- Email: [email protected]
Dynamic light scattering and electrokinetic potential of bis(quarternary ammonium bromide) surfactant micelles as the function of the alkyl chain length
Colloids and Surfaces A: Physicochemical and Engineering Aspects 143 (1998) 69–75
Dynamic light scattering and electrokinetic potential of bis(quarternary ammonium bromide) surfactant micelles as the function of the alkyl chain length Martin Pisa´rcˇik a,*, Martina Dubnicˇkova´ a, Ferdinand Devı´nsky a, Ivan Lacko a, Jirˇ´ı Sˇkvarla b a Department of Chemical Theory of Drugs, Faculty of Pharmacy, Comenius University, Kalincˇiakova 8, SK-832 32 Bratislava, Slovakia b Department of Mineral Processing and Enviromental Protection, Technical University, Park Komenske´ho 19, SK-043 84 Kosˇice, Slovakia Received 7 January 1998; accepted 29 April 1998
Abstract The micellar properties of bis(quarternary ammonium bromide) surfactants were studied by means of dynamic light scattering and electrokinetic measurements. The coexistence of small spherical micelles and large aggregates for surfactants with eight to 12 carbon atoms in the alkyl chain is observed. Below this region, only spherical micelles appear and above 12 carbon atoms, large aggregates are present. The electrokinetic measurements of zeta potential correspond with the light scattering results. Bimodality in the peaks was observed in the region of nine to 12 carbon atoms in the alkyl chain. As follows from the light scattering results, solubilization with a monomeric surfactant showed that, in the case of dodecyltrimethylammonium bromide (DTAB) and 12-4-12 surfactant, the presence of large aggregates decreases strongly with increasing DTAB concentration in the system. No such observation was made when hexadecyltrimethylammonium bromide (CTAB) was present in the system with 16-4-16 surfactant, where only unimodal size spectra were registered. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Bis(quaternary ammonium bromide) surfactants; Micellar properties; Alkyl chain length
1. Introduction Bis(quarternary ammonium bromide) surfactants, also called dimeric surfactants, attract the interest of researchers because of their unusual physical properties. Monomers of the formulaare formed into micelles, the physical properties of which depend mainly on the alkanendiyl chain connecting the polar heads (also called a spacer) which is usually a polymethylene chain [1,2]. * Corresponding author.
Research interest has focused on determination of the critical micelle concentration [3–5] by means of different methods, aggregation and structural properties [6 ], lyotropic mesophases [7], a theoret-
0927-7757/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7- 7 7 5 7 ( 9 8 ) 0 04 9 7 - X
70
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
ical explanation of processes occurring at the air/water interface [8], and small-angle neutron scattering measurements on micelles [9]. Several works were oriented on the interactions between monomeric and dimeric surfactants [10,11], as well as on the cationic–anionic surfactant interactions [12–14]. Light scattering techniques are widely used to study micelle size and shape. This has been done in the case of conventional monomeric ammonium surfactants [15–19], where the micelle aggregation number, radius of gyration, concentration of the sphere–rod transition and hydrodynamic micelle radius in the case of dynamic light scattering are the most frequently reported parameters. The electrophoretic properties of monomeric surfactant micellar systems have been investigated in the case of alkylammonium salicylate micelles [20,21] and amine oxide micelles [22]. However, there is lack of light scattering data for systems containing dimeric surfactants. Micelle size distributions were investigated by dynamic light scattering on systems containing two carboxylate head groups [23]. The aim of this contribution is to complete the physical micelle properties of dimeric quarternary ammonium surfactants, by using light scattering to determine micellar properties as a function of the number of carbon atoms in the alkyl chain, as well as to investigate their interactions with conventional monomeric surfactants.
intensity was registered at the scattering angle of 90° and temperature of 298.15 K. Brookhaven Zeta-Plus electrophoretic light scattering measurement equipment (wavelength 670 nm, sampling time 256 ms, modulator frequency 250 Hz) was used for determination of the zeta potential. The zeta potential distributions reported represent the result of averaging three to five consecutive runs.
