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Fluorescence studies on the characterization of mixed micelles
Journal of Molecular Liquids, 45 (1990) 95-100
95
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FLUORESCENCE STUDIES ON THE CHARACTERIZATION OF MIXED MICELLES M.M. Vel~izquez, I. Garcfa-Mateos, F. Lorente, M. Valero and L.J. Rodrfguez Dpto de Qufmica ffsica. Universidad de Salamanca. Apdo 449. E-37080. Salamanca. Spain
ABSTRACT The characterization and study of the polarity of mixed micelles of sodium decyl and sodium dodecyl sulfate and sodium cholate by fluorescence probing have been carded out. Pyrene and Pyrene-l-carboxaldehyde were used. The obtained results allow the discussion of the changes of micelle structure with composition. INTRODUCTION Knowledge of the properties and behaviour of systems containing two or more surfactants is of great interest from the technological point of view because most surfactant substances employed in daily applications are formed by mixtures of surfactants. A fundamental aspect in the knowledge of these systems is their characterization and later thermodynamic study with a view to gaining insight into the distribution of the surfactant monomers between the water and micelle phases since the structure and behaviour of these aggregates depends on this distribution. This kind of study is the basis of many works carried out on these systems (ref. 1). After their characterization, it is necessary to gain information on the structure of mixed micelles and their variation with composition. To do so, photophysical and photochemical methods can be used.These have been successful in the study of the properties of macromolecular systems (ref.2), and of homomicelles, and are now beginning to be used to examine the nature of mixed micellar aggregates (refs 3 and 4). The present work studies the polarity of the inside and of the micelle-water interface of mixed micelles of sodium decyl and sodium dodecyl sulfate (SdeS/SDS) and sodium dodecyl sulfate and sodium cholate (SDS/NaC) by fluorescence probing. The probes used were Pyrene and Pyrene1-carboxaldehyde.
EXPERIMENTAL SEC'rlON Materials.Sodium decyl and sodium dodecyl sulfate (Merk) and Sodium cholate (Merk) were used as received. Pyrene (Merk) was purified by gel chromatography and Pyrene-l-carboxaldehyde (Aldrich) was recrystallized several times from 95% ethanol. Solutions of SDS/NaC were prepared with aqueous solutions of phosphate buffer and were adjusted to pH=8.0. The total concentration of sodium ion of the buffer components was 8.4 raM. Methods. Introduction of Pyrene and Pyrene-1-carboxaldehyde into the mixed surfactant solutions was as follows: appropriate quantities of a concentration of these probes in ethanol were placed in a
0167-7322/90/$03.50
© 1990 Elsevier Science Publishers B.V.
96 volumetric flask and the solvent was evaporated off by passing N2 through. The solution formed by binary mixtures of the above surfactants at a constant mole fraction was added and the resulting solution was shaken until the probes had been solubilized. The solubilization of the probes in the aggregates was periodically monitored by the absorption spectrum of the solutions. Absorption spectra were recorded on a Beckman DU-7 spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-540 spectrofluorimeter. Fluorescence decays were obtained using time-correlated single photon counting. Excitation was made with a lamp filled with N2 (kexc = 337 nm) (Edimburg Instruments) and the emission of the sample was measured at 375 nm with a Philips XP2020Q photomultipiier. Alternate measurements of the pulse profile (1000 counts at the maximum per sample) and sample emission were performed until 2 104 counts at the maximum were reached. The fluorescence decays were deconvolved using a program based on the modulation functions method ( Software package SANDBOX for digital PDP 11/73 and VAX 780). Previous characterization of the mixtures was performed by a conductimetric method. Conductivity was measured with a Crison 533 conductivity bridge with an Ingold cell of 0.991 + 0.001 cm"1 cell constant. All measurements were carried out at 30 _+0.1° C.
