110
Langmuir 1993,9, 11&112
Electrochemical Studies Associated with the Micellization of Cationic Surfactants in Aqueous Mixtures of Ethylene Glycol and Glycerol R.Palepu, H.Gharibi, D.M.Bloor, and E.Wyn-Jones' Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT,U.K. Received April 15,1992. In Finial Form: September 30,1992 Membrane electrodes selective to the cationic surfactante cetylpyridiniumbromide, tetradecyltrimethylammoniumbromide, .andddecyltrimethylammoniumbromide have been used to studythe micellization of the surfactante in various water/glycerol and water/ethylene glycol mixtures. It has been found that the logarithm of the criticalmicellar concentrationof the surfactants is directlyproportionalto the solvent/ water ratio expressed in weight percent. The degree of micellar dissociation has also been evaluated for all the systems studied.
Introduction Experimentalstudieson the aqueoussolution properties of surfactante are used to underpin efforts to establish a correlation between the surfactant chemical structure and the different phase behavior of the system. As a result, the behavior of these systems has become better understood and the molecular features necessary to produce specific effecte are at least partially known for common materials. In recent years solutions of surfactante in nonaqueous polar solventahave also received considerable attention and a general understanding of the relative behavior of the same surfactant in different solventa is now emerging.lB2 For example, in terms of critical micellar concentration, degree of micellar dissociation, and aggregation numbers the behavior of a CIScationic surfactant in ethylene glycol or formamide resembles that of the CS homologue in water. These studies are also useful in comparing the mechanism of the so-called 'solvophobic" effect as opposed to the 'hydrophobic" effect. In an attempt to gain more information on these systems, we report here our electrochemicalstudies on the micellization of dodecyl- (DTAB) and tetradecylmethylammonium ("TAB)bromides in glyceroVwater mixtures and cetylpyridinium bromide (CPyBr) in ethylene glycol/water mixtures. In the present studies of micellization in mixed solventa, ethylene glycol and glycerol are regarded exclusively as cosolvents rather than the usual tertiary surfactant systemswhich contain cosurfactant or a solubilizate which essentially form mixed micelles. Experimental Section Surfactant membrane electrodes selective to the cationic surfactantswere constructed using the same procedures as used in earlier publications.2 The membrane comprises a specially conditioned poly(viny1 chloride) (PVC) and a commercially availableplaaticiser. The PVC used in the present work contains negatively charged groups which are neutralized by the cationic surfactant before use. All surfactantsolutions were doped with lo-'mol dm-3sodium bromide. In these circumstances the free concentrationof the sodium ions ia constant becausethe cationic surfactant and sodium ions carry the same charge. During the experiment the emf of the surfactant electrode was measured relative to a commercial sodium ion electrode (Corning476210) whose function is to act as a standard reference electrode. In order to obtain information about counterion binding, we also (1) Sjoberg, M.; H e n r i h n , U.; Warnheim, T.Longmuir 1990,6,1205, and referencea quoted therein. (2) Gharibi, H.;Palepu, R.;Bloor, D.M.;Hell,D. G.; Wyn-Jones, E. Longmuir 1992,8, 782, and referencen quoted therein.
0743-7463/93/24O9-0110$04.00/0
carried out simultaneous measurements of the emf of the surfactant electrode relative to a commercial bromide ion electrode (Corning solid state B E 30-36-00). The emf measurements of the surfactant selective electrode relative to the sodium electrode are ueed to evaluate monomer surfactant concentration. Since the surfactant and sodium ion are both univalent and positive, measurements using thia cell give the ratio (surfactant activity)/(sodium ion activity). The activity coefficients of the surfactant monomer and its co-ions are expected to be approximately equal, in which case the surfactant monomer concentration/sodiumion concentration is measured. Since [Na+]is constant, the emf, E,,, of this cell is given by E,, Eo +-2.303RT log (m,) 5
where Eo is a constant and ml is the monomer surfactant concentration. Typical experimental data taken from thia cell are shown in Figure 1 where E,, is plotted against log (total surfactant concentration) for different solvent mixtures. The emf of the surfactant ion relative to the bromide ion Es+pr-is given by
whereEoS+pris constant,7+is the mean ion activity coefficient, and m2 the concentration of free counterions. Mid plota of emf against log ( C I ~ / ~ Care ~W shown in Figure 2. In the plots C1 is the total surfactant concentration and CZ the total concentration of bromide ion (=C1 + mS), where ms is the concentration of added sodium bromide.
