J . Phys. Chem. 1992, 96, 871-876
871
Hydrodynamic and Thermodynamic Aspects of the SDS-EHEC-Water System Christina Holmberg, Stefan Nilsson, Satish K. Singb,t and Lars-Olof Sundeliif* Department of Physical and Inorganic Pharmaceutical Chemistry, Uppsala University, Uppsala Biomedical Center, P.O. Box 574, S-751 23 Uppsala. Sweden (Received: February 8, 1991, In Final Form: September 5, 1991)
The specific interaction between a strongly hydrophobic but still water-soluble cellulose derivative, ethyl(hydroxyethy1)cellulose (EHEC), and a low molecular weight anionic surfactant, sodium dodecyl sulfate (SDS), is studied in dilute solution without added neutral electrolyte by means of hydrodynamic (viscosity) and thermodynamic (dialysis equilibrium) measurements. The dialysis equilibrium shows strong adsorption of SDS at concentrations far below the normal cmc for SDS. The dialysis equilibrium is sensitive to the polymer concentration at higher SDS concentrations where the adsorption seems to decrease. In the region of increasing adsorption of SDS to EHEC the hydrodynamic measurements reveal a drastic reconformation of the polymer in dilute solution leading to a 4-fold reduction in hydrodynamic volume. At higher polymer concentrations the viscosity passes through a very marked maximum when the SDS concentration is increased. The results are interpreted in terms of a clustering theory for the SDS adsorption.
Introduction It is a well-known fact’-3 that certain additives, in particular charged surfactants, have a tendency to give a strong increase in the viscosity of water solutions in ethyl(hydroxyethy1)cellulose (EHEC). Similar effects have been observed for other polymers, but the temperature dependence of these effects for the EHEC (and similar polysaccharide) systems seems to be speciales in the sense that the solution viscosity passes through a maximum upon raising the temperature from ambient toward the so-called cloud point (CP). Furthermore, various additives (e.g., salts) have a marked and intricate influence on the system proper tie^.^-'^ So far, it has not been possible to describe the effect in terms of molecular properties. The present investigation is an attempt to provide the outlines of such a description. The basic process is the following. EHEC is an uncharged water-soluble macromolecule with a CP normally in the range 35-70 “C,the exact value depending upon the degree and type of substitution. Certain types and degrees of substitution give the polymer rather marked hydrophobic properties (CP = 40 “C), thus making water a less good solvent. This might lead to associative interaction between polymer chains even in the absence of additives. Upon addition of surfactant, e.g., sodium dodecyl sulfate (SDS),to the solution, adsorption of the surfactant molecules on the polymer chains occurs and the properties of the polymer molecules are altered. This affects in turn the interaction between polymer chains which become pronounced, in particular a t elevated concentrations. Evidently, the hydrodynamic effects observed are closely connected to the thermodynamics of the adsorption process. Hence, it could be anticipated that a combination of hydrodynamic (viscosity) and thermodynamic (equilibrium dialysis) measurements would provide some insight into the problem. (1) Jullander, I. Chim. Ind. (Paris) 1954, 71, 288. (2) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (3) Gcddard, E. D. Colloids Surf. 1986, 19, 255. (4) Jullander, I. Sven. Papperstidn. 1952, 55, 197. (5) Bass, S. L.; Barry, A. J.; Young, A. E. Cellulose Ethers. In Cellulose and Cellulose Derivatives; Ott, E., Ed.; Interscience: New York, 1943; p 758. (6) Manley, R. St. J. Ark. Kemi 1956, 9, 519. (7) Sonnerskog, S. Sven. Papperstidn. 1945, 48, 413. (8) Carlsson, A.; Karlstrijm, G.; Lindman, B. Colloids Surf. 1990,47, 147. (9) Heyman, E. Trans. Faraday Soc. 1935, 31, 846. (10) Heyman, E.; Bleakley, H. G.; Docking, A. R. J. Phys. Chem. 1938, 42, 353. (1 1) SBnnerskog, S. Some Ethers of Cellulose and Starch. Dissertation, Stockholms Hijgskola, Stockholm, 1952. (12) Jullander, I. Sven. Papperstidn. 1953, 56, 443. (13) Carlsson, A.; Karlstrom, G.; Lindman, B. Langmuir 1986, 2, 536. (14) Carlsson, A. Nonionic Cellulose Ethers: Interactions with Surfactants, Solubility and Other Aspects. Dissertation, Lund, 1989. (15) Kratky, 0.; Leopold, H.; Stabinger, H. In Methods in Enzymology; Hirs, C. H. W., Timasheff. S. N., Eds.; Academic Press: New York, 1973; Vol. 27.
