Langmuir 1993,9, 3422-3426
3422
Thermodynamic Properties of Micellization and Adsorption and Electrochemical Studies of Hexadecylpyridinium Bromide in Binary Mixtures of 1,a-Ethanediol with Water Allison Callaghan, Ronald Doyle, Edward Alexander, and R. Palepu’ Department of Chemistry, University College of Cape Breton, P.O. Box 5300, Sydney, Nova Scotia, Canada B l P 6L2 Received July 29, 1993. I n Final Form: September 1 5 , 1 9 9 P
Micellar and surfacepropertiesof hexadecylpyridinium bromide in variouswater/lJ2-ethanediolmixtures were determined using membrane electrodes selective to the surfactant ion and surface tension and conductivity measurements at different temperatures. The critical micellar concentrations (CMC) and surfaceproperties, such as surface excess concentrations,minimum area per molecule, and surfacepressure at the CMC, as well as the thermodynamic quantities of adsorption and micellization were calculated. The effective dissociation of the micelles (a)was evaluated by three different methods. From the dependency of the thermodynamic properties of micellization and adsorption on the concentration of l,a-ethanediol, it can be concluded that micelles do form in these systems. The aggregation number and the size of the micelles decrease in a 1,2-ethanediol-richmixture with water. Aggregation of several amphiphiles in a nonaqueous polar solvent has received considerable attention recently in the 1iterature.l-l7 Micelle or liquid crystal formation has been reported in solvents such as ethylene g l y ~ e r o l ,formamide,+l4 ~?~ and hydrazine.15J6 All these solvents have high cohesive energies and dielectric constantsand considerable hydrogen-bonding ability. Evans et al.” have suggested that the hydrogen-bonding ability of a solvent is a prerequisite for micellization to occur. However, it has been established from the study of formation of lamellar phases of lecithin in a variety of polar solvents1a20 and even in ethylammonium nitrate, a fused salt,2l that the unique structural feature of liquid water (H-bonding) is not a prerequisite for micelles or surfactant aggregates to form. Also, micellizationof several * To whom correspondence should be addressed. FAX: (902)562-0119. Telephone: (902)539-5300. E-mail:
[email protected]. @Abstractpublished in Advance ACS Abstracts, November 1, ~~~
~~
1993.
.-,
---.
(1)-__*, Rav. . A. Nature 1971.231. 313 .. -.. . -. ---I
(2)Ray, A. J. Am. Chem. SOC.1969,91,6511. (3)Friberg, S.E.; Liang, Y. C. Colloid Surf. 1987,24,325. (4)Evans, D.F.; Yamauchi, A.; Wei, G. J.; Bloomfield, V. A. J. Phys. Chem. 1983,87,3537. (5)Sjaberg,M.; Henrikeson,U.; W h h e i m , T. Langmuir 1990,6,1205. (6)Lattes. A.: Rico. I. Colloid Surf. 1989. 35. 221. (7) Das, K.P’;Cegfie, A.; Monduzzi, M.; SMerman, 0.;Lindman, B. Prog. Colloid Polym. Sci. 1987,73,167. (8) Rico, I.; Lattes, A. J. Phys. Chem. 1986,90,5870. (9)Belmajdoub, A,; Marchal, A. P.; Canet, D.;Rico, I.; Lattes, ANouu. J . Chim. 1987,11, 415. (10)Auvray, X.;Anthore, R.; Petipas, C.; Rico, I.; Lattes, A. C. R. Acad. Sci.. Ser. 2 1988.306. 695. (11)Aukray, X.;Petipas,‘C.;Anthore, R.; Rico, I.; Lattes, A.; AhmahZedeh, Samii.; de Savignac, A. Colloid Polym. Sci. 1987,265, 925. (12)Binana-Limbele, W.; h a , R. Colloid Polym. Sci. 1989,267,440. (13)Almgren, M.; Swarup, S.; LofrGth, J. J . Phys. Chem. 1985,89, 4621. (14)Gopal, R.; Singh, J. R. J. Phys. Chem. 1973,77,554. (15)Ramadan, M.; Evans, D.F.; Lumry, R. J . Phys. Chem. 1983,87, 4538. (16)Ramadan,M.;Evans, D.F.; Lumry,R.; Philion, S.J.Phys. Chem. 1985,89,3405. (17)Beesley, A.; Evans, D.F.; Laughlin, R. G. J . Phys. Chem. 1988, 92.791. (18)El Nokaly, M.; Ford, L. D.;Friberg, S. E.; Larsen, D.W. J. Colloid Interface Sci. 1981,84,228. (19)Larsen, D.W.; Friberg, S.E.; Christenson, H. J. Am. Chem. SOC. 1980,102,6565. (20)Bergenem, B.; Stenius, P. J . Phys. Chem. 1987,91,5944. (21)Evans, D.F.;Kaler, E. W.; Benton, W. J. J.Phys. Chem. 1987, 87,533.
