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. A sulfonated polymer of (15[1])PTFE is used to produce a type of these structures used to separate the compartments of a (*) fuel cell, and is named for facilitating the exchange of protons. Changing the pore size of these structures prevents deposition onto them in their namesake “fouling.” Cellulose acetate is used to make a type of these structures used in dialysis, (-5[1])and the degree of movement through them is determined by tonicity. For 10 points, osmosis can occur (-5[1])across what semi-permeable structures? ■END■ (10[2])