Surface Properties of Gelatin Films

Surface Properties of Gelatin Films

Tomasz Białopiotrowicz* and Bronisław Janczuk

Department of Interfacial Phenomena, Faculty of Chemistry, Maria Curie-SkłodowskaUniversity, Maria Curie-Skłodowska Sq. 3, 20-031 Lublin, Poland

Received February 13, 2002. In Final Form: July 22, 2002

Contact angle measurements for water, glycerol, formamide, ethylene glycol, diiodomethane, R-bromo-naphthalene, tricresyl phosphate, dimethylsulfoxide (DMSO), and bromoform on polymethyl methacrylate(PMMA) covered with adsorptive and gelatinized gelatin films were made. Adsorption was performed fromsolutions in the 0-25 g/L concentration range. A gelatinized gelatin film was created from solutions of40-100 g/L concentrations. It was found that the biggest changes of the contact angles were up to monolayercoverage of the PMMA surface. For all liquids (besides water) the contact angle was almost constant abovethe gelatin concentration 50 g/L. Very high contact angles (∼134°) were obtained for water on a gelatinizedgelatin film obtained from a solution of 100 g/L concentration. From the obtained contact angles, theLifshitz-van der Waals components and the values of the electron-acceptor and electron-donor parametersof the acid-base components of the films were calculated for the glycerol-ethylene glycol-diiodomethanethree-liquid system. It was found that the water contact angle strongly influenced the Lifshitz-van derWaals component and acid-base parameters of the gelatin film surface free energy. The systems involvingwater gave different results than those without water. It was found that the values of the gelatin filmsurface free energy components and parameters cannot be explained only on the basis of the functionalgroup’s orientation in gelatin molecules. To find an explanation, back-calculations of the contact angle forwater, formamide, R-bromonaphthalene, tricresyl phosphate, dimethylsulfoxide, and bromoform were made.The authors found a good agreement between the calculated and measured values of the contact anglefor all liquids studied, besides water and, partially, DMSO. To explain the obtained results of the measuredcontact angles, four models of hydratation of gelatin films were presented.

Introduction

Gelatin is a fibrous protein with wide applications inpharmaceutical, food, and photographic industries.1,2 Itis a mixture of single and double unfolded chains ofhydrophilic character.3,4 This hydrophilicity means thatin highly hydrated states gelatin can create a three-dimensional network called a hydrogel. However, unex-pectedly, high contact angles of water are observed onsuch a surface. Holly and Refojo4 connected this phe-nomenon with a preferred orientation of hydrophobicmoieties at the hydrogel-air interface. Many otherresearchers5-8 have also obtained very high contact anglevalues on a gelatin gel surface. Braudo5 et al. obtainedvery big values of the contact angle: 124° for water, 34°for bromoform, 43° for dimethylformamide on 14% gel.They noticed that such a high contact angle had to beconnected with water content in gelatin gel, but they didnot give a deeper explanation of this fact. Very similarvalues were obtained by Summ6 et al. Wolfram andStergiopoulos7 determined contact angles for 15% gelatinegels for water (111°), formamide (74°), and diiodomethane

(59°). Yasuda8 et al. obtained lower (around 90°) valuesof the contact angle for 15% gelatin gel.

Much less attention was paid to adsorptive gelatinefilms and contact angle measurements on such substrata.Gelatin adsorption on various surfaces was studied by afew authors.9-11

This paper is an extension of our previous paper12 inwhich we described the measurement results for fourliquids on gelatin films obtained on a PMMA surface byadsorption in the 0-100 mg/mL concentration range ofthe solution.

In our previous paper we also noticed such a strangebehavior of water; therefore, we decided to study thatproblem more closely. We measured contact angles formore liquids to obtain more information. Taking intoaccount the above-mentioned results of Wolfram andStergiopoulos,7 their contact angle for water was the sameas ours13,14 for PTFE (Teflon); however, our results forformamide and diiodomethane are considerably higher(102.7° and 72°, respectively). The explanation given byWolfram and Stergiopoulos7 that during gelificationhydrophobic gelatine groups are oriented into air, creatinga very hydrophobic surface, seems to be not quite adequate.If it is the case, also the contact angles for formamide anddiiodomethane should be similar to those for PTFE. Theonly explanation of those results is that water behaves

* E-mail: [email protected]. Phone: +48-81-53756 03. Fax: +48-81-533 33 48.

(1) Courts, A. In Applied Protein Chemistry; Grant, P., Ed.; AppliedSci. Publishers Ltd.: London, 1973; p 78.

(2) James, T. H. In The Theory of the Photographic Process;Macmillan: New York, 1977; p 25.

(3) Stryjer, L. Biochemistry; W. H. Freeman: San Francisco, CA,1988; Chapter 7.

(4) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315-322.