--.
amphiphiles was reported141~~1~~ in pure solvents such as acetonitrile, dimethyl sulfoxide, and acetone where hydrogen-bonding ability is either nonexistent or very minimal. In an attempt to compare the mechanism of the so-called solvophobic effect as opposed to the hydrophobic effect, we have recently reported the micelle formation and other micellar properties of several cationic surfactants in pure 1,2-ethanedioland its aqueous mixtures through electrochemical m e t h 0 d s . ~ ~ ? ~ 5 In the present study, the aggregation behavior of hexadecylpyridinium bromide ((CP)Br)in 1,2-ethanediol/ water mixtures was investigated using techniques such as conductivity, surface tension, and density measurements and electrochemicalmethods as a function of temperature. In the present investigation 1,2-ethanediol(EG) is regarded as a cosolvent rather than the usual ternary surfactant systems containing cosurfactants forming mixed micelles.
Experimental Section Hexadecylpyridiniumbromide was crystallizedtwice from hot dry acetone, and the crystals were washed with cold dry ether and dried under vacuum. Freshly openedbottles of 1,2-ethanediol (Aldrich 99.5%) were used. Water was nanopure. Surface tension measurements were performed on a Fisher surfacetensiometer, equipped with a 13-mm-diameterplatinumiridium DuNouy ring. The solutions were transferred slowly into a double-walledvessel, around which thermostated liquid was circulated to maintain constant temperature. Adsorption processes at the air-aqueous solution interface were generally completed in about 10 min, and the repetition rate of individual readings was 15min. The aboveprocedureimprovesthe accuracy on an individiual surface tension reading up to i0.2 “em-1. The surface tension values (7)were corrected as described in the manual.2s The surface tension of water was measured periodically to check that the technique was being properly carried out. Conductivity measurements were made in a thermostated jacketed beaker with a dip cell having a cell constant of 1.02 cm-1 and an automatic conductivity CDM 83 bridge operating at lo00 Hz. Specific conductivity values of each set containing 15-20 (22)Gopal, R.;Singh, J. R. Kolloid 2.2.Polym. 1970,239,699. (23)Gopal, R.; Singh, J. R. J. Indian Chem. SOC.1972,49,4. (24)Gharibi, H.;Palepu, R.; Bloor, D.M.; Hall,D.G.; Wyn-Jones, E. Langmuir 1992,8,782. (25)Palepu, R.; Gharibi, H.; Bloor, D.M.; Wyn-Jones, E. Langmuir 1993,9,110. (26)Instruction Manual (5-0506-17), Fisher Surface Tensiomat, Allies Fisher Scientific, p 9.
0743-746319312409-3422$04.00/0 0 1993 American Chemical Society
Micellar and Surface Properties of (CP)Br
Langmuir, Vol. 9, No. 12,1993 3423 Table 11. Surface Excess Concentration (r-), Minimum Area per Molecule ( A h ) ,and Surface Pressure at the cmc (rCmc) for Hexadecylpyridinium Bromide in Aqueous Mixtures of If-Ethanediol composition/ rmL.x '4mill X (wt % of EG) T/K 108/ (mol.m-*) l P / m z Le/(mN-m-1)
40
oooE
35
0
30
-3.5
-3.3
-3.1
-2.9
-2.7
10 C
m
45
45
40% E.G.
30% E.G.
