Langmuir 1993,9,43&443
Solubilization and Aggregation Numbers in Micellar Mixtures of Anionic and Cationic Surfactants with Tetraethylene Glycol and Tetraethylene Glycol Dimethyl Ether D. Gerrard Marangoni,t Andrew P. Rodenhiser, Jill M. Thomas, and Jan C . T. Kwak' Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 453 Received June 22,1992. In Final Form: November 16,1992
Criticalmicelle concentrations,apparent degreesof counterionbinding, additivedistributioncoefficients, and surfactant and additive aggregation numbers are reported for aqueous mixtures of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB), with tetraethylene glycol (TEG) and tetraethylene glycol dimethyl ether (TEGDM). Critical micelle concentration (cmc)values and apparent degrees of counterion binding (8) were determined via a conductance method. The mole fraction of the solubilizate(eitherTEGor TEGDM)in the micelle was determinedusingthe NMR paramagneticrelaxation method. Surfactantaggregationnumbers were obtained from static fluorescencequenchingexperiments, using pyrene as the fluorescent probe and cetylpyridinium ion (CP+)as the quencher. The results show a remarkabledifferencein the solubilizationbehavior in SDSand DTAB micelles for these two ethoxylates. TEG is not incorporated appreciablyin either SDS or DTAB micelles, while TEGDM ia solubilizedeasily by SDS micelles (p = 0.44)but hardly incorporated in DTAB micelles (p = 0.06). Addition of TEGDM to a SDS micellar solution decreases both the cmc and 8 values of the micelles. When TEGDM is added to a 0.06 m SDSsolution,the surfactantaggregationnumber,N,, decreasesfrom 66 to 40,while the additive (TEGDM) aggregation number, N,, increases rapidly from 0 to 37, as the total TEGDM concentration is increased to 0.30 m. Thew results are discussed in terms of the difference in the solubilizationbehavior of theae ethoxylatee in anionic and cationic micelles.
Introduction
hydrophobic interaction between the polymer and surfactant micelles. A number of studies have dealt with the interaction of poly(ethy1eneoxide) (PEO)of varying chain lengths with anionic micelles, typically SDS. On the other hand, it appears that the interaction of hydrophobically modified polymers with surfactants has only recently been receiving attention. As an example, two recent articles examine the interaction with SDS as a function of concentration of a PEO in which the end groups were replaced by pyrenyl groups,usingfluorescenceemissionto follow the formation of aggregates.81~The results indicate a significant interaction of the polymer with SDS, at concentrations well below the cmc of SDS micelles in the absence of additives. At low micelle concentrations, the polymer ia solubilized in SDS with two pyrenyl fragments occupying the same micelle, allowing excimer formation and measurable emission. As the number of micelles ia increased, this effect disappears due to the solubilization of the luminescent end groups in different micelles. Additionally, Winnik et al.lOJ1 have examined the interaction with SDS and CTAC of poly(N-isopropylacrylamides)hydrophobically modified by the addition of alkyl chains and the clustering of the alkyl chains of the polymer itaelfto form micelles. Their results clearly indicate that the ionic surfactants bind to the polymer but that the binding is not cooperative; however, the binding of short chain nonionic surfactants is cooperative in nature. In a number of recent papers, we have determined the interaction of water-soluble, nonionic polymers and alcoholswith both anionicand cationicmicellar system8,11-18
Recently, an extensive amount of research has dealt with the use of polymer-surfactant systems in such diverse applicationsas tertiary oil recovery, drug delivery,paints, and pulp and paper production.' The broad and varied usage of polymer-surfactant systems necessitates a thorough unraveling of the interactions (i.e., the binding) between the primary components, the polymer and surfa~tant.~-~ The binding of surfactants to polymers can be electrostatic and site specific, as in the case of an oppositelycharged polyelectrolyteand surfactant.6 In the case of neutral polymers with anionic surfactants, the binding is thought to be due to the contribution of the repeating group to the hydrophobic interactiom2-8 For polymer-surfactant systems where the polymer contains water-soluble end groups and few repeating units, the binding decreasessignificantlyas the interaction of watersoluble end groups with the solvent dominates the
* To whom correspondence should be addressed. + Present address: Research Chemistry Branch, AECL Research Whitashell Labs, Pinawa, MB, ROE 1LO. (1) Polymers in Agueorur Media; Advances in Chemistry Series, No. 233; G h , M. J., Ed.;Americnn Chemical Society: Washington, DC, 1989. (2) Robb, I. D. In Anionic Surfactants: Physical Chemistry of Surfactant Action; Lucaseen Reynders, E. H., Ed.;Marcel Dekker: New York, 1981; Vol. 11, p 109. (3) Goddard, E. D. Colloids Surf. 1986, 19, 255, 301. (4) Saito, S. In Nonionic surfactants: Physical Chemistry; Schick, M. J., Ed.;Marcel Dekker: New York, 1987; Vol. 23, p 881. (5) Hayakawa, K.; Kwak, J. C. T. In Cationic Surfactants; Holland, P., Rubingh, D., Eds.;Marcel Dekker: New York, 1991; Vol. 37, Chapter 0.
