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The question of micelle formation in non-aqueous polar solvents: Positron annihilation results
Volume 144, number 1
CHEMICAL PHYSICS LETTERS
12 February 1988
THE QUJZSTIONOF MICELLE FORMATION IN NON-AQUEOUS POLAR SOLVENTS: POSITRON ANNIHILATION RESULTS Z.B. ALFASSI and W.G. FILBY Departmentof NuclearEnergy, Ben Gurion University,Beersheva84105 Israel and Kernforschungszentrum, D-7500 Karlsruhe,FederalRepublicof Germany Received 29 May 1987; in final form 7 December 1987
Positron annihilation studies show that while the surfactants CTAR and NaLS form micelles in aqueous solution, they are not formed in dipolar aprotic solvents.
The remarkable properties of aqueous amphiphilic substances have been the subject of a voluminous literature [ 11. For example various macroscopic and microscopic properties of these so-’ lutions show a marked concentration dependence at about the critical micelle concentration (CMC) - the saturation concentration for the unaggregated surfactant. Above the CMC however, solution properties change in such a way as to suggest a growing selfassociation to various kinds of aggregates (micelles) . The driving force for aggregation resides mainly in the tendency of the non-polar groups to avoid contact with water at the same time as the polar group is strongly solvated. In contrast to the situation with aqueous solutions, micelle formation in non-aqueous polar solvents has attracted only meagre attention, largely due to restrictions on available experimental methods [ 11. Thus only a few reports are available dealing with the surfactants studied here (CTAB and NaLS). Gopal and Singh [ 21 measured the refractive index of various surfactants in FA, NMAC, DMF and DMAC #*and compared their results with results obtained conductimetrically. They concluded that there is micelle formation in FA and NMAC and none in DMF and DMAC. Comparison of the measured +I’ CTAB,cetyl trimethyl ammonium bromide; NaLS, sodium
CMCs was, however, poor. Singh et al. [ 3,4] partially repeated the work of Gopal and Singh, measuring the CMCs of NaLS and CTAB in a series of dipolar aprotic solvents at various temperatures from 20 to 55°C conductimetrically. The CMC values in NMAC, FA and NMF were much lower and in DMSO, DMF and DMAC much higher than in water. Whereas in general the CMC increased with decreasing dielectric constant, DMSO and Hz0 were exceptions in that their CMC values were respectively somewhat lower and considerably higher than expected. It was suggested, on the basis of thermodynamic data, that micelle formation involved destruction or promotion of structured solvent species. However it must be pointed out that in the graphical data presented, except for the aqueous solutions, the discontinuities supposedly indicating the onset of micellisation were vague and liable to equivocal interpretation. In fact, replotting the data presented in ref. [4] for sodium dodecyl sulphate in DMF and DMSO as recommended by Mukerjee and Cardinal [ 51 reveals no discontinuity interpretable as the onset of micelle formation. Further, replotting the raw data below 60 mM shows that very different “CMC” values can be obtained depending on the section of data used and its density. For true micelleforming surfactants, variations in the CMC of rarely more than l-2% are observed when using data higher
lauryl sulphate; FA, formamide; NMAC, N-methylacetamide; DMF, dimethylformamide; DMAC, dimethylacetamide; DMSO, dimethylsulphoxide; NMF, N-methylformamide.
0 009-26141881%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
83
Volume 144,number 1
Table 1 Positron annihilation parameters for solvents used in this work Solvent
t2 (ns)
12($6)
Ref.