3. Results and discussion 3.1. Dynamic light scattering The particle size spectra of m-4-m dimeric surfactants (m=7–16) are shown in Figs. 1 and 2. The surfactant concentrations used in the scattering measurements were chosen to be slightly larger than the critical micelle concentration (CMC ). It
2. Experimental Bisquarternary ammonium salts were prepared by reaction of tertiary diamines with 1-bromoalkanes as described [24]. The surfactants were purified by manifold crystallization from a mixture of acetone and methanol. Sample identity was confirmed by thin-layer chromatography and elemental analysis. A Brookhaven BI 9000 light scattering system with an argon laser (wavelength 514.5 nm) was used for dynamic light scattering measurements. The CONTIN [25] algorithm was used for determination of the particle size spectra. The scattered
Fig. 1. Particle size spectra obtained from dynamic light scattering measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 7 to 12.
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
Fig. 2. Particle size spectra obtained from dynamic light scattering measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 13 to 16.
is seen that at m=7 only small, probably spherical aggregates are present. Starting with m=8, a population of large aggregates of diameter around 200 nm is observed. The population of small spherical micelles decreases as the number of carbon atoms in the alkyl chain is increased and, with m=13, only large aggregates are present. In the case of dimeric surfactants, a bimodal particle size distribution has been observed for double-chain surfactants with carboxylate head groups [23]; the major population was assigned to vesicles whereas no physical significance was attributed to the minor population. The results obtained probably indicate the increasing interactions of hydrophobic alkyl tails with increasing carbon number in the alkyl chain, which can lead to a decrease of the mean distance between the polar dimeric heads and hence to an increase in size. It is interesting to note that the small aggregates present in the system for m=7–12 have the same, approximately con-
71
stant diameter of 2–3 nm which is determined mainly by the spacer properties. Only one type of aggregate appears for seven carbon atoms in the alkyl chain. At m=7 the alkyl tails may be too short to interact, which leads to the presence of spherical micelles. It may be assumed that with increasing number of carbon atoms the hydrophobic interactions become more intensive and a second phase of large aggregates with diameter 100–200 nm can be observed. However, this coexistence of two micelle types requires comment. The present results can be compared with the wellknown behaviour of dimeric surfactants as a function of the number of carbon atoms (s) in the spacer [4,8] in the case of 12-4-12 surfactant. In our case, the aggregates formed by the s=4 surfactants lie between long, rod-like micelles (s=2 and 3) and spherical micelles formed by s=5 to 10 surfactants due to the maximum value of area per surfactant for these spacer lengths [8]. Thus, this conclusions for s=4 surfactants can lead us to predict the coexistence of the two types of aggregate (spherical micelles and vesicles) which can be understood as a transition state from spherical micelles to vesicles. 3.2. Electrokinetic measurements The zeta potential distributions as a function of m=9–16 are given in Figs. 3 and 4. The distributions for m=7 and 8 were not determined because of the high conductivity, which was greater than the measurement limit. The main peak was found to be positive in the region m=13–16 and slightly above zero for m=9, 10 and 11. This peak should correspond with the positive charge of the head group in the dimeric surfactant. It is interesting to note that for surfactants with m=9–12, the second, negative peak appears with a strong maximum at m=12. Similar examples have been found in the literature. The sign of the electrophoretic mobility changed from positive to negative in the case of some cationic micelles (tetradecyl- and hexadecyltrimethylammonium salicylate micelles) with the addition of 0–0.5 M sodium salicylate, indicating specific adsorption and penetration of salicylate ions [20,21].
72
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
Fig. 3. Zeta potential distributions obtained from electrokinetic measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 9 to 12.
Fig. 4. Zeta potential distributions obtained from electrokinetic measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 13 to 16.
