- Email: [email protected]
SANS study of micelle formation in aqueous mixed solutions of sodium and magnesium dodecylsulfates: Part II
Physica B 276}278 (2000) 341}342
SANS study of micelle formation in aqueous mixed solutions of sodium and magnesium dodecylsulfates: Part II M. Avdeev , V. Garamus *, L. Rosta , I. Smirnova, N. Smirnova RISSP, KFKI P.O. Box 49, Budapest H-1525, Hungary FLNP, JINR, 141980 Dubna, Moscow region, Russia GKSS, Abt. WFS-Geb. 03, Max Planck Str., D-21502, Geesthacht, Germany St. Petersburg State University, 198904 St. Petersburg, Russia
Abstract Mixtures of two anionic surfactants } sodium and magnesium dodecyl sulfates } in heavy water were studied by small-angle neutron scattering (SANS) at various values of the total concentration of the surfactants. In part I of these proceedings, the experimental conditions and the model "tted to data were described. In this part the obtained dependence of the average aggregation number and e!ective fractional charge of mixed micelles on the relative concentration of the surfactants is discussed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Small-angle neutron scattering; Surfactants; Micelles
1. Results and discussion The mean aggregation number N and the e!ective fractional charge a obtained from the "ts are shown as a dependence on the relative concentration in Fig. 1. For the f parameter the values between 1 and 2 were obtained. One can see (Fig. 1(a)) that at x'0.5 the dependence N (x) di!ers signi"cantly for the values studied of the total concentration. This is an unexpected result since according to the previous experimental data [1] the aggregation number in solutions of Mg dodecyl sulfate does not change signi"cantly with concentration and, if so, all graphs N (x) must converge in one point at x"1. Instead, we have obtained the inversion in the concentration dependence of N for pure Mg(DS) (x"1 in Fig. 1(a)). The reason of the di!erence between the present and previous results is unclear for us at the moment. Still, from Fig. 3(a) we are inclined to think that
* Corresponding author. Tel.: #49-4152-871290; fax: #494152-871356. E-mail address: [email protected] (V. Garamus)
deviations from the linear increase of the aggregation number with the relative concentration are not signi"cant. As one can see in Fig. 1(b) the e!ective fractional charge decreases with growing x. Above x+0.2 this decrease obeys the linear law and does not depend on the total concentration. The deviation from the linear behaviour below x"0.2 may be explained by a stronger bonding of Mg> counterions in comparison with Na>. Due to this fact the relative concentration of Mg> on the micellar surface is much higher than in the bulk. This e!ect is responsible for the sharp decrease of CMC in the region of small x (Part I, Fig. 1).
2. Conclusions The quantitative estimates of the parameters of the mixed micelles in the sodium and magnesium dodecyl sulfates solutions were obtained from SANS data. The complex dependence of the mixed micelle formation on the total concentration was observed. A detailed study of this dependence as well as a comparison of the used model with other approaches [2}5] is the aim of further investigations. The e!ect of the enrichment of the
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 5 4 5 - 8
342
M. Avdeev et al. / Physica B 276}278 (2000) 341}342
micellar surface found with Mg> cations explains the behaviour of CMC for small concentrations of Mg> in the mixed solutions.
References [1] [2] [3] [4] [5]
Fig. 1. Dependence of aggregation number (a) and e!ective fractional charge (b) on relative concentration x.
S. Borbely et al., Physica B 213 & 214 (1995) 600. K.M. Lusvardi et al., Langmuir 11 (1995) 487. M. BergstroK m, J. Skov Pederesen, Langmuir 14 (1998) 3754. M. BergstroK m, J. Skov Pederesen, Langmuir 15 (1999) 2250. J. Penfold et al., J. Phys. Chem. B 103 (1999) 5204.
SANS study of micelle formation in aqueous mixed solutions of sodium and magnesium dodecylsulfates: Part II M. Avdeev , V. Garamus *, L. Rosta , I. Smirnova, N. Smirnova RISSP, KFKI P.O. Box 49, Budapest H-1525, Hungary FLNP, JINR, 141980 Dubna, Moscow region, Russia GKSS, Abt. WFS-Geb. 03, Max Planck Str., D-21502, Geesthacht, Germany St. Petersburg State University, 198904 St. Petersburg, Russia
Abstract Mixtures of two anionic surfactants } sodium and magnesium dodecyl sulfates } in heavy water were studied by small-angle neutron scattering (SANS) at various values of the total concentration of the surfactants. In part I of these proceedings, the experimental conditions and the model "tted to data were described. In this part the obtained dependence of the average aggregation number and e!ective fractional charge of mixed micelles on the relative concentration of the surfactants is discussed. 2000 Elsevier Science B.V. All rights reserved. Keywords: Small-angle neutron scattering; Surfactants; Micelles
1. Results and discussion The mean aggregation number N and the e!ective fractional charge a obtained from the "ts are shown as a dependence on the relative concentration in Fig. 1. For the f parameter the values between 1 and 2 were obtained. One can see (Fig. 1(a)) that at x'0.5 the dependence N (x) di!ers signi"cantly for the values studied of the total concentration. This is an unexpected result since according to the previous experimental data [1] the aggregation number in solutions of Mg dodecyl sulfate does not change signi"cantly with concentration and, if so, all graphs N (x) must converge in one point at x"1. Instead, we have obtained the inversion in the concentration dependence of N for pure Mg(DS) (x"1 in Fig. 1(a)). The reason of the di!erence between the present and previous results is unclear for us at the moment. Still, from Fig. 3(a) we are inclined to think that
* Corresponding author. Tel.: #49-4152-871290; fax: #494152-871356. E-mail address: [email protected] (V. Garamus)
deviations from the linear increase of the aggregation number with the relative concentration are not signi"cant. As one can see in Fig. 1(b) the e!ective fractional charge decreases with growing x. Above x+0.2 this decrease obeys the linear law and does not depend on the total concentration. The deviation from the linear behaviour below x"0.2 may be explained by a stronger bonding of Mg> counterions in comparison with Na>. Due to this fact the relative concentration of Mg> on the micellar surface is much higher than in the bulk. This e!ect is responsible for the sharp decrease of CMC in the region of small x (Part I, Fig. 1).
2. Conclusions The quantitative estimates of the parameters of the mixed micelles in the sodium and magnesium dodecyl sulfates solutions were obtained from SANS data. The complex dependence of the mixed micelle formation on the total concentration was observed. A detailed study of this dependence as well as a comparison of the used model with other approaches [2}5] is the aim of further investigations. The e!ect of the enrichment of the
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 5 4 5 - 8
342
M. Avdeev et al. / Physica B 276}278 (2000) 341}342
micellar surface found with Mg> cations explains the behaviour of CMC for small concentrations of Mg> in the mixed solutions.
References [1] [2] [3] [4] [5]
Fig. 1. Dependence of aggregation number (a) and e!ective fractional charge (b) on relative concentration x.
S. Borbely et al., Physica B 213 & 214 (1995) 600. K.M. Lusvardi et al., Langmuir 11 (1995) 487. M. BergstroK m, J. Skov Pederesen, Langmuir 14 (1998) 3754. M. BergstroK m, J. Skov Pederesen, Langmuir 15 (1999) 2250. J. Penfold et al., J. Phys. Chem. B 103 (1999) 5204.