La ‘ola de violencia’ es a nivel nacional

emulsion droplet size

The concentrations of monoolein and silica particles in the mixed-emulsifier systems

were also found to affect the droplet size of the prepared O/W emulsions. The effect of

each of the components’ concentration (e.g. monoolein) in these mixed-emulsifier systems is discussed in isolation by considering those systems where the concentration

of the other component (e.g. silica particles) is kept constant.

Mixed-emulsifier systems where the concentration of monoolein was varied, but the

concentration of silica particles was kept constant are firstly discussed. The droplet sizes

of O/W emulsions stabilised by such mixed-emulsifier systems were found to initially

decrease with increasing monoolein concentrations of up to 3% (Figure 5-1). Further

increases of the monoolein concentration seem to have little effect on the emulsion

droplet size, which remain more or less constant. What was additionally observed is that

the effect of monoolein concentration on the emulsion droplet size follows the same

pattern regardless of the silica concentration in the mixed-emulsifier systems. Similar

findings can be reported for mixed-emulsifier systems where the concentration of silica

particles is varied but the concentration of monoolein is now kept constant (Figure 5-2).

The droplet size of O/W emulsions, stabilised by such mixed-emulsifier systems,

initially decreases with increasing concentrations (of up to 1.2%) of silica particles, but

further increases of the particles’ concentration (up to 2%) seem to have little effect on

the emulsion droplet size. What can be similarly concluded is that the effect of silica

concentration on the emulsion droplet size follows the same pattern regardless of the

Chapter 5. Effect of monoolein and hydrophilic silica particle mixtures on the stability of O/W emulsions

concentrations of both monoolein and silica particles in the mixed-emulsifier systems

affect the droplet size of the resulting emulsion up to a set of respective “threshold” concentrations, above which the droplet size is then limited by the emulsification

process.

Figure 5-1: (a) Average size D(4,3) of emulsion droplets (after emulsification) as a function of monoolein concentration and for various concentrations of colloidal particle; where not visible, error bars are smaller than the symbols. (b/c) Droplet size distributions of emulsions prepared at pH2, for 2%/8% of monoolein for various silica particle concentrations.

Chapter 5. Effect of monoolein and hydrophilic silica particle mixtures on the stability of O/W emulsions

Figure 5-2: Average size D(4,3) of emulsion droplets (after emulsification) as a function of silica concentration and for various concentrations of surfactant in the system; where not visible, error bars are smaller than the symbols.

The effect of the monoolein and silica particle concentrations, in the mixed-emulsifier

systems, on the droplet size of the prepared O/W emulsions can be explained by

considering the mechanism by which these systems induce stability. As previously

discussed, the final droplet size of the formed emulsion is determined by both the break-

up and re-coalescence phenomena taking place during the emulsification process. Both

phenomena can be affected by variation in the concentration of monoolein and/or silica

particles in the mixed-emulsifier systems. For instance, re-coalescence events in the

system should be further delayed by increasing the concentration of monoolein in the

mixed-emulsifier systems due to more efficient surface coverage. Furthermore, this

could facilitate/promote the droplet break-up phenomena during emulsification due to a

more rapid interfacial tension reduction induced by the increasing monoolein

concentration. On the other hand, by increasing the concentration of silica particles in

Chapter 5. Effect of monoolein and hydrophilic silica particle mixtures on the stability of O/W emulsions

an increasingly faster process. Therefore, in both cases, emulsion microstructure is

stabilised at increasingly earlier stages, after its formation, and thus the final emulsion

droplet size decreases. Eventually, after a certain set of threshold concentrations for the

monoolein and silica particles in the mixed-emulsifier systems is reached, the emulsion

droplet size becomes limited by the process and remains constant (Figures 5-1 and 5-2).

Another noteworthy observation lays with the difference in size between emulsion

droplets stabilised solely by colloidal particles and those stabilised by mixed-emulsifier

systems containing monoolein in concentrations below 1%. It was observed that

emulsion droplets stabilised by the mixed-emulsifier systems (in these concentration

regimes) were larger than those stabilised solely by colloidal particles, with both

containing the same concentration of colloidal particles. This difference is suggested to

be a result of what was previously described as an “additional step”, in the stabilisation process of O/W emulsion droplets by the mixed-emulsifier systems, which is the

displacement of monoolein from the oil-water interface and its replacement by silica

particles. Earlier in this section it was argued that such an additional step, which does

not take place for the stabilisation process of O/W emulsions by colloidal particles,

should delay the assembly of silica particles from the mixed-emulsifier systems, onto

the oil-water interfaces. Following the same line of reasoning it can be concluded that

this “delay” should also account for the difference in size between emulsion droplets stabilised solely by colloidal particles and those stabilised by mixed-emulsifier systems

of low monoolein concentrations (≤ 1%).

