Behavior of Sulfonephthalein and Azo dyes as effective pH sensors in hybrid materials

The behavior of pH indicators belonging to two different families, Sulfonephthalein and Azo in the Organically modified silicates (OrMoSil) polymer matrix, was investigated. The fraction and the dimension of the OrMoSil organic moiety were co-modulated with the cationic surfactant Hexadecyltrimethylammonium p-toluenesulfonate concentration. Two matrices were tested: the former based on Tetraethyl orthosilicate/ Trimethoxyphenylsilane (TEOS/FTMS), the latter, more lipophilic, based on TEOS/Dodecyl-TEOS. The working interval of Bromothymol Blue (BB) and Nitrazine Yellow (NY), with the co-variation of cationic surfactant amount, C, was wider for the matrix based on TEOS/Dodecyl-TEOS. The overall working interval of BB was between pH 5.80 and 12.50 and between 6.00 and 11.30 for NY. Among the pH indicators characterized by transitions between non-complementary colors, the most important results could be achieved with Methyl Red (MR). With the addition of Tetrabromophenol Blue (TBB), the sensitivity gradually rises with a linear hue profile between pH = 4.20 and pH = 11.90 with a small amount of dye for only one spot. In correspondence of the molar ratio nTBB/nMR = 0.233, there is the inversion point characterizing a spot working as an “on-off” sensor centered at pH = 4.15 suitable for pH measurement of acidic soils.