Controlling the Microstructure of Reverse Micelles and Their

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Controlling the Microstructure of Reverse Micelles and their Templating Effect on Shaping Nanostructures Soma Sharma, Nitin Yadav, Pramit K. Chowdhury, and Ashok Kumar Ganguli J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b03063 • Publication Date (Web): 06 Jul 2015 Downloaded from http://pubs.acs.org on July 7, 2015

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The Journal of Physical Chemistry

Controlling the Microstructure of Reverse Micelles and their Templating Effect on Shaping Nanostructures

Soma Sharma,a,b Nitin Yadava , Pramit K. Chowdhurya and Ashok K. Gangulia, c* a

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India b

Department of Chemistry, Central University of Haryana, Jant-Pali, Mahendragarh, Haryana 123029, India c

Institute of Nano Science & Technology, Phase X, Mohali, Punjab 160062, India

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Abstract Reverse micelles as nanoreactors have been most successful in designing nanostructures of different size and shape. Nevertheless, important questions regarding the explicit roles of intrinsic parameters in modifying soft colloid templates which eventually give rise to variety of nanostructures are still unresolved. In this paper, we have focused on this challenging aspect of microemulsion based synthesis of nanostructures i.e. how the tunable parameters like water to surfactant molar ratio, solvent properties and surfactant structure modifies the microstructure (size/shape) of reverse micelles (surfactant/cosurfactant/oil/water). Further we have elucidated the correlation between these nanoreactors with the size and morphology of the evolving nanostructures within the aqueous core (using insitu studies) as well as the finally obtained nanostructures. We have employed Fluorescence correlation spectroscopy (FCS), small angle Xray scattering (SAXS), dynamic light scattering (DLS) and transmission electron microscopy (TEM) to obtain details on the microstructural transformation of reverse micelles and their templating behavior on designing nanostructures, at (near) single droplet level and in an ensemble. The structure (size/shape) of nanoreactors i.e. reverse micelles finally guides the size and shape of nanostructures. As the water content increases it induces the micellar growth and subsequently the growth of nanostructures develops linearly up to a critical value beyond which the finite bending modulus of surfactant film triggers the structural rearrangement of microemulsion droplets (MEDs) and the linear plot shows deviation. Bulkiness of the solvent molecules modulates the ME droplets and MEDs encapsulated nanostructures by influencing the curvature and rigidity of surfactant film and results in smaller dimensions of the micellar core which leads to nanostructures with large aspect ratio. The origin of this structural evolution may be explained in terms of solvent molecular structure which affects the penetrability of solvent molecules into the surfactant tail region. Interestingly, MEDs with cationic surfactants feature the onset of one dimensional micellar growth and the shape evolves into a nearly prolate spheriod. Consequently the growth of micellar core and dynamical exchange in an anisotropic manner leads to formation of nanorods. The implication of such studies could be far reaching due to the geometry-dependent novel properties and potential applications of anisotropic nanostructures.


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Introduction The remarkable transition in the properties of nanomaterials with the variation in their size and shape and their wide variety of applications has triggered enormous interest of researchers in nanoscience and nanotechnology.

Owing to the geometry-dependent novel properties and

potential applications of anisotropic nanostructures such as nanospheres, nanorods, nanowires, nanobelts, nanoplates and nanohelix, in chemical/optical sensing,1,2 nanoelectronics,3,4 surface enhanced raman scattering,5 metal enhanced fluorescence6 and catalysis,7 researchers put tremendous efforts on the theme to design the nanostructures with controlled size and morphology and also engineered the nanomaterials with different functionality to mediate targeted drug delivery,8 in vivo imaging,9 multimodal MRI,10 bioprobes,11 energy harvesting12 and so forth.

Numerous preparation methods such as sol-gel processing, chemical vapor

deposition, coprecipitation, hydrothermal/solvothermal methods, and biomimetic synthesis, have been established and documented for fabricating nanostructures. However, among them reverse micelle emerged as one of the most versatile and successful method for precisely tailoring the nanostructures. Our group has successfully synthesized metal nanoparticles,13 metal borates14 and rare-earth hexaboride anisotropic nanostructures,15 dielectric16 and magnetic oxides17, 18 and ternary and quaternary oxides19 over relatively narrow size distribution by employing reverse micelles as a synthetic route. Reverse Micelles, considered as shaping vesicles for inorganic nanostructures, are formed at certain composition of water in oil (w/o) and is a microemulsion which is an isotropic, thermodynamically stable dispersion of water droplets in a continuous oil phase stabilized by a surfactant and cosurfactant (generally a short chain alcohol and amine). The nanometer sized aqueous core of reverse micelles act as “nanoreactor” within which controlled reactions leading to the formation of wide array of nanostructures over a relatively narrow size distribution. The significant advantage of this approach is that nanostructures are homogenous and monodisperse and one could precisely tailor the size and morphology of nanostructures by controlling the structure of reverse micelles.

There is a considerable amount of literature available that

investigated the role of controlling variables e.g. surfactant structure,20 cosurfactant,21,22 molar ratio of water to surfactant molecules, W0 = [H2O/Surfactant],23,24 solvent23,24 and other factors such as intermicellar exchange rate,25 elastic constant of surfactant film26 and reactant 3 ACS Paragon Plus Environment


