The pH-triggered transitional phase behaviour of Pickering emulsions stabilised by hydrophobised bacterial cellulose (BC) is reported in this work. Neat BC was hydrophobised via esterification with acetic (C 2 -), hexanoic (C 6 -) and dodecanoic (C 12 -) acids, respectively. We observed that C 6 -and C 12 -BC stabilised emulsions exhibited a pH-triggered reversible transitional phase separation. Water-in-toluene emulsions consisting of 60 vol.-% dispersed phase stabilised by C 6 -and C 12 -BC

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... ter-in-oil (w/o) emulsions were produced at pH 5. Lowering the pH of the aqueous phase to 1 did not affect the emulsion type. Increasing the pH to 14, however, caused the emulsions to phase separate. This phase separation was caused by electrostatic repulsion between modified BC as a result of dissociable acidic surface groups at high pH, which lowered the surface coverage of the water droplets. When the pH was readjusted to 1 again, w/o emulsions re-formed for C 6 -and C 12 -BC stabilised Pickering emulsions. C 2 -BC stabilised emulsions, on the other hand, underwent an irreversible pH-triggered transitional phase separation and inversion. This difference in phase behaviour between C 2 -BC and C 6 -/C 12 -BC was attributed to the hydrolysis of the ester bonds of C 2 -BC at high pH. This hypothesis is in good agreement with the measured degree of surface substitution (DSS) of modified BC after the pH-triggered experiments. The DSS of C 2 -BC decreased by 20% whilst the DSS remained constant for C 6 -and C 12 -BC.