The interaction of sodium dodecyl sulfate with gelatin

The interaction of sodium dodecyl sulfate with gelatin

The Interaction of Sodium Dodecyl Sulfate with Gelatin W I L L I A M J. KNOX, JR. AND T E R E N C E O. PARSHALL Research Laboratories, Eastman Kodak C...

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The Interaction of Sodium Dodecyl Sulfate with Gelatin W I L L I A M J. KNOX, JR. AND T E R E N C E O. PARSHALL Research Laboratories, Eastman Kodak Company, Rochester, New York 14650

Received August 7, 1969 The interaction of sodium dodecyl sulfate with gelatin at pH 7 has been investigated by measuring the surface tension of aqueous SDS/gelatin mixtures. Comparison of the resulting data with comparable precipitation data obtained at pH 4.1 indicates that the compositions of the complexes formed above and below the isoelectric point are quite similar. INTRODUCTION

reflected in the surface tensions of aqueous SDS/gelatin mixtures at pH values above the isoelectric point. Such measurments not only should be useful in detecting complex formation but also should give some indication as to the composition of the complexes. It was of interest for us to explore this possibility for SDS in aqueous solutions of lime-processed gelatin at pH 7.

The formation of complexes between anionic surfactants in aqueous protein solutions has been well established (1-6). In an earlier communication (6) from these Laboratories, precipitation studies involving the interaction of sodium dodeeyl sulfate (SDS) with lime-processed gelatin at a pI-I value (4.1) below the isoelectrie point were described. It was determined that the SDS/ gelatin interaction was characterized by four distinct regions of concentration, each of which involved complexes of different SDS content. The relationship of these results, however, to the SDS/gelatin interaction above the isoelectric point was not clear. Kragh (7) has studied the effect of gelatin on the dynamic and static surface tension of Aerosol OT (sodium di(iso-octyl) succin1-sulfonate) at a pH (7) above the isoelectrie point. On the basis of the dynamic surface tension measurements in 1.0% gelatin, he concluded that "gelatin exhibited only the typical effects of a monovalent electrolyte." The static surface tension measurements were interpreted as indicating the absence of complex formation with the implication that other anionic surfactants might not, except at very high concentrations, form complexes with gelatin at pH 7. It seems reasonable to expect the formation of gelatin complexes, as the result of the interaction between lime-processed gelatin and certain anionic surfactants, to be

EXPERIMENTAL PROCEDURE Materials. The gelatin used was a highgrade lime-processed bone gelatin which had been deashed and then deionized by two mixed resin treatments. Its ash content was found to be 0.01% and its water content, as determined by heating to constant weight at 103°C, was found to be 7.82%. Its isoelectric point, as determined by mixed bed ion exchange, was 4.8. The concentrations indicated in this communication refer to the dry gelatin. The SDS used was obtained from Distillation Products Industries and was purified by recrystallization from methyl alcohol. Its purity was indicated by the absence of any minimum in the ~/ vs. log[SDS] curve. The Aerosol OT (AOT) employed in comparison measurements was obtained from E. I. DuPont de Nemours and Company, and was used as received. Its water content was found to be 4 %. The inorganic agents used were reagent grade. The surface tensions were measured by a Wilhelmy balance technique with a Dognon-