RESULTS AND DISCUSSION. Characterization of mixed micellar a~wre~ates. Before starting the study with photochemical techniques of the structure of the mixed rnicelles formed by SdeS/SDS and SDS,rNaC, it was necessary to determine the critical micelle concentration (CMC) of the mixed aggregates. A conductimetric technique based on a deconvolution methodology of the conductivity/concentrationcurves was used. This was developed to determine the CMC of binary mixtures of ionic surfactants and has been successfully applied to mixtures of cationic surfactants (ref.5). The CMC at different mole fractions are shown in Table 1. From these values it was possible to obtain a quantitative description of the micellar aggregates formed, using the pseudophase separation model that relates the CMC of the mixed aggregate, CMC*, to that of the pure components, Ci, by the expression (ref.6): 1/CMC* = • xi / fi Ci
(1)
where xi is the mole fraction and fi is the activity coefficient of each component. For solutions with ideal behaviour, fi=l. This is the case of the system formed by sodium decyl and sodium dodecyl sulfate. However, the mixture formed of SDS/NaC deviates from ideal behaviour. In this case the activity coefficients are defined according to the regular solutions model (ref.6) thus: fi = exp [(Wij/RT) (1-yi)2]
(2)
where Wij is the energy parameter of the interaction and Yi is the mole fraction of the surfactant in the mixed micelle. The indexes refer to each of the surfactants forming the mixture.
97 The experimental results for the SDS/NaC system fit equations 1 and 2, with a value for the interaction energy parameter Wij = -0.61. From this model it is possible to determine the mole fraction of each of the surfactants in the mixed miceUe, as shown in Table 1. TABLE 1 XNaC CMC*/mM CMC*(a)/mM YNaC 0.193 0.627 0.644 0.130 0.398 0.695 0.708 0.263 0.495 0.755 0.751 0.335 0.601 0.810 0.809 0.413 0.799 1.000 0.983 0.609 (a) calculated from eqs 1 and 2. In the fight of these findings, it is possible to observe a marked difference in the behaviour of the two binary mixtures studied. This difference could be explained in terms of the notion that surfactants with a similar structure (SdeS/SDS) mix ideally, because the environment of the hydrophobic and hydrophilic groups in the mixed micelle is similar to that of the homomiceUe. Conversely, on substituting SdeS by sodium cholate, the structural differences between these two surfactants may lead to disturbances in the system, which would be responsible for the kind of deviation in behaviour observed. The results of the photophysical study detailed below may corroborate this interpretation. Fluorescence of Pyrene in mixed anionic micelles. The polarity inside the micelle was obtained from the ratio of the intensities of the ftrst and third vibration bands of the fluorescence spectrum of Pyrene I1/I 3 (ref.7) and from their fluorescence lifetime (ref.8). Low values of the I1/I3 ratio correspond to a non-polar environment and this ratio increases as the polarity of the solvent rises (teL7). Regarding the fluorescence lifetime, it decreases as the polarity of the medium rises. Since the Pyrene is solubilized in a zone of the micelle between the interface and the inside of it, that is the micelle palisade layer (ref.9), the information obtained from the fluorescence of Pyrene refers to this zone of the micelle. This methodology has been applied to several homomicellar systems (ref.2), but only in recent years it has been applied to certain studies on surfactant mixtures (refs.3 and 4). The plot of the ratio of the intensities of the emission bands of Pyrene against total concentration shows a typical profile that does not vary when working with mixtures containing a constant mole fraction of surfactant. At concentrations much lower than the CMC the 1:3 peak ratio has a value of 1.80, corresponding to an aqueous solution of Pyrene. As the total concentration of surfactant increases, this ratio decreases, indicating that the Pyrene passes to a more hydrophobic environment until a particular concentration is reached at which the 1:3 peak ratio remains constant. Therefore, that points to stabilization of the mixed micelle. These values would correspond to the Pyrene in the mixed micelle and their variation with the mole fraction is shown in Figure 1 for the SdeS/SDS and SDS/NaC systems, respectively.
