Modification of chemical performance of dopants in xerogel films with entrapped ionic liquid

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Abstract
The effect of ionic liquid (IL), 1-butyl-3-methylimidazolium chloride [C4mim][Cl], entrapment on the properties of the xerogel films is described. This has been investigated by evaluation of the behavior of sulfonephthalein pH indicators entrapped in a xerogel matrix formed by tetraethoxysilane (TEOS) and vinyltriethoxysilane (VTEOS) with co-entrapped ionic liquid. We report that co-doping of the ionic liquid with pH indicators within the xerogel matrix significantly modifies the performance of primary dopants such as acidic constants and pH indicating ranges but the spectral intensity depends on the structure of the indicator. Increase in pH indicating ranges in the presence of ionic liquid can be advantageously used for designing pH optodes with a wider determination range. IL–xerogel modified films showed increased wettability, hydrophilicity, reduced gel shrinkage and faster response times compared to the unmodified silica matrix. The reason for such modification is explained mainly based upon changes in the environment of the indicator caused by an increase in hydrophilicity in the presence of IL as well as ionic interactions between the indicator and IL. The long term stability of the xerogel films was investigated over at least fifteen months.