Journal of Colloid and Interface Science, ¥ol. 33, No. 1, May 1970

16

INTEI~ACTION OF SODIUM DODECYL SULFATE WITH GELATIN Abribat balance improved by Padday (8). The surfaetant/gelatin solutions in distilled water were adjusted to pH 7 with sodium hydroxide. All measurements were made after the surface had reached equilibrium. This was usually achieved in about 60 see. Companion precipitation experiments involving the same SDS and deionized gelatin in appropriate mixtures were carried out at 35°C and pH 4.1. The procedure followed has been outlined in an earlier communication (6). The supernatant solutions obtained from three or more independent experiments were analyzed for both gelatin and SDS. These results were used to determine the composition of the precipitated compIexes. DISCUSSION OF I~ESULTS There are several complicating factors associated with the system under investigation. Gelatin is a polyelectrolyte, the molecules of which in aqueous solution are characterized by a distribution of positive and negative charges. It should be emphasized, however, that gelatin carries a net negative charge at pH 7 and exists in equilibrium with sodium cations. These cation may act as gegen ions to reduce the critical mieelle concentration of free SDS and also to reduce the potential barrier at the air/solution interface resulting from the adsorption of surface-active anions, thereby increasing the extent of the adsorption and the lowering of the surface tension. Because of the small amount of sodium hydroxide required to bring the pH of the SDS/gelatin solutions to 7, both these effects were found to be small. The effects which might be attributable to the anionic and cationic charges on the gelatin are complicated by the random-coil configuration of the gelatin molecules with the constrained positions of these charges. This should modify the influence of any cationic charges associated with the gelatin or its possible complexes on the adsorption of dodecyl sulfate ions at the air/solution interface. Furthermore, since the cationic sites on the gelatin, which might be occupied by surfactant, cannot distribute themselves as if they were separate entities,

17

the extent to which close packing can be achieved in the air/solution interface by dodeeyl sulfate groups on any existing SDS/gelatin complexes will be seriously limited. In addition, the interpretation of the surface tension data for SDS/gelatin solutions involves the assumption that some of the complexes which might be formed will be surface active and may exist in equilibrium with free SDS. This situation entails competition for the surface by a variety of complexes varying both in composition and in surface activity and by free SDS. The interpretation of the surface tension data obtained in this investigation is based upon these considerations. The Surface Tension of SDS in 0.5% Gelatin Solutions. The data illustrating the effect of 0.5 % gelatin on the surface tension of SDS in distilled water and in 0.2 M sodium chloride solutions and comparable data for AOT are summarized in Figs. 1 and 2. Comparison of these data indicates clearly that the effect of gelatin on the

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FIG. 1. The effect of gelatin on the surface tension of SDS in distilled water and 0.2 M sodium chloride: • SDS in distilled water; O SDS in 0.5% gelatin solution; • SDS in 0.2 M sodium chloride; £x SDS in 0.5% gelatin solution containing sodium chloride at 0.2 M. Journal of Colloid and Interface Science, Vol. 33, No. 1, M a y 1970

18

KNOX AND PARSHALL

77

b-

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Log [AoT] FZG. 2. T h e effect of g e l a t i n on t h e s u r f a c e t e n s i o n of AOT in distilled water and 0.2 M sodium

ehioride: • AOT in distilled water; O AOT in 0.5% gelatin solution; A AOT in 0.2 M sodium chloride; A AOT in 0.5% gelatin solution containing sodium chloride at 0.2 M. surface tensions of aqueous SDS solutions in both the presence and absence of sodium

chloride is significantly different from that involving comparable AOT solutions. These variations warrant a different interpretation for SDS solutions from that proposed by Kragh (7) for AOT. For 0.5 % gelatin solutions, the slope of the v vs. log[SDS] curve undergoes a number of significant changes over the entire surfaetant concentration range investigated. This is in marked contrast to the comparable curves obtained for AOT. In the 0.2 M sodium chloride containing 0.5% gelatin, SDS does not behave as if gelatin were absent, whereas AOT does. Although the data for AOT could be interpreted as indieating the absence of complex formation, the data for SDS clearly indicate the formation of complexes. Nonionie surfactants do not interact with gelatin to form complexes (9). The surface tension data (Fig. 3) obtained for Triton X100 (a nonionie surfaetant described by the manufacturer as an octylJournal of Colloid and Interface Science, Vo]. 33, No. 1, May 1970