98 On the other hand, the fluorescence lifetime of Pyrene was determined at different mole fractions of surfactant and at a total concentration of the surfactant at which formation of the corresponding mixed micelle is guaranteed. These concentrations were 20.0mM for the SdeS/SDS mixture, (40.0mM for homomicellar SdeS) and 16.0mM for SDS/NaC. The fluorescence lifetime was obtained by a single photon counting technique. In most cases the fluorescence decay curves fitted three exponentials. To assign them to a given process, the kinetic curve corresponding to Pyrene in homomicellar SDS was considered, as this decay kinetics is well known. The exponential corresponding to the longest lifetime, Xo= 142.3 ns, coincides with the value assigned in the literature to the emission of the Pyrene monomer in homomicellar SDS solutions with oxygen (ref. 10). The second exponential of intermediate lifetime, XE=62.2 ns, was assigned to the formation of the excimer in SDS (ref.11). The formation of excimer at the working Pyrene concentration (=10 -6 M) is not observed in the fluorescence emission spectrum where a characteristic emission maximum should appear between 440 and 450 nm. On analyzing this kind of behaviour in terms of the percentage of intensity emitted, this emission is of the order of 5%, indicating the presence of a very small amount of excimer formed, for this reason it could not be detected in the emission spectrum. The third exponential, with the shortest lifetime (x3= 1 ns), with a percentage of fluorescence intensity of less than 1%, is found in many probe kinetic curves in micellar systems and is ascribed to disperse radiation. Under these conditions, the mean lifetime was determined at different mole fraction of surfactant. Figure 1 shows the plots of these values together with the I1/I3 intensity ratio for the SdeS/SDS and SDS/NaC mixtures, respectively. 160
~'5t11/13
QSdeS/SDS
1,4
280
/ns
SDS/NaC
140 1,4
I1/I
•
120
x
Ins
240
1,2
1,3
1,2
/°
80
200
?°
100
1,0 '
m
•
m
160
1'1t
60 X~C ,
1,0
•
0,0
,
=
0,2
,
.
•
,
.
.X,s_S.D
40
•
0,4
0,6
0,8
,0
0,8
• 0,0
• = , 0,2
• , • 0,4
•
, • 0,6
Fig. 1. Variation of the 11/13ratio and of the lifetime of Pyrene with mole fraction
•
, • 0,8
120
• ,0
99 Joint observation of these results leads to the following considerations: i) The Ilfl 3 ratio and lifetime values in the SdeS/SDS mixture do not seem to be affected by the composition of the mixture and are close to the values found for homomicellar surfactants. These values (I1/13= 1.2; x= 139 ns) indicate that the Pyrene is within a zone close to the interface, i.e. the micelle palisade layer. This kind of behavjour has been observed in SDS homomicelles (ref. 10). ii) In the SDS/NaC mixture, both the intensity ratio and fluorescence lifetime vary with composition, their values lying within those corresponding to homomicellar surfactants. O n increasing the mole fraction of NaC, the lifetime of fluorescence increases and the 11/I3 ratio decreases. This behaviour is due to the fact that as the proportion of sodium chelate increases in the micelle, the Pyrene moves from the palisade layer to more internal hydrophobic layers. These results are in agreement with the thermodynamic study of the previous section showing that the SDS/NaC mixture exhibits non-ideal behaviour. The results of the photochemical study indicate the differences in the structure of both surfactants forming the mixture. Such differences are so important that on mixing the two surfactants the new structure of the mixed miceUe ranges between two limiting structures corresponding to both homomicelles. Fluorescence of Pvrene-l-carboxaldehvde in mixed anionic micelles. The polarity of the micellar interface was determined from the emission maximum of Pyrene-l-carboxaldehyde, which displays a red shift on increasing the polarity of the solvent. The shift of the absorption maximum is linear with the dielectric constant for values of the latter ranging between 10 and 80 (ref.12). Studies carried out with NMR spectroscopy show that this probe is solubilized on the micellar surface (ref.13), in