20 39
30
35 36
30 -32
-3.0
-2.8
-2.6
-24
Log
-30
-2.9
-2.8
-2.7
-2.6
40
-2.5
c
Figure 1. Surface tension of (CP)Br in ethylene glycol + water mixtures. Table I. Micellar Properties of (CP)Br in 1,2-Ethanediol + Ha0 Mixtures at 298 K X 10' cmc X lOV(mol.dm-8) a EG in surface method method method themixture emf tension conductivity 1 2 3 0 0.67 0.68 0.68 0.29 0.26 0.27 10 0.79 0.76 0.29 20 1.30 1.01 1.20 0.28 0.27 0.30 30 1.30 1.50 0.31 40 2.50 2.09 2.30 0.29 0.33 0.32 50 3.31 3.70 0.32 60 8.80 8.40 8.50 0.32 0.34 0.32 80 32.00 0.34 0.36 0.42 1 w 100.00 w t % of
a
Reference 24.
different concentrations of the surfactant at a fixed solvent composition were measured at different temperatures. The density measurements at various temperatures were obtained using an Anton-Parr digital densimeter DMA 45 in the static mode. The instrument was calibrated with nanopure water and dry nitrogen. Electrochemicalmeasurements were obtained using a surfactant membrane electrode selectiveto CP+ions. The preparation of the membrane electrode was described in our earlier publications.n.28 The membrane consists of a specially conditioned poly(viny1 chloride) and a commercially available plasticizer. During the experimental measurements the electromotive force (emf)of the surfactant electrodewasmeasuredrelative to a double junction reference electrode (Orion904200)to obtain the values of the activity of surfactant ions in solution. In order to obtain information about counterion binding, simultaneous measurementa of the emf of the bromideelectrode (Orion94-3500) against the double junction (DJ)reference electrode were also carried out. All solutionsof surfactant were prepared in the mixed solvent system on a molal basis and converted to other concentration scales using the density data at different temperatures.
Results Surface Tension. Plots of the surface tension (7) at 298 K of solutions of (CP)Br in binary mixtures of EG + HzO versus the log of the bulk phase concentration of surfactant are shown in Figure 1. The critical micellar (27)Gharibi, H.;Palepu, R.; Tiddy, G. J. T.; Hall,D. G.; Wyn-Jones, E. J. Chem. SOC.,Chem. Commun. 1990,115. (28)Palepu, R.; Hall,D. G.; Wyn-Jones, E. J. Chem. SOC., Faraday Trans. 1990,86,1535.
298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313
3.22 3.02 2.83 2.65 3.07 2.89 2.71 2.53 2.86 2.72 2.58 2.44 2.72 2.54 2.37 2.20 2.55 2.38 2.22 2.06
51.6 55.0 58.7 62.6 54.0 57.5 61.4 65.6 58.1 61.1 64.5 68.0 61.0 62.3 70.0 75.4 65.1 69.7 74.8 80.6
39.5 38.3 37.0 36.1 34.8 33.7 32.4 31.4 33.3 32.8 31.4 30.6 30.2 28.6 27.5 26.9 27.2 26.0 24.6 23.7
concentration (cmc)was taken as the concentration at the point of intersection of two linear portions of the plots. Experimental cmc values are listed in Table I. The maximum surface excess concentration, F-, the minimum area per surfactant molecule, A- (m2),at the air/solvent interface, and the value of a at the cmc were obtained using the surface tension data and the following equations: 29
and
-- YO - Ycmc
(3) where u is the number of ions per surfactant molecule, R is the gas constant, N A is Avogardro's number, and yo is the surface tension of the pure solvent. It should be noted that, in eq 1,the concentration term C was used instead of activity. It is assumed that for dilute solutions (such is the case in the present study),the values of mean activity coefficients of (CP)Br are approximately equal to 1. The calculated values of rma,Amin, and ll a t the cmc are presented in Table 11. Conductivity. Plots of specific conductance ( K ) vs the concentration of surfactant indicate a break at critical micelle concentrations (Figure 2). The apparent degree of micelle ionization (a)was taken as the ratio of the slopes of the lines above and below the cmc in the plots of K vs C. In order to distinguish between a proper or cooperative micelle formation and a more gradual association (premicellar aggregation) before the cmc, it has been advised to plot differential equivalent cond~ctance:30~~~ rcmc
AA = lOOO[
where ~
-1
(4)
C1- Cl 1 ~l~ - is the increment in conductivity and C1- C1'
(29)Rosen, M. J. Surfactantsand~nterfaciol~henome~; John Wiley
& Sone: New York, 1978; Chapters 2 and 3. (30)Backlund, S.; Bergenstahl, B.; Molander, 0.; Warnheim, T. J.