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concentration27 merely on the size and morphology of nanostructured materials synthesized by microemulsion based synthesis route. Vaidya et al.28 varied surfactants in the composition of reverse micelles and obtained different morphology, nanospheres, nanocubes and nanorods of nickel oxalate. They also showed bulkiness of the solvents leads to larger dimensions of nanorods. Ranjan et al.29 observed decrease in the particle size and change in morphology of copper oxalate from nanorods to nanocubes and nanoparticles as cosurfactant chain length increases. Effect of intermicellar exchange rate has been studied by varying different parameters including water to surfactant molar ratio.25 It was found that with increase in W0, the particle size initially increases and then decreases, also average particle size and polydispersity found to be dependent on intermicellar exchange rate. The growth kinetics of iron oxalate nanorods inside the water pool of CTAB led to a comprehensive insight on the growth mechanism.30 Interestingly, three distinct periods were observed: a nucleation-dominant nanoparticle growth period, followed by anisotropic growth of nanoparticles to nanorods and further elongation of nanorods. Thus the unequivocal evidences provided by the vast scientific literature construct a reliable picture of how tunable parameters and other aforementioned factors influence the growth of nanostructures in an anisotropic manner within reverse micelles. However, it is particularly noteworthy here that a clear picture on the role of tunable parameters in controlling the structure (size/shape) of nanoreactors i.e. reverse micelles which eventually guides the size and shape of nanostructures, is still missing and emergence of this conclusive picture requires intensive investigation. Previous work has been limited to the influence of parameters on the size and shape of the nanoscale final materials. Very few researchers have addressed this challenging issue which is still not understood. Shrestha et al.31 studied structural control of reverse micelles in surfactant/oil binary systems (w.r.t alkyl chain length of oil, temperature, surfactant concentration, added water), have shown evidence of anisotropic micellar growth upon increase of the alkyl chain length of oil from decane to hexadecane, decrease in micellar size as temperature increases, one dimensional micellar growth with surfactant concentration and formation of swollen micellar core upon adding of water i.e. induced two dimensional micellar growth.

Nevertheless the shortcoming of the study is that their

investigation was limited to the structural control of reverse micelles (binary and ternary system). They make no attempt to correlate the induced structure transition of reverse micelles by tunable parameters with the structure (size and shape) of nanostructures. The present study aims to 4 ACS Paragon Plus Environment

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answer an important issue, how the controlling variables i.e. surfactant structure, alkyl chain length of oil phase, water to surfactant molar ratio influence the structure (size/shape) of quaternary (surfactant/cosurfactant/oil/water) water in oil microemulsion droplets (nanoreactors) which we correlate with the size and morphology of evolving (insitu) nanostructure within the aqueous core as well as the final nanostructures. In order to get a complete understanding, we have carried out a detailed and comprehensive study on the synthesis of nickel oxalate nanostructures as a case study by varying controlling variables, surfactant structure, solvent in terms of bulkiness, water to surfactant molar ratio. We have used FCS, SAXS along with DLS and TEM characterization techniques in order to acquire an in-depth knowledge on the microstructural transformation of reverse micelles by tuning compositional parameters and their templating behavior on designing nanostructures of desired shape and size. Owing to the limited resolution, FCS could detect only the small sized MEDs with nanoparticles, though, insitu DLS could detect large droplets/aggregates with enclosed nanostructures, (likely nanorods) after completion of reaction (15 hr). So we have employed FCS and DLS techniques to monitor the micellar growth of small and large sized droplets/aggregates present in the microemulsion system respectively. DLS and TEM are also used to follow the growth of obtained nanoparticle/nanorods. In fact, by combining FCS and SAXS with DLS and TEM techniques one can obtain a comprehensive understanding of the nanostructure growth mechanism and the templating effect of ME systems in a ME-based reaction scheme. Note that we have earlier reported the synthesis of nickel oxalate using CTAB as surfactant, 1butanol as cosurfactant and isooctane as the oil phase at W0 12. We obtained uniform and smooth nanorods of nickel oxalate hydrate (225 nm diameter and 2.5 µm length). We anticipate that the present study would provide a deeper insight in to the understanding of how the structural transitions of reverse micelles, stimulated by varying intrinsic parameters influence the growth of anisotropic nanostructures within the micellar core at single droplet level and enable researchers to precisely architect the nanostructures with different size and morphology by controlling the structure of reverse micelles which considered as shaping vesicles.

Materials and Methods 5 ACS Paragon Plus Environment


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Commercially available Cetyltrimethylammonium bromide (CTAB, Sigma Aldrich, 99%), Cetylpyridinium bromide (CPB, Sigma Aldrich, 98%) and Triton-100 (TX-100, Sigma Aldrich) were employed as surfactant in the reverse micellar based synthesis of nickel oxalate nanostructures. Isooctane (UV-grade, Spectrochem, 99%), n-hexane (UV-grade, Spectrochem, 99%) and cyclohexane (UV-grade, Spectrochem, 99%) used as solvent and 1-butanol (UV-grade, Spectrochem, >99%) employed as cosurfactant in the synthesis. Nickel nitrate (Fisher Scientific, 98%) and ammonium oxalate (CDH, 99%) were used as reactants and HPLC water (Merck) employed as aqueous medium in synthesis. Mixture of Chloroform (SRL, 99.5%) and methanol (Qualigens, 99.8%) (1:1) used for washing the product and obtained nanostructures were redispersed in ethanol (Merck, 99.9%) for further analysis. All the solvents were dried before use and the purity of CTAB, 1-butanol, isooctane, nickel nitrate (in water) and ammonium oxalate (in water) was confirmed by taking their correlation curves in FCS. Absence of the correlation curves ascertained the purity of the reagents. Sulphorhodamine-B (SRhB, Sigma Aldrich) was used as fluorescent probe. For

the

synthesis

of

nickel

oxalate

nanostructures

two

microemulsions

(surfactant/cosurfactant/oil phase/aqueous phase) were prepared, one containing Ni2+ ions (0.1 M nickel nitrate) and the other containing C2O42- ions (0.1 M ammonium oxalate) as reactants. Each microemulsion solution containing independent reactants was stirred continuously at 25C till the solution become clear. Equal volumes of two microemulsion solutions with individual reactants were mixed to start the reaction. This is a fast reaction which takes 15 hr for its completion. A dye (sulphorhodamine-B, 3nm) was added to act as a reporter molecule. SRhB being a polar molecule (insoluble in isooctane) stays inside the aqueous core and being negatively charged stays near the interface of the CTAB/1-butanol/isooctane reverse micelles (due to the positively charged surfactant). Insitu FCS and DLS measurements were done at the starting of the reaction i.e. at 0 hr and after completion of the reaction i.e. after 15 hr of reaction. SAXS measurements have been carried out on bare microemulsion droplets (MEDs) and MEDs containing reactants i.e. ammonium oxalate (AmOx) and nickel nitrate (Ninit). DLS and TEM measurements were carried out on the nickel oxalate nanostructures redispersed in ethanol after completion of the reaction. Characterization Techniques 6 ACS Paragon Plus Environment

