Engineering the reverse micellar-templated Co(OH)2 nanospheres based Pickering emulsion and unveiling the mechanism of ambiguous phase separation

Abstract

The present study employs reverse micellar-templated Co(OH)2 nanospheres, in situ hydrophobized with SDS in Pickering emulsion formulations and unlocking the mechanism of ambiguous phase separation at a certain surfactant concentration (4.3 mM). Co(OH)2 is synthesised in the core of reverse micelle: SPAN 80/1-butanol/water/toluene. Novel O/W Pickering emulsions have been developed with the help of positively charged (ς = 4.47 mV) Co(OH)2 (0.05 wt%) and an adequate amount of an anionic surfactant, SDS. An ambiguous phase separation of Pickering emulsion at 4.3 mM SDS is noticed within just 4 hr of standings, whereas, surprisingly stable O/W Pickering emulsions are found above and below these SDS concentrations. Upon addition of 2.6 mM SDS to the aqueous colloidal dispersion of Co(OH)2, the particles converted to amphiphilic one and stabilized the Pickering emulsion. However, these monolayer adsorbed surfactants face a hydrophobic interaction through tail-to-tail mode after adding 4.3 mM SDS to the colloidal dispersion. Consequently, they achieve a nanoplates-like architect and regain the extreme hydrophilic surface from both ends and eventually leads to the ambiguous phase separation through inter-droplet coalescence. The contact angle value altering from 44.9° (for 2.6 mM SDS) to 19.6° (for 4.3 mM SDS) confirms that the surface has changed from being amphiphilic to becoming extremely hydrophilic. Yet, again an increase in SDS (6.06 mM and onwards) suggest that surfactant molecules further adsorbed in either any of the two hydrophilic ends and revolving the particles to its previous amphiphilic state and eventually started to stabilise the emulsion droplets.