J. Phys. Chem. 1991, 95, 5664-5673
5664
observed in the experimental curve for the birefringence (eq 8b) in high-field conditions. This information can be combined with the field dependence of the steady birefringence to determine individually the permanent dipole and/or the polarizability difference, thus completing the determination of the electrooptic parameters ( R and s, or e and g). The only remaining unknown, the rotational diffusion coefficient, could be determined from the simulated birefringence vs time curve. Indeed, that curve may contain information enough to determine simultaneously the second electrooptic parameter and the rotational coefficient. In this work we have been mainly concerned with the determination of the validity of analytical theories by comparison with Brownian dynamics simulations, implicitly assuming that the latter are essentially correct. It would be desirable to compare the simulation results with systematic measurementsof reversing-pulse birefringence at varying, moderate, or high fields. Some related experimental work has been done for rodlike for which the analysis is anyhow complicated by eventual polydis-
persity and field-induced trandtions. While polydispersity could be easily introduced in the simulations, field effects should be removed beforc a quantitative comparison between simulation and experiment is made. From another point of view, if one relies in the validity of the Brownian dynamics technique, simulation results at varying field strength can be useful to ascertain whether some experimental features are not related to dynamics and related aspects and may therefore be due to field-induced structural changes. It has been assumed in this paper that the induced dipole responds instantaneously to the applied field, as happens for the electronic polarization in the classical Debye treatment. There are relevant cases where this is not true. A most interesting example is DNA in a low ionic strength salt solution, for which the polarization of the ionic atmosphere is not instantaneous. The Brownian dynamics technique studied in this paper is a promising approach for such cases, since time-dependent induced dipoles can be easily introduced in the simulations.
(11) Yamaoka, K.;Yamamoto, S.;Ueda, K. J. Phys. Chem. 1985,89, 5192. (12)Yamaoka, K.;Yamamoto, S.; Ueda, K. Biopolymers 1987,26,673.
Acknowledgment. This work has been supported by g;ant PB87-0694 from the Duecci6n General de Investigacih Cientifica y TQnica to J.G.T.
Dodecyltrimethyiammoium Bromide in Water-Urea Mixtures. Volumes, Heat Capacities, and Conductivities S.C a d , R. De Lid,* S.Milioto, and N.Tirone Dipartimento di Chimica Fisica, Universitb di Palermo, via Archirafi 26, 90123 Palermo, Italy (Received: November 26, 1990)
Densities, heat capacities, and conductivities of dodecyltrimethylammonium bromide (DTAB)-water-urea mixtures over a wide range of urea and surfactant amcentrations were measured at 298 K. The partialmolar volumes'V( and heat capacities (Ch) of DTAB in urea solutions at fmed compositions were derived. The profiles of C as a function of DTAB concentration show anomalies at about 0.30-0.35 mol kg',depending on the urea molality (mu),&ereas those of V2do not. According to literature findings, this peculiarity has been attributed to a micelle structural transition. The partial molar volume of monomeric surfactant at the cmc as well as that at infinite dilution increases linearly with m,,while the heat capacity decresses in a nonlinear manner. As far as the surfactant in the micellized form is concerned, on increasing mu the partial molar heat capacity increases linearly while the partial molar volume increases with a curvature. Consequently, both volume and heat capacity of micellization, evaluated on the basis of the pseudophase transition model, do not change linearly with mu. The effect of added urea on DTAB micellization is due to the change of the physicochemical properties of the solvent mixture since urea does not solubilize in the micellar phase. The cmc and the degree of counterion dissociation (6)of DTAB micelles were derived from conductivity measurements in a large range of urea concentration (up to ]Om,). The cmc increases linearly with m, while j3 changes in a nonlinear manner.
