856
J. Phys. Chem. 1980, 84, 856-859
Physicochemical Studies on the Characterization of Triton X 100 Micelles in an Aqueous Environment and in the Presence of Additives A. B. Mandai, S. Ray, A. M. Biswas, and S. P. Mouiik" Department of Chemistry, Jadavpur University, Calcutta-700032, India (Received November 19, 1979)
From the measurements of conductance of TX 100 micellar solution in 5 mM NaC1, the extent of micellar hydration has been directly determined. This coupled with the intrinsic viscosity has helped in evaluating the asymmetry of the micellar particles and their average axial ratios. The shape of the micelles has been observed to be oblate spheroids, whose sizes are variable with temperature. From the absolute values of the axes, the micellar volume and hence the hydrated radius and radius of gyration as well as the diffusional coefficients have been calculated. The partial molal volume of the TX 100 micelles has been also determined. Its comparison with the molar volume of pure TX 100 suggested a volume contraction due to immobilization of the water phase by the hydrophilic head groups of TX 100. While both urea and tetraethylammonium bromide have decreased the intrinsic viscosity significantly,Na2S04has increased it significantlyat low concentration and has finally decreased it progressively with further addition of salt. Thermodynamic activation parameters for viscous flow are in favor of the formation of water structure by the presence of TX 100 micelles.
Introduction other physical features of n-alkyl poly(oxyethy1ene) mono ethers. All these works have considered the micelles of T X The nonionic surfactant, p-tert-alkylphenoxy poly(ox100 to be oblate spheroids which is supported from the yethylene) ether (Triton X 100, abbreviated as T X 100) reasonably low intrinsic viscosity obtained for the micelles is widely used in biological work, such as separation of whose aggregation number16 at ordinary temperature is proteins from cell membranes,1$2and mixed with phosaround 143. pholipids, it produces effective substrates for studying Recently, hydration of proteins and carbohydrates and enzymes of phospholipid metaboli~m.~"Solubilization of complexation of carbohydrates with metal ions over and lipidlike bodies by TX 100 through mixed micelle formation can be utilized in chemical and industrial ~ o r k . ~ - ~above the hydration have been quantitatively calculated from measurements of the conductance of their soluInformation about physicochemical properties of mixed tions.17Js The basic equation (eq 2) has been shown to hold micelles of T X 100 is therefore necessary for an understanding of the system itself. It is reported that mixed k' = 1 - 1.93V,,( F C ) micelles of lipids with T X 100 may be formed with little k change of shape and size of the pure micelles.1° The study for particles (large compared to the ions and solvent of shape, size, and hydration of pure T X 100 micelles is molecules) of any shape (prolate, oblate, spheres, and therefore important from a fundamental as well as pracneedles), where k'and k are the specific conductance in tical standpoint.1° Micelles formed by n-alkyl poly(oxythe presence and absence of obstructants, v h is the hyethy1ene)mono ethers and alkylphenoxy poly(oxydrated specific volume, and C is the concentration of the ethy1ene)mono ethers (Triton X group of compounds) have obstructant, expressed in g/mL. been recently considered to be oblate spheroids with a A direct estimate of v h is thus possible without assuming considerable degrees of hydrati~n.~Jl Earlier reports have any shape. We shall use this relation and couple the results considered T X 100 micelles to be ~ p h e r i c a l . ~ JAlthough ~J~ with eq 1 to find the particle asymmetry factor v. Other this controversy has yet to be solved the evaluation of the information regarding the micelle of T X 100 will then exact geometry is barred for want of independent follow. knowledge of micellar hydration. It will be further shown that the viscosity equation of The intrinsic viscosity [ q ] is related to the shape factor Vandlg holds for T X 100 micelles. The knowledge of V , v and hydration factor 6 (g of water bound/g of micelle) can be also used there to obtain v , Moreover, the influas ences of the additives like urea, sodium sulfate, and tetraethylammonium bromide on the