,18 surfactant polarhead,8 and the solvent system as found in this study. These parameters determine the micellar structure, specifically the extent of the hydrocarbon (HC)-water contact region.18 In previous s t ~ d i e s , 'the ~ +solvent ~~ system was kept constant (water) and the surfactant chain length was varied. It was found for example that in sodium alkyl sulfates (da/dym),m_gdecreased from 0.23 in Cloto 0.07 in CI6,with the coaggregation tendency decreasing at the same time, keeping in mind that the micellar size increases on going from C l o to CI6. It was concluded that the ability of an amphiphilic additive to penetrate the micelle was decreasing as the surfactant chain length was increased. For NaLS in urea solutions, the surfactant chain length is kept constant and the solvent system is varied. In this case, however, the micellar size decreases as urea concentration increases3' and the micelle become less compact than micelles in H 2 0 . This is evident also from the increased value of ao with urea concentration as seen in Table 11, suggesting a decreasing micellar surface charge density with increasing urea concentration. These observations together with the increasing value of cmco with increasing urea concentration suggest that increasing urea concentration is equivalent to decreasing the effective surfactant chain length. According to a recently proposed micellar model,I8 the micellar structure may be looked upon as made up to two regions, an inner region comprising the hydrocarbon core not in contact with water and an outer region comprising the polar heads plus portions of (30) Abu-Hamdiyyah, M.;Kumari, K. J . Phys. Chem.1990, 94, 2518. M.;Swarup, S. J. Colloid Interface Sci. 1984, 91, 256.
(31) Almgren,
Solubilization Tendency of I-Alkanols
The Journal of Physical Chemistry, Vol. 94, No. 16, 1990 6451
the hydrocarbon chains which are in contact with the solvent in D20and thus a decrease in the extent of the HCsolvent contact (HC-solvent contact region), with the former region increasing region compared to that in ordinary water. Thus, it follows that at the expense of the latter with increasing suractant chain length. the ability of an alcohol to penetrate the micelles will be less in Thus, our result in aqueous urea suggest that the HC-solvent D 2 0 than in H 2 0 which explains the higher value of f3 and the contact region in the micelle of NaLS increases at the expense correspondingly lower value of (da/dy,,JYm+ in D20.18330 of the inner core with increasing urea concentration. However, Transfer Free Energy. The results of transfer free energy of to explain the decreasing (da/dy,.,JYm+ with increasing urea alcohols from water to aqueous urea or D 2 0 illustrated in Figure concentration, the micelles must not only be decreasing in size 6 and 8 are in general agreement of what is known about the but also becoming much more open and disorganized so that solubilization power of urea solutions and of D 2 0 for nonpolar insertion of a nonionic polar head between the ionic heads in the or amphiphilic substances as a function of the nonpolar moiety.693'39 It is now well recognized that urea solutions tend to be micelle becomes less effective in screening the ionic charges on the micellar surface. Partial solubilization of the alcohol in the a better solvent than water for nonpolar substances with relatively "center" of the would also show a decreased effect on large size and vice versa for the very small size members of these the micellar degree of ionization. However, this is unlikely for substances. Our results show the transfer from H 2 0 to urea short-chain alcohols at low concentrations. solution is unfavorable for ethanol and to a lesser extent for Thus, unlike the case of a given solvent and a homologous series propanol, and this tendency increases with urea concentration for of surfactants18*mwhere (da/dy,)ym4 decreases due to increasing both alcohols. For the larger alcohols butanol, hexanol, and cohesion between the hydrocarbon chains and a decrease in the octanol the transfer is favorable with a tendency that increases HC-solvent contact region, the decrease in (da/dy,,JYm+, noted with alcohol chain length and with increasing urea concentration. in this study with increasing urea concentration is explained as These trends are related to the weakening of the hydrogen-bonded