Results (a)CriticalMicellarConcentrationDetermination. An inspection of the emf plots in Figure 1shows that a t low concentrations of the surfactant Eml is directly proportional to log ml and the Nernstian slopes of the electrodes used in this work were within the acceptable range 6 7 4 mV/decade. At higher concentrations of surfactant there is a definite break in the emf/log concentration plots which is characteristic of a critical micellar concentration (cmc). The emf plots shown in Figure 2 again display typical Nemstian behavior below the cmc with a sharp break at the cmc. At concentrations above the cmc the electrode with the emf conmonitors the product (ytm11/2m21/2) tinuing to increase with a small but definite positive slope-this being a neceeearycondition for thermodynamic stability. T h e cmc's are listed in Table I. (b) Degree of Micellar Disrocriation. In order to evaluate the degree of micellar dissociation (a)the fmt Q 1993 American Chemical Society
Micellization of Cationic Surfactants
Langmuir, Vol. 9, No. 1, 1993 111 solutionisestimated (neglectingthe micelleconcentration) and a new estimate of y+ is found from the Debye-Huckel equation in the form (3)
1%-
1W-5Q.-
0, 106
"""":
"""':
'
10s
'
''1""':
1O4
"'"":
'
"""':
10.1
10.3
'
101
'"'Y
100
T d CPyBr Conocnrration (CI)[mol dm"1
Figure 1. Plot of emfvereus log (Cd for cetylpyridiniumbromide in ethylene glycol/water mixturea at 26 O C using Na+ reference electrode. Concentrationof ethylene glycol: (0 20%;(A)40% ; ( 0 )60%; (0)81%.
-
d
where A is a constant. Substitution of this y+ in (2) leade to a new estimate of m2. This cycle is then repeated until the values of m2 and y+ converge. This procedure is then carried out at all concentrationsin the micellar range and a is estimatedfrom the slope of the plot log (mly+)against log (m2y+)according to log (mly+) = K - (1- a)log (m2yi) (4) Unfortunately this method does not work succeeefully in the present work since very often m2 values greater than the total added salt are obtained and sometimes m2 and y+ fail to converge. In this work we have adopted a slightly different method using a trial and error approach to estimate y+ and m2. In these circumstances the value of A in the Debye-Huckel equation (3) is estimated for a given mixture,' and a value of y+ can be estimated. Once this is achieved m2 then follows from (2). The range of surfactant concentrations spanning the micellar region is small enough to make the reasonable assumption that y+ can be regarded as a linear function of concentration. By adopting an iterative trial and error approach using a computer program, it is then possible to evaluate a series of m2 and y+ values in the micellar range and then evaluate a from the graphical solution of (4). This procedure is carried out with a range of initial yi values. In many cases values of y+ lead to an m2 value greater than added total salt, therefore several constraints on the poesible range of y+ must be applied. Even if we allow for a very generous band of y+ values at different concentrations in the micellar region, the value of a from the graphical solution of eq 4 remains almost unchanged. Finally if we take into consideration the error limits on the slopes and intercepts of the emf data, the final value of the degree of micellar dieassociation is very little affected. The a values derived in this work are listed in Table I.
