When a soap water bubble is given a positive charge it expands. If it is given a negative charge, then it

• Due to the uniform distribution of charged particles on it, which causes them to reject one another due to the electrostatic force, the bubble will grow. 
• Due to the charge on it, this will occur to both positively and negatively charged bubbles.

Hence the bubble expands whenever a charge is given to it, irrespective of the sign.