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TEM studies were carried out on a FEI Technai G2 20 electron microscope operated at 200 kV. The obtained nickel oxalate nanostructures were washed with a mixture of chloroform and methanol with 1:1 ratio then dried at room temperature and redispersed in ethanol before carrying out the TEM measurements. The size of MEDs with and without the nanostructures of nickel oxalate was measured using a Malvern Zeta Sizer ZS 90 DLS spectrometer with a 633 nm CW laser. DLS studies were carried out on the MEDs containing reactants (nickel nitrate and ammonium oxalate) and the product at the starting of the reaction (0 hr) and after completion of the reaction (after 15 hr).

An aliquot of the reaction mixture was taken for in situ DLS

measurements. To obtain the size of the particles, an aliquot of the reaction mixture was centrifuged and the surfactant layer was washed with the mixture of chloroform and methanol and then DLS measurements were carried out on the redispersed nanostructures in ethanol. FCS measurements were carried out using a home-built FCS setup as discussed earlier.30 In FCS, the fluorescence fluctuations from a dye inside tiny observation volume (~1 femtoliter) are correlated.

The fluctuations are time-correlated to obtain normalized auto-correlation

function, G(), as shown in equation 1

G   

F t F (t   ) F t 

(1)

2

Where, F(t) is fluorescence intensity at time t, and  is time shift. F is the fluctuation in fluorescence relative to average, . For a 3-D Gaussian observation volume with radial (r) and axial length (l), G() can be related to diffusion time (D) of fluorophore by equation (2).

1  G   1  N D 

   

1/ 2 1 2    r      1   l          D   

(2)

Here, N is average number of fluorophores present in the observation volume. The diffusion coefficient (D) is then related to the diffusion time (D) as

D 

r2 4D 7 ACS Paragon Plus Environment


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For the present microemulsion solution, a modified version of equation (2) i.e. equation (3) is used to model the correlation curves, which incorporates a single term for reaction-time and a bimodal Gaussian distribution for diffusion times.  m  G     a 1  i i  1   Di

Where,

   

 1 2 1   r        l     Di 

    

1

2

(3)

  2 

  ln    ln  Di 1p  ai  D i   B1i exp   b1   

   

  2 

  ln    ln  Di 2p   B2i exp   b2   

   

B1i and B2i are the relative contributions of components of the two distributions, 1p and 2p are the peak diffusion times, and b1, b2 are related to the widths of distributions. Least-square fitting is performed using equation (3). Figure 6 shows the data fitted with equation (3). The hydrodynamic radii (Rh) of the particles were then calculated from Stokes-Einstein equation, 4

k T R  B h 6D

(4)

where, kB is Boltzmann’s constant, T is temperature (25°C in the present case), and η is the viscosity of solvent (bulk solvent isooctane/cyclohexane/n-hexane in this case). SAXS measurements were carried with SAXSess mc², Anton Paar GmbH instrument. We used a SAXSess camera (Anton Paar, Austria) attached to an ID3003 X-ray generator, equipped with a sealed X-ray tube which was operated at 40 kV and 50 mA. An equipped Göbel mirror and a Kratky block collimation system enabled us to convert the divergent polychromatic X-ray beam into a focused monochromatic X-ray beam of Cu Kα radiation (λ = 0.1542 nm) with a welldefined line shape.

Samples are introduced into a quartz capillary and placed inside an

evacuated chamber. The 2D scattering patterns were measured recorded by an imaging-plate (IP) detector (a Cyclone, Perkin-Elmer) and integrated into to one dimensional scattered intensities I(q) as a function of the magnitude of the scattering vector q = (4π/λ) sin(θ/2) using 8 ACS Paragon Plus Environment

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SAXS Quant software (Anton Paar), where θ is the total scattering angle. All I(q) data were corrected for the background scattering from the capillary and the solvents. The background scattering (solvent-ﬁlled capillary) was measured separately and subtracted from the scattering curve. The SAXS data was analyzed to obtain the real space structure function, the so called pair-distance distribution functions, p(r) via a virtually model free routine, we used the generalized indirect Fourier Transformation (GIFT) method.

Results and Discussion In our previous studies, we have reported the microemulsion based synthesis of nickel oxalate nanostructures with CTAB as surfactant, 1-butanol as cosurfactant and isooctane as continuous phase. Smooth, homogenous and mono dispersed nanorods of nickel oxalate were obtained with water to surfactant molar ratio of W0=12. We have also investigated the role of solvent and surfactant on the size and morphology of synthesized nickel oxalate nanostructures. The present study is an attempt to understand the role of tunable parameters in influencing the structure of quaternary reverse micelles (Surfactant/cosurfactant/oil phase/aqueous phase) and their correlation with the size, morphology and growth rate of nanostructures (nickel oxalate). Effect of Surfactant Structure In order to unravel the influence of surfactant structure on the structure (size/shape) and micellar growth of MEDs and further their effect on the nanostructures, we have studied three different surfactant structure w.r.t the charge and size of polar head group. Three different surfactants were employed, CTAB, CPB as cationic surfactant and TX-100 as nonionic surfactant to carry out the microemulsion synthesis. The other controlling variables namely aqueous content (W0 =12), oil phase and cosurfactant, were kept constant. Thus, the systems studied were, as follows: CTAB/Isooctane/butanol/aqueous

phase,

CPB/Isooctane/butanol/aqueous

phase,

TX-100/

Cyclohexane/butanol/aqueous phase. Figure 1 (A, B, C) shows the nanostructures synthesized with variation of surfactant (CTAB, CPB and Tx-100 respectively) as observed by TEM. Interestingly, spherical morphology of particles, dimensions ∼30 nm were obtained in case of non-ionic surfactants while using the cationic surfactant CTAB and CPB, nickel oxalate nanoparticles assembled in to the nanorod 9 ACS Paragon Plus Environment