consequence the study of the shift of the emission band in micellar aggregates provides information about the characteristics of the micelle-water interface. This methodology was employed to study the micellar interface of homomicellar systems (ref. 2) but no information is available concerning its application in systems formed by mixtures of surfactants. The emission spectra of Pyrene 1-carboxaldehyde were obtained in solutions of constant mole fraction, but varying the total surfactant concentration. The excitation wavelength in all cases was 370 nm. The dependence of the wavelength of the emission maximum on the total concentration of surfactant at a constant mole fraction was similar to that obtained for the intensity ratio (11/13) in the case of Pyrene and its interpretation is also similar. From these curves it is possible to obtain the ~-max of Pyrene-l-carboxaldehyde in the mixed micelles formed at different mole fractions of surfactant. From the linearity between ~'maxand the dielectric constant of the solvent (rcf.12), the polarity of the interface was determined. The results are shown in Fig 2 for the SdeS/SDS and SDS/NaC mixtures, respectively. From this behaviour, it may be seen that in the SdeS/SDS mixture, the dielectric constant does not depend on composition, and the value of the respective homomicelles is maintained. This
100
result is similar to that obtained for the palisade layer of these mixed micelles. All that indicates that the structure of such mixed micelles is very similar to that of the corresponding homomicelles. Conversely, the dielectric constant of the interface of the mixed micelles of SDS/NaC decreases considerably as the mole fraction of sodium cholate increases. This decrease is also in agreement with the decrease in the polarity of the internal part of these mixed micelles. Finally, since the dielectric constant decreases on increasing the mole fraction of sodium cholate, thus reducing charge density, it could be speculated that less work would be required to insert a monomer into the mixed micelle, so that the CMC should decrease when the mole fraction of sodium cholate is increased. However, the results obtained indicate the opposite kind of behaviour. That may confh-m the fact already reported that hydrophobic interactions mainly participate in the micellar association of bile salts being~electrostatic interactions of less importance (ref. 14). 52
48
SdeS/SDS
SDS/NaC
46!
.
50 m
[]
I~
[]
i
m []
rn
[]
[]
44 m
48
m
42 2
x 46
•
0,0
i
0,2
•
•
i
0,4
•
•
!
0,6
•
'
i
0,8
XN~
SDS 40
•
,0
• 0,0
•
= 0,2
•
•
, 0,4
•
•
~ 0,6
•
•
~ 0,8
• 1,0
Fig.2. Variation of the dielectric constant of the micellar interface with the mole fraction. Acknowledgement We are grateful to Dr. Silvia B. Costa and Dr. A. Ma~anita who have put the single photon counting at our disposal. We also acknowledge their suggestions in the discussion of results. This work has been in part supported by Excma Diputaci6n Provincial de Salamanca. REFERENCES 1 J.F. Scamehorn ed. Phenomena in Mixed Surfactant Systems, A.C.S. Washington, 1977. 2 K.Kalyanasundaram Photochemistry in Microheterogeneous Systems. Academic Press,. London, 1987. 3 I. Muto, K. Esumi, K. Meguro and R. Zana, J. Colloid Interface Sci., 120 (1987) 162-171. 4 K.Kalyanasundaram, Langmuir, 4 (1988) 942-945. 5 I. Garcfa-Mateos, M.M. Vel~izquez and L.J.Rodn'guez, Langmuir submitted 6 D.N. Rubing in Solution Chemistry of Surfactants vol 1, K.L.Mittal ed, Plenum Press, New York, 1979 7 K.Kalyanasundaram and J.K.Thomas, J.Am.Chem.Soc 99 (1977) 2039-2044 8 K.A.Zachariasse, B.Kozankiewicz and W.Kuhnle in Surfactants in Solution vol. 1 K.L.Mittal ed. Plenum Press, New York 1984. 9 J. Ulmius, B. Lindman, G. Lindblom and T. Drakenberg J.Colloid Interface Sci 65 (1978) 88-97 10 M.W. Geiger and N.J. Turro Photochem Photobiol. 22 (1975) 273-276 11 S.S. Atik, M. Nam and L. Singer, Chem Phys Letter 67 (1979) 75-80 12 K.Kalyanasundaram and J.K. Thomas, J.Phys. Chem. 81 (1977) 2176-2180 13 J.C. Erikkson and G. Gillbert, Acta Chem. Scan 20 (1966) 2019-2024 14 N.A Mazer, M.C.Carey, R.F.Kuvasnick and G.B.Benedek,Biochemistry 18 (1979) 3064-3068