The electrophoretic mobility was found to be negative for amine oxide micelles (dodecyldimethylamine oxide and tetradecyldimethylamine oxide) in NaCl solution at zero degree of protonation, a [22]. With increasing value of a, the negative mobility peak decreased and the second, positive peak appeared. In the region of a=0.41–0.61, two peaks — positive and negative — were observed. It was assumed that, at a=0, the micelles adsorbed the Cl− ions and with increasing a the adsorption is restrained. In order to explain the bimodality in the mobility distributions, one of the possible interpretations was the existence of the two kinds of micelle species with different counterion binding resulting from the two different micelle shapes — spherical and rod-like [22]. We can compare the above explanation with the present bimodality for the dimeric cationic surfactants with number of alkyl carbons, m, from nine 9 to 12 (Figs. 3 and 4). The main, positive peak
is probably attributed to the large aggregates with a low degree of curvature and a weak adsorption of Br− counterions at the micelle surface. The small, negative peak probably follows from the coexistence of the small spherical micelles with adsorbed bromide counterions. This investigation corresponds approximately with the dynamic light scattering size distributions ( Figs. 1 and 2) where spherical micelles were found for m up to 12. 3.3. Solubilization with a monomeric surfactant In Fig. 5, the particle size spectra of 12-4-12 surfactant are shown as a function of the concentration of the monomeric ammonium surfactant dodecyltrimethylammonium bromide (DTAB), the dimeric surfactant concentration lying slightly above its CMC. It is evident that increased DTAB concentration destroys the large aggregates formed by the dimeric surfactant. Very similar results were
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
73
Fig. 6. Plot of two peaks from Fig. 5 as a function of DTAB concentration.
Fig. 5. Solubilization of 12-4-12 surfactant with the monomeric surfactant DTAB. Particle size spectra obtained from dynamic light scattering measurements for different DTAB concentrations (concentration of 12-4-12 is c=0.0015 mol l−1).
obtained in the system 12-10-12 surfactant/DTAB as documented by optical light and cryo-transmission electron micrographs [10]. The concentration dependence of the two peaks ( Fig. 6) shows that the size of the large aggregates is nearly independent of the DTAB concentration, whereas the size of the small micelles decreases as the DTAB concentration is increased. However, the system of 16-4-16 surfactant with hexadecyltrimethylammonium bromide (CTAB) indicates no solubilization as the CTAB concentration increases. Only large unimodal aggregates are present through the whole concentration region of added CTAB and their size increases with increasing CTAB concentration (Fig. 7). The shape of the normalized autocorrelation function in the logarithmic representation indicates the bimodality in the case of the 12-4-12 surfactant/DTAB system ( Fig. 8). The exponential part representing the small aggregates is more
Fig. 7. Solubilization of 16-4-16 surfactant with the monomeric surfactant CTAB. Plot of the single peak as a function of CTAB concentration (concentration of 16-4-16 is c=2×10−5 mol l−1).
intensive at high DTAB concentration, opposite to the diminishing exponential corresponding to the large aggregates. No bimodality is observed in the system 16-4-16 surfactant/CTAB (Fig. 9).
74
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
only spherical micelles appear and, for m>12, the large aggregates are present. Electrokinetic measurements of zeta potential correspond with the light scattering results. Bimodality in the peaks was observed in the region m=9 to 12. Solubilization with a monomeric surfactant showed from light scattering results that, in the case of dodecyltrimethylammonium bromide (DTAB) and 12-4-12 surfactant, the presence of the large aggregates decreases strongly with increasing DTAB concentration in the system. No such observation was made when hexadecyltrimethylammonium bromide (CTAB) was present in the system with 16-4-16 surfactant, where only unimodal size spectra were registered. Fig. 8. Plot of the normalized autocorrelation function of 124-12 surfactant in the presence and absence of DTAB.
Acknowledgment M.P. gratefully acknowledges the Comenius University, Bratislava (grants UK/3875/98 and UK/1527/97) for support of this research.
References
Fig. 9. Plot of the normalized autocorrelation function of 16-4-16 surfactant in the presence and absence of CTAB.