due to the increasingly more open and probably more disorganized skeletal structure in water by urea which tends to destroy the micelle. The spectroscopic results obtained by Grieser et al.15 for natural cavities capable of accommodating the relatively small the effect of pentanol and of urea reinforce the above picture for nonpolar m~ieties!J~-~~However, for the larger nonpolar moieties the micellar structure in urea solution. The alcohol and urea have that require work to make cavities for their accommodation, it opposite effects on the structure of NaLS micelles; the former becomes easier to create such cavities in urea solutions (requires reduces and the latter increases the HC-solvent contact region. less work because of the weakening in the skeletal hydrogenThe overall effect on 0 is represented by the term d In O/d[u], bonded structures), hence the increased solubilization power!^^'-^^ which as in the case of d In cmco/d[u] term works against This tendency is reversed for the transfer of these alcohols with coaggregation. The contribution of this term to the free energy relatively large nonpolar moieties from H 2 0 to D 2 0 where a of coaggregation is 0.15RT per 1 M urea. greater amount of work is required to create a cavity in the solvent Ability To Depress the cmc. This effect depends on the alcohol D 2 0 with stronger hydrogen-bonded skeletal structure than in chain length,7 the extent of the HC-solvent contact r e g i ~ n , l ~ ~ H20. ~ ~ For the short-chain alcohol, ethanol, the two solvents H 2 0 and the free monomer concentration in the s o l ~ t i o n . As ~ the and D20appear to be equally favorable. It is well-established alcohol chain length increases, the ability to depress the cmc of that for very small nonpolar groups, e.g., methane or argon where a given surfactant increases. For a given alcohol the ability to they tend to be accommodated in the natural cavities of the solvent, depress the cmc increases with the free monomer concentration the transfer free energy from H 2 0 to D 2 0 is more favorable6 as of the surfactant, the so-called "phobic-out" or "push" effect in the natural cavities would be more stable in the latter solvent. the coaggregation process. The other factor that is of importance The transfer free energy of alcohols from NaLS micelles in H 2 0 in the coaggregation process may be termed the "pull" effect, to micelles in urea solutions corresponds to the transfer from a which is the tendency of the amphiphilic additive to reduce the larger more compact micelle to a smaller much less compact and extent of HC-solvent region in the micelle. Thus, for a given more disorganized micelle where the HC-water contact region alcohol the ability to depress the cmc increases in going from H 2 0 becomes larger and the inner H C core smaller than in water. As to urea solutions as observed experimentally. the reduction of the HC-solvent contact region by the amphiphilic The contributions of this term per 1 M urea are 0.09,0.14,0.22, additive is one of the driving forces for the coaggregation process, and 0.17 for propanol, butanol, hexanol, and octanol, respectively. it is clear that for proanol and to a lesser extent butanol the efficacy Whether the value obtained for octanol is real, signaling a size of reducing the HC-solvent contact region on coaggregation effect, remains to be seen. This suggests that the whole octanol becomes increasingly unfavorale with increasing extent of the chain is not used in reducing the HC-solvent contact area. HC-solvent contact region. On the other hand, for the larger D20. Upon comparison of the cmc of NaLS in D 2 0 and in alcohols hexanol and octanol the reduction in the HC-solvent H 2 0 , it appears as if NaLS effectively has a longer hydrocarbon contact region becomes more efficient with increasing urea conchain in D 2 0 than in H 2 0 since the cmc decreases with increasing centration until about 4 M for hexanol and 3 M for octanol, after surfactant chain length. As has been noted earlier, the ability which it appears that the reduction of the HC-solvent contact to depress the cmc produced by a given nonionic amphiphile region becomes less efficient. This tendency is just visible in decreases with increasing surfactant chain length18pm(or decreases butanol as the minimum appears at 2 M urea. with decreasing free monomer concentration in the