Carboxymethyl Chitosan and Its Hydrophobically Modified Derivative as pH-Switchable Emulsifiers

Abstract

The emulsification properties of carboxymethyl chitosan (CMChi) and hydrophobically modified carboxymethyl chitosan (h-CMChi) were studied as a function of pH and dodecane/water ratio. The pH was varied between 6-10, and the oil/water ratio between 0.1-2.0. In CMChi solution, the emulsion stability increased as the pH was lowered from 10 to 7, and the phase inversion was shifted from oil/water ratio 1.0 to 1.8, respectively. The system behaved differently in pH 6 due to the aggregation of CMChi and the formation of nanoparticles (∼200-300 nm). No phase inversion was observed and the maximum amount of emulsified oil was reached at oil/water ratio 1.2. The h-CMChi showed similar behavior as a function of pH but, due to hydrophobic modification, the phase inversion was shifted to higher values in pH 7-10. In pH 6, the behavior was similar, but the maximum amount of emulsified oil was higher compared to CMChi. The amount of adsorbed particles correlated with the emulsified amount of oil. Reversible emulsification of dodecane was demonstrated by pH adjustment using CMChi and h-CMChi solutions. The formed emulsions were gel-like, suggesting particle-particle interaction.

Figure 1

Synthesis of CMChi from native chitosan using chloroacetic acid, and the following synthesis of h-CMChi using dodecanal.

Figure 2

Aggregation and precipitation of CMChi and h-CMChi detected by DLS measurements. Upper graph represents the hydrodynamic diameter of the aggregates and the lower graph presents the PDI. Average values until precipitation with standard deviations are presented based on two measurements.

Figure 3

Determination of pI for CMChi and h-CMChi by measuring the zeta potential of the CMChi and h-CMChi precipitates as a function of pH. Average values with standard deviations are presented based on three measurements.

Figure 4

TEM images of CMChi and h-CMChi precipitates in a dried state prepared by drying the sample at the presented pH (6.0–6.8). A magnification of the particles is shown at the corner of each image. The scale bar equals 2000 nm.

Figure 5

Emulsion stability using (a) CMChi solution and (b) h-CMChi solutions. Emulsion stability measured as a fraction of separated dodecane after centrifuging. Lower dodecane fraction indicates a more stable emulsion. The emulsion stability is presented in varying pH as a function of dodecane to CMChi or h-CMChi solution volume ratio. Average values with standard deviations are presented based on two measurements for CMChi and h-CMChi particles at pH 6.0 and 6.2, respectively.

Figure 6

Effect of pH (6–10) on CMChi and h-CMChi in preventing the coalescence of oil droplets and stabilizing emulsions.

Figure 7

Amount of adsorbed CMChi and h-CMChi particles, and the amount of emulsified dodecane after centrifuging as a function of oil/water ratio. Average values with standard deviations are presented based on two measurements. Photographs show the decrease in opalescence of the water phase as a function of oil/water ratio in the prepared emulsions before centrifuging.

Figure 8

(a) pH-switchable emulsification of dodecane by using CMChi (0.5 mg/mL pH 6.0) solution and varying the pH using aqueous NaOH and HCl solutions. (b) pH-switchable emulsification of dodecane using h-CMChi (0.5 mg/mL pH 6.2) solution and varying the pH using aqueous NaOH and HCl solutions.