Microstructure Determination in a Self Assembled Surfacant System Formed by AOT/Lecithin/Isooctane and Water

We have observed a novel transformation from a microemulsion to a gel phase by increasing the water content of a system consisting of bis(2-ethylhexyl) sodium sulfosuccinate(AOT), phosphatidycholine(lecithin), and 2,2,4 -trimethylpentane (isooctane). The viscosity increases by six orders of magnitude and a rigid gel forms as the water content is increased above a specific threshold. Small angle neutron scattering (SANS) experiments on this surfactant mesophase reveal the microstructure as columnar hexagonal at lower water contents and temperatures, and lamellar at higher water contents and temperatures. SANS is also able to capture the transition between these two structures. These structures are thermally reversible. Differential scanning calorimetery results show no abrupt phase transitions in the gel system with increasing temperature. For low water and surfactant contents (W0=20, total surfactant concentration in isooctane = O.OlM), microemulsion droplets are formed. SANS experimental data fits well with the polydisperse sphere morphology and a gradual increase in the droplet size is observed as the lecithin/ AOT ratio is increased. The different packing parameters for AOT and lecithin drive the observed changes in droplet sizes. For lecithin/ AOT molar ratios approaching unity, both SANS and static light scattering measurements reveal an attractive interaction develops between these droplets, leading to the formation of clusters. Hydrodynamic radii obtained from dynamic light scattering measurements show results that are consistent with the SANS data.

as the water content is increased above a specific threshold. Small angle neutron scattering (SANS) experiments on this surfactant mesophase reveal the microstructure as columnar hexagonal at lower water contents and temperatures, and lamellar at higher water contents and temperatures. SANS is also able to capture the transition between these two structures. These structures are thermally reversible. Differential scanning calorimetery results show no abrupt phase transitions in the gel system with increasing temperature.
For low water and surfactant contents (W 0 =20, total surfactant concentration in isooctane = O.OlM), microemulsion droplets are formed. SANS experimental data fits well with the polydisperse sphere morphology and a gradual increase in the droplet size is observed as the lecithin/ AOT ratio is increased. The different packing parameters for AOT and lecithin drive the observed changes in droplet sizes. For lecithin/ AOT molar ratios approaching unity, both SANS and static light scattering measurements reveal an attractive interaction develops between these droplets, leading to the formation of clusters. Hydrodynamic radii obtained from dynamic light scattering measurements show results that are consistent with the SANS data.

ACKNOWLEDGEMENTS
Good things in one's life don't happen just like that. It requires a lot of support and a lot of sacrifices from the people who care about you. It would not be possible for me, to be where I am today, without the love and support of the people around me.
First of all, I would like paying my sincere thanks to my parents. They are just amazing, the way they encouraged me all life long. Their love is precious to me.   Tables   TABLE  PAGE   CHAPTER II 11-1 Compositions of all the components in the microemulsion, for all the 22 samples.

II-2A
Results from the fit of polydisperse sphere form factor for samples 23 1, 2 and 3 and polydisperse sphere form factor with square well structure factor for sample 4 and 5, to the SANS experimental data.

II-2B
Results from the fit of ellipsoid form factor, in the appropriate 24 q region for all the samples, to the experimental data 11-3 The radius of gyration obtained from a Guinier analysis and the 25 radius of gyration calculated from the dimensions of the two different morphologies under consideration.

11-4 'a'
the average area/head group calculated for the SANS data using 26 Eq. II-8. ' a ' decreases systematically from sample 1 to sample 5 indicating a more dense surfactant film and greater rigidity at the interface as the lecithin content is increased.

111-1
Results from the experimental data for all the five samples. 57 Column 2 contains the hydrodynamic radii information from dynamic light scattering experiments. Column 3 reports the radius of gyration obtained from the Guinier analysis of the static light scattering data. Column 4 reports the calculated radius of gyration (using Eq.IIl-3) based on the hydrodynamic radius tabulated in colurnn2.