of view, on the contrary, to the best of our knowledge only a few examples of systematic investigations in water-addfiive dxtures have been published. In fact, volumes and compressibilitiesof sodium decanoate12 at 25 OC in several water-alcohol mixtures and of sodium dodecanoate13 and octanoete14in aqueous solutions as of the surfactant and the concentrations have been measured. Volumetric measurements of tetradbcyltrimethylammonium bromide (TTAB) in water-1,Zethanediol mixtures as a function of TTAB concentration have betn reported by Backlund et aLI5 Osmotic coefficients4of a series of alkyltrimethylammonium bromides in SO-
Introduction In the past few years direct thermodynamic investigations of surfactant solutions have betn performed' the effects Of the length, the nature Of the pdar head, and the counterion binding of the surfactant on the micellization process in water at different temperatures have been extensively by different meth& such as volumes,~-3 heat enthalpies of dilution,l compressibilities,u osmotic coeficients,l,4 and so on. The dependena ofthe and the desree of of the micelle on temperature, pnssure, and additive concentration point has been also well investigated.w' From a th-,,dynamic ~
(8) Bhattacharya, P.; &rumall& I. N. IdIan 1. Chem. 1987,26,25. (9)h t t . A. B.; Tartar, V. J. Am. Chem. Soc. 1943.65.692. (10)Salthawat Shah, S.;Ejrz-ur-Rtbmm. In InteroctW of water in lonlc Mdnonloni~h y d m w H.,Ed.; SpfhgW-VCrhg: Berlin, 1987. (1 1) Trcintr, C.J. Colloid Inter ace Sei. 1982,92,444. (12)Vikingatad, E.;Kvammen, J. Colloid Intet$we Scl. 1989,74,16. (13) Hoilaad, H.; Vikingatad, E. 1. Colloid Intetjiwe Scl. 1978,61,126. ( 14) k n , 0.;De LY, R.; Davidron, I.; Otntreux, S.;Dcanoycra, J. E. J . Colloid Interfm Scl. 1981. 79. 432. (15) Backluh, S.;Bar&&], B.;Mohndcr, 0.;Wamheim, T. J. W l d d Inlaface Scl. 1989,131, 393.
~~~~~~
(1) De Liri, R.; Fhiaro, E.; Milim, S. J. Sdutlon Chem. lUW,17,1015. (2)Archer, D.0.:Majcr, V.; I n g h A.; Wood, R. H.J. Colloid Inlet$Scl. 1968,124,591. (3) Tanaka, M.;Kanerhina, S.;Shin-No, K.; Okajima, T.; Tomida, T. J. Colloid Interface Scl. 1974,46, 132. (4)Dea,rden, L. V.; Woollcy, E. M. J. Phys. Chem. 1987,91,2404. (5) Schick, M.J. J . Phys. Chem. 1964,68,3585. ( 6 ) Tuddmhamm, R. F.; Alcxandcr, A. E.J. Phys. C h m . 1%2,66,1839. (7)Emonon, M. F.; Holtur, A. J. Phys. Chem. 1%7, 71,3320.
0022-3654/91/2095-5664$02.50/0
6.
(6
1991 American Chemical Society
DTAB in Water-Urea Mixtures
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5665
dium bromide aqueous solutions have been measured at 55 OC. As well, there are sparse thermodynamic data for other additives. The effect of sodium bromide on the enthalpies of micellization of dodecyltrimethylammonium bromide16 (DTAB) and sodium dodecyl sulfate17has been also studied. Musbally et al." report volumes and heat capacities of nonyltrimethylammoniumbromide in 3 mol d w 3 urea (MJ while Singh et al.I9 report heat capacities of sodium dodecyl sulfate in 3Mu and of hexadecylpyridinium chloride and bromide in 2Mu. Since it is commonly thought urea disrupts the protein structures, we felt it would have been interesting to study the thermodynamics of micellization in water-urea mixtures since micelle formation can be considered a simple model system containing hydrophobic bonds. For this reason, conductivity, density, and heat capacity measurements of DTAB-water-urea system as functions of the surfactant and the urea concentrations have been carried out at 298
-
o$ ** +*
+** m
4:* * * +o*
r"
K.
1
Experimental Section Materiala Dodecyltrimethylammonium bromide (DTAB) (Sigma, product) was crystallized twice from ethyl acetate-ethanol mixture (7/1 v/v) and then dried in a vacuum oven at 60 OC for at least 48 h before the use. Urea (Fluka) was crystallized from a water-ethanol mixture (6/4 v/v) and then dried in a vacuum Oven at 40 O C . Deuterium oxide 99.95%(C. Erba, RPE product) was used as received. All solutions were prepared by mass using degassed conductance water and their concentrations were expressed as the number of solute moles per kilogram of solvent. Equipment. The solutions densities were measured at 298 K with a vibrating tube flow densimeter (Model 03D. Sodev Inc.) sensitive to 3 ppm or better. The temperature was maintained constant within 0.001 OC by using a closed-loop temperature controller (Model CT-L, Sodev Inc.). The calibration of the densimeter was made with water (d = 0.997047 g and deuterium oxide (d = 1.10445 g ~ m ' - ~ ) . ~ ' Care was takenn to avoid H 2 0contamination of deuterium oxide. The relative differen- in heat capacities per unit volume Au/uo = ( a uo)/uowerc determined with a Picker flow microcalorimeter (Setaram) at 298 K. With a flow rate of about 0.01 cm3 s-I and a basic power of 21.2 mW, the temperature increment was approximately 0.5 OC. Heat capacity measurements of the aqueous urea solutions were camed out by taking water as reference solvent and, in turn, they were used as reference solvent for the waterurea-surfactant ternary systems. The specific heat capacities (c,,) of solutions of density dare related to Au/uo through the equation cp = cp,ol1 + Aa/uoldo/d (1) where cp,oand do correspond to the specific heat capacity and density of water for measurements of water-urea mixtures and those of the water-urea mixtures for measurements of DTABwater-urea systems. The value of 4.1792 J K-I g-I for specific heat capacity of water was takenz3 For the conductance measurements a cell similar to that described by Daggett et al.,% with unplatinized electrodes was used. The cell constant (2.834 & 0.001) was determined at 298 K by measuring the conductance of dilute solutions of aqueous KCl and using the equation of Lind et alaz5 All measurements were
-
1.
