Electric birefringence relaxation spectroscopy of linear micelles in aqueous cetyltrimethylammonium bromide solutions with sodium salicylate

Oizumi, Junichi; Kimura, Yasuyuki; Ito, Kohzo; Hayakawa, Ritsuko

doi:10.1063/1.471446

Cited by 10 publications

(5 citation statements)

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“…This paper reports the effect of salts on the structure of micelles when the hydrophilicity of the aromatic counterions is varied. The effect of salt sodium 2-hydroxy benzoate (Na2-HB), also referred to as NaSal, on the structure of cationic micelles is well studied in the literature. − The micelles grow strongly with the addition of a small amount of Na2-HB. We compare the effect of different salts, which have either a smaller or higher hydrophilicity than Na2-HB.…”

Section: Introductionmentioning

confidence: 99%

Effect of the Hydrophilicity of Aromatic Counterions on the Structure of Ionic Micelles

Aswal

2003

J. Phys. Chem. B

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Small-angle neutron scattering experiments have been carried out on aqueous micellar solutions of cationic surfactant cetyltrimethylammonium bromide (CTABr) in the presence of different aromatic salts. The salts with different hydrophilicity of the counterions that have been used are sodium benzoate (NaB), sodium 2-hydroxy benzoate (Na2-HB), sodium 2,6-dihydroxy benzoate (Na2,6-DHB), and sodium 4-hydroxy benzoate (Na4-HB). It is found that cationic CTABr micelles grow strongly with the addition of small amount of NaB, Na2-HB, and Na2,6-DHB salts. The effect of addition of salts on the growth of the micelles is significantly higher for Na2-HB and Na2,6-DHB as compared to that for NaB. On the other hand, the addition of salt Na4-HB is relatively much less effective similar to the effect of inorganic salts (e.g., NaBr) on the growth of the micelles. We explain the propensity of strong growth of the micelles in the presence of NaB, Na2-HB, and Na2,6-DHB because of the high charge neutralization by these salts as the aromatic counterions are adsorbed on the surface of the micelle. In these counterions, the hydrophilicity seems to be important in deciding the location of the charged group of the counterions with respect to the level of the oppositely charged micellar surface. The counterions of Na4-HB are not adsorbed on the micellar surface, and hence, it behaves differently than the above other salts.

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Section: Introductionmentioning

confidence: 99%

Effect of the Hydrophilicity of Aromatic Counterions on the Structure of Ionic Micelles

Aswal

2003

J. Phys. Chem. B

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“…In conclusion, we have shown that when the ionic strength of linearly aggregating surfactants is fairly low, the growth of the micelles is strongly influenced by an inhomogeneous charge distribution on the aggregates. Although the theory presented in this Note applies only to systems with a low net charge, it seems reasonable to expect similar effects on highly charged systems. − It would be interesting to test the theory on suitable experimental systems, which presumably include weakly ionized surfactants, such as cetyltrimethylammonium tosylate, or cationic surfactants, the net surface charges of which have been dimished by strongly adsorbed salicylate counterions. , …”

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confidence: 97%

“…8-10 It would be interesting to test the theory on suitable experimental systems, which presumably include weakly ionized surfactants, such as cetyltrimethylammonium tosylate, 24 or cationic surfactants, the net surface charges of which have been dimished by strongly adsorbed salicylate counterions. 16,25…”

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Equilibrium Charge Distribution on Linear Micelles

Schoot

1997

Langmuir

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“…In a recent study we have investigated the influence of NaSal concentration C S on the dynamics of linear micelles in CTAB solutions by using the frequency-domain electric birefringence relaxation (FEB) spectroscopy. 7 The FEB spectroscopy gives the information on the relaxation time for the micelle rotation as well as the relaxation time of the low-frequency (LF) dielectric relaxation due to the induced dipole moment produced by Salcounterions which are tightly bound to the linear micelle and can fluctuate along the micelle contour. The dependence of the contour length L of linear micelles on the NaSal concentration C S has been obtained from the both relaxation times, which shows that Salions play an decisive role in the surfactant aggregation.…”

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High-Frequency Dielectric Relaxation of Linear Micelles in Aqueous Solutions of Cetyltrimethylammonium Bromide with Sodium Salicylate

et al. 1997

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It is well-known that ionic surfactants in aqueous solutions with added salts aggregate to form a linear (rodlike or wormlike) micelle with electric charges. 1-3 This kind of linear micelle can be called a quasi-polyion because it has a charge distribution and linear conformation similar to the polyion. It is noted, however, that the linear micelle is different from the polyion in that the number of surfactant molecules (i.e., "monomers") constituting the micelle is changeable with environmental conditions. For example, when salts of usual kinds are added to the ionic surfactant solutions, the aggregation of surfactants proceeds and the contour length L of a linear micelle becomes longer. An exceptional salt is sodium salicylate (NaSal) 4-6 added to aqueous solutions of cetyltrimethylammonium bromide (CTAB): when the ratio of the NaSal concentration C S to the surfactant concentration C CTAB is less than unity, the viscoelastic measurement indicates that the relaxation time gets longer with increasing C S , which implies that the aggregation of surfactant proceeds. When the ratio of C S to the C CTAB is larger than unity, the relaxation time decreases with increasing C S , which is peculiar to NaSal.In a recent study we have investigated the influence of NaSal concentration C S on the dynamics of linear micelles in CTAB solutions by using the frequency-domain electric birefringence relaxation (FEB) spectroscopy. 7 The FEB spectroscopy gives the information on the relaxation time for the micelle rotation as well as the relaxation time of the low-frequency (LF) dielectric relaxation due to the induced dipole moment produced by Salcounterions which are tightly bound to the linear micelle and can fluctuate along the micelle contour. The dependence of the contour length L of linear micelles on the NaSal concentration C S has been obtained from the both relaxation times, which shows that Salions play an decisive role in the surfactant aggregation. We have also found that the diffusion constant of tightly bound Salions along the micelle contour is smaller than the diffusion constant of Salions in a free state. This hindered diffusion of Salions along the micelle contour is consistent with an NMR result 5 which suggests that Salions put their aromatic rings half into the micelle. The fluctuation of Salions along the micelle contour causes the fluctuation of charge density along the contour and produces positively and negatively charged local areas on the micelle, even if the micelle is neutral in the average. This situation can be treated by Oosawa's theory 8 for the polarizability of polyelectrolytes due to the fluctuation of counterions along the polyions. In the present case, the other counterions, Brand Na + ions, are expected to be loosely bound to the local areas with positive and negative charges, respectively, and fluctuate in the direction perpendicular to the micelle contour much faster than the charge fluctuation due to Salions. This faster fluctuation of loosely bound counterions is expected to produce the h...

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Electric birefringence relaxation spectroscopy of linear micelles in aqueous cetyltrimethylammonium bromide solutions with sodium salicylate

Cited by 10 publications

References 17 publications

Effect of the Hydrophilicity of Aromatic Counterions on the Structure of Ionic Micelles

Effect of the Hydrophilicity of Aromatic Counterions on the Structure of Ionic Micelles

Equilibrium Charge Distribution on Linear Micelles

High-Frequency Dielectric Relaxation of Linear Micelles in Aqueous Solutions of Cetyltrimethylammonium Bromide with Sodium Salicylate

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