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morphology with aspect ratio ~ 10:1 (length ~ 3.0 µm, dia. ~ 300 nm) and ~ 4:1 (length ~ 1.6 µm, dia. ~ 400 nm) respectively. DLS studies were performed on extracted nickel oxalate nanostructures, results (figure 2) revealed that small nanostructures of dimensions ~ 30 nm were obtained with nonionic surfactant, TX-100 and larger nanostructures in case of cationic surfactant, CTAB and CPB. Thus, MEDs with nonionic surfactants give spherical morphology of nanostructures and with cationic surfactants, nanorod morphology of nickel oxalate nanostructures obtained, as ascertained by the TEM studies. It is noteworthy here that using nonionic surfactants, isotropic nanostructures were obtained whereas nanoparticles assembled in an anisotropic manner and give rise to nanorods in case of cationic surfactants. Furthermore, insitu FCS and DLS studies were carried out on MEDs at the beginning (0 hr) and at the completion (15 hr) of reaction in order to obtain a qualitative picture of how surfactant structure modified the initial MEDs (bare, reactant ions, nucleation dominant particles), perturbed MEDs encapsulated nanostructures, further their correlation with synthesized nanostructures and unravel their contribution in guiding a particular morphology. The dimensions of MEDs, bare and with solubilized reactant ions (ammonium oxalate, AmOx) and nickel nitrate, Ninit), was found to be almost same for a particular surfactant structure as shown in figure 3 (a,b,c) measured by DLS. Similar trend is observed in FCS and the data matches well with the dimensions obtained from DLS data (see Figure 4b). The size of bare MEDs with CTAB, CPB and Tx100 as surfactant was around 4 nm, 5 nm and 11 nm respectively. The insitu FCS and DLS studies revealed the nature of droplets/aggregates present in the microemulsion solution during synthesis. The smaller MEDs dimensions were observed at the starting of the reaction, may be mostly due to the reactants and nucleation dominant nanoparticles, approximately 3-4 nm, 6-7 nm, ~ 11 nm for CTAB, CPB and TX-100 respectively and larger droplets/aggregates were obtained after completion of reaction (15 hr) which correspond to grown nanostructures. Interestingly, the sequence gets reversed i.e. smaller size of MEDs with nanostructures in case of nonionic surfactant and larger MEDs with cationic surfactant. When the reaction on the edge of completion, along with large MEDs, small MEDs also observed by FCS (figure 4b) may be ascribed to the droplets with very short nucleation dominant particles. We have rationalized the above results of the effect of surfactant structure on the initial MEDs (bare, reactant ions, nucleation dominant particles) and perturbed MEDs with encapsulated nanostructures in terms of interaction energy () between the water and surfactant 10 ACS Paragon Plus Environment

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head, an important parameter which strongly influence the surfactant efficiency. Owing to the negative interaction energy between water and surfactant head, the surfactant molecules position themselves at the water-oil interface with their head pointing to water and tail towards oil. Addition of surfactant molecules remarkably decrease the water-oil interfacial energy, thereby, F, change in interfacial free energy becomes negative (F = E1 - TS, where E, change in interfacial energy, S, change in interfacial entropy and T, temperature of system).

This

signifies larger value of - (negative sign implies the interaction between water and surfactant head is negative) results in larger decrease in F, interfacial free energy. In other words, the stronger interaction (-) between water and surfactants head group leads to large water-oil interface and thereby smaller dimensions of MEDs. Owing to the charge on surfactant head group, the cationic surfactants possess strong interaction between water and surfactant head group (CTAB and CPB) as compared to the nonionic surfactants. Therefore, the MEDs (bare, with solubilized reactant ions and nucleation dominant nanoparticles) of cationic surfactant system were of smaller size while MEDs with nonionic surfactants were large in dimension. In cationic surfactants, CPB has pyridinium ion as the polar head group while ammonium ion in case of CTAB. The restricted orientation of pyridinium ions due to its large size provide steric hindrance to the interacting water molecules results in less interaction between water and head group as compared to CTAB. Thus the order of the size of initial MEDs was observed as CTAB < CPB. Our experimental findings are consistent with the Parbhakar et al.32 stimulated model of surfactant effects using Monto Carlo technique. It was stated that the size of the water droplet decreases with increase in the strength of the interaction energy between water and the surfactant head. In our earlier studies,28,29 we have explained that the cationic surfactants carrying positive head groups arrange themselves into an assembly on the surface of growing metal (nickel) oxalate nanoparticles (because of negative  potential) and subsequently retard the growth along the diameter of nanoparticles. As a consequence, the cationic surfactants guides the micellar growth anisotropically due to increased intermicellar exchange rate along the axis and results in elongated reverse micelles. These elongated MEDs dynamically exchange with other droplets at both ends resulting in the formation of nanorods. However, such an assembly of surfactants on the surface is unlikely with nonionic surfactants since the surfactant head groups are not charged.



