(400e) Interfacial and Molecular Interactions between Cellulose Nanocrystals and Surfactants in Brine and Responsible for Pickering Emulsion Stability

Stable O/W emulsions were prepared using dodecane as organic phase and aqueous phases containing a combination of cellulose nanocrystals (CNC) and surfactants in high salinity brines. In addition, interfacial and molecular interactions between negatively charged CNC and different surfactants were analyzed in the presence of high ionic strength (I) brines; American Petroleum Institute (API: I=1.9M) brine and synthetic seawater (SSW; I=0.65M). The surface properties of cellulose nanocrystals in brine were studied in terms of surface/interfacial tension as a function of CNC concentration in brine. At concentrations higher than 0.5 wt.%, CNC exhibited surface activity and decreased the surface tension of brine. In addition, the interfacial tension between dodecane and API brine was decreased at CNC concentrations as low as 4x10^-3 wt.%. Adsorption of surfactants onto the CNC surface was studied by surface tension, spectroscopy and surfactant concentration measurements. The surface tension vs concentration plot of coco-alkylamines shifted to the right (higher concentration) in the presence of CNC, while there was no appreciable change for dodecyltrimethylammonium bromide (DTAB) and octyl glucopyranoside (OGP) in either SSW or API brine. Adsorption isotherms for various surfactants were generated indicating the adsorption of both CAA and DTAB onto CNC in both API brines and SSW; OGP did not significantly adsorbed. FTIR and Raman spectroscopy confirmed the –OH groups on the surface of the CNC are involved on the adsorption of both surfactants onto the CNC surface. Contact angles between surfactant solutions and CNC films submerged in dodecane were measured to quantify wettability of CNC by surfactant solution in brine; the wettability was decreased by the presence of DTAB, whereas CAA instead increased it. Based on the results from surface properties and molecular interactions, O/W emulsions were prepared using dodecane as organic phase and CNC-surfactants mixtures in the aqueous phase. Emulsion stability was then measured over the duration of 24 hours by monitoring the creaming rate of emulsions and droplet size distribution to detect presence/absence of droplet coalescence. At oil fractions lower than 0.5, CNC without surfactants stabilized the emulsions against creaming for longer times compared to surfactants mixed with CNC. The former emulsions showed smaller average droplet size with narrower distributions indicating they are stable to coalescence as well. The findings of the study indicate that both ionic and non-ionic interactions like H-bonding, are important for synergistic interactions between CNC and surfactants in brine.

Keywords: Cellulose nanocrystals, adsorption, Pickering emulsions, surfactants, surface, and molecular interactions