On the adsorption of polyoxyethylene p- tert-octylphenyl ether on silica

Abstract

The adsorption of a series of ethoxylated p- tert-octylphenols on various silica substrates has been investigated to yield experimental data on the selectivity of adsorption. Therefore the oligomer distribution before and after adsorption has been determined, A comparison of the adsorption under static and dynamic conditions has also been included. Treatment with hydrofluoric acid, as proposed in the literature for removing the disturbed surface layer of comminuted quartz samples, leads to a significant change in the adsorption properties of the adsorbents. Below a certain degree of ethoxylation, which is characterized by the cloud point being near the measuring temperature, adsorption increases drastically. There is another definite degree of ethoxylation below which the selective character of adsorption is enhanced significantly. This value coincides with literature values for the degree of ethoxylation below which the aggregation numbers of micelles increase rapidly. In contrast to the literature data, the data obtained led to the conclusion that the selectivity of adsorption is determined not only by the hydrophobic/hydrophilic balance of the individual oligomers. Hence, after adsorption, oligomers with short polar groups may be enriched in the remaining solution. At concentrations well above the CMC a striking difference between the adsorption behaviour under dynamic and static conditions has been observed. Thus under dynamic conditions the amount or surfactant bound to the column continues to rise without reaching a saturation value. The selectivity behaviour and other adsorption phenomena have been discussed on the basis of the mixed adsorption mechanism proposed, reflecting the influence of specific surface characteristics. The mixed adsorption mechanism includes primary adsorption via the terminal hydroxyl groups as well as via hydrogen bonding between undissociated silanol groups and oligoether chains of the surfactants. In addition, at a sufficiently high surface coverage, interactions between hydrophobic chains become important.