- Email: [email protected]
Determination of Critical Micelle Concentration of Anionic Surfactants: Comparison of Internal and External Fluorescent Probes
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
189, 177–180 (1997)
CS974804
NOTE Determination of Critical Micelle Concentration of Anionic Surfactants: Comparison of Internal and External Fluorescent Probes
Fluorescence spectroscopy has been utilized to determine the critical micelle concentrations (CMC) of anionic surfactants by external and internal probes and the results have been compared to those obtained by the surface tension method. The selection of anionic surfactants gave us the flexibility of having the probe within the surfactant molecule and having it reside at the palisade layer or in the micellar core. It is shown that the CMC value is not affected by the nature (whether part of the surfactant molecule or externally added) and location of the probe within the micelle. CMC values determined by surface tension and fluorescence techniques using external and internal probes agree quite well in linear alkylbenzene sulfonate (LAS) and secondary alkyl sulfonate (SAS) systems. q 1997 Academic Press Key Words: LAS; SAS; CMC; surface tension; fluorescence.
INTRODUCTION Anionic surfactants have a wide range of household and industrial applications (1). In recent years, the aggregation behavior of linear alkylbenzene sulfonate (LAS) isomers and their micelle structure have been investigated by a variety of techniques, such as NMR spectroscopy (2–4), fluorescence spectroscopy (5–10), and small angle neutron scattering (11). In the determination of the CMC by fluorescence spectroscopy, the nature of the probe, whether it is external or internal (present within the molecule under investigation), and the exact location of the probe, whether it resides in the micellar core or at the palisade layer of the micelle, are of paramount importance. In either case, it is essential that the probe is ideally distributed in the micelle such that the spectroscopic and molecular dynamical parameters of all probe molecules are identical. This would require (i) a sufficiently high partition coefficient for the probe for micelles so that there is a negligible amount of the probe in the aqueous phase, (ii) a single site of solubilization for the probe in the micelle, and (iii) absence of alternative structures (dimers, aggregates, acid–base forms, etc. which have different fluorescent and dynamical properties) for the probe (8, 9). Fluorescent methods using external probes, specifically using large aromatic molecules such as pyrene, have often been criticized for failing to reveal the exact location of the probe (8). In the present paper, fluorescence spectroscopy has been utilized to determine the critical micelle concentration (CMC) of anionic surfactants by external and internal probes and the results have been compared to those obtained by the surface tension method. The selection of anionic surfactants gave us the flexibility of having the probe within the surfactant molecule. The anionic surfactants selected are LAS and secondary alkyl sulfonates (SAS). The only difference between the former and the latter is the absence of a phenyl ring in the latter class. The structures of these surfactants are given in Fig. 1. The phenyl ring served as the internal fluorescent probe in
the study of LAS micellar systems and earlier we have shown by NMR studies that the phenyl ring resides at the palisade layer with the water boundary between ortho and meta protons (to the sulfonate group) (3). In the case of SAS micellar systems, we used externally added phenol as the fluorescent probe which resided at the palisade layer of these micelles. In both cases, pyrene is used as an external fluorescent probe located at the micellar core. In Fig. 1 we show the micelle structures of LAS and SAS / Phenol systems.
MATERIALS AND METHODS A sample of sodium salt of LAS was prepared in the laboratory by neutralizing commercial linear alkylbenzene sulfonic acid (ex. Reliance industries, Bombay) with sodium hydroxide solution in the Z-blade (Sigma) mixture. The paste of the Na–LAS obtained was dried in an oven at around 607C, followed by vacuum drying. This dried commercial Na–LAS was subjected to Soxhlet solvent extraction using hexane to remove nondetergent organic matter. This paste was once again dried in vacuum followed by another round of Soxhlet extraction with methanol. The solvent was evaporated to obtain pure sodium salt of LAS. The distribution of the alkyl chain length was determined by fast atom bombardment mass spectroscopy. The molecular weight was 338 and its chain length distribution was C9 Å 1%, C10 Å 25%, C11 Å 34%, C12 Å 26%, C13 Å 12%, C14 Å 1%, and C15 Å 1%. Sodium salt of commercial secondary alkyl sulfonate (ex: Hoechst, Germany) was also analyzed by fast atom bombardment mass spectroscopy to determine its molecular weight and chain length distribution. The SAS was found to have a molecular weight of 306 and chain length distribution as C13 Å 4%, C14 Å 56%, C15 Å 33%, C14 Å 5% and C17 Å 2%. The sample was used after drying in the oven at Ç1107C. Surface tension measurements were carried out on a Kruss tensiometer K10, Kruss GmbH, Germany at 257C using the Nuoy’s ring method. The fluorescence measurements were carried out on a Hitachi F2000 spectrofluorimeter. In some cases, the results were reconfirmed on a Shimadzu spectrofluorimeter Model RF 510. The glassware used was washed with chromic acid and double distilled water. Solutions involving the use of an extrinsic probe, pyrene, were prepared by the addition of 0.25 ml of 2 1 10 04 M pyrene stock solution prepared in CHCl3 to each 25 ml volumetric flask. The flasks were left in the oven overnight to evaporate the CHCl3 . The required amount of respective surfactant solution was added and diluted to 25 ml. The final concentration of pyrene was 2 1 10 06 M. Phenol had a solubility of 6.7% in aqueous medium. Studies of SAS with phenol as the extrinsic probe were carried out maintaining the SAS:phenol mole ratio at 1:0.14. The water used for all the experiments was double distilled.