CHAPTER IV
IV-1 Comparison of the experimental average specific heat capacity for 74 all the samples with the specific heat capacity calculated using simple mixing rule. AOT (sample 4), and ( + ) 0.005M Lecithin /0.005M AOT (sample 5). Sample 1, 2 and 3 show a classical scattering profile, a low q asymptote (the Guinier region), followed by a drop (fractal region), and then a decay at large q. Sample 4 and 5 shows an upturn in intensity at low q indicative of interactions between the particles.

II-2(a)
Cross-section Guinier plots for O.OlM AOT sample, (0) x = 0, (0) 28 x = 1, and(~) x = 2. The solid line represents the straight line fit to the experimental data. These data clearly suggests the potential morphologies of spheres rather than long cylinders or lamellar shapes.

Il-2(b)
Cross section Guinier plots for 0.00125 M lecithin/0.00875 M 29 AOT sample, (0) x = 0, (D) x = 1, and (~) x = 2. The solid line represents the straight line fit to the experimental data. These data clearly suggests the potential morphologies of spheres rather than long cylinders or lamellar shapes.

II-2(c)
Cross section Guinier plots for 0.0025 M lecithin/0.0075 M AOT 30 sample, (0) x = 0, (D) x = 1, and (~) x = 2. The solid line represents the straight line fit to the experimental data. These data x clearly suggests the potential morphologies of spheres rather than long cylinders or lamellar shapes.

II-2(d)
Cross section Guinier plots for 0.00375 M lecithin/0.00625 M 31 AOT sample, (0) x = 0, (D) x = 1, and (~) x = 2. The solid line represents the straight line fit to the experimental data. These data clearly suggests the potential morphologies of spheres rather than long cylinders or lamellar shapes. II-2(e) Cross section Guinier plots for 0.005 M lecithin/0.005 M AOT 32 sample, (0) x = 0, (D) x = 1, and (~) x = 2. The solid line represents the straight line fit to the experimental data. These data clearly suggests the potential morphologies of spheres rather than long cylinders or lamellar shapes. II-4(d)Non-linear least square fit (solid line) to the (EB) 0.00375M 41 lecithin/0.00625M AOT (sample 4) data with an ellipsoid form factor. Fit is done in an appropriate q region (q>0.018A-1 ) to obtain particle shape information only.

II-4(e)
Non-linear least square fit (solid line) to the (0) 0.005 M lecithin/0.005 42 M AOT (sample 5) data with an ellipsoid form factor. Fit is done in an appropriate q region (q>O.Ol 8A-1 ) to obtain particle shape information only.

111-2
Straight-line fit (solid line) to the Ln(Rq) versus q2 data for 59 samples, (0) O.OlMAOT (samplel) and (D) 0.00125M Lecithin /0.00875M AOT (sample2). Slope of the fit decreases from samplel to sample2 indicates increase in the radius of gyration with increasing lecithin content in the system.

Ill-3(b)
Cryo-TEM image showing micelles (arrow heads) of sizes 5-10 nm 61 in a thin vitrified film of the system consisting of n-pentane, ndodecane, octaethylene glycol dodecyl ether (C12 (EO) 8) and water, mixed in a fixed ratio of 1: 1 :2:22.67(by wt). In recent years, there has been a significant interest in the use of self assembled surfactant microstructures, as a templated media for the synthesis of novel materials. A much studied surfactant system for the synthesis of nanoclusters is the AOT water irI oil microemulsion system. Here, the anionic surfactant, AOT (bis-2ethylhexyl sodium sulfosuccinate) (Figure 1-1) stabilizes rnicroaqueous droplets in a hydrocarbon phase. The water pools serve as microreactors for the synthesis of inorganic materials, and restrict particle growth to the nanoscale. The AOT water in oil microemulsion 1 system has been used successfully in the synthesis of semiconductor quantum dots 7 and super paramagnetic ferrites 8 with novel optical and magnetic properties.