(5) Braudo, E.; Tolstoguzov, V. B.; Nikitina, E. A. Kolloidn. Zh. 1974,38, 208-213.

(6) Summ, B. D.; Mashnina, N. V.; Goriunov, J. V. Kolloidn. Zh.1986, 48, 188-191.

(7) Wolfram, E.; Stergiopoulos, Ch. Acta Chim. Acad. Sci. Hung. 1977,92, 157-167.

(8) Yasuda, T.; Okuno, T.; Yasuda, H. Langmuir 1994, 10, 2435-2439.

(9) Bajpai, A. K. J. Macromol. Sci., Pure Appl. Chem. 1995, A32,467-478.

(10) Curme, H. G.; Natale, C. C. J. Phys. Chem. 1964, 68, 3009-3016.

(11) Samanta, A.; Chattoraj, D. K. J. Colloid Interface Sci. 1987,116, 168-179.

(12) Białopiotrowicz, T.; Janczuk, B. Eur. Polym. J. 2001, 37, 1047-1051.

(13) Janczuk, B.; Białopiotrowicz, T. J. Colloid Interface Sci. 1989,127, 189-198.

(14) Janczuk, B.; Białopiotrowicz, T.; Wojcik, W. J. Colloid InterfaceSci. 1989, 127, 59-68.

9462 Langmuir 2002, 18, 9462-9468

10.1021/la0201624 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 10/31/2002

quite differently than formamide, ethylene glycol, anddiiodomethane, which is also connected with the com-pactness of gelatin films. Now, since we better understandmolecular interactions at an interface, we can give a moreconvincing and true explanation of this phenomenon. Weshall try to give a deeper explanation of it using van Oss-Good15,16 (vOG) theory.

The main goal of this paper is to find an explanationwhy there exists a big discrepancy between contact anglesfor water and other liquids on the studied gelatin films.Very high contact angles for water suggest that gelatinbehaves as a very hydrophobic substance, but that is notconfirmed by contact angles for other liquids. Therefore,in this paper we have described contact angles on gelatinfilms formed in a wide range of compactness from veryloose adsorptive films to highly compressed gelatinizedones. Because in our previous studies and those of otherresearchers PMMA was used as a model polymer sub-strate, it was more reasonable to use the PMMA also inthe present studies to avoid the influence of the kind ofpolymer substrate on the properties of the gelatin filmand to better compare all results.

Experimental Section

Materials.Polymethylmethacrylateplates (PMMA,OswiecimChemical Works, Oswiecim, Poland) were used as solid substratafor covering by gelatin films. Commercial plates were cut into20 × 30 mm ones, which were cleaned ultrasonically in a de-tergent solution for 10 min. Next, they were sonicated in tap,distilled, and finally MilliQ (Millipore) water. In these experi-ments, gelatin (Sigma G-6650, Type B; from bovine skin,approximately 75 Bloom) was used without further purification.Gelatin was dissolved in MilliQ water to obtain concentrationsfrom 0.005 to 100 mg/mL. We used water of room temperaturefor lower concentrations, warm water (ca. 40 °C) for higherconcentrations (up to 25 mg/mL), but hot water (ca. 60 °C) forconcentrations higher than 25 mg/mL. A fresh solution wasprepared each time.

Contact angles were measured on dried gelatin films. Thefilms were prepared in two wayssby adsorption and gelatiniza-tion. Adsorption was performed in glass Petri dishes (10-15 cmin diameter) at 20 °C for solution concentrations from 0.005 to25 mg/mL. After 2 h the plates were taken from the dishes, andthe remaining solution drops were removed with filter paper.Next, the plates with an adsorbed gelatin layer were kept in adesiccator filled with silica gel for at least 48 h at roomtemperature (20-25 °C). Gelatinized gelatin films were preparedby dipping PMMA plates in a hot gelatin solution. Then, theplates were kept in the desiccator, like those with the adsorbedfilm.

MilliQ water, glycerol p.a. POCh Gliwice (Poland), formamidepure Fluka AG (Switzerland), ethylene glycol p.a. POCh Gliwice(Poland), diiodomethane p.a POCh Gliwice (Poland), R-bromo-naphthalene Fluka AG (Switzerland), tricresyl phosphate FlukaAG (Switzerland), dimethylsulfoxide Aldrich (USA), and bro-moform Aldrich (USA) were the liquids used.

Methods. Advancing contact angle measurements were madeusing the sessile drop method with the Kruss (Hamburg,Germany) G10 computer image analysis system.

Results and Discussion

The contact angles (θ) for water (W), glycerol (G),formamide (F), ethylene glycol (E), and diiodomethane(D) on polymethyl methacrylate (PMMA) covered with agelatin film are shown in Figure 1, and those for R-bromo-naphthalene (B), tricresyl phosphate (T), dimethylsul-foxide (DM), and bromoform (BR) are shown in Figure 2.