Colloid Interface Sci. 1989,131,393. (31)Guveli, D.Colloids Surf. 1989,39,349.
3424 Langmuir, Vol. 9,No. 12, 1993
Callaghan et al.
100
_g_BBooooo
70
0 0
60
::j
*O0 0
30
5
O 0 O 00
0 0
5
10 15 20 25 30 55 40
0 205:
v 40% 0 SOX
A
io-'
''01
10-2
10-3
log
80%
I 10"
c
Figure 4. Emfplota of the CP+/DJreference electrode system for (CP)Br in ethylene glycol + HzO mixtures. 60% EG
0
25
50 75 100 125 150 175
0
100
ZOO
300
400
mol L - ~(10')
Figure 2. Specificconductance ( K ) vs concentration of (CP)Br in the aqueous mixtures of ethylene glycol. 202 407. 60X 80% 40
-70
20
10-4
so
0
15
loE 0
20
20X EG 10
0.0
0.5
1.0
1.5
2.0
2 .5
N
E
10-3
0.0
1.0
log
ooo
2.0
30
20.0
30.0
I
lo-'
10-2
c
Figure 5. Emf plots of the Br/DJ reference electrode system for (CP)Br in ethylene glycol + HzO mixtures.
12
0 - , 52
8
IO
80% EG
60% EG 0.0
5.0
10.0
15.0
C'/M
;
0.0
10.0
A
(10')
Figure 3. Differential equivalent conddctivity vs mean concentration. is the increment in concentration vs the mean concentration C' = (C1- C19/2. It is observed that for a normal micellization process AA decreases vertically over a narrow range of concentration (Figure 3). It is observed that when the EG content exceeds 30 % the AAdecreasesover a wider range of concentration,suggestingthat gradual association of CP+ ions is taking place before the micelles are formed. Electrochemical Studies. Plots of emf data as a function of log C of the surfactant at different compositions of the aqueous mixtures of EG are presented in Figure 4 and 5. At low concentrations of the surfactant, the emf is directly proportional to log C of the surfactant at 298 K and the slopes are in the range of 57-60 mV/decade (Nernstian slope). A t higher concentrations of the surfactant there is a definite break in the emf vs log C plots which is characteristic of the critical micellar concentration. In the case where the micellization process is not cooperative, the break point is not sharp and the slopes deviate from Nernstian behavior over a narrow range just before the cmc (Figures 4 and 5).
0.4
0.5
0.6
07
E-/Sz
Figure 6. E + / & vs EJSz for CPBr in a 40% EG + 60% Ha0 mixture.
Effective Dissociation. The degree of effective dissociation of the micelle (a)was determined by three different methods as a function of ethylene glycol concentration. In method one,= we made use of the emf measurements of the surfactant-selectiveelectrode (E+)and bromide ion electrode (E-) against a double function reference electrode. In the micellar region the following equation was used to evaluate the value of a:
E+IS, = constant - (1- a)EJS2
(5) where SIand S2 are the slopes of the respective plots of E+and E- against log C of the surfactant below the cmc. From the slope of the plot of E+/& vs EJS2 in the micellar region the value of a was calculated and presented in Table I as a function of the ethylene glycol concentration. A typical plot is presented in Figure 6.
-
Langmuir, Vol. 9, No. 12, 1993 3425
Micellar and Surface Properties of (CP)Br -O"O
Table 111. Thermodynamic Parameters of Micellization of Hexadecylpyridinium Bromide in Aqueous Solutions of If-Ethanediol at 298 K
composition1 (wt % of EG)
0.000 0.00
0.05
log [(cmc
0.10
0.15
0.20
+ C,)/cmc,]
Figure 7. log (cmc/cmcd vs log[(cmc + CJcmql for (CP)Br in a 40% EG 60% HzO mixture.