RESULTS AND DISCUSSION The LAS molecule contains a phenyl ring which served as an internal probe (See Fig. 1). The fluorescence spectrum of LAS consists of a primary
177
AID
JCIS 4804
/
6g22$$$401
04-08-97 22:44:18
0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
coidas
178
NOTE
FIG. 1. Structures of (a) linear alkylbenzene sulfonates (LAS); (b) secondary alkyl sulfonates (SAS); and (c) LAS micelle and (d) SAS micelle in the presence of phenol. The water boundary in both of the micelles is indicated.
emission peak at 289 nm and an excimer emission peak at a longer wavelength, 350 nm. The excimer to monomer ratio was shown to undergo a marked change at CMC and has been used as an indicator of the onset of micellization (5, 7) of LAS as well as comicellization of LAS with other anionic surfactants (8). The fluorescence intensity measurements of monomer and excimer peaks of LAS solutions over a range of concentrations below and above its CMC were carried out. The phenyl ring was excited at 260 nm and the monomer emission intensities at 289 nm and excimer emission intensities at 350 nm were measured. Figure 2A shows the plot of excimer to monomer intensity ratio versus the log of LAS concentration. It can be seen that the excimer to monomer intensity ratio initially remains almost constant followed by a steep rise at 2.7 mM concentration. This indicates the point at which there is a sudden increase in excimer formation due to aggregation or micellization. The CMC of this system by surface tension measurement (see Fig. 2C) is found to be 1.8 m M. Fluorescence studies on LAS solutions were also carried out using pyrene as an external probe. Pyrene is one of the fluorescent probes that has been extensively used to study aggregation of surfactants and solubilization in micellar systems (12). Recently it has been shown that pyrene molecules are located in the inner core of micelles where no water molecules exist (10). Hence the location of pyrene contrasts the location of the phenyl ring (in LAS micellar solutions) which exists at the palisade layer. The emission spectra of pyrene in micellar solutions of surfactants without a phenyl ring show a series of bands between 225 and 280 nm. However, these bands were shown to disappear in micellar solutions of surfactants with a phenyl ring such as LAS (6). We also observed a similar phenomena. In our case pyrene was excited at 339 nm and the intensities of emission at 377 ( I1 ), 383 (I3 ) and 392 nm (I5 ) were recorded. Figure 2B shows the plot of intensity ratios (I3 /I1 and I5 /I1 ) as a function of LAS concentration. Both curves show a gradual rise in the ratios with concentration. The curves are seen to reach a plateau at a concentration of 2.8 m M. The results presented in Fig. 2 indicate that the location of the probe (either in the micelle interior or at the palisade layer) or the nature of the probe (whether it is part of the surfactant molecule itself or externally added) does not alter the CMC to any significant extent, in LAS micellar systems.