4. Conclusions On the basis of size distributions determined by dynamic light scattering, the coexistence of small spherical micelles and large aggregates for surfactants with m=8–12 is observed. Below this region,
[1] F. Devı´nsky, I. Lacko, Tensides Surf. Det. 27 (1990) 344. [2] F. Devı´nsky, I. Lacko, T. Imam, J. Colloid Interface Sci. 143 (1991) 336. [3] R. Zana, M. Benrraou, R. Rueff, Langmuir 7 (1991) 1072. [4] E. Alami, G. Beinert, P. Marie, R. Zana, Langmuir 9 (1993) 1465. [5] M. Frindi, B. Michels, Langmuir 10 (1994) 1140. [6 ] D. Danino, Y. Talmon, R. Zana, Langmuir 11 (1995) 1448. [7] E. Alami, H. Levy, R. Zana, Langmuir 9 (1993) 940. [8] H. Diamant, D. Andelmann, Langmuir 10 (1994) 2910. [9] H. Hirata, N. Hattori, M. Ishida, H. Okabayashi, M. Frusaka, R. Zana, J. Phys. Chem. 99 (1995) 17778. [10] D. Danino, Y. Talmon, R. Zana, J. Colloid Interface Sci. 185 (1997) 84. [11] R. Zana, H. Levy, D. Danino, Y. Talmon, K. Kwetkat, Langmuir 13 (1997) 402. [12] D.A. Jaeger, B. Li, T. Clark, Langmuir 12 (1996) 4314. [13] Y. Kondo, H. Uchiyama, N. Yoshino, K. Nishyama, M. Abe, Langmuir 11 (1995) 2380. [14] L. Liu, M.J. Rosen, J. Colloid Interface Sci. 179 (1996) 179. [15] M. Pisa´rcˇik, F. Devı´nsky, E. Sˇvajdlenka, Colloids Surf. 119 (1996) 115.
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75 [16 ] R.B. Dorshow, J. Briggs, C.A. Bunton, D.F. Nicoli, J. Phys. Chem. 86 (1982) 2388. [17] S. Ozeki, S. Ikeda, J. Colloid Interface Sci. 87 (1982) 424. [18] T. Imae, R. Kamiya, S. Ikeda, J. Colloid Interface Sci. 108 (1985) 215. [19] T. Imae, S. Ikeda, J. Phys. Chem. 90 (1986) 5216. [20] T. Imae, J. Phys. Chem. 94 (1990) 5953.
[21] [22] [23] [24]
75
T. Imae, T. Kohsaka, J. Phys. Chem. 96 (1992) 10030. T. Imae, N. Hayashi, Langmuir 9 (1993) 3385. D.A. Jaeger, E.L.G. Brown, Langmuir 12 (1996) 1976. T. Imam, F. Devı´nsky, I. Lacko, L. Krasnec, Pharmazie 38 (1983) 308. [25] S.W. Provencher, Makromol. Chem. 180 (1979) 201.
Dynamic light scattering and electrokinetic potential of bis(quarternary ammonium bromide) surfactant micelles as the function of the alkyl chain length Martin Pisa´rcˇik a,*, Martina Dubnicˇkova´ a, Ferdinand Devı´nsky a, Ivan Lacko a, Jirˇ´ı Sˇkvarla b a Department of Chemical Theory of Drugs, Faculty of Pharmacy, Comenius University, Kalincˇiakova 8, SK-832 32 Bratislava, Slovakia b Department of Mineral Processing and Enviromental Protection, Technical University, Park Komenske´ho 19, SK-043 84 Kosˇice, Slovakia Received 7 January 1998; accepted 29 April 1998
Abstract The micellar properties of bis(quarternary ammonium bromide) surfactants were studied by means of dynamic light scattering and electrokinetic measurements. The coexistence of small spherical micelles and large aggregates for surfactants with eight to 12 carbon atoms in the alkyl chain is observed. Below this region, only spherical micelles appear and above 12 carbon atoms, large aggregates are present. The electrokinetic measurements of zeta potential correspond with the light scattering results. Bimodality in the peaks was observed in the region of nine to 12 carbon atoms in the alkyl chain. As follows from the light scattering results, solubilization with a monomeric surfactant showed that, in the case of dodecyltrimethylammonium bromide (DTAB) and 12-4-12 surfactant, the presence of large aggregates decreases strongly with increasing DTAB concentration in the system. No such observation was made when hexadecyltrimethylammonium bromide (CTAB) was present in the system with 16-4-16 surfactant, where only unimodal size spectra were registered. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Bis(quaternary ammonium bromide) surfactants; Micellar properties; Alkyl chain length
1. Introduction Bis(quarternary ammonium bromide) surfactants, also called dimeric surfactants, attract the interest of researchers because of their unusual physical properties. Monomers of the formulaare formed into micelles, the physical properties of which depend mainly on the alkanendiyl chain connecting the polar heads (also called a spacer) which is usually a polymethylene chain [1,2]. * Corresponding author.