phenoxypolyethoxy alcohol) in 1.0 % gelatin solutions show that the presence of gelatin has a negligible effect on the surface tension. Comparison of these results with those for comparable SDS solutions also suggests SDS/gelatin interaction over the entire range of SDS concentrations studied. Additional evidence for SDS/gelatin interaetion resides in the pH change which accompanies the addition of increasing amounts of SDS to aqueous gelatin. In the absence of pH adjustment, the pH of the SDS/gelatin solution rises with increasing surfactant concentration (Fig. 4). The SDS/protein interaction is indicated by this rise in pII (10) and has been attributed to the uptake of protons by the protein as a result of complex formation (11). The surfaee tension data for SDS in distilled water, in aqueous solutions of sodium chloride at various concentrations, and in 0.5% gelatin solutions (Fig. 5), unlike those for AOT, reveal that gelatin and monovalent electrolytes do not have similar effects on surface tension. T h e 1' vs. log[SDS] curve for 0.5 % gelatin shows,, three distinct regions of behavior. F o r 7c 60

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s i o n of T r i t o n X100 a n d S D S : • S D S i n d i s t i l l e d w a t e r ; O S D S i n 1.0% g e l a t i n ; A T r i t o n X100;;

Triton X100 in 1.0% gelatin.

INTEI~ACTION 85[

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concentrations below 1.0 X 10 -3 M, the curve shows a rapid decline in surface tension to a value of 43.5 dynes/cm. Between 1.0 X 10 -'~ and 5 X 10 .3 M, the surface tension remains essentially constant. Beyond 5.0 X 10 -3 M, the curve shows another rapid decline to a surface tension value approaching that of SDS in distilled water. A somewhat similar -/ vs. In [SDS] curve obtained by Jones (12) in his investigation of the interaction between SDS and polyethylene oxide (PEO) has been interpreted as indicating S D S / P E O complex formation. Concentration Region below 1.0 X 10 -3 M.

The total number of basic groups per gram of gelatin according to Chibnall (13) is equivalent to 0.95 millimole. If we assume that all these groups are ionized at p H 7 and take into account the amount of sodium hydroxide required to adjust the p i t to 7, the cationic concentration in 0.5 % gelatin approximates 0.006 M. Gelatin lowers the surface tension of SDS to a degree far in excess of that expected in a solution 0.006 M in monovalent ions despite the fact that gelatin is weakly surface active. Furthermore, it is reasonable to expect gelatin at 0.5 % to dominate the surface in the absence of complex formation at low SDS concentrations. The surface tension of the 0.5% gelatin solution was found to be 62.5 dynes/ cm. If one assumes that there is no interaction between gelatin and SDS at con-

SULFATE

WITH

GELATIN

19

centrations in the range 3.16 X 10 -a to 10-a M, it might be reasonable to expect the surface tension lowerings due to gelatin and SDS to be approximately additive. However, the observed lowering (21.6 dynes/era at 3.16 X 10 -4 M SDS) is significantly greater than the sum (14.4 dynes/ cm) of the surface tension lowerings due to 0.5% gelatin and SDS at 3.16 X 10 -4 M in 0.006 M sodium chloride. SDS in 0.006 M sodium chloride is used in this comparison in order to account for any lowerings which might be attributable to the presence of cations. The failure of the surface tension lowerings for SDS in 0.5 % gelatin solutions to resemble those for SDS in distilled water and in aqueous solutions of sodium chloride at various concentrations, coupled with the pronounced lowering of the surface tension by the presence of 0.5% gelatin, suggests the presence of SDS/gelatin complexes that are more surface active than SDS. The postulated presence of SDS/gelatin complexes in this region is consistent with the observations of Isemura and his co-workers (5), who have reported equilibrium dialysis