AID
JCIS 4804
/
6g22$$$402
04-08-97 22:44:18
As mentioned earlier, SAS is a surfactant similar to LAS except that SAS does not contain a phenyl ring. We have observed (results to be published elsewhere) that the SAS / phenol system behaves similarly to the LAS system in many respects. For example, in NMR experiments the two terminal methyl groups of the hydrocarbon chains of LAS show a single triplet below CMC but give rise to two well-resolved triplets above the CMC (see Fig. 2 of reference 3). We attribute this to the folding back of one of the chains (possibly the shorter chain) toward the micelle surface and to experiencing the phenyl ring effect. In the case of SAS, the two terminal methyl groups of hydrocarbon chains give rise to a single triplet below and above CMC. However, addition of phenol gives rise to two wellresolved triplets. The chemical shifts of phenol neat and phenol inserted into the SAS micelle indicate that phenol inserts itself into the micelle and the water boundary lies between the ortho and meta (to the hydroxyl group) protons. This also supports earlier observations on LAS isomers by fluorescence polarization and excimer emission measurements using two different probes to investigate the micellar interface and micellar core. These studies showed that the benzene moiety resists both translational and rotational diffusion due to the preferential location of the short alkyl chain near the micelle–water interface and the long alkyl chain in the micellar core (13). In Fig. 3, we present the surface tension measurement results of SAS, without any additive, as a pure system and also in the presence of external probes, phenol and pyrene, at concentrations well below the CMC to well above the CMC. In the case of phenol, initial studies were carried out on a pure phenol system to determine the effect of phenol in lowering the surface tension of water. The surface tension of water did not show any appreciable change as a function of increasing phenol concentration up to 10 mM phenol concentrations. At higher concentrations, a very gradual decline in the surface tension was noticed and the surface tension decreased from 71.1 to 58.4 mN m01 in the 10–100 m M concentration range. This confirms the fact that like other compounds with short or no alkyl chain, phenol also does not undergo any self aggregation. Fluorescence intensity, I296 , with excitation wavelength 274 nm showed that phenol exhibits its fluorescence behavior between 1 and 3 mM after which it undergoes self quenching showing a significant decrease in the I296 values at high phenol concentration. Therefore, to monitor the effect of phenol on the behavior of SAS, the phenol concentration was kept below 2 mM. It can be seen from Fig. 3 that in the absence of any fluorescent probe the CMC of SAS determined by the surface tension method is 2.2 mM (Fig. 3A) and the
FIG. 2. CMC of LAS. (a) Excimer to monomer fluorescence intensity measurements of the internal probe (phenyl ring) showing enhancement at CMC. (b) Fluorescence intensity measurements of the external probe, pyrene, also showing enhancement at CMC. (c) Surface tension measurement confirms CMC values in (a) and (b).
coidas
179
NOTE
TABLE 1 CMC Values (mM) of Surfactant Systems by Surface Tension and Fluorescence Measurements
FIG. 3. CMC of SAS obtained by surface tension. (a) No fluorescent probe; (b) with pyrene; and (c) with phenol.
equilibrium surface tension at CMC is 34.1 mN m01 . Addition of pyrene (Fig. 3B) and phenol (Fig. 3C) did not have any appreciable effect on the CMC (Table 1) but the surface tension at the CMC decreased to 31.8 and 31.1 mN m01 , respectively. In Fig. 4, we present the fluorescence intensity measurements of SAS solutions with pyrene as an external probe residing at the micelle core and phenol as an external probe residing at the palisade layer (see Fig. 1). The intensities I5 /I1 and I3 /I1 in the case of pyrene are given in Fig. 4A. Both these values remain constant up to an SAS concentration of 2.2 m M and increase sharply above this concentration. This is the CMC and is comparable to the value obtained by the surface tension method (see Fig. 3A). The I299 intensity of phenol in the case of the SAS / phenol system is presented in Fig. 4B and the I299 maximum is seen around 4.1 mM. A compilation of the CMC values determined by surface tension and fluorescence using internal and external probes is given in Table 1. The curves obtained from the plots of surface tension measurements as a function of the logarithm of the concentration show a roughly constant slope at the beginning, followed by a flat portion (see Figs. 2C and 3). Applying the Gibbs adsorption isotherm, the surface excess is considered to be constant in the concentration range where the curve shows constant inclination, corresponding to the formation of a monomolecular layer. Above the CMC it is reasonable to suppose that the activity increases only slightly with an increase in concentration and thus the surface tension stays at a nearly constant value. From this standpoint, the departure from a straight line
