dissipate更有点扩散的意思，有梯度差别的。比如说热量从高温处慢慢扩散到低温处，能量慢慢扩散到四周，直到消失。。。
disperse 有点太阳把水汽驱散这样的感觉。无序的扩散。
我也将不好，抄两端词典的解释给你看看。
1.Main Entry: dis·si·pate
1 a : to break up and drive off (as a crowd) b : to cause to spread thin or scatter and gradually vanish <one's sympathy is eventually dissipated — Andrew Feinberg> c : to lose (as heat or electricity) irrecoverably
2 : to spend or use up wastefully or foolishly <dissipated the family fortune in reckless business ventures>
intransitive verb
1 : to break up and scatter or vanish <the clouds soon dissipated> <the team's early momentum has dissipated>
2 : to be extravagant or dissolute in the pursuit of pleasure; especially : to drink to excess
2. Main Entry: dis·perse
1 a : to cause to break up <police dispersed the crowd> b : to cause to become spread widely c : to cause to evaporate or vanish <sunlight dispersing the mist>
2 : to spread or distribute from a fixed or constant source: as a archaic : disseminate b : to subject (as light) to dispersion c : to distribute (as fine particles) more or less evenly throughout a medium
intransitive verb
1 : to break up in random fashion <the crowd dispersed on request>
2 a : to become dispersed b : dissipate, vanish <the fog dispersed toward morning>