Indian Journal of ChemistryVol. 23A. May 1984. pp. 372-374

Interaction of Polyvinyl Alcohol with Protein at Air/Water & Oil/WaterInterfaces & Its Effect on Emulsion Stability

V M SABEP, SH M ZOURAB. M M EL-SAYED & S M KOTBUniversity of Alexandria. Research Centre UNARC Alexandria. Egypt

Received 25 July 1983; revised 14 November 1983; accepted 2 January 1984

The interaction of polyvinyl alcohol (PVA) with the protein extracted from cicer at pH:::::4(the isoelectric point of theprotein) has been studied at the air/water and toluene/water interfaces. Electrophoretic mobility data indicate partialreplacement of the protein molecules by PVA. As a result of this interaction the mechanical properties of the protein film arehighly reduced and a less stable emulsion is obtained.

Proteins have been used as emulsifiers and/orstabilizers and are often essential ingredients in sprays,cosmetics, pharmaceutics, food-stuffs and many otherindustrial products. Such useful properties of proteinshave led to many investigations of protein films at theair/water and oil/water interfaces 1 . Recently Grahamand Phillips 2 studied the adsorption and properties ofwell-characterized protein films at the air/water andoil/water interfaces using surface pressure, directadsorption and rheological measurements. Theproperties were then related to the behaviour andstability of some model foams and emulsions stabilisedwith these proteins.

The use of synthetic polymers to control thestability/flocculation behaviour of colloidal systems isof growing technological importance. Such polymersstabilise the colloidal dispersions not only in water butalso in the presence of high concentration ofelectrolyte.' due to the powerful steric barriers whichthey produce and which prevent the close approach ofthe particles. Moreover, polymer films producemechanical barriers due to the viscoelastic properties.The effect of addition of polyvinyl alcohol (PVA) onthe interfacial properties of the protein film at theair/water and toluene/water interfaces and its effect onthe resulting emulsion have now been studied usinginterfacial tension and rheological measurements.

Materials and MethodsThe isolation and characterization of the protein

extracted from cicer were described elsewhere" . Pure,dry and dialyzed protein was used as an emulsifier. Itwas slightly soluble in water, therefore, its aqueoussolution was prepared by dissolving a known weight ofprotein in distilled water (2 litres), the insolubleprotein left after 24 hr removed by centrifugation anddried by washing several times with ethanol, acetoneand ether and then weighed. The concentration of thesoluble protein was then determined by weight

372

difference. The solution was kept at 4"e. Doublydistilled water was used for the preparation of all thesolutions. Analar toluene (Merck) was further purifiedby passing it through an alumina column to removeany surface active impurity. Polyvinyl alcohol (PVA)(BDH), molecular weight 49,000, contained 11-14%acetate groups. As indicated by the supplier a 4%solution had a viscosity of 13 cp at 20°e. All otherchemicals were of analytical grade and used as such.The oil/water (O/W) emulsions were prepared bystirring 0.4% by volume of oil in the aqueous proteinsolution in a homogenizer for a total period 10 min;stirring in the homogenizer was done for 1 minfollowed by an interval of 30 sec. The creamy layer wasskimmed off after 30 min.

Interfacial tension, electrophoretic mobility.elasticity and interfacial viscosity as well as the rate ofcoalescence were measured as described previously" .The isoelectric point of the protein (PH ~4J) wasdetermined from electrophoretic mobilities of toluenedroplets stabilised with the protein as a function of pH.The size of the emulsion droplet was calculated bytaking microphotographs of the emulsion using aPentax camera attached to an optical microscope. Theradii of about 800 particles were determined by acathetometer. From the size distribution curve theaverage radius of the emulsion stabilised by proteinwas found to be 0.25 u .

Results and DiscussionThe surface tension (Y,J of the protein solution and

the interfacial tension (}'J of toluene in aqueous proteinsolution, initially changed with time, but after 60 minof ageing there was no change in i's and I'i' Therefore,all measurements were made within I hr.

