Properties of silicas chemically modified by monodentate amines studied by conductometry

Abstract

The protonation of aminoalkyl groups covalently bonded on the silica surface was studied by the conductometric titration method. Porous varieties of silica can adsorb HCl from an aqueous solution. Conductometric titration was proposed for the determination of concentra tions and constants of protolytic equilibrium of grafted amino groups. During HCl chemisorp tion the effect of temperature on the electric conductivity of suspensions of the modified silicas was studied.The main method for studying acid base equilibria at the liquid-solid interface is pH metry. 1 This method re vealed specific features of protonation of organic bases covalently bonded on the surface of chemically modified silica (CMS). 2 However, the use of pH metry to study protolytic equilibria of CMS does not allow one to distin guish processes that occur in solution from those occur ring both in solution and at the interface. For instance, when a silica weighed sample (m = 0.1 g) with groups of alkylsulfonic acid (C H = 0.1 mmol g -1 ) covalently bonded on the surface is suspended in water (V = 0.01 L), the pH of the suspension decreases to 3 ([H]•10 -3 = C H •m/V ). 3 The suspension of the nonmodified silica in a 10 -3 М solution of sulfonic acid has the same pH. Therefore, when using pH metry to study CMS, it seems important to be positive that the measured values reflect the interac tion of the acid with immobilized groups rather than with products of desorption or CMS destruction. Ions capable of migrating make the main contribution to the electric conduction of aqueous suspensions of CMS. Therefore, adsorbed (mobile) and covalently bonded (immobile) molecules can easily be discriminated during conducto metric titration. 1 In conductometric studies, adsorbed sub stances are protonated due to the interaction with acid and loose their ability to form hydrogen bonds with silanol groups. This results in their desorption into a solution and increases the electric conduction of the solution. At the same time, molecules covalently bonded on the support surface cannot migrate to the bulk even after protonation and, hence, the addition of acid to these CMS does not considerably increase the conduction of the suspension up to the equivalence point. The potential to use conduc tometric titration to study CMS is exemplified by deter mination of the concentration of aminopropyl groups grafted on SiO 2 . 4The purpose of this work is to evaluate an ability of conductometric titration for studying protolytic equilibria at the water-CMS interface. For the study, we chose silicas with covalently bonded aliphatic amines, which differ by the concentration of immobilized groups and by the support nature and have known protolytic prop erties. 4-6 ExperimentalConductometric titration of aqueous suspensions of CMS was carried out with an aqueous solution of HCl. To measure the electric conduction of CMS suspensions, a vessel with two parallel plates of platinum tin (Arrhenius cell) was used. A con ductometric cell was temperature controlled with an accuracy of ±0....

Organosilicon ion-exchange and complexing adsorbents

“…32 It was established that adsorbents synthesized from porous silicas (silica gel, silochrome) can adsorb the acid from aqueous solutions.…”