Optical observation of O/W emulsions stabilised by the mixed-emulsifier systems

Chapter 5. Effect of monoolein and hydrophilic silica particle mixtures on the stability of O/W emulsions

by a small quantity of “free” silica particles, which eventually sediments. Preliminary

experiments showed that the aqueous phases of these systems, regardless of whether

they appear cloudy or not, do not contain oil droplets or monoolein. This clearly

indicates that it is indeed the silica (aggregated) particles that cause the observed

cloudiness and not a “secondary emulsion” formed in the aqueous phase. What can also be concluded is that the observed phenomenon does not arise as a result of any

interactions between the silica particles and monoolein, which would affect the

aggregation of the former thus causing the observed “cloudiness”. The occurrence of a cloudy aqueous phase seems to be determined only by the concentration of monoolein

in the mixed-emulsifier systems; regardless of the silica concentration, systems of

monoolein concentrations below 3% develop cloudy aqueous phases while in those

containing higher monoolein concentrations the aqueous phases are clear. An

explanation for this phenomenon is that the surface area (at the cream layer) becomes

large enough, due to the increasing monoolein concentration. This leads to smaller

emulsion droplets, so as to hold all the available silica particles, which otherwise would

collect in the aqueous phase where they would become visible (cloudiness).

Figure 5-3: Effect of increasing monoolein concentrations on the cream layer of 20% oil-in-80% water emulsions at pH 2 and for a constant concentration of silica particles (1%).

Chapter 5. Effect of monoolein and hydrophilic silica particle mixtures on the stability of O/W emulsions

In terms of the volume (fraction) of the cream layer of O/W droplets, developed shortly

after emulsification, it was observed that it increases with increasing concentrations of

monoolein (evident in Figure 5-3) and silica particles in the mixed-emulsifier systems

(see Table 5-1). The increase in cream layer volume can be partly attributed to the

reduction in emulsion droplet size associated with increasing monoolein and/or silica

concentrations (see Table 5-1). However, this effect seems to persist even for those

concentrations of mixed-emulsifier systems for which the size of the emulsion droplets

remains unchanged; that is mixed-emulsifier systems of monoolein concentrations over

3% and silica particles concentrations over 1.2%. This change in phase volume

(especially at high concentrations of monoolein and silica particles) is most likely due to

the way that the droplets interact/pack for the cream layer. One explanation for this

would be that the droplets are aggregating and starting to form a three dimensional

network thus changing the inter-droplet arrangement in the cream layer. This

aggregation would seem to be a consequence of having uncharged silica particles

positioned in the interface by the monoolein in such a way as to stick together on

contact. This is probably due to capillary forces between particles as they approach each

other.

5.3

Conclusion

The stability of O/W emulsions prepared at pH 2 in the presence of both hydrophilic

silica particles and monoolein was investigated in this chapter. The hypothesis proposed

here is that emulsion stability is induced via a mechanism in which both components in

Chapter 5. Effect of monoolein and hydrophilic silica particle mixtures on the stability of O/W emulsions

monoolein is to initially “delay” the re-coalescence phenomena and induce further droplet break-up, during emulsification, by rapidly covering the newly created interface

and reducing interfacial tension, in order to allow for the silica particles to assemble at

the oil-water interface and provide long-term stability after the end of the emulsification

process. This dual manner by which mixed-emulsifier systems induced stability was

found to depend on the concentrations of both monoolein and silica particles.

In order to promote such a stabilisation mechanism, oil, water, particles and monoolein

have to be emulsified following a specific protocol. In addition to the long-term

emulsion stability provided by the mixture of particles and monoolein, emulsion droplet

size also decreased compared to droplet size of emulsions stabilised only by silica

particles. This is also due to the 2-step mechanism and particularly the fact that droplet

In document Entre bloqueos viales, mesas de negociación y arengas : El Paro Nacional Agrario (Colombia, 2013) como acontecimiento mediático (página 75-82)