(6) Perron, G.; Francoeur, J.; Dwnoyers, J. E.; Kwak, J. C. T. Can. J. Chem. 1987,65,990. (7) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Colloid Interface Sci. 1990.137. 137. 6 Hu;Y.-Z.;Zhao, C.-L.;Winnik,M. A.;Sundararajan,P. R. Langmuir 1990,6,880.
(9) Winnik, F. M.; Winnik, M. A.; Ringsdorf, H.; Venzmer, J. J. fhys. Chem. 1991,95, 2583. (10) Winnik, F. M.; Rinpdorf, H.;Venzmer, J. Lcrngmuir 1991,7,906. (11) Winnik, F. M.; Ringsdorf, H.; Venzmer, J. Langmuir 1991,7,912. (12) Gao, Z.; Kwak, Labont4, R.; Kwak, J. C. T.; Maraugoni, D. G.; Wasylishen, R. E. colloids Surf. 1990,45, 269. (0
1993 American Chemical Society
Solubilization and Aggregation Number8
Langmuir, Vol. 9, No. 2, 1993 439
with an emphasis on additives (alcohols or polymers) containingethylene oxide (EO)groups, using the recently developedNh4R paramagneticrelaxation e~periment.l2-~5J7 For example, Gao et al. have examined the interaction of poly(ethy1ene oxide) (PEO) with SDS micellar solutions as a function of the polymer chain length.17 Their results indicate that the maximum ratio of solubilized EO monomers to micellized surfactant ions approaches 2:l. Additionally, these authors report that in excess SDS, the mole fraction of polymer repeating units in the micellar phase,thep value, increaseswith an increasein the number of EO units to a maximum of about 0.85 0.02 for PEO 4000 and above. The p value is defined as
*
P = na,mic/na,t
(1)
where na,mic is the number of moles of additive (in this case, PEO monomers) in the micellar phase and n , t is the totalnumber of moles of additive. For polymers containing large numbers of repeating units, the contribution of the EO groups to the hydrophobic interactions dominates the formation of the mixed micelles. When the number of repeating units is decreased, as in the case of tetraethylene glycol (TEG), the p value drops drastically, indicating a negligible interaction with anionic micelles. The interaction of the glycol with the micelles is dominated by the hydrogen bonding of the two end OH groups. When the OH end group of TEG was replaced by the more hydrophobic methoxy group, OCH3, in tetraethylene glycol dimethyl ether (TEGDM), the p value in 0.243 m SDS micelles increases to 0.44, which is similar to the value observed for 1-butanol in SDS micelles.12-14 Nakayama et al.l9 have examined the heat of solution when low molecular weight PEO's and PEOs with substituted ethoxy and methoxy groups are dissolved in water. They report that for low molecular weight PEO's, the change in the heat of solution with an increase in the number of EO groups is small, indicating that the interaction of the polymers is dominatedby the interaction of the hydroxyl groups with water, and hardly affected by the hydrophobic effect of the EO backbone. With the end OH groups replactd with methoxy and ethoxy groups, a large increase in AC was observed, indicating a strong hydrophobic effect. %he lack of interaction with low molecularweight PEOs was attributed to the participation of the end OH groups in hydrogen bonding, therefore depressing the hydrophobic effect of the methylene backbone. Bender and Pecora20 have determined the mutual diffusion coefficients in water of PEO 600 and PEO 8O00, and of PEOs that were hydrophobically modified by replacing one or both of the end OH groups with methoxy or ethoxy groups. The mutual diffusion coefficients of the PEOs with alkyl end groups differed significantlywith those with OH end groups, implying notable differences in the aqueous solution behavior. Recently, we have explored the composition of ionic surfactant/alcohol mixed micelles. The cmc values of (13) Marangoni, D. G.; Kwak, J. C. T. Langmuir 1991, 7, 2083. (14) Marangoni,D. G.; Thomas, J. M.; Rodenhiker,A. P.; Kwak, J. C. T. ACS Symp. Ser. 1992,501, 194. (15) Gao, Z.; Wasylishen, R.E.; Kwak, J. C. T. J. Phys. Chem. 1989,
93.2190. , ~~. . (16) Yamashita, F:; Perron, G.; Desnoyers, J. E.; Kwak, J. C. T. J. Colloid Interface Scr. 1986, 114, 548. (17) Gao, Z.; Wasylishen, R. E.; Kwak, J. C. T. J. Phys. Chem. 1991, 95,462. (18) Yamaehita, F.; Kwak, J. C. T. To be submitted for publication. (19) Nakayama, H. Bull. Chem. SOC.Jpn. 1970,43, 1683. (20) Bender, T. M.; Pecora, R.J. J . Colloid Interface Sci. 1988, 126, 638.
