Investigations of Highly Concentrated Emulsions Incorporating Multi-walled Carbon Nanotubes

2018-08-03T05:38:57Z (GMT) by Sharu Bhagavathi Kandy
<p>The refining characteristics of highly concentrated water-in-oil (w/o) emulsions, wherein the dispersed phase constitutes greater than 90 wt% of the total emulsion, have been investigated. The dispersed phase of the emulsion comprises a supersaturated solution of inorganic salts, and the continuous phase consists of a mixture of an emulsifier in a blend of two oils. The development of microstructure at various stages of the emulsification process has been studied in detail and an empirical correlation between the characteristic droplet size and refining time has been proposed. As the refining time was increased, the Sauter mean diameter (d32) of the aqueous phase droplets decreased exponentially and the width of the droplet-size distribution reduced. The evolution of rheological characteristics of the emulsion during the refinement of the microstructure has also been investigated through different protocols of the dynamic and steady-state rheology. The increase in the refining time led to an increase in the elastic modulus, the yield stress and the viscosity of the emulsion. The network structure of the dispersed phase, the droplet-size distribution and the corresponding interdroplet interactions all govern the rheological characteristics of the final emulsion. The dependence of the elastic modulus and the yield stress on the characteristic droplet-size has also been discussed.<br>Multi-walled carbon nanotubes (MWCNTs) were incorporated into the oil phase of highly concentrated w/o emulsions with the aim of achieving ‘network-like’ structure of MWCNTs throughout the entire continuous phase of the emulsion, which can The refining characteristics of highly concentrated water-in-oil (w/o) emulsions, wherein the dispersed phase constitutes greater than 90 wt% of the total emulsion, have been investigated. The dispersed phase of the emulsion comprises a supersaturated solution of inorganic salts, and the continuous phase consists of a mixture of an emulsifier in a blend of two oils. The development of microstructure at various stages of the emulsification process has been studied in detail and an empirical correlation between the characteristic droplet size and refining time has been proposed. As the refining time was increased, the Sauter mean diameter (d32) of the aqueous phase droplets decreased exponentially and the width of the droplet-size distribution reduced. The evolution of rheological characteristics of the emulsion during the refinement of the microstructure has also been investigated through different protocols of the dynamic and steady-state rheology. The increase in the refining time led to an increase in the elastic modulus, the yield stress and the viscosity of the emulsion. The network structure of the dispersed phase, the droplet-size distribution and the corresponding interdroplet interactions all govern the rheological characteristics of the final emulsion. The dependence of the elastic modulus and the yield stress on the characteristic droplet-size has also been discussed.<br>Multi-walled carbon nanotubes (MWCNTs) were incorporated into the oil phase of highly concentrated w/o emulsions with the aim of achieving ‘network-like’ structure of MWCNTs throughout the entire continuous phase of the emulsion, which can ultimately modify the emulsion characteristics. By keeping the same aqueous-to-oil phase ratio, the amount of MWCNTs in the oil phase was systematically adjusted to investigate their effects on the refining characteristics, microstructure and rheology of the emulsion. The concentration of the MWCNTs in the emulsions for the investigation has been varied from 0.5 to 4 wt% of the oil phase of the emulsion; the corresponding concentration of the MWCNTs in the emulsion varied from 0.0325 to 0.26 wt% of the total emulsion. The refining characteristics of nanotube-incorporated emulsions have been investigated. The incorporation of MWCNTs led to a finer emulsion microstructure with reduced droplet size and narrowed droplet-size distribution. The decrease in droplet size with the addition of MWCNTs is mainly due to the increase in the viscosity of the oil phase which, in turn, results in an increased applied stress during emulsion refining. However, the state of dispersion of MWCNTs within the emulsion also plays a crucial role in determining the final microstructure of the nanotube-incorporated emulsions.