System
Surface tension
Fluorescence
LAS LAS / pyrene SAS SAS / pyrene SAS / phenol
1.8 — 2.2 1.9 1.8
2.7 2.8 — 2.2 4.1
indicates the beginning of micelle formation, and the arrival at a horizontal line corresponds to its completion; this means that all of the amount added beyond this concentration dissolves in micellar state (14). Thus, CMC values from surface tension measurements at the intersection of the two lines described above precisely indicate the concentration of the onset of micellization. Plots of the fluorescence intensity ratio as a function of surfactant concentration appear as sigmoidal curves as in Fig. 4A. The sigmoidal curve shows that the pyrene is transferred from water to the nonpolar environment. The CMC may be defined in three ways: (i) the onset of formation of aggregate structures, (ii) the concentration at which the monomer surfactant concentration levels off, and (iii) the concentration of monomer at which the total concentration of the surfactant equals unity. The three sections in Fig. 4A are indicated by two critical concentrations. The region between the two concentrations where there is a steep rise in the intensity ratio can be ascribed to the start of micelle formation. The lower or starting concentration is just the onset of formation of aggregate structures while the higher concentration indicates the completion of the aggregation phenomena. In our present studies on the SAS / pyrene system by fluorescence measurements, Fig. 4A gives the lower concentration at 2.2 mM (onset of micellization) while the upper concentration (indicating the completion of micellization) is seen at 4.1 mM. This wide difference in the two concentrations is due to the presence of the different homologs and their isomers present in the commercial SAS sample. Reports in literature show that some consider the lower concentration value as the CMC (10) while others consider the upper concentration as the CMC (15). As mentioned earlier, the CMC value of the SAS / phenol system measured by fluorescence showed a higher value and we attribute this to the concentration at which micellization is complete (Fig. 4B). The results presented in Figs. 2–4 and Table 1 indicate that the CMC value is not affected by the nature (whether part of the surfactant molecule or externally added) and location of the probe within the LAS and SAS micelles.
CONCLUSION It is shown that CMC values determined by surface tension and fluorescence techniques using external and internal probes agree well in linear alkylbenzene sulfonate (LAS) and secondary alkyl sulfonate (SAS) systems.
REFERENCES
FIG. 4. CMC of SAS by fluorescence, (a) pyrene and (b) phenol. Note the fluorescence quenching in phenol system.
AID
JCIS 4804
/
6g22$$$402
04-08-97 22:44:18
1. Berth, P., and Jeschke, P., Tenside 2, 75 (1989). 2. Tezak, D., Hertel, G., and Hoffmann, H., Liquid Crystals 10, 15 (1991). 3. Das, S., Bhirud, R. G., Nayyar, N., Narayan, K. S., and Kumar, V. V., J. Phys. Chem. 96, 7454 (1992). 4. Goon, P., and Kumar, V. V., Ind. J. Chem. A 35, 182 (1996).
coidas
180
NOTE
5. Aoudia, M., and Rodgers, M. A. J., J. Am. Chem. Soc. 101(22), 6777 (1979). 6. Binana-Limbele´, W., van Os, N. M., Rupert, L. A. M., and Zana, R., J. Colloid Interface Sci. 141, 157 (1991). 7. Aoudia, M., Wade, W. H., and Rodgers, M. A. J., J. Colloid Interface Sci. 145, 493 (1991). 8. Nayyar, N., Das, S., Bhirud, R. G., Narayan, K. S., Singh, A. P., and Kumar, V. V., J. Colloid Interface Sci. 160, 496 (1993). 9. Maiti, N. C., Mazumdar, S., and Periasamy, N., J. Phys. Chem. 99, 10708 (1995). 10. Itoh, H., Ishido, S., Nomura, M., Hayakawa, T., and Mitaku, S., J. Phys. Chem. 100, 9047 (1996). 11. Caponetti, E., Trilo, R., Patience, C. H. O., Johnson, J. S., Magid, L. J., Butler, P., and Payne, K. A., J. Colloid Interface Sci. 116, 200 (1987). 12. Gratzel, M., Kalyanasundaram, K., and Thomas, J. K., J. Am. Chem. Soc. 96, 7869 (1974). 13. Aoudia, M., Rodgers, M. A. J., and Wade, W. H., J. Colloid Interface Sci. 101, 472 (1984).