95
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
FLUORESCENCE STUDIES ON THE CHARACTERIZATION OF MIXED MICELLES M.M. Vel~izquez, I. Garcfa-Mateos, F. Lorente, M. Valero and L.J. Rodrfguez Dpto de Qufmica ffsica. Universidad de Salamanca. Apdo 449. E-37080. Salamanca. Spain
ABSTRACT The characterization and study of the polarity of mixed micelles of sodium decyl and sodium dodecyl sulfate and sodium cholate by fluorescence probing have been carded out. Pyrene and Pyrene-l-carboxaldehyde were used. The obtained results allow the discussion of the changes of micelle structure with composition. INTRODUCTION Knowledge of the properties and behaviour of systems containing two or more surfactants is of great interest from the technological point of view because most surfactant substances employed in daily applications are formed by mixtures of surfactants. A fundamental aspect in the knowledge of these systems is their characterization and later thermodynamic study with a view to gaining insight into the distribution of the surfactant monomers between the water and micelle phases since the structure and behaviour of these aggregates depends on this distribution. This kind of study is the basis of many works carried out on these systems (ref. 1). After their characterization, it is necessary to gain information on the structure of mixed micelles and their variation with composition. To do so, photophysical and photochemical methods can be used.These have been successful in the study of the properties of macromolecular systems (ref.2), and of homomicelles, and are now beginning to be used to examine the nature of mixed micellar aggregates (refs 3 and 4). The present work studies the polarity of the inside and of the micelle-water interface of mixed micelles of sodium decyl and sodium dodecyl sulfate (SdeS/SDS) and sodium dodecyl sulfate and sodium cholate (SDS/NaC) by fluorescence probing. The probes used were Pyrene and Pyrene1-carboxaldehyde.
EXPERIMENTAL SEC'rlON Materials.Sodium decyl and sodium dodecyl sulfate (Merk) and Sodium cholate (Merk) were used as received. Pyrene (Merk) was purified by gel chromatography and Pyrene-l-carboxaldehyde (Aldrich) was recrystallized several times from 95% ethanol. Solutions of SDS/NaC were prepared with aqueous solutions of phosphate buffer and were adjusted to pH=8.0. The total concentration of sodium ion of the buffer components was 8.4 raM. Methods. Introduction of Pyrene and Pyrene-1-carboxaldehyde into the mixed surfactant solutions was as follows: appropriate quantities of a concentration of these probes in ethanol were placed in a
0167-7322/90/$03.50
© 1990 Elsevier Science Publishers B.V.
96 volumetric flask and the solvent was evaporated off by passing N2 through. The solution formed by binary mixtures of the above surfactants at a constant mole fraction was added and the resulting solution was shaken until the probes had been solubilized. The solubilization of the probes in the aggregates was periodically monitored by the absorption spectrum of the solutions. Absorption spectra were recorded on a Beckman DU-7 spectrophotometer. Fluorescence spectra were recorded on a Shimadzu RF-540 spectrofluorimeter. Fluorescence decays were obtained using time-correlated single photon counting. Excitation was made with a lamp filled with N2 (kexc = 337 nm) (Edimburg Instruments) and the emission of the sample was measured at 375 nm with a Philips XP2020Q photomultipiier. Alternate measurements of the pulse profile (1000 counts at the maximum per sample) and sample emission were performed until 2 104 counts at the maximum were reached. The fluorescence decays were deconvolved using a program based on the modulation functions method ( Software package SANDBOX for digital PDP 11/73 and VAX 780). Previous characterization of the mixtures was performed by a conductimetric method. Conductivity was measured with a Crison 533 conductivity bridge with an Ingold cell of 0.991 + 0.001 cm"1 cell constant. All measurements were carried out at 30 _+0.1° C.
