Distinctive Effect of Maleic Acid and Fumaric Acid on Structural Transitions in Cationic Micellar Solution

Linet Rose J.a, B. V. R. Tatab, Yeshayahu Talmonc,V. K. Aswald, P. A. Hassane and Lisa Sreejith*a

Supporting Information

Figure S1: Viscosity profile of 0.1M CTAB/0.04M MA and 0.1M CTAB/0.04M FA system in presence of different alcohols. (a) n-hexanol (b) n-octanol (c) n-decanol and (d) n-dodecanol. (Measured at 30 °C using Brook field DV II cone and plate viscometer.)

Fig S2: (a) Conductivity and (b) pH of 0.1M CTAB/xM MA and 0.1M CTAB/xM FA systems as a function of acid concentration.

Fig. S3: Viscosity profile of (a) 0.1M CTAB/0.042M octanol/xM MA and (b) 0.1M CTAB/0.042M octanol/xM FA system as a function of acid concentration.

Details of SANS analysis

The SANS data was analyzed using the method of Hayter-Penfold for an assembly of ellipsoidal micelles. For mono disperse micelles, the coherent differential scattering cross section dS/dW is given by the equation [37, 38]

dS/dW=n(rm-rs)2V2[⟨F(q)2⟩+⟨F(q)⟩2(S(q)-1)]+B (1)

Where n is the number density of micelles, rm and rs are scattering length densities of micelle and solvent respectively and V is the volume of the micelle. F(q) is the single particle form factor and S(q) is the inter-particle structure factor. B is a constant that represents the incoherent scattering background. For ellipsoidal micelles, the single particle form factor can be obtained from [39, 40]

Fq2=01[Fq,μ2dμ] (2)

Fq2=[01Fq,μdμ]2 (3)

Fq,μ=3(sinx-x cos x)x3 (4)

x=qa2μ2+b21-μ21/2 (5)

where a and b are the semi-major and semi-minor axis of the ellipsoidal micelle respectively and m is the cosine of the angle between the directions of a and the wave vector transfer q. The inter-particle structure factor S(q) specifies the correlation between the centers of different micelles and is the Fourier transform of the radial distribution function g(r) for the mass centre of the micelle. The calculation of S(q) for any shapes other than spheres is complicated. Hence, for S(q) calculations, prolate ellipsoidal micelles are assumed to be a rigid equivalent spheres of diameter σ=2ab21/3 interacting through a screened Coulomb potential. The dimensions of micelle were determined from the model fitting of SANS data. It is a common feature of SANS spectra from ionic micellar solution that they exhibit strong correlation peak, which is attributed to the sharpening of intermicellar structure factor when the effective coulomb interaction between micelles become long ranged. The correlation peak observed in the low q region of both 0.1M CTAB/0.04M MA and 0.1M CTAB/0.04M FA samples indicates repulsive interaction between charged micellar heads. However the correlation peak of 0.1M CTAB/0.04M MA sample diminishes in presence of 0.035 M octanol. This suggests the presence of effectively screened, elongated micelles in CTAB/MA/octanol system. The value of semi-major axis (a) has increased from 51 Å to 111 Å upon addition of 0.035 M octanol to 0.1M CTAB/0.04M MA sample. But the value of semi-minor axis (b) remains almost unchanged. This strongly suggests one dimensional elongation of micelles in CTAB/MA/octanol system. When TLMs with length longer than a few tens of nanometers are formed by one-dimensional elongation, the main feature of SANS spectra is limited to the cross-sectional radius of the micelle [41]. So the values obtained for semi-major axis may not match with the exact length of the TLM. However, the observed spectral features and trend of calculated micellar dimensions, strongly support the notion of octanol induced structural transitions in maleic acid containing system. In contrast, the inability of octanol to induce considerable micellar elongation in CTAB/FA system is clear from the retention of correlation peak in the SANS spectrum of 0.1M CTAB/0.04M FA sample even after the addition of 0.035 M octanol.

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37 Hayter JB, Penfold J (1983) Determination of micelle structure and charge by neutron small angle scattering.
Colloid Polym Sci 261:1022-1030.

38 Sheu EY, Wu CF, Chen SH (1986) Effect of ion size on the aggregation and surface charge of ionic micelles in
1:1 electrolyte solutions. J Phys Chem 90:4179-4187.

39 Hayter JB, Penfold J (1980) An analytic structure factor for macro ion solutions. Mol Phys 42:109-118.

40 Hansen JP, Hayter JB (1982) A rescaled MSA structure factor for dilute charged colloidal dispersion. Mol Phys
46:651-656.

41 Verma G, Aswal VK, Hassan PA (2009) pH-Responsive self-assembly in an aqueous mixture of surfactant and
hydrophobic amino acid mimic. Soft Matter 5:2919-2927.