FA
2.26f0.11 1.90+0.30 2.58+0.08 2.29f0.31
9.97 kO.72 13.55kO.26 11.2 f0.6 14.95k0.22
[ lo] this work
DMF
2.66f0.13
10.7 kO.7
[lOI
Hz0
2.11f0.26 1.80
16.9 +0.24 27.8
thiswork [ill
1.79kO.16 2.36f0.08
27.14kO.33 19.6 f 1.5
this work
DMAC
2.14+0.14 2.34 2.4OkO.32
16.2 f1.2
this work
12.60f 1.0
I121
NMF
DMSO
1101 this work
[lOI
13.6OkO.33 this work 14.30 [I31
than the CMC to locate it #*. Thus information on the subject is rare’and the overall picture is unclear. In this light the positron annihilation technique recently introduced by Handel and Ache [ 71 to aqueous and non-polar micellar systems seemed promising. The experimental and theoretical background has been described elsewhere [ 8 1. Its application to micelle studies is based on the fact that the annihilation characteristics of the Ps atom are determined by its microscopic chemical and physical environment. Thus abrupt decreases in the percentage of positrons forming positronium (I2 %), apparently indicative of the onset of aggregation, have been observed in many systems close to the CMC value. Similarly, sh&p changes in the (FPSlifetime occur in liquid crystals and polymorphic systems [ 91, In this work we report Ps annihilation results on solutions of two ionic surfactants, CTAB and NaLS, in water and the dipolar aprotic solvents DMF, NMF and FA. Experiments were conducted up to the limits of surfactant solubility and a second series overlapping the published CMC values. Lifetime and intensity parameters for the pure solvents are also presented (table 1) for comparison. The curves for NaLS and CTAB in aqueous solution are those calculated for partial inhibition as suggested by Alfassi and Ache [ 141. Clearly while for the nonamide solvents agreement between our data and two previous sets of measurements is good, our intensity results for the amides are consistently higher than y2For further comment on the deduction of CMC values from experimental data see refs. [ 5,6].
84
12 February 1988
CHEMICALPHYSICSLETTERS
Such discrepancies have already been pointed out by Kirkegaard et al. [ 15 ] and generally arise because of differing spectral deconvolution techniques and/or incorrect source corrections employed in the earlier work. Varying solvent purity may also be involved. The more recent work of AbbB et al. [ 131 employing glass foil sources (source correction 2%) and the Positronfit deconvolution program [ 151 for lifetime determinations in DMSO shows good agreement with our data for this substance. Plots of the ortho-positronium yield ( Z2W) versus concentration for CTAB and NaLS solutions are shown in fig. 1. The measured “CMC” values are included for comparison [ 2-41. Immediately apparent is their radically different appearance in water as compared with the other solvents. Whereas addition of ~600 mM CTAB or NaI.S to water results in an almost equivalent diminution of Z, from 27% to z 17%, a case of limited inhibition, the same amount (where possible) added to the dipolar aprotic solvents results in only minimal change. Additionally no discontinuity is observed for the latter solvents although in water the CMC is accurately reflected #3. Thus on the basis of these results we conclude that positron annihilation accurately confirms the formation of micelles of CTAB and NaLS in aqueous solution. It seems likely therefore that the discontinuities reported in nonaqueous solutions [2-41 are not indicative of true micelle formation but rather of non-cooperative, perhaps stepwise, association to oligomers of undefined, though probably low, aggregation numbers and/or possibly fluctuating structures [ 161. Muller has provided an effective theoretical basis for this proposal [ 171. For surfactants in various solvents for which the product dielectric constant x the sum of the radii of the headgroups is large, the association proceeds stepwise to open chain aligomers. These cannot be revealed by positronium intensity measurements probably because of their inability to interfere with the positronium formation mechanism. It is interesting to consider the origin of the different patterns of micellisation in the two groups of solvents. According to Reich’s model for micelle formation [ 18 ] the driving force for aggregation is the those
of ltho
and Tabata
[lo].
p3The clear discontinuity at the CMC is easily visible in the concentration range studied in ref. [ 71.
12 February 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number 1
mM
mM
Fig. 1. Positron annihilation data for CTAB and NaLS in aqueous and non-aqueous solution: 0, H20, v, NMF; q ,FA; A, DMF.
tendency to form micelles in which the polar head group is efficiently shielded from hostile solvent. A more polar solvent will thus be less hostile to the head groups and will require fewer hydrocarbon chains to achieve a given amount of screening. Consequently in alcohols common surfactants are known to be monomeric, forming solvated ion pairs [ 191. Indeed, since the positive charge generally associated with the surfactant head group in micelles will be almost completely neutralised by counterions contained within the ion pair there will be no double layer in the usual sense and consequently Coulombic interactions will be diminished. This may be the origin of their lack of inhibiting effect on Ps formation in non-aqueous solvents; clearly however solvent polarity is not the only factor. The ability of preformed structures to participate, or not, in micelle formation will also be of prime importance.
