A solid surfactant wouldn't be able to accomplish much-- surfactants work by going between molecules and forming a chemical bridge between them. While oil and water cannot easily bond to one another, a surfactant that contains a nonpolar/oily side joined to a polar group can act as a bridge between the oil molecules and water molecules,surrounding the oil and allowing it to be soluble in water. A solid chunk of surfactant doesn't have free molecules to get between molecules and form an interface, and would not function.