In one embodiment, the composition according to the invention is characterized in that the co-polyamino acid bearing carboxylate charges and at least one hydrophobic radical -Hy is chosen among the co-polyamino acids according to formula XXXb hereinafter:formula XXXbwherein,· D represents, independently, either a group -CH2- (aspartic acid) or a group -CH2-CH2- (glutamic acid),· X represents a cationic entity chosen in the group comprising alkali cations,· Rband Rb’, identical or different, are either a hydrophobic radical -Hy, or a radical chosen in the group consisting of an H, a C2 to C10 linear acyl group, a C3 to C10 branched acyl group, a benzyl, a terminal "amino acid" unit and a pyroglutamate,at least one of Rb and R’b is a hydrophobic radical -Hy,· Q and Hy are as defined above.· n + m represents the degree of polymerization DP of the co-polyamino acid, namely the mean number of monomeric units per co-polyamino acid chain and 5 ≤ n + m ≤ 250.在一個實施例中，根據本發明之組成物之特徵在於帶有羧酸鹽電荷及至少一疏水基-Hy之共聚胺基酸係選自下文中根據式XXXb之共聚胺基酸：式XXXb其中，· D獨立地表示基團-CH2-(天冬胺酸)或基團-CH2-CH2-(麩胺酸)，· X表示在包含鹼金屬陽離子之群中選擇的陽離子實體，· 相同或不同的Rb及Rb'為疏水基-Hy或在由以下組成之群中選擇的基團：H、C2至C10直鏈醯基、C3至C10分支鏈醯基、苯甲基、末端「胺基酸」單元及焦麩胺酸根，Rb及R'b中之至少一者為疏水基-Hy，· Q及Hy係如上文所定義。· n + m表示該共聚胺基酸之聚合度DP，亦即每共聚胺基酸鏈的單體單元之平均數目，且5 ≤ n + m ≤ 250。