(16) Espda, L.; Jones, M.N.; Pilcher, G. J. Chem. Thermodyn. 1970,2,
(17) Pilcher, G.; Jones, M. N.;Espda, L.; Skinner, H. A. 1. Chem. Thermodyn. 1%9,1,38 1 . (IS) Murklly, 0. M.;Perron, 0.;Dcsnoyen, J. E. J. Colloid Inrerfuce Sei. 1976, 54, 80. (19) Singh, P. K.; Ahluwalia, J. C. In Sur/oetrmrs in Sofurlon;Mittal, K. L., Ed.; Plenum Prw: New York, 1989. (20) Kell, G. S.J . Chem. Eng. Dura 1967, 12, 66. (21) Hundbk of Chemistry und Physics, 67th 4.; CRC Press: B o a Racon, FL, 1986-198?., (22) De LY, R.; Mdioto, S.;I QO,A. J . Solurion Chem. 1990,19,767. (23) Stinuon, M. F. Am. 1. P ys. 1955, 29,614. (24) Daggett, H. N.; Bair, E. S.; Kraus, C. A. J. Am. Chem. Soc. 1951,
Y
I? -,7oQ rl. I
(25) Lind Jr., J. E.; Zwolenick, J. J.; Fuou, R. M.J . Am. Chem. Soc. 1959.81, 1557.
0.03
+ mu=O.l 0 m,=3
+ mu= 10 .
1
.
1
.
0.09
0.06
1
.
0.12
ms /mol kg-1 Figure 1. Specific conductivities (corrected for solvent) of dodecyltri-
methylammonium bromide in water-urea mixtures vs surfactant m e lality.
0.34
t
0.28
0.26 1
2
3
4
5
6
7
8
9
m, /mol kg-l
Figure 2. Dependence of the degree of ionization of dodecyltrimethylammonium bromide in the micellized form on the urea molality.
performed with the cell in a constant temperature oil-bath-controlled within 0.005 OC by a Hewlett-Packard 2804 A quartz thermometer. The electrical resistance measurements were made with a calibrated ac bridgez6at a frequency of 2 kHz. Recluits The specific conductivities corrected for those of the solvent ( x - xo) as functions of the surfactant molality arc reported in Table I while some examples are shown in Figure 1 . It is well-known that the cmc is given by the intersection point of the s l o p of the plots of specific conductivitiesvs concentration above and below the cmc. As the plots in Figure 1 show, the cmc shifts toward larger surfactant concentration with increasing urea molality. In addition, as observed for DTAB in water,' in the micellar region there are positive deviations at about 0.09 mol kg-' DTAB. The cmc values as functions of urea concentration (summarized in Table 11) are in agreement with those that we have evaluated from the graph reported by Emerson and Holtzer.' The degree of the counterion dissociation (8) was calculated as the ratio of the s l o p of the specific conductivity vs concentration above and below the An alternative approach is given by the Evans equationa based on the ionic conductance, (26) Janz, 0. J.; Mclntym, J. D. E. J. Electrochem. Soc. 1%1,108,72. (27) zlna. R. J. Colloid Inrer/acr Sci. 1980,78, 330. (28) Evans, H. C. 1. Chem. Soc. 1956, 579.