Research interest has focused on determination of the critical micelle concentration [3–5] by means of different methods, aggregation and structural properties [6 ], lyotropic mesophases [7], a theoret-
0927-7757/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7- 7 7 5 7 ( 9 8 ) 0 04 9 7 - X
70
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
ical explanation of processes occurring at the air/water interface [8], and small-angle neutron scattering measurements on micelles [9]. Several works were oriented on the interactions between monomeric and dimeric surfactants [10,11], as well as on the cationic–anionic surfactant interactions [12–14]. Light scattering techniques are widely used to study micelle size and shape. This has been done in the case of conventional monomeric ammonium surfactants [15–19], where the micelle aggregation number, radius of gyration, concentration of the sphere–rod transition and hydrodynamic micelle radius in the case of dynamic light scattering are the most frequently reported parameters. The electrophoretic properties of monomeric surfactant micellar systems have been investigated in the case of alkylammonium salicylate micelles [20,21] and amine oxide micelles [22]. However, there is lack of light scattering data for systems containing dimeric surfactants. Micelle size distributions were investigated by dynamic light scattering on systems containing two carboxylate head groups [23]. The aim of this contribution is to complete the physical micelle properties of dimeric quarternary ammonium surfactants, by using light scattering to determine micellar properties as a function of the number of carbon atoms in the alkyl chain, as well as to investigate their interactions with conventional monomeric surfactants.
intensity was registered at the scattering angle of 90° and temperature of 298.15 K. Brookhaven Zeta-Plus electrophoretic light scattering measurement equipment (wavelength 670 nm, sampling time 256 ms, modulator frequency 250 Hz) was used for determination of the zeta potential. The zeta potential distributions reported represent the result of averaging three to five consecutive runs.
3. Results and discussion 3.1. Dynamic light scattering The particle size spectra of m-4-m dimeric surfactants (m=7–16) are shown in Figs. 1 and 2. The surfactant concentrations used in the scattering measurements were chosen to be slightly larger than the critical micelle concentration (CMC ). It
2. Experimental Bisquarternary ammonium salts were prepared by reaction of tertiary diamines with 1-bromoalkanes as described [24]. The surfactants were purified by manifold crystallization from a mixture of acetone and methanol. Sample identity was confirmed by thin-layer chromatography and elemental analysis. A Brookhaven BI 9000 light scattering system with an argon laser (wavelength 514.5 nm) was used for dynamic light scattering measurements. The CONTIN [25] algorithm was used for determination of the particle size spectra. The scattered
Fig. 1. Particle size spectra obtained from dynamic light scattering measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 7 to 12.
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
Fig. 2. Particle size spectra obtained from dynamic light scattering measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 13 to 16.
is seen that at m=7 only small, probably spherical aggregates are present. Starting with m=8, a population of large aggregates of diameter around 200 nm is observed. The population of small spherical micelles decreases as the number of carbon atoms in the alkyl chain is increased and, with m=13, only large aggregates are present. In the case of dimeric surfactants, a bimodal particle size distribution has been observed for double-chain surfactants with carboxylate head groups [23]; the major population was assigned to vesicles whereas no physical significance was attributed to the minor population. The results obtained probably indicate the increasing interactions of hydrophobic alkyl tails with increasing carbon number in the alkyl chain, which can lead to a decrease of the mean distance between the polar dimeric heads and hence to an increase in size. It is interesting to note that the small aggregates present in the system for m=7–12 have the same, approximately con-
71
stant diameter of 2–3 nm which is determined mainly by the spacer properties. Only one type of aggregate appears for seven carbon atoms in the alkyl chain. At m=7 the alkyl tails may be too short to interact, which leads to the presence of spherical micelles. It may be assumed that with increasing number of carbon atoms the hydrophobic interactions become more intensive and a second phase of large aggregates with diameter 100–200 nm can be observed. However, this coexistence of two micelle types requires comment. The present results can be compared with the wellknown behaviour of dimeric surfactants as a function of the number of carbon atoms (s) in the spacer [4,8] in the case of 12-4-12 surfactant. In our case, the aggregates formed by the s=4 surfactants lie between long, rod-like micelles (s=2 and 3) and spherical micelles formed by s=5 to 10 surfactants due to the maximum value of area per surfactant for these spacer lengths [8]. Thus, this conclusions for s=4 surfactants can lead us to predict the coexistence of the two types of aggregate (spherical micelles and vesicles) which can be understood as a transition state from spherical micelles to vesicles. 3.2. Electrokinetic measurements The zeta potential distributions as a function of m=9–16 are given in Figs. 3 and 4. The distributions for m=7 and 8 were not determined because of the high conductivity, which was greater than the measurement limit. The main peak was found to be positive in the region m=13–16 and slightly above zero for m=9, 10 and 11. This peak should correspond with the positive charge of the head group in the dimeric surfactant. It is interesting to note that for surfactants with m=9–12, the second, negative peak appears with a strong maximum at m=12. Similar examples have been found in the literature. The sign of the electrophoretic mobility changed from positive to negative in the case of some cationic micelles (tetradecyl- and hexadecyltrimethylammonium salicylate micelles) with the addition of 0–0.5 M sodium salicylate, indicating specific adsorption and penetration of salicylate ions [20,21].