intrinsic viscosity of T X = v(OTx + '%IzO) (1) 100 will be presented. Partial molal volumes of TX 100 where OTX and OHzO are the partial specific volumes of T X above the micellar points at different temperatures will 100 and water, respectively. The shape factor v is a also be presented to ascertain a detailed picture of the function of the axial ratio; v is 2.5 for spheres and increases micellar solution. Activation parameters for solvent flow both for oblate and prolate spheroids.14J5 Now, [VI, I J T ~ , in the presence of micelles will be documented to realize and OH20are measurable quantities but v and 6 are insethe influence of the micelles on the net water structure. parable. A prior knowledge of any one of the two is reExperimental Section quired to estimate the other from [VI, Robson and Dennisg Materials and Method. The T X 100 sample used was have considered a reasonable model for the micelles and, the same as described earlier.16 The salts NaC1, Na2S04, from the conformation of the hydrophobic and hydrophilic (C2H6)4NBr,and urea were BDH analar grade and were groups, have calculated the factor v and hence the extent used without further purification. Double distilled conof hydration. An endeavor of this type has also been ductivity water with a specific conductance of 1.5 X adopted by Tanford et al.ll to solve the hydrodynamic and mho cm-l at 25 "C was used as the solvent. Viscosity measurements were taken in a Ostwald vis*Address correspondence to this author a t the Department of cometer calibrated at different temperatures with sucrose Chemistry, Georgia State University, Atlanta, GA 30303. 0022-3654/80/2084-0856$0 1.OO/O
0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980' 857
Physicochemical Studies of Triton X 100
'?'OO
$0
310
310 T('K)
3!40
3kO
3k0
i
Figure 1. Differential viscosity-temperature plot for TX 100 at different concenfrations (circle) 0.005 M; (circle with cross), 0.0 1 M; (triangle) 0.05 M.
I
I
0-
340
1
0 io
005
0 19'
( f'4Figure 2. Application of eq 2 to TX 100 micelles in 5 mM NaCl solution at different temperatures.
TABLE I: Intrinsic Viscosity, Particle Asymmetry, and Hydration of TX 100 Micelles at Different Temperatures hydra- hydration no. tion no. [VI, axial from from dL/g T,"C: u ratio 8 , g/g eq 2 eq 3 26.13 25.62 0.37 0.754 15 3.85 6.5 13.72 14.67 0.397 0.35 25 3.96 5.3 7.92 10.73 0.33 0.230 35 4.07 4.8 12.19 18.77 0.355 0.31 45 4.18 5.5
solutions. Densities of solutions were measured in a calibrated pycnometer. The temperatures of measurements were accurate to within k0.05"C. Uncertainties in temperature, density measurement, and flow detection imparted a maximum error of k0.7% to the viscosity. Conductaiice measurements were taken in a Elico conductivity brndge made a t Hyderabad, India. A dip type cell was used with a cell constant 0.7 crn-l. The uncertainty in the conductance measurement was within f0.5'3'0. In actual experiments, solutions having different concentrations (of T X 100 were prepared and thermostated for a t least 1 h at a constant temperature. A definite volume of am individual solution was then taken in the dried viscometer whose time of flow was measured at least four times after being allowed to come to thermal equilibriurn for 20 min. The densities of the corresponding solutions were also measured under identical environmental conditions. In the conductance experiment, the TX 100 solution in 5 mM NaCl was thermostated for 30 min to which a thermostated 5 mM NaCl solution was progressively added. Conductance was measured at each dilution after mixing the riolution thoroughly with a magnetic stirrer placed inside the container and allowing few minutes to regain therrnial equilibrium. Conductance of 5 mM NaCl was also measured. T X 100 imparted negligible conductance t o the solution. Results In Figure 1, the (ArlAT) vs. T plot shows a distinct change around 62 "C. The TX 100-water system has thus undergone a change in hydrodynamic properties at this temperature. In Figure 2, the validity of eq 2 has been presented. The hydration numbers calculated from the slopes of the lines a t different temperatures are presented in Table I. The
-4-Y -6-
Figure 3. Vand's plot for TX 100 at different temperatures.
TABLE 11: Diffusional and Dimensional Properties of TX 100 Micelles at Different Temperaturesa
lo6*
lozo M.,.