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Thus, surfactant molecules arrange themselves isotropically leading to more uniform growth resulting in the formation of nanoparticles with spherical morphology. Also, as explained in our earlier reports30 the droplets containing growing nanostructures should achieve a critical size of ~55 nm before switching their shape and symmetry and grow into larger dimensions, which is achieved here as in case of cationic surfactants but not in case of Tx-100. This indicates that the nanostructures can grow further in case of cationic surfactants and lead to nanorods of larger dimensions while no such anisotropic growth of the nanostructures is observed when oxalate nanostructures were synthesized using Tx-100 as a surfactant system. We have performed SAXS studies on MEDs with CTAB and TX-100 as surfactants to complement the above data (FCS, DLS, and TEM). The SAXS data was analyzed to obtain the pair-distance distribution functions, p(r) via GIFT method. Figure 5 shows the pair-distance distribution functions of MEDs (water, ammonium oxalate, nickel nitrate) with CTAB and Tx-100 as surfactant, obtained from SAXS. The distribution profile of MEDs of Tx-100 (Figure 5 d, e, f) are symmetrical functions which evident the spherical shape of the droplets. However, with cationic surfactant (CTAB), an asymmetric size distribution profile (Figure 5b) suggests that reverse micelle containing AmOx ions has undergone elongation and evolves into nearly prolate spheroid shape.31 These findings feature the onset of one dimensional micellar growth in the presence of cationic surfactant (CTAB). Furthermore, In our earlier work, ZnOx nanostructures containing MEDs (with cationic surfactant, CTAB), as a function of reaction time shown the characteristic of rod like or cylindrical micelles, having size curve asymmetrical in shape with a pronounced peak on the lower-d side and an extended long tail on the higher-d side.33 The above study manifests the effect of surfactant structure on the structure of reverse micelles and subsequently show how the modified structure of reverse micelles give rise to different morphology and size of nanostructures.

Effect of Water to Surfactant Ratio, W0 Water to Surfactant molar ratio, W0 = [H2O/Surfactant], has been considered as one of the key parameters in modulating the structure of reverse micellar templates and subsequently affects the yield of nanoparticles, average particle size and particle size distribution. Several studies have been conducted on the effect of increasing W0 on the particle size. However, an important 12 ACS Paragon Plus Environment

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subject that still requires investigation is how the variation in water content affects the micellar growth and subsequently nanoparticle parameters and in what range of W0 the effect is significant. Such studies are of considerable interest to obtain reproducibly a large quantity of nanostructures with narrow size distribution and monodispersity. With this motivation, we have investigated the effect of water to surfactant molar ratio, W0 = [H2O/Surfactant] on the reverse micellar template and further we examined the influence on nanostructures, synthesized at different W0. We have studied the reverse micellar system at following W0 values 4, 8, 12, 16, 20 and 30 while keeping all other controlling parameters (surfactant, cosurfactant, oil phase) constant. We have found that at higher W0 (> 30 ) value, the transparent microemulsion solution gradually turns into turbid emulsions. This onset of turbidity is apparent to the naked eye as a dramatic decrease of transparency. FCS and DLS studies were performed on MEDs (bare, with solubilized reactant ions and nickel oxalate nanostructures, aliquot of the sample was taken out from the reaction mixture at the starting of reaction, 0 hr and on the completion of reaction i.e. 15 hr with varying W0 values 4, 8, 12, 16, 20 and 30) and the obtained data was analysed to determine the size of MEDs near single droplet level and in an ensemble. Figure 6 shows the FCS data for MEDs containing oxalate nanostructures at the starting of the reaction (W 0=8) measured for the reverse micelles and was fit to the equation (modified form of equation 2) by using a Gaussian distribution function. Figure 7 and 8 show plots of the size of the droplets with variation of W0 measured by FCS and DLS studies respectively. Interestingly, we have found that the average size of bare MEDs and the droplets containing solubilized reactants (AmOx and NiNt) varies from ~3 to ~ 12 nm and show a linear increment up to W0 of 20. The size of MEDs deviates from linearity beyond W0 of 20 and show a marked decrease in size from ~12 to ~5 nm at W0 of 30. The results obtained with MEDs containing nickel oxalate nanostructures during the progress of the reaction (0 hr and 15 hr) revealed the nature of droplets/aggregates present in the microemulsion solution. Two kinds of MEDs were observed, one with small dimensions contain nucleation dominant nanoparticles which increases in size up to W0 of 20 then decreases and other one with larger size enclosed growing nanostructures that shows linear increment in size till W0 of 16 afterwards the size decreased. A linear increase in the size of the MEDs up to a certain value, W0 of 20 in case of bare MEDs and MEDs with solubilized reactant ions and W0 of 16 with MEDs containing growing nanostructures can be attributed to the increased free water concentration inside the water pool, 13 ACS Paragon Plus Environment


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results in the swollen micellar core. At low water content, inside the micellar core, the structure of water molecules modifies into bound water. Owing to the scarcity of water molecules w.r.t per surfactant molecule to hydrate the counter ions and polar head group, the polar head group arrest the water molecules in its vicinity which leads to decrease in the inner water content within the core and makes the environment rigid.34 However, as the water content inside the MEDs gradually increases, the quantity of free water also increases which induces the micellar growth. Owing to the finite bending modulus of the surfactant film which covers the MEDs, the size of MEDs increases up to a certain extent on increasing the water content beyond which the micellar core size decreases. In other words, the bigger droplets formed at higher W0 are not stable, since they undergo a structural rearrangement and break down into smaller droplets due to the finite bending modulus of surfactant film. Nevertheless, further investigations with similar systems would be necessary to make unequivocal assertion of this structural rearrangement phenomenon. In order to explore the correlation between the sizes of templating droplets with the growth of the nanostructures synthesized in them as a function of W0 values, we have investigated the size of nickel oxalate nanostructures at different W0 value with DLS and TEM. These studies were performed on the nanostructures which were extracted from the micellar solution and then redispersed in ethanol for measurements. The results obtained with DLS studies revealed an increase in size of nanostructures up to W0 of 16 and then decreases till 30, as shown in figure 9, thereby findings is in well agreement with the insitu DLS and FCS results on the templating MEDs containing oxalate nanostructures (see Figure 7 and 8). TEM studies (figure 10) show the formation of a mixture of nanoparticles and nanorods at W0 value of 4 and then the dimension of these nanorods increases till W0 of 16. Nanorods of smaller dimension were obtained as W0 increased from 16 to 20 and then mixture of smaller nanoparticles and nanorods were obtained at W0 of 30 which resembles the nanostructures obtained with W0 of 4. TEM results suggest that mean size of the oxalate nanostructures is governed by the initial MEDs size till W0 of 16 and beyond that it deviates from their behaviour and thus supported the findings of FCS and DLS studies. Therefore, our results clearly evident the linear relation between the size of reverse micellar templates and the nanoparticles synthesized in them. The process of Nanorod formation from dissolved reactant ions can be represented in the following sequence: ions  monomers  nuclei  nanoparticles  growing nanostructures 14 ACS Paragon Plus Environment