The behavior of the system is determined by a combination of factors including CP (hydrophobicity and substituent structure) and polymer molecular weight, type, and concentration of surfactant, temperature, and presence/absence of salt. However, it is preferable to analyze these factors one at a time. The present investigation has therefore been limited to the SDS/EHEC/water system a t a given temperature (20 “C) with a given EHEC fraction (CST-103, CP in the range 28-37 “C) and in the absence of salt over a concentration range 0-1 5 m m SDS and 0-3 g/L EHEC.
”ry Association Equilibrium in Surfactant-Polymer Systems. A macromolecule in solution interacts with solvent and other low molecular weight components in a way that to a large degree is decisive for the physical properties of the macromolecule.16 The solvent itself can be ‘good” in the sense that the polymer molecules prefer the solvent molecules in their surrounding to their own segments, or it can be ‘poor” in the sense that it prefers polymer chains over the solvent molecules. Thus, the average physical dimensions of the coil structure will be affected and become extended in a “good” solvent and contracted in a “poor” one. Furthermore, preferential adsorption often takes place in a mixed solvent” which can lead to a considerable local difference in composition between the solvent in the bulk and in the region of a coil. This difference can affect the physical properties of the polymer considerably. The amphiphilic character of surfactants makes them interesting agents in polymer chemistry. For instance, the hydrophobic part of such a molecule will have a marked tendency to interact in an associative sense with hydrophobic polymers or rather with hydrophobic groups of water-soluble polymers and at the same time give the polymer somewhat new properties due to the hydrophilic ‘coating” introduced. Phenomena of this type have been extensively studied, and the literature on the subject is vast.18-20 In the present paper the interaction between an ionic surfactant and a nonionic polymer is studied. One of the most common features of such an interaction, so far discussed in the literature, is the existence of two critical concentrations of the surfactant, T1 and T2. T1 represents the concentration for the ‘onset” of interaction or binding, while T2 is the concentration at which the polymer becomes ‘saturated” with the surfa~tant.~JA third critical point T2’ has also been defined representing the apparent (16) Flory, P. J. Principles of Polymer Chemistry; Cornel1 University Press: Ithaca, NY, 1953. (17) Kratochvh, P.;SundelBf. L.-0.Acta Pharm. Suec. 1986, 23, 31. (18) Nagarajan, R. Colloids Surf. 1985, 13, 1. (19) GilHny, T.; Wolfram, E. Complex formation between ionic surfactants and polymers-in aqueous solution. In Microdomains in Polymer Solutions; Dubin, P., Ed.; Plenum Press: New York, 1985; p 383. (20) Cabane, B. J . Phys. Chem. 1977,81, 1639.
0022-365419212096-871%03.00/0 0 1992 American Chemical Society
872 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
Holmberg et al.