5666 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
Causi et al.
mu = 0.09995
.am
0.02276 0.02698
1.332 1.432 1.483 1.575 I .669
0.033 13 0.04259 0.05379 0.06427 0.07608
1 2.019 2.275 2.524 2.785
0.08629 0.09677 0.1075 0.1186 0.1290
3.042 3.295 3.556 3.827 4.080
0. I 86 0.589 0.909 1.241 1.450
0.02105 0.02519 0.03403 0.043 13 0.05275
1.546 1.639 1.836 2.041 2.261
mu = 0.1998 0.06228 0.07310 0.08403 0.09488 0.106 IO
2.482 2.737 2.998 3.259 3.530
0.1277 0.1394 0.1523 0.1679
4.058 4.345 4.661 5.038
0.00159 0.00642 0.01040 0.01354
0.147 0.589 0.934 1.199
0.01556 0.01 804 0.01977 0.02204
I 368 1.497 I3 8 1.599
mu = 0.4946 0.02429 0.02686 0.03227 0.04045
1.651 I .7oa 1.831 2.016
0.04986 0.05924 0.06840 0.07918
2.231 2.450 2.672 2.919
0.00151 0.00303 0.00448 0.00627 0.008 I 5
0.142 0.280 0.4 10 0.567 0.729
0.00985 0.01 269 0.0162 1 0.021 79 0.02775
0.874 1.111 1.389 1.640 1.781
mu = 0.9985 0.03363 0.04741 0.05855 0.07046
1.913 2.228 2.491 2.773
0.08430 0.09635 0.1 101 0.1 332
3.100 3.393 3.370 4.302
0.00407 0.00703 0.00974 0.01294
0.362 0.615 0.840 1.099
0.01532 0.01770 0.02074 0.02503
I 288 1.473 1.675 1,802
0.05459
1.93 1 2.096 2.270 2.489
0.06543 0.08016 0.09786 0.1173
2.744 3.096 3.523 3.996
0.00163 0.0041 3 0.00621 0.00839 0.01 342 0.01588
0. I42 0.354 0.526 0.702 1.097 I 284
0.01873 0.02083 0.02374 0.02735 0.03230
1.498 1.652 1.816 1.920 2.040
mu = 2.9980 0.03762 0.04498 0.05430 0.06396 0.07464
2.164 2.333 2.549 2.774 3.026
0.08710 0.0991 1 0.1 113 0.1243 0.1368
3.322 3.61 1 3.905 4.216 4.516
0.00249 0.00510 0.00778 0.01052 0.01253 0.01494
0.201 0.405 0.608 0.812 0.959 1.131
0.01 722 0.01991 0.02244 0.02542 0.02853 0.03152
1.293 1.480 1.653 1.852 2.017 2.106
mu = 5.oooO 0.03622 0.04288 0.04848 0.05183 0.06085 0.06 184
2.224 2.380 2.515 2.582 2.787 2.818
0.07023 0.09580 0.1034 0.1211 0.1377
3.001 3.325 3.505 3.921 4.3 13
0.00638 0.01297 0.01962
0.476 0.934 1.374
0.02894 0.03679 0.04447
1.957 2.292 2.474
mu = 6.9994 0.05255 0.061 37
2.653 2.846
0.08210 0.1008
3.302 3.715
0.00587 0.01 185 0.01868
0.396 0.776 1.190
0.02541 0.03272 0.03976
1.580 I ,984 2.341
mu = 9.9886 0.05093 0.06448 0.07901
2.629 2.913 3.210
0.09306 0.1 116 0.1288
3.497 3.877 4.230
0.00508 0.009 1 1 0.0 1044 0.01 186 0.01 194 0.013 14
0.478 0.835 0.95 1 1.073 1.080 1.182
0.00198 0.00634 0.01Ooo 0.01391 0.01727
0.01495 0.01691
0.01881
mu = 2.0004 0.03048 0.03761
0.04518
Units arc mol kg-' for concentrations; cm-I ohm-l for specific conductivities. TABLE II: Critical Mkeik Concentration and Degree of Ionization of Mi& of D0dceylMmetLyl.wnal.a Bromide in Water-Urea Mixturd at 298 K m. cmc B mu cmc B
ob 0.09995 0.1998 0.4946 0.9985
0.0155 0.0158 0.0159 0.0166 0.0179
0.25 0.26 0.26 0.26 0.27
2.0004 2.9980 5.oooO 6.9994 9.9886
0.0203 0.0227 0.0276 0.0326 0.0394
0.29 0.30 0.32 0.34 0.35
On increasing the urea concentration (Figure 2), 6 increases tending to a maximum value at high m,. In spite of the extensive literature on the effect of additives*)' on /3 values, t h m arc few data concerning additives nonpenetrating in micelles, like urea, and, as far we know, no data are available for DTAB. However, the change of /3 for DTAB with urea concentration is comparable with that reported in the same concentration range for sodium decanoate in water-methanolz9 and about 5 times smaller than that for dodecylammonium chloride in ~ater-acetone~'while 6
'Units arc mu and cmc, mol kg-l. bCalculatcd from data in ref 1.
which, unfortunately, cannot be used for this system because the aggregation number and the counterion conductivity as functions of urea concentration are not known. However, the present 0 values are consistent with that for DTAB in water where both methods gave close values.'
