8/8/2019 7 Water- Estructura y funcion a nivel 1 molecular

1/2

Le us suppose that the dissolved meth-ane molecule is surrcunded by an en-velope of 10 or 20 water inolecules. Theformation of such a structure would ac-count fcr the heat iiberated. In he spaceoccupied by the methane molecule theattractve forc on the water rnclecules,and henee the inward pressure. would

disappear. Under these conditions, as wehave seen. water will freeze ai s. hishertemperatura. Thus the molecules at theinterface bstween the methane and wa termolecules may crystallize into "ice." Thefrozen hydrates may accurnulate andseprate out of the solution.

This hypothesis is known as the "ice -berg'"' theory, It is supported by,the fac that practicay all the nonelectrolyticsubstances tested have beeri round tofonn solid crystailine hydrates. in con-tras, electrolytes show littie tendency toform thera.

All this leads to an entirely new con-cept of solubiiity. Chemists have lon supposed that soluhility always involve:atractive forces. 3u i now appears thathe1 dissoiution of a nonelectrolyte is duno to an attraction between he substance and water but to a lack of attraction. The nonionic substances corabinwith water because hey remo ve intern;

pressure and thereby permit rormatio ofa crystailine compound.

In order to unders and the forrnaticof these hydrates, it is necessarr to cosider their molecular structure in detaThey tend to fali into sroups accordi:

to the number o water molecules thcontara.The groun work for.he study of i

hvdrates structure was laid by M. \Stackelberg in Germanv 10 years age.Ke showed by X-ray studies that thisstructure was eubic. in contrast to thehexagonal structure of ice. W. F. Claus-sen of our laboratory recently attaokedthe problem of buiiding such cubicstructures. each ccntainng a gas mole -cule. into a repeatins lattice, It turns outthatthere are wo possible cubic lapices,ene of which was proposed and workedout bv Linus Paulins of the California

w

Institute of Technology. This has s. spac-in2 of 12 Ansrstroms between moleculesv/hiie the other has 17 Angsiroms. Thesraalier lattice conains 46 water mole-cules and the iarger ene 136, The lilesfor gas molecuies in he sraalier latticehave 12 or 14 walls. v/hiie those in thelargar one have 12 or 16 sides. Theseheles are of aifferent size; and make

possible a be^viiderins arrav cf hvaratss.The differsnt sized hole ca.n be fllledOiv wii; diSsreni sizeci rrioiscuiss. r.ncnot ail the heles ir a lafcice need be niied.The mode! extains the &crual

composition ofhvdrares withremarkabie

precisin.Theimportan ceof this typ. ohy-drate tothe processesof life cannot

be

overercTihasized. These processes occurmainly at the interaces between waterand protein inolecules. Water has a ven"strong tendenc 1.- to crystallize there, forthe protein molecule eontains larjie non -

ionic. or nonpoiar 2rcups. Anv hvarateso forrned has a iower densitrv than ice.-'consequently its formation car: cause E.Itr^e. desructive expansin.

The rsezinc of corn ai r. temceraiureof 40 desrees '^. becornes understano-abie in terrns of the formation o zhv--.i, Winter \vhea;. on he other hana.

RAPID MIGEATTO?! OF HYDROGEN IONS through water is esplained by the assunip- tiothat the ion? do not actual y tra Tel throtigh the water bul are pnsfec irom one molecule to thenext by a proeess of exchanie. Here hydrogen ion is repreented by colorea dot snrrouned by abroken circle. In he first"horizontal row the hydrozeE 9n approaches a water moieenle. In thesecon row the ion has taken the place of one hydrogen atom of the molecnle. expelling theatom as a nev,- ion. In the third row tlie new ion repeats this p

8/8/2019 7 Water- Estructura y funcion a nivel 1 molecular

2/2

forrns hvdrates slowly as emperaruresdrop thrcugh he fali. and under theseconditions the hydrate acts as an effec-ive antfreeze proiecin the cells frorndaniage.

The frozen fooc industrv uses rapidfreezing to avoid the formation of iargecrvstals which would damage tie placells. But i miffht be well o explore the

possibility of the ooposite approach. Ver-.'slow cooline of h'%'ins piant foods mightorrn hydrates which would pre-ventdamase from ice crystais when tne piantwas frozen.

Le: UE return now to see how niestructure of water may be modified whenar; ee:rroivte. sav a salt. Eoes into soiu-