From Figure 1 it is shown that the contact angle for bipolarliquids (W, G, F, E) (curves 1-4) decreased with increasedconcentration (C) of the gelatin aqueous solution used forgelatin film formation to a minimum value at 1 g/L, andthen it increased in the range of gelatin concentrationfrom 5 to 100 g/L. However, a considerable contact angleincrease for glycerol, formamide, and ethylene glycol(curves 2-4) was observed in the range of gelatinconcentration from 5 to 40 g/L. The contact angle for theseliquids was almost constant above the concentration 40g/L. The changes of the contact angle for monopolar liquids(DM and BR) (Figure 2, curve 3), as a function of gelatinconcentration, were similar to those for bipolar liquids(Figure 1, curves 1-4). However, in the case of apolarliquids (D, B, T) the contact angle increased in the C rangefrom 0 to 0.5 g/L and then decreased to a minimum valueat 2 g/L. In the range of gelatin concentration from 2 to100 g/L, continuous increase of the contact angle tookplace. The changes of the contact angle for bipolar liquidsas a function of gelatin solution concentration (C) pre-sented in Figure 1 are somewhat different than thosedescribed in our previous paper.12 However, it is commonlyknown that gelatin properties depend on its origin,production technology, and its molecular weight. It should

(15) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Sep. Sci. Technol.1989, 24, 15-29.

(16) van Oss, C. J. Interfacial Forces in Aqueous Media; Dekker:New York, 1994; Chapter 8.

Figure 1. Relationship between the contact angle (θ) for water(curve 1, W), glycerol (curve 2, G), formamide (curve 3, F),ethylene glycol (curve 4, E), and diiodomethane (curve 5, D)and concentration of gelatin in the aqueous solution (C) fromwhich the gelatin film was formed on the polymethyl meth-acrylate plate.

Figure 2. Relationship between the contact angle (θ) forR-bromonaphthalene (curve 1, B), tricresyl phosphate (curve 2,T), dimethylsulfoxide (curve 3, DM), and bromoform (curve 4,BR) and concentration of gelatin in the aqueous solution (C)from which the gelatin film was formed on the polymethylmethacrylate plate.

Surface Properties of Gelatin Films Langmuir, Vol. 18, No. 24, 2002 9463

be emphasized that in the present studies gelatin was ina gel form above 25 g/L concentration.

From the literature data concerning gelatin adsorptionfrom aqueous solution on AgBr and Al2O3,9-11 it is shownthat in the gelatin concentration range 1-2 g/L probablya full monolayer film of gelatin is formed. The minimalvalue of the contact angle of bipolar liquids in thisconcentration range suggests that the hydrophobic partof the gelatin molecule should be oriented toward thePMMA surface, and thus hydrophilic groups should bepresent on the gelatin film surface. The presence of thesegroups on the gelatin film surface caused its wettabilityby bipolar liquids to increase in comparison with that ofbare PMMA.

From their studies on gelatin adsorption on silverbromide sol, Curme and Natale10 concluded that gelatinmolecules in the film should have been laterally com-pressed or, alternatively, the adsorbed gelatin film shouldhave been expanded to explain the observed higher gelatinsaturation as compared to the value predicted geo-metrically using the molecular weight and the gelatinradius of gyration. The studies of Vaynberg et al.17 showedthat the adsorbed gelatin film at saturation on acrylic-based latex had approximately a double average densityof free gelatin compared to that in a solution. Taking intoaccount these conclusions, it is possible that the densityof the gelatin film formed from solution was continuouslygrowing in the gelatin concentration range from 2 to 100g/L, and the contact angle for the studied liquids increasedin this concentration range. The contact angle increase ofbipolar liquids indicates a density decrease of polar groupson the gelatin film surface.

It is interesting that the values of the contact angle forwater on a gelatinized film surface in the gelatin con-centration range from 45 to 100 g/L were higher thanthose on paraffin or a Teflon surface.13 These values areeven bigger than those obtained by Wolfram et al.7 forwater on a gelatin gel. However, in the case of other liquidsstudied, the maximal values of the contact angle (Figures1 and 2) were lower than those measured on paraffin andTeflon.13

Using the approach of van Oss at al.16 to interfaces, itis possible to explain more exactly the contact anglechanges for different liquids and the changes of the gelatinfilm structure by calculating its surface free energycomponents and parameters.