CTAB and NaLS in FA were reported. Measurements conducted above the Krafft temperature - the fusion temperature of the solvated surfactants - showed cleardiscontinuities in their surface tension-log( concentration) plots, indicating the formation of truemicelles. The CMC values were z 1OOfold higher than for both CTAB and NaLS in water. The authors emphasise the necessity of working above the Krafft temperature if micelle formation is to be observed. surements
on
Acknowledgement We are grateful to a referee for bringing the paper of Rico and Lattes [ 2 1 ] to >ourattention.
References Note added Recent publications also support our findings. Almgren et al. [ 201 examined the behaviour of NaLS in mixtures of water and some organic solvents including FA, NMF, DMAC and DMSO. Using a variety of tebhniques (conductance, surface tension, solubilisation and fluorescence) no evidence for the formation of normal micellar aggregates was found. In a second publication [ 2 1 ] surface tension mea-
[ I ] C. Tanford, The hydrophobic effect ( Wiley-Interscience, New York, 1973): H. Wennerstriim and B. Lindmann, Phys. Rept. 52 (1978) 1; H.F. Eicke, in: Micellization, solubilization and microemulsion, Vol. 1, ed. K.L. Mittal (Plenum Press, New York, 1977) p. 429. [2] R. Gopal and J.R. Sin&J. Phys. Chem. 77 (1973) 554. [3] H.N. Singh,S.M. Saleem,R.P.SinghandK.S. Birdi, J.Phys. Chem. 84 (1980) 2191. [ 41 H.N. Singh, S. Singh and K.C. Tawari, J. Am. Oil Chem. sot. 52 (1975) 436.
Volume 144, number 1
CHEMICAL PHYSICS LETTERS
[5] P. Mukerjee and JR. Cardinal, J. Pharm. Sci. 65 (1976) 882. [ 61 AS. Kertcs and H. Gutman, in: Surface and colloid science, Vol. 8, ed. E. Matijevic, (Wiley, New York, 1976). [ 71 E.D. Handel and H.J. Ache, J. Chem. Phys. 71 (1979) 2083. [ 81 Y.C. Jean, B. Djermouni and H.J. Ache, in: Solution chemistry of sutfactants, ed. K.L. Mittal (Plenum Press, New York, 1978). [ 91 J.B. Nicholas and H.J. Ache, J. Chem. Phys. 57 (1972) 1597. [ lo] Y. Ito and Y. Tabata, Chem. Phys. 15 (1972) 584. [ 111 H.J. Ache, ed., Positronium and muonium chemistry, Advan. Chem. Ser. No. 175 (1979). [ 121 P.R. Gray, C.F. Cook and G.P. Sturm, J. Chem. Phys. 48 (1968) 1145.
86
12 February 1988
[ 131 JCh. Abbe, G. DupHtre, A.G. Maddock and A. Haessler, Radiat. Phys. Chem. 15 (1980) 6 17. [ 141 Z.B. Alfassiand H.J. Ache, J. Phys. Chem. 88 (1984) 4347. [ 151 P. Kirkegaard, M. Eldrup, O.E. M0gensenandN.J. Pedersen, Computer Phys. Commun. 23 (198 1) 307. [ 16] H.F. Eicke, private communication. [ 171 N. Muller, J. Colloid Interface Sci. 63 (1978) 383. [ 181 I. Reich, J. Phys. Chem. 60 (1956) 257. [19lA.W.RalstonandC.W.Hoerr,J.Am.Chem.Soc.68(1946) 851. [ 20] M. Almgren, S. Swarup and J.E. tifroth, J. Phys. Chem. 89 (1985) 4621. [ 211 I. Rico and A. Lattes, J. Phys. Chem. 90 (1986) 5870.