various surfactant/alkoxyethanol mixtures were determined by conductance and emf measurements, and the concentration of micelles in solution was obtained from static luminescence quenching experiments. The NMR paramagneticrelaxation experimentwas used to determine the mole fraction, or p value, of alkoxyethanol in the micelles.12J3 By combination of these results, the alcohol and surfactant aggregation numbers for a series of alkoxyethanols in DTAB and SDS micellar solutions have been determined as a function of the alcohol concentration.14 The use of luminescence quenching to determine surfactant aggregation numbers is well e~tablished.~l-2' The main advantage of luminescence probing techniques is that they can be used to directly determine the number of micelles in solution, at any surfactant concentration rather than at the cmc. These techniques do not require a priori knowledge of the micellar shape or volume. One disadvantage with luminescence probing is the need for solubilizing probes and quenchers in the micelles, which may alter the micellar structure and concentration. The static quenching method, proposed originally by Turro and Yekta,= has been used extensively in the literature for the determination of the surfactant aggregation number, NE, in the presence of additives. These authors derived a simple Stern-Volmer type relationship between the bulk quencher concentration and the logarithm of the fluorescence intensities at varying quencher concentrations
where IO is the fluorescence intensity without quencher, I is the intensity at bulk quencher concentration [&I, and [MI is the micelle concentration. The static quenching method has an advantage in its ease of implementation. However, the application of the Turro-Yekta method depends on compliance with a number of experimental criteria:21-29(1)both the luminescent probe and quencher must be solubilized in the micelle and be immobile, remaining within the micelle during the luminescent lifetime of the probe; (2) the quenching process is much faster than the decay of the luminescenceintensity so that luminescence is observed only from micelles containing a solubilized probe and no quencher. The criteria for the successful application of the static quenching method to the determination of the surfactant aggregation number have been discussed previously.14921-249289m It is generally accepted that application of the Turro-Yekta equation is valid for the probe/quencher pairs ruthenium tris(bipyridy1) chloride/9-methylanthracene (9-MA) and pyrene/cetylpyridinium ion (CP+) in micellar systems where the aggregation number is less than 80.243Pt29 A number of studies have examined the effects of using the static quenching method. In solubilizateson NE, a series of paper~,2l-~~ Almgren and Swarup determined the surfactant aggregation number of SDS micelles as a function of the concentration of both the surfactant and (21) Almgren, 5.;Swarup, S. J. Colloid Interface Sci. 19&3,91,256. (22) Almgren, S.; Swarup, S. J. Phys. Chem. 1982,86,4212. (23) Almgren, 5.;Swarup, S. J. Phys. Chem. 198S, 87,876. (24) Malliaris, A. Adu. Colloid Interface Sci. 1987,27, 253. (25) Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 72, 151. (26) Zana,R. InSurfactant Solutions: New Methodsof Investigation; h a , R.,Ed.;Marcel Dekker: New York, 1987. (27) Thomas, J. K. The Chemistry of Excitation at Interfaces; American Chemical Society: Washington, DC, 1984, Chapter0 6-7. (28) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978,100,5951. (29) Almgren,M.;Ufroth, J.-E. J. Colloid InterfaceSci. 1981,81,486.
Marangoni et al.