III-4(a)
The work in this thesis is based on our finding of two-surfactant system, which demonstrates a novel transformation from a microemulsion to a rigid gel state.
Specifically, we have found that the combination of zwitterionic surfactant is added to the system, the viscosity increases, and the solution turns to a rigid gel phase when W 0=50-70(W 0 is defined as the molar ratio of water to AOT). The gel stays rigid for W 0 levels of up to 200 and typical zero shear viscosities over this range are on the order of 10 6 Pa.s. 9 Thus these surfactant gels may be sufficiently rigid to serve as templates for the synthesis of nanostructured materials, with the hypothesis, that the spatial immobilization of the hydrophilic and the hydrophobic regions would allow the formation of extended structures that are organized over multiple length scales.
In order to exploit these systems for templated material synthesis and also from the fundamental perspective of self-assembly in mixed surfactant systems, it 1s important to completely understand the microstructures involved in these systems.
Numerous methods have been reported in literature for microstructure studies of complex fluids. A wide variety of techniques can be used to analyze the microstructures in self-assembled systems, including small angle neutron scattering (SANS), static and dynamic light scattering. All of these techniques involve modeling of the experimental data to determine particle shape and size. Imaging techniques like TEM, Cryo-TEM, SEM or Freeze fracture TEM provides high-resolution direct images. Thus the interpretation of data is model independent in this case. The excellent study on the microstructure of nonionic rnicroemulsion using microscopy, neutron magnetic resonance (NMR), small angle X-ray scattering (SAXS) and small angle neutron scattering (SANS) reported by Regev et al. 10 has been cited throughout this work.
The gel phase (W 0 =70 onwards) of these systems has already been characterized using small angle neutron scattering (SANS). 11 As either the water content or temperature is varied, the microstructure undergoes a transition from columnar hexagonal to lamellar ( Figure I -2). This change in curvature is only present in the mixed surfactant system at these overall concentrations. It was hypothesized that the different surfactant packing parameters, P (P = vial, where v is the volume occupied by the tails, a the effective head group cross-sectional area, and l the maximum effective tail length), displayed by AOT (P = 1.1) and lecithin (P = 0.6) prompts a deformation of the spherical AOT reverse micelles as lecithin is added, producing structures of lower curvature.
In this dissertation SANS, light scattering and cryogenic transmission electron microscopy have been done at low surfactant and low water content (W 0 =20, total surfactant concentration in isooctane = O.OlM). In order to reveal the respective roles of the two surfactants in determining the morphology of the water droplets, different ratios of lecithin to AOT (0, 1/3, 1/7, 3/5, 1) were used.
Chapter II of this dissertation reveals the effect of two surfactants, on the droplet morphology, using small angle neutron scattering. Light scattering and cryogenic transmission electron microscopy (Cryo-TEM) studies on the similar system are reported in chapter III. . The idea is to combine these techniques together for a better understanding of the microstructure of the system under consideration.
An effort has also been made to further explore the gel system. Freeze fracture transmission electron microscopy and differential scanning calorimetery techniques were used and those results are reported in chapter IV.