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FIG. 5. Comparison of the effect of gelatin and sodium chloride on the surface tension of SDS: • SDS in distilled water; O SDS in 0.006 M sodium chloride; [] SDS in 0.012 M sodium chloride; [] SDS in 0.5% gelatin; • SDS in 0.04 N

sodium chloride; A SDS in 0.2 N sodium chloride. Jour~zal of Colloid and Interface Science, VoI. 33, No. 1, ~ a y

i970

20

KNOX AND PARSHALL

studies which indicate that gelatin, even in the alkaline side (pH 8) of the isoelectric point, has a strong affinity for long-chain alkyl sulfates at low concentrations. Concentration Region between 1 and 5 X 10-s M. In this region, the ~, vs. log[SDS] curve exhibits a constant value of 43.5 dynes/cm for the surface tension. This is interpreted as indicating the presence of complexes having higher surfactant composition than those formed at lower SDS concentrations. The constant value is thought to reflect the influence of the random-coil configuration of the gelatin in preventing close packing in the surface of dodecyl groups attached to the cationic sites on the gelatin. Concentrations beyond 5 X 10 -3 M. As the concentration of SDS is increased beyond 5 X 10-s M, the ~/vs. log[SDS] curve falls off rather sharply and the surface tension approaches that of a pure SDS solution. It is conceivable that in this region a second layer is bound to the existing gelatin complex with the polar groups oriented toward the water (14). This should endow the resulting complex with increased hydrophilicity (15) and low surface activity, thereby enabling the free SDS in equilibrium with the complex ultimately to dominate the surface. The Composition of the SDS/Gelatin Complexes in 0.5% Gelatin Solutions. The changes in slope of the ~/vs. log [SDS] curve in Fig. 5 have been interpreted as indicating SDS/gelatin complex formation. These changes in slope are reflected in the 7 vs. molar SDS concentration curve presented in Fig. 6. Comparison of this curve with the companion curve for the fraction of gelatin precipitated at pI-I 4.1 as a function of the molar SDS concentration suggests that at the indicated pIl values above and below the isoelectric point for lime-processed gelatin, the compositions of the complexes formed and probably the mechanisms of their formation are quite similar. The gelatin precipitation data (Curve I, Fig. 6) show that the SDS/gelatin interaction at pH 4.1 is characterized by four distinct concentration regions. The composition of the complexes as determined Journal of Colloid and Interface Science, Vol. 33, No. 1, May 1970

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F~G. 6. Comparison of the precipitation data at pH 4.1 and surface tension data at pit 7 for SDS in 0.5% gelatin: Curve I--Fraction of gelatin precipitated vs. SDS concentration; Curve II-Surface tension of SDS gelatin solutions vs. SDS concentration. from the analysis of the supernatant solution for both gelatin and SDS is summarized in Table I. The SDS/gelatin ratios obtained with deionized gelatin are generally in good agreement with those reported earlier (6) for untreated gelatin (ash content 2.2%) except at those SDS concentrations where the determination of the composition of the complex involves small differences between the surfactant and gelatin concentrations originally present and those found in the supernatant solution. The significance of these regions has already been described (6). In the region A B (0.0 to 1.0 X 10.3 M), soluble complexes of low SDS content are believed to exist. This possibility is supported by the results of equilibrium dialysis studies (5) involving gelatin and dodecyl benzene sulfonate at low pH values and low surfactant concentrations. Gelatin precipitation begins and achieves its maximum in region BC (1 X 10.3 to 3 × 10.3 M). The SDS/ gelatin complex presented in this region has an essentially constant composition (0.48 millimole/gram of gelatin). This is believed

INTERACTION OF SODIUM DODECYL SULFATE WITH GELATIN

21

TABLE I S D S / G E L A T I N RATIOS IN PRECIPITATES ~ND IN THE SUPERN3-TANT SOLUTIONS FROM AQUEOUS M I X T U R E S C O N T A I N I N G VARIOUS AMOUNTS OF S D S IN 0 . 5 % GELATIN SOLUTIONS AT P H 4.1 X : : ;_:

MiI!imoIes Fraction of X 10-8 SDS SD rna- SDS in precip- gelatin in itate from 1 in mixture rant solu- liter of mixture suDernatant solution tion

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0

. . 0.76 0.67 0.52 0.35 0.31 O.42 O.50 0.59 O.87 1.55 2.1 4.0 5.1 6.4 8.0

. .