440 Langmuir, Vol. 9, No. 2, 1993 Table I. cmc Values (h0.3 mm)and Degmr of Counterion Binding (B i0.02) for SDS/TEG and SDS/TEGDM Mixed Miceller an a Function of the Total Concentration of Added Alcohol TEG TEGDM CJm cmc B cmc B O.Oo0
0.025 0.050
0.075 0.100
8.16 7.83 7.77 7.66 7.60
0.62 0.61 0.59 0.58 0.57
8.16 7.53 7.40 7.20 6.99
0.56
0.53 0.50 0.47
0
(3)
Nawaa observed to increaseslowlyfor the less hydrophobic additives; for the more hydrophobic solubilizates, Na increased more quickly, so that the total aggregation number, Nt = NB+ Na, remained relatively constant or increased slowly. Similar results for 0.0400 m SDS/ additive mixed micelles were found by M a l l i a r i ~using ,~~ the static quenching method with pyrene/CP+ ion as the probe/quencher pair. In this paper, we have been motivated by the reported unusual solubilization behavior of TEG and TEGDM in anionic micelles" to examine the interaction of these glycols with anionic and cationic micelles. Our emphasis will be on a description of the equilibrium properties of the mixed micelles in terms of the free energy of micelle formation (cmcvalues), the fraction of additive contained in the micellar phase (the p value), and the aggregation number of both the surfactant and the additive as a function of the glycol concentration. We will discuss the large differencesin the solubilizationbehavior of the glycols in anionic and cationic micelles. Experimental Section Sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB) were obtained from Sigma and purified by repeated recrystallizations from ethanol and an acetone/ ethanol mixture, respectively. Tetraethylene glycol (TEG) and tetraethylene glycol dimethyl ether (TEGDM)were both obtained from Aldrich and were used without further purification. The deionized water had a resistivity of (1.5-2.5) X 10s fl cm. All alcohol/water mixed solvent systems were prepared on a molality basis. The surfactant solutions were made up directly in the mixed solvent; the molalities are reported as the number of moles of surfactant per kilogram of mixed solvent. The cmc values of SDS/glycol and DTAB/glycol mixed micellar systems were obtainedfrom conductancemeasurements,usinga resistance bridge (Industrial Instruments) operating at lo00 Hz. A diptype conductance cell (cell constant 1.00 cm-') was used together with an automated solution addition system. All spin-lattice relaxationtimes (2'1's) were measuredon freshly prepared solutions. Details of the NMR experimentsand solution preparation have been described previouely.1*-15 (30)Stilbn, P.J. Colloid Interface Sci. 1982,87, 386. (31)Stilbe, P.J. Colloid Interjace Sci. 1982,89, 547.
8.6
0.62
solubilizate for a wide variety of solubilizates. A number of trends were reported in these studies, including the decrease in N8brought about by the addition of alcohols and polar additives. For a given concentration of surfactant, the rate of the decrease in N, with an increase in the concentration of polar additive was observed to be larger as the hydrophobicchain length of the additive was using increased. From the p values obtained by Stilbs3013~ the FT-PGSE experiment,the additive concentration, Ca, and the micellar concentration from the static quenching method, these authors calculatedthe additive aggregation number, Ne, as follows Na = pCJM1
CMC / mmolal
7.0 1
8.6
\
0
.
0.000
0.026
0.060
0.076
0.100
0.126
Ca,t / molal Figure 1. cmc values (*0.3 mm, conductance measurements) for SDS/TEG ( 0 )and SDS/TEGDM ( 0 )mixed micelles. Solution preparation for the luminescence quenching experiments was as follows. A small amount of a 0.0060 m pyrene/ ethanol solutionwas placed in a small flaskand the solvent allowed to evaporate, depositing the pyrene as a thin f i i on the bottom of the vessel. The stock solution of the surfactant/mixed solvent system was prepared directly in the flask containing the pyrene; it was stirred for at least 4 h to ensure complete dissolution of the pyrene in the surfactant solution. This method of solution preparation was found previouslywn-29to be a very effectivemeans of dissolving hydrophobic fluorescent probes (Le., pyrene) into a surfactant solution. The quencher solutions were prepared from one-halfof the stockSurfactantJprobe solutionsby dissolving the CP+ in the stock surfactant solution and stirring for 1-2 h. Mixtures at specific concentrations of quencher were prepared by mixing portions of the two stock solutions by mass. Steadystate pyrene fluorescenceemission spectra were recorded at room temperature (=23 "C) on a Perkin-Elmer MPF-66spectrophotometer, using an excitation wavelength of 338 nm and scanning the emission from 350 to 500 nm. The Zl/ZS ratios were measured directly from the spectra. The intensity of the pyrene emission at 373 nm (11) was used in the plots of In (Za/l) versus the quencher concentration.