The contact angle value in a solid-liquid drop-airsystem resulted from the surface free energies of the solidand the liquid and the solid-liquid interfacial free energy.The equilibrium state of the solid-liquid drop-air systemis described by the Young equation:18

where γS is the surface free energy of a solid, γSL is thesolid-liquid interface free energy, γL is a liquid surfacetension, ΠeL is the film pressure of the liquid, and θ is thecontact angle. Van Oss and co-workers15 suggested thatthe surface free energy of a solid or liquid could be dividedinto two components:

The γLW component results from noncovalent long-rangeLifshitz-van der Waals interactions (sum of dispersion,

dipole-dipole, and dipole-induced dipole interactions),and γAB from Lewis acid-base interactions.15

The γAB component can be expressed as a function ofthe geometric mean of the electron-acceptor (γ+) and theelectron-donor (γ-) parameters:

According to van Oss and co-workers,15 the interfacialsolid-liquid free energy (γSL) can be written in the form

Combining eq 1 with eq 4 gives

For determination of the components and parameters ofthe surface free energy of a solid, the contact angle valuesmust be measured for at least three liquids, among themtwo bipolar and one apolar, and a set of three equationssuch as eq 5 must be simultaneously solved if ΠeL is knownor equal to 0. Furthermore, it is possible to calculate thevalues of γS

AB and γS from eqs 2 and 3. Because the contactangles of nine liquids were measured on gelatin film, thereare many possibilities for determination of the values ofγS

LW, γS+, γS

-, γSAB, and γS for that film.

According to the papers of van Oss15 et al. and DellaVolpe and Siboni,19 when LW components and acid-baseparameters for the three liquids used are not very different,a matrix corresponding to the system of three equationsis almost singular and the system is ill-conditioned.However, as it was found by Janczuk et al.,20 the contactangles only for three-liquid systems including two polarand one apolar liquid gave reasonable values of thecomponents and parameters of the surface free energy ofa solid. On the other hand, the system of three liquidsincluding water can give quite different results of γS

LW, γS+,

and γS- than the system based on only organic liquids

(without water), particularly when the solid surface orfilm surface can be strongly hydrated. However, in oursituation it was impossible to exclude the hydrationprocess; thus, for this reason the surface properties of asolid under a liquid drop can be different then those outsidethe drop. Taking this into account, the contact angles forglycerol, ethylene glycol, and diiodomethane and theirγL

LW, γL+, γL

-, γLAB, and γL values taken from the literature21

(Table 1) were used for calculation of the Lifshitz-vander Waals component and electron-acceptor and electron-donor parameters of the acid-base component of thegelatin film surface free energy. For these calculations itwas assumed that ΠeL was equal to zero.16

The values of γSLW, γS

+, γS-, γS

AB, and γS calculated fromeqs 2, 3, and 5 are presented in Figure 3, from which itresults that the γS

LW values (curve 1) decreased from 36.7to 33.3 mJ/m2 in the gelatin concentration range from 0(bare PMMA) to 0.5 g/L and then increased to 34.9 mJ/m2

at 2 g/L concentration. In the range of the gelatinconcentration from 2 to 100 g/L, the γS

LW values decreased

(17) Vaynberg, K. A.; Wagner, N. J.; Sharma, R.; Martic, P. J. ColloidInterface Sci. 1998, 205, 131-140.

(18) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley-Interscience: New York, 1990; Chapter 4.

(19) Della Volpe, C.; Siboni, S. J. Adhesion Sci. Technol. 2000, 14,235-272.

(20) Janczuk, B.; Chibowski, E.; Bruque, J. M.; Kerkeb, M. L.;Gonzalez-Caballero, F. J. Colloid Interface. Sci. 1993, 159, 421-427.

(21) Janczuk, B.; Białopiotrowicz, T.; Zdziennicka, A. J. ColloidInterface. Sci. 1999, 211, 96-105.

γS - ΠeL - γSL ) γL cos θ (1)

γ ) γLW + γAB (2)

γAB ) 2xγ+γ - (3)

γSL ) γS + γL - 2xγSLWγL

LW - 2xγS+γL

- - 2xγS-γL

+ (4)

γL(cos θ + 1) + ΠeL )

2xγSLWγL

LW + 2xγS+γL

- + 2xγS-γL

+ (5)

9464 Langmuir, Vol. 18, No. 24, 2002 Białopiotrowicz and Janczuk

from 34.9 to 31.73 mJ/m2. The minimal value of theLifshitz-van der Waals component corresponds to thatof the gelatinized film.

The shapes of curves 2, 4, and 5, representing changesof γS

+, γSAB, and γS as a function of the gelatin concentra-

tion in solution, are similar with the exception of thegelatin concentration range from 40 to 100 g/L. In thisrange the electron-acceptor parameter and acid-basecomponent were almost constant, but γS values continu-ously decreased. On all these curves maximum valueswere observed at 1 g/L gelatin concentration. At thisconcentration probably a monolayer film of gelatin isformed.9,10,12

In the case of the electron-acceptor parameter, its valuedecrease was observed in the gelatin concentration rangefrom 0 to 40 g/L (curve 3), and a further increase of thegelatin concentration used for film formation did notinfluence the γS

- values.The changes of the electron-acceptor and electron-

donor parameters of the gelatin film surface indicate thatgelatin molecules adsorbed on PMMA at a low concentra-tion assumed such conformations that the polar groupswere oriented toward the air side and the hydrophobicparts were oriented toward the PMMA surface. At aconcentration of gelatin higher than 1 g/L, the influence

of the polar groups on the surface free energy parametersdecreased because the γS

+ and γS- values decreased, and

in the gelatin concentration range from 40 to 100 g/L,they became minimal.

