Welcoming Remarks

Many pharmaceutical topical ointments are water in oil type Pickering emulsions. The liquid-liquid interfaces are stabilized by microcrystals of fatty acid-based excipients (Ali et al., 2022). Figure 1 depicts an optical micrograph of a typical emulsion of PEG-400 in heavy paraffin oil, stabilized by monoglyceride (MG) crystals. The crystals encapsulate the micro droplets of PEG and hinder droplet-droplet coalescence and subsequent phase separation.

Quite commonly in the industry, the excipient amount is decided by a trial-and-error method as there is a dearth of a systematic protocol to decide upon the best composition. This leads to added un-necessary manufacturing cost, and also possibly hinders the process of drug delivery in some formulations.

Here we ask the question: “What is the criteria to predict the minimum amount of excipient required to form a stable formulation?”. It becomes necessary then to quantify the strength of the interfacial excipient layer, considering the stabilizer partitioning between the two immiscible phases and comparing it with the probable shelf life it can provide.

In the present study, we use monoglyceride (MG) as a model excipient and characterise the properties of the interfaces formed for different polar liquid - oil combinations. The interfacial moduli are measured for varying concentrations of MG. For these compositions, we then perform standard Accelerated Stress Testing (AST) procedures to predict the stability of the formulation. The experimental data is compared with the predictions of a model that uses interfacial rheology measurements to predict emulsion stability as a function of MG concentration. We also investigate the strength of the interface under different simulated process conditions such as interfacial shear and equilibration time.

This study demonstrates the potential use of interfacial shear rheology to arrive at the feasible composition and procedure to obtain emulsions which cater to the industrial standards.

Samim, A. et al., Langmuir 2022, 38, 28, 8502–8512, DOI : https://doi.org/10.1021/acs.langmuir.2c00202

Figure 1: Monoglyceride (MG) crystals encapsulate PEG-400 (polar phase) droplets in PEG -Oil emulsion, composition: MG 1.1%, PEG-400 10%, Paraffin Heavy Oil 88.9%.