AID
JCIS 4804
/
6g22$$$403
04-08-97 22:44:18
14. Shinoda, K., Nakagawa, T., Tamamushi, B., and Isemura, T., ‘‘Colloidal Surfactants.’’ Academic Press, New York, 1963. 15. Kalyanasundaram, K., and Thomas, J. K., J. Am. Chem. Soc. 99, 2039 (1977). PIYALI . GOON * C. MANOHAR† V. V. KUMAR * ,1 *Hindustan Lever Research Centre I C T Link Road, Andheri (E) Mumbai 400 099, India †Bhabha Atomic Research Centre Trombay Mumbai 400 085, India Received November 4, 1996; accepted January 30, 1997 1
To whom correspondence should be addressed.
coidas
189, 177–180 (1997)
CS974804
NOTE Determination of Critical Micelle Concentration of Anionic Surfactants: Comparison of Internal and External Fluorescent Probes
Fluorescence spectroscopy has been utilized to determine the critical micelle concentrations (CMC) of anionic surfactants by external and internal probes and the results have been compared to those obtained by the surface tension method. The selection of anionic surfactants gave us the flexibility of having the probe within the surfactant molecule and having it reside at the palisade layer or in the micellar core. It is shown that the CMC value is not affected by the nature (whether part of the surfactant molecule or externally added) and location of the probe within the micelle. CMC values determined by surface tension and fluorescence techniques using external and internal probes agree quite well in linear alkylbenzene sulfonate (LAS) and secondary alkyl sulfonate (SAS) systems. q 1997 Academic Press Key Words: LAS; SAS; CMC; surface tension; fluorescence.
INTRODUCTION Anionic surfactants have a wide range of household and industrial applications (1). In recent years, the aggregation behavior of linear alkylbenzene sulfonate (LAS) isomers and their micelle structure have been investigated by a variety of techniques, such as NMR spectroscopy (2–4), fluorescence spectroscopy (5–10), and small angle neutron scattering (11). In the determination of the CMC by fluorescence spectroscopy, the nature of the probe, whether it is external or internal (present within the molecule under investigation), and the exact location of the probe, whether it resides in the micellar core or at the palisade layer of the micelle, are of paramount importance. In either case, it is essential that the probe is ideally distributed in the micelle such that the spectroscopic and molecular dynamical parameters of all probe molecules are identical. This would require (i) a sufficiently high partition coefficient for the probe for micelles so that there is a negligible amount of the probe in the aqueous phase, (ii) a single site of solubilization for the probe in the micelle, and (iii) absence of alternative structures (dimers, aggregates, acid–base forms, etc. which have different fluorescent and dynamical properties) for the probe (8, 9). Fluorescent methods using external probes, specifically using large aromatic molecules such as pyrene, have often been criticized for failing to reveal the exact location of the probe (8). In the present paper, fluorescence spectroscopy has been utilized to determine the critical micelle concentration (CMC) of anionic surfactants by external and internal probes and the results have been compared to those obtained by the surface tension method. The selection of anionic surfactants gave us the flexibility of having the probe within the surfactant molecule. The anionic surfactants selected are LAS and secondary alkyl sulfonates (SAS). The only difference between the former and the latter is the absence of a phenyl ring in the latter class. The structures of these surfactants are given in Fig. 1. The phenyl ring served as the internal fluorescent probe in
the study of LAS micellar systems and earlier we have shown by NMR studies that the phenyl ring resides at the palisade layer with the water boundary between ortho and meta protons (to the sulfonate group) (3). In the case of SAS micellar systems, we used externally added phenol as the fluorescent probe which resided at the palisade layer of these micelles. In both cases, pyrene is used as an external fluorescent probe located at the micellar core. In Fig. 1 we show the micelle structures of LAS and SAS / Phenol systems.
MATERIALS AND METHODS A sample of sodium salt of LAS was prepared in the laboratory by neutralizing commercial linear alkylbenzene sulfonic acid (ex. Reliance industries, Bombay) with sodium hydroxide solution in the Z-blade (Sigma) mixture. The paste of the Na–LAS obtained was dried in an oven at around 607C, followed by vacuum drying. This dried commercial Na–LAS was subjected to Soxhlet solvent extraction using hexane to remove nondetergent organic matter. This paste was once again dried in vacuum followed by another round of Soxhlet extraction with methanol. The solvent was evaporated to obtain pure sodium salt of LAS. The distribution of the alkyl chain length was determined by fast atom bombardment mass spectroscopy. The molecular weight was 338 and its chain length distribution was C9 Å 1%, C10 Å 25%, C11 Å 34%, C12 Å 26%, C13 Å 12%, C14 Å 1%, and C15 Å 1%. Sodium salt of commercial secondary alkyl sulfonate (ex: Hoechst, Germany) was also analyzed by fast atom bombardment mass spectroscopy to determine its molecular weight and chain length distribution. The SAS was found to have a molecular weight of 306 and chain length distribution as C13 Å 4%, C14 Å 56%, C15 Å 33%, C14 Å 5% and C17 Å 2%. The sample was used after drying in the oven at Ç1107C. Surface tension measurements were carried out on a Kruss tensiometer K10, Kruss GmbH, Germany at 257C using the Nuoy’s ring method. The fluorescence measurements were carried out on a Hitachi F2000 spectrofluorimeter. In some cases, the results were reconfirmed on a Shimadzu spectrofluorimeter Model RF 510. The glassware used was washed with chromic acid and double distilled water. Solutions involving the use of an extrinsic probe, pyrene, were prepared by the addition of 0.25 ml of 2 1 10 04 M pyrene stock solution prepared in CHCl3 to each 25 ml volumetric flask. The flasks were left in the oven overnight to evaporate the CHCl3 . The required amount of respective surfactant solution was added and diluted to 25 ml. The final concentration of pyrene was 2 1 10 06 M. Phenol had a solubility of 6.7% in aqueous medium. Studies of SAS with phenol as the extrinsic probe were carried out maintaining the SAS:phenol mole ratio at 1:0.14. The water used for all the experiments was double distilled.