critical micelle concentration of the surfactant in the presence of the polymer, i.e., the point at which free micelles begin to form in the system.14 T1 has been found to be relatively independent of the polymer concentration while T2 is linearly dependent on it. The principal driving force for the binding between an ionic surfactant and a nonionic polymer is hydrophobic interaction and the process is quite similar to that of self-association of surfactant into micelle^.^ The interaction between a nonionic polymer and an ionic surfactant could take one or more of the following forms: (a) redistribution of the surfactant between the bulk solution and the coil regions as described above for a polymer in a mixed solvent; (b) surfactant molecules bound individually along the polymer chain; (c) surfactant molecules clustered around hydrophobic sites on the polymer; (d) polymer molecules wrapped around surfactant micelles in such a way that the polymer segments partially penetrate and wrap around the polar head group regions of the micelles. If there exists a critical concentration T1, as discussed above, this would rule out an interaction as envisaged in (b), at least as a single cause of observed effects. The interaction types (c) and (d) are fairly similar as far as the final effect of the interaction is concerned. However, there are conceptual differences in the initiation step between the two. The “site clustering” (c) implies a strongly cooperative surfactant-polymer interaction, starting well below the normal cmc and occurring over a range of surfactant concentrations. The “mixed micelle” approach (d) implies the formation of a micelle in a normal fashion, albeit at a bulk concentration lower than cmc and with an aggregation number lower but of the same order of magnitude as that of a normal micelle. The polymer is wrapped around this micelle, with its hydrophobic parts inserted between the surfactant molecules.2o The PEO/SDS/water system is welldescribed by this approach?J’ Other systems may well involve a combination of models (c) and (d), with (d) as the second step in the interaction process: Micelle-liie clusters are first formed on hydrophobic sites, and then other hydrophobic sites of the chain attach to these clusters. At higher polymer concentrations, hydrophobic parts of neighboring chains can compete for binding with these clusters. This will lead to formation of cross-links and, eventually, of three-dimensional structures accompanied by a considerable increase in viscosity. From equilibrium dialysis experiments, to be discussed in the section Results and Discussion, it appears as if the redistribution of surfactant to EHEC is not a clear-cut cooperative phenomenon. The results could, however, possibly be explained by the following general model based on a combination of the processes (a), (c), and (d) above, and possibly also (b). Suppose the initial step in the surfactant-polymer interaction consists in a redistribution of surfactant with preference for the polymer coil regions over the bulk solution. Consequently, the local surfactant concentration in the coil region might reach values of the order of normal cmc values long before the bulk concentration corresponds to cmc conditions. Thus, the formation of surfactant micelles at bulk concentration values lower than the normal cmc can be understood. Furthermore, this process should depend mainly on the type of polymer present and be insensitive to the polymer concentration. Once formed the micelles can interact with the polymer. For instance, the polymer chain might fold itself around the micelle, thereby anchoring its most hydrophobic group in the micellar phase. This would induce a strong reduction in the polymer hydrodynamic volume and at the same time reduce the total preferential redistribution of surfactant between the bulk and the coil regions. Thus,the cooperative part of the interaction between micelles and polymer is counterbalanced by a corresponding reduction in the preferential redistribution. The Hill equationz2has been normally used to describe polymersurfactant binding when accompanied by cluster formation?’ This expression, which predates Langmuir’s equation, can be considered a more general form of Langmuir’s adsorption equation
with provision for mperativity of degree n in a system with equilibrium surfactant concentration C. The degree of binding, 8, is, according to the Hill approach, given by the expression
(21) Shirahama, K. Colloid Polym. Sci. 1974, 252, 918. (22) Hill, A. V. J . Physiol. (London) 1910, 40, IV-VIII.
e = QC/(I
+ QC)
(1)
where Q is an empirical constant. This equation is adequate to describe the type of binding envisaged in (c) where clusters of size n attach to hydrophobic sites on the polymer molecule until saturation is achieved. However, an interaction visualized as a combination of (a) and (d) as described above would make an interpretation in terms of eq 1 less clear-cut. A more detailed analysis,will be presented in a forthcoming paper.23 Analysis of Equilibrium Dialysis Data. The difference in concentration of DS- ions between the two sides of the membrane corresponds to the amount of surfactant bound to the polymer under the assumption that the bound ion makes no contribution to the ionic activities. However, the “raw” difference must be corrected for the Donnan effectz4due to the charged polymer complex. Electrostatic balance at equilibrium within the two solutions in dialysis equilibrium denoted by the subscripts A (i.e., the side containing polymer) and B (i.e., the solvent side) requires C,Y
+ [DS-].