D, T. cm2 15 25 35 45
0.44 0.62 0.82 0.97
54.44 51.31 50.16 53.09
20.14 17.96 16.55 16.46
42.44 39.62 38.35 40.20
35.59 33.43 32.57 34.37
39.10 36.10 34.66 35.93
1.09 1.10 1.11 1.12
25,OO 19.80 17.43 19..42
mL; a = 52 A , b = a TX 100: M,(oblate) = 30.6 x 27 A , 6 = 1.0 g/g (ref 9). C,,H,, n-alkyl poly(oxyethg1ene) ether; aggregation no. = 120. a = 42.7 A , b = 26.0 A (ref 11).
deviations of experimental points beyond the concentration 0.11 g/mL is par with the earlier observations on different systems.17J8 The validity of Vands equation at different temperature is depicted in Figure 3. Calculated shape parameters from eq 3 agree with the intrinsic viscosity derived values (eq 2.303 2.3036 - -1 (3) log ?Ir uv,c Y
858
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980
TABLE 111: Partial Molal Volume and Excess Volume of TX 100 Micelles
15 25 35 45 55
583.19 587.06 591.70 594.30 602.90
-VE,
-
T, " C V, mL/mol
V, mL/mol
568.12 f 572.37 f 570.74 f 564.55 f 578.30 f
mL/mol
0.39 0.44 0.60 1.55 0.92
15.90 14.69 20.96 29.75 24.60
TABLE IV: Effects of Additives on the Intrinsic Viscosity of TX 100 at 25 " C urea
+ TX 100
additive, g/mL 0 0.03 0.06 0.12 0.18 0.24 0.30 0.36
[Ol
5.30 3.20 1.54 0.78 0.45 0.36 0.28 0.23
(C,H,), NBr + TX 100 additive, g/mL 0 0.013 0.026 0.039 0.051 0.077
[Ol
5.30 4.93 3.03 1.91 1.23 0.74
Na,SO,
+ TX 100
additive, g/mL 0 0.0071 0.0142 0.0213 0.0284 0.0426
LO1
5.30 14.84 9.05 6.24 5.43 5.13
TABLE V: Thermodynamic Activation Parameters for Viscous Flow of TX 100 Solution at 25 "C
TX 100 concn, M 0 0.031 0.041 0.051 0.061
kJ mol-'
AGO,
9.15 9.46 9.58 9.70 9.84
A H " , kJ mol-'
AS", J deg-'
15.01 17.83 17.83 17.83 17.83
17.05 28.08 27.65 27.25 26.79
mo1-l
1) and are presented in Table I, where qr is the relative viscosity, Q is a constant, and the other terms have their usual significance. In Table I1 are given the other physicochemical parameters for the micellar system. These include the lengths of the semimajor and semiminor axes, micellar volume, ratios of frictional coefficients (f/fo),etc. at different temperatures. Partial molal volumes of the micelles determined from density measurements are given in Table 111. The recorded V are the mean of at least five values which have been observed to be practically independent of T X 100 concentrations above the cmc. The probable errors of the mean V values are also shown in the table. The effect of additives like Na2SO4, (C2HJ4NBr,and urea are shown in Table IV. Remarkable effects of both urea and the tetraalkyl salt on the [77] can be observed. Both of them have been effective at low concentrations. The thermodynamic activation parameters for viscous flow are recorded in Table V.
Discussion Viscosity of TX 100 micelles can be well described by the equation of Vand. Normal viscosity behaviors are ensured below 60 "C, beyond which phase separation leading to turbidity can be visually witnessed (Figure 1). The shape factor v can be found knowing the hydration of the micelles. On an average, this hydration has been observed to be 26,14, and 8 at 1 5 , 2 5 , and 35 "C, respectively. The method is an independent one. At temperatures higher than 35 "C, the intrinsic viscosity has increased. If it is not due to increased hydration, the shape has changed toward more asymmetry. The reasonable low values of [ q ] have spoken in favor of oblate shape of TX 100 micelles," and the direct
Mandal et al.