RESULTS AND DISCUSSION. Characterization of mixed micellar a~wre~ates. Before starting the study with photochemical techniques of the structure of the mixed rnicelles formed by SdeS/SDS and SDS,rNaC, it was necessary to determine the critical micelle concentration (CMC) of the mixed aggregates. A conductimetric technique based on a deconvolution methodology of the conductivity/concentrationcurves was used. This was developed to determine the CMC of binary mixtures of ionic surfactants and has been successfully applied to mixtures of cationic surfactants (ref.5). The CMC at different mole fractions are shown in Table 1. From these values it was possible to obtain a quantitative description of the micellar aggregates formed, using the pseudophase separation model that relates the CMC of the mixed aggregate, CMC*, to that of the pure components, Ci, by the expression (ref.6): 1/CMC* = • xi / fi Ci
(1)
where xi is the mole fraction and fi is the activity coefficient of each component. For solutions with ideal behaviour, fi=l. This is the case of the system formed by sodium decyl and sodium dodecyl sulfate. However, the mixture formed of SDS/NaC deviates from ideal behaviour. In this case the activity coefficients are defined according to the regular solutions model (ref.6) thus: fi = exp [(Wij/RT) (1-yi)2]
(2)
where Wij is the energy parameter of the interaction and Yi is the mole fraction of the surfactant in the mixed micelle. The indexes refer to each of the surfactants forming the mixture.
97 The experimental results for the SDS/NaC system fit equations 1 and 2, with a value for the interaction energy parameter Wij = -0.61. From this model it is possible to determine the mole fraction of each of the surfactants in the mixed miceUe, as shown in Table 1. TABLE 1 XNaC CMC*/mM CMC*(a)/mM YNaC 0.193 0.627 0.644 0.130 0.398 0.695 0.708 0.263 0.495 0.755 0.751 0.335 0.601 0.810 0.809 0.413 0.799 1.000 0.983 0.609 (a) calculated from eqs 1 and 2. In the fight of these findings, it is possible to observe a marked difference in the behaviour of the two binary mixtures studied. This difference could be explained in terms of the notion that surfactants with a similar structure (SdeS/SDS) mix ideally, because the environment of the hydrophobic and hydrophilic groups in the mixed micelle is similar to that of the homomiceUe. Conversely, on substituting SdeS by sodium cholate, the structural differences between these two surfactants may lead to disturbances in the system, which would be responsible for the kind of deviation in behaviour observed. The results of the photophysical study detailed below may corroborate this interpretation. Fluorescence of Pyrene in mixed anionic micelles. The polarity inside the micelle was obtained from the ratio of the intensities of the ftrst and third vibration bands of the fluorescence spectrum of Pyrene I1/I 3 (ref.7) and from their fluorescence lifetime (ref.8). Low values of the I1/I3 ratio correspond to a non-polar environment and this ratio increases as the polarity of the solvent rises (teL7). Regarding the fluorescence lifetime, it decreases as the polarity of the medium rises. Since the Pyrene is solubilized in a zone of the micelle between the interface and the inside of it, that is the micelle palisade layer (ref.9), the information obtained from the fluorescence of Pyrene refers to this zone of the micelle. This methodology has been applied to several homomicellar systems (ref.2), but only in recent years it has been applied to certain studies on surfactant mixtures (refs.3 and 4). The plot of the ratio of the intensities of the emission bands of Pyrene against total concentration shows a typical profile that does not vary when working with mixtures containing a constant mole fraction of surfactant. At concentrations much lower than the CMC the 1:3 peak ratio has a value of 1.80, corresponding to an aqueous solution of Pyrene. As the total concentration of surfactant increases, this ratio decreases, indicating that the Pyrene passes to a more hydrophobic environment until a particular concentration is reached at which the 1:3 peak ratio remains constant. Therefore, that points to stabilization of the mixed micelle. These values would correspond to the Pyrene in the mixed micelle and their variation with the mole fraction is shown in Figure 1 for the SdeS/SDS and SDS/NaC systems, respectively.
