815 ILCS 205/2
Creditors shall be allowed to receive at the rate of five (5) per centum per annum for all moneys after they become due on any bond, bill, promissory note, or other instrument of writing; on money lent or advanced for the use of another; on money due on the settlement of account from the day of liquidating accounts between the parties and ascertaining the balance; on money received to the use of another and retained without the owner's knowledge; and on money withheld by an unreasonable and vexatious delay of payment. In the absence of an agreement between the creditor and debtor governing interest charges, upon 30 days' written notice to the debtor, an assignee or agent of the creditor may charge and collect interest as provided in this Section on behalf of a creditor.
815 ILCS 205/2