72
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
Fig. 3. Zeta potential distributions obtained from electrokinetic measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 9 to 12.
Fig. 4. Zeta potential distributions obtained from electrokinetic measurements on micelles of bis(quarternary ammonium bromide) surfactants with number of carbon atoms in the alkyl chain, m, from 13 to 16.
The electrophoretic mobility was found to be negative for amine oxide micelles (dodecyldimethylamine oxide and tetradecyldimethylamine oxide) in NaCl solution at zero degree of protonation, a [22]. With increasing value of a, the negative mobility peak decreased and the second, positive peak appeared. In the region of a=0.41–0.61, two peaks — positive and negative — were observed. It was assumed that, at a=0, the micelles adsorbed the Cl− ions and with increasing a the adsorption is restrained. In order to explain the bimodality in the mobility distributions, one of the possible interpretations was the existence of the two kinds of micelle species with different counterion binding resulting from the two different micelle shapes — spherical and rod-like [22]. We can compare the above explanation with the present bimodality for the dimeric cationic surfactants with number of alkyl carbons, m, from nine 9 to 12 (Figs. 3 and 4). The main, positive peak
is probably attributed to the large aggregates with a low degree of curvature and a weak adsorption of Br− counterions at the micelle surface. The small, negative peak probably follows from the coexistence of the small spherical micelles with adsorbed bromide counterions. This investigation corresponds approximately with the dynamic light scattering size distributions ( Figs. 1 and 2) where spherical micelles were found for m up to 12. 3.3. Solubilization with a monomeric surfactant In Fig. 5, the particle size spectra of 12-4-12 surfactant are shown as a function of the concentration of the monomeric ammonium surfactant dodecyltrimethylammonium bromide (DTAB), the dimeric surfactant concentration lying slightly above its CMC. It is evident that increased DTAB concentration destroys the large aggregates formed by the dimeric surfactant. Very similar results were
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
73
Fig. 6. Plot of two peaks from Fig. 5 as a function of DTAB concentration.
Fig. 5. Solubilization of 12-4-12 surfactant with the monomeric surfactant DTAB. Particle size spectra obtained from dynamic light scattering measurements for different DTAB concentrations (concentration of 12-4-12 is c=0.0015 mol l−1).
obtained in the system 12-10-12 surfactant/DTAB as documented by optical light and cryo-transmission electron micrographs [10]. The concentration dependence of the two peaks ( Fig. 6) shows that the size of the large aggregates is nearly independent of the DTAB concentration, whereas the size of the small micelles decreases as the DTAB concentration is increased. However, the system of 16-4-16 surfactant with hexadecyltrimethylammonium bromide (CTAB) indicates no solubilization as the CTAB concentration increases. Only large unimodal aggregates are present through the whole concentration region of added CTAB and their size increases with increasing CTAB concentration (Fig. 7). The shape of the normalized autocorrelation function in the logarithmic representation indicates the bimodality in the case of the 12-4-12 surfactant/DTAB system ( Fig. 8). The exponential part representing the small aggregates is more
Fig. 7. Solubilization of 16-4-16 surfactant with the monomeric surfactant CTAB. Plot of the single peak as a function of CTAB concentration (concentration of 16-4-16 is c=2×10−5 mol l−1).
intensive at high DTAB concentration, opposite to the diminishing exponential corresponding to the large aggregates. No bimodality is observed in the system 16-4-16 surfactant/CTAB (Fig. 9).