The values of γSLW, γS

+, γS-, γS

AB, and γS of the gelatin filmresult from the contribution of the structural groupspresent in the gelatin film to the components andparameters of its surface free energy. The contributionsof the structural groups depend not only on their surfacefree energy but also on the density of these groups on thegelatin film surface. Of course, the influence of the watermolecules adsorbed on the gelatin film on its surface freeenergy components and parameters cannot be excluded.

Gelatin is built of the same amino acids as collagen.22

The most important amino acids of collagen are glycine,alanine, valine, leucine, phenylalanine, serine, treonine,tyrosine, proline, hydroxyproline, asparghinic acid,glutaminic acid, arginnine, histydine, lysine, hydroxy-lysine, and methionine. Such amino acids as arginnineand hydroxylysine have two -NH2 groups; asparghinicand glutaminic acids have two -COOH groups. In thecase of hydroxyproline, arginnine, and histydine, anadditional -NH group is also present. The only aminoacid of collagen containing sulfur is methionine, havinga -SCH3 group. It results from the composition of collagenthat, apart from -CH3 and -CH2- groups, also -NH2,-NH, -SCH3, and -COOH groups could be present onthe gelatin film surface. Because in gelatin there arepeptide bonds, the presence of the -NHsCdO groupscannot be excluded on the gelatin film surface. In all, thesurface free energy of the gelatin film and its componentsand parameters should result mainly from the surfacefree energy of the above-mentioned groups and probablyfrom water molecules adsorbed and properly oriented onthe gelatin film. The values of the surface free energy andits components and parameters of some structural groupsavailable in the literature23,24 are given in Table 2. Fromthis table and Figure 3 it results that the minimal valueof the Lifshitz-van der Waals component of the gelatinfilm is even lower than the Lifshitz-van der Waalscomponent of the bare PMMA surface free energy (36.7mJ/m2)24 and higher than the γS

LW components of the-CH3, -CH2-, -NH2, and -COOH groups.23,24 However,it should be stated that the surface free energy of the CH3,-CH2-, and -COOH groups23,24 was determined fromthe contact angle measurements, and in the case of-COOH groups, the contribution of water molecules totheir surface free energy cannot be excluded. On the otherhand, the surface free energy of polyethylene13 (32-36

(22) Nenitescu, C. D. Organic chemistry vol. II; PWN: Warsaw, 1969;(polish translation) p 392.

(23) Zisman, W. A. Contact Angle, Wettability and Adhesion; ACSSymp. Ser. No. 43; American Chemical Society: Washington, DC, 1964;pp 1-22.

(24) Janczuk, B.; Mendez-Sierra, J. A.; Gonzalez-Martın, M. L.;Bruque, J. M.; Wojcik, W. J. Colloid Interface Sci. 1996, 184, 607-612.

Table 1. Values of the Lifshitz-van Der Waals (γLW) andAcid-Base (γAB) Components, and Electron-Acceptor

(γ+) and Electron-Donor (γ-) Parameters of theAcid-Base Component of the Surface Free Energy (γ) of

Liquids, in mJ/m2, Taken from the Literature16,21

liquid γLW γ+ γ- γAB γ

water 21.8 25.5 25.5 51 72.8glycerol 33.46 7.20 32.39 30.54 64formamide 31.84b 8.97b 19.07b 26.16b 58

39a 2.28a 39.6a 19a 58ethylene glycol 29.76 2.76 30.13 18.24 48diiodomethane 50.8 0.72 0 0 50.8R-bromonaphthalene 44.0 0.39 0.48 0.8 44.8dimethylsulfoxide 36 0.5 32 8 44tricresilphosphate 40.9 0 0 0 40.9bromoform 41.5 1.72 0 0 41.5

a Determined from contact angle measurements.16 b Determinedfrom liquid-liquid interfacial tension measurements.21

Figure 3. Relationship between the gelatin film surface freeenergy and its components (γ) calculated from eq 5 andconcentration of gelatin in the aqueous solution (C) from whichthe gelatin film was formed on the polymethyl methacrylateplate: curve 1, Lifshitz-van de Waals component (γS

LW); curve2, electron-acceptor parameter of the acid-base component(γS

+); curve 3, electron-donor parameter of the acid-basecomponent (γS

-); curve 4, acid-base component (γSAB); curve 5,

total surface free energy (γS).