surfactant in Thus, it appears that on one end of the scale an amphiphile solution), hence the decreased ability to depress the cmc in DzO with a small nonpolar moiety (propanol) is not efficient for the compared to that in H 2 0 . reduction of a large HC-solvent contact region (in 5 M urea) and The experimentally observed lower coaggregation tendency in at the other end only a portion of the nonpolar moiety of a large D 2 0 compared to that in ordinary water as seen in Table I11 amphiphile (octanol) appears to be used for the reduction of the follows from the observation that NaLS has an effectively longer HC-solvent contact region (in 5 M urea). hydrocarbon chain in DzO than in H 2 0 as it has been observed The transfer free energy of an alcohol from H 2 0 to D20 experimentally in the homologous ionic surfactant system in water; corresponds to the transfer from a less to a more compact micelle, namely, In K decreases with increasing surfactant chain length.18s30 and thus the transfer is from a larger to a relatively smaller The increased effective chain length of NaLS in D 2 0 compared to that in ordinary water also predicts a larger and more compact (34) Wetlaufer, D. M.; Malik, S. K.; Stoller, L.; Coffin, R. L. J . Am. micelle in D 2 0 . Furthermore, the smaller (YO values in D,O Chem. SOC.1964, 86, 508. reinforce the above picture as it indicates a higher miceller surface (35) Nozaki, Y.; Tanford, C. J . Biol. Chem. 1963, 238, 4074. charge density in DzO than in ordinary H20. These observations (36) Kresheck, G. C.; Schneider, H.; Scheraga, H. A. J. Phys. Chem. 1%5, suggest an increase in the cohesion of the chains in the micelles 69, 3132. (32) Zana, R. J . Colloid Interface Sci. 1980, 78, 330. (33) Lianos, P.; Lang, J. J . Colloid Interface Sei. 1983, 96, 222.
(37) Kuharski, R. A.; Rossky, P. J. J . Am. Chem. Soc. 1984,106,5794. (38) Robinson, M.; Jencks, W. P. J . Am. Chem. Soc. 1975, 97, 631. (39) Tanaka, H.; Touhara, H.; Nakanishi, K.; Watanabg, N . J . Chem. Phys. 1984.80, 5170.
J . Phys. Chem. 1990, 94, 6452-6457
6452
HC-solvent contact region. Thus, as the alcohol chain length increases the transfer process becomes increasingly unfavorable. Summary and Conclusion
We have investigated the solubilization tendency of several alcohols in NaLS in D20, ordinary water, and aqueous urea solutions by eq 1 relating the distribution coefficient to the initial relative lowering ability to depress the cmc and the corresponding initial relative ability to increase the effective degree of micellar ionization. It was found that the coaggregation tendency decreases in going from H 2 0 to D 2 0 and also on going from H 2 0 to 5 M urea. This trend has been confirmed by measuring the coaggregation tendency of the alcohols in these solvent systems by head space gas chromatography. We have also determined by the latter technique the transfer free energy of alcohols from H 2 0 to D 2 0 and from H 2 0 to aqueous urea solutions and then estimated the transfer free energy of an alcohol from micelles in H 2 0 to micelles in D 2 0 and to micelles in urea solutions. Apo(H20-D20) is about zero for ethanol and increases linearly with alcohol chain length. Ag0(H20-u) is positive for ethanol and propanol and
becomes more positive with urea concentration. It is negative for butanol, hexanol, and octanol and tends to become more negative with urea concentration. At a given urea concentration Ap0(H2+u) decreases with increasing alcohol chain length. The transfer free energy of an alcohol from micelles in H 2 0 to micelles in D 2 0 is positive and increases with alcohol chain length. On the other hand, Apom(H20+u) is positive for propanol and increases with urea concentration. For butanol it appears to be favorable at 2 M urea after which Apom(H20-u) becomes positive and increases with urea concentration. For hexanol and to a greater extent octanol Ag0,(H20-u) is negative which becomes more negative with urea concentration, the decrease reaching a maximum at 4 and 3 M urea, respectively. The results have been interpreted in terms of the effect of the "structuredness" of the solvent system on the extent of the HCsolvent region of the micelle and the ability of an alcohol to reduce the extent of this region in the NaLS micelles. Acknowledgment. This research was supported by the University of Kuwait, Project No. SC027 and SC042.
