What is the difference between propylene glycol and propylene glycol monostearate?

propylene glycol is a dihydroxy compound

        1,2 propane diol  CH2OH-CHOH-CH3 while the monosteartae is a monoester of proylene glycol witht he organic acid stearic acid ( C18)   CH3(CH2)16 -COOH

water is lost between the acid and one of the OH groups of propylene glycol and you get the monoester

     CH2OH-CH-CH3
                     I
                    O
                     I
                    CO-(CH2)16-CH3
Because of the long CH containing tail and the presense of the OH on the other end  this monoester is an amphipath and can be used to emulsify and help solubilize compounds into a water based mixture because it can bring the phases together as it has a polar and non-polar end