Results and Discussion (i)cmc Values, Degrees of Counterion Binding,and p Values. The cmc values for mixed micelles composed of SDS and tetraethylene glycol (TEG) and tetraethylene glycoldimethyl ether (TEGDM),obtained from the breaks in the curves of conductance vs total surfactant concentration (molality units), are presented in Table I and plotted in Figure 1 as a function of the additive concentration. Comparisonof the slopeof the linear conductance plots before and after the cmc yields the apparent degree of counterion binding, also reported in Table I. In the case of TEG, there is only a small dependence of the cmc values with increasing additive concentration. The cmc values in SDS/TEGDM mixed solutione are much more strongly dependent on the additive concentration, indicating a stronger interaction of TEGDM with anionicSDS micelles. For SDS/TEGDM mixed micelles, the cmc decreases in a linear fashion with the additive concentration, and the value for the sloped(cmc)/dc, is calculated to be 0.0152, comparable to the value obtained for SDS/ 1-butanol mixed micelles."
Langmuir, Vol. 9, No. 2, 1993 441
Solubilization and Aggregation Numbers Table 11. cmc Values (f0.3 mm) and Degrees of Counterion Binding (B f 0.02) for DTAB Micelles in Aqueous Solutions of TEG and TEGDM as a Function of the Total Concentration of Added Glycol TEG TEGDM Cdm cmc B cmc B 16.3 0.74 16.3 0.74 O.OO0 16.6 0.74 16.6 0.74 0.060 17.2 0.73 16.8 0.74 0.100 0.150 16.9 0.74 17.3 0.73 0.200 17.3 0.74 17.4 0.73 0.300 17.9 0.72 17.9 0.72
The decrease in the apparent fl values for SDS/TEGDM mixed micelleswith an increase in the glycolconcentration indicates a significant reduction of the surface charge density of the mixed micelle formed between SDS and the hydrophobically modified glycol. The reduction in the surface charge density of the mixed micelles can be explained as follows. Due to the polar nature of both the EO groups and the OCH3group, it would be expected that a fraction of the glycolmoleculesare located in the palisade region of the mixedmicelle, dispersedamongthe surfactant headgroups. This would reduce the electrostatic interactions between neighboring SDS headgroupa, thus the surface charge density of the mixed micelle would be lowered, and fl would be diminished. The cmc’s for DTAB/TEG and DTAB/TEGDM mixed micelles are presented in Table 11, together with the apparent degrees of counterion binding. It can be readily seen from Table I1 that the trends in the cmc values, as a function of the glycol concentration, differ between the cationic micelles and anionic micelles. These cmc results can be explained as follows. The contribution of the EO groups to the hydrophobicinteractions has been Shown to be negligible in cationic surfactant/alkoxyethanolmixed micelles.13J4 This would indicate that the distribution of these glycols in cationic DTAB micelles would be vanishingly sd,i.e.,theglycolremainsin the aqueousphase. Both TEG and TEGDM may be classed as structurebreaking solutes in water.1g Similar to what has been observed for the addition of urea to micellar solutions,the addition of the structure-breaking solute in micellar solutions diminishes the hydrophobic effect, which is the driving force for micellization of surfactmts.3*-% Increasing the amount of the structure-breaking solute in water would therefore shift the cmc to a higher surfactant concentration. It is interesting to note that the degree of counterion binding is constant, indicatingthat the micelles may be free of added glycol,and are simply DTAB micelles formed at slightly higher surfactant concentrations. This observation is in agreement with the results, described below, of the paramagnetic relaxation experiment used to obtain the degree of solubilization of the additive. The degrees of solubilization, p, of the glycols in surfactant micelles, defined by eq 1, were obtained from NMR relaxation experiments as described in previous papers.12-16 The results are presented in Table 111for the glycols in both SDSand DTAB micelles. The mole fraction based distribution coefficient, K,,is defined as follows Kx X m i d x a q (4) Xmic and Xaq are the mole fractions of solubilizate in the Abu-Himidiyah, M.;Kumari, K.J . Phys. Chem. ISSO,94,6445. Shgh, H. N.; Swarup,S. Bull. Chem. SOC.Jpn. 1978,51,1534. Sibbern. - -. M.:. Henrikmon. U.: Warnheim, T. Langmuir 1990,6,
2105. (36)Backlund, S.;,Bergex”, B.; Molander, 0.;Warnheim, T. J. Colloid Interface S a . ISSO,131, 393. (36)W e n , M.;Swarup,S.; Lefroth, J. J. Phys. Chem. 1986,89, 4621.
