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In this paper, we describe the design, construction and performance of a new high resolution neutron powder diffractometer that has been installed at the Dhruva reactor, Trombay, India. The instrument employs novel design concepts like the use of bent, perfect crystal monochromator and open beam geometry, enabling the use of smaller samples. The resolution curve of the instrument was found to have little variation over a wide angular region and a Δd/d ∼ 0.3% has been achieved. The instrument provides sample environment of very low temperatures and high magnetic fields using a 7 Tesla cryogen-free superconducting magnet with a VTI having a temperature range of 1.5-320 K. The special sample environment and high resolution make this neutron powder diffractometer a very powerful facility for studying magnetic properties of materials.

Combined small-angle neutron scattering (SANS) and small-angle x-ray scattering (SAXS) studies have been performed on charged micelles of anionic surfactants of alkali metal dodecyl sulphate (MDS, M = Li, Na, Rb and Cs) in aqueous solution. The dimensions of the micelles have been obtained using SANS as the neutrons are mostly scattered by the hydrogenous core of the micelles. The contrast for the head groups and the counterions is very small, so they are not seen with the neutrons. On the other hand, x-rays are scattered by the electron density fluctuation, which is proportional to the atomic number, and thus x-rays have been used to see the high-Z components of the micelles directly. SAXS from LiDS gives the headgroup thickness, and the counterion condensation is measured directly by the scattering from the high-Z counterions of MDS (M = Na, Rb and Cs) micelles. Based on the results of these two techniques, the role of the different counterions in deciding the structure of micelles has been explained in terms of counterion condensation and the thickness over which they are condensed around the charged micelle. It is found that as the hydrated size of the counterion decreases, the counterion condensation increases and counterions are condensed over a smaller thickness, which results in larger charge neutralization and hence a larger size of the micelles.

Combined studies of small-angle neutron scattering and small-angle x-ray scattering have been carried out for the direct observation of counterion condensation on charged micelles of two cationic surfactants, cetyltrimethylammonium bromide (CTABr) and cetyltrimethylammonium chloride (CTACl) in aqueous electrolyte solutions. The addition of the electrolytes KCl and KBr in the CTABr and CTACl micellar solutions, respectively, yields an increase in the counterion condensation as well as the exchange of the counterions. It is found that the counterions in CTABr/KCl and CTACl/KBr micellar solutions are exchanged to the extent of maintaining the same concentration gradient for both the counterions (Br − and Cl − ) around the charged micelles with respect to the solution.