Table 2. Values of the Lifshitz-van Der Waals (γLW) andAcid-Base (γAB) Components, and Electron-Acceptor

(γ+) and Electron-Donor (γ-) Parameters of theAcid-Base Component of the Surface Free Energy (γ) of

the Functional Groups Present on Gelatin and PMMASurfaces, in mJ/m2, Taken from the Literature21,23,24

group γLW γ+ γ- γAB γ

-CH3 22 22-CH2- 26 26-NH2 29.14 63.5 0.11 5.29 34.43-COOH 20.09 0.0 87.54 0.0 20.09PMMA 36.68 0.16 10.01 2.53 39.21


mJ/m2) suggests that for maximal tightly packed -CH2groups23 the surface free energy is higher than 26 mJ/m2.Taking into account the studies of Donnet25 et al. andCarre and Vial26 as well as the surface free energy ofPMMA,21 it is possible that the surface free energy ofnonhydrated -COOH groups is considerably higher andrather close to that of ester groups. Taking into accountthese values of surface free energy of -CH2 and COOHgroups, it can be stated that the minimal value of γS

LW forthe gelatin film is lower than their Lifshitz-van der Waalscomponents. This means that only water moleculesadsorbed on the gelatin film surface could have decreasedits Lifshitz-van der Waals components of the surface freeenergy because the Lifshitz-van der Waals component ofthe water surface free energy is only equal to 21.8 mJ/m2.In reality, changes of the surface properties of the gelatinfilm as a function of the concentration of gelatin in solutionfrom which the film was formed probably depend onadsorption of water molecules, the surface free energy ofthe functional groups, and their average molecular areas26

in the gelatin film.To explain the influence of the adsorbed water molecules

on the gelatin film properties, back-calculations of thecontact angle from eq 5 for water, formamide, R-bromo-naphthalene, tricresyl phosphate, dimethylsulfoxide, andbromoform were made.

For this purpose the values of γSLW, γS

+, γS-, γS

AB, and γSdetermined from contact angles for glycerol, ethyleneglycol, and diiodomethane and the literature data of γL

LW,γL

+, γL-, γL

AB, and γL16,19 were used. For these calculations

it was assumed that ΠeL was equal to zero.16

The contact angle values calculated in this way for W,F, B, T, DM, and BR are shown in Figure 4 (solid lines)together with the experimental ones (marker points).

From Figure 4 it results that for F, B, T, and BR thereis a good agreement between the calculated and measuredcontact angle values. In the case of formamide, a good

agreement was obtained if for contact angle calculationsfrom eq 5 the γL

LW, γL+, and γL

- values determined from thecontact angle measurements for formamide on the solidsincluding those having a hydrated surface16 (Table 1) wereused. This agreement indicates that the values of thecomponents and parameters of the surface free energy ofthe gelatin film calculated from the contact angles ofglycerol, ethylene glycol, and diiodomethane are reason-able or, alternatively, that the same factors influence theequilibrium state of the gelatin film-liquid drop-airsystem involving these liquids.

Unfortunately, there is a weaker agreement betweenthe contact angle of DM calculated from eq 5 and thatmeasured, in comparison to the case for other organicliquids. As regards water, the calculated values are higherthan those measured in the gelatin concentration rangefrom 0 to 40 g/L, and they are lower in the range from 40to 100 g/L. There is a considerable difference between themeasured and calculated contact angles depending on themethod of gelatin film formation (i.e. adsorption orgelatinization).

To show more clearly the difference between the contactangles calculated from eq 5 and determined ones, therelationship between these contact angles is presented inFigure 5. The straight line in this figure corresponds tothe calculated contact angles being equal to the measuredones. From Figure 5 it results that the marker points forall organic liquids, with the exception of DM, are layingalmost on the line. This means that there are very smalldifferences between the calculated and measured contactangles. For DM in the range of low gelatin concentrations,some differences between these contact angles are ob-served. In the case of water there is practically nocorrelation between the calculated and measured contactangles.

Such a big difference between the measured andcalculated values of the contact angle for water suggeststhat the components and parameters of the gelatin filmsurface free energy determined from the contact angle ofglycerol, ethylene glycol, and diiodomethane result not

(25) Donnet, J. B.; Qin, R. Y.; Wang, M. J. J. Colloid Interface Sci.1992, 153, 572-577.

(26) Carre, A.; Vial, J. J. Colloid Interface Sci. 1993, 160, 491-492.

Figure 4. Relationship between the contact angle (θ) deter-mined experimentally (marker points) and calculated from eq5 (solid lines) for R-bromonaphthalene (points 1 and curve 1′,B), tricresyl phosphate (points 2 and curve 2′, T), dimethyl-sulfoxide (points 3 and curve 3′, DM), bromoform (points 4 andcurve 4′, BR), formamide (points 5 and curves 5′ and 5′′, F), andwater (points 6 and curve 6′, W) and concentration of gelatinin the aqueous solution (C) from which the gelatin film wasformed on the polymethyl methacrylate plate. Note: Curves 5′and 5′′ were calculated on the basis of different values of theformamide surface tension components and parameters (seeTable 1).