(29) Vikingltad, E.; Kvammen, 0.J . Colloid Inter oca SEI.1980,74, 16. (30) Lawn, J. W.; Tepley, L. B. J. Colloid Inlet& Scl. 1974,49, 113. (31) Boatram, 0.;Backlund, S.; Blobus, A. M.; Hoiland, H. J. Colloid 'nterfacr lM9* 128v 179* (32) Zana, R.; Yiv, S.;Straziclle, C.; Liana, P.J . Colloid I n t e r , i Scl. lsl,80, 208. ( 3 3 ) Abu-Hamdiyyah, M.;Kumari, K. J . Phys. Chrm. 1990, 94, 6445.
DTAB in Water-Urea Mixtures TABLE III: V o h and H u t Capacities of Urea in Water at 298 K" m 1@M V+ V2 -103Au/uo C+ 0.09301 1.414 44.25 44.28 2.06 91.5 0.2010 3.172 44.26 44.31 4.50 90.0 0.3000 4.700 44.31 44.33 6.38 94.1 0.7392 11.333 44.35 44.45 15.95 91.1 1.0002 15.093 44.43 44.51 20.59 95.3 1.9598 38.92 28.046 44.51 44.73 95.2 2.9941 41.153 44.61 44.91 56.44 96.6 4.1211 53.936 44.1 1 45.06 70.35 102.0 4.9902 62.877 44.78 45.14 83.60 101.0 5.9551 11.549 44.85 45.20 93.81 103.5 6.9995 8 1.422 44.91 45.21 105.6 104.3
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5667
~
C, 91.9 92.4 92.9 94.9 96.1 100.1 104.0 101.6 109.9 112.1 113.9
"Units are mol kg-' for m;g cm-3 for densities; cm3 mo1-I for volumes; J K-I mol-' for heat capacities. TABLE I V Volumes and Heat Capacities of l M e c y ~ y l a " o n i u mBromide in Aqueous Urea Solution 0.3OOo 0 at 298 K" m 103M V, VI -l@Au/uo C. C, 0.01002 0.195 288.32 288.35 0.01085 0.210 288.31 288.41 0.36 1051 1061 0.0 I 190 0.232 288.31 288.39 0.39 1058 1062 0.42 0.01214 0.247 288.39 288.38 1057 1062 0.47 0.01441 0.280 288.32 288.35 1058 1063 1.26 0.01192 0.318 289.99 294.98 908.5 134 0.01997 0.338 290.80 295.04 1.54 883.3 715 0.03001 0.41 1 29 1.98 295.20 2.53 851.4 680 0.03998 0.514 293.29 295.28 4.21 714.0 651 0.05993 0.822 293.85 295.30 6.84 136.6 630 0.08003 1.063 294.21 295.30 9.85 697.5 61 1 0.09988 1.293 294.48 295.30 12.40 691.0 596 0.1500 1.889 294.66 295.30 19.79 650.5 511 0.1998 2.438 294.88 295.30 21.38 621.0 560 0.2492 2.991 294.92 295.30 33.90 611.4 558 0.2998 3.5 13 295.05 295.30 41.16 580.8 561 0.3491 4.048 295.02 295.30 41.62 599.5 548 0.3991 55.45 4.564 295.04 295.30 580.8 534 0.4957 5.51 1 295.06 295.30 67.91 512.4 530 0.6000 6.470 295.1 1 295.30 80.37 570.0 530
For units see Table 111.
"For units see Table 111. for sodium dodecyl sulfate in H20-D20 does not depend on The increase of the release of the counterion binding with increasing additive concentration can be ascribed to a decrease of the charge density at the micellar surface due to the decrease of the aggregation number of the micelle. This correlation accounts also for the temperature effect9J2on a given surfactant and for the alkyl chain lengthMin a homologous series. The @ values as functions of urea concentration are reported in Table 11. The apparent molar volumes (V,) and heat capacities (C,) of urea in water and of DTAB in water-urea mixtures of fixed composition were calculated by means of the following equations
M V*=-d
103(d - do) mddo (3)
where m and M a r e the molality and the molecular weight of the solute, respectively. The V, and C, values of urea in water together with the excess densities (Ad = d do) and the relative differences in heat capacities per unit volume are collected in Table 111 while the comsponding properties for DTAB in the aqueous urea solutions are listed in Tables IV-VIII.
-
~~
~
Miyaghhi, S. Bull. Chem. Soc. Jpn. 1975, 48, 2349. Chang, N.J.; Kaler, E. W. J . Phys. Chem. 1985,89,2996. (36) De Liri, R.;Milioto, S.;Triolo, R.J . Solullon Chem. 1988, 17,673.