The equilibrium values of }'., }'i are given in Table I.Zeta-potentials m at various pHs for the emulsionstabilised with protein are given in Table 1.The resultsshow a gradual decrease of 1'. and Yi with increase in pH

SABET et al.: INTERACTION OF PYA WITH PROTEIN

Table I-Variation of Zeta-potential, Surface Tension ofProtein Solution (yJ and Surface Tension of Toluene in

Aqueous Protein Solution (y;) with pH

pH , Y. )Ii

mV mNm -I mNm-1

2.5 +42.5 63.94 20.202.9 + 36.0 63.27 18.873.5 + 22.5 56.94 15.984.0 + 11.5 55.50 15.324.77 -14.2 61.49 17.434.93 -19.4 61.80 17.506.23 -51.1 62.10 17.70

-2Qr-------------,

>E•.. -15

•;:z~ -10e

o PVA

• PVA •. etc •.r Prol •.in

-a -2 -I

IOQ C

Fig. I-Variation of zeta-potential with PYA concentration in thepresence (e--e) and absence (0-0) of cicer protein at pH 4

till a minimum is reached at pH 4, which is theisoelectric point of the protein. Thereafter the values ofIs and I'i rise again with further increase in pH. In thepresence and absence of protein and at pH 4.0 both 'isand )Ii decrease gradually on increasing theconcentration of the polymer. A similar trend has beenobserved earlier":" . Further at all concentrations ofPYA the values of v, and y, are lower in the presence ofprotein as compared to those obtained in its absence.At high PYA concentration 'is and ,'i values for thePYA-protein system approach the correspondingvalues for the PYA alone. The variation of zeta-potential as a function of the polymer concentration (cin g litre -1) in the presence and absence of protein atpH 4 is shown in Fig. 1. In the absence of protein, thegradual decrease of zeta-potential with increase inextent of PYA adsorption is to be expected fornon ionic macromolecules. Similar results wereobtained at the solid-liquid" and liquid-liquid 9

interfaces. At low PYA concentration, the macromo-lecule attains a flatter conformation which asadsorption progresses leads to a thicker adsorbedlayer. As a result of this adsorption and build-up ofthicker adsorbed layers, a shift in shear plane towardsthe solution occurs and this leads to a gradual loweringof zeta-potential. Eventually, a limiting value isreached at the maximum thickness of the adsorbedlayer. However, in the presence of protein the zeta-potential is slightly affected by the addition of PV Aand its value is close to the limiting value reached in the

Table 2- Variation of Gi , '1iand K with PVA Concentrationat pH 4 in the Absence and Presence of Protein

Cone. PYA Gi 'Ii Kg litre-I dyne cm -I poise hr -,

In the absence of protein0.001 0.0 0.02 0.120.005 0.0 0.030.010 0.3 0.05 0.130.050 0.7 0.06 0.090.100 0.4 0.07 0.081.000 0.1 0.09 0.08

In the presence of protein0.00 9.5 9.89 0.050.001 5.6 9.10 0.100.005 5.5 9.000.010 5.3 8.00 0.100.050 2.8 4.60 0.100.100 2.7 2.50 0.121.000 0.3 0.08 0.12

presence of PV A alone. This is not surprising since atpH4, i.e. at the isoelectric point the particles coated withthe protein film carry a very low charge.

The values of the elasticity G; and interfacialviscosity '1; for PV A in the presence and absence ofprotein atpH 4are given in Table 2. It is evident that inthe absence of protein both G; and 'I; values at allpolymer concentrations are very low. However, in thepresence of protein G; and 'I; decrease gradually from9.5 dyne em I and 9.X9 poise respectively at zero PYAconcentration to very low values with increasingpolymer concentration. At high PVA concentration G;and 'I; approach the corresponding values for the PYAalone. The reduction of the rheological parameter ofthe protein films caused by the addition of surfactanthas been explained by many workers I () 13.