-/ lo"
IO4
10"
IO'
10.2
100
(CPCY) [mol dm-31
Figure 2. Plot of emfvereus log (CI'/~C21/2)for cetylpyridinium bromide in ethylene glycoVwater mixtures at 26 "C using B r
referenceelectrode. Concentrationof ethylene glycol: (0 ) 20 7%; (A)40%; ( 0 )60%; (0)81%. Table I. Critical Micellar Concentrations and Degree of lbficellar Disasrociation at 26 OC surfactant solvent ratio (wt %) cmc (mol dm-3) a CPyBr ethylene glycoVwater 10010 81/19 60140 a0160 20180 Ol100
CllTAB
0.0088 0.0026 0.0013 0.00067
0.6 0.26 0.30 0.23 0.30 0.30
glyceroVwater 70130 30170 10190 01100
&TAB
0.1 0.033
0.0136 0.00372 0.00366
0.00360
0.22 0.24 0.26 0.16
glycerollwater 70130 30170 10190 0/100
0.044 0.02 0.016 0.016
0.26 0.16 0.16 0.2
Discussion
An inspection of Figure 1shows that at concentrations on both sides of the cmc the emf data deviate from those predicted by the Nemst equation ementially representing nonideal behavior. At concentrationsimmediately below the cmc the nonideal behavior is associated with premicellar aggregation whose effect is more pronounced as the fraction of coeolvent is increased and also as the chain length of the surfactant is decreased. At concentrations above the cmc ml decreases with increasing surfactant concentration-again a characteristic behavior of ionic surfactants. The cmc of the surfactantsin the solvent "sfollow a logarithm dependence (Figure 3) according to the following equation:
step in this exercise is to evaluate m2 and yt. In aqueous solutions the starting point is to estimate ml close to the cmc in the intermicellar region.3 At the same surfactant concentration m2 is estimated from (2) by assuming y+ = 1. From this information the ionic strength (I) of the
log (cmc) = log (cmc),,, + KC (6) where C is the solvent/water ratio in w t 95 and K is a constant. It is interesting to note that Sjoberg et al.' reported that the relative aggregation numbers of Cle TAB in formamide/water and ethylene/glycol water mixtures varied in a s i m i i way. From the limited data in
(3) Palepu, R.; Hall, D.G.; WynJonm, E. J. Chem. Soc., Faraday Trona. 19)0,86,1636.
Prm: Oxford, 1988.
(4) Atkina,
P. W.Phyuical ChemLtry, 3rd ed.; Oxford U d v a n i ~
112 Langmuir, Vol. 9, No. 1, 1993
Palepu et al. Table 11. K Valuer (Eauation 6) for Cationic Surf.okntm ~~
surfactant
CloTAB' &TAB' ClrTAB'
&TABb
0 . q ,,_,,_..
Ci-zP~Bfi C i S W C i P W
,.-*.-
I..-
0.Wd 0
ethylene glycoVwater
alvceroVwater
0.8 1.4 1.7 2.0 1.7 1.8 2.2
0.7 0.8
a Thie work. B t h t 8 d from ref 2. We have not specified the unite of K since the C h expressed in % wt/wt and the cmc's are quoted in mol dm-3.
10
20
M
40
50
60
70
80
90
100
Codvent Concentration [WE&]
Figure 8. Plot of log (cmc) vereu concentration of coeolvent in water at 26 OC: ( 0 )ClaTAB in glyceroUwater; (0)CIdTAB in glyceroUwater; (A) CPyBr in ethylene glycol/water.
thiswork it is very difficult to commenton the significance of K. However if we refer to our earlier publication2 reportingthe cmc's of a series of alkyltrimethylammonium bromides and alkylpyridinium bromides in pure water and pure ethylene glycol, we can estimate K from these data for each surfactant as follows. In eq 5 the cmc in the pure coeolvent is given by logtcmc) = log(cmc),,,,
+ lOOK
(6)
The values of K at 25 OC are listed in Table 11. The most striking features concerning the values K are as follows: (a) In all systems K increases as the chain length of the surfactant increases. (b) For the CIZ-and ClrTABs the values of K for glycerovwater are lees than the correspondingvalues in ethylene/glycolwater "3. (c) For the two homologous cationic series the values of K are roughly similar for the same hydrophobic group. The significanceof these conclusionsin terms of molecular interactions must await further developments in theoretical treatments.
Finally the most striking features concerning the determination of the a values is that once the correctrange of activity coefficienta have been chosen,the r d t i n g a values that emerge from the graphical treatment are very constant. Another noteworthy feature is that once water is added to ethylene glycol, the a value of CPyBr is reasonably constant and close to ita value in water. At 25 OC both &TAB and C12TAB were insoluble in pure glycerol; however in glyceroUwater mixtures the a values are to all intenta and purposes fairly constant and c l w to the values found in aqueous solutions. Although the alkyltrimethylammoniumbromides are soluble in glycerol at 50-60 OC, it is very difficult and tedious to carry out accurate emfmeasurementaat these temperatures. These results imply that once water is added to the nonaqueous polar solvent, the local environment leading up to the binding of counterions is very similar to that found in pure water. In other words it is likely that both the micelle and counterion bind water molecules preferentially to the alternative solvent.
Acknowledgment. R. Palepu thanks the authorities of the University College of Cape Breton, Sydney, Canada, for granting sabbatical leave and H. Gharibi wish- to thank the Iranian government for a postgraduate studentahip.