(34) (35)
TABLE V Volumes a d H u t Capacities of Dodecyltriwtbyhmmoaium Bromide in Aqaeoua Urea solation 1.0002 m a t 298 K ' m 103M V, -103Au/uo V+ C+ Ca 0.01199 0.192 288.91 288.90 0.46 1023 1032 0.01398 0.221 289.10 288.84 0.53 1026 1033 0.01500 0.238 289.12 288.81 0.01608 0.251 288.93 288.18 0.60 1026 1034 0.01699 0.271 288.95 288.15 0.65 1030 1035 0.02002 0.293 290.23 295.00 0.87 1008 693 0.02990 0.402 291.36 295.30 2.32 813.4 666 0.03985 0.462 293.15 295.45 3.68 819.6 652 0.05999 0.642 293.91 295.60 6.52 153.6 630 0.07991 0.810 294.41 295.60 9.58 711.8 615 0.1094 1.079 294.66 295.60 13.10 680.9 600 1.335 294.90 295.60 0.1400 18.16 658.6 590 0.1699 1.596 294.91 295.60 22.14 651.5 584 26.51 0.1998 1.827 295.14 295.60 631.6 519 0.2493 2.235 295.20 295.60 33.14 628.9 571 39.61 0.2990 2.645 295.20 295.60 622.0 516 3.043 295.23 295.60 0.3502 46.40 614.7 564 0.4OoO 3.414 295.21 295.60 53.13 605.5 554 0.4493 3.184 295.28 295.60 59.16 602.9 550 0.4998 4.148 295.29 295.60 65.60 596.8 546 0.5488 4.481 295.31 295.60 71.63 592.5 545 0.5913 4.845 295.21 295.60 11.21 590.3 545
TABLE VI: Volumes rad Heat Capacities of Dodccyltrimetbyhmmonium Bromide in Aqueous Urea Solution 2.9941 m at 298 Kg m 103M V+ V, -103Au/uo C+ C, 0.01095 0.072 290.84 290.41 0.53 960.9 951 0.0 1499 0.100 290.74 290.10 0.72 960.4 956 0.01193 0.125 290.45 289.87 0.87 957.2 954 0.01919 0.135 290.58 289.13 959.6 953 0.95 1.30 0.02451 0.150 291.24 295.26 942.1 696 0.02938 0.153 292.08 295.48 1.86 906.5 678 0.03915 0.163 293.05 295.10 3.24 835.0 641 4.54 0.04889 0.171 293.67 295.95 191.1 621 7.33 0.06863 0.184 294.42 296.12 744.6 606 11.81 0,09891 0.208 294.95 296.16 691.4 592 0.1294 0.236 295.20 296.18 15.95 615.3 593 19.14 0.1509 0.249 295.38 296.18 658.1 596 0.1749 22.64 0.282 295.38 296.16 644.8 601 0.1982 0.280 295.51 296.12 25.60 643.2 606 26.90 0.2085 0.294 295.56 296.12 642.2 609 0.2344 0.281 295.14 296.10 3 1.53 616.1 410 0.2462 0.339 295.58 296.08 610.4 504 33.33 0.2794 0.331 295.14 296.06 0.2915 0.319 295.67 296.04 40.51 599.1 583 0.3194 0.380 295.14 296.02 42.85 604.9 595 0.3491 0.416 295.13 296.00 599.9 602 46.86 0.3950 51.61 0.469 295.12 295.94 607.9 602 0.4356 0.496 295.16 295.90 56.69 604.0 602 0.4931 0.571 295.72 295.80 63.10 605.3 602 0.5443 69.13 0.601 295.76 295.15 596.6 602 0.60 11 0.651 295.16 295.10 15.36 600.9 602 0.6181 0.660 295.18 295.68 18.02 594.1 602
For units see Table 111. V, and C, of urea in water were fitted by means of the following equation
V, Y O 2 + B f l , + C*: (4) where Yo2is the standard (infinite dilution) partial molar property and BY and Cy are fitting parameters which, according to the McMillan-Mayer ap~roach,~' account for the pair and triplet soluttsolute interactions. The following equations were obtained V, (cm3 mol-') = 44.25 (0.01) [0.14 (0.01)]mU - [0.007(O.OOl)]m,2 ( 5 ) C, (J K-'mol-') = 91.4 (0.9) + [2.5 (0.8)]mU- [0.08 (0.1 l)]m,2 ( 6 )
+
(37) McMillan, W.;Mayer, J. J. Phys. Chem. 1945, 13, 176.