+
In method two, the cmc values of (CP)Brin the binary solvent mixtures were determined by the surface tension method at different electrode concentrations (NaBr).The values were analyzed using the equationz8 cmc = -(1- a)log cmc + C, log cmcO cmcO
(6)
where cmQ refers to the value in the absence of salt and 9is the salt concentration. When the left-hand side is plotted against log [(cmc + Cl)/cmcol, a straight line passing through the origin (Figure 7) with the slope value equal to -(1 - a) is obtained. The values of a are also presented in Table I. Finally, the method three, a values were obtained from the ratio of the slopes of the plots of specific conductance vs concentration as described earlier, and the values are included in Table I. Thermodynamic Parameters of Micellization and Adsorption. The Gibbs free energy of micellization for the surfactant was calculated using the equationB
0 10 20 30 40 50 60 70 80 100
-AGodJ
kJ
48.4 47.0 45.5 43.5 41.0 38.0 34.9 31.7 28.4 19.7
-AHodJ
ASodJ
kJ 12.9 13.3 16.0 23.5 27.0 30.2 32.8 34.1 35.8 46.5
(J-K-1)
119 113 99 67 47 26 7 -8 -25
-90
AGO& kJ
1.4 2.9 4.9 7.4 10.4 13.5 16.7 20.0 28.7
Table IV. Thermodynamic Properties of Adsorption of Hexadecylpyridinium Bromide in Aqueous Solutions of If-Ethanediol at 298 K
composition1
(wt % of EG)
0 10 20 30 40
AGO& kJ
-60.7 -58.3 -57.1 -54.6 -51.9
AHO.d
kJ
1.9 -1.7 -9.4 -12.9 -13.2
As0& (JeK-') 210 190 160 140 130
AHo,& were obtained from the dependency of AGOa&on temperature. All these parameters at appropriate temperatures are given in Table IV.
Discussion The values of the critical micellar concentration and effective degree of micellar dissociation values determined by three different techniques are in good agreement (Table I). In the present study, ethylene glycol is acting as a cosolvent and as a structure-breaking solute. In micellar solutions, structure-breaking solutes lower the hydrophobic effect which is considered to be the driving force for AGOmic = (2 - a)RT In a+,cmc (7) micellization. The presence of structure breakers in the aqueous phase may disrupt the organization of the water The above equation is valid for the ionic surfactants, and produced by the dissolved hydrophobic group, thereby in the present study ai,,,, was replaced by Xmc. The decreasing the entropy increase on micellization. Since temperature dependency of a was taken into account in the entropy increase favoring micellization is decreased, the calculations of AGO,, a t different temperatures. The a higher bulk concentration of surfactant is required for entropy and enthalpy of micellization (ASOmic and AHo,,,ic) and free energy of transfer (AGokans) were ~alculated3~ micellar formation. Therefore, the cmc increases with ethylene glycol content. The increase in the cmc values employing the following equations: with EG content can also be explained on the basis of a decrease in the cohesive energy density or solubility parameter of water with the addition of EG, thereby increasing the solubility of a hydrocarbon chain of AHomic = AGomic + TASomic surfactant monomers. Adecrease in the dielectric constant of the aqueous phase would cause an increase in repulsions between the ionic head groups,thus opposingmicellization. All these effects lead to the increase in cmc values with ethylene glycol concentration in the solvent mixture. Also, The values thus obtained a t appropriate temperatures are with the increase in ethylene glycol content, the formation listed in Table 111. of small aggregates (or premicellar aggregates) before the The thermodynamic parameters of adsorption33~~ were cmc is evident from the plots of E+vs log cmc (Figure 4), obtained using the equation where the slopes just below the cmc exhibit nonideal (11) A G o a b = AGo,ic - rcmc/r,, behavior. This behavior is further substantiated in the plots of AA vs mean concentrations (Figure 3). Extensive where AGoads is the standard free energy of adsorption. gradual association of the monomers before the actual The standard state in the surface phase is defihed as the micelle formation is well established for various cationic surface covered with a monolayer and surface-active agent surfactants in pure ethylene glycol from electrochemical a t a surface pressure of zero. The values of AGO,& and studiesVz4 (32) Misra, P. K.; Mishra, B. K.; Behera, G. B. Colloids Surf. 1991,57, The free energy of micellization is negative and becomes 1. less negative as the EG content in the mixed solvent system (33) Roeen, M. J.; Dahamayake, M.;Cohen, A. W. Colloids Surf. 1982, increases (Table 111), indicating that the formation of 5,159. (34) Sesta, B.; La Mesa, C. Colloid Polym. Sci. 1989,267,148. micelles becomes 1ess.spontaneousat higher EG contents.
