Of course, you can mix both ionic and nonionic surfactants together. Also, you can mix cationic with nonionic , but you can't do it with anionic and cationic. Indeed! You can replace your vehicle system with a buffer, but you've to take into your considerations that strong ions may suppress the activity of surfactants, definitely charged ones.

Cationic and anionic surfactants can precipitate their salts. Anionic surfactants are well mixed with alkaline buffers. There may be problems of cationic or anionic surfactants with amino acids as a function of pH. Non-ionic surfactants are well mixed with all buffers, but can be salted out with a large buffer concentration.

Thank you Mr. Ahmed and Mr. Yuri. 

For buffer system, for example, I wanna use phosphate buffer that has range 5.8 to 8.0. I want to make buffer solution at pH 10, can phosphate buffer solution still can be used as the range already out ?? Or I need to use other buffer system?

Thank you.