Surface Molecular Orientations Determined by Electronic Linear Dichroism in Optical Waveguide Structures D.M. Cropek and P. W. Bohn* Department of Chemistry and Beckman Institute, 1209 W. California St., Urbana, Illinois 61801 (Received: January 17. 1990; I n Final Form: April 16, 1990)
Electronic linear dichroism measurements were performed on monolayers of monophenyldimethylsilanes to determine average orientations of the phenyl rings. The very weak absorption of these silanes at the excitation wavelengths available makes them excellent candidates to test the lower detection limits of this novel approach to orientation studies. By use of thick glass waveguides (150 pm thick), the interaction between the absorbate monolayer and the electric field is maximized, and absorption loss coefficients for the silanes are obtained with high precision. Orientation angles for the long axis of the phenyl groups, measured from normal, are between 58' and 75', in good agreement with an expected tetrahedral angle of 7 0 . 5 O . The exact angle appears to be strongly dependent on the monolayer coating procedure.
Introduction
Orientational studies on aligned molecular assemblies is an area which is receiving increased attention because of growing interest in the characterization of ordered structures with thicknesses on the order of several monolayers and below. These thin film structures include Langmuir-Blodgett self-assembled monolayer^,^.^ surface-modified electrode^,^,^ and biomolecular assemblies* for various purposes such as monitoring electrode reactions, surface-catalyzed reactions, construction of layers with specific optical and physical properties, chemical sensors, and electronic components. Linear dichroism (LD) techniques allow for the investigation of these aligned samples.9-" LD is the anisotropic response of a partially or completely oriented sample to different polarizations of an excitation source. Information which can be obtained from an LD experiment falls into three main categories: ( 1 ) molecular optical properties, (2) dynamics, and (3) orientation information.I2 LD can be done in numerous experimental modes,I3 including absorption, fluorescence, electron spin resonance, X-ray scattering, and Raman scattering; however, the first two methods are the most prevalent. Traditionally, transmission LD measurements have been used to study either of two kinds of oriented molecular assemblies. The first kind are relatively thick films or even bulk samples which have large concentrations of the oriented structure, for example, guest molecules in liquid crystals14 or stretched polymer films.I5
* 4htFIu
10
\\
hom correqpondence should be addressed
The second kind consists of thin layers of molecules with large absorption cross sections (or fluorescence quantum yields) such as phthalocyanines." These experiments are designed to generate appreciable absorption when passing the excitation radiation perpendicular to the film plane. In the present experiments, we have extended the LD measurements, through the use of integrated optical structures, to the study of submonolayer amounts of even ( I ) Kuhn, H. J . Photochem. 1979, 10, 1 1 1. (2) Swalen, J. D. J . Mol. Electron. 1986, 2, 155. (3) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A,; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (4) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. ( 5 ) Maoz, R.; Sagiv, J. Langmuir 1987, 3, 1034. (6) Murray, R. W. Annu. Rev. Marer. Sci. 1984, 1 4 , 145. (7) Wrighton, M. S. Science 1986, 231, 32. (8) Blankenburg, R.; Meller, P.; Ringsdorf, H.; Salesse, C. Biochemistry 1989, 28, 8214. (9) Norden, B. Appl. Spectrosc. Rev. 1978, 14, 157. (IO) Schellman, J.; Jensen, H. P. Chem. Reu. 1978, 87, 1359. ( I I ) Palacin, S.; Lesieur, P.; Stefanelli, I.; Barraud, A. Thin Solid Films 1988, 159, 83. (12) Thulstrup, E. W.; Michl, J. In Polarized Spectroscopy of Ordered Systems; Samori, B., Thulstrup, E. W., Eds.; Kluwer Academic Publishers:
Boston, 1988; pp 1-24. (13) Kuball, H. G.; Friesenhan, H.; Schonhofer, A. In Polarized Spectroscopy of Ordered Systems; Samori, B., Thulstrup, E W.. Eds.: Kluwer Academic Publishers: Boston, 1988; pp 85-104. (14) Johansson, L. B. A. Chem. Phys. Len. 1985, 118, 516. ( I S ) Tempczyk, A,; Gryczynski, Z . ; Kawski, A.; Grzonka, Z . Z . Natur,forsch. 1988, 43A. 363.
0022-365419012094-6452$02.50/0 C 1990 American Chemical Society