74
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75
only spherical micelles appear and, for m>12, the large aggregates are present. Electrokinetic measurements of zeta potential correspond with the light scattering results. Bimodality in the peaks was observed in the region m=9 to 12. Solubilization with a monomeric surfactant showed from light scattering results that, in the case of dodecyltrimethylammonium bromide (DTAB) and 12-4-12 surfactant, the presence of the large aggregates decreases strongly with increasing DTAB concentration in the system. No such observation was made when hexadecyltrimethylammonium bromide (CTAB) was present in the system with 16-4-16 surfactant, where only unimodal size spectra were registered. Fig. 8. Plot of the normalized autocorrelation function of 124-12 surfactant in the presence and absence of DTAB.
Acknowledgment M.P. gratefully acknowledges the Comenius University, Bratislava (grants UK/3875/98 and UK/1527/97) for support of this research.
References
Fig. 9. Plot of the normalized autocorrelation function of 16-4-16 surfactant in the presence and absence of CTAB.
4. Conclusions On the basis of size distributions determined by dynamic light scattering, the coexistence of small spherical micelles and large aggregates for surfactants with m=8–12 is observed. Below this region,
[1] F. Devı´nsky, I. Lacko, Tensides Surf. Det. 27 (1990) 344. [2] F. Devı´nsky, I. Lacko, T. Imam, J. Colloid Interface Sci. 143 (1991) 336. [3] R. Zana, M. Benrraou, R. Rueff, Langmuir 7 (1991) 1072. [4] E. Alami, G. Beinert, P. Marie, R. Zana, Langmuir 9 (1993) 1465. [5] M. Frindi, B. Michels, Langmuir 10 (1994) 1140. [6 ] D. Danino, Y. Talmon, R. Zana, Langmuir 11 (1995) 1448. [7] E. Alami, H. Levy, R. Zana, Langmuir 9 (1993) 940. [8] H. Diamant, D. Andelmann, Langmuir 10 (1994) 2910. [9] H. Hirata, N. Hattori, M. Ishida, H. Okabayashi, M. Frusaka, R. Zana, J. Phys. Chem. 99 (1995) 17778. [10] D. Danino, Y. Talmon, R. Zana, J. Colloid Interface Sci. 185 (1997) 84. [11] R. Zana, H. Levy, D. Danino, Y. Talmon, K. Kwetkat, Langmuir 13 (1997) 402. [12] D.A. Jaeger, B. Li, T. Clark, Langmuir 12 (1996) 4314. [13] Y. Kondo, H. Uchiyama, N. Yoshino, K. Nishyama, M. Abe, Langmuir 11 (1995) 2380. [14] L. Liu, M.J. Rosen, J. Colloid Interface Sci. 179 (1996) 179. [15] M. Pisa´rcˇik, F. Devı´nsky, E. Sˇvajdlenka, Colloids Surf. 119 (1996) 115.
M. Pisa´rcˇik et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 143 (1998) 69–75 [16 ] R.B. Dorshow, J. Briggs, C.A. Bunton, D.F. Nicoli, J. Phys. Chem. 86 (1982) 2388. [17] S. Ozeki, S. Ikeda, J. Colloid Interface Sci. 87 (1982) 424. [18] T. Imae, R. Kamiya, S. Ikeda, J. Colloid Interface Sci. 108 (1985) 215. [19] T. Imae, S. Ikeda, J. Phys. Chem. 90 (1986) 5216. [20] T. Imae, J. Phys. Chem. 94 (1990) 5953.
[21] [22] [23] [24]
75
T. Imae, T. Kohsaka, J. Phys. Chem. 96 (1992) 10030. T. Imae, N. Hayashi, Langmuir 9 (1993) 3385. D.A. Jaeger, E.L.G. Brown, Langmuir 12 (1996) 1976. T. Imam, F. Devı´nsky, I. Lacko, L. Krasnec, Pharmazie 38 (1983) 308. [25] S.W. Provencher, Makromol. Chem. 180 (1979) 201.