Date of Award

6-1940

Document Type

Open Access

Degree Name

Bachelor of Science

Department

Chemistry

Language

English

Abstract

Gelation is that phenomenon in which the entire system of both dispersion medium and dispersed substances solidify into an apparently homogeneous, solid mass. Upon the assumption that a jelly consists of myriads of hydrous particles that have become enmeshed into a network that entrains liquids, it follows that any substance should form a jelly when a suitable amount of a highly dispersed substance is precipitated provided that the particles absorb the dispersing medium very strongly. The process of gelation differs macroscopically from flocculation in that flocculation presents a separation of the dispersed substance. It has been proposed that gelation is flocculation in which the dispersed substance is highly hydrated. We will differentiate two types of gelation processes. Gelatin and agar-agar gelatinize through a decrease in temperature, and we will refer to this as temperature-gelation. Aluminum oxide, cerium dioxide, and silica gelatinize upon addition of a sufficient amount of electrolyte, and we will refer to this as electrolyte-gelation. The factors governing the gelation of hydrated silica or colloidal silicic acid have been summarized by Flemming, Hurd and Carver and Hurd and Hallstrom, as: 1. Acid concentration and type of acid 2. pH 3. silica concentration 4. temperature 5. agitation 6. added soluble material 7. type of silicate We have investigated factors (1) and (2) while keeping the other factors as nearly constant as possible. It is, of course, impossible to vary the acid concentration without also varying factor (6) since the vary-ing of the acid also varies the content of the sodium salt of that acid. For this investigation the Ostwald viscometer was used. Prasad, Mehta, and Desai have reported the successful use of the viscometer in gelation processes, and Hurd and Santora also used the instrument with success. However, in order to come a decision on the general applicability of the viscometer in following the gelation process, a careful re-examination of the question with a view to the structure of the gel became inperative.

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