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.sub.2-M.sup.+, SO.sub.3.sup.-M.sup.+, SO.sub.4.sup.-M.sup.+, PO.sub.3.sup.-M.sup.+, PO.sub.4.sup.-M.sup.+.sub.2, N(R).sub.4.sup.+, a pyridinium or substituted pyridinium group, and a zwitterionic group; R is selected from the group consisting of --H, --CH.sub.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.