98 On the other hand, the fluorescence lifetime of Pyrene was determined at different mole fractions of surfactant and at a total concentration of the surfactant at which formation of the corresponding mixed micelle is guaranteed. These concentrations were 20.0mM for the SdeS/SDS mixture, (40.0mM for homomicellar SdeS) and 16.0mM for SDS/NaC. The fluorescence lifetime was obtained by a single photon counting technique. In most cases the fluorescence decay curves fitted three exponentials. To assign them to a given process, the kinetic curve corresponding to Pyrene in homomicellar SDS was considered, as this decay kinetics is well known. The exponential corresponding to the longest lifetime, Xo= 142.3 ns, coincides with the value assigned in the literature to the emission of the Pyrene monomer in homomicellar SDS solutions with oxygen (ref. 10). The second exponential of intermediate lifetime, XE=62.2 ns, was assigned to the formation of the excimer in SDS (ref.11). The formation of excimer at the working Pyrene concentration (=10 -6 M) is not observed in the fluorescence emission spectrum where a characteristic emission maximum should appear between 440 and 450 nm. On analyzing this kind of behaviour in terms of the percentage of intensity emitted, this emission is of the order of 5%, indicating the presence of a very small amount of excimer formed, for this reason it could not be detected in the emission spectrum. The third exponential, with the shortest lifetime (x3= 1 ns), with a percentage of fluorescence intensity of less than 1%, is found in many probe kinetic curves in micellar systems and is ascribed to disperse radiation. Under these conditions, the mean lifetime was determined at different mole fraction of surfactant. Figure 1 shows the plots of these values together with the I1/I3 intensity ratio for the SdeS/SDS and SDS/NaC mixtures, respectively. 160
~'5t11/13
QSdeS/SDS
1,4
280
/ns
SDS/NaC
140 1,4
I1/I
•
120
x
Ins
240
1,2
1,3
1,2
/°
80
200
?°
100
1,0 '
m
•
m
160
1'1t
60 X~C ,
1,0
•
0,0
,
=
0,2
,
.
•
,
.
.X,s_S.D
40
•
0,4
0,6
0,8
,0
0,8
• 0,0
• = , 0,2
• , • 0,4
•
, • 0,6
Fig. 1. Variation of the 11/13ratio and of the lifetime of Pyrene with mole fraction
•
, • 0,8
120
• ,0
99 Joint observation of these results leads to the following considerations: i) The Ilfl 3 ratio and lifetime values in the SdeS/SDS mixture do not seem to be affected by the composition of the mixture and are close to the values found for homomicellar surfactants. These values (I1/13= 1.2; x= 139 ns) indicate that the Pyrene is within a zone close to the interface, i.e. the micelle palisade layer. This kind of behavjour has been observed in SDS homomicelles (ref. 10). ii) In the SDS/NaC mixture, both the intensity ratio and fluorescence lifetime vary with composition, their values lying within those corresponding to homomicellar surfactants. O n increasing the mole fraction of NaC, the lifetime of fluorescence increases and the 11/I3 ratio decreases. This behaviour is due to the fact that as the proportion of sodium chelate increases in the micelle, the Pyrene moves from the palisade layer to more internal hydrophobic layers. These results are in agreement with the thermodynamic study of the previous section showing that the SDS/NaC mixture exhibits non-ideal behaviour. The results of the photochemical study indicate the differences in the structure of both surfactants forming the mixture. Such differences are so important that on mixing the two surfactants the new structure of the mixed miceUe ranges between two limiting structures corresponding to both homomicelles. Fluorescence of Pvrene-l-carboxaldehvde in mixed anionic micelles. The polarity of the micellar interface was determined from the emission maximum of Pyrene-l-carboxaldehyde, which displays a red shift on increasing the polarity of the solvent. The shift of the absorption maximum is linear with the dielectric constant for values of the latter ranging between 10 and 80 (ref.12). Studies carried out with NMR spectroscopy show that this probe is solubilized on the micellar surface (ref.13), in consequence the study