5668 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 TABLE VU: Volume8 and Heat C8paclties of I h u k c y b h t h y h d m m BromMe in Aqrcola Urea SdVtiOn 4.9902 m at 298 Ka m -10)M .V V, -103Au/u~ C. C,,. 910.8 909 0.00999 0.020 292.66 292.20 0.57 0.75 0.01296 0.022 292.39 291.95 906.6 909 0.88 0.01500 0.028 292.54 291.77 906.6 899 0.01999 0.026 292.03 291.33 1.11 913.8 907 1.15 0.02000 0.033 292.36 291.33 908.0 907 0.02398 0.027 291.88 290.99 1.44 906.0 906 0.02498 0.034 292.10 290.90 1.89 0.03000 0.052 292.43 295.40 886.8 820 2.47 0.03492 0.095 293.32 295.65 862.4 685 3.21 0.122 293.62 295.80 827.9 660 0.04002 4.51 0.04992 0.183 294.19 296.00 792.8 637 5.92 0.238 294.47 296.20 762.0 624 0.05992 8.71 0.07985 0.374 295.14 296.35 724.4 608 11.61 0.09999 0.489 295.36 296.45 696.5 600 18.38 0.1495 0.778 295.72 296.40 665.7 592 24.95 0.1999 1.056 295.87 296.40 651.8 591 3 1.53 640.5 594 1.317 295.92 296.38 0.2503 32.90 0.2581 1.390 296.05 296.38 634.1 594 37.91 0.2994 1.591 296.05 296.35 631.3 598 44.12 626.8 605 0.3501 1.861 296.12 296.32 0.3747 1.982 296.13 296.30 0.3998 2.074 296.07 296.30 51.53 607.9 524 2.269 296.00 296.27 0.4490 603.3 548 57.56 0.4500 2.335 296.14 296.27 0.5018 2.554 296.11 296.25 63.60 602.1 567 68.75 0.5493 2.723 296.04 296.25 601.8 580 74.88 0.5983 2.966 296.11 296.20 595.1 591
.For units see Table Ill. TABLE VIII: Vdracr and H u t Caprcitiea of IbdecylMrctLyhmmoeiamBromide in Aqwole Urea Solution 6.9995 m at 298 K ' m -103M V+ -lO.'Aa/oo C+ VZ cn 0.59 892.8 895 0.01000 0.098 294.32 293.97 0.01498 0.143 294.11 293.65 0.88 892.1 892 1.16 892.0 889 0.01986 0.183 293.84 293.33 1.18 890.7 889 0.01999 0.190 294.09 293.32 0.02492 0.228 293.80 293.00 1.47 889.5 888 1.75 890.5 882 0.02996 0.269 293.66 292.68 0.03490 0.319 293.91 293.64 2.51 841.4 720 2.17 878.5 720 0.03500 0.321 293.84 293.64 3.08 824.9 650 0.03998 0.364 293.80 295.30 3.41 830.3 635 0.04495 0.426 294.13 295.60 5.78 759.7 620 0.05988 0.626 295.02 296.10 0.06619 0.687 294.98 296.20 8.32 722.5 612 0.07803 0.826 295.20 296.40 0.1001 1.080 295.44 296.50 11.17 703.1 606 0.1496 1.651 295.81 296.50 17.88 669.6 602 0.1993 2.212 296.02 296.50 24.60 649.5 602 2.606 295.98 296.50 0.2384 0.2701 2.985 296.19 296.50 33.32 640.8 602 35.68 638.4 602 0.2895 3.170 296.15 296.50 0.3252 3.492 296.06 296.45 40.29 630.8 440 44.70 612.5 490 0.3493 3.791 296.26 296.45 0.3597 3.887 296.23 296.45 46.33 607.7 550 0.4018 4.223 296.06 296.45 51.15 607.1 586 0.4500 4.730 296.20 296.45 57.19 601.2 580 0.4980 5.194 296.26 296.42 59.93 601.6 574 0.5968 6.062 296.21 296.40 71.15 592.9 572
'For units see Table Ill.
where the values in parentheses indicate the uncertainties. Whenever it is possible to make mparisons, these results agree very well with those reported in the literature." The partial molar of urea in water, reported in Table 111, were properties (Y2) calculated as functions of the concentration by means of eqs 5 and 6 according to Y, = a(mY,)/am (7)
Causi et al.
x
t
44 4
*
sb 30
t
1 .
I
.
I
.
0.08
0.04
.+
I
0.12
xu
Figure 3. Debyc-Hiickel limiting slope for heat capacity as a function of urea mole fraction. Closed symbol, from experimental data; open symbol, from interpolated data (see text).