= [N~+]A
[DS-]g = [ N ~ + ] B
(2) (3)
where c = concentration of EHEC in solution A expressed as grams of substance per gram of solution, [DS-Ii = concentration of free DS- ions in solutions i = A, B expressed as millimoles per gram of solution, and [Na+Ii = concentration of Na+ ions in solutions i = A, B also expressed as millimoles per gram of solution. According to (2) y is defined as an experimental binding ratio in the sense that.fAy gives the excess amount of DS- ions with respect to the equhbrium free ion concentration in the A solution. From the ampodtion definitions above it follows that y is obtained as millimoles of DS- ions in exccss (“bound”) per gram of EHEC. A chemical potential balance at equilibrium (approximating activities with concentrations) leads to the condition [N~+]A D[s - 1 ~ [Na+IB[DS-lB
(4)
Combining eqs 2 4 gives [DS-] A = c ~/ CA ’
(5)
where CA = C , Y
+ [DS-],
(5’)
is the total concentration of DS- ions in solution A and CB = [DS-],. Both CAand CBare measured directly in the experiment through scintillation counting. According to these definitions y is calculated from Y = (CA2 - CB’)/(CACA)
(6)
Viscosity. The properties of the uncharged polymer EHEC are changed considerably when SDS is redistributed or bound to it. Charges are added which will render the molecule a polyelectrolytic character. Part of the hydrophobic groups on the macromolecularbackbone will be shielded by the added surfactant which will change the polymer-solvent interaction. The hydrodynamic volume is a sensitive parameter by which to monitor these chemical changes. Furthermore, the system in question is known to show interactions which depend on polymer concentration and temperature and sometimes lead to strong rheological effects and gel formation. z ~ l Viscosity measurement is a dependable and straightforward tool to study the hydrodynamic volume of polymers in so1ution,l6 the essential quantity to following being the reduced viscosity qsp/c. (23) SundelBf, L.-0.; Singh,S.K.; Holmberg, C.; Nilsson, S.Some theoretical remarks on the interaction surfactant/uncharged polymer in solution. Manuscript in preparation. (24) Donnan, F. G. 2.Elektrochem. 1911, 17, 1554. (25) Einstein, A. Ann. Phys. 1906, 19, 371.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 873
Thermodynamics of the SDS-EHEC-Water System Here c is the polymer concentration expressed as mass per unit volume, and qsp is defined by (7) llsp = (v - l l o ) / t o Here q and qo are the viscosities of solution and solvent, respectively. Extrapolating qSp/cto zero concentration gives the intrinsic viscosity [VI,which is a measure of the hydrodynamic volume per mass unit of the polymer (at infinite dilution). At finite concentrations the polymer-polymer interaction must be taken into account. The first-order effect is given by the following expression % P / ~ = 171 + kH[q12C +
(8)
e * *
where the higher order terms have been omitted. The Huggins’ constant kH is a measure of the intensity of the polymer-polymer interaction. The above theory is well-known’6J6and straightforward for most normal polymer solutions composed of single solvents. However, in the present case there is always an equilibrium between the free surfactant and the polymer/surfactant complex. Changing the total concentration of either the polymer or the surfactant will change the solution properties of the polymer, and this will be immediately reflected in viscosity changes. Hence, to give the above approach a clear meaning the quantities [ ] and kH must be determined for a polymer which is well-defined with respect to the amount of surfactant “adsorbed”. To this end viscosity measurements have been performed in sets of constant polymer concentration allowing the overall SDS concentration to vary. Various such sets have been measured with different polymer concentrations. From these results, data values of qSp/cfor constant amount of adsorbed SDS,i.e.,for constant y , can be constructed in the following way. The total concentration of SDS is according to (Sa) given by ISDSI,,, = CY
+ 1DS-1,
(9)
For a constant value of y and a given polymer concentration c, the value of [DS-1, is obtained from the dialysis equilibrium curves. [SDS],,,is calculated according to (9), and the corresponding value of llSp/ccan be read from the viscosity curve for the polymer concentration in question. Repeating the procedure for different polymer concentrations will give sets of reduced viscosity data which then can be utilized in the normal way according to eq 8 to give [q] and kH for constant y . A very extensive discussion of how to interpret viscosity data can be found in ref 26.
Materials The following chemicals were used: sodium dodecyl sulfate (SDS), 99.9% pure, used as supplied, Lot 733L488460 (Batch 1.8553-loo), Merck, Spinga, Sweden; radioactive SDS,35S,Lot JC 1981, Batch 8803, Amersham, England; ethyl(hydroxyethy1)cellulose (EHEC), fraction CST-103, MW, = 80000 (as given by manufacturer; more accurate molecular weight determination in progress), MSeo = 0.7, DSethyl = 1.5, L920 32, Berol Kemi, Stenungsund, Sweden. The stock solution of EHEC and the doubly distilled water used in the experiments were filtered through 0.8- and 0.45-pm Millex-AA filters (Millipore SA, Molsheim, France), respectively. The EHEC stock solution was freed from remaining salt by dialysis in tube membranes (MW cutoff lOO00) from Union Carbide, Chicago, IL. The equilibrium dialysis experimentswere carried out in a specially designed cell. The membrane used was CUROPAN flat membrane (molecular weight cutoff approximately 10Om), Part No. 933-0225-01, Technicon AB. Experiments Preparation of Sol~tion~.The stock solution of SDS was prepared by dissolving SDS in doubly distilled and filtered water ~
~~
-
(26) See e.g.: Bohdanecky, M.; Kovir, J. Viscosiiy of Polymer Soluiions; Elsevier: Amsterdam, 1982.