II-A Introduction
Surfactants have the ability to self-assemble into a wide variety of supramolecular structures such as micelles, bilayers, vesicles, liquid crystals, and emulsions. 1 Factors that determine the specific type of structure that is formed by a surfactant include the alkyl chain length, 2 the nature of the solvent, surfactant concentration, 3 temperature, salt concentration, 4 and the presence of a co-surfactant. 5 The additional compositional degree of freedom available in mixed surfactant solutions allows a wider range of aggregate structures and morphologies than is possible in single surfactant systems. For example, aqueous mixtures of cationic and anionic surfactants form a rich variety of composition-dependent microstructures in solution including spherical micelles, worm-like mixed micelles, vesicles, and lamellar phases. 6 The expanded range of micellar geometries and functionalities available in mixed surfactant systems not only makes them useful for applications, but also provides opportunities for fundamental studies of self-assembly. Typical length scales for these micelles are of the order of several nanometers. Thus neutron scattering, where the probe wavelength is several angstroms, is a useful way to probe micellar structures. 6 • 7 • 8 Additional advantages include the ability to selectively deuterate desired components in a mixture, thus allowing specific domains to be 9 observed, and the insensitivity to dust.
In the experiments reported here, the 'aqueous' phase is deuterium oxide (D20).
Our interest lies in the micelles formed by mixing the anionic surfactant AOT and the zwitterionic phospholipid lecithin with water and isooctane. We have previously reported on the rigid mesophase formed when water is added to a solution containing 0.8M AOT and 0.4M lecithin in isooctane for W 0 (molar ratio of water to AOT) values greater than 60. 9 The micro structure of this 'gel' has been characterized extensively via SANS . 10 As either the water content or temperature is varied, the microstructure undergoes a transition from columnar hexagonal to lamellar. This change in curvature is only present in the mixed surfactant system at these overall concentrations. It was hypothesized that the different surfactant packing parameters, P (P = vial, where v is the volume occupied by the tails, a the effective head group cross-sectional area, and l the maximum effective tail length), displayed by AOT (P = 1.1) and lecithin (P = 0.6) prompts a deformation of the spherical AOT reverse micelles as lecithin is added, producing structures of lower curvature.
Cadmium sulfide nanoparticles formed by precipitation in the AOT I lecithin I isooctane I aqueous solution microemulsion change shape from spherical to acicular as lecithin to AOT ratio in the microemulsion is increased from 0: 1 to 1: 1. 11 This dramatic change in nanoparticle morphology is potentially a templating effect, produced by changes in droplet shape when lecithin is added to the system. In this paper, we look at mixtures of these surfactants formed at compositions just above the critical micelle concentration to uncover the role of lecithin on the configuration of the reverse micelles. By probing the effects of binary surfactant interactions on the resultant shape of the reverse micelles, a better understanding of self-assembly and microstructure may be obtained. From an applied perspective, these experiments may also help elucidate templating effects on nanoparticle synthesis.
Deuterium oxide (99.9%) was purchased from Cambridge Isotopes Laboratory. All chemicals were used without further treatment or purification. Reverse micellar solutions were obtained by adding the exact amount of D 2 0 necessary to bring the W 1 = 20 (W 1 = moles D 2 0 I moles total surfactant). Since the total molar concentration of surfactant in isooctane does not vary, a fixed Wi ensures that the fraction of water in each of the samples is also identical. Observed changes in scattering profiles can then be assigned only to variations in size and morphology of the deuterated water droplets, and interactions between them.

II-B3. Experimental details
Measurements were carried out on the 30 m NG3 bearnline at the NIST Center for Neutron Research (Gaithersburg, MD). The instrument utilizes a mechanical velocity selector as a monochromator, a circular pinhole collimator, and a twodimensional position sensitive detector (65 x 65 cm 2 ). The SANS intensity, I, was recorded as a function of the scattering vector q (= 4nsin(8/2)/J .. , where 8 is the scattering angle and 'A the neutron wavelength). The neutron wavelength was 6 A.
The detector angle was set at 2° and the sample-to-detector distance was set to 2m and 13m to cover the widest possible range of q (0.004 -0.6 A-1 ) . Samples were contained in closed stainless steel cells with quartz windows that provided a path length of 2 mm. The sample temperature was maintained at 25°C. The raw data was corrected for the scattering from an empty quartz cell, the detector sensitivity and background, the transmission of each sample, and placed on an absolute scale using software provided by NCNR. Since the major components of incoherent scatter are the spin contribution from hydrogen atoms and compositional contributions arising from differences in hydrogen and carbon cross-sections in isooctane, this number 12 was subtracted from all our intensity data.