CHEMICAL PHYSICS LETTERS
12 February 1988
THE QUJZSTIONOF MICELLE FORMATION IN NON-AQUEOUS POLAR SOLVENTS: POSITRON ANNIHILATION RESULTS Z.B. ALFASSI and W.G. FILBY Departmentof NuclearEnergy, Ben Gurion University,Beersheva84105 Israel and Kernforschungszentrum, D-7500 Karlsruhe,FederalRepublicof Germany Received 29 May 1987; in final form 7 December 1987
Positron annihilation studies show that while the surfactants CTAR and NaLS form micelles in aqueous solution, they are not formed in dipolar aprotic solvents.
The remarkable properties of aqueous amphiphilic substances have been the subject of a voluminous literature [ 11. For example various macroscopic and microscopic properties of these so-’ lutions show a marked concentration dependence at about the critical micelle concentration (CMC) - the saturation concentration for the unaggregated surfactant. Above the CMC however, solution properties change in such a way as to suggest a growing selfassociation to various kinds of aggregates (micelles) . The driving force for aggregation resides mainly in the tendency of the non-polar groups to avoid contact with water at the same time as the polar group is strongly solvated. In contrast to the situation with aqueous solutions, micelle formation in non-aqueous polar solvents has attracted only meagre attention, largely due to restrictions on available experimental methods [ 11. Thus only a few reports are available dealing with the surfactants studied here (CTAB and NaLS). Gopal and Singh [ 21 measured the refractive index of various surfactants in FA, NMAC, DMF and DMAC #*and compared their results with results obtained conductimetrically. They concluded that there is micelle formation in FA and NMAC and none in DMF and DMAC. Comparison of the measured +I’ CTAB,cetyl trimethyl ammonium bromide; NaLS, sodium
CMCs was, however, poor. Singh et al. [ 3,4] partially repeated the work of Gopal and Singh, measuring the CMCs of NaLS and CTAB in a series of dipolar aprotic solvents at various temperatures from 20 to 55°C conductimetrically. The CMC values in NMAC, FA and NMF were much lower and in DMSO, DMF and DMAC much higher than in water. Whereas in general the CMC increased with decreasing dielectric constant, DMSO and Hz0 were exceptions in that their CMC values were respectively somewhat lower and considerably higher than expected. It was suggested, on the basis of thermodynamic data, that micelle formation involved destruction or promotion of structured solvent species. However it must be pointed out that in the graphical data presented, except for the aqueous solutions, the discontinuities supposedly indicating the onset of micellisation were vague and liable to equivocal interpretation. In fact, replotting the data presented in ref. [4] for sodium dodecyl sulphate in DMF and DMSO as recommended by Mukerjee and Cardinal [ 51 reveals no discontinuity interpretable as the onset of micelle formation. Further, replotting the raw data below 60 mM shows that very different “CMC” values can be obtained depending on the section of data used and its density. For true micelleforming surfactants, variations in the CMC of rarely more than l-2% are observed when using data higher
lauryl sulphate; FA, formamide; NMAC, N-methylacetamide; DMF, dimethylformamide; DMAC, dimethylacetamide; DMSO, dimethylsulphoxide; NMF, N-methylformamide.
0 009-26141881%03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
83
Volume 144,number 1
Table 1 Positron annihilation parameters for solvents used in this work Solvent
t2 (ns)
12($6)
Ref.