to a concentration of 0.1 m. Radioactive SDS was dissolved in water and diluted to provide the required activity for the experiments. A stock solution of EHEC was prepared by the following standard techniquea6 A weighed amount of EHEC was mixed with approximately 1/4 of the total amount of water, heated to 80 OC. This solution was shaken in a warm water bath for about 30 min. The rest of the water (cooled to 8 “C) was then added, and the mixture was stirred in an icebath for about 4 h in order to get a homogeneous solution. To obtain salt-free conditions for the experiments, the EHEC stock solution was carefully dialyzed against doubly distilled water (4 days). The solution was then filtered through Millex-AA filters (0.8 pm) to remove possible large aggregates or particles. Finally, the EHEC concentration was determinated by drying samples to constant weight at 105 OC. This method is suitable for determination of EHEC concentration shce EHEC loses its water of hydration and precipitates out of solution upon heating. The stock solution of EHEC was stored at 8 “C. Solutions for viscosity and equilibrium dialysis studies, were prepared by weighing the required amounts of the EHEC stock solution into appropriately diluted SDS stock solutions. The order of adding the components is of importance since it greatly affects the properties of the solutions. After mixing, large changes in the solution viscosity were observed over extended times (of the order of 10 h). To make sure that experimental data apply to an equilibrium situation, all samples for viscosity studies were prepared about 1 day in advance to let this timedependent process settle (cf. Figure 6). The samples for the viscosity time dependence studies were also prepared as described above. The moment when EHEC was added to the diluted SDS stock equals the starting time for these experiments. Dialysis samples were also prepared the day before the start of the experiments. Equilibrium Dialysis. Equilibrium dialysis experiments were carried out between SDS and EHEC-SDS solutions containing initially equal concentrations of SDS plus a small and equal amount of radioactive SDS. Approximately 25 000 dpm/mL of activity was employed in the experiments. The studies were carried out in a dialysis cell made up of two compartments divided by a membrane (MW cutoff approximately 10000). The cell was flushed with doubly distilled water at least 1 h before use. The water was removed, and the solutions for dialysis (EHEC-SDS solution and SDS solution) were slowly pumped into their respective cell compartments by a peristaltic pump. Care was taken to ensure that no air was trapped in the system. The volumes of the two compartments are identical and approximately 2.5 mL. A dialysis experiment was run for a minimum of 3 h. A peristaltic pump transported the solutions through the dialysis cell continuously while equilibrium was established. Preliminary experiments indicated that true equilibrium normally was reached within 2-2.5 h. Three aliquots were taken from each compartment before flushing the system with doubly distilled water and starting a new experiment. The concentrations of DS- ions in the EHEC-SDS and SDS solutions were determined by scintillation counting. Preliminary experiments indicated that the presence of EHEC did not cause an extinction of the scintillation. Viity Measurements. The viscometric measurements were performed in ordinary Ostwald capillary viscometers with a solvent (appropriate SDS solutions) flow time of approximately 100 s at 20 OC. The samples were thermostat4 in the viscometer in a water bath for 15 min before measurements were made. Corrections for kinetic and end effects were considered unnecessary. Density Dete”tiom, Densities were determined in a digital densitometer DMA 02C from Anton Paar K.G., A-8054 Gratz, Austria, according to Kratky et al.I5 The accuracy exceeds 0.0015 kg/m3.
Results and Discussion The system EHEC/SDS/water is known to exhibit strong interaction in certain composition intervals leading to enhanced viscosity. At elevated polymer concentrations and/or with addition of salt even gel formation is observed. However, at higher polymer
Holmberg et al.