11-C Results and discussion
A summary of all the compositions of the samples used in this study is presented in Table II- The scattering intensity obtained from an assembly of particles is given by 13 where N is the number of scattering centers, K; is the contrast factor resulting from radiation of type i, P( q) is the intraparticle form factor describing scattering for a single particle, and S( q) is the structure factor accounting for interparticle interactions.
In dilute systems the intraparticle function P(q) dominates the scattered intensity and S(q) goes to unity, leading to At low q, the scattering intensity from a dilute system of non-interacting particles is independent of morphology, and is given by Here Rg is the radius of gyration of the scattering objects. Rg can be obtained from a plot of ln[I(q)] versus q2. For spheres of radius R, the radius of gyration, Rg, is given by Eq. II-4 needs to be modified if the spheres are polydispersed. For a Schultz size distribution of spheres 14 with a mean particle radius R and a standard deviation cr , the scattering intensity as q ---)-0 is given  leading to the following expression for the radius of gyration (Eq. II-6) Here (Eq. II-7) Figure II-2(a) shows a plot of ln(I(q)) versus q 2 for sample 1, as well as a Guinier fit, using Eq. II-5 to fit the data. The radius of gyration, obtained from Eq. II-6 is 31.3 A. This procedure is repeated for samples 2 and 3. Samples 4 and 5 display upturns in intensity at low q (for sample 3 this is observed in the light scattering data at q ~ 0.002 A-1 , not reported here. Samples 1 and 2 do not show this upturn in the q space probed by light scattering). However, the length scale corresponding to the upturn is at least an order of magnitude larger than the droplet sizes, validating a Guinier fit for these samples in the appropriate region. All the radii of gyration produced this way are shown in Table II samples, x = 0, providing further evidence that the droplets are spheres, and eliminating the possibility that the droplets are long rods (x = 1) or that the scattering is from lamellae (x=2).
Each of the I(q) data sets were fitted using expressions for the form factors for polydispersed hard spheres, and shown in figure b,c,d,e). The mean sizes as well as the polydispersities (defined as the standard deviation cr divided by the mean radius R) are shown in Table II-2A. As the lecithin content is increased, the mean size goes up, while the polydispersity decreases. For water-in-oil microemulsions, the droplet radius as a function of water content is described by 16 (Eq. II -8) where vis the molecular volume of water and 'a' the average area/head group occupied by the surfactants at the interface. Table II-4 shows the values of 'a' calculated for our data using Eq. II-8 decreases systematically from 52.5 (A 2 ) for sample 1 to 39 (A 2 ) for sample 5, indicating a more dense surfactant film and greater rigidity at the interface as the lecithin content is increased. The observed decrease in polydispersity is also a consequence of this increased rigidity at the isooctane/water interface.
The twin tailed aruoruc surfactant bis(2-ethylhexyl) sodium sulfosuccinate (AOT) typically forms spherical water-in-oil microemulsions. 17 AOT has a surfactant packing parameter, P, of 1.1 (P = vial, where v is the volume occupied by the tails, a the effective head group area, and l the maximum effective tail length), and tends to have a spontaneous curvature that is concave towards water. 18 • 19 This causes AOT to spontaneously form spherical reverse micelles once above the critical micelle concentration. The zwitterionic surfactant phosphatidylcholine (lecithin) has a significantly larger head group and a smaller packing parameter of 0.6. 20 Thus lecithin would form structures of lower curvature than AOT. Indeed, in nonpolar solvents, lecithin forms wormlike cylindrical reverse micelles that can incorporate water up to a water/lecithin molar ratio of 20.

