SUMMARY OF DOCTORAL DISSERTATION

INVESTIGATIONOF NOVEL SURFACTANT SYSTEMS CONTAININGHYDROTROPES

Designing supramolecular structures,utilizing amphiphilic molecules is a frontier area of research. This topic has developed to such an extent that during the past few years, formation of rods, ribbons, tubules, etc. made up of properly designed amphiphiles has been reported. The resulting systems may have many potential applications in high-tech areas.

It is known that a solution of a surfactant like CTAB may show viscous, viscoelastic and liquid crystallinephases depending upon the concentration of the surfactant. This profile is dramatically changed when a hydrotrope is mixed with a surfactant. Such a mixed system produces a viscoelastic solution at a very low concentrationof 10^-3 to 10^-2 of the surfactant. In the last fewyears, reports have started appearing which show that the viscoelasticsystems are closely related to vesicles and liquid crystalline structures.Their formation at low concentrations is very interesting and understandingthem can lead to designing of such systems at an extremely low cost. Adetailed review of the viscoelastic solutions has been published recently,and it appears that several aspects of these systems can be understoodonly when we take into account the relation between viscoelastic solutions,vesicles and liquid crystals. While some of these systems have been studiedextensively, several features of these are puzzling and have not been properlyattended. For example, strongly viscoelastic solutions are produced whenan aromatic anionic additive, like sodium salicylate (SS), is added toa CTAB solution. Shifting the hydroxyl group from the o-position in SSto the m- or p-position produces a drastic reduction in the ability toproduce viscoelasticity. Thus, the nature of the hydrotrope considerablyaffects the profile of the solution.

When SS is added to a CTABsolution, the zero shear viscosity shows a double peak nature when plottedas a function of concentration of SS. In between the peaks, the viscositygoes to a minimum at approximately equimolar concentrations of CTAB andSS. Such an effect is observed not only for SS, but for other additivestoo.

Recently, we have demonstratedthat a solution containing Na salt of 3-hydroxy naphthalene-2-carboxylicacid (SHNC) and CTAB goes through viscolestic gel, liquid crystalline andvesicular phases at low concentrations . Surprisingly, it also exhibitedstrong flow birefringence. This provide the impetus for the presentwork.

In the present project, systemscontaining CTAB and anionic hydrotropes have been studied to evolve a coherentpicture of such systems and explain the formation of suchphases.

In the first phase of the project,a procedure was developed for the preparation of the salts of CTAB andvarious hydrotropes. The salts of CTAB and various hydrotropes were thusprepared. The hydrotropes selected were the sodium salts of salicylic acid,HNC and its positional isomers, 4-toluene sulphonic acid, 4-xylene sulphonicacid, ethyl benzene sulphonic acid, propyl benzene sulphonic acid, isopropylbenzene (cumene) sulphonic acid, heptanoic acid, octanoic acid, nonanoicacid, hexane sulphonic acid, heptane sulphonic acid and octane sulphonicacid.

¹H NMR and¹³CNMR spectroscopy were used to identify the different phases and also witha view to study the mobility of the CTAB and hydrotrope molecules in thesolution.

In the second phase, effortswere mainly directed towards the characterization of CTAHNC, which is asalt of CTAB and SHNC.

The viscosity - concentrationprofile for the (CTAB + SHNC) system was plotted for a fixed concentrationof CTAB and varying concentrations of SHNC, where a dual hump was observed.A turbid solution was formed at equimolar concentrations of CTAB and SHNC,while at other concentrations of SHNC, the solutions wereviscoelastic.

The phase diagram at differentconcentrations and temperatures for CTAHNC showed a turbid vesicular phaseat lower concentrations, whereas at concentrations above 3 % w/v, a stronglybirefringent liquid crystalline phase was observed. At elevated temperatures,there appeared a biphasic region which is partly birefringent and partlyisotropic.

The aqueous solutions of CTAHNCwere turbid in nature. The turbidity cleared off when the solutions wereheated to ~90 degrees C. The turbidity also cleared off when an excessof CTAB or SHNC was added to CTAHNC. The viscosity-temperature profileof the system exhibited a rise in the viscosity beyond the clearingtemperature.

¹H NMR spectrumof this system was studied with respect to temperature in order to studythe effect of temperature on the aggregational behaviour of thecomponents.

The existence of multilayeredvesicles has been demonstrated using the freeze-fracture Cryo-SEM techniqueon a 0.6 % CTAHNC solution at room temperature. Light scattering studyof CTAHNC in the concentration range of 0.07 - 0.6 mM suggested the structureof the micellar aggregates to be vesicles of size ~ 100 nm. DSC studieson the CTAHNC system suggested a solid-fluid transition occurring withinthe system. Hence, from all these evidences, it is concluded that CTAHNCforms vesicles at room temperatures. Thus, there was a transition froma solid type surfactant to ion pairs, either by heating or on the additionof charge defects. The solid showed a transition to vesicles, while theion pairs showed a transition to worm-like micelles. A new theory suggestinga transition from solid to fluid micelles has been put forth to explainthese observations. The solid micelles formed can be 'melted' either byheating or by lowering the melting point to room temperature by additionof 'charge defects' such as CTAB or SHNC. On melting, worm-like micellesare formed and the viscosity shoots up.

¹H NMR studies werealso done on the Cetyl trimethylammonium p-toluene sulphonate (CTApTS)-Sodiumdodecylbenzene sulfonate system (NaDBS). Previous reports confirm the existenceof spontaneous single walled equilibrium vesicles of controlled size andsurface charge in aqueous mixtures of NaDBS/CTApTS. ¹H NMR studiesof the NaDBS/CTApTS system indicated the rigidity of the aromatic ringof NaDBS on the CTA micellar surface in the vesicular solutions. Strongcounterion association is also present within the system . ¹HNMR studies on this system too indicates the hindered rotation of the dodecylsulphate counterion on the CTA micellar surface.

Dye solubilization was carriedout in aqueous solution of CTApTS using a water-insoluble dye, Orange OT.The solubilisation effeciency was found to increase for CTApTS, and thesolubilization occurred at a much lower concentration for CTApTS than thatobserved for CTAB or for Na-pTS.

The thesis consists of threechapters. The first chapter presents a brief state-of-the-art report onthe aggregational behaviour of different surfactant-hydrotrope systems.The second chapter results obtained while the third chapter gives theexperimentaldetails.