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(critical size ~55 nm, switch the growth anisotropically30)  Nanorods. A stable nucleus can be formed after a cluster containing a critical no. of monomers (Nc) formed35 which further grow by the addition of ions and monomers and aggregation of primary particles or nuclei led to bigger particles. It is interesting to note here that a nucleus is formed if the ion occupancy number is greater than Nc, which is an important parameter, defined as the number of reactant species in reverse micelle. The surfactant aggregation number increases as W0 increases,36, 37 as a result, the micellar concentration decreases which led to the increase in ion occupancy number. Furthermore, increase in flexibility of the surfactant film with increasing aqueous content, results in a higher intermicellar exchange rate.

Therefore, increase in W0 affects two important

parameters, average ion occupancy number and intermicellar exchange, both play crucial role in the formation and growth of particles. At low W0 value, water is mainly present as bound water which decreases the micellar size and makes the interface rigid. Consequently both parameters ion occupancy number and intermicellar exchange rate decreases, resulting in the decrease in the particle size. On the contrary, at higher W0, the increased free water induces swelling in micellar core, thus MEDs size increases, as a result, the average ion occupancy number increases and intermicellar exchange rate also increases due to increase in flexibility leading to the increased size of nanostructures. As W0 value increases further, the bigger MEDs split into the smaller MEDs due to the finite bending modulus of surfactant film and results in smaller nanostructures. Our findings are consistent with the earlier observations, Bagwe et al.,25 investigated the role of aqueous content on the size of AgCl nanoparticles.

They concluded that the particle size

increased with increase in W0 from 5 to 10, but at W0 of 15 the size of the particles decreased. Atik and Thomas38 presented W0 values from 5.5 to 11, they showed that increase in water content decreases the rigidity of interface that leads to increased reaction rate, and however above W0 value of 11 they observed a decrease in reaction rate. Roberts et al.24 studied the W0 effect on copper nanoparticles synthesis in the range W0 = 3 to 15 in isooctane/water/AOT system. They reported maximum particle growth rates at approximately W0 of 12 beyond which the growth rate decreased. However, most of these studies were based on the effect of the aqueous content in reverse micelles on the product (nanoparticles) synthesized in them. In our studies we have seen the effect of variation of aqueous content both on the size of the templating droplets and also on the growth of the nanostructures synthesized in them and have established a correlation between the two. 15 ACS Paragon Plus Environment


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Effect of Continuous Phase Solvent properties significantly affect the micellar dynamics because the degree of penetration of solvent molecules (oils) into the surfactant chain region of reverse micellar systems differs depending on the chain length of solvents. The properties of solvents influences the attractive interaction between micelles by affecting the bending elasticity of surfactant films. 39

The

micellar exchange rate constant decreased by a factor of 10 when solvent is changed from isooctane to cyclohexane.40 Owing to the above important facts, we intended to study how interactions of surfactant lipophilic tail-solvent molecules modify the structure of reverse micelles and then correlate these structures with the size and morphology of synthesized nanostructures.

In this study, Nickel oxalate nanostructures were synthesized using CTAB

microemulsions (1-butanol as cosufactant, W0 of 12) with cyclohexane, n-hexane and isooctane as the oil phase. Figure 11 (a, b) shows the size of bare MEDs and MEDs with solubilized ions and growing nickel oxalate nanostructures, obtained by insitu DLS and FCS measurements in different solvent systems. The hydrodynamic size of bare MEDs and the droplets containing solubilized ions is found to remain almost constant as observed from DLS and FCS data (Figure 11 a, b), in each solvent. The insitu FCS and DLS measurements were also carried out with MEDs containing nanostructures at starting (0 hr) and after completion of the reaction (15 hr) to examine the structural transition of perturbed MEDs template with the growth of nanostructures. Analysis of data revealed the size of droplets/aggregates present in microemulsion solution (Figure 11a & b). As expected the droplet dimensions were smaller at the starting of the reaction, approximately 3 - 4 nm, 6-7 nm, 9-11 nm for isooctane, cyclohexane and n-hexane respectively, used as bulk (oil) phase and larger droplets/aggregates were obtained after completion of reaction (15 hr) which enclosed the nanostructures. However, interestingly, the order got reversed for the nanostructures i.e. smallest size of MEDs with nanostructures in case of nhexane and largest in case of isooctane. In the initial hours of reaction along with the small droplets, insitu DLS and FCS measurements could detect the very short nucleation-dominant nanoparticle growth of nanostructures present in the droplets. Beyond this DLS could not detect the smaller droplets, which could be due to the availability of very few MDs containing the nucleated particles which have very low (total) scattered intensity to be detected by DLS (Figure 11 a). Hence, using DLS and FCS one can easily follow the growth of nanorods within MEDs. As evident by the above results, the MEDs (bare, containing solubilized ions and nucleation 16 ACS Paragon Plus Environment