RESULTS AND DISCUSSION The LAS molecule contains a phenyl ring which served as an internal probe (See Fig. 1). The fluorescence spectrum of LAS consists of a primary
177
AID
JCIS 4804
/
6g22$$$401
04-08-97 22:44:18
0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
coidas
178
NOTE
FIG. 1. Structures of (a) linear alkylbenzene sulfonates (LAS); (b) secondary alkyl sulfonates (SAS); and (c) LAS micelle and (d) SAS micelle in the presence of phenol. The water boundary in both of the micelles is indicated.
emission peak at 289 nm and an excimer emission peak at a longer wavelength, 350 nm. The excimer to monomer ratio was shown to undergo a marked change at CMC and has been used as an indicator of the onset of micellization (5, 7) of LAS as well as comicellization of LAS with other anionic surfactants (8). The fluorescence intensity measurements of monomer and excimer peaks of LAS solutions over a range of concentrations below and above its CMC were carried out. The phenyl ring was excited at 260 nm and the monomer emission intensities at 289 nm and excimer emission intensities at 350 nm were measured. Figure 2A shows the plot of excimer to monomer intensity ratio versus the log of LAS concentration. It can be seen that the excimer to monomer intensity ratio initially remains almost constant followed by a steep rise at 2.7 mM concentration. This indicates the point at which there is a sudden increase in excimer formation due to aggregation or micellization. The CMC of this system by surface tension measurement (see Fig. 2C) is found to be 1.8 m M. Fluorescence studies on LAS solutions were also carried out using pyrene as an external probe. Pyrene is one of the fluorescent probes that has been extensively used to study aggregation of surfactants and solubilization in micellar systems (12). Recently it has been shown that pyrene molecules are located in the inner core of micelles where no water molecules exist (10). Hence the location of pyrene contrasts the location of the phenyl ring (in LAS micellar solutions) which exists at the palisade layer. The emission spectra of pyrene in micellar solutions of surfactants without a phenyl ring show a series of bands between 225 and 280 nm. However, these bands were shown to disappear in micellar solutions of surfactants with a phenyl ring such as LAS (6). We also observed a similar phenomena. In our case pyrene was excited at 339 nm and the intensities of emission at 377 ( I1 ), 383 (I3 ) and 392 nm (I5 ) were recorded. Figure 2B shows the plot of intensity ratios (I3 /I1 and I5 /I1 ) as a function of LAS concentration. Both curves show a gradual rise in the ratios with concentration. The curves are seen to reach a plateau at a concentration of 2.8 m M. The results presented in Fig. 2 indicate that the location of the probe (either in the micelle interior or at the palisade layer) or the nature of the probe (whether it is part of the surfactant molecule itself or externally added) does not alter the CMC to any significant extent, in LAS micellar systems.