Table 111. Distribution Coefficients and Free E n e d e r of Transfer for TEG and TEGDM in SDS. and DTABb Micellar Solutions
K*
P
alcohol
* 0.07 * 0.04 0.00 * 0.14
TEG TEGDM
0.00 0.44
TEG TEGDM
0.05 0.09
a
CSDS =
- AG&J mol-’
SDS 161 33 DTAB
12.6 f 0.6
16 i16
0.243 m; T = 298 K;c(proxy1) = 10 mm; .C = 0.060 m.
* CDTAB = 0.162 m; T = 308 K;c(MnCl2) = 1.0 mm; c. = 0.050 m.
micellar and aqueousphases, respectively;Le., X b = n-J (na,mic + nsurf,mic) and Xaq = na,aq/(na,aq+ niurf,aq + h), where n, is the number of moles of water in 1 kg of solvent. Note that in these experiments, Ra,t is much less than h. Xmic and Xaq can be calculated easily from the known concentration of glycol and surfactant, the cmc value, and the p value, since naJnic= pna,t and naaq (1 - P)na,bThe free energy of transfer of the glycolfrom the aqueous phase to the micellar phase can then be calculated from the relation
AG; = -RT In K,
(5)
The calculated values of the distribution coefficients,along with the free energy of transfer of the glycols from the aqueousto the micellar phase, are also presented in Table 111. It can be seen from Table I11 that changing the hydrophilic OH group for the more hydrophobic OCH3 group has a dramatic effect on the distribution of these ethers in anionic SDS micellar solutions (see also ref 17). For TEG, the results from the NMR paramagnetic relaxation experiment indicate a negligible interaction for both SDS and DTAB micelles. This is not unexpected since TEG contains two OH groups and a small number of EO chains; Le., the driving force for the transfer of TEG to the micelle interior (the transfer free energy of the EO group) would be overwhelmed by the interaction of the two OH groups with water. However, a much more favorable interaction is observed for TEGDM with SDS micelles. This is, in part, due to the contribution of the EO groups to the hydrophobic interactions13J4 and a hydrophobic effect due to the OCH3 groups; i.e., substituting the methoxy groups for the OH group resulta in a loss of favorable hydrogen bonding. The transfer of both TEGDM or TEG from water to the cationic micelles is still energeticallyunfavorable (Le., p(TEGDM) p(TEG) 0). This behavior for TEG is in agreement with earlier observation^,^^^^^ but the fact that the end-group capping does not lead to increased solubilizationin DTAB micelles is remarkable. (ii) Surfactantand Additive AggregationNumbers and Pyrene I1/& Ratios. The emission intensities of micellar solubilizedpyrene at 373 nm were plotted against the quencher concentration, [Q], according to eq 2. The slope of these plots is the reciprocal of the micelle concentration, [MI. The slope and the relative error in the slope were calculatedby least-squaresmethods. Good linearity was found for all plots of In (Idl)versus [Ql(the relative error in the slope was generally around 2-3%). From the micellar concentrations,the aggregationnumbers of the surfactant were calculated from the relationship
-
-
with the cmc values obtained above. Aggregation numbers of the surfactant, NE,for 0.0500 m SDS/glycol mixed
442 Langmuir, Vol. 9, No. 2, 1993
Marangoni et al.
Table IV. Aggregation Numbers of Surfactant and Alcohol. for 0.0600 m SDS/"EG and SDWTEGDM Mixed Micelles as a Function of the Concentration of Alcohol
O.OO0
0.025 0.050 0.075 0.100 0.150 0.200 0.250 0.300 a
66
0
1.26
59
0
1.26
61 55 52 53 47
0 0 0 0 0
1.25 1.26 1.24 1.25 1.24
N , i 3,N .