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dominant nanoparticles) size is found to be smallest in case of isooctane and then dimensions increases as oil phase changed from isooctane to cyclohexane and largest droplets were obtained when nanostructures were synthesized using n-hexane as the bulk phase, while the size of MEDs with nanostructures follows the size reverse order as; n-hexane < cyclohexane < isooctane. DLS measurements were performed on nickel oxalate nanostructures, extracted from the reaction mixture and redispersed in ethanol, to see the correlation between the template and the nanostructures synthesized in them. Interestingly, we found that the size of nanostructures (extracted from the microemulsions) follows the same order as the dimensions of MEDs containing the grown nanostructures (15 hr), as measured by insitu DLS i.e. size in n-hexane < cyclohexane < isooctane (figure 12). The nanostructures are slightly larger than that observe by insitu DLS, which might be due to agglomeration of nanostructures. TEM studies were also performed on the extracted nickel oxalate nanostructures to ensure the correlation between structure of template and obtained nanostructures. Results show the formation of nanorod like morphology with varying aspect ratio, 10:1 (an average diameter of 300 nm and average length of 3 μm, figure 13a), 6:1 (an average diameter of 300 nm and a length of ∼ 1.8 μm, figure 13b) and 5:1 (diameter of 110 nm and length of 565 nm, Figure 13c) with the solvent (continuous bulk phase) as isooctane, cyclohexane and hexane respectively. Thus, TEM studies clearly evident the remarkable decrease in the aspect ratio of nickel oxalate nanorods with the variation of continuous phase as isooctane, cyclohexane and n- hexane. In an interesting manner, the order of decrease in the aspect ratio of nanorods i.e. isooctane > cyclohexane > n-hexane, matched with findings of insitu DLS studies on MEDs with encapsulated nanostructures. It is noteworthy here that nature of solvent structure also influences the uniformity of nanorods. The findings revealed that nanorods synthesized with n- hexane and cyclohexane were not as uniform as those obtained with isooctane as bulk oil phase. We have measured the size of MEDs and the synthesized nickel oxalate nanostructures by using DLS, FCS and TEM, near the single droplet level and in an ensemble.

The underlying

mechanism of the structural evolution of MEDs during the progress of reaction, as discussed above, may be rationalized in terms of different degree of penetration of oil into the surfactant tail region, resulting in different extent of solvation into the micellar tail region which profoundly influences the surfactant curvature and rigidity of the reverse micellar system. 17 ACS Paragon Plus Environment


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Owing to the entropy of mixing, the penetration of shorter chain alkanes into the surfactant layers is more effective in comparison to longer chain homologues. The hexane molecules because of shorter chain length tend to easily penetrate into the surfactant tail region and lead to increased effective cross-section area of the tail region. As a consequence, hexane molecules are efficiently packed in the alkyl tail region and increase the negative curvature of the surfactant film (surfactant polar head groups on the interior of the aggregate and the apolar tails on the exterior surface, defined here as negative curvature). Cyclic alkanes as solvents, in our case cyclohexane, are more effective than the straight-chain oil in terms of their penetrability into the micellar alkyl tails and consequently favor increased negative curvature of the surfactant interface by packing in the alkyl-tail region.40 However isooctane being bulkier with a larger molecular volume cannot penetrate and solvate the surfactant tails effectively. This signifies that the effect of isooctane as solvent on the curvature of surfactant interface is not to a significant degree. The order of increased curvature of surfactant film, isooctane < cyclohexane < hexane resembles the successive increase of MEDs size (bare, with solubilized ions and nucleation dominant particles), isooctane < cyclohexane < hexane and both the above parameters are inversely related to the bulkiness of solvents, hexane < cyclohexane < isooctane, at a particular W0. Thus, the above discussion provides a picture that bulkiness of the solvents directs the order of increased curvature of the surfactant film i.e. isooctane < cyclohexane < hexane which guides the size of the bare MEDs and MEDs with solubilized ions and nucleation dominant nanoparticles which increased in this successive order as isooctane < cyclohexane < hexane. The size of MEDs with nanostructures and the nanostructures obtained after extraction, followed the reverse trend where size increased in the solvents in a manner as, hexane< cyclohexane< isooctane. The observed deviation in the size trend between initial MEDs and MEDs with encapsulated nanostructures can be rationalized in terms of rigidity of the surfactant monolayers, stimulated by different degree of penetrability of solvent molecules in the surfactant tail region. We have earlier explained the order of the ability of penetration of oil into the tail region on the basis of chain length and bulkiness of solvent used in this study i.e. isooctane < cyclohexane < hexane. In the reverse micellar reaction, intermicellar exchange rate as rate determining step40 profoundly influences the growth kinetics of the nanostructures inside the MEDs.

These

intermicellar exchanges between MEDs are governed by attractive interactions (interdroplet tailtail interactions) between the micelles. The hexane and cyclohexane molecules being easily 18 ACS Paragon Plus Environment

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penetrable between the surfactant tails increased the packing density of reverse micelles and provide a rigid interface for intermicellar exchange. Consequently, the retarded intermicellar exchange rate results in decreased size of MEDs with enclosed nanostructures and also reduced aspect ratio of the nanorods extracted. On the other hand, owing to the long chain length and bulky structure of isooctane, it is difficult to penetrate for these molecules into the surfactant tails and provide flexible interface (less rigid) for intermicellar exchange. The interdroplet tail-tail interactions between the surfactant molecules increases as packing density decreases because of weak presence of isooctane molecules in tail region, resulting in increase in the collision frequency and intermicellar exchange which leads to an increase in the growth rate of nanostructures. Consequently, the size of MEDs containing nanostructures and the aspect ratio of synthesized nickel oxalate nanorods increased in the order of decreased rigidity of the interface, i.e for reactions using hexane < cyclohexane < isooctane. Earlier, it has been explicitly stated that decrease in rigidity of inverted (reverse) micelles is linked to the penetrability of oil into surfactant tail region. Binks et al.39 studied the bending elasticity constant (or rigidity) of AOT monolayers by varying chain-length of n-alkanes from C7 to C14. Their findings revealed that the rigidity of microemulsions decreases marginally up to decane, C10 chain length (~ 1 kT) and above C10, sharply decreases to about ~ 0.6 kT for tetradecane. The authors stated that the differing degrees of oil penetration into the surfactant tail led to decrease in rigidity with increasing oil chain length. Furthermore, Eastoe et al.41 measured the film rigidities in dichain phospocholine based microemulsion system with C6-C16 n-alkanes. Their results also suggest decrease in rigidity as n-alkanes chain length increased and related it to penetration of alkanes. MEDs droplets have less negative curvature and less rigid surfactant film interface results in high intermicellar exchange rate that leads to the large dimensions of MEDs encapsulated nanostructures which finally gives large aspect ratio of synthesized nickel oxalate nanorods. This study presented a clear picture of how solvent properties guide the structure of the template MEDs which finally govern the structure of the extracted nanostructures.