21
When both surfactants are present, the droplets grow as lecithin is added, reflecting this geometric packing effect.
The upturn in intensity at low q for samples 4 and 5 ( Figure  If each of the two surfactants distributes themselves locally on the interface in regions of preferred curvature, then the spherical AOT reverse micelles can be deformed into prolate ellipsoids as lecithin is added to the system .. Therefore, our SANS data was fitted to ellipsoid morphologies, and the fits are shown in Figure II -4(a, b, c, d, e) and the lengths of the major and minor axes resulting from these fits are reported in Table II-2B. The ratio of the major to minor axis changes from 2 for sample 1 to 1.5 for sample 5. The approach of this ratio towards 1 as the lecithin content is increased is consistent with the increased rigidity of the interface from samples 1 -5. Since form factors for polydispersed spheres and prolate ellipsoids can be quite similar over the q range examined here, it is not surprising that this alternate model also fits the data. The picture that develops then is either polydispersed spheres with AOT/lecithin ratios varying between spheres, or ellipsoids where the AOT and lecithin occupy different regions of an ellipsoidal interface. On the basis of this SANS data alone, it is thus not possible to discriminate between these droplet morphologies.
The radius of gyration of an ellipsoid with principal axes radii ' a' and 'b', where 'a ' is the radii of the rotation axis, is given by (Eq. II-10) Using the morphology and size data from the fits, the calculated radii of gyration are compared to the measured Rg in Table II-3 . The maximum deviation is 10%, pointing to the consistency in the model being applied.
We therefore summarize the interpretations to the scattering patterns in these dilute surfactant systems. With pure AOT as the surfactant, our observations are consistent with the literature that these are spherical droplets with a measure of polydispersity, and with negligible droplet-droplet interactions at these low concentrations. 17 The partial substitution of lecithin for AOT, keeping overall concentration the same, leads to a larger size and greater interfacial rigidity. While we do not have direct evidence, there could also a deformation of the droplets to prolate ellipsoids. The tendency of AOT to occupy regions of higher curvature (about water) and the tendency of lecithin to form structures of lower curvatures, imply that the major axis of the ellipsoids with the higher curvatures regions being predominantly occupied by AOT, while lecithin may predominantly occupy the low curvature regions at the ends of the minor axis. The substitution of lecithin also leads to "sticky" droplets with very long-range interactions of the order of 50 nm.

11-D Conclusions
Reverse We note a very interesting observation here. In the synthesis of CdS nanoparticles in these mixed surfactant systems at a higher overall surfactant concentration (O. lM), we have seen a dramatic change in particle morphology, from quantum dots in the pure AOT system, to highly acicular quantum rods in the 1: 1 AOT:lecithin system. 11 Additionally, the structure of these CdS crystallites changes 20 from the face-centered cubic structure in the quantum dot morphology to the hexagonal structure in the quantum rod morphology. It is interesting to note that the length of these acicular particles is of the order of 50-100 nm, while the width is [4][5] nm. Perhaps the length extension of these quantum rods is a function of the interaction and cluster formation of the ellipsoidal droplets in the mixed surfactant system. Thus, it may be possible, by the use of mixed surfactants, to modify nanoparticle crystal structure and to template nanoparticle shape characteristics.   The solid line represents the straight line fit to the data. These data clearly suggests the potential morphologies of spheres rather than long cylinders or lamellar shapes.   I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I '  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  I  Transmission electron microscopy (TEM) is potentially one of the most useful techniques in the study of micro structured fluid system, providing direct images of the microstructure. The high vacuum in the microscope column, typically less than  Pa, imposes a very strict limitation on the vapor pressure of the sample to be examined. It dictates that almost every fluid system to be examined requires pretreatment. Also, in these systems one typically finds rapid motion on the supramolecular scale, which needs to be arrested to prevent blurring of the image, as it is recorded by a slow scan CCD camera. The process of making the specimen satisfying the conditions of low vapor pressure and arrested motion is called 'fixation'. Fixation can be done either chemically (by addition of an alien chemical compound to the system, which selectively absorbs to certain part of the specimen, thus stabilizing them) or thermally (by ultra fast cooling of the liquid to a solid or a quasi solid state, at which its vapor pressure is extremely low and supra molecular motion is stopped).
Most complex fluids are very sensitive to the chemical fixation so the only alternative is thermal fixation, which is also called direct imaging Cryo-TEM 5 . An attempt has been made to understand the microstructure in the low concentration mixed surfactant system with varying Lecithin/ AOT ratios using Cryo-TEM as a direct imaging technique.
Distilled and deionized water was used. All chemicals were used without further treatment and/or purification.

