Polysaccharide Applications - ACS Publications - American Chemical

Chapter 16

Creaming and Rheology of Flocculated Emulsions Pretima Manoj, Andrew D. Watson, Annette J. Fillery-Travis, David J. Hibberd, and Margaret M. Robins

Downloaded by MONASH UNIV on August 29, 2013 | http://pubs.acs.org Publication Date: August 20, 1999 | doi: 10.1021/bk-1999-0737.ch016

Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, United Kingdom

In many successful oil-in-water emulsion formulations, polymers are added to the aqueous phase. This can result in the flocculation of the oil droplets by a depletion mechanism. The flocculated droplets form a space forming structure which extends throughout the container, and if strong enough, may hold the droplets in suspension which is stable for a certain period of time. Creaming then starts with the upward movement of the droplets and proceeds by the collapse of the structure under the influence of gravity. We have found the time prior to the creaming, called the 'delay period', is affected by both the polymer and oil concentrations and consequently, is related to the strength of the flocs formed. We present here an attempt to understand the processes the emulsions undergo before and during the collapsing of the flocculated structure.

In many successful formulations, polymers are added to the aqueous phase of the emulsions to enhance the stability to creaming. However, the presence of nonadsorbing polymers in colloidal dispersions can induce phase separation and flocculation by a depletion mechanism. This behaviour has been investigated for the past two decades and various theoretical treatments have been successfully applied to this system, for instance the statistical mechanical approach of Russell and Gast (1). In this laboratory we have been investigating the formation of space-forming structures within these flocculated systems. It is thought that the presence of the structure can induce a delay time prior to the onset of creaming (2). The time delay phenomenon has already been reported by this laboratory by Parker et al (3) and in literature where the delay period is referred to as induction phase and transient gelation.

234

© 1999 American Chemical Society

In Polysaccharide Applications; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

235 The presence of the delay period prior to creaming presents an interesting question which was addressed by Dickinson and co-workers (4). Does the delay originate from the rheology of the continuous phase or does the space filling structure change the dispersion rheology sufficiently to produce a yield stress effectively stopping flow or creaming of the complete emulsions? To address these questions, these issues require an extensive rheological investigation and we present here the results of creaming and rheological measurements on related oil-in-water emulsions.


METHODS. Emulsion Preparation and Characterisation: The emulsions were prepared from a premix. An oil phase was added to a solution of the non-ionic surfactant Brij35 (polyoxyethylene 23-lauryl ether, Sigma Chemical Company) and the emulsion prepared in a Waring blender using a predetermined cycle. The resulting premix was stable to coalescence and disproportionation over the timescale of the experiments. Three oil phases were used within the experimental protocol: Premix 1 for visual characterisation of creaming: Premix 1 contained nhexadecane (Cetane - Ci6H ; density 773.4 kg.m" at 20°C, Sigma Chemical Company) in a surfactant solution. 3

34

Premix 2 for ultrasonic monitoring of creaming: Premix 2 contained a mixture (density 693.2 kg.m" at 20°C) of n-heptane ( C H ; density 683.8 kg.m" at 20°C; SLR, Fisons) and hexadecane in the volume ratio 9:1 in surfactant solution. The purpose of mixing hexadecane with heptane was to increase the creaming rate. 3

3

7

16

Premix 3 for Viscometry: Premix 3 contained 1-bromohexadecane (Cetyl Bromide - (CH (CH) Br); density 1000 kg.m" at 20°C; Fluka) as the dispersed phase 3

3

i5

in a surfactant solution. The purpose of using the density-matched system was to eliminate the effects responsible for creaming of the droplets during viscometry measurements. The experimental emulsions were prepared by diluting the premixes with a polymer solution to obtain the desired concentrations of oil and polymer. The diluent was a mixture of an aqueous solution of the high molecular weight polymer hydroxyethylcellulose (HEC, Natrosol 250HR, Aqualon, mean r (radius of gyration) = 58 nm) and the preservative sodium azide (NaN , Sigma Chemical Company). Prior to use the polymer was purified by dialysis and then freeze-dried. The polymer was dispersed and hydrated by stirring together the ingredients whilst heating to 80°C, then allowing the solution to cool to room temperature. After preparation: • the Premix 1 emulsions were transferred to measuring cylinders for visual monitoring; • the Premix 2 emulsions were transferred to measurement cells for ultrasonic monitoring; g

3


236

•

the Premix 3 emulsions were transferred to a double gap geometry in Bohlin rheometer for viscometry measurements.


For ease of emulsion reference, a coding system has been adopted here as follows: • a 20% (v/v) hexadecane (Premix 1) in the presence of 0.10% (w/w) H E C (with respect to aqueous phase), is labelled as [20H:0.10]; • a 34% heptane/hexadecane (Premix 2) emulsion in presence of 0.35% (w/w) HEC (with respect to aqueous phase), is labelled as [34HH:0.35] and finally, • a 25% (v/v) bromohexadecane (Premix 3) emulsion in presence of 0.20% (w/w) HEC (with respect to aqueous phase), is labelled as [25B:0.20]. Droplet Size Distribution: The droplet size distribution of the premix emulsion was measured using a Malvern Mastersizer laser diffraction sizer (fitted with a small sample handling unit). The formation of the emulsions was found to be highly reproducible giving a size distribution of volume-mean diameter (0I43) of 1.91 jam for Premix 1; 2.01 \xm for Premix 2; and 1.98 jum for Premix 3 (Figure 1). Our previous studies using the same systems (3, 5, 6) have shown the primary size distribution to remain unchanged in the course of the experiments, i.e. coalescence of the droplets was negligible.

14 j

particle size (microns)

Figure 1. Size distributions for Bromohexadecane premix (—); Hexadecane premix (—) and Heptane/hexadecane premix (—).

Creaming Experiments: Visual assessment of creaming: Premix 1 dispersions and polymer diluents were poured into 100 ml measuring cylinders immediately after preparation. The cylinders were inverted ten times to ensure thorough mixing. The sample


237


emulsions were kept at 25°C and the movement of any creaming boundaries was followed with time. The turbidity of the serum layers below the developing creams was noted. The experiments were repeated between 3 and 5 times to obtain mean delay times. The standard deviation within each set of results was calculated to be less than 12%. Ultrasonic Monitor measurements: The IFR ultrasonic monitor (7) was used to characterise the creaming behaviour of Premix 2 emulsions by measuring the speed of ultrasound in emulsions at a series of heights of the container. The oil volumefraction () as a function of height was then calculated using a simple mixing theory (8). At each sampling time the validity of the calibration was verified by calculating the integrated oil volume-fraction over sample height. These oil volume-fraction profiles allowed early detection of creaming, and gave detailed information on the creaming process. Rheological Measurements: Viscosity (steady stress (T) - shear rate ( / ) ) measurements were carried out on a Bohlin Controlled Stress (CS) rheometer using a double gap geometry at 25°C. Solvent traps were used to minimise evaporation and dust settlement. The viscometry measurements were carried out on; H E C solutions alone, emulsions without polymer and emulsions with polymer. Measurements were found to be highly consistent (

Polysaccharide Applications - ACS Publications - American Chemical

Recommend Documents