Microstructure of ethylcellulose oleogels and its relationship to mechanical properties

• Rheological properties of a variety of ethylcellulose (EC) oleogels were investigated.
• A de-oiling procedure is reported for imaging the internal structure of EC oleogels.
• Pore sizes were characterized for a variety of partially de-oiled oleogels.
• De-oiled oleogels imaged by AFM confirmed the porous morphology observed by cryo-SEM.
• EC xerogels were successfully imaged by SEM, also showing evidence of porous regions.

Small deformation rheology was used at a controlled strain of 0.05% and frequency from 1 to 100 Hz to obtain G′, G″, and tan(δ) values for ethylcellulose oleogels of various compositions. Significant increases in both G′ and G″ were noted in oleogels prepared with 100 cP ethylcellulose as compared to 45 cP, yet no significant differences in tan(δ) were observed among gels containing the same type of vegetable oil. Oleogels prepared with increasing polymer concentration showed a significant increase in G′ only when the concentration exceeded 13%. When the surfactant sorbitan monostearate was incorporated at a level of 3.33%, a significant increase in tan(δ) was seen, indicating a decrease in elasticity of these oleogels. Cryo-scanning electron microscopy was successfully utilized to image the internal polymer network of ethylcellulose oleogels, which contained small oil-filled pores of approximately 3.0–4.5 μm in size lying between the interconnected polymer chains. Decreases in pore sizes were observed with increasing polymer concentration, oil unsaturation, and high levels of surfactant incorporation. No significant change in pore size was seen among gels made with different polymer viscosities, signifying an alternate mechanism for changing oleogel hardness. De-oiled ethylcellulose oleogels were also successfully imaged using fluid-phase contact mode atomic force microscopy. The striking similarity in the images obtained by both these methods strongly suggests the morphological features of the oleogels are preserved through the sample preparation procedure. Ethylcellulose xerogel micrographs were shown for the first time, and also contain some regions with pores similar in morphology to those identified in micrographs of the partially de-oiled oleogels.

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