<br>The state of dispersion of MWCNTs in the emulsions was investigated through cryo-FEG-SEM analysis, however, from the fractured surface morphology, it was hard to unequivocally conclude the selective dispersion of MWCNTs in the continuous phase of the emulsion. Rheological properties of nanotube-incorporated were characterised as a function of the emulsification time, as well as the MWCNTs concentration. The rheological behaviour of the nanotube-incorporated emulsions was identical to that of the neat emulsions, and primarily governed by the droplet drop size and droplet-size distribution. However, the strain behaviour, especially the yield strain and crossover stain are independent droplet size of the droplet size and the polydispersity of the emulsion. Emulsions that have smaller droplets exhibited higher storage modulus (G^'), yield stress (τ_Y) and apparent viscosity (η). For all the studied refining times, nanotube-incorporated emulsions have higher G^', τ_Y, and η values when compared to the neat emulsion, and these values further increased with the MWCNTs concentration. This is primarily due to the decrease in droplet size with the addition of MWCNTs. Furthermore, our findings suggest that the incorporated MWCNTs did not induce any significant changes in the rheological behaviour of emulsions with identical droplet sizes and it remained essentially unchanged with the MWCNTs concentration. However, the nanotube-incorporated emulsions possessed the solid-like behaviour up to a higher applied stress when compared to the neat emulsion of identical droplet size.<br>Two tetra-alkylated pyrenes have been designed and synthesized for the noncovalent surface modification of MWCNTs, namely, 1,3,6,8-tetra(oct-1-yn-1-yl)pyrene (TOPy) and 1,3,6,8-tetra(dodec-1-yn-1-yl)pyrene (TDPy). The modifier molecules were designed in such a way that they could facilitate better dispersion of individualised MWCNTs in the continuous phase of the emulsion. Moreover, the adsorbed modifiers facilitate the MWCNTs, which are incorporated in the emulsions, to be localised in the continuous phase of the emulsion through the interaction between oil and the alkyl chains of the modifiers. Scanning electron microscopic and transmission electron microscopic analyses suggested that the modifier molecules have been adsorbed on the MWCNT surface, which subsequently resulted in the ‘debundling’ of MWCNT ‘agglomerates.' The red-shift in the C‒H wagging vibrational bands in the FTIR spectroscopy and the G-band shift in Raman spectroscopic analysis for the modified MWCNTs, and the fluorescence quenching of the alkylated pyrene molecule in the presence of the MWCNTs, have confirmed the π–π interaction between the modifier molecules and MWCNTs.<br>The modified MWCNTs were then incorporated into highly concentrated water-in-oil emulsions, and the effect of the noncovalent surface modification on the emulsion morphology was investigated. The concentration of modified MWCNTs was varied between 0.25 ‒ 2 wt% of the oil phase of the emulsion while maintaining the identical droplet size. In the modified MWCNT-incorporated emulsion, there was a significant reduction in the average agglomerate size and the area ratio of the remaining MWCNT agglomerates in the emulsion matrix when compared to the corresponding emulsions that comprise unmodified MWCNTs.<br>The dispersion and localisation of modified and unmodified MWCNTs in the oil phase was assessed through the electrical conductivity measurements. For the MWCNT‒oil blend dispersions, there was a significant improvement in the electrical conductivity (an increase of the order of ~106 in the DC electrical conductivity with 1 wt% MWCNTs). Emulsions with 1 wt% and 2 wt% MWCNTs exhibited a low DC electrical conductivity as opposed to the purely insulating behaviour of the neat emulsion. This change could be an indication of the change in emulsion morphology due to the presence of incorporated MWCNTs. However, the enhancement in the electrical conductivity of the emulsions was very low when compared to the enhancement in oil blend with the addition of MWCNTs. The electrical conductivity measurements of the emulsions did not suggest the formation of a complete and effective percolation network up to an MWCNT content of 2 wt% of the oil phase.<br>In the present study, the first of its kind, an attempt has been made to investigate the effect of incorporation of MWCNTs into the highly concentrated w/o emulsions. A significant level of understanding has been gleaned about the effect of MWCNT incorporation on the morphology and rheology of the HCEs through different microscopic techniques, rheological analysis and various spectroscopic analyses.</p>