III-B2 Sample preparation
Five stock solutions with differing molar ratios of AOT and lecithin were synthesized by adding the measured amount of either surfactant to a specific volume of isooctane and sonicating the mixture until a clear isotropic solution was obtained.
The five stock solutions had AOT/lecithin molar compositions of (1)

III-B3.1 Static light scattering
Samples were contained in a 26mm outside diameter glass vials with Teflon stoppers. The vials were rinsed thoroughly in tap water followed by several rinses with 49 filtered water and dried upside down in an oven in order to prevent dust from collecting inside the cell. All the samples were filtered several times using 0.22micrometer filter to get rid off any dust contamination.
An argon ion laser from Brookhaven Instruments Corporation (BIC) was used for the light scattering measurements. The scattering intensity, I(q), was recorded as a function of the scattering vector q (= 4nsin(8/2)/A. with 8 defined as the scattering angle and A. the laser wavelength). All data collection was carried out at 25 °C. The range of the q-values used for the static light scattering measurements was 0.0008 A to 0.003 A.

ITI-B3.3 Cryogenic transmission electron microscopy
A few (2)(3) microliters of a sample stored at 25 °c in a controlled environment vitrification system were withdrawn from a pipette, deposited on a lacey carbon electron microscope grid, and blotted to remove excess liquid. The grid bearing the sample was plunged into a liquid ethane reservoir cooled by liquid nitrogen to a temperature close to its freezing point. The rapid heat transfer away from the grid vitrified the sample. The specimen was transferred under liquid nitrogen to the cooled tip of a cryotransfer stage (Oxford CT3500), which was then inserted under positive dry nitrogen pressure into the JOEL 1200 TEM and imaged at slight underfocus. The sample temperature was maintained at -165 °c at all times during imaging to prevent any transformation from amorphous to crystalline phase in isooctane.

III-Cl Light scattering
To obtain a value from the measured intensity that depends only on the property of the scatterer and not on the source or the detector optics we used the Rayleigh ratio which is defined as follows, (Eq. III-1) where Ie(=time averaged solution intensity-time averaged solvent intensity), le is the time averaged toluene intensity at 90°, 9 is the scattering angle and Rc(90°) is the Rayleigh ratio of toluene at 90° (= 32xl0-6 cm-1 ).
Using the Guinier approximation, at low q the scattering intensity R(q) is given by, where Rg is defined as the radius of gyration of the particle.
We can therefore extract a value for Rg by plotting lnRe( q) versus q 2 and measuring the slope of the line that best fits the data. If the micelles are spherical, the radius R, is given by the equation The DLS experimental data was analyzed using a multiple pass non-negatively constrained least squares analysis. The mean hydrodynamic radius (Rh) was calculated from DT using the Stokes-Einstein relation for spherical particles. which is an oil (dispersed phase) in water (continuous phase) microemulsion system, was used as the second control in order to make sure the feasibility of imaging micelles of sizes 5-20 nm.

111-D Results and discussion
Mean hydrodynamic radii of all the five samples, determined by the dynamic light scattering measurements, are reported in Table III

111-E Conclusions
Light scattering studies on the reverse micelles formed by a mixture of the anionic surfactant AOT and the zwitterionic lecithin are in accord with all the findings from the SANS analysis. The differences in the calculated radius of gyration from a Guinier region analysis for both techniques accounted for the differences in probing the water core in case of SANS versus probing the whole particle (water core +

IV-A Introduction
The structural study of microstructured fluids is a major area of research in colloid science. As a working definition, a microstructured fluid is one in which one or more components of the fluid have organized on supermolecular scales, from one to hundreds of nanometer. Microstructured fluids arise in a number of physiological, biological, chemical, and physical systems as interfaces, films, foams, emulsions, suspensions, membranes, gels, and liquid crystals.
A system consists of two surfactants, 0.8M AOT(bis-2ethylhexyl sodium sulfosuccinate) and 0.42M lecithin(phosphatidylcholine) in isooctane, which demonstrates a novel transformation from a microemulsion to a rigid gel state with increasing water content of the system, falls in this category of a microstructured fluid.
The microstructure of the gel state of this system has been characterized extensively via SANS. 1 As either the water content or temperature is varied, the rnicrostructure undergoes a transition from columnar hexagonal to lamellar phase.
Modem rapid cooling methods followed by freeze fracture replication techniques provides a direct visualization of the three dimensional structure of the particles or units that make up the system, while simultaneously revealing their 68 orientation and distribution with molecular resolution. This is the direct imaging technique used to characterize the mixed surfactant gel system that is described in this chapter.
The other part of this chapter deals with the thermal properties of the system under observation. Differential scanning calorimetery (DSC) is used to detect any thermal transitions in the system as it goes from one phase to another.
Distilled and deionized water was used. All chemicals were used without further treatment and/or purification.