of the shift of the emission band in micellar aggregates provides information about the characteristics of the micelle-water interface. This methodology was employed to study the micellar interface of homomicellar systems (ref. 2) but no information is available concerning its application in systems formed by mixtures of surfactants. The emission spectra of Pyrene 1-carboxaldehyde were obtained in solutions of constant mole fraction, but varying the total surfactant concentration. The excitation wavelength in all cases was 370 nm. The dependence of the wavelength of the emission maximum on the total concentration of surfactant at a constant mole fraction was similar to that obtained for the intensity ratio (11/13) in the case of Pyrene and its interpretation is also similar. From these curves it is possible to obtain the ~-max of Pyrene-l-carboxaldehyde in the mixed micelles formed at different mole fractions of surfactant. From the linearity between ~'maxand the dielectric constant of the solvent (rcf.12), the polarity of the interface was determined. The results are shown in Fig 2 for the SdeS/SDS and SDS/NaC mixtures, respectively. From this behaviour, it may be seen that in the SdeS/SDS mixture, the dielectric constant does not depend on composition, and the value of the respective homomicelles is maintained. This
100
result is similar to that obtained for the palisade layer of these mixed micelles. All that indicates that the structure of such mixed micelles is very similar to that of the corresponding homomicelles. Conversely, the dielectric constant of the interface of the mixed micelles of SDS/NaC decreases considerably as the mole fraction of sodium cholate increases. This decrease is also in agreement with the decrease in the polarity of the internal part of these mixed micelles. Finally, since the dielectric constant decreases on increasing the mole fraction of sodium cholate, thus reducing charge density, it could be speculated that less work would be required to insert a monomer into the mixed micelle, so that the CMC should decrease when the mole fraction of sodium cholate is increased. However, the results obtained indicate the opposite kind of behaviour. That may confh-m the fact already reported that hydrophobic interactions mainly participate in the micellar association of bile salts being~electrostatic interactions of less importance (ref. 14). 52
48
SdeS/SDS
SDS/NaC
46!
.
50 m
[]
I~
[]
i
m []
rn
[]
[]
44 m
48
m
42 2
x 46
•
0,0
i
0,2
•
•
i
0,4
•
•
!
0,6
•
'
i
0,8
XN~
SDS 40
•
,0
• 0,0
•
= 0,2
•
•
, 0,4
•
•
~ 0,6
•
•
~ 0,8
• 1,0
Fig.2. Variation of the dielectric constant of the micellar interface with the mole fraction. Acknowledgement We are grateful to Dr. Silvia B. Costa and Dr. A. Ma~anita who have put the single photon counting at our disposal. We also acknowledge their suggestions in the discussion of results. This work has been in part supported by Excma Diputaci6n Provincial de Salamanca. REFERENCES 1 J.F. Scamehorn ed. Phenomena in Mixed Surfactant Systems, A.C.S. Washington, 1977. 2 K.Kalyanasundaram Photochemistry in Microheterogeneous Systems. Academic Press,. London, 1987. 3 I. Muto, K. Esumi, K. Meguro and R. Zana, J. Colloid Interface Sci., 120 (1987) 162-171. 4 K.Kalyanasundaram, Langmuir, 4 (1988) 942-945. 5 I. Garcfa-Mateos, M.M. Vel~izquez and L.J.Rodn'guez, Langmuir submitted 6 D.N. Rubing in Solution Chemistry of Surfactants vol 1, K.L.Mittal ed, Plenum Press, New York, 1979 7 K.Kalyanasundaram and J.K.Thomas, J.Am.Chem.Soc 99 (1977) 2039-2044 8 K.A.Zachariasse, B.Kozankiewicz and W.Kuhnle in Surfactants in Solution vol. 1 K.L.Mittal ed. Plenum Press, New York 1984. 9 J. Ulmius, B. Lindman, G. Lindblom and T. Drakenberg J.Colloid Interface Sci 65 (1978) 88-97 10 M.W. Geiger and N.J. Turro Photochem Photobiol. 22 (1975) 273-276 11 S.S. Atik, M. Nam and L. Singer, Chem Phys Letter 67 (1979) 75-80 12 K.Kalyanasundaram and J.K. Thomas, J.Phys. Chem. 81 (1977) 2176-2180 13 J.C. Erikkson and G. Gillbert, Acta Chem. Scan 20 (1966) 2019-2024 14 N.A Mazer, M.C.Carey, R.F.Kuvasnick and G.B.Benedek,Biochemistry 18 (1979) 3064-3068