Figure 5. Relationship between the contact angle calculatedfrom eq 5 (θc) and that determined experimentally (θd) forR-bromonaphthalene (marker points B), tricresyl phosphate(marker points T), dimethylsulfoxide (marker points DM),bromoform (marker points BR), formamide (points F), and water(points W). The solid line represents the case when thecalculated contact angle is equal the determined one for eachvalue of gelatin concentration in aqueous solution from whichthe gelatin film was formed on polymethyl methacrylate plate.Note: The angles for formamide calculated only on the basisof its components and parameters of surface tension determinedby van Oss16 et al. are shown.


only from the functional groups present in gelatin butalso from hydration of the gelatin film. Glycerol, ethyleneglycol, formamide, diiodomethane, R-bromonaphthalene,tricresyl phosphate, bromoform, and, partially, dimeth-ylsulfoxide do not change the properties of the hydratedgelatin film during their contact with this film. In thecase of water there are some changes of the gelatin filmsurface free energy components and parameters as a resultof water drop contact with the gelatin film surface.

This statement is confirmed by the contact anglescalculated from eq 5 for CH3, -CH2-, -NH2, and COOHfunctional groups (Table 3). For calculations the compo-nents and parameters for these groups determined fromcontact angle measurements were used. It should beemphasized that these values can be different from “real”values; however, they are useful for comparison of thecontact angle data measured in the systems involving-CH3, -CH2-, -NH2, and -COOH functional groups.Comparing the measured maximal values with calculatedones (Figures 1 and 2; Table 3) it is seen that, except forwater, the measured contact angles are lower than thosecalculated for hydrophobic groups. For water the maximalvalue of the measured contact angle on the gelatin filmis almost 20° higher than that calculated for the -CH3group. In the studied system it is impossible that functionalgroups exist for which the surface free energy results onlyfrom Lifshitz-van der Waals forces for which the contactangle would be lower than the value for the -CH3 group(e.g. CF2 groups). Thus, only specific gelatin film hydrationand its changes during water drop settling on the gelatinfilm surface could explain high values of the contact anglefor water.

In Figure 6 a scheme of the gelatin film-water drop-air equilibrium is presented. It shows different possibilitiesof water film influence on the water contact angle values.Diagram a presents the hydrated gelatin film-waterdrop-air equilibrium when a water drop does not changethe film surface free energy components. In diagram b anadditional water film is present behind a water drop settledon a hydrated gelatin film. Diagram c deals with thehydrated gelatin film-water drop-air equilibrium inwhich the interfacial free energy of the hydrated gelatinfilm-water is lower than that calculated from eq 4, whichis caused by a specific orientation of water molecules fromunder the water drop. In diagram d it is assumed thatgelatin molecules are oriented in such a way that the -CH3and -CH2 groups are only directed toward air and thata water film is present both behind and under the waterdrop.

The equilibrium state of three-phase systems in thesefour cases can be expressed by the Young equation in thefollowing forms, respectively

where S and W refer to a solid and water, respectively,and γSF(W) is the solid/water film surface free energy andγSW(f) is the solid/water film-water interfacial free energy.

According to Fowkes27 and van Oss et al.,16 the surfacefree energy of a solid in the presence of a water film canbe determined from the following relationship:

Taking into account eqs 4 and 7, it is possible to calculaterespectively from eqs 6a, 6b, 6c, and 6d the contact angleof water for four cases of the equilibrium state shown inFigure 6.

The contact angle values for water calculated in thisway for cases a, b, and c are presented in Figure 7 (curves2, 3, and 4, respectively) together with those measured(curve 1).

For case a (Figure 7, curve 2), as mentioned above(Figures 4 and 5), the values of the calculated contactangledonotagreewith thosemeasured.Thecontactanglesdetermined for case b (Figure 7, curve 3) are considerablyhigher than those measured in the gelatin concentrationrange from 0 to 40 g/L, whereas those at concentrationshigher than 40 g/L are somewhat lower than thosemeasured.

Case c deals with the hydrated gelatin film-waterdrop-air equilibrium in which the interfacial free energyof the hydrated gelatin film-water is lower than thatcalculated from eq 4, which is caused by a specificorientation of water molecules from under the water drop.In calculations it was assumed that the minimal value ofthis interfacial tension equals zero. In the studied gelatinconcentration range the calculated values of the contactangle for water (Figure 4, curve 4) were lower than thosemeasured. Thus, the measured values of the contact angleat each gelatin concentration in the range from 0 to 40 g/Lwere between the values calculated from models b and c.This means that the water film behind and under thewater drop influences the contact angle values, but on thebasis of the presented models, the contact angles at gelatinconcentration higher than 40 g/L cannot be completelyexplained.

