T H E DISTRIBUTION OF SOLUTES IN SILICA GgL‘ SUTTOh’ REDFERN* AND W. A. PATRICK
Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland Received January 8.2, 1038
It has been shown that within a hydrogel of silicic acid a solute is associated with a greater quantity of water than outside the hydrogel. If the concentrations are expressed as molalities, then it is found that the concentration inside the gel is less than the concentration outside. This conclusion was reached on the basis of the experimental studies of the diffusion of electrolytes in hydrogels by Allan (1) and the analytical studies of Love (7). These papers gave results for sodium sulfate and hydrochloric acid. Xllan and Love were also led to believe t h a t the activity coefficient of the solute in the gel was different from the activity coefficient outside, since the activities a t equilibrium must be equal. However, this conclusion is based on the assumption that all of the water in the gel is acting in the rdle of solvent. This is probably not true, because there is considerable evidence for the existence of what is called bound water. The hydration of molecules or ions in solution is one example. Substances such as gelatin, gum arabic, silica gel, and alumina have the property of adsorbing water. This water does not exhibit its normal vapor pressure or other colligative properties. Neuhausen and Patrick (11) have shown that a silica gel can be heated to 300°C. in a vacuum produced by a Gaede pump for a period of 6 hours without reducing the water content below 4.8 per cent. In recent years the extensive series of measurements of bound water initiated by the studies of Newton and Gortner (12) on bound water in plant saps leave little doubt that such a thing as bound water exists. A satisfactory, absolute definition of bound water has never been made. Briggs (2) says that “there are nearly as many definitions of bound water as there are methods for determining it. The methods are consistent in that they remove or otherwise change the state of a fraction of the water leaving a portion unaccounted for, this being designated as bound. The This article is based upon the dissertation submitted by Sutton Redfern to the Faculty of Philosophy of the Johns Hopkins University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1936. a Present address: The Fleischmann Laboratories, 810 Grand Concourse, New York, New York. 497
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SUTTON REDFERK AND W" A . PATRICK
usual idea of bound water carries wit,h it a picture of a portion of the water in the system as being associated with the colloid phase with such strength t h a t it is no longer free t o exhibit those properties which are characteristic of water, i.e. it is no longer available t>oact as a solvent, or it cannot be separated from the colloid by freezing it or by subjecting the system t o pressure, as in an ult,rafilter." 1-cry probably there is: a definite bound water f free water equilibrium which is measured at a different point in each different method. This makes it difficult to get check results, although Sayre (35) has obtained check result's by three different methods on a guin arabic sol. The purpose of this study is t o test accurately the whole question from an experimental standpoint, and furthermore to formulate if possible a rational explanation of the phenomena involved. BRIEF S T A T E M E N T O F AIETHOD
The method of studying t h e distribution of the various solutes betwecn the gel and its surromiding equilibrium solution is in brief as follows: A cylinder of silica gel, of previously determined water content, was placed in a flask with the desired solutf solution; the flask was placed in the thermostat, and allowed t o stand, with occasional shaking, for two weeks or longer. Then duplicate samples of the external solution were removed t o tared rveighing bottles, and the molal concentration of the solute determined by the appropriate niethod. The gel war removed from thfx solution and carefully wiped \\ith filter paper to remove any excess solution. Four samples of the gel were then cut off and t h p silicon dioxide content of two of them and the solute content of t'he other two were determined. The above analyses gave sufficient data from which the concentration of solute in the gel and the concentration of solutc in the external solution could be calculated. PREP.4RATIQS
OF GELS
Gels prepared from sulfurous acid (1) h a w a greater nicchanical strength than gels prepared from sulfuric or hydrochloric acids and so were used. The sodium silicate used t o prepare t h e gels was a commercial product, eoiitaining NazO and SiOn in the ratio 1:3.51. I t was diluted until it contained 6 per cent XaZO. The gels were prepared by adding t'he diluted *odium silieate solution t o a saturated aqueous solution of sulfur dioxide alid pouring t,he resulting sol into paraffined g l a s molds 3.4 x 10 c m . .Ifter the sol set t o a gel, it ivas removed and washed overnight in tap lvater in order to remove most of the solublc aalta. The gel cylinders \\'ere t,hen ivashed alternately in 6 N and 12 A' hydrochloric acid until no Inore iron was extracted, and fiiially washed in distilled water uiitil f r w o f chloride. Gel. w a a h d in thi.3 niannrr with liydrocliloric acid did not
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rontain more than 0.1 per cent residue based on the weight of silica after t h e silica was removed n i t h hydrofluoric acid and sulfuric acid After being washed, the gels were dried superficially in t h e open air. They were now approxiniately 85 per cent water and could be used directly as representing this water content. For lower Rater contents, the gel cylinderi were dried slowly under raised bell jars. The drying procesq \$as followed by weighing t h e cylinders. Since these dried gels would ciack when placed in ITater, they were resaturated with water vapor by Allan’s method (1). EQUILIBRATION AXD SAMPLING O F GELS
The iolutes uyed in this investigation were potassium chloride, potaqiium bromide, sodium chloride, sodium bromide, lithium chloride, magnesium chloride, barium chloride, calcium chloride, strontium chloride, and dextrose. Theqe qolutec: were all of c P grad? and were used without further purification. L% gel cylinder a a s placed in a 250-ml wide-mouth Erlennieyer flask and covered with a solution of t h e solute The flaqk \$as tightly stoppered v i t h a rubber stopper, placed in a thermostat maintained a t 25.00”C. 1 0 0 5 O , and left for two weeks or longer. The dextrose solutions were covered x i t h a layer of toluene to prevent the formation of mold. It is a safe aqsumption that t h e toluene did not interfere. Jones and Gortncr ( 5 ) also used toluene “because of its inactivity ton ards aqueous colloidal \ystems.” When the gel had come to equilibrium, duplicate qamples of the external solution were removed, \\eighed, and the amount of dissolved .elute determined. The gel cylinder \vas then reinoi ed and immediately viped off carefully with filter paper Two samples of gel \\ere then broken off with a knife and placed in neighing bottles The silica was determined in these samples The reqt of the gel was split into t n o pieces, and the solute removed and analyzed by an appropriate method. METHODS O F EXTR4CTION AKD ANALYSIS
The gel
