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An electrophoretic medium including a mixture of charge control agents, for example quaternary amine salts of polyisobutylene combined with quaternary amine salts of polyesters. The described electrophoretic medium exhibits improved color saturation as compared to similar electrophoretic media havin

An electrophoretic medium including a mixture of charge control agents, for example quaternary amine salts of polyisobutylene combined with quaternary amine salts of polyesters. The described electrophoretic medium exhibits improved color saturation as compared to similar electrophoretic media having only one of the charge control agents.

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1. An electrophoretic medium comprising: a first set of charged particles;a non-polar fluid;a first charge control agent comprising a quaternary amine and a branched hydrocarbon tail of between 20 and 1000 carbon atoms in length; anda second charge control agent comprising a quaternary amine and a l

1. An electrophoretic medium comprising: a first set of charged particles;a non-polar fluid;a first charge control agent comprising a quaternary amine and a branched hydrocarbon tail of between 20 and 1000 carbon atoms in length; anda second charge control agent comprising a quaternary amine and a linear polyisobutylene tail of between 12 and 500 carbon atoms in length,wherein the branched hydrocarbon tail of the first charge control agent comprises a polyester or a poly secondary alcohol;wherein the second charge control agent comprises a species of (CH3)3C—[—CH2—C(CH3)2—]n-L-NR1R2R3+ X−, wherein L is a linking moiety, R1, R2 and R3 are independently alkyl having 1-4 carbon atoms, n is an integer from 2 to 40, and X− is a counter ion; andwherein the electrophoretic medium exhibits color saturation for red and yellow color states in an electrophoretic display. 2. The electrophoretic medium of claim 1, wherein n is between 5 and 20. 3. The electrophoretic medium of claim 1, wherein L is a saturated or unsaturated alkylene or amide-alkylene chain of 2 to 6 carbon atoms. 4. The electrophoretic medium of claim 1, wherein L is —CRR—CR═CR—CRR—, —CRR—CRR—CRR—, or —NHC(O)—CRR—CRR—CRR—, wherein the R's are independently hydrogen or an alkyl of 1-4 carbon atoms. 5. The electrophoretic medium of claim 1, wherein L is —CH2—CH2—CH2—, —CH2—CH═CH—CH2—, —CH2—CH2—CH(CH3)—, —CH2—CH(CH3)—CH2—, or —NHC(O)—CH2—CH2—CH2—. 6. The electrophoretic medium of claim 1, wherein L is —CH2—CH2—CH(CH3)—. 7. The electrophoretic medium of claim 1, wherein R1, R2 and R3 are methyl. 8. The electrophoretic medium of claim 1, wherein X− is F−, Cl−, Br−, I−, R′SO3−, R'SO4− or R′CO3−, wherein R′ is an alkyl of 1-4 carbon atoms or an aryl of 6 to 18 carbon atoms optionally substituted with an alkyl of 1-4 carbon atoms. 9. The electrophoretic medium of claim 1, wherein X− is R'SO3− wherein R′ is a phenyl optionally substituted with an alkyl of 1-4 carbon atoms. 10. The electrophoretic medium of claim 1, wherein L is —CH2—CH2—CH(CH3)—, R1, R2, and R3 are methyl, and the counter ion is CH3SO4−. 11. The electrophoretic medium of claim 1, wherein a concentration of the first charge control agent is 0.05% to 0.6% by weight of the electrophoretic medium. 12. The electrophoretic medium of claim 1, wherein a concentration of the second charge control agent is 0.05% to 0.6% by weight of the electrophoretic medium. 13. The electrophoretic medium of claim 1, wherein a total concentration of both the first and the second charge control agents is between 0.1% and 0.4% by weight of the electrophoretic medium. 14. The electrophoretic medium of claim 1, wherein a ratio of the first charge control agent to the second charge control agent is from 5:1 to 1:5. 15. The electrophoretic medium of claim 14, wherein the ratio of the first charge control agent to the second charge control agent is from 2:1 to 1:2. 16. The electrophoretic medium of claim 1, further comprising a second set of charged particles dispersed in the non-polar fluid, wherein the first and the second sets of charged particles have opposite charges. 17. The electrophoretic medium of claim 16, further comprising a third set of charged particles dispersed in the non-polar fluid, wherein the third set of charged particles is not white and not black. 18. The electrophoretic medium of claim 17, wherein the third set of charged particles is yellow or red.