Chopra, S. L.

Sethi, R. K.

English

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Indian Journal of ChemistryVol. IfA, February 1976, pp. 94·96Sorption of Water & Some Straight- & Branched-chain AliphaticAlcohols by CaseinR. K. SETHI & S. L. CHOPRADepartment of Chemistry & Biochemistry, Punjab Agricultural University, LudhianaReceived 24 A ugust 1974; accepted 4 A pril 1975Sorption isotherms of water, methanol, ethanol, n-propanol, n-butanol, n-pentanol, iso-propanol, isobutanol, s-butanol, t-butanol and isopentanol on casein have been determined at35°, using quartz fibre spring technique. The sorption capacity of the protein is ~reater forbranched chain alcohols than that for normal alcohols and the amount of sorbate taken up de-creases with increase in the Iength of the hydrocarbon chain. The specific surface area of caseinhas been calculated by the application of the BET theory. The values of the thermodynamicfunctions indicate the absence of chemical interactions between the adsorbate and adsorbent.SORPTION of water vapour by a variety ofproteins-:" including caseins"? has beenextensively studied. However, little workhas been done on the sorption of alcohols andrelated compounds by these materials. The im-portance of water sorption as a possible tool forthe estimation of surface area of proteins, in eluci-dating varietal differences and for indicating possibleinteraction with the ionic groups in these substanceshas rightly been emphasized by several investi-gators5,6,9,lO. A systematic study of the sorptionbehaviour of a series of polar sorbates of equalcapacity for binding but of varying size has nowbeen undertaken. This is expected to throw somelight on the variation of sorptive behaviour ofproteins with change in the hydrocarbon chainof the alcohol molecules. Of particular importanceare the studies on the sorption of alcohols of lowmolecular weight on casein because unlike waterthese alcohols which are similar to water in certainchemical properties do not cause swelling of theprotein network.Materials and MethodsThe vitamin-free, crystallized and lyophilizedsample of casein (purity >99%) was obtained fromMIs Calibiochem, Los Angeles, USA. The particlesize was that of 100 mesh British standard sieve.The sorbates used were water, methanol, ethanol,n-propanol, isopropanol, n-butanol, s-butanol, iso-butanol, t-butanol, n-pentanol and isopentanol.Alcohols were of AR grade (BDH).The quartz fibre spring technique described byRaoll was employed in the present work. Thesorption apparatus was kept at 35° inside an airthermostat. The apparatus was evacuated to 10-2mm. A cathetometer reading up to 10-2 mm wasemployed for measuring the length of the spring.The rate of sorption of water and alcohols on caseinwas slow and the equilibrium was established gene-rally after 4 to 5 hr.Results and DiscussionSorption process - The adsorption isotherms forwater and normal alcohols are shown in Fig. 1a94while those for isomeric alcohols are plotted inFig. lb. The isotherms for both types of alcoholsare similar except that the sorption values forbranched-chain alcohols are slightly higher incomparison to those for the normal alcohols. Theamounts of sorbates taken by casein at the satu-ration pressure of water, methyl, ethyl, n-propyl,isopropyl, n-butyl, s-butyl, z-butyl, n-pentyl andisopentyl alcohols are 35·6, 30·2, 27·0, 25'0, 32,6,17·5, 24·5, 26·2, 28'8, 12·2 and 12·5% respectively.It is observed that the amount of sorbate takenup by casein decreases with the increase in the lengthof the hydrocarbon chain. The amount sorbedstarts falling off much faster as soon as the length36,-----------------,.,I ,WATER2 ,METHANOL3 J ETHANOL4, I)-PROPANOL5 • II~ BUTANOL6 • 11-PENTANOLa1 ,ISOPROPANOL2, tert-BU TANOL·3.150 BUTANOL4, see-BUTANOL5 , ISO PENTANOLb0'2 0'3 O-t,. 