A “mixed-matrix” approach to creating these structures overcomes an upper limit on their selectivity called the Robeson limit. The presence of many different particles around these structures creates an uneven charge distribution across them in the Gibbs-Donnan effect. (15[1]-5[1])A sulfonated polymer of (15[1])PTFE is used to produce a type of these structures used to separate the compartments (15[1])of a (*) fuel cell, (-5[1])and is named for facilitating the exchange of protons. (10[5])Changing (10[1])the (10[1])pore size (10[2])of these (10[1])structures (10[1]-5[1])prevents (10[2])deposition (10[1])onto them (10[1])in their namesake “fouling.” Cellulose acetate (10[1])is used to make a type of these structures (-5[1])used in dialysis, (-5[1])and the degree (10[1])of movement through them (10[2]-5[1])is determined by (10[1])tonicity. (10[2])For 10 points, (10[1])osmosis (10[1])can occur (10[2]-5[1])across what (10[1])semi-permeable (10[1])structures? (10[2])■END■ (10[4]0[1])