66 59 53 49 46 43 43 40 40
0 4 8 11 14 19 26 30 37
1.26 1.23 1.25 1.26 1.27 1.28 1.28 1.29 1.29
* 5.
micelles are presented in Table IV and plotted in Figure 2. The errors reported here are obtained from the relative error in the slope of the plot of In (Io/I) vs [QIcalculated at the 90% confidencelevel,regarding all deviationsfrom Nsas random errors. Our result for the aggregation number of 0.0500 m SDS, in the absence of added glycol, is in excellent agreement with both time-resolved and static experiments involving the same probe/quencher pair.21-24*2*12s*n' The glycol aggregation numbers, Na, presented in Table IV,were calculated from eq 3 above, using the micellar concentrationsdetermined via the static fluorescencequenchingexperiment and the p values from the NMR paramagnetic relaxation experiment in 0.243 m SDS micelles. The reported errors in Ne reflect the sum of the relative errors in the micellar concentration, [MI, and the distribution constants (pvalues) calculated at the 90% confidence interval, regarding all deviations from the calculated value of Na as random errors. The ratios of the intensities of the first and third peaks of the pyrene emission spectrum, 11/13,which are indicative of the micropolarity of the average pyrene probe environment, are also presented in Table IV. The errors in the 11/13 ratios are estimated to be on the order of 1-2 % . In the above section, the idea was advanced that for TEG, the interaction of the EO groups in the alcohol with the surfactant methylene chains for SDS was dominated by the interaction of the OH group with water, with the overall effect being that TEG resides exclusively in the aqueous phase; Le., p is effectively zero. However, when the OH groups are replaced with OCH3 gr?ups, as in the case of TEGDM, a large decrease in AC, is observed. These observations appear to account for the trends in the aggregation numbers for both the surfactant (N,)and the additive (NJ.In Table IV, the aggregation numbers for 0.0600 m SDS/TEG mixed micelles are compared with the aggregation numbers of 0.0500 m SDS/TEGDMmixed micelles. It can be seen from Table IV and the plotted values of the aggregation numbers (Figure 2) that the N , values of 0.0500 m SDS/TEG mixed micelles show only a minor decrease, in good agreement with the results of previous author~.~t3'This indicates, again, a negligible interactionof TEG with SDSmicelles. However,for 0.0500 m SDS/TEGDMmixed micelles, N8decreasesrapidlywith an increase in the concentration of TEGDM. In fact, the rate of the decreasein N,for SDS/TEGDMmixed micelles, as a function of the concentration of TEGDM, is identical with the rate of decream for SDS/l-butanolmixed micelles, two alcohols with similar distribution con~tants.1~ Additional information about the structure of these mixed micelles can be obtained from an analysis of the pyrene 11/13ratios in Table IV. A remarkable feature of (37) Hashimoto,S.;Thomas, J. K.J. Am.Chem. SOC.1985,105,5230.
'O
t
so 20
-
10
-
0 0
I
I
1
I
I
I
I
0.06
0.1
0.l6
0.2
0.26
0.8
036
0.4
C,/ molal Figure 2. Surfactant aggregation numbers for SDS/TEG ( 0 ) and SDS/TEGDM ( 0) mixed micelles.
the SDS/TEGDM mixed micellar system is that the 11/13 ratios of solubilized pyrene increase, as opposed to the decrease observed when 1-butanol or 2-butoxyethanol is solubilized in SDS micelles." Normally, a decrease in 11/13is aesociatedwith a lower polarity sensed by the pyrene probe. Zanax and Thomas2' have interpreted this trend in terms of a more disordered micelle structure when the alcohol is incorporated in the micelle, allowing the luminescent probe (e.g., pyrene) to penetrate further into the micelle, thereby sensing a less polar environment. A 13C NMR relaxation study by Monduzzi et ala%concludes that the addition of 1-pentanol to SDS micelles results in more disorder, especially in the region near the micellar surface. Gao et have examined the distribution of aromatic probe molecules, like pyrene, in anionic and cationic micelles. Their findings indicated that SDSsolubilizedprobe molecules, e.g., pyrene and naphthalene, are evenly distributed throughout the micelle. When a shortchain alcohol (e.g., 1-butanol)is added to the micelles, this dwtribution is not expected to be altered, and hence, the aromatic probe molecule would still sample the entire micelle volume, including the region close to the surface of the SDS micelle. If this is the case, an alternative explanation of the 11/18 decrease for pyrene in SDS/1butanol and SDS/2-butoxyethanol mixed micelles could be lower water penetration into the micelle, due to the presence of additives at the micellar surface. If a similar argument is made for SDS/TEGDM mixed micelles, the unusual increase in the 1 1 / 1 3 ratio could not be the result either of the presence of the glycol altering the way in which the pyrene samples the micellar interior or of bringing additional water into the micellar interior. However, it may indicate the presence of the polar EO groups and OCH3 groups from the glycol in the micellar interior. It is unlikely that these polar groups would be buried deep in the micellar core;the most probable location (38) Monduzzi, M.: Ceglie, A.: Lindmnn, B.:S&ler". 0. J. Colloid Interface sci. ISSO, 136,113. (39) Wasylishen, R. E.; Kwak, J. C. T.; Gao, Z.; Verpoorte, E.; MacDonald, J. B.;Dickson, R. M. Can. J. Chem. 1991,69,822.