It is possible that the measured contact angle corre-sponding to the gelatin concentration range from 40 to100 g/L fulfills the conditions given by the fourth model(d). In such a case the contact angle value is determinedby the free energy of paraffin in the presence of a waterfilm and the paraffin-water interfacial free energy insteadof the surface free energy of the hydrated gelatin film inthe presence of a water film and the hydrated gelatinfilm-water interfacial free energy.

Taking into account the density of -CH3 and -CH2-groups, it was assumed that the average value of thesurface free energy (Table 2) was close to the surface freeenergy of paraffin13 (25.5 mJ/m2). The paraffin surfacefree energy in the presence of a water film can be calculatedfrom eq 6, and as a first approximation, the paraffin-water interfacial free energy (including water film) canbe assumed as equal to 51 mJ/m.2 13

The contact angle values calculated in this way cor-respond to the gelatin concentration range from 45 to 100g/L and are in the range from 127 to 128°, being very closeto those measured.

The calculations presented above suggest that at lowgelatin concentrations its hydrated film on a PMMAsurface can be formed in which polar and nonpolar groups

(27) Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40-53.

Table 3. Values of the Contact Angle Calculated fromEq 5

group θW θG θF θE θD θB θDS θT θBR

-CH3 113.5 98.8 95 86.2 71.6 67.1 73.8 62.2 62.9-CH2- 110.2 94.5 90.4 80.8 64.5 59.3 67 53.5 54.3-NH2 32.4 0 0 0 58.3 31.3 0 46.5 45.8-COOH 29.2 53.5 33 48.2 55.2 54 58.5 66.3 10.6PMMA 74.25 64.45 55.56 46.58 36.37 26.6 31.1 26.6 0

γS - γSW ) γW cos θW (6a)

γSF(W) - γSW ) γW cos θW (6b)

γS - γSW(f) ) γW cos θW (6c)

γSF(W) - γSW(f) ) γW cos θW (6d)

γSF(w) ) γS - xγSLWγW

LW - xγS+γW

- - xγS-γW

+ (7)


are directed toward the air side. In the hydrated gelatinfilm at a higher gelatin concentration the apolar groupsare only oriented toward air as a result of reorientationof gelatin molecules. The water molecules are probablyoriented in the same way as near the hydrophobic part ofa surfactant in solution.28

We think that such high values of the contact angle areconnected not only with the existence of water but alsowith a specific structure of its films at the gelatin gelsurface. This phenomenon must be connected with a high

density of gelatin molecules at the gelatin gel-airinterface. This high density of gelatin molecules mustcause a specific arrangement of water molecules. Theremust be a strong repulsion between water molecules ina film and in a drop. It is impossible to obtain such highvalues of the contact angle only by the action of attractiveLifshitz-van der Waals forces.

ConclusionsOn the basis of the obtained contact angle data, it is

suggested that the adsorptive gelatin film structuredepends on the concentration of aqueous gelatine solutionfrom which adsorption proceeds.

The values of the gelatin film surface free energycomponents and parameters calculated from the contactangles of glycerol, ethylene glycol, and diiodomethaneallow us to predict the contact angles for other studiedorganic liquids.

The surface free energy components and parameters ofgelatin films determined from the contact angles fororganic liquids do not explain the very high contact anglesfor water.

For an explanation of the very high values of the watercontact angle, it was assumed that the surface free energyof the gelatin film results not only from the existence ofgelatin functional groups on the surface but also fromspecific hydration of the gelatin film.

Calculations of the contact angle for water on a gelatinfilm indicate that the degree of its hydration in the three-phase system gelatin film-liquid drop-air is different inthe presence of an organic liquid than in water.

LA0201624

(28) Rosen, J. M. Surfactants and Interfacial Phenomena; Wiley-Interscience: New York, 1989.

Figure 6. Four cases of the equilibrium state of the gelatin film-water drop-air system: case a, water drop does not change thehydrated gelatin film surface free energy components (under and behind the water drop no additional water film is present); caseb, an additional water film is present behind a water drop settled on a hydrated gelatin film; case c, an additional water film ispresent under a water drop settled on a hydrated gelatin film; case d, an additional water film is present under and behind a waterdrop settled on a hydrated gelatin film.

Figure 7. Relationship between the contact angle (θ) for waterdetermined experimentally and calculated on the basis of eqs6a, 6b, and 6c, respectively, and the concentration of gelatin inthe aqueous solution (C) from which the gelatin film was formedon the polymethyl methacrylate plate: curve 1, measured values(Wd); curve 2, calculated values for case a in Figure 6; curve 3,calculated values for case b in Figure 6: curve 4, calculatedvalues for case b in Figure 6.


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Surface Properties of Gelatin Films