Dynamic Mechanical Freezing Points of Cosmetic O/W Emulsions and Their Stability at Low Temperatures

Abstract

To develop cosmetic products quickly and effectively, developers need reliable test methods for evaluating product stability. Phenomena that determine the stability of cosmetic emulsions include phase transitions taking place in the relevant application and storage temperature range. The tendency of ingredients to crystallize influences the stability of emulsions especially at low temperatures. In this article a Differential Scanning Calorimetry (DSC) was used to determine the temperature of the water/ice phase transition of the continuous water phase for oil in water emulsions. A rheological, oscillating test method was developed to measure the phase transition temperature and the results were compared with those of the DSC and standard cold storage tests. There was a correlation of the crystallization temperature of the external water phase with the storage stability at –10°C. The accompanying increase in viscosity with cold storage due to structural alteration of the emulsions could be confirmed by TEM micrographs.

Rheological Characterisation of Dairy Emulsions For Cold Foam Applications

“…This critical temperature, however, could be affected by the adopted temperature ramp rate, oscillation frequency, and gap between plates. According to Hetzel et al, [15] the dynamic freezing point shows no dependence on frequency and gap, whilst an effect of the ramp rate was found only for values greater than 5 • C/min. The ramp adopted in this work (1 • C/min), lower than that limit, should be adequate; however, to ensure that the measured property did not depend on the ramp rate, time sweep tests were performed at different increasing temperatures, starting from the critical value previously detected by the time cure.…”

“…The values obtained by this rheological method are usually different from the classic freezing points determined in static conditions by thermal analysis, [15] probably because of the different test conditions (surface to volume ratio, sample amount) and to the transition criterion: a sharp change in enthalpy owing to the melting latent heat is used for thermal analysis, a sharp change in rheological properties is used in the present case. The latter, being related to mechanical properties, probably could be more useful when the fluid dynamic behaviour of these materials is studied.…”

“…Time cure tests at 1 Hz with a ramp rate of 1 • C/min (decreasing temperature from 4 • C to emulsion freezing) and time sweep tests were used to detect a "dynamic" freezing temperature by rheological measurements. [15] All tests were not affected by water loss problems due to the low temperatures adopted. The preparation of all samples was carried out in triplicate and all results presented are average values; differences of measurement are shown by standard deviations, typically lower than 10%.…”

“…In addition, the dynamic freezing point should be less affected by crystal growth inhibition problems (small volume and extremely smooth clean surface in differential scanning calorimeter, DSC), than the DSC transition temperature; therefore, it should be much closer to the real situation and much more practice-oriented when rheology or mechanical characteristics are involved. [15] All samples were subjected to the illustrated sequence of rheological tests and the freezing temperatures (FP), reported in Table 1, evidenced how different formulations affect them. It is well-known that dextrose is able to decrease water activity, giving a higher cryoscopic depression than sucrose, [3,16] therefore, sample E4 (−6 • C) has a FP higher than sample E1.…”