0-5 0-6 0-7 0-8 os 1-0RELATIVE VAPOUR PRESSUREFig. I - Adsorption isotherms of water and differentalcoholsSETHI & CHOPRA: SORPTION OF WATER & ALCOHOLS BY CASEIN0·061 , ETHANOL2 , METHANOLJ , WATER4, n -PROPANOL5 , n-BUTANOL•0-0441\so 001 0·3Fig. 2 - Application of BET equationof the alkyl groups increases beyond three carbonatoms. The alcohols under study have essentiallythe same polarity (for example, the dipole momentvalues of methanol, ethanol, n-propanol and n-butanol are 1·70, 1·69, 1·66 and 1·66 respectively),and therefore the decrease in the extent of sorptionis mainly due to the increase in the molecular size.The amount of branched-chain alcohols sorbed iscomparatively more than that of normal alcohols.This is due essentially to shorter chain lengths ofthe former. The adsorption is speculated- to beon the freely exposed surface, followed by pene-tration and is promoted by swelling to some extent.Therefore, the longer the chain, higher will be theenergy barrier for the penetration. This may bedue to a rapid increase in steric effects at this length.Our observations on the mechanism of sorptionand diffusion of water vapour by albumins stronglysupport the above hypothesis.Surface area of casein - The sorption isotherms(Fig. 1, a and b) are sigmoid in shape and belongto type-If of the BET classification, indicatingthat the sorption of water and aliphatic alcoholsby casein is multimolecular. The BET plots areshown in Fig. 2. The plots for n-pentanol andbranched-chain alcohols are not given for reasonsof space. From the slope and intercept of eachplot, the values of monolayer capacity have beencalculated'>. These values along with the cross-sectional area of the sorbate molecule13,14 havebeen employed to calculate the surface area12 ofcasein. The results are recorded in Table 1.The value of the specific surface of casein deter-mined using liquids of various vapours are notcoincident. This casts a doubt on the applicationof BET equation to systems in which there is swell-ing of the sorbent. Casein certainly swells betterwith water and it may swell to different extentswith various alcohols. However, it does not seemlikely that the variation in the surface area due toswelling would be large enough to account for allthese differences.Pore-size distribution - Following the procedureof Foster12,15. the pore-size distribution has beendetermined from the isotherms. No allowance ismade for the changing thickness of the adsorbedlayer on the walls of the pores. The isotherms ofwater, methanol and ethanol have been chosen forthese calculations as the contact angle is assumedto be zero for these sorbates. The predominantpore size for casein has been found to be 4·2, 5·8~nd 5·~ A respectively for these systems. It isinteresting to note that there are micropores in theprotein according to the classification of adsorptionIsotherms bv Dubinin'",Thermodynamic constants - The values of thenet heat of adsorption in the BET equation hasbeen calculated from Eq. (1)13,c = e(E,-EL)/RT ••• (1)where E1 is the heat of adsorption in the first layerand EL is the heat of liquefaction of the adsorbate(Table 2). The low magnitude of the heat of ad-sorption is indicative of the weaker interactionof alcohols with casein and rules out the possibilityof any chemical interaction. Moreover, the valuesare of the same order for all the alcohols showingthat the interacting forces are of the same kind.There appears to be no precise relation, however,between the energy liberated and the length of thecarbon chain.Further, the free energy change accompanyingthe sorption of vapour by casein has been evaluatedusing Eq. (2),RTJf dfAG = M 0 a . 