The values of coalescence rate are also summarisedin Table 2. In the absence of protein the rate ofcoalescence decreases gradually with increasingpolymer concentration. Stability here may beattributed to the steric effect of the adsorbed polymerlayer. However, addition of PYA to the protein filmincreases the values of K relative to those obtained forthe protein alone. It seems therefore that the proteinfilm at its isoelectric point is more effective in thestabilisation of the emulsion against coalescence thaneither PYA or the mixed PV A-protein films.

Surface and interfacial tension measurements as afunction of PYA concentration in the presence andabsence of protein indicate competitive adsorption ofthe two molecules at the interface. A t very low PYAconcentration, both }" and 1'; are dominated by thepresence of protein since the values of ,,, and ,'; areclose to those obtained for protein alone.

373

INDIAN J. CHEM., VOL. 23A, MAY 1984

The gradual decrease of the mechanical propertiesof the protein film with increasing PVA concentration(Table 2) points towards replacement of the proteinmolecules by PVA and formation of a mixed film at theinterface. It is interesting to note that protein alone ismore effective in stabilising the oil/water emulsionsthan either PVA or PVA +protein. Clearly in thepresent system, electrostatic interaction plays a minorrole and the stability is mainly governed by themechanical properties of the protein film. The stericinteraction produced by the adsorbed polymer seemsalso to be less effective in the stabilisation of theemulsion against coalescence than the rheologicalproperties of the protein film themselves. Correlationbetween film elasticity and the rate of coalescence ofoil/water emulsion has been observed by manyworkers 1~ 1". It seems that the increase in G i preventslateral displacement of molecules in the thin filmbetween approaching oil droplets, thus reducing thecoalescence considerably.

References

Miller I R & Bach D. Surface and colloid science. Vol 6, edited byE Matijeric (Wiley, New York). 1973. 185. Miller I R. inProgress in surface and membrane science. Vol 4. edited by DA Cadenhead, J F Danielli & M D Rosenberg (AcademicPress. New York), 1971, 298.

374

2 Graham D C & Phillips M C, J Colloid Interface Sci, 70 (1979)403,415,427; 76 (1980) 227, 240.

3 Tadros Th F & Winn P D, Unpublished data.4 EI-Sayed M M, Sabel V M, Zourab Sh M & Kotb S M,

Unpublished data.5 Sabet V M, Zourab Sh M & Said W, J Dispersion Sci Techn, 3(4)

(\ 982) 419.6 Tadros Th F & Vincent B, Emulsion stability Encyclopedia of

emulsion technology, Vol I, edited by P Becher (MarcelDekker), In press.

7 Lankveld J M V & Lyklema J, J Colloid Interface Sci. 41 (1972)454.

8 Tadros Th F. J Colloid Interface Sci, 46 (1974) 528.9 Tadros Th F, Theory and practice of emulsion technology,

Proceedings of a symposium organised by Society ofChemistry and Industry, September (1974) (AcademicPress), 1976, 281-299.

10 Cumper C W N & Alexander A E. Trans Faraday Soc. 46 (1950)235.

II Biswas B & Haydon D A. Proc R Sac (AI, 271 (1963) 296.12 Moilliet J T, Collie B & Black W, Surface actirity (E and F N

Spon Ltd. London), 1961. 2b7.13 Pearson J T. J Colloid Sci, 27 (1968) 64.14 Srivastava S N & Haydon D A, Proc 4th Int Congress Surface

Actiritv, (Gordon and Breach, London), 1967. 1221.15 Boyd J. Parkinson C & Sherman P. J Colloid Interface Sci. 41

(1972) 359.16 Wasan DT,ShahS M,Aderangi N' Chan 1\1S& McNamaraJ J.

Soc Pet Eng J. IS( 1978)409; Wasan D T. Shah SM. Chan MS. Sampath K & Shah R. ReI' D H' Pet Chern Alii Chcm Sac,23 (1978) 705: Wasan D T, Sarnpath- K & Aderangi N,AICHI:' Svmp S('/'. 76 (\980) 93.