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Transition from glass- to gel-like states in clay at a liquid interface

View Article: PubMed Central - PubMed

ABSTRACT

Colloidal clay in water suspensions are known to exhibit a multitude of bulk phases depending on initial colloidal concentration and ionic strength, and examples of this include repulsive Wigner colloidal glasses at low ionic strength and attractive gels at higher ionic strength due to screened electrostatic forces by the electrolyte. From confocal Raman microscopy combined with elasticity measurements, we infer that clay trapped at quasi two-dimensional interfaces between oil and water also exhibit confined glass-like or gel-like states. The results can be important for the preparation of particles stabilized colloidal emulsions or colloidal capsules, and a better understanding of this phenomenon may lead to new emulsion or encapsulation technologies.

No MeSH data available.


Confocal Raman microscopy of a single oil drop (a) in Laponite 1.5 wt% dispersion in DI-water; and (b) in Laponite 0.75 wt% dispersion in 0.1 M NaCl obtained by dilution of Laponite 1.5 wt% with the same salt solution (more details in Supplementary Figure 4b). The pictures display a two-dimensional Raman mapping of the oil (green area to the left), Laponite tactoids (yellow/red/white) and water (black) positions. The 2–4 μm thick film of nanostructured Laponite tactoids adsorbed at the oil water interface cannot be seen unless the aqueous phase is diluted with NaCl solution as shown experimentally in Fig. 2c.
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f1: Confocal Raman microscopy of a single oil drop (a) in Laponite 1.5 wt% dispersion in DI-water; and (b) in Laponite 0.75 wt% dispersion in 0.1 M NaCl obtained by dilution of Laponite 1.5 wt% with the same salt solution (more details in Supplementary Figure 4b). The pictures display a two-dimensional Raman mapping of the oil (green area to the left), Laponite tactoids (yellow/red/white) and water (black) positions. The 2–4 μm thick film of nanostructured Laponite tactoids adsorbed at the oil water interface cannot be seen unless the aqueous phase is diluted with NaCl solution as shown experimentally in Fig. 2c.

Mentions: Accordingly, confocal Raman microscopy was used to reveal the Pickering interfacial films, and images scanning a horizontal plane of 13 × 10 μm2 crossing the interface between an oil drop dispersed in different water phases were obtained, as displayed in Fig. 1, revealing the presence not of individual clay platelets (laponite is constituted by disc shaped about 30 nm diameter and 1 nm thick nanoparticles) but rather of tactoids, aggregated laponite particles. For detailed investigations, normalized linearly scanned concentration profiles of each phase near and across the oil-water interface were obtained before and after diluting the water phase and consequently reducing the bulk Laponite concentration by 50%, from 1.5 to 0.75 wt.%. We monitored the main Raman peaks in the 630 to 1710 cm−1 range for Laponite, oil and water, appearing at 680, 1445 and 1630 cm−1 respectively, corresponding to symmetric Si-O-Si stretching in laponite, CH2 bending in mineral oil and HOH bending in water (Supplementary Figures 2 and 3). Clearly, the Laponite tactoids are more or less homogeneously dispersed in pure water arranged in structures that resemble a house of cards, and has a tendency to avoid the oil/water interface, as confirmed by the dark interface indicating the presence of a water layer. At this point, it is important to stress that no significant change was observed upon dilution of suspension with pure water. In contrast, when salt was added into the aqueous phase and the emulsion diluted, a nanostructured layer of Laponite tactoids remained strongly adsorbed at the oil/water interface generating a stable Pickering shell.


Transition from glass- to gel-like states in clay at a liquid interface
Confocal Raman microscopy of a single oil drop (a) in Laponite 1.5 wt% dispersion in DI-water; and (b) in Laponite 0.75 wt% dispersion in 0.1 M NaCl obtained by dilution of Laponite 1.5 wt% with the same salt solution (more details in Supplementary Figure 4b). The pictures display a two-dimensional Raman mapping of the oil (green area to the left), Laponite tactoids (yellow/red/white) and water (black) positions. The 2–4 μm thick film of nanostructured Laponite tactoids adsorbed at the oil water interface cannot be seen unless the aqueous phase is diluted with NaCl solution as shown experimentally in Fig. 2c.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121648&req=5

f1: Confocal Raman microscopy of a single oil drop (a) in Laponite 1.5 wt% dispersion in DI-water; and (b) in Laponite 0.75 wt% dispersion in 0.1 M NaCl obtained by dilution of Laponite 1.5 wt% with the same salt solution (more details in Supplementary Figure 4b). The pictures display a two-dimensional Raman mapping of the oil (green area to the left), Laponite tactoids (yellow/red/white) and water (black) positions. The 2–4 μm thick film of nanostructured Laponite tactoids adsorbed at the oil water interface cannot be seen unless the aqueous phase is diluted with NaCl solution as shown experimentally in Fig. 2c.
Mentions: Accordingly, confocal Raman microscopy was used to reveal the Pickering interfacial films, and images scanning a horizontal plane of 13 × 10 μm2 crossing the interface between an oil drop dispersed in different water phases were obtained, as displayed in Fig. 1, revealing the presence not of individual clay platelets (laponite is constituted by disc shaped about 30 nm diameter and 1 nm thick nanoparticles) but rather of tactoids, aggregated laponite particles. For detailed investigations, normalized linearly scanned concentration profiles of each phase near and across the oil-water interface were obtained before and after diluting the water phase and consequently reducing the bulk Laponite concentration by 50%, from 1.5 to 0.75 wt.%. We monitored the main Raman peaks in the 630 to 1710 cm−1 range for Laponite, oil and water, appearing at 680, 1445 and 1630 cm−1 respectively, corresponding to symmetric Si-O-Si stretching in laponite, CH2 bending in mineral oil and HOH bending in water (Supplementary Figures 2 and 3). Clearly, the Laponite tactoids are more or less homogeneously dispersed in pure water arranged in structures that resemble a house of cards, and has a tendency to avoid the oil/water interface, as confirmed by the dark interface indicating the presence of a water layer. At this point, it is important to stress that no significant change was observed upon dilution of suspension with pure water. In contrast, when salt was added into the aqueous phase and the emulsion diluted, a nanostructured layer of Laponite tactoids remained strongly adsorbed at the oil/water interface generating a stable Pickering shell.

View Article: PubMed Central - PubMed

ABSTRACT

Colloidal clay in water suspensions are known to exhibit a multitude of bulk phases depending on initial colloidal concentration and ionic strength, and examples of this include repulsive Wigner colloidal glasses at low ionic strength and attractive gels at higher ionic strength due to screened electrostatic forces by the electrolyte. From confocal Raman microscopy combined with elasticity measurements, we infer that clay trapped at quasi two-dimensional interfaces between oil and water also exhibit confined glass-like or gel-like states. The results can be important for the preparation of particles stabilized colloidal emulsions or colloidal capsules, and a better understanding of this phenomenon may lead to new emulsion or encapsulation technologies.

No MeSH data available.