Solubilization and Aggregation Numbers Table V. Aggrqration Numbers of DTAB and Alcohol. for 0.0750 m DTAB in Aqueous Solutions of TEG and TEGDM as e Function of the Concentration of Alcohol TEG TEGDM CJm N, Nn 11/13 N, Nn 11/13 O.Oo0 50 0 1.46 52 0 1.47 0.050 60 0 1.46 49 1 1.46 0.100 51 0 1.46 49 2 1.46 0.150 45 0 1.46 40 2 1.47 0.200 51 0 1.45 45 3 1.46 0.250 44 4 1.47 0.300 45 4 1.46 ON, 3,Nn 5.
*
*
of these polar groups would be still near the surfactant headgroups. Note that the 11/13value for pyrene in SDS/ TEG mixtures does not change, as expected from the zero p value for TEG in SDS micelles. These observationsare also consistent with the light scattering results of Bender and Pecoramand the thermodynamic studies of Nakayamal9 indicating a greater degree of self-association (i.e., hydrophobicity) in aqueous solution for TEGDM over TEG, due to the presence of the more hydrophobicOCH3 group. The aggregation numbers of 0.0750m DTAB/TEG and DTAB/TEGDM mixed micelles are presented in Table V. Unlike what was observed for 0.0500 m SDS/TEGDM mixed micelles, there is no large decrease in the N8values as the concentrationof either TEG or TEGDM is increased. As well, the 11/13ratios remain constant with increasing glycol concentration. This information, coupled with the low distribution constant of TEGDM and TEG in DTAB micelles (seeabove),again indicatesa negligible association of either glycol with DTAB micelles. This is similar to our resulta from a previous study in which we noted that the decrease in the cmc values as a function of the additive concentrationin DTAB/alkoxyethanolmixed micelles, and the distribution constantaof alkoxyethanolsin DTAB and DPC micelles (the p values), was independent of the number of EO groups in the eth0xy1ates.l~
Conclusions The cmc values of mixed micelles formed in solutions of the anionic surfactant SDS and TEG or TEGDM
Langmuir, Vol. 9, No. 2, 1993 443
indicatethat the interaction of small, water-solubleglycols can be enhanced greatly by altering the hydrophobicity of the end groups. The cmc values of SDS micelles in mixed water/TEG solventa are consistent with a negligible interactionof this small polyglycolwith SDSmicelles,while the cmc values of SDS micelles in the presence of an increasing amount of TEGDM in the solvent mixture can be explained in terms of a high degree of interaction with the micelles. From the dependence of the 6values of the mixed micelles on the glycol concentration,it appears that the glycol is solubilized in the micellar palisade region, near the surfactant headgroup. The cmc values in DTAB/ TEG or DTAB/TEGDM mixtures do not show any evidence of mixed micelle formation. NMR resulta for the solubilization of TEG and TEGDM in anionic and cationic micelles also show that TEG is not solubilized in SDS or DTAB, while TEGDM interacts very strongly with SDS but weakly with DTAB. This interaction is due to both the hydrophobic effect of the OCHs group and the favorable transfer free energy of the EO groups to the interior of anionic micelles. By combining resulta from the surfactant aggregation numbers (determined from static fluorescencequenching methods) and the distribution coefficienta (p values) of the additive determined by the NMR paramagnetic relaxationmethod, we find that in SDS/TEGDMsystems, the surfactant aggregation number, N,,decreasesfrom 66 to 40 as the total solution concentration of TEGDM is increased to 0.30m;at the same time, N,,the number of TEGDM molecules in the mixed micelle, increases from 0 to 37. Both TEG and TEGDM are not solubilized by cationic DTAB micelles, showing, once again, the remarkable difference in the interaction of ethoxylates with anionic and cationic micelles.
Acknowledgment. D.G.M. and J.M.T. are grateful to the Natural Sciences and Engineering Rssearch Council of Canada (NSERC)for scholarshipsupport. The authors would like to thank Dr. ZhishengGao for his contributions to this research. This work was supported by NSERC. All NMR measurements were carried out at the Atlantic Region Magnetic Resonance Centre (ARMRC) at Dalhousie University.