FA
2.26f0.11 1.90+0.30 2.58+0.08 2.29f0.31
9.97 kO.72 13.55kO.26 11.2 f0.6 14.95k0.22
[ lo] this work
DMF
2.66f0.13
10.7 kO.7
[lOI
Hz0
2.11f0.26 1.80
16.9 +0.24 27.8
thiswork [ill
1.79kO.16 2.36f0.08
27.14kO.33 19.6 f 1.5
this work
DMAC
2.14+0.14 2.34 2.4OkO.32
16.2 f1.2
this work
12.60f 1.0
I121
NMF
DMSO
1101 this work
[lOI
13.6OkO.33 this work 14.30 [I31
than the CMC to locate it #*. Thus information on the subject is rare’and the overall picture is unclear. In this light the positron annihilation technique recently introduced by Handel and Ache [ 71 to aqueous and non-polar micellar systems seemed promising. The experimental and theoretical background has been described elsewhere [ 8 1. Its application to micelle studies is based on the fact that the annihilation characteristics of the Ps atom are determined by its microscopic chemical and physical environment. Thus abrupt decreases in the percentage of positrons forming positronium (I2 %), apparently indicative of the onset of aggregation, have been observed in many systems close to the CMC value. Similarly, sh&p changes in the (FPSlifetime occur in liquid crystals and polymorphic systems [ 91, In this work we report Ps annihilation results on solutions of two ionic surfactants, CTAB and NaLS, in water and the dipolar aprotic solvents DMF, NMF and FA. Experiments were conducted up to the limits of surfactant solubility and a second series overlapping the published CMC values. Lifetime and intensity parameters for the pure solvents are also presented (table 1) for comparison. The curves for NaLS and CTAB in aqueous solution are those calculated for partial inhibition as suggested by Alfassi and Ache [ 141. Clearly while for the nonamide solvents agreement between our data and two previous sets of measurements is good, our intensity results for the amides are consistently higher than y2For further comment on the deduction of CMC values from experimental data see refs. [ 5,6].
84
12 February 1988
CHEMICALPHYSICSLETTERS
Such discrepancies have already been pointed out by Kirkegaard et al. [ 15 ] and generally arise because of differing spectral deconvolution techniques and/or incorrect source corrections employed in the earlier work. Varying solvent purity may also be involved. The more recent work of AbbB et al. [ 131 employing glass foil sources (source correction 2%) and the Positronfit deconvolution program [ 151 for lifetime determinations in DMSO shows good agreement with our data for this substance. Plots of the ortho-positronium yield ( Z2W) versus concentration for CTAB and NaLS solutions are shown in fig. 1. The measured “CMC” values are included for comparison [ 2-41. Immediately apparent is their radically different appearance in water as compared with the other solvents. Whereas addition of ~600 mM CTAB or NaI.S to water results in an almost equivalent diminution of Z, from 27% to z 17%, a case of limited inhibition, the same amount (where possible) added to the dipolar aprotic solvents results in only minimal change. Additionally no discontinuity is observed for the latter solvents although in water the CMC is accurately reflected #3. Thus on the basis of these results we conclude that positron annihilation accurately confirms the formation of micelles of CTAB and NaLS in aqueous solution. It seems likely therefore that the discontinuities reported in nonaqueous solutions [2-41 are not indicative of true micelle formation but rather of non-cooperative, perhaps stepwise, association to oligomers of undefined, though probably low, aggregation numbers and/or possibly fluctuating structures [ 161. Muller has provided an effective theoretical basis for this proposal [ 171. For surfactants in various solvents for which the product dielectric constant x the sum of the radii of the headgroups is large, the association proceeds stepwise to open chain aligomers. These cannot be revealed by positronium intensity measurements probably because of their inability to interfere with the positronium formation mechanism. It is interesting to consider the origin of the different patterns of micellisation in the two groups of solvents. According to Reich’s model for micelle formation [ 18 ] the driving force for aggregation is the those
of ltho
and Tabata
[lo].
p3The clear discontinuity at the CMC is easily visible in the concentration range studied in ref. [ 71.
12 February 1988
CHEMICAL PHYSICS LETTERS
Volume 144, number 1
mM
mM
Fig. 1. Positron annihilation data for CTAB and NaLS in aqueous and non-aqueous solution: 0, H20, v, NMF; q ,FA; A, DMF.
tendency to form micelles in which the polar head group is efficiently shielded from hostile solvent. A more polar solvent will thus be less hostile to the head groups and will require fewer hydrocarbon chains to achieve a given amount of screening. Consequently in alcohols common surfactants are known to be monomeric, forming solvated ion pairs [ 191. Indeed, since the positive charge generally associated with the surfactant head group in micelles will be almost completely neutralised by counterions contained within the ion pair there will be no double layer in the usual sense and consequently Coulombic interactions will be diminished. This may be the origin of their lack of inhibiting effect on Ps formation in non-aqueous solvents; clearly however solvent polarity is not the only factor. The ability of preformed structures to participate, or not, in micelle formation will also be of prime importance.