874 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 e
3800
i ;I
I n
34001 L\
e
t *
E
*
: 5
*
3000
.
h
-0E
Y
2600 e e
2200 meem. 0
1800
I
5
I
10 15 Total SDS concentration (mmolal)
I
20
Figure 2. Results from dialysis experiments at 20 OC for the EHEC/ SDS/water system at a constant EHEC concentration c = 0.0010 g/mL. Ordinate values y (=mmoles of SDS bound per gram of EHEC) have been calculated according to eq 6 (see the Theory section) and plotted versus the total concentrationof DS-ions, corresponding to CA. From diagrams like this, values of CAfor fixed values of y can be found, and by combination with Figure 1 values of qsp/cfor constant values of y can be extracted as plotted in Figure 3.
1400
1000 600 200 0‘
0
I
1
1
I
2
4
6
8
I
I
1 0 1 2
Total SDS concentration (mmolal) Figure 1. Experimental results at 20 OC for qsp/c in the EHEC/SDS/ water system as a function of the total SDS concentration (corresponding to C, in the Theory section) for different EHEC concentrations c (g/
+,
c = 0.0005; 0, c = 0.0010 0 , c = 0.0015; 0,0.0020; U, c = 0.0025; V, c = 0.0029. mL):
e
concentrations the rheology becomes rather complex to interpret. To this end it was decided to begin the studies at low polymer concentrations in order to determine the properties of individual polymer molecules and first-order coil-coil interactions. It was felt that from such results conclusions could possibly be drawn concerning the effects at higher concentrations. In Figure 1 are shown the results from viscosity measurements on the EHEC/SDS/water system where for each set of measurements the polymer concentration has been kept constant while varying the total SDS concentration. From Figure 1 one may classify the SDS concentration axis into three parts. (I) At low SDS concentrations, the reduced viscosity is quite insensitive to the amounts of SDS present. (11) In a middle range of SDS concentrations, the reduced viscosity becomes very sensitive to the amount of SDS present. The curves for low polymer concentrations show an inflection or a shallow minimum whereas those for higher polymer concentrations exhibit a very marked maximum. (111) At still higher SDS concentrations the curves again become rather insensitive to changes in the SDS concentration. Region I1 coincides in SDS composition with the one where very stiff solutions (‘gels”) are formed at high polymer contents.14 Obviously, the polymer properties change along the curves in Figure 1 due to variation in amount of ‘bound” surfactant. The equilibrium dialysis experiments provide information about this redistribution (“binding”) of SDS to EHEC as a function of composition. Figure 2 shows a plot of an extensive amount of dialysis data for a constant polymer concentration of 0.0020 g/mL. The ordinate axis gives the excess amount of SDS in the polymer solution (expressed as millimoles of SDS per gram of EHEC) and the abscissa axis the total SDS concentration in the system. Even Figure 2 (and Figure 5) may be divided into regions according to the position along the SDS concentration axis: a first part with very slight redistribution of surfactant between coil and bulk region, a “foot point” (at approximately 3.5 mm SDS; see Figure 2), and finally a region where binding increases almost linearly. At about 14 mm SDS the curve drops dramatically due to desorption.
If dialysis experiments are repeated at different polymer concentrations and the results corrected for the Donnan effect, the ‘binding” is found to be almost independent of polymer concentration up to a certain SDS concentration (see Figure 5). By combining the results from Figure 1 and the dialysis results for different polymer concentrations as given in Figure 5, it is possible to recalculate the viscosity data to sets of values of the reduced viscosity as a function of polymer concentration for a constant amount of bound surfactant (constant y); see Figure 3 and the Theory section. From such a plot intrinsic viscosities, [TI, and initial slopes, k,, can be calculated as a function of y. Figure 3 shows that there is a large variation in both hydrodynamic volume (intrinsic viscosity) and polymer-polymer interaction (initial slope) as y increases from 0 to 6 mmol of SDS per gram of EHEC. Furthermore, in an intermediate range of y values, the macromolecular interaction becomes very pronounced and complex as seen from the strong curvature of some of the curves in Figure 3. This behavior is most likely due to a cooperative polymer-polymer interaction in this composition range which leads to extensive network formation. The basic results extracted from Figure 3 have been plotted in Figure 4, the most interesting parameters being the intrinsic viscosity [ q ] and the Huggins’ constant kH. Figure 4 shows that the hydrodynamic volume decrease is about 5-fold when the adsorbed amount of surfactant increases from 0 to 2.5 mmol of SDS per gram of EHEC and then passes through a minimum after which a moderate increase in hydrodynamic volume sets in. Evidently, these results can be understood in terms of the arguments presented in the Theory section. Initially, surfactant clusters are formed at a hydrophobic site and onto which the rest of the polymer coil shrinks, thus strongly reducing its hydrodynamic volume. However, as the number of clusters per polymer molecule becomes larger, and since they are highly charged, the coils will expand due to Coulombic repulsion. Furthermore, the polymer structure has on an average become more hydrophilic which will affect the polymer-solvent interaction and hence the conformation. The conclusions as the polymer-polymer interaction which can be drawn from Figure 4 are also consistent with this picture. When two coils, not saturated by surfactant, approach each other, they will show a strong tendency to share clusters due to the gain in free energy. This is equivalent to a high intensity in hydrodynamic interaction as revealed by the rather large values of the Huggins’ constant in the intermediate range of y values. (kHpasses through a pronounced maximum.) As the individual coils become more saturated, this interaction should decrease; the increasing Coulombic repulsion probably also contributes to this reduction.
