New ultrasound contrast agents that comprise microbubbles encapsulating a gas within a shell made from a blend of bipolar compounds having mixed carbon chain length. The compounds have hydrophobic groups selected from the group consisting of straight-chained alkyls, alkylethers, alkylthiolethers, alkyldisulfides, polyfluoroalkyls, and polyfluoroalkylethers having a carbon chain length greater than or equal to 16 and less than or equal to 32. The polar groups are connected to the hydrophobic groups by means of a linker. The polar head groups are selected from the group consisting of CO 2 -M + , SO 3 − M + , SO 4 − M + , PO 3 − M + , PO 4 − M + 2 , N(R) 4 + , a pyridinium or substituted pyridinium group, and a zwitterionic group; R is selected from the group consisting of —H, —CH 3 , alkyl, cycloalkyl, substituted cycloalkyls containing one or more heteroatoms, and benzyl and can be the same or different; and Z′ is a nonionic and M is a cation.