Conclusions Nickel

oxalate

nanostructures

were

synthesized

in

CTAB/isooctane/1-butanol/water

microemulsions with variation of W0 from 4 to 30. The results indicate that careful control of 19 ACS Paragon Plus Environment


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water to surfactant ratio (W0), surfactants and bulk oil phases is vital in designing the optimal conditions needed to make particles of desired size and shape. Results suggest that size of MEDs, kinetics and growth mechanism of nanostructures formation follows a linear relation with W0 till 16 and deviates beyond that. At higher W0 (beyond 16), there are some structural changes in reverse micelles and due to this difference in the structural makeup of the water cores, the MEDs and the nanostructures size are smaller at higher W0. TEM images show that smooth nanorods with high aspect ratio are obtained when isooctane was used as an oil and aspect ratio of these rods decreased in the decreasing order of their intermicellar exchange rates. Bulkiness of the solvent molecules leads to smaller reverse micelles, higher curvature, larger exchange rates and larger dimension of nanorods.

As confirmed by DLS, TEM and SAXS studies

nonionic surfactants favor the formation of spherical MEDs and isotropic growth of particles which lead to smaller nanostructures with cube-like morphology as compared to cationic surfactants which favor the formation of cylindrical MEDs and produces anisotropic rod-like morphology. Initial microemulsion droplet size/shape is the vital factor in controlling the final particle size synthesized in the droplet. AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] ; Phone: +91 011 2659 1511. Acknowledgements: This work is supported by DST (Nano Mission) (SR/NM/NF95/2010), DeitY (12(4)/2007-PDD), Govt. of India. We thank Dr. Sobhan Sen and his student Moirangthem Kiran Singh for helping us in FCS experiments. Soma Sharma thank CSIR for providing fellowship.

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Figure Captions:

Figure 1 TEM micrographs of nickel oxalate nanorods synthesized using (a) CTAB, (b) CPB, and (c) Tx-100 as surfactants. Figure 2 DLS measurements of nickel oxalate nanostructures synthesized by variation of surfactant from CTAB to CPB to Tx-100. Figure 3 Distributions of hydrodynamic size of MEDs for different surfactant systems containing (a) water (b) ammonium oxalate (AmOx) and (c) nickel nitrate (Ninit) plotted on logarithmic size-scale. Figure 4 Size of MEDs, with different surfactant systems which contain water, reactant species ammonium oxalate (AmOx) and nickel nitrate (Ninit) and nickel oxalate nanostructures synthesized at 0h (NiOx_0h) and after 15 h (NiOx_15h) of reaction obtained by (a) DLS and (b) FCS studies. Figure 5 The pair-distance distribution functions, p(r) of CTAB MEDs containing (a) water (b) ammonium oxalate (c) nickel nitrate, and TX-100 MEDs (d) water (e) ammonium oxalate (f) nickel nitrate, obtained as an output of GIFT analysis. Figure 6 Fit to the correlation curve and residuals of the fit for CTAB MEDs containing nickel oxalate nanostructures synthesized with W0 value of 8. Figure 7 Size of MEDs (obtained by FCS) of CTAB RMs which contain water, ammonium oxalate (AmOx) and nickel nitrate (Ninit) and nickel oxalate nanostructures synthesized at 0 hr and after 15 hr of reaction. (● &

represent the bigger droplets containing nickel oxalate

nanostructures synthesized at 0h and at 15h). Figure 8 (A). Size of MEDs (obtained by DLS) of CTAB reverse micellar system which contain water, ammonium oxalate (AmOx) and nickel nitrate (Ninit) and nickel oxalate nanostructures synthesized at 0 hr and after 15 hr of reaction. (B). Expanded part of encircled area of (A). Figure 9 DLS measurements of nickel oxalate nanostructures synthesized at different W0 and redispersed in ethanol. Figure 10 TEM images for the nickel oxalate nanostructures as obtained at different W0. 25 ACS Paragon Plus Environment


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Figure 11 Size of MEDs of CTAB reverse micellar system which contain water, reactants (ammonium oxalate, AmOx and nickel nitrate, Ninit) and nickel oxalate nanostructures synthesized at 0 hr (immediately after mixing) and after 15 h of reaction in different bulk (oil) phase, as measured by (a) DLS & (b) FCS. Figure 12 DLS measurements of nickel oxalate nanostructures (extracted from reaction mixture and redispersed in ethanol) synthesized with varying solvents. Figure 13 TEM micrographs of nickel oxalate nanorods synthesized by using CTAB reverse micelles by variation of bulk oil phase (A) isooctane, (B) cyclohexane (C) n-hexane. Scheme 1 Shape of the MEDs made up of CTAB is cylindrical and that of TX_100 is spherical and so are the nickel oxalate nanostructures synthesized in them.


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Figures:

Figure 1

Figure 2



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(a)

(b)

(c)

Figure 3


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(a)

(b)

RMs containing reactants/product


Figure 4

Figure 5



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Figure 6

Figure 7


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Figure 8

Figure 9



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Figure 10 (a)


(b)


Figure 11 32 ACS Paragon Plus Environment

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Figure 12

Figure 13



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Scheme 1


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For TOC Graphic Only


Controlling the Microstructure of Reverse Micelles and Their

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