J. Phys. Chem. 1986, 90, 4167-4174
4167
Ultrasonic Relaxation Studies of 2-ButoxyethanoCWater and 2-Butoxyethanol-Water-Cetyltrimethylammonium Bromide Solutions as a Function of Composition S. Kato,? D. Jobe, N. P. Rae,$ C. H. H O , and ~ Ronald E. Verrall* Department of Chemistry, University of Saskatchewan, Saskatoon. Saskatchewan, Canada S7N 0 WO (Received: December 2, 1985; In Final Form: March 26, 1986)
Ultrasonic absorption measurements of aqueous solutions of 2-butoxyethanol (BE) and ternary aqueous solutions of cetyltrimethylammoniumbromide (C16TAB)and BE in the presence of relatively high concentrationsof the alcohol were carried out in the frequency range 3-210 MHz and between 15 and 35 OC. A single ultrasonic relaxation process is observed in both binary and ternary systems at XBE< 0.016; two relaxation processes are generally present in these systems at Xae > 0.016, although large sound absorption does not allow for accurate analysis of their frequencies except when Xse > 0.15. Photon correlation spectroscopy (PCS) and methyl red dye solubility measurementswere made in an attempt to obtain additional information on the nature and size of some of the aggregates present. The single relaxation process observed at low BE compositions in the binary systems is believed to be due to the formation of 'clathrate-like" aggregates of alcohol with water. In ternary systems at low BE concentrations, mixed micelles of C,,TAB and BE are the predominant aggregate, and hence the single relaxation is attributed to the exchange of alcohol molecules between mixed micelles and the bulk phase. The two relaxation processes that occur in the binary systems at high compositions of BE are assigned to merging of clathrate-like structures and to self-association of alcohol whereas in the ternary systems they are assigned, tentatively, to the exchange of alcohol molecules between the bulk phase and water-alcohol aggregates, stabilized by surfactant, and to the self-association of alcohol. Analysis of the ultrasonic data for the binary systems in terms of this mechanistic scheme gives estimates of the molecular composition of these aggregates which are consistent with previously reported results obtained from PCS measurements. The temperature dependence of the relaxation frequencies has been analyzed, and estimates of the activation enthalpies range from 10 to 30 kJ mol-I for both processes. At high alcohol concentrations the relaxation frequencies in ternary systems are observed to decrease with increasing C16TABconcentration whereas they are invariant with changing surfactant concentration at intermediate alcohol concentrations. It is postulated that the relative insolubility of CI6TAB in BE forces the alcohol and alcohol-water aggregates to dissociate at high C16TABconcentrations in order to provide free water to solubilize the surfactant. Studies focusing on the surfactant itself are required to confirm this speculation.
Introduction The relative stability of microemulsions is an important factor in their use in in situ oil recovery processes. The results of previous studies carried out on these systems suggest that the formation and stability of microemulsions may, in part, result from their dynamic character enhanced by the presence of cosurfactant (alcohol). The role of alcohols in ternary systems of water-surfactantalcohol has been investigated by using a variety of techniques.'" Generally, the results show that the alcohols influence the micellization process by either modifying the properties of water or, in the case of long-chained alcohols, participating directly in the micellization process by forming mixed micelles with the surfactant. In these instances the alcohols are considered to be cosolvents. However, thermodynamic studies of certain binary alcohol-water systems have shown alcohol properties to be similar to those of surfactants in water-surfactant systems and oil in ternary systems of water-surfactant-oil and water-alc~hol-oil.~ The experimental evidence suggests that alcohols are capable of forming microheterogeneities like micelles except that their structure is more poorly understood. Few studies have been reported concerning the dynamical properties of cosurfactants. Chemical relaxation methods, including the use of ultrasonic relaxation techniques, recently have proved to be a useful tool to investigate the fast dynamic properties of the cosurfactant molecule. Several ultrasonic relaxation studies,8-11 including one from this laboratory: have been carried out to investigate the dynamics of mixed micellar systems in order to obtain quantitative information on the kinetic and thermody'On leave from the Department of Chemical Engineering, Faculty of En ineering, Nagoya University, Nagoya 464, Japan. fOn leave from the Department of Physics, Sri Venkateswara University, Post Graduate Centre, Kavali 524 202, India. SResearch Chemistry Branch, Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada ROE 1LO.
0022-3654/86/2090-4167$01.50/0
namic parameters of the cosurfactant exchange process between the micelles and bulk solvent. The majority of previous studies have focused primarily on dilute solutions of cosurfactant and not on relatively higher concentrations of cosurfactant which are likely to be used in tertiary oil recovery processes. Studies of these systems covering a wider range of concentrations of cosurfactants are needed to determine whether there is any change in the role of the cosurfactant under these conditions. In this paper we report studies of laboratory prepared ternary systems composed of surfactant (cetyltrimethylammonium bromide, C16TAB)-cosurfactant (2-butoxyethanol; BE)-water with a view to assessing the kinetics of the exchange processes of cosurfactant between the bulk phase and aggregates present at high alcohol concentrations. In light of the observations that the long-chained alcohols exhibit surfactant-like properties in aqueous binary systems, the latter were examined to provide appropriate background information for the analysis of the ternary systems. One of the major problems in the interpretation of relaxation data is the identification of the dynamic processes responsible for the observed relaxation. In complex systems such as those being (1) Zana, R.; Yiv, S.; Strazielle, C.; Lianos, P. J. Colloid Interface Sci. 1981, 80, 208.