7 '" (2)where a is the weight of the liquid sorbed at a relativevapour pressure f and other parameters have theirTABLE 1- VALUES OF MONOLAYER CAPACITY (Xm),MOLECULAR CROSS-SECTION (A tn) AND SPECIFICSURFACE (5) FOR VARIOUS SORBATESSor bate Xtn Am 5tst« of adsorbent) (A2) (m"/g)Water 0'1393 10'6 487'9Methanol 0'0898 21'2 358'4Ethanol 0'0726 27'0 256'5n-Propanol 0'1592 31·4 501'5n-Butanol 0'2272 36·0 631'8n-Pentanol 0'0606 40·2 166·7Isopropanol 0'0896 14'5 130·4Isobutanol 0'0519 25·0 105'8s-Butanol 0'0500 17'4 70'8I-Butanol 0·0723 55'5 326·4Isopen tanol 0·0947 28'8 186'6TABLE 2 - THERMODYNAMIC PARAMETERS AT 35°Sorbate Heat of adsorption(E1-EL)(cal/mole)545579636471473345654636644637750-!1G(cal/mole)WaterMethanolEthanoln-Propanoln-Butanoln-PentanolIsopropanolIsobutanols-Bu tanolt-ButanolIsopen tanol1197288302203107323231661562123695INDIAN J. CHEM., VOL. 14A, FEBRUARY 1976usual significance. The values of free energy(Table 2) are again quite low and indicate theinvolvement of physical interactions. In a seriesof analogous reactions, the rates of reactions areoften roughly in the order of free energy change.Thus a comparison between the straight-chain andthe branched-chain alcohols reveals that the latterhave a comparatively greater affinity for the caseinsurface.Contact ang~ and the sorption isothe,ms - Accordingto Gregg12, the contact angle is assumed to be zerowhen the surface is pure and the liquid wets thesurface; as a result the isotherm is asymptotic tothe saturation pressure ordinate. But with liquidswhich have a definite contact angle, the isothermsintersect the ordinate at an angle. In the presentexperiment, the asymptotic nature of water, metha-nol, ethanol and n-propanol isotherms leads to theconclusion that the contact angle is zero. Foxand Zisman-? have shown that for many liquidson solids, the contact angle decreases with thedecreasing surface tension of the liquid. Thevalues of the surface tension'" of n-butanol andn-pentanol are 23·35and 24·30 dynes/em respectively(at 35°) showing thereby that contact angle increasesfrom n-butanol to n-pentanol. As regards branched-chain alcohols, the values of surface tension18,19of t-butyl, isopropyl, isobutyl, s-butyl and isoamylalcohols are 18·8, 20·5, 21·8, 22·2 and 22·7 dynes/emrespectively (at 35°). In the light of Fox andZisman's conclusion, the contact angle of thesealcohols should be in the order: t-butyl<isopropyl< isobutyl < s-butyl and <isopentyl alcohol. Theshapes of the adsorption isotherms (Fig. 1) supportthese' observations.96AcknowledgementOne of the authors (RK.S.) thanks the CSIR,New Delhi, for the grant of a research fellowship.References1. ELEY, D. D. & LESILE, R. B., Trans. Faraday Soc., 62(1966), 1002.2. RAo, K. S. & DAS, B., J. phys. Chem., 72 (1968),1223.3. DAS, B., SETHI, R. K. & CHOPRA, S. L., J. Indian chem,Soc., 50 (1973), 119.4. SEEHOF, J. M., KOILlN, B. & BENSON, S. W., J. Am.chem, Soc., 75 (1953), 2427.5. McLAREN, A. D. & ROWEN, J. W., J. polymer Sci., 7(1951), 289.6. BULL, H. B., J. Am. chem . Soc., 66 (1944), 1499.7. BERLIN, E., ANDERSON, B. A. & PALLANSCH, M. J.,J. phys. Cbem., 73 (1969), 303.8. PURl, B. R., TOTE]A, K. K. & MALIK, R. C., J. Indianchem. Soc., 46 (1969), 554.9. GREEN, R. W., Proc. R. Soc. (New Zealand), 77 (1949),313.10. MELLON, E. F., KORN, A. H. & HOOVER, S. R., J. Am.chem, Soc., 70 (1948), 3040.11. RAo, K. S., J. phys. Chern., 45 (1941), 500.12. GREGG, S. J. & SING, K. S. W., Adsorption, surfacearea and porosity (Academic Press, London and NewYork). 1967.13. RAo, K. S. & DAS, B., J. Colloid 6- Interface Sci., 32(1970), 24.14. RAo. K. S. & DAS, B., Proc . Indian Acad. Sci .• 70 (1969),18.15. FOSTER. A. G., J. chem, Soc., (1952). 1806.16. DUBININ, M. M .• Q. Rev. chem, Soc .• 9 (1959). 101;Chern. Reu., 60 (1960). 235.17. Fox. H. W. & ZISMAN. W. A .• J. Colloid Sci .• 5 (1950).514; 7 (1952), 109.18. TIMMERMANS. J., Physico-chemical constants of pureorganic compounds (Elsevier. Amsterdam), 1950.19. International critical tables, edited by W. WashburnVol. 4 (McGraw-Hili. New York). 1928. 453. '

Sorption of Water & Some Straight- & Branched-chain Aliphatic Alcohols by Casein

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Sorption of Water & Some Straight- & Branched-chain Aliphatic Alcohols by Casein

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