IV-B2 Sample preparation:
Typical and 170. Once the H 2 0 was added, the samples were sonicated in a warm water bath and vortexed until a clear yellow liquid was obtained. This process was repeated until the total required amount of H 2 0 was added for the corresponding W 0 value. The samples were then removed from the water bath and allowed to cool to ambient conditions. The samples were all in the gel state.

IV-B3.1 Freeze fracture transmission electron microscopy (FFTEM):
A small amount of sample (maintained at 25°C and 40°C) was put on a sample pan and rapidly frozen in liquid nitrogen. The frozen material is then :fractured, at low temperature (-l 65°C) and at ultra high vacuum, with the aid of the glass knife, inside the freeze :fracture equipment (Central microscopy facility, University of Massachusetts, Amherst). The :fractured surface was then replicated by a platinum coating ( 4-5nm) followed by a carbon backing (15-20 nm). The remaining sample was then dissolved, using methanol, and the platinum replica supported by a carbon film was collected on the electron microscope grid for direct imaging via transmission electron microscopy (TEM). Imaging was done using JOEL 1200 TEM.

IV-B3.2 Differential scanning calorimetery (DSC):
Measurements were done using Differential Scanning Calorimeter (v. DSCQlOO series) from TA Instruments. The instrument calibration was performed using an indium standard. For each sample a single stainless steel sample pan from Cole Parmer was designated. Baselines were generated for all the samples; each empty sample pan was equilibrated at 10 Degrees Celsius than kept at that temperature for 3 minutes then heated at a constant ramp rate of 5°C/minute. Weighed amounts of the sample were then sealed in the stainless steel pans and equilibrated at the starting temperature of 10°C for 3 minutes prior to the initiation of the heating scans. Heat flow differences between the sample and reference pans were recorded over the temperature range of 10-60 °C.

IV-C Analysis:
The DSC works by measuring the temperature of two pans, one reference and one containing the sample. In order to increase the temperature of either pan, heat must be added. Intuitively, the pan containing the sample will require more heat to be added to it than the empty pan. The DSC measures this heat flux and records it.
Amount of heat added to the system per unit time can be related to its temperature change by the following equation,

Ml liT
--=me -lit p lit (Eq. IV-1) where 1'.1H/ l' .1t is the rate of heat added to the system, m is the mass of the system, Cp is the specific heat capacity of the system and 1' .1T/1' .1t is the temperature ramp rate.
Knowing the ramp rate with heat flow information and the mass of the system, specific heat capacity (cp) of the system can be determined.

IV-D Results and discussion:
Figure IV-1 shows a freeze :fracture TEM image of W 0 = 170 samples at 40°C.
Parallel lines running throughout the image clearly show the evidence of the lamellar phase in the system at this water content. The other samples including Wo = 130 and W 0 = 150 did not provide any adequate details of the microstructure and that is why those images are not reported here.
DSC experimental results of the specific heat capacities for all the three samples are presented in FigureIV-2. The experimental error associated with the specific heat capacity hides out any major changes in the heat capacity of the system with the temperature. The experimental average specific heat capacity, for the observed temperature range, for all the samples are reported in Table IV-1 and compared with the specific heat capacities, calculated using simple mixing rule.
Within the experimental error all the samples shows a good accord with the theoretical values.

IV-E Conclusions:
Freeze :fracture transmission electron microscopy has proven to be a successful technique in terms of imaging out this system. Further efforts in this field may give proper details of the microstructure in the gel system. DSC, on the other hand, may 72 provide evidence of the heat changes involved with the phase transitions in the gel phase with increasing temperature of the system.