3426 Langmuir, Vol. 9, No. 12, 1993 The equation used in the calculation of AGO,,, stictly applies when the mean aggregation number is large, and it is not likely to be the case at higher EG ~ o n t e n t sAlso .~ the valuesof enthalpy, if directly measured, will not depend on the choice of ideal dilute solution standard state (whether based on mole fraction, molality, or molarity), whereas the entropy change depends on the choice of standard state. Therefore, the method employed in evaluating AHOmie and ASo,ic should be viewed only as approximate. However, some generalities can be postulated when analyzing the present data. The values of AHomic become increasingly negative, where as the values of ASOmic change from positive to negative with increasing EG content, indicating that AHomic becomes more dominant in 1,2-ethanediol-rich mixtures with water. The positive values of AGOt, are due to the solvation of the hydrophobic part of the surfactant molecule by the hydrophobic part of the cosolvent, the other end being solvated by water. Thus, the ability of the cosolvent to undergo association is decreased. The values of rmsand rcmc decrease with ethylene glycol content, indicating that the hydrocarbon chain of the surfactant interacts with EG, thereby leading to a shifting of surfactant molecules from the interface to the bulk solution. The temperature effect is due to an increase in molecular motion, resulting in a poorer packing at the liquid/air interface. The free energy of adsorption (AGoads) represents the free energy of transfer of 1mol of surfactant in solution to the surface at unit pressure. The values of AGO,& are more negative than AGomic values at all EG weight fractions, suggesting that when micelles are formed, work has to be done to transfer the surfactant molecules from the monomericform at the surface to the micellar state through the mixed solvent media. The higher values of ASoads indicate that greater freedom of motion of hydrocarbon chains occurs at the planar air/aqueous solution interface than in the relatively cramped interior of the micelle. The values of AGO,& become less negative with the addition of EG to the surfactant solution below the cmc, suggesting that the adsorption of surfactant molecules at the interface becomes less favorable. This is possibly due to a change in the molecular orientation of the surfactant near the surface involving flattening of the hydrophobic part of the surfactant.
Callaghan et al.
Finally, the values of the cmc and the size of the ionic spherical micelles are determined by the balance between the solvophobic effect, Le., the difference in free energy for a hydrocarbon chain in oil, and in the solvent in question and the electrostatic effect. If we assume that the interfacial tension at a surface aggregate in a solvent is related to the solvent/oil interface tension, a lower interfacial tension value of the solvent/oil interface means a more favorable exposition of the surfactant alkyl chain to the polar solvent and a larger area per polar group. The interfacial tension of dodecane with water decreases from a value of 50 to 18 mN-m-l with pure ethylene glycol.30 It is therefore more favorable for the hydrocarbon chain of the amphiphile to be in contact with ethylene glycol than with water as reflected by the interfacial tension. The solvophobic interactions will increase when the water content of the solvent mixture is increased, leading to the formation of micelles at a lower concentration with an increase in micellar size. Recently Sjdberg et al.5 established from 2Hspin-lattice and spin-spin nuclear magnetic relaxation rate studies in the micellar solutions of EG + HzO of a-deuterated CTAB that the micelles are indeed formed and the aggregation number and micellar radii decrease with an increase in EG content. Following on the same lines, we conclude from the dependency of the values of the cmc, a,rms,A&, rmC, and the thermodynamic parameters of micellization and adsorption on the ethylene glycol content that the aggregation number and the size decrease in the present study with an increase in EG concentration. The increase in the effective degree of dissociation at higher ethylene glycol concentrations can be ascribed to a decrease of the charge density at the micellar surface due to the decrease in the aggregation number and the size of the micelles.
Acknowledgment. Financial support by the Natural Sciences and Engineering Research Council of Canada and the University College of Cape Breton in the form of operating grants is greatly appreciated. We would like to acknowledge the partial funding in the form of salary supplement from the Nova Scotia Department of Economic Development (A.C. and R.D.) and Enterprise Cape Breton (E.A.).
