Choosing the right emulsifier

Choosing the right emulsifier
is crucial. It not only promotes emulsion formation, facilitating the formation of fine particles and improving emulsion stability, but also allows for control over the type of emulsion.

1. General principles for selecting emulsifiers

Due to the variability of the oil and aqueous phase compositions and the different types of emulsions required, it is practically impossible to have a single universally effective emulsifier. Therefore, the selection of an emulsifier is always specific to a particular system, emulsification method, and type of emulsion required. Nevertheless, there are generally some principles to consider when selecting an emulsifier:
( 1 ) Most emulsifiers have good surface activity and can reduce surface tension;
( 2 ) Emulsifier molecules can form a stable and tightly packed condensed film at the interface;
( 3 ) The emulsifying performance of an emulsifier is related to its affinity with the oil or aqueous phase. Oil-soluble emulsifiers readily produce W/O type emulsions, while water-soluble emulsifiers readily produce O/W emulsions. The mixed use of water-soluble and oil-soluble emulsifiers can create multi-emulsion composite systems.
( 4 ) The emulsifier should be able to appropriately increase the viscosity of the external phase in order to reduce the aggregation rate of droplets;
( 5 ) It should meet the special requirements of the emulsion system, such as the requirement that the emulsifier of some systems be non-toxic and have certain pharmacological properties;
( 6 ) It should be able to achieve the emulsification effect with the lowest concentration and the lowest cost, and the emulsification process should be simple.

2. Methods for selecting emulsifiers

The most common methods for selecting emulsifiers are HLB (hydrophilic-lipophile balance) and PIT (phase inversion temperature). The former is applicable to all types of surfactants, while the latter is a supplement to the former and is only applicable to nonionic surfactants.
(1) HLB method

① Determination of emulsifier.

Surfactants are all amphiphilic molecules, exhibiting both hydrophilic and lipophilic properties. The HLB value indicates the relative attraction of a surfactant to both water and oil: a high HLB value indicates strong hydrophilicity, while a low HLB value indicates strong lipophilicity. The appearance of aqueous solutions of surfactants with different HLB values and their suitable applications are listed in the table below. The relationship between HLB values and applications in the table is only a general range and does not strictly adhere to this limit in practical applications.

HLB value	Appearance of aqueous solution	HLB value	Application areas
1-4	Not dispersed	1.5-3.0	Defoamer
3-6	Poor dispersion	3-6	W/o type emulsifier
6-8	Emulsion dispersion after stirring	7-9	wetting agent
8-10	Stable emulsion dispersion	8-18	O/W type emulsifier
10-13	Translucent to transparent	13-15	Detergent
13-20	clear solution	15-18	Solubilizer

As can be seen from the table above, only surfactants (or mixtures) with HLB values between 3 and 6 are suitable as W/O type emulsifiers; only surfactants (or mixtures) with HLB values between 8 and 18 are suitable as O/W type emulsifiers.

② Determination of the optimal emulsifier for oil-water systems.

First, determine the HLB value required for emulsification of the system, and then find the most efficient emulsifier mixture.

• A pair of emulsifiers with significantly different HLB values were selected and mixed in different proportions. Using these mixtures as emulsifiers, emulsions of a specified system were prepared. The stability of the resulting emulsion waves was measured, and a bell-shaped curve was obtained by plotting the results against the calculated HLB values of the mixed emulsifiers, as shown in the figure. The HLB value corresponding to the highest peak of this curve is the HLB value required for emulsifying the specified system (the system in the figure has HLB = 10.5). Clearly, while the selected mixed surfactants can achieve the required HLB value for the specified system, they are not necessarily the most efficient.
  

The effect of emulsifiers

the HLB value of the emulsifier mixture

The determination and selection of emulsifiers: The bell curve is determined using a pair of emulsifiers (O);

It is determined using different mixed emulsifiers, and the emulsifier used at the highest point is the most suitable.

Second, after determining the required HLB value for the system to be emulsified, several pairs of emulsifiers were mixed so that the HLB values of each mixed emulsifier were the values determined by the above method (such as the value of 10.5 in the figure above). The specified system was emulsified using these emulsifiers, and its stability was measured. Clearly, some later-formulated emulsifiers are more efficient than the original mixed emulsifiers used in determining the bell curve. In summary, the method for determining the required emulsifier formulation for a given system is as follows: arbitrarily select a pair of emulsifiers, change their HLB values within a certain range, find the HLB value with the highest efficiency, then change the types and proportions of the compounded emulsifiers, while still maintaining the required HLB values, until the pair of compounded emulsifiers with the highest efficiency is found.

(2) PIT method

The term PIT refers to the temperature at which the HLB of an emulsifier changes drastically in a specific system, and at the same time, the emulsion system undergoes a phase transition. This temperature is characteristic of the emulsion system and is called the phase transition temperature (PIT).

The phase transition temperature can be considered as the temperature at which the hydrophilic and lipophilic properties of the emulsifier are just balanced. Near the PIT, the stability of the emulsion and the change in HLB are very sensitive. Therefore, the PIT method can not only measure the HLB value, but also obtain a more accurate value.

The specific operation of selecting nonionic surfactants using the PIT method is to take equal amounts of oil and water phases, add 3% to 5% surfactant to prepare an emulsion, continuously shake and heat the system, and observe the change in the type of emulsion. The temperature at which the emulsion changes from O/W type to W/O type is the PIT. For this oil phase, if you want to prepare an O/W type emulsion, you should choose a surfactant with a PIT 20 to 60°C higher than the emulsion storage temperature; if you want to prepare a W/O type emulsion, you should choose a surfactant with a PIT 10 to 40°C lower than the emulsion storage temperature. PIT cannot be measured below 0°C.
(3) Relationship between HLB and PIT
Both HLB value and PIT are characterizations of the hydrophilic-lipophilic equilibrium number of surfactants. The former is experimentally cumbersome and time-consuming, so it is mostly calculated based on the characteristics of molecular structure. HLB value cannot reflect the influence of factors such as temperature, surfactant concentration, additives, oil phase properties and oil-water phase ratio. These influences are manifested as:
① For a certain emulsifier, the PIT of the oil phase polarity increases. For binary oil blends, PIT is additive, meaning it is the sum of the products of the PIT of each individual oil and the volume fraction of its component.
② When a single polyoxyethylene ionic surfactant is present at a concentration of 3%-5%, the PIT is constant. However, considering the distribution of polyoxyethylene chain lengths within the surfactant molecules, if there are more components with short polyoxyethylene chains, the PIT decreases sharply with increasing surfactant concentration; conversely, if there are more components with long polyoxyethylene chains, the PIT decreases more slowly.
③ When the surfactant concentration is constant, an increase in the oil-to-water ratio also increases PIT. However, when the oil-to-surfactant ratio is fixed, changing the oil-to-water ratio does not change the PIT. The lower the oil-to-surfactant ratio, the lower the PIT.
④ Adding additives that change the polarity of the oil phase will alter the PIT; decreasing the polarity of the oil phase lowers the PIT, and vice versa. PIT is related to HLB values; generally, PIT increases with increasing HLB values. A high HLB value indicates good water-resistance of the emulsifier, thus resulting in a higher PIT and higher stability of the prepared O/W emulsion.