(2) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 76, 62. (3) Rosenholm, J. B.; Drakenberg, T.; Lindman, B. J. Colloid Interface Sci. 1978, 63, 538. (4) Makala, M. R.; Rosenholm, J. B.; Stenius, P. J. Chem. Soc., Faraday Trans. I 1980, 76, 473. ( 5 ) Vikingstad, E. J. Colloid Interface Sci. 1979, 72, 75. (6) Rao, N. P.; Verrall, R. E. J. Phys. Chem. 1982, 86, 4777. (7) Desnoyers, J. E. Pure Appl. Chem. 1982, 54, 1469. (8) Zana, R.; Lang, J. Adv. Mol. Relax. Processes 1975, 7 , 21. (9) Hall, D.; Jobling, P. L.; Wyn-Jones, E.; Rassing, J.-E. J. Chem. SOC., Faraday Trans. 2 1977, 73, 1582. (10) Yiv, S.; Zana, R. J. Colloid Interface Sci. 1978, 65, 286. (11) Gettings, J.; Hall, D.; Jobling, P. L.; Rassing, J.-E.; Wyn-Jones, E. J . Chem. Soc., Faraday Trans 2 1978, 74, 1957.
0 1986 American Chemical Society
Kat0 et al.
4168 The Journal of Physical Chemistry, Vol. 90,No. 17, 1986
studied in this work it is imperative that one be able to draw upon other sources of information concerning the physical properties of these systems in order to carry out the analysis. To this end, estimates of the aggregate sizes were determined using photon correlation spectroscopy (PCS). This technique has been successfully used12 in the study of inverted micelles and microemulsions. Diffusion coefficients and Stoke's radii can be determined with some precision for very small particles, although for the aggregates discussed here the relative trends in these parameters are perhaps of greater significance. To further aid in the identification of the nature of the aggregates, we carried out solubilization studies of methyl red dye in these systems using absorption spectrophotometry. These data can provide some information about the degree of aggregate formation as the composition of the components is varied. In the first stage6 of this comprehensive study experimental conditions were maintained such that the surfactant concentration was kept constant while the cosurfactant concentration was varied. This methodology is somewhat poor because it makes it more difficult to interpret data when micellar properties (e.g., composition, aggregation number, etc.) as well as the concentration of free cosurfactant and surfactant are modified simultaneously. For systems in which the ratio of cosurfactant to surfactant remains constant, the surfactant aggregation number and mixed micelle composition remain fairly constant. However, at very low concentrations of cosurfactant it was shown that the relaxation frequencies observed at fixed mole ratios of cosurfactant to surfactant are similar to those observed when there is simultaneous variation of cosurfactant and surfactant compositions6 The majority of the ultrasonic absorption results of ternary systems reported here were obtained at fixed mole ratios of cosurfactant to surfactant for higher compositions of cosurfactant. The observed high-frequency relaxation process is now believed to be associated with alcohol exchange between the bulk phase and water-alcohol microphases stabilized by C16TAB. The results of these ultrasonic absorption and photon correlation spectroscopy studies of binary systems of BE-water and ternary systems of BE-water-C16TAB show that the cosurfactant BE plays a primary role in the formation of microheterogeneous phases at all alcohol concentrations. At high alcohol to water mole ratios the solubilization of surfactant in these systems causes a decrease in the concentration of alcohol and alcohol-water aggregates so as to allow for dispersion of the surfactant. Experimental Section
Chemicals. 2-Butoxyethanol (BE) and cetyltrimethylammonium bromide (Ci6TAB) were obtained from Aldrich Chemical Co. and from Sigma Chemical Co., respectively. All chemicals are used without further purification. Solutions were prepared on a mass basis with deionized water obtained from a Millipore Super-Q system. Ultrasonic Absorption and Ultrasonic Velocity. A previously d e ~ c r i b e d ' ~apparatus *'~ based on the pulse method was used for ultrasonic absorption measurements in the frequency range of 3-210 MHz. Three cells were used to measure the ultrasonic absorption: a single-crystal reflector with crystal of fundamental frequency 1 M H z for the frequency range 3-15 MHz, a sendreceive type with two crystals of fundamental frequency 5 MHz for the frequency range 5-65 MHz, and another send-receive type with two crystals of fundamental frequency 10 M H z for the frequency range 10-210 MHz. Crystals were matched within 0.5%. The sound absorption coefficient was measured by varying the sound path length and observing the resulting attenuation to the intensity of the ultrasonic wave by monitoring the first sound echo. Ultrasonic velocity measurements were carried out using an ultrasonic interferometer operating at a frequency of 4.00 MHz. All measurements were made under temperature conditions maintained constant to *0.01 O C of the reported values. (12) Zulauf, M.; Eicke, H.-F. J. Phys. Chem. 1979, 73, 480. (13) Verrall, R. E.; Nomura, H. J. Solution Chem. 1977, 6, 217. (14) Verrall, R. E.; Nomura, H. J. Solution Chem. 1977, 6, 541.
Photon Correlation Spectroscopy. Measurements were made with a Malvern 4600 photon correlation spectrometer. An argon ion laser (100 mW) was used as the light source, and the scattering angle was always 90'. Samples were contained in 1- X 1- X 4-cm glass cells which were thermostated to 25 f 0.1 OC during the measurement. All solutions were filtered prior to use. The rate at which particles move or diffuse through a liquid is a direct function of their size. The larger the particles, the smaller is the diffusion rate. The latter is characterized by the diffusion coefficient (0) of the particles. The diffusion coefficient can be obtained from the fluctuation time constant 7, of the diffraction pattern scattered by the particles according to
D = (T$)-'
(1)
where K is the scattering vector [K= (4.rr/X)n sin (0/2)], 0 is the scattering angle, n is the refractive index of the liquid, and h is the wavelength of the light source. For spherical particles, D is also related to the Stokes radius, rh, by D = kT/6.rrqrh where 7 is the viscosity of the liquid, Tis the absolute temperature, and k is the Boltzmann constant. In a sample of monodispersed particles the correlation function decay rate of the scattered light intensity follows a single exponential. The correlator measures this time constant, and one can determine particle size assuming spherical geometry of the aggregates. The presence of a single exponential was confirmed by the values of the near to far point ratios which were between 0.9 and 1.0 for all measurements. Visible Absorption Measurements. Visible absorption measurements of methyl red (MR) in the mixed solvent systems containing BE-water-C16TAB and BE-water were carried out using a Cary 118C spectrophotometer at 25 O C . Solubilities were estimated from this data in the usual way. Results
Ultrasonic Absorption and Data Analysis. Table I shows the composition of binary and ternary systems investigated in this work. As previously mentioned, the ratio of the concentration of alcohol (BE) to surfactant (C,,TAB) was kept constant in most cases when changing both the alcohol and surfactant compositions in the ternary systems. The sound absorption coefficient a divided by the square of the ultrasonic frequency f was analyzed as a function off. The accuracy in the absorption measurement is better than &3%. The analyses of the absorption data were carried out according to eq 3 and 4, assuming that the results indicated either one or two discrete ultrasonic relaxation processes (3) (4)
wherefrepresents the measured frequency, f,,is the relaxation frequency, Ai is the relaxation amplitude, B is the background contribution to sound absorption arising from any other processes that may be occurring at higher frequencies beyond the experimental frequency range, u is the ultrasonic velocity, and p' is the excess sound absorption coefficient per wavelength. In the case of double relaxation processes, suffixes 1 and 2 refer to the lowand high-frequency processes, respectively. Experimental data in the frequency range investigated were computer fit to eq 3, and the best values of the parameters, Ai,h,,and B were obtained assuming either single or double relaxation processes were present. A nonlinear least-mean-squares fit was carried out by using the complete grid search technique. The initial values of the relaxation parameters were chosen by inspection of ultrasonic absorption spectra, and then the quantity
was minimized. In this relation n represents the number of data points and the suffixes obsd and calcd refer to the observed and
The Journal of Physical Chemistry, Vol. 90, No. 17, I986 4169
Aqueous Solutions of BE and C16TAB-BE TABLE I: Composition of Systems Investigated cosurfactant BE wt % 14.1 18.5 50.0 65.2 13.8 73.8 60.0 83.0 9.5 33.0 50.0 84.2 9.2 30.6 44.7 51.2 60.9 67.1 65.5 72.7 46.1 53.7 68.8 76.9 49.1 6.61 8.62
system T1 T2 T3 T4 T5 T6 T12 T26 T34 T3 5 T36 T31 T38 T39 T40 T4 1 T42 T4 3 T44 T45 T46 T47 T48 T49 T50 TS 1 T52
water
surfactant CI6TAB mol 96 96.8 95.9 85.3 15.8 67.7 70.0 81.4 51.3 98.38 92.7 86.0 47.4 98.3 92.2 85.1 80.0 68.1 55.1 68.9 56.0 79.0 65.5 69.4 56.6 87.2 98.93 98.58
wt % 78.1 74.1 45.5 31.7 23.8 26.2 40.0 17.0 90.0 65.4 47.6 11.8 88.2 60.1 42.6 34.2 21.7 13.1 23.3 14.9 30.7 19.1 24.6 15.8 50.9 93.39 91.38
mol % 2.1 3.6 14.3 23.8 32.0 30.0 18.6 42.7 1.59 7.2 13.8 51.8 1.6 7.1 13.6 18.3 29.2 41.1 29.5 41.7 18.1 29.6 29.6 42.1 12.8 1.07 1.42
wt % 7.8 7.4 4.5 3.1 2.4
mol % 0.5 0.5 0.4 0.4 0.3
0.5 1.6 2.4 4.0 2.6 8.7 12.7 14.6 17.4 19.2 11.2 12.4 23.2 27.2 6.6 7.3
0.025 0.1 1 0.22 0.8 0.1 0.65 1.3 1.7 2.1 3.8 1.6 2.3 2.9 4.9 0.9 1.3
~40
20 -
m
2
-
x
a
10:
-
c
4-
2-
I
3
5
IO
30
100
50
f MHz
200
Figure 1. a/f data as a function of frequency for ternary systems composed of C,6TAB-BE-water at 25 OC. Mole ratio BE/CI6TAB = 10.8.
N
-
u)
I
E
400-
0
k
0 X
4°C
200-
I
3
I
l , # t # l
5
IO
I
30
I
I
1
50
1
1
1
I
100
200
1
J
f MHz
Figure 2. u/f data at 25 OC as a function of frequency for ternary systems T42-T48 composed of C,,TAB-BE-water. T6 is a binary alcohol-water system. Mole ratio BE/water = 0.43.
I
I
I
I
I
I
I
I
I
I
1
1
1
1
1
I
Kat0 et al.
4170 The Journal of Physical Chemistry, Vol, 90, No. 17, 1986
TABLE 11: Ultrasonic Relaxation Parameters and Sound Velocity Data for Binary BE-H20 and Ternary C,,TAB-BE-Water Systems at 25 O C A, X IO", AI, A2 X lOI7 fi, B x ioi7, u, mole ratio system cm-I s2 MHz cm" s2 MHz cm-' s2 m SKI 103~~,,, 103fimar2 BE/C,~TAB T6 158 13 85 I40 117 1399 1.44 8.32 172 89 158 1430 3.31 10.9 610 7.6 TI2 6.70 1374 0.62 150 65 65 1s 60 T26 6 T50" 24.2 5.75 179 T5 1 26.8 11.1 T52 383 1546 6.00 372 25 7.19 36.8 1293 6.0 T34 1488 7.35 31.3 59 715 116 7.6 T35 1299 63 1448 3.94 19.8 83 130 330 640 8.5 T36 33 1360 0.88 2.78 124 104 118 11 T37 38.1 1532 3.99 9.94 590 22 1041 5 .O T38 104 1484 3.40 25.3 61 7.2 560 636 T39 1449 1.56 80 18.0 130 8.3 259 311 T40 10.8 87 1435 1.13 15.4 134 7.7 246 205 T4 1 1406 1.08 108 11.4 127 T42 8.9 173 150 9.48 t 386 1.17 10.9 114 130 120 T43 155 1404 0.95 10.1 130 10 115 125 T44 135 18.1 8.16 87 1380 1.13 136 116 12 137 T45 1440 1.41 16.1 7.4 240 93 130 T46 265 6.1 1414 1.24 14.6 134 7.3 200 103 24 1 T47 1401 1.29 9.59 128 128 11.1 107 166 T48 32.4 107 70 1378 1.25 6.95 12.6 I44 T49 144 _I
"Values cannot be accurately determined. bBinary mixtures of BE and water.
TABLE III: Temperature Dependence of Ultrasonic Relaxation Parameters and Sound Velocity Data for Ternary Systems Tl-T5, T42, and T26" (ClrTAB-BE-Water) A, x 1017, temp, A, x IO", A,? fr23 B x 1017, u, system OC cm-' s2 MHz cm-l s2 MHz cm-' s2 m s-I 1o3fimax, 1O3fimaX2 4 20 TI 15 1070 1600 70 1517 3.25 24.3 8 800 35 60 1516 21.2 1000 25 6.06 8 440 45 50 1514 4.85 15.0 800 35 73 T3 15 475 7.6 460 165 1464 2.64 24.6 77 315 143 1441 17.5 25 387 8.3 2.31 11 278 220 92 112 1422 2.17 14.4 35 171 T4 15 5.6 420 249 76 1440 1.70 13.6 88 134 1413 1.28 8.0 25 226 170 10.6 10 106 130 105 1388 1.10 9.6 35 155 T5 15 9 100 206 150 144 1426 1.32 10.7 121 1393 6.7 80 25 11.6 158 128 1.28 74 70 13 140 98 1362 0.66 6.7 35 1437 T42 15 297 218 6.8 87 160 1.45 13.6 I50 108 1.08 8.9 127 1406 11.4 25 173 111 I1 116 133 0.84 106 1379 10.6 35 77 110 62 1404 T26 1Sb 12 0.65 4.8 115 55 I70 23 0.62 1342 45 40 35 6.3
'Binary mixture of BE and water. bRelaxation parameters at 25
OC
the calculated relaxation spectra, ([A,/l + cf/fr,)21 + [A,/1 + (f/fi2)2]ifu, and they are in good agreement with experimental data. The dotted lines in Figure 3 show the estimated contribution from each relaxation process to the total p' for the system T 4 3 . This independent analysis of the data is in terms of the maximum excess sound absorption coefficient per wavelength, pmx,,as defined by pmax, = (Afr,u)/2 (i = 1, 2 ) (5) Tables I1 and I11 list values of the estimated relaxation parameters from eq 3 as well as some values of pmax,for both the binary alcohol-water systems and the ternary systems composed of surfactant-alcohol-water at 25 O C . The temperature dependence of the ultrasonic absorption of several ternary systems, Tl-T5 and T42, and the binary system, T26, was carefully measured to obtain estimates of the apparent activation enthalpies of the relaxation processes in these solutions. In systems Tl-T5 the mole ratios of water to surfactant were maintained approximately constant. Values of the enthalpies of activation were obtained by means of the Eyring rate expression
f,,= kT/2rh{exp(-AHZ*/R7') exp(A.S*/R))
(6)
where AHi* and ASi* are the enthalpy and entropy of activation, respectively, with respect to the ith relaxation process and all the
are presented in Table 11. TABLE I V Activation Enthalpiesa for Ternary Systems of C,,TAB-BE-Water AH,*,
system T1 T3 T4 T5 T42 T26b
kJ mol-' 25 f 23 9.9 i 8.5 22.8 f 3.2 12.7 f 4.6 17.7 f 1.4 25.1 f 1 1
AH2',
kJ mol-' 32 f 9.8 7.1 f 4.7 11.5 f 1.8 11.5 f 1.1 15.2 f 0.2 15.4 f 7.3
"From plots of In (2.rrfr1/T) vs. 1/T. *Binary BE-water systems.
other factors have their usual meaning. Values of AHi' (Table IV) were obtained from the slope of the plot of In ( 2 r j J T ) vs. 1/ T . Figure 4 shows a typical example (system T 4 ) of the temperature variation of a/f vs. J and Table 111 summarizes the values of the relaxation parameters derived from these temperature studies. The magnitude of the activation enthalpies are consistent with that expected for diffusion-controlled processes. More importantly, they are approximately the same for both the binary and ternary systems. Both the binary and ternary systems show a single relaxation curve at compositions of XeE < 0.016. On the other hand, for values of X,, > 0.016, double relaxation curves are obtained for
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4171
Aqueous Solutions of BE and C16TAB-BE
T4
H
'
N L
T34 'T38
oi
Ib
20
3b
40
io
BE mol %
T 43
N
r
T37
z
T42
-
100
L
c I
3
I
I I l l l l
5
7 IO
I
I
20 30
I 1 1 1 1 1 1
50
100
I
4
200
f MHz Figure 4. a/f data as a function of frequency for ternary system T4 at 15, 25, and 35 OC.
I
Ib
o;
do
BE m o l
40
'
io
X
Figure 6. Relaxation frequenciesf,, and& as a function of mole percent BE for ternary systems with constant mole ratio of BE/CI6TAB: 0 , 10.8; 0, 63.
1100
f r 2 MHz
Figure 7. Relationship between& and& for ternary systems composed of C16TAB-BE-water at 25 OC.
fMHz Figure 5. a/f data as a function of frequency for binary systems of BE-water at 25 OC.
both systems. The binary systems exhibit very large sound absorption in the composition range 0.016 C XBE C 0.15, and it was not possible to analyze the data from these systems with a high degree of accuracy (see system T50, Figure 5 ) . This high abJ~ sorption may be related to the concentration f l ~ c t u a t i o n s ' ~that occur with the formation of merged 'clathrate-like" structures. Table I1 shows the relaxation frequencies obtained from the analysis of the binary systems. In the composition ranges below XBE= 0.016 and above XBE = 0.15 there is a general increase in the magnitudes of the relaxation frequencies with increasing BE concentration for systems showing the presence of both single and double relaxation processes. Previous work6 with the ternary systems showed that a single relaxation frequency occurred at compositions of XBE C 0.016. The relaxation frequencies observed for all ternary systems investigated in this work show an increase with increasing BE concentration (Figure 6). As well, Figure 7 shows that a linear correlation exists between f,,and f,,. Figure 8 shows the dependence of relaxation frequenciesf,, and f,, on CI6TABconcentration (molality per kilogram of mixed solvent) for ternary systems with constant mole ratios ( R ) of BE/H20. For R values less than 0.25, the relaxation frequencies (15) Tamura, K.; Maekawa, M.; Yasunaga, T. J . Phys. Chem. 1977,81, 2122. (16) Blandamer, M. J.; Clarke, D. E.; Hidden, N. J.; Symons, M. C . R. J . Chem. SOC.,Faraday Trans. I 1968, 64, 2691.
cr 0 '
0'
A
I
I
I
I
0.4
08
C16TAB (mol kg-l)
I
I1
12
.,
Figure 8. Dependence of relaxation frequenciesf,,and f,, on C16TAB concentrations (molality per kilogram of mixed solvent) for systems with constant mole ratios of BE/water: A,0.016; A, 0.077; 0,0.16; 0.23; 0 , 0.43; 0, 0.75. Note: m = 0 data refer to binary alcohol-water
systems. are generally independent of the surfactant concentration. In systems having a relatively higher concentration of BE, Le., R > 0.25, the relaxation frequencies show a negative dependence with increasing CI6TABconcentration. Solubility. The solubility of MR as a function of CI6TAB concentration in both binary and ternary solutions of CI6TABBE-H20 having different values of R is shown in Figure 9. There are three distinct types of solubility behavior shown in this graph. The first two curves (XBE = 0.00 and XBE= 0.016) exhibit an
Kat0 et al.
4172 The Journal of Physical Chemistry, Vol, 90, No. 17, I986 A
-a
-------'/
a], 0
a== 0
0.05
0.10
ClsTAB (mol L-'1
Solubility of methyl red ( S , in mol L-I) vs. the concentration of C16TABat various mole ratios of BE/H20 ( R ) . 0,0.00; A, 0.016; 0,0.23;m, 0.43;0 , 0.75;A, 1.00. Note: zero on abscissa scale refers to binary alcohol-water systems; X refers to pure BE. Figure 9.
increase in the solubility of MR with increasing C16TABconcentration. At concentrations of XB, = 0.187 and 0.354, there is relatively little dependence of dye solubility with increasing C16TABconcentration relative to the aqueous binary solution with the same mole fraction of alcohol. Finally, at mixed solvent compositions of X,, = 0.428 and 0.500 there is an initial sharp increase in dye solubility at very low C16TABconcentrations to a plateau region where the MR solubility, thereafter, remains virtually constant with increasing surfactant concentration. The curves for XBE = 0.00 and 0.016 are representative of the solubility dependence expected of water-insoluble dyes in micellar solutions. The crossover of the solubility curves at higher C16TAB concentrations, leading to lower dye solubility for solutions of XBE = 0.016, is not unusual and has been observed for other systems." The addition of some organic compounds to micelles has been found to result in a lowering of the surface potential at the micelle surface due to the incorporation of the solubilizate near the micelle surface. This reduction of the potential leads to a decrease in the attraction of the dye for the micellar phase." The pair of lines in Figure 9 showing little or no variation of dye solubility with surfactant concentration indicate that normal micelles may no longer exist and the liquid-phase structures are similar to the BE-H20 binary systems. The surfactant is dispersed through the aggregates of BE and H,O, resulting in small variation of the dye solubility with addition of C16TAB. The final type of dye solubility behavior illustrated in Figure 9 is the most unusual. The sharp increase in dye solubility that occurs at low surfactant concentrations plateaus abruptly to a value slightly greater in magnitude than the solubility of the dye in pure BE. It should be noted that the solubility of the dye in pure BE is ca. 2 orders of magnitude greater than in pure water and that C16TAB is virtually insoluble in pure BE. The impact of the surfactant a t low concentrations appears to be one of stabilizing wateralcohol structures that increase the dye solubility, as though C16TABwere playing the role of a cosurfactant under these conditions. However, increasing the surfactant concentration in a relatively rich alcohol medium creates solubilization problems for the added C16TAB. Whatever structures it is able to occupy at lower concentrations are no longer available. It appears the C16TABmay force reorganization of the water-alcohol aggregates so as to allow for localization of the surfactant in a more "water-rich" region, perhaps creating "pools" of BE in which the dye may locate to give solubility values not too different from that obtained in pure BE. These data in themselves offer nothing more than a qualitative picture of changes in the microheterogeneities (17) Fletcher, P. D.I.; Robinson, B. H. J . Chem. SOC.,Furaduy Truns. 1 1983, 79, 1959.
R
Figure 10. Values of the product Ds vs. the mole ratio of BE/H20 ( R ) in the absence (0)and the presence (A) of 0.100 m CI6TAB. TABLE V Values for the Product Dq and r h at Various Mole Ratios ( R ) of BE/H*O in the Binary Systems BE-H20 and Ternary Systems BE-H20-0.1 m CI6TAB system R ~ n cm2 . CPs-' rh. A BE-H,O 0.0091 2.11 x 10-8 1000 0.016 1.04 X lo-* 2100 0.050 4.62 x 10-7 47 0.128 2.54 X lod 8.6 0.150 3.94 x 10-6 5.9 BE-H2O-O. 10 0.00 1.1 x 10-6 20 4.2 x 10-7 m CI6TAB 0.016 52 2.8 X lod 7.9 0.050 0.128 3.0 X IOd 7.1 0.293 2.5 X 10" 8.6
that exist in these systems. However, in conjunction with the PCS and ultrasonic results, they allow for a minimum rationalization of these systems which will be presented in the Discussion section. Photon Correlation Spectroscopy. Figure 10 shows the variation in the magnitude of the product Dq, which is inversely proportional to the hydrodynamic radius, r,, of the aggregate, as a function of mole ratio R for the binary and ternary systems. In the latter the concentration of C16TABwas always fixed at 0.1 m. Table V lists the values of rh for these systems calculated from eq 2. No resolvable scattering decay was observed with the present equipment for systems having values of R > 0.30. The minimum in the Dv values or a maximum in the hydrodynamic radius of the aggregate occurs at R N= 0.02. In the case of the binary BE-H20 system, the data beyond the minimum correspond to an almost linear decrease in the hydrodynamic radius of the particles with increasing ratios of B E / H 2 0 in the composition range investigated. However, beyond the sharp minimum, the ternary systems show only slight variation of the magnitude of the product Dq with increasing R; this means that there is very little change of the hydrodynamic radii of the aggregates. In Figure i o the minimum for this binary system BE-H20 occurs at a mole fraction of BE zz 0.016. This is the composition that clathrate-like structures have been postulated to occur in these systems.'* As well, solutions of composition around this value are observed to exhibit varying degrees of opalescence in their physical appearance. The opalescence quickly disappears with increasing mole fraction of alcohol. A similar observation has been reportedlg for solutions of isobutoxyethanol and water near this composition. (18) Roux, G.;Perron, G.; Desnoyers, J. E. J . Solution Chem. 1978, 7, 639. (19) Fanning, R. J.; Kruus, P. Can. J . Chem. 1970, 48, 2052.
Aqueous Solutions of BE and C16TAB-BE
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 4173
The assignment of the relaxation frequencies to a given process has been done on the basis of the relative magnitudes of the peak excess sound absorption coefficient per wavelength, wmx,. Tables I1 and I11 show that the value for the high-frequency relaxation is always greater than that for the lower frequency. Figure 3 also shows a similar trend in the relative contributions of the two processes to the absorption coefficient per wavelength. Although the results for this figure are for a ternary system, they are typical Discussion of the results obtained in both the binary and ternary systems. is proportional to the volume change associated with Since pmax The results of the ultrasonic relaxation studies of the binary the process, the high-frequency relaxation has been assigned to systems of BE-HzO show that, depending on the amount of BE the merging of the clathrate-like structures since this process is present, one or two relaxation processes are observed within the expected to have a larger volume change. The low-frequency experimental frequency range studied in this work. Binary systems relaxation process is assigned to the self-association of the alcohol containing BE in the composition range 0.00 I XBE I 0.016 show molecules. These assignments are in agreement with those made a single ultrasonic relaxation process. This is believed to be due by Tamura et al.I5 for water-tert-butyl alcohol (TBA) binary to the formation of clathrate-like structures observed in light mixtures. scattering studies of these systems and other alcohol-water One obvious difference between the results of the BE-water m i x t ~ r e s . At ~ ~compositions ~~ of XBE > 0.016 two relaxation studies reported here and those of TBA-waterlS is the magnitude processes are likely to be present although this is only readily of the single relaxation frequency obtained at low alcohol comapparent from the present work at compositions of XBE > 0.15. positions. Tamura et al. found the relaxation frequency for the The large sound absorption that occurs in the composition range formation of TBA-water clathrate-like structures to be ca. 68 0.016 < XBE < 0.15 does not permit an accurate analysis of the MHz whereas that for BE-water is found to be ca. 8 MHz. If data in terms of two processes. Nevertheless, on the basis of light the assignment is correct, the difference may be interpreted as scattering results, the sharp rise in & values beyond the minimum reflecting the greater relative stability of BE-water clathrate-like suggests that the clathrate-like structures of BE and water begin structures as compared to those of TBA-water. Whether the to break down and form other types of aggregates. presence of the additional oxygen atom in BE with its two potential Two relaxation processes have been observed in a number of hydrogen bonding sites is the critical factor contributing to this binary alcohol-water systems of relatively high alcohol compodifference is not clear. PCS dataz3do show that clathrate-like sition.I5*l6 They have been attributed to (1) the merging of structures in BE-water systems require a greater stoichiometric clathrate-like structures to form further alcohol-water aggregates ratio of water to alcohol than for the case of water-TBA. and (2) the association of alcohol molecules to form microagThe ternary system results at high concentrations can be ingregates of alcohol. The binary systems exhibiting two relaxation terpreted on the basis of the impact of the surfactant on the processes were analyzed according to the method outlined by structures believed present in the binary system. The possibility Tamura et al.I5 using the following reaction scheme: that the relaxation processes observed are those associated with kll KII the exchange of the surfactant species between the bulk phase BE(H20)i BE * C * C, (7) and the aggregates is ruled out on the basis that the alkyl chain k-I I length of C16TABshould be sufficiently long so that the relaxation k21 frequency associated with this process is at much lower frequencies, 2BE e(BE)2 (BE),, (8) k-21 Le., below the megahertz region.24 The single relaxation frequency reported previously6 for these where C i s the clathrate-like structure, I is the number of water ternary systems in the composition range 0 < XBE < 0.02 is molecules in the clathrate-like aggregate, m is the number of believed to be due to the exchange of the alcohol between the bulk merged clathrates, and n is the number of alcohol molecules in phase and the mixed micelle. The solubility data and the PCS the associated alcohol aggregate. Rate expressions were generated results appear to confirm this assignment. The solubilities of the as previously shown'5 by varying the concentration of the species dye, methyl red, in ternary systems having a mole ratio of BE/H20 in the above reaction scheme through the use of the overall = 0.016 are very similar to aqueous micellar solutions of C16TAB, equilibrium expressions and 02) Le., in the absence of alcohol. The slight decrease in the solubility of the dye in the ternary system relative to the binary aqueous micellar system is due to the presence of the alcohol at the surface of the mixed micelle competing with the dye for solubilization and sites. A similar explanation has been used to account for the effect of alcohols on the micellar-induced catalysis of Ni2+-PADA rea c t i o n ~ . ' The ~ PCS results indicate a swelling of the micelle in this composition region, possibly to accommodate the alcohol, and this observation supports the argument that mixed micelles are and also by varying the magnitude of I , m, and n. A least-squares present in the ternary systems at low BE concentrations. The method was applied to computer fit the relaxation frequencies to swelling of micelles also has been observed2s when additives such the stoichiometric concentrations of alcohol in the binary systems. as alcohols are added to micelles. The best fit values of I, m,and n (55,2, and 5, respectively) along Beyond XBE = 0.016 distinct changes occur in the ultrasonic, and /?I2 are given in Table VI. The with the values of kll, kzl,@,, solubility, and PCS properties of the ternary system. The most minimum error of fit was &8% for both relaxation frequencies. notable features are the occurrence of a second relaxation process, Consequently, the reaction scheme shown in eq 7 and 8 appears which is readily apparent beyond the composition range 0.016 < to be a possible description of dynamic processes occurring in XBEC 0.15, and the invariance of these relaxation frequencies binary systems at high alcohol compositions. This scheme is also with surfactant concentration over a broad intermediate compoconsistent with observations from light scattering s t ~ d i e s of * ~ ~ ~ sition ~ range of BE. The PCS data show that the particle radius BE-water mixtures. decreases to an approximate constant value and the solubility of the MR dye changes very little from that in the binary system.
TABLE VI: Values for the Best Fit Parameters for the Linear Regression of fr,and f, with the Stoichiometric Concentration of Alcohol for the Blnarv Systems BE-H,O composn of Bc(HzO),, C, and (BE), I = 5 5 , rn =2, n = 5 overall equilibrium constants 81 = 1.36,82 0.0043 k l l = 1.62 X lo8, forward rate constants, M-I s-l kzl = 1.7 x 105
+
2
(20) Ito, N.; Fujiyama, T.; Udagawa Y . Bull. Chem. SOC.Jpn. 1983,56, 319. (21) Iwasaki, k.;Fujiyama, T. J . Phys. Chem. 1977, 81, 1408. (22) Iwasaki, K.;Fujiyama, T. J . Phys. Chem. 1979, 83, 463. (23) Ito, N.; Saito, K.;Fujiyama, T. Bull. Chem. Soc. Jpn. 1981, 54, 991.
(24) Yiv, S.; a n a , R.; Ulbricht, W.; Hoffman, H. J. Colloid Interface Sci. 1981, 81, 224. (25) Almgren, M.; Swarup, S.J . Phys. Chem. 1982, 86, 4212.
4174
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
These results indicate that the formal mixed micelles present at the low BE compositions are broken down into smaller units or transferred into other structures. Reports that the mean aggregation number of micellar systems decrease markedly in the presence of moderate quantities of a l ~ o h o lwould ~ ~ , ~appear ~ to support this argument. However, the magnitudes of the two relaxation frequencies are very similar to those found in the binary systems, and this result a t first sight suggests that the dynamic processes being probed ultrasonically are similar to those observed in the binary system. Taken together, these results lead us to propose that the low-frequency relaxation is due to self-association of molecules, as argued in the case of the binary systems. On the other hand, the high relaxation frequency process reflects merging of alcohol-water aggregates; however, the stoichiometry of these structures is expected to be different from that of binary systems in the absence of Ci6TAB. Since CI6TABis virtually insoluble in BE, it appears that it must be solubilized in a relatively "water-rich" environment in these systems. At higher mole ratios of BE/HzO, both the solubility of the dye M R and the relaxation frequencies, f,,and f,*,show a dependence upon the concentration of C16TAB. PCS analysis of these systems did not show the presence of any measurable correlation functions. The solubility of the dye increases rapidly from the binary solution values with small additions of CI6TABbut levels off to a value near to that for the dye in pure BE. Furthermore, the relaxation frequencies are observed to decrease with increasing surfactant concentration to magnitudes observed at lower ratios of BE/H20, Le., the intermediate region where CI6TABis found to have no effect on the ultrasonic relaxation frequencies. The interpretation of the data in the absence of specific information concerning properties of the surfactant is difficult. A possible, though somewhat speculative, qualitative explanation of these results can be derived from the present results. It is based on the fact that solubilization of C16TABrequires a water-rich environment, and this may promote the formation of aggregates containing Ci6TAB at the expense of water-BE aggregates. Therefore, in addition to the mechanistic scheme (eq 7 and 8) postulated for the binary system, the following step occurs in the presence of C16TAB. yCi6TAB + xBE zBE(H2O)SS [(c16TAB),(BE),+,(H20)551 (9)
+
*
The increased presence of CI6TABwould cause eq 9 to shift to the right and necessitate concomitant backward shifts of both processes in eq 7 and 8. This would result in a decrease in the concentration of both the merged clathrate-like and alcohol aggregates and hence a decrease in the values offr, andf,,. This shift appears to stop when mole ratios of B E / H 2 0 < 0.4 are reached, presumably, where the dispersion of CI6TABin the binary (26) Almgren, M.; Lofroth, J.-E.J . Colloid Interface Sci. 1981,81,486. (27) Lianos, P.; Zana, R. Chem. Phys. h i t . 1980, 72, 171.
Kat0 et al. system is favorable and does not require any breakdown of the aggregates formed in eq 7 and 8. Using a regression formula, we found data for R = 0.43 and R = 0.75 to intersect that of R = 0.23 at CI6TABconcentrations of 1.37 and 1.16 m, respectively. The concentration of water at these concentrations is 14.5 and 8.38 m , respectively. Using the average mole ratio of H 2 0 / C16TAB 9.1, we estimated the smallest possible unit of which is C16TAB-BE-water to be [(C16TAB)6(BE),+1(H20)55] not inconsistent with low aggregation numbers found for surfactants in alcohol-water mixtures of relatively high alcohol c ~ n c e n t r a t i o n s .It~ is ~ ~difficult ~ ~ to estimate the number of alcohols that are present in such aggregates. Confirmation of these types of structures requires more information about aggregation numbers, particle sizes, and possible relaxation processes involving the surfactant. Steady-state fluorescence and time-resolved fluorescence anisotropy studies are currently in progress to provide insight into both the chemical and physical characteristics of these microphases. Conclusion The results of studies presented in this paper indicate that the type of aggregate species formed in both binary water-BE and ternary water-BE-CI6TAB solutions are strongly dependent upon the water to alcohol mole ratios. Below XBE = 0.016 clathrate-like aggregates are postulated in the binary water-BE systems while mixed micelles of C16TAB and BE are believed to exist in the ternary systems. When the mole ratio,of BE to water exceeds XBE = 0.016, both the binary and ternary systems undergo changes leading to different types of aggregates. In the binary systems merged clathrate structures and associated alcohol structures are suggested as the predominant aggregate species. These same aggregate species are believed to be present in the ternary systems at intermediate mole ratios of BE to water, Le., 0.016 < R < 0.43. The surfactant appears to be dispersed at these compositions. At mole ratios of R greater than 0.43 the surfactant role becomes one of competing for water with the alcohol-water aggregates. This leads to a decrease in the concentration of both the alcohol aggregates and the alcohol-water aggregates and hence a decrease in the ultrasonic relaxation frequencies of dynamic processes involving these species. The impact of added CI6TABappears to be limited to causing a shift in the concentration of aggregates to a condition realized in ternary systems where R < 0.43.
Acknowledgment. Financial support from the Alberta Oil Sands Technology and Research Authority and NSERC is acknowledged. S.K. thanks Nagoya University for a leave of absence to perform this work and Yoshida Foundation of Science and Technology for financial support in the form of a travel grant. Registry No. C,,TAB, 57-09-0; BE, 111-76-2.
Supplementary Material Available: Tables of ultrasonic data for various systems (7 pages). Ordering information is given on any current masthead page.
