There is no difference between so and so that.

extracted from http://www.bartleby.com/64/C001/058.html

1. Grammar: Traditional Rules, Word Order, Agreement, and Case
§ 58. so

so or so that in purpose clauses. Many people insist that so must be followed by that in formal writing when used to introduce a clause giving the reason for or the purpose of an action: He stayed so that he could see the movie a second time. But since many respected writers use so for so that in formal writing, it seems best to consider the issue one of stylistic preference: The store stays open late so (or so that) people who work all day can buy groceries.
1
so or so that in result clauses. It is acceptable to use either so or so that to introduce clauses that state a result or consequence: The Bay Bridge was still closed, so (or so that) the drive from San Francisco to the Berkeley campus took an hour and a half.
2
so as a connector in a narrative. So the guy sits down at our table and pulls up his chair so he can be closer to me. So he starts telling us about his uncle…. So is frequently used in informal speech to string together the elements of a narrative. But readers of formal writing generally expect connections to be made more explicit.
3
so as intensive. People sometimes object to the use of so as an intensive meaning “to a great degree or extent”: We were so relieved to learn that the deadline had been extended. This usage is most common in informal contexts, perhaps because unlike very, it presumes that the listener or reader will be sympathetic with the speaker’s evaluation of the situation. Thus you would be more likely to say It was so unfair of them not to invite you than to say It was so fortunate that I didn’t have to put up with your company. For just this reason, you can sometimes put intensive so to good use in more formal contexts to invite the reader to take the point of view of the speaker or subject: The request seemed to her to be quite reasonable; it was so unfair of the manager to refuse. Just remember not to overdo it.
4
More at as.