. . O.24 O.83 1.48 2.15 2.69 3.08 3.50 3.91 4.13 3.95 3.9 2.5 1.9 1.1 --

. . 0.89 0.65 0.40 O. 15 0.07 O.03 0.03 0.03 0.03 0.09 0.21 0.40 0.59 0.78 1.00

Fraction of gelatin in precipitate

. . O. 11 0.35 0.60 0.85 0.93 O.97 0.97 0.97 0.97 0.91 0.79 0.60 0.41 0.22 --

to result from an Ml-or-none mechanism in which SDS interacts only with t h a t fraction of gelatin necessary for the formation of an insoluble complex in which about one-half of the available sites on the gelatin are involved. I n region CD (3 X 10 .3 to 5 N 10 .3 M), m a x i m u m gelatin precipitation is maintained as the composition of the complex increases continuously from 0.48 to 0.87 millimole/gram of gelatin. I t is in this concentration range t h a t essentially all the cationic sites on the gelatin become surfaetant-occupied. Region DE (5 X 10.3 to 8 X 10.3 M) has been described as the region of solubilization. P a n k h u r s t (3) has postulated t h a t the solubilization of insoluble SDS/gelatin complexes in excess SDS is achieved b y the physical adsorption of SDS on insoluble SDS/gelatin complexes, with the physically adsorbed SDS oriented toward the water, thereby endowing the insoluble complex with solubility. These solubilized complexes appear to exist in equilibrium with free SDS. T h e ~/vs. SDS concentration curve (Curve I I , Fig. 6) for 0.5 % gelatin solutions reveals concentration regions which suggest t h a t at p H 7 the compositions of the indicated

Grams of gelatin in precipitate

. .

Millimolesof SDS/gram geIatin in precipitate

Grams of gelatin in supernatant solution

. . 0.55 1.75 3.00 4.25 4.65 4.85 4.85 4.85 4.85 4.55 3.95 3.00 2.05 1.10 --

0.44 0.47 0.49 0.51 0.58 O.64 O.72 0.81 0.85 0.87 0.99 0.83 0.93 1.00 --

4.45 3.25 2.00 0.75 0.35 O. 15 O.15 0.15 O. 15 0.45 1.05 2.00 2.95 3.90 5.00

Millimoles of SDS/gram gelatin inof supernatant solution

0.0 0.1 O. 17 0.20 0.26 0.47 0.88 2.78 3.33 3.93 5.80 3.44 2.00 2.00 1.72 1.64 1.60

complexes are similar to those formed at p H 4.1. The region A B corresponds to the concentration range in which the pronounced lowering of the surface tension has been attributed to the presence of SDS/gelatin complexes of low SDS content. Beyond the region AB the surface tension drops rapidly to a value of 43.5 d y n e s / c m and remains

essentially constant throughout the regions BC and CD assigned to the precipitation curve. The correspondence of the concentration range over which the constancy of the surface tension persists with that in which precipitation is initiated and maxim u m precipitation is maintained at p H 4.1 suggests similarity in the composition of the complexes formed above and below the isoelectric point. I t is conceivable t h a t the failure of the surface tension curve to distinguish between complexes involving only one-half of the cationic sites on the gelatin and those involving higher surfaetant content is attributable to the random-coil configuration of the gelatin. I n the region of solubilization (region DE) for the complexes precipitated at p H 4.1, the surface tension of comparable S D S / gelatin solutions at p H 7 gradually falls off Journal of Colloid and Interface Science, Vo]. 33, No. 1, ~Iay 1970

22

KNOX AND PARSItALL

and finally approaches that of pure SDS solutions. This behavior is consistent with the formation of complexes similar to those proposed for this region of solubilization in the precipitation studies. Coekbain (15) studied the interracial tension between benzene and aqueous mixtures of SDS and bovine serum albumin (BSA). He obtained ~ vs. SDS concentration curves quite similar to those presented here, and interpreted them as indicating SDS/ BSA complex formation. The Surface Tension of S D S in 1.0% Gelatin Solutions. The surface tension data for SDS in 1.0% gelatin solutions lend themselves to the same interpretation as that proposed for SDS in 0.5% gelatin solutions. Comparison of the 7 vs. log [SDS] curve for 1.0% gelatin (Fig. 3) with the corresponding curve for 0.5% gelatin reveals that, in addition to similarities in shape, the concentration range over which the constancy of surface tension prevails for 1.0 % gelatin solution is approximately twice that for 0.5 % gelatin solutions. The