Thermodynamics of the SDS-EHEC-Water System
P
, ,
1ooy v-
0
0.05 0 . 1 0 ~ 0 . 1 50.20 0 . 2 5 EHEC concentration (%)
Figure 3. Sets of viscosity a t 20 O C for constant values of y extracted from Figure 1 by utilizing Figures 2 and 5 (see legend of Figure 2 and the main text, especially shown Theory): (---)y = 0;( 0 )y = 0.3;( 0 ) y = 0.5; (0) y = 1; (A)y = 2; (B) y = 3; (X) y = 6. The E H E C concentration scale is c X 100. Each curve in this figure thus relates to a well-defined adsorption state of the polymer.
The initial slope, k,, determined directly in Figure 3, shows a more complex variation with y than does the theoreticlly more understandable parameter kH. It could be remarked at this point, however, that the results obtained depend in a critical way on the accuracy of the dialysis data. Dialysis data obtained at slightly higher temperatures (approximately 25 “C)and to be published in connection with a forthcoming paper on the temperature dependence of the EHEC-SDS interaction27m e r only slightly from Figures 2 and 5. There seem to be indications of certain specific features in Figure 4, but a more detailed analysis must await results over a more extended temperature region. At higher temperatures (presently under investigationz7)there is a clear tendency of a second region of viscosity increase. However, the essential features of Figure 4 remain. From these observations it can be concluded that, within the limits of experimental accuracy, Figure 4 gives a fairly accurate correlation of the data at 20 OC. Finally, in Figure 4 the critical concentration c*, here defined as 1/[7], has been plotted. Obviously, c* can be utilized as an estimate of the polymer concentration at which the segment concentration begins to become uniform throughout the entire bulk. At [SDS] = 0 we find c* = 0.002g/mL, while c* = 0.009 g/mL for maximum in [VI, corresponding to 0.2% and 0.9% polymer concentration, respectively. According to the original Einstein theory,2showever, c* should be multiplied by the factor of 2.5 to give the true critical concentration. While the factor the Einstein factor that should be really used has been seems to be of the correct order of magnitude. Referring to (27) Holmberg, C.; N i e n , S.; Singh, S . K.; Sundekf, L.-0. Temperature dependence of hydrodynamic volume and surfactant-polymer interaction in solution. The EHEC/SDS/water system. Manuscript in preparation.