The standard partial molar volume and heat capacity of DTAB in the given water-urea mixture were derived from the apparent molar properties in the premicellar region according to eq 8, where
AYis the DebyeHUckel parameter and ms the surfactant concentration. The DebyeHUckel limiting slope values for heat capacity (Ac) have been calculated from the ATvalues, Le., the DebytHUckel limiting slope for activity coeffkient, at different reported by K u d u and temperatures and urea mole fraction (Xu) M a ~ u m d a r .At ~ ~a given urea concentration,data have been fitted as A, = a
+ bT + cT2
(9)
where the fitting parameters (I,b, and c are functions of urea concentration. Therefore, Ac values were calculated by means of the following equations" aA, 4 X 2.3Rl"' AL=(TF) 3 Ac = aAL/aT
(11)
where AL is the limiting Debye-Hackel slope for the apparent molar relative enthalpy. The 4values at 298 K as a function of Xuare shown in Figure 3. The peculiar trend of Ac vs Xusuggested us to calculate Ac at Xuvalues lower than the experimentalones. Therefore, A, were interpolated from the plot of A, vs Xuat all temperatures. The Ac values calculated as above are shown in Figure 3 (open symbols). As can be seen, a maximum is present at about 0.03Xu; the amplitude of the maximum excludes that it is due to the uncertainties involved in the calculation method. From the plot shown in Figure 3, the Ac values at the urea composition of interest have been interpolated; they are reported in Table X. The limiting DebytHUckel slope for the volume (Av) cannot be evaluated since the dependence of A, on pressure and on the urea concentration is unknown. Desnoyers et a1.18 observed that the effect of the addition of 3.4 mol of urea on the standard partial molar volume of nonyltrimethylammonium bromide (NTAB) is equivalent to the increase of temperature by 8 O C . According to this idea, which does not agree with the Ac vs Xuplot, the AV values for the water-urea compositions of intereat at 298 K were calculated from the dependence of AVin pure water on temper~~
(38) Derrorierr, N.;Perron, 0.;Mathieson, J. 0.;Conway, 9. E.; Desnoyen, J. E. J. Solutlon Chem. 1974, 3, 789.
40
__
~~~
~
(39) Kundu, K. K.; Mazumdar. K. J. Chem. Soc..Faraday Trans.11973,
69, -806.
(40)Woolley, E. M.;Burchfield, T. E. J. Phys. Chem. 1984,88, 2155.
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5669
DTAB in Water-Urea Mixtures
44.8
-
44.6
0 m, = 0.05 (exp)
44.4
m,=0.08 A m,=0.12 X
+ *=0.20 A m, = 0.20 (exp)
44.2 I
.
I
.
I
2
1
I
.
3
4
.
I
I
5
6
I
7
mu / m d kg-1 Figure 4. Apparent molar volume of urea in dodecyltrimethylammoniumbromide micellar solutions at fixed concentrations vs urea concentration.
A ms =0.12
+
i'
ms=0.20 A ms = 0.20 (exp)
1
2
3
5
4
6
7
mu /mol kg-1 Figwe 5. Apparent molar heat capacity of urea in dodecyltrimethylammonium bromide micellar solutions fixed concentrations vs urea concentration. ature." Within the uncertainties, the AVvalues so evaluated do not depend on the added urea. Therefore, in eq 7 the value of AV in water'l was introduced. The values of Yo2 and BY obtained for the volumes and heat capacities of DTAB in the water-urea mixtures are listed in Table
X. In the premicellar region the partial molar properties have been calculated by means of the following equation
Y2 = Y o 2 + 1.5Afls'/2
+2Bvs
(12)
while in the pastmicellar region they were obtained from the plots of Y4 as a function of the molality by drawing the best curve and, then, by calculating the partial molar quantities from points interpolated at regular intervals as Y2= A(mY4)/Amaccording to q 7. The partial molar properties are collected in Tables IV-VIII. The apparent molar volumes (V,") and heat capacities (C4,J of urea in water-DTAB mixtures at tied surfactant concentration have been calculated as functions of urea concentration by using
the procedure and data reported Some Y4,uvalues are collected in Table IX and shown in Figures 4 and 5. As can be seen, by excluding C4,uvalues at low mu for some ms values, vs mu at fixed ms linear correlations have been obtained for Yo," and for Y4,uvs ms at fixed mu (not shown) in the entire range of concentration analyzed. Deviations from the linear trend of C,,,, vs mu can be ascribed to the complexity of C4 vs mS curve involving uncertainties which become more important by decreasing additive and surfactant concentrations. For this reason, density and heat capacity measurements of urea in 0.05 and 0.2 mol kg-' DTAB as functions of concentration were carