Langmuir 1992,8, 702-707
782
Articles Electrochemical Studies Associated with Micellization of Cationic Surfactants in Ethylene Glycol H. Gharibi, R. Palepu, D. M. Bloor, D. G. Hall, and E. Wyn-Jones* Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4 WT, U.K. Received August 14, 1991.I n Final Form: November 12,1991 Membrane electrodes selective to a series of alkyltrimethylammonium bromides and also alkylpyridinium bromides have been constructed and used to investigate the aggregation properties of the surfactants in ethyleneglycol. During these electrochemicalmeasurementsthe emf of the surfactant selective electrode was measured relative to a sodium electrode and also a bromide ion selective electrode. As a result of these measurements it is possible in principle to evaluate the monomer surfactant and counterion concentrations. The data for all the surfactants reveal nonideality which is interpreted as a critical micellar concentrationwhose value increasesas the chain length of the surfactant decreases. At concentrations immediately below this critical micellar concentration nonideality is interpreted as the existence of premicellar aggregation. The data are analyzed to yield the effective degree of dissociation of the micelles and also equilibrium and thermodynamic information concerning aggregation in the surfactants.
Introduction Surfactants are widely used in both industry and everyday life and the properties of aqueous solutions of surfactants have received considerable attention.' As a result of extensive experimental studies over a long period of time and covering a wide variety of different experimental techniques, the relationship between surfactant chemical structure and both micellar and lyotropic liquid crystalline structures is fairly well understood for aqueous systems, and their practical applications, which require a knowledgeand control of their aqueous solution properties, have enhanced cor~siderably.'-~In many applications of surfactant the fluid medium is a polar nonaqueous solvent rather than water and at present these systems are not as well understood as those in aqueous solutions. Studies on nonaqueous polar solvents are extremely important in the sense that they underpin current and anticipated development in the technological use of these system^.^^^ In addition at the molecular level a range of nonaqueous polar solvents can be used which gives the experimentalist the opportunity to gain information on the molecular mechanisms of the so-called solvophobic effect. Although surfactant association structures in nonaqueous polar solvents have been known for some time, there are still many controversial issues which remain unsolved. There is currently much interest in the solution properties of surfactants in these nonaqueous polar solvents. In particular there are certain controversies associated with the experimental data on these systems especially related to estimating critical micellar concentrations.'-20 One of the (1) Attwood, D.; Florence, A. T. Surfactant Systems; Chapman and Hall: London, 1983. (2)Wyn-Jones, E.; Gormally, J. Aggregation Processes in Solution; Elsevier: Amsterdam; 1983;Chapters 2,3, and 7. (3)Tiddy, G. J. T. Modern Trends of Colloid Science in Chemistry and Biology, Berkhauer Verlag, 1985;Chapter 7. (4)Bloor, D. M.; Wyn-Jones, E. The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Kluwer: Dordrecht, 1990. (5)Friberg, S. E.; Liang, Y. C. Microemulsions;CRC Press: Cleveland, OH, 1988 Chapter 3. (6)Evans, D. E. Langmuir 1988,4,3.
key issues in this area is the design of experimental methods which can lead to unambiguous conclusions. In connection with the understanding of aggregation behavior, one of the fundamental measurements which can lead to a better understanding of these systems is the measurement of surfactant monomer concentration as a function of total concentration. In previous studies of the aggregation behavior of aqueous solutions of a series of trimethylammonium bromides21 and also alkylpyridinium bromides,21p22we constructed surfactant-selective electrodes in order to measure monomer concentration as a function of total surfactant concentration. In all cases these electrodes displayed almost ideal Nernstian behavior and could be used in a routine fashion to determine the concentration of monomer surfactants in these systems. As a result of the interest shown in the aggregation behavior of these surfactants in nonaqueous polar solvents and in particular ethylene glycol, we decided to investigate whether or not these electrodes could operate in this (7)Backlund, S.;Bergenstah, B.; Molander, 0.;Warnheim, T. Colloid Interface Sci. 1989,131,393. (8)Biname-Linbe, W.;Zana, R. Colloid Polym. Sci. 1989,267, 440. (9)Ray, A. Nature 1971,231,313. (10)Ray, A. J. J. Am. Chem. SOC.1969,91,6511. (11)Singh, H. N.;Salem, S.M.; Singh, R. P.; Birdi, K. S. J. Phys. Chem. 1980,84,2191. (12)Das, K. P.; Ceglie, A.; Lindmann, B. J. Phys. Chem. 1987,91, 2938. (13)Jopal, R.; Singh, J. R. Kolloid 2.Z. Polym. 1970,239,699. (14)Belmydonts, A.; El Bayed, K.; Brondeau, J.; Canet, D.; Rico, U.; Lattes, A. J . Phys. Chem. 1988,92,3569. (15)Lattes, A.; Rico, I. Colloid Surf. 1989,35, 221. Lofroth, J. E. J.Phys. Chem. 1985,89, (16)Almgren, M.; Swamp, S.; 4621. (17)Alfass, 2.B.; Filby, W. G. Chem. Phys. Lett. 1988,144,83. (18)Sjoberg, M.; Henriksoon, U.; Warnheim, T. Langmuir 1990,6, 1205. (19)Gharibi, H.; Palepu, R.; Tiddy, G. J. T.; Hall, D. G.; Wyn-Jones, E.J. Chem. Soc., Chem. Commun. 1990,115. (20)Jonstromer, M.; Sjoberg, M.; Warnheim, T. J.Phys. Chem. 1990, 94,7549. (21)Gharibi, H.; Takisawa, N.; Brown, P.; Thomason, M. A,; Painter, D. M.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Chem. SOC.,Faraday Trans 1991,87,707. (22)Palepu, R.; Hall, D. G.; Wyn-Jones, E. J . Chem. Soc., Faraday Trans. 1990,86, 1535.
0743-7463/92/240~-0782$03.00/0 0 1992 American Chemical Society
Micellization of Cationic Surfactants "
-?M
:aa
Langmuir, Vol. 8, No. 3, 1992 703
T
t p/ 104
10.)
10.1
IO.'
100
T o 3 CmernV~ion
Figure 1. Typical emf data for the ClePyBr electrode as a function of surfactant concentration in IO4 sodium bromide solution at 25 "C. The emf was measured relative to a sodium
ion commercial electrode.
solvent. As we have pointed out in a preliminary communicationlg the electrodes performed efficiently and could be used equally efficiently in ethylene glycol as they were in water. The purpose of this communication is to report electrochemical data on the micellization of a series of alkyltrimethylammonium bromides and also alkylpyridinium bromide in ethylene glycol.
Experimental Section The surfactant electrodes selective to a series of alkyltrimethylammonium bromides and also pyridinium bromides used in this work were originally constructed to investigabthe aqueous solution behavior of the surfactants. The procedures used to construct these electrodesare now well d o c ~ m e n t e d . ~In~the -~~ present work, emf measurementson all the surfactant solutions were studied in ethylene glycol containing a constant amount of sodium bromide (lo4 mol dm-9. The emf measurements of the surfactant-selectiveelectrode were measured relative to a commercial sodium ion electrode (Kent, EIL). During these experiments we also carried out simultaneous emf measurements of the surfactant-selective electrode relative to the bromide ion selective electrode (Kent,EIL)to obtain information about counterion binding. In addition to the above measurements we also carried out further measurements of solutions of cetyltrimethylammonium bromide in ethylene glycol in differentbut constant amounts of sodium bromide ranging from lo-* to 2 x 10-l mol dm-3. The surfactants used in this work were decyl-, dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromides and also dodecyl-, tetradecyl-,hexadecyl-, and octadecylpyridinium bromides. The alkyltrimethylammonium bromides were purified commercial samples and the pyridinium bromides were synthesized by treating the corresponding l-bromoalkane with dried pyridine and extracting with n-hexane. The crude product was then recrystallized several times from dry acetone. The critical micelle concentration (cmc) of the material in aqueous solutions agreed well with the literature values. The ethylene glycol used was a purified commercial sample (Aldrich Chemical Co., Ltd)which was also carefully dried before use. The cmc's of decyltrimethylammonium bromide, tetradecyltrimethylammonium bromide, and also hexadecylpyridinium bromides were measured in the temperature range 15-30 "C. Results A typical plot of the emf data as a function of surfactant concentration is shown in Figure 1. In the presence of mol dm-3 sodium bromide the general behavior of all the emf plots is very similar showing three characteristic regions. A t low concentrations of the surfactant, the emf is directly proportional to log [surfactant concentration], (23) Wan-Bahdi, W. A.; Palepu, R.; Bloor, D. M.; Hall, D. G.;WynJones, E.J . Phys. Chem. 1991,95, 6642.
and with the exception of the measurements on &TAB/ ethylene glycol at 298 K, the Nernstian characteristic of the surfactant-selective electrodes used in this work was within the acceptable range in the sense that the numbers obtained from the slopes of the emftlog concentration plots were in the range 55-62 mvldecade which is consistent with the expected behaviour of such electrodes. The slope of the emf/log concentration plot for C12TAB is 52 mV per decade-although this is a rather low figure the results were repeated several times, checked for reversibility by increasing and also decreasing the surfactant concentration, and in all cases found to be consistent. A t the higher concentrations of surfactants there is a definite break in the emftconcentration plots. This break is distinctive and is taken as the point where the slope of the emf/surfactant concentration plots changes sign. This type of behavior is typical of micelle formation, and in the present work, as in previous reports of surfactants in aqueous solution,' the cmc is not sharp, rather it occurs over a small range of concentrations around the point at which the emf/log concentration plots change sign. In practice the concentration at which this break occurs can be estimated by plotting the monomer concentration against the total surfactant concentration rather than the log of surfactant concentration.lg In between the concentration region in which the electrode displays Nernstian behavior and the cmc, and in particular at concentrations immediately below the cmc, the emf data deviate from those predicted by the Nernst equation, essentially representing nonideal behavior. The degree of this nonideal behavior as measured by the extent of deviation of the measured emf from the Nernst equation increases as the chain length of the surfactant is decreased. In addition the concentration range over which this nonideal behavior occurs also increases as the chain length of the surfactant is increased. However in the measurements on cetylpyridinium bromide in the presence of constant but different amounts of salt up to 0.1 mol dm-3,this nonideal behavior is suppressed as more salt is added. On this basis we attribute the origin of the nonideal behavior to premicellar aggregation. It could be argued that this might occur as a result of some impurities (especiallywater) in the ethylene glycol solvent. Although careful attempts were made to ensure that all the samples were dry, this cannot be guaranteed. However in the case of cetylpyridinium bromide, we tested this possibility by doping the ethylene glycol solutions with a small amount of water and found that any deviations from the actual measurements require a substantial amount of water which would be far more than any left in the solvent after careful drying. We therefore conclude that the premicellar aggregation is a characteristic property of the surfactants in these solutions. The data are summarized in Table I and Table I1 which list the cmc and electrode characteristics of the electrodes used in this work. Another noteworthy feature which was observed in the data on cetylpyridinium bromide is that addition of salt not only suppressed the premicellar aggregation region but also tends to flatten out the variation of the monomer surfactant concentration with overall surfactant concentration in the micellar region. This is typical of the behavior of ionic surfactants and makes the cmc of the surfactant easier to estimate.
Analysis of the Emf Data in the Premicellar Region In the premicellar region the experimental information available from the emf data is the variation of surfactant monomer concentration with the total surfactant concentration. In this premicellar region we will assume that
784 Langmuir, Vol. 8, No. 3, 1992
Gharibi et al. mol dm-3NaBr for the
Table I. Electrode Characteristics, Degree of Micellar Dissociation, and Cmc Values in Surfactants Studied in This Work electrode characteristics
cmc, mol dm-3 ethylene glycol
surfactant
temp, K
E,, mV
slope, mV decade-'
water
CinTAB -"
298 288 298 303 288 298 298 298 298 298 303 308 308
146
55 55 51 60 57 59 58 57 58 60 61 62 62
0.066 0.015 0.014 0.015 0.0027 0.0036 8.16 x 10-4 0.012 0.00285 6.7 X 10"' 7.0 X 10"' 7.6 X 10"'
ClzTAB C14TAB C16TAB ClzPyBr CI~PYB~ Cd'YBr ClePyBr a
23 98 183 88 214 140 187 204 223 241 171
degree of micellar dissociation a water ethylene glycol*
0.44 0.25 0.32 0.35 0.13 0.17 0.10 0.65 0.18 0.10 0.12 0.15 0.10
0.3
0.82 0.73 0.89 0.97 0.72 0.60 0.42 0.43 0.56 0.60 0.31 0.53 0.23
0.2 0.1
0.2 0.2 0.3
The values of a using the iteration procedures via eqs 5 and 6 are identical to within two significant figures.
Table 11. Electrode Characteristics, Cmc Values, Activity Coefficients, and Degree of Micellar Dissociation (a)for ClgyBr in Ethylene Glycol at 298 K cOncn of NaBr, moldm-3 O.OOO1 o.OOO1 o.OOO1 0.01 0.05 0.10 0.20
electrode characteristics E,, slope, mV mV decade-' 203.7 59.8 -39.2 -100.5 -132.2 -180.4
59.8 59.4 59.5 56.4
cmc, moldm-3 0.10
rangeofy, 0.73-0.71
0.0975 0.0850 0.0770 0.0600
0.74-0.72 0.89-0.72 0.95-0.88 0.92-0.83
a
0.61 0.48O 0.41b 0.42 0.36 0.48 0.63
Table 111. Equilibrium Constants Kl and Kz Associated with Dimerization and Trimerization at Premicellar Concentrationsof Surfactant in lo-' mol dm-3NaBr surfactant temp, K K1, mol-' dm-3 Kz, mol-' dm-3 CloTAB 298 5 f l 88 f 15 C1zTAB 298 0.6 f 0.2 88 f 15 2 f 0.5 Cl4TAB 288 100 f 25 298 1f 0.2 200 f 20 C16TAB 298 1.5 f 0.5 900 f 200 0.2 f 0.10 ClzPyBr 298 112 f 20 0.7 f 0.4 ClrPyBr 298 161 f 30 C&Br 298 0.5 f 0.3 185 f 60 ClsPyBr 308 1 f 0.3 250 f 80
Using eq 8. Using eq 9.
the observed nonideal behavior is associated with the buildup of small aggregates A, from monomers A1 taking place via a bimolecular stepwise mechanism A,
+ A, 7~ A,,,
( n = 1,2, 3, ...)
or via the randon association method A,, + A,
=.A,+,
(n,m = 1 , 2 , 3 , ...)
(2) which postulates the formation of aggregates by the association of any two other aggregates. The next step to consider is whether the experimental data available, that is the variation of monomer concentration with total surfactant concentration, are consistent with either or both of the above models. If, in the first instance, we limit the aggregationprocess to monomers, dimers, and trimers only, both the above schemes are identical and involve only the two steps, namely the formation of dimers and the formation of trimers according to the scheme KI
A,+A,+A, Kz
A,
+ A, + A,
(3)
where K1 and K2 are the equilibrium constants associated with the two steps. C1 denotes the total surfactant concentration and [All is the monomer concentration. The following equations now apply KI = [A21/[A1I2,K2 = [Ad/ [Azl[A11 and C = A1 2[A2] + 3[A31. Thus for the formation of trimers, a plot of (C1 - [A1I)/[A1l2 against [Ad will be linear with a slope of 3K1K2and intercept of 2K1. If the process is limited to dimers only, then the quantity (C1 - [A1I/[A1l2 will be constant. If the concentration of (CI - [All/ [All2 does not obey either of the
+
I
0
n
om.
001
ooii
002
00:s
001
ooli
om
OW
1A11
Figure 2. A plot of (C1- [A1])/[A1lZagainst A1 for the analysis of premicellar aggregation involving scheme 3 in Cl$yBr containing lo4 NaBr at 25 OC.
above predictions, then we recognize that further analysis is necessary and the existence of higher aggregates such as tetramers, etc., will be required. On the other hand if the experimental data are consistent with the above schemes limited to dimers/or trimers, only there is no need for further analysis to consider higher aggregates. In the present work all the emf data on the surfactants in the premicellar region were consistent with the formation dimers and trimers only. In the present analysis no data were taken over the narrow concentration range over which the cmc occurs. Summaries of the equilibrium constants K1 and Kz,together with their statistical errors, are given in Table I11 with a typical plot shown in Figure 2. At this stage it must be emphasised that the purpose of the above exercise is not really to claim that this is a method to evaluate equilibrium constants associated with the aggregation scheme in the premicellar region (although we are not aware at present of any other method which gives similar direct information on the aggregation scheme) but rather to see if the numbers that emerge from such an analysis are reasonable. In this context we have used the
Langmuir, Vol. 8, No. 3, 1992 785
Micellization of Cationic Surfactants derived equilibrium constants to evaluate the typical relative concentrations of monomers dimers and trimers and verified that [A11 >> [A21 [A31. One of the interesting observations that emerge from these considerations of premicellar aggregation is that the above model involving monomers, dimers, and trimers works over the majority of the premicellar range but always breaks down if data are analyzed in the narrow range of surfactant concentration immediately below the cmc. At these concentrations, the deviation in the measured emf is larger than expected from monomers, dimers, and trimers. This confirms what is already well documented in the literature, that is that the cmc occurs over a narrow concentration range rather than being a sharp change. Presumably at these concentrations immediately below the cmc, higher aggregates than trimers are formed before micelles proper occur.
.I I7
N
Degree of Micellar Dissociation a In order to make use of the data listed in Table I so that information concerning the thermodynamics of the micellization of these cationic surfactants in ethylene glycol can be determined, it is necessary to evaluate the degree of micellar dissociation, a. Recently we have shown22that this micellar parameter can be evaluated from emf measurements such as those described in this work. Once micellization takes place, the surfactant counterions will start binding to the micelles and it is possible in principle to evaluate the "free" counterion concentration in the intermicellar region from measurements of the emf from the surfactant-selective electrode relative to the bromide electrode. The emf of this cell is given by the following equation
(4) where A 1is the surfactant monomer concentration evaluated from the emf measurements of the surfactant electrode relative to the sodium ion, m2 the free counterion concentration, and y+ the mean activity coefficient. In an earlier communication we described an iterative procedure to determine m2 and y+using the above equation and the A1 values found from the surfactant electrode.22 Unfortunately this method failed in the present work. The exact reason for this failure is not known; all we know is that the method always seems to fail when an appreciable amount of premicellar aggregation occurs. This is unfortunate, especially since the emf data of the surfactant electrode relative to the bromide electrode have been measured for all the surfactants studied in this work and in principle these data contain information about m2 and Y+. As a result of this failure to evaluate m2 and y+ directly from the electrode data, we have attempted an alternative approach to detemine whether a values can be derived from the present measurements. In this approach we refer to the following two equations? log Alya = K - (1- a)log m2y,
(5)
and m2 = A,
+ C3 + a(C1- m,)
(6)
In eqs 5 and 6 K is a constant and Ca is the concentration of addedsalt. In theory if A,, m2, and yt could be evaluated for each total surfactant concentration C1 in the intermicellar range, then analysis of eq 5 in graphical form is
pr
\
a
1
'('
.I 44
140
I
1 0 1
I5
I?
It5
1,
in
I
8 91
49
IWmlh)
Figure 3. A plot of log (Alyd against log (m2ya) in the analysis of eq 5 for the degree of micellar dissociation a for ClayBr in sodium bromide a t 25 "C. a plot of log (Aly+) against log (m2y+) which sould be a straight line with a slope -(1 - a). Similarly analysis of
eq 6 is a plot of (m2 - AI) against C1- AI, which should be a line straight with a slope of a and intercept of C3 (which is known). In the following exercise we have used an iterative computer procedure in an attempt to evaluate y+, m2, and a well in the micellar range. We have neglected the data over a small range of concentrations immediately above the cmc mainly because it has been shown even in the most ideal conditions22that eq 6 only holds when measurements are carried out well in the micellar range. The procedure is as follows. First of all, a value of y+ is guessed and its ionic strength dependence assumed to be consistent with the Debye-Huckel equation of the form
log (Ti)= -AI1I2 + BI (7) where A and B are adjustable parameters and I is the ionic strength of the solution defined as I = '/2(m1+ C3 + m2). At every surfactant concentration above the cmc, A1 is known from the surfactant selective electrode data, and using our assumed value of y+ it is possible to calculate m2 from eq 4. These data are then used to analyze eqs 5 and 6 graphically so that a can be estimated. By adjusting our initial estimate for y i and also using 7 for the concentration dependence of yt, it is possible to iterate through this cycle until the a values that emerge from the plots using eqs 5 and 6 are reasonably close. Once this point is reached, there is a very narrow range of y values which will yield a values from eqs 5 and 6 which are acceptably close. Beyond this narrow range of y i values further changes in y+ make the two a values from the above procedures diverge. Further criteria that we have used in order to back up the a values quoted in Table I is to check on the linearities of the respective plots in the analysis of eqs 5 and 6 and also to ensure that the plot using eq 6 extrapolates to the value of added salt, Cs, which is known. Typical plots that we have experienced in the analysis of eqs 5 and 6 are at the point where the two a values are identical, as shown in Figures 3 and 4. Our results for cetylpyridinium bromide in different amounts of sodium bromide are listed in Table 11. In these different solutions all the a values should be the same. The next step is to check these a values using other independent methods. This we have done in two ways: a. For cetylpyridinium bromide we have evaluated the cmc values form the emf data at different salt concentrations. By using the equation22 cmc + C3 (8) log cmcO = -(1- a)log cmc,,
[=]
[
]
where the subscript 0 refers to zero salt concentration, it
Gharibi et al.
786 Langmuir, Vol. 8, No. 3, 1992 h
I
I
0 01
01:
0 I.
0 16
018
115
0:r
0::
0:
11
lil
11
115
E. E* I
,
,I,
Figure 4. A plot of m2 - A1 against C1- ml for the analysis of eq 6 for the determination of degree of micellar dissociation a NaBr at 25 O C . for C1ayBr in
Figure 6. Determinationof the degree of micellar dissociation of ClayBr in M sodium bromide using a double junction electrode and eq 9.
c
I
I
4 10 0
00s
"I
"I'
n: 1°C
111'
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