These expressions are not interchangeably all the time: Mientras is used for expressing 2 or more actions being carried out simultaneously, or to indicate a length of time in which one is going to do something: Mientras estemos en este País acataremos sus leyes. Me gusta leer mientras escucho música. and regarding mientras que it is the equivalent to whereas in english, so it is used for giving an opposite point of view, or feature of something: Por este lugar llueve todo el año, mientras que en el norte no llueve nunca. Mientras can not be used for giving opposite sides of something, if i had to use mientras in the last sentence i wrote it would change the meaning, it wouldn't convey the same. Yet. using mientras que to replace "mientras" wouldn't affect the meaning at all, let's check this out: Prefiero comer mientras veo tele. Prefiero comer mientras que veo la tele (though que sounds redudant for many, for others it sounds completely natural) Btw, i'll leave u a link in which you are gonna find a further explanation about this topic: https://ellapizrojo.wordpress.com/2017/05/23/mientras-y-mientras-que/ Let me know if it isn't still clear for u.

I would say that no, you can't, each one has different meaning. I agree with Federico Astudillo in his explanation about the meaning and use of each one. "Mientras" is like "while" and "mientras que" is like "whereas". Could you give examples where you want to use one or the other? Because "mientras" can have also other meanings ("as long as" and "if") but I'm not sure sure if you are asking for them.

Mientras: It can function as an adverb of time or as a conjunction, expressing simultaneity between two actions. As an adverb, it is a tonic word and is written isolated by commas from the rest of the statement: Desarmaron los instrumentos y los secaron. Ronald, mientras, se acariciaba la mejilla con el reverso de los dedos. The same sense has the adverbial phrase "mientras tanto": La señora, mientras tanto, permanecía impávida y con cara de pocos amigos. As a conjunction, on the other hand, "mientras" is an atonic word and does not separate with a comma from the verb that introduces: Todo esto lo revive Andrés mientras mezcla colores y aceites. When it introduce a verb in subjunctive, it often acquires a conditional nuance: La polémica no se extinguirá mientras persista la palabra escrita. "mientras" is also used to oppose two simultaneous actions, although in this case it is more frequent and advisable to use the conjunctive phrase "mientras que" Mientras que: Conjunctive phrase used to counterpose two simultaneous actions, with a sense equivalent to "in the other hand": Muchos deplorarán esta revolución, mientras que otros la celebrarán.

Mientras y mientras que son intercambiables. En el español de España "mientras que" es menos aceptado en el habla culta.

Mientras Mientras es más formal en mi opinión.