AID
JCIS 4804
/
6g22$$$402
04-08-97 22:44:18
As mentioned earlier, SAS is a surfactant similar to LAS except that SAS does not contain a phenyl ring. We have observed (results to be published elsewhere) that the SAS / phenol system behaves similarly to the LAS system in many respects. For example, in NMR experiments the two terminal methyl groups of the hydrocarbon chains of LAS show a single triplet below CMC but give rise to two well-resolved triplets above the CMC (see Fig. 2 of reference 3). We attribute this to the folding back of one of the chains (possibly the shorter chain) toward the micelle surface and to experiencing the phenyl ring effect. In the case of SAS, the two terminal methyl groups of hydrocarbon chains give rise to a single triplet below and above CMC. However, addition of phenol gives rise to two wellresolved triplets. The chemical shifts of phenol neat and phenol inserted into the SAS micelle indicate that phenol inserts itself into the micelle and the water boundary lies between the ortho and meta (to the hydroxyl group) protons. This also supports earlier observations on LAS isomers by fluorescence polarization and excimer emission measurements using two different probes to investigate the micellar interface and micellar core. These studies showed that the benzene moiety resists both translational and rotational diffusion due to the preferential location of the short alkyl chain near the micelle–water interface and the long alkyl chain in the micellar core (13). In Fig. 3, we present the surface tension measurement results of SAS, without any additive, as a pure system and also in the presence of external probes, phenol and pyrene, at concentrations well below the CMC to well above the CMC. In the case of phenol, initial studies were carried out on a pure phenol system to determine the effect of phenol in lowering the surface tension of water. The surface tension of water did not show any appreciable change as a function of increasing phenol concentration up to 10 mM phenol concentrations. At higher concentrations, a very gradual decline in the surface tension was noticed and the surface tension decreased from 71.1 to 58.4 mN m01 in the 10–100 m M concentration range. This confirms the fact that like other compounds with short or no alkyl chain, phenol also does not undergo any self aggregation. Fluorescence intensity, I296 , with excitation wavelength 274 nm showed that phenol exhibits its fluorescence behavior between 1 and 3 mM after which it undergoes self quenching showing a significant decrease in the I296 values at high phenol concentration. Therefore, to monitor the effect of phenol on the behavior of SAS, the phenol concentration was kept below 2 mM. It can be seen from Fig. 3 that in the absence of any fluorescent probe the CMC of SAS determined by the surface tension method is 2.2 mM (Fig. 3A) and the
FIG. 2. CMC of LAS. (a) Excimer to monomer fluorescence intensity measurements of the internal probe (phenyl ring) showing enhancement at CMC. (b) Fluorescence intensity measurements of the external probe, pyrene, also showing enhancement at CMC. (c) Surface tension measurement confirms CMC values in (a) and (b).
coidas
179
NOTE
TABLE 1 CMC Values (mM) of Surfactant Systems by Surface Tension and Fluorescence Measurements
FIG. 3. CMC of SAS obtained by surface tension. (a) No fluorescent probe; (b) with pyrene; and (c) with phenol.
equilibrium surface tension at CMC is 34.1 mN m01 . Addition of pyrene (Fig. 3B) and phenol (Fig. 3C) did not have any appreciable effect on the CMC (Table 1) but the surface tension at the CMC decreased to 31.8 and 31.1 mN m01 , respectively. In Fig. 4, we present the fluorescence intensity measurements of SAS solutions with pyrene as an external probe residing at the micelle core and phenol as an external probe residing at the palisade layer (see Fig. 1). The intensities I5 /I1 and I3 /I1 in the case of pyrene are given in Fig. 4A. Both these values remain constant up to an SAS concentration of 2.2 m M and increase sharply above this concentration. This is the CMC and is comparable to the value obtained by the surface tension method (see Fig. 3A). The I299 intensity of phenol in the case of the SAS / phenol system is presented in Fig. 4B and the I299 maximum is seen around 4.1 mM. A compilation of the CMC values determined by surface tension and fluorescence using internal and external probes is given in Table 1. The curves obtained from the plots of surface tension measurements as a function of the logarithm of the concentration show a roughly constant slope at the beginning, followed by a flat portion (see Figs. 2C and 3). Applying the Gibbs adsorption isotherm, the surface excess is considered to be constant in the concentration range where the curve shows constant inclination, corresponding to the formation of a monomolecular layer. Above the CMC it is reasonable to suppose that the activity increases only slightly with an increase in concentration and thus the surface tension stays at a nearly constant value. From this standpoint, the departure from a straight line