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C

D

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precipitation data for the interaction of SDS with gelatin in 1.0 % solutions at pll 4.1 and the corresponding surface tension data are summarized in Fig. 7. The concentration regions which characterize the SDS/gelatin interaction in 1.0% gelatin solution at pH 4.1 (Curve I, Fig. 7) correspond closely with those indicated for the comparable interaction at pH 7. These results support the hypothesis presented earlier for the SDS/gelatin interaction in 0.5 % gelatin solutions. CONCLUSIONS The changes in surface tension associated with increasing concentrations of SDS in gelatin solutions are quite different from those observed by Kragh for AOT. Although Kragh's results suggest that gelatin behaves as a monovalent electrolyte in aqueous AOT, no such behavior in aqueous SDS has been observed. Surface tension measurements indicate that SDS/gelatin complexes are found at pH 7 throughout the whole range of concentrations studied (3 X i0 -~ to 2 N 10 -2 M). Comparison of surface tension vs. SDS molar concentration curves with comparable curves representing the fraction of gelatin precipitated at pH 4.1 as a function of the molar SDS concentration indicates similarity i11 complex compositions and probably reaction mechanisms at pH values above and below the isoeleetric point. REFERENCES 1. PUTNAM, F. W., AND NEURATI-I, I-I., J. Am.

Chem. Soe. 66, 692 (1944). o~

Curve

rr

°.2

A B

I

40

[

e.o M x IO s

12.0

E [6.0

2

055

SDS

FIG. 7. Comparison of the precipitation data at pll 4.1 and surface tension data at pH 7 for SDS in 1.0% gelatin: Curve I--Fraction of gelatin precipitated vs. SDS concentration; Curve II-Surface tension of SDS gelatin solutions vs. SDS concentration. Journal of Colloid and Interface Science, Vot. 33, No. 1, May 1970

2. "~ANG, J. T., ANn FOSTER, J. F., J. Am. Chem. Soc. 75, 5560 (1953). 3. PANKHU:aST, K. G. A., Surface Chem. Research (London) Suppl., !09 116 (1949). 4. ISEMURA, T., TOEIWA, F., AND IKEDA, S., Bull. Chem. Soc. Japan 28, 555 (1955). 5. ISEMURA, T., ToI(IwA, F., AND IKEDA, S.,

Bull. Chem. Soc. Japan 35, 240 (1962). 6. KNOX, W. J°, JR., ANDWRIGHT, J. F., J. Colloid Sei. 29, 177 (1965). 7. KniGHt, A. M., Trans. Faraday Soe. 60, 225

(1964). 8. PADDAY, J. F., Proe. Intern. Congr. Surface Activity 2nd London 1, 1 (1957).

INTERACTION OF SODIUM DODECYL SULFATE WITH GELATIN 9. LUNDGREN,H. P., Textile Res. J. 15,335 (1945). 10. SCATCHARD, G., AND BLACK, E. S., J. Phys. & Colloid Chem., 53, 88 (1949). 11. KLOTZ, I. M., Proteins B2, 740 (1954). 12. JONES, M. N., or. Colloid de Interface Sci. 23, 36 (1967).

'-)3

13. CHIBNALL, A. C., J . Soe. Leather Trades' Chemists 30, 1 (1946). 14. ISEMCRA, T., .~:','D IMANISHI, A., Mere. Inst. Sci. Ind. Res. Osaka Univ. 15, 173 (1958). 15. COCI~BAIN,E. G., Trans. Faraday Soc. 49, 104

(1953).

Journal of Colloid and Interface Science, Vol. 33, No. i, May 1970