CTAB and NaLS in FA were reported. Measurements conducted above the Krafft temperature - the fusion temperature of the solvated surfactants - showed cleardiscontinuities in their surface tension-log( concentration) plots, indicating the formation of truemicelles. The CMC values were z 1OOfold higher than for both CTAB and NaLS in water. The authors emphasise the necessity of working above the Krafft temperature if micelle formation is to be observed. surements
on
Acknowledgement We are grateful to a referee for bringing the paper of Rico and Lattes [ 2 1 ] to >ourattention.
References Note added Recent publications also support our findings. Almgren et al. [ 201 examined the behaviour of NaLS in mixtures of water and some organic solvents including FA, NMF, DMAC and DMSO. Using a variety of tebhniques (conductance, surface tension, solubilisation and fluorescence) no evidence for the formation of normal micellar aggregates was found. In a second publication [ 2 1 ] surface tension mea-
[ I ] C. Tanford, The hydrophobic effect ( Wiley-Interscience, New York, 1973): H. Wennerstriim and B. Lindmann, Phys. Rept. 52 (1978) 1; H.F. Eicke, in: Micellization, solubilization and microemulsion, Vol. 1, ed. K.L. Mittal (Plenum Press, New York, 1977) p. 429. [2] R. Gopal and J.R. Sin&J. Phys. Chem. 77 (1973) 554. [3] H.N. Singh,S.M. Saleem,R.P.SinghandK.S. Birdi, J.Phys. Chem. 84 (1980) 2191. [ 41 H.N. Singh, S. Singh and K.C. Tawari, J. Am. Oil Chem. sot. 52 (1975) 436.
Volume 144, number 1
CHEMICAL PHYSICS LETTERS
[5] P. Mukerjee and JR. Cardinal, J. Pharm. Sci. 65 (1976) 882. [ 61 AS. Kertcs and H. Gutman, in: Surface and colloid science, Vol. 8, ed. E. Matijevic, (Wiley, New York, 1976). [ 71 E.D. Handel and H.J. Ache, J. Chem. Phys. 71 (1979) 2083. [ 81 Y.C. Jean, B. Djermouni and H.J. Ache, in: Solution chemistry of sutfactants, ed. K.L. Mittal (Plenum Press, New York, 1978). [ 91 J.B. Nicholas and H.J. Ache, J. Chem. Phys. 57 (1972) 1597. [ lo] Y. Ito and Y. Tabata, Chem. Phys. 15 (1972) 584. [ 111 H.J. Ache, ed., Positronium and muonium chemistry, Advan. Chem. Ser. No. 175 (1979). [ 121 P.R. Gray, C.F. Cook and G.P. Sturm, J. Chem. Phys. 48 (1968) 1145.
86
12 February 1988
[ 131 JCh. Abbe, G. DupHtre, A.G. Maddock and A. Haessler, Radiat. Phys. Chem. 15 (1980) 6 17. [ 141 Z.B. Alfassiand H.J. Ache, J. Phys. Chem. 88 (1984) 4347. [ 151 P. Kirkegaard, M. Eldrup, O.E. M0gensenandN.J. Pedersen, Computer Phys. Commun. 23 (198 1) 307. [ 16] H.F. Eicke, private communication. [ 171 N. Muller, J. Colloid Interface Sci. 63 (1978) 383. [ 181 I. Reich, J. Phys. Chem. 60 (1956) 257. [19lA.W.RalstonandC.W.Hoerr,J.Am.Chem.Soc.68(1946) 851. [ 20] M. Almgren, S. Swarup and J.E. tifroth, J. Phys. Chem. 89 (1985) 4621. [ 211 I. Rico and A. Lattes, J. Phys. Chem. 90 (1986) 5870.