System
Surface tension
Fluorescence
LAS LAS / pyrene SAS SAS / pyrene SAS / phenol
1.8 — 2.2 1.9 1.8
2.7 2.8 — 2.2 4.1
indicates the beginning of micelle formation, and the arrival at a horizontal line corresponds to its completion; this means that all of the amount added beyond this concentration dissolves in micellar state (14). Thus, CMC values from surface tension measurements at the intersection of the two lines described above precisely indicate the concentration of the onset of micellization. Plots of the fluorescence intensity ratio as a function of surfactant concentration appear as sigmoidal curves as in Fig. 4A. The sigmoidal curve shows that the pyrene is transferred from water to the nonpolar environment. The CMC may be defined in three ways: (i) the onset of formation of aggregate structures, (ii) the concentration at which the monomer surfactant concentration levels off, and (iii) the concentration of monomer at which the total concentration of the surfactant equals unity. The three sections in Fig. 4A are indicated by two critical concentrations. The region between the two concentrations where there is a steep rise in the intensity ratio can be ascribed to the start of micelle formation. The lower or starting concentration is just the onset of formation of aggregate structures while the higher concentration indicates the completion of the aggregation phenomena. In our present studies on the SAS / pyrene system by fluorescence measurements, Fig. 4A gives the lower concentration at 2.2 mM (onset of micellization) while the upper concentration (indicating the completion of micellization) is seen at 4.1 mM. This wide difference in the two concentrations is due to the presence of the different homologs and their isomers present in the commercial SAS sample. Reports in literature show that some consider the lower concentration value as the CMC (10) while others consider the upper concentration as the CMC (15). As mentioned earlier, the CMC value of the SAS / phenol system measured by fluorescence showed a higher value and we attribute this to the concentration at which micellization is complete (Fig. 4B). The results presented in Figs. 2–4 and Table 1 indicate that the CMC value is not affected by the nature (whether part of the surfactant molecule or externally added) and location of the probe within the LAS and SAS micelles.
CONCLUSION It is shown that CMC values determined by surface tension and fluorescence techniques using external and internal probes agree well in linear alkylbenzene sulfonate (LAS) and secondary alkyl sulfonate (SAS) systems.
REFERENCES
FIG. 4. CMC of SAS by fluorescence, (a) pyrene and (b) phenol. Note the fluorescence quenching in phenol system.
AID
JCIS 4804
/
6g22$$$402
04-08-97 22:44:18
1. Berth, P., and Jeschke, P., Tenside 2, 75 (1989). 2. Tezak, D., Hertel, G., and Hoffmann, H., Liquid Crystals 10, 15 (1991). 3. Das, S., Bhirud, R. G., Nayyar, N., Narayan, K. S., and Kumar, V. V., J. Phys. Chem. 96, 7454 (1992). 4. Goon, P., and Kumar, V. V., Ind. J. Chem. A 35, 182 (1996).
coidas
180
NOTE
5. Aoudia, M., and Rodgers, M. A. J., J. Am. Chem. Soc. 101(22), 6777 (1979). 6. Binana-Limbele´, W., van Os, N. M., Rupert, L. A. M., and Zana, R., J. Colloid Interface Sci. 141, 157 (1991). 7. Aoudia, M., Wade, W. H., and Rodgers, M. A. J., J. Colloid Interface Sci. 145, 493 (1991). 8. Nayyar, N., Das, S., Bhirud, R. G., Narayan, K. S., Singh, A. P., and Kumar, V. V., J. Colloid Interface Sci. 160, 496 (1993). 9. Maiti, N. C., Mazumdar, S., and Periasamy, N., J. Phys. Chem. 99, 10708 (1995). 10. Itoh, H., Ishido, S., Nomura, M., Hayakawa, T., and Mitaku, S., J. Phys. Chem. 100, 9047 (1996). 11. Caponetti, E., Trilo, R., Patience, C. H. O., Johnson, J. S., Magid, L. J., Butler, P., and Payne, K. A., J. Colloid Interface Sci. 116, 200 (1987). 12. Gratzel, M., Kalyanasundaram, K., and Thomas, J. K., J. Am. Chem. Soc. 96, 7869 (1974). 13. Aoudia, M., Rodgers, M. A. J., and Wade, W. H., J. Colloid Interface Sci. 101, 472 (1984).
AID
JCIS 4804
/
6g22$$$403
04-08-97 22:44:18
14. Shinoda, K., Nakagawa, T., Tamamushi, B., and Isemura, T., ‘‘Colloidal Surfactants.’’ Academic Press, New York, 1963. 15. Kalyanasundaram, K., and Thomas, J. K., J. Am. Chem. Soc. 99, 2039 (1977). PIYALI . GOON * C. MANOHAR† V. V. KUMAR * ,1 *Hindustan Lever Research Centre I C T Link Road, Andheri (E) Mumbai 400 099, India †Bhabha Atomic Research Centre Trombay Mumbai 400 085, India Received November 4, 1996; accepted January 30, 1997 1
To whom correspondence should be addressed.
coidas