in order to form a W/O emulsion, the water phase must be dispersed into the oil; this is complicated by the fact that water droplets have a much stronger tendency to aggregate than oil droplets do
the purpose of magnesium sulphate is to increase the ionic strength of the water phase, hence increase its surface tension, and enable it to be dispersed more easily, allowing the emulsion to be formed more easily
magnesium sulphate is generally used because it's a cheap, readily soluble and non-toxic salt with a divalent cation; divalent ions increase ionic strength more than monovalent ones such as sodium do
in O/W emulsions, it serves no purpose whatsoever
UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative)
I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
Comments
the purpose of magnesium sulphate is to increase the ionic strength of the water phase, hence increase its surface tension, and enable it to be dispersed more easily, allowing the emulsion to be formed more easily
magnesium sulphate is generally used because it's a cheap, readily soluble and non-toxic salt with a divalent cation; divalent ions increase ionic strength more than monovalent ones such as sodium do
in O/W emulsions, it serves no purpose whatsoever
Thank you very much for explaining!