evaluation of hydration17Jshas enabled an estimation of the micellar geometry. The molecular asymmetry at different temperatures as observed in Table I has revealed the dimensionsto be changing with temperature. This has meant that the conformation of the poly(ethy1eneoxide) chain becomes more compact with increased temperature. We mention here the trial of an alternative method for resolving the shape factor. This has been through the utilization of Fricke's equation.20 The method has been applied on montmorilonite clay micelles.21 Such an analysis has not been of any real help to us; the overall shape resembled at best a sphere. (It is quite likely that an oblate figure of minimum asymmetry may turn out to be a sphere if the method is not enough sensitive.) From the extent of hydration and the aggregation number (143), the volume of the T X 100 micelles has been computed (the presence of extra space due to geometrical and thermal consequences has been neglected). From the oblate spheroid volume 4/37r(a2b)and the axial ratio a / b , the semimajor axis a has been computed which has subsequently yielded b. The hydrodynamic radius Rh and the radius of gyration R, have been calculated from Perrin's formulations.22 Finally, the ratios of the frictional coefficients, relative to spherical shape, f / f ohave been calculated from Rh/Ro,where Ro is the radius of an equivalent sphere. f / f ohas been observed to vary as ( U / ~ ) O . ~ oblate ~; spheroids normally show a variation as ( ~ z / b ) O . Results ~~. in Table I1 reveal that the semiaxes are comparatively lower than the reported values. According to Dennis et al.? the poly(ethy1eneoxide)chain has a contribution of 17 A on the consideration that is has a meandering conformation. The semiminor axis is thus 27 A with 10 A attributed to the hydrophobic part. Our results nearly conform with this at 15 "C. At higher temperature, the length of the head group is remarkably low. A t 35 "C it is only 7 A instead of 17 A. (The semimajor axis has a value close to 52 A, as reported by Dennis et al.,9 and remains invariant with temperature.) A great degree of coiling of the poly(ethy1eneoxide)chain (head group of the surfactant) has been envisaged; the geometry has tended toward spherical through decreased hydration. The chain (random coil or meander) has collapsed at higher temperature through loss of water and the dehydrated micelles have undergone a secondary aggregationll imparting turbidity to the solution. Examination of the Huggins constant k has also revealed an almost spherical shape at 15 "C ( k = 2.0). k has steadily increased with temperature indicating that the medium has tended to become a poor solvent for TX 100 and phase separation has occurred at a moderately high temperature. For the effects of the additives on the intrinsic viscosity, both urea and tetraethylammonium bromide have remarkably decreased [VI. Na2S04 has an increasing effect at lower concentrations, and a diminishing effect afterward. The hydration in these environments at a fixed u (say 2.5) as reflected from [v] has shown mostly negative values in the cases of urea and Et4NBr. Since urea breaks down water structure and decreases the hydrophobic interaction (micelles are broken down by urea), the shape factor v must not be constant or wrong idea of hydration would be revealed. Et4NBr may as well affect the micelles and nonpolar interaction, but compared to urea it is recognized to be less effective, It is further remarkable that at concentration of 0.5 M (well below denaturation of proteins and demicellization of surfactants) urea is very effective. Some special property of urea must be invoked to rationalize this, which at present is uncertain. If we recognize that both urea and EtdNBr desolvate the head group of the TX 100
J. Phys. Chem. 1980, 84, 859-863
micelles, decreasing [17] a t 6 = 0 in eq 1 means decreasing Y, Le., an approach toward a spherical shape. Such a calculation also leads to unusual results and change of shape cannoi, also account for the low [VI. Na,SO,, on the other hand, may have imposed further hydration to the head group and has increased [TI. A t a higher salt concentration a salting-out type phenomenon may have dehydrated the micelles and reduced [VI. The latter is in line with the observed clouding of T X 100 at lower temperaNa2S04. t ~ r with e ~increasing ~ The partial molal volume above the cmc at a particular temperature has been observed to be almost constant. The volume contribution at micellar point and above is therefore the same. Since the excess volume P is negative, some kind of organization of the water molecules by the poly(oxyethy1eneoxide) chains around the micelles has been revealed. Hydrogen bonding of water through such chains can be a very common occurrence. Each oxygen center has roughly been observed to fix, on the average, 2-3 water molecules. Entrapping, over and above hydrogen bonding, is therefore possible in the network of the many flexible head groups on the outer mantle of the micelles. With increasing temperature, although the partial molail volume has increased, VE has decreased. Dehydration of the water embedded chains might have brought them closer (since the hydration barrier is reduced) and the increment due to the temperature rise has compensated for this effect with a net decrease of VE. Above 45 “C this is not orderly. It points to the complex nature above 35 “C (cf. Table I). The activation parameters for solution fluidity are in favor of more ordering of the e n v i r ~ n m e n tboth ; ~ ~ A € P and
a59
AS* have higheir magnitudes in the presence of T X 100 than in its absence. Acknowledgment. Thanks are due to the authorities of the Jadavpur University for the use of laboratory facilities by S. Ray and A. M. Biswas.
References and Notes (1) A. Helenius and K. Slmons, Biochim. Biphys. Acta, 415, 29 (1975). (2) S. Razin, Biochim. Biophys. Acta, 265, 241 (1972). (3) R. A. Deems, B. R. Eaton, and E. A. Dennis, J . Bioi. Chem , 250, 9013 (1975). (4) T. G. Warner and E. A. Dennis, J . Bo/. Chem., 250, 8003 (1975). (5) E. A. Dennis, ,Arch. Biochem. Biophys., 158, 485 (1973). (6) R. J. Robson and E. A. Dennis, Biochim. Biophys. Acta, 5016, 513 (1978). (7) E. H.Crook, D. 6.Fordyce, and G. F. Trebbi, J. Phys. Chem., 67, 1987 (1963). (8) A. Ray and G. Nemethy, J. Am. Chem. Soc., 03, 6787 (1971). (9) R. J. Robson end E. A. Dennis, J . Phys. Chem., 81, 1075 (1977). (10) C. Tanford and J. A. Reynolds, Biochim. Siophys. Acta, 45’7, 133 (1976). (11) C. Tanford, Y. Nozaki, and M. F. Rohde, J . Phys. Chem., 81, 1555 (1977). (12) L. M. Kushner and W. D. Hubbard, J. Phys. Chem., 58, 1163 (1954). (13) A. A. Ribeiro and E. A. Dennis, Biochemistry, 14, 3746 (1975). (14) G. 6.Jeffery, Proc. R. Soc. London, Ser. A , 102, 161 (1923). (15) R. Simha, J . Phys. Chem., 44, 25 (1940). (16) F. Podo, A. Ray, and G. Nemethy, J. Am. Chem. SOC.,05, 6164 (1973). (17) S.P. Moulik, lbctrochim. Acta, 16, 981 (1973). 118) S. P. Moulik and D. P. Khan. Carbohvd. Res.. 36. 147 11974). i19i V. Vand, J. Rbys. Colloid Chem., 52,’277 (1948). (20) (a) H. Fricke, Phys. Rev., 24, 575 (1924); (b) ibid., 26, 682 (1925). 121) H. C. Thomas and A. Cremers. J . Phvs. Chem., 74, 1072 (1970). (22) F. Perrin, J . Phys. Radium, 7, 1 (1936). (23) K. Shinoda, T. Nakagawa, 6.Tamamushi, and T. Isemura, “Colloidal Surfactants: Some Physicochemical Properties”, Academic Press, New York, 1963. (24) S. P. Moulik and D. P. Khan, Indian J . Chem., 16A, 16 (1978). >
,
Viscous Liquids and the Glass Transition. 9. Nonconfigurational Contributions to the Excess Entropy of Disordered Phases P. D. Gujrati’ and Martin Goldstein” Division of Natural Science and Mathematics, Yeshiva University, New York, New York 10033 (Received August 27, 1979)
An analysis of calorimetric data on crystal, liquid, and glassy phases of a single substance permits an estim.ation of the relative contributions of configurationaland nonconfigurational (e.g., vibrational) factors to the excess entropy of the supercooled liquid at Tg. Calorimetric data on three substances were previously analyzed; we report here an analysis of data on eight additional substances. We find in the 11 substances that there is an average decrease of the entropy difference between glass and crystal on cooling from Tgto the vicinity of 0 K of 5.8 J/(K mol), representing a fraction, on the average, of 0.30 of the excess entropy at Tg. A comparable analysis on orientationally disordered crystals below the “glass” transitions shows that these substances show larger absolute and relative losses: 7.9 J/(K mol) and 0.58, respectively. It is concluded that communal entropy in the liquid state does not play a major role in the glass transition.
Introduction In an earlier paper from this laboratory1 we considered two questions: (1)How much of the excess entropy A S of the amorphous phase at Tgmay arise from nonconfigurationa] degrees of freedom (e.g., vibrational differences between amorphous and crystalline phases)? + Physics Department, Carnegie -Mellon University, Pittsburgh, P A 15213.
0022-3654/80/2084-0859$0 1.OO/O
(2) How much of the “configurational” specific heat AC, a t T gis truly configurational? We showed that the first question could be answered if specificheat daki on crystal and glass phases were available from Tgto the vicinity of 0 K, and the second if specific heat data on glasses of different fictive or structural temperatures were available over the same range. Data of the latter sort had been obtained for six substances by Chang and associates a t the National Bureau of Standards.2-6 Three of these substances had also been 0 1980 American
Chemical Society