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 875
J . Phys. Chem. 1992, 96, 876-883
876
-0
E
0.80
R
0
200
400
600
800 t (mid
1000
1200
1400
Figure 6. Viscosity results obtained at 20 OC concerning the aging of
solutions (time dependence) presented as the ratio of relative viscosity at time t divided by the relative viscasity at time zero plotted versus time for the following compositions: 0.05% EHEC + 2 mm SDS 0,0.20% EHEC + 2 m m SDS;A, 0.05% EHEC + 8 mm SDS;A, 0.20% EHEC + 8 mm SDS. to the foot point and the linear slope, this will be postponed until more extensive data are available a t different temperatures. It may here be sufficient to note that in Figures 2 and 5 the foot print falls in the region 3.5-4 m m SDS, which is the “total” SDS concentration where the large increase in viscosity sets in (see Figure l), which indicata that the two phenomena are interrelated. It seems as if the experimental data just discussed are compatible with the ideas presented in the Theory section, a t least in a qualitative sense. At the same time there are indications that the mechanisms are more complex. Preliminary results from the temperature dependence of intrinsic viscosity and classical light
scattering have already given hints in this dire~tion.~’Studies of the surface tension in the EHEC/SDS/water system in progress28also seem to support many of the features of the interaction process presented here. All data so far discussed have been obtained for almost salt-free conditions. Salt is known to have very remarkable effects on the system in q ~ e s t i o n . ~Work ~ . ’ ~ is in progress on such salt-containing systems along the lines presented in this paper. It should finally be observed that freshly prepared solutions show drastic changes in properties with time. Figure 6 gives a few examples of this for different compositions. The surface tension studies2* support these findings. To ensure equilibrium in the solutions under study, it is evident from Figure 6 that one has to wait for up to 10 h prior to measurement in certain composition regions. All data discussed above have been obtained at the equilibrium situation so defined. The cause of this time dependence is still unclear, but it could possibly be explained by the change in polymer-solvent interaction induced by redistribution and binding of the surfactant. Conformational changes in the EHEC backbone might contribute to the effects as might also the molecular state of EHEC (for instance, presence of an aggregated form) prior to the addition of SDS. Even this time dependence is now subjected to a more detailed study.29
Acknowledgment. The present work has been financially supported by grants from the Swedish Natural Science Research Council and from Kabi Invent AB, Novum, Huddinge, and Kabi Pharma AB, Solna. We acknowledge fruitful discussions of the material presented here with Prof. BjBrn Lindman, Dr. Conny Bogentoft, Dr. Anders Carlsson, and Mr. Peter Sponbergs. Registry NO. SDS, 151-21-3; EHEC, 9004-58-4. Nahringbauer, I. Private communication. (29) Nilsson, S.;Holmberg, C.; Sundelbf, L.-0.; Singh, S.K. On the time dependence of macroscopic and molecular properties of the EHEC/SDS/ water system. Manuscript in preparation. (28)
Ultrasonic Study of Proton-Transfer Reactions In Aqueous Solutions of Amino Acids T. V. Chalikian,*.t D. P. Kharakoz,t A. P. Sarvazyan,’ C. A. Cain,* R. J. McGough,s I. V. Pogosova,t and T. N. Gareginiant Department of Physics, Yerevan Medical Institute, Kirova 2, Yerevan, 375025, Armenia, USSR, Institute of Biological Physics, Academy of Sciences, USSR, Pushchino. Moscow Region, 142292, USSR, and Department of Electrical Engineering and Computer Science, The University of Michigan, Ann Arbor, Michigan 48109-2122 (Received: April 3, 1991; In Final Form: July 22, 1991)
Ultrasonic velocity and absorption in dilute solutions of 12 amino acids have been measured over a wide pH range. Ultrasound velocity, volume, and compressibilitychanges in amino acids solutions due to ionization of amino and carboxyl groups were evaluated by means of a single-frequency method recently developed. The mutual influence of the amino and carboxyl groups on the hydrational volume and compressibility has been estimated quantitatively. An abnormal reverse sign of the compressibility change during ionization of the amino group in the amino acid skeleton has been found and leads to a question about the reliability of a previously published method of separation of the individual partial compressibilities of oppositely charged ions.
Introduction The state of ionization of amino acid residues on the surface of proteins is one of the most important factors influencing hydration of these biomacromolecules. Hydration of proteins plays a significant role in the stability, dynamics, structural characteristics, and functional activity of these biopolymers. The im+Yerevan Medical Institute. *Academy of Sciences. USSR. #The University of Michigan.
portance of this problem is reflected, for example, in the review by Nemethy et a1.l Hydration of biopolymers in solution is determined by the accessibility of the polymer chain to water molecules and the nature of the exposed atomic groups. During the past 15 years, a considerable number of papers have been devoted to the investigation of protein hydration by means of ultrasonic velocity measurements?-” It was shown that hydration (1) Nemethy, G.; Peer, W. J.; Sheraga, H. A. Annu. Rev. Eiophys. Eioeng. 1981, 10, 459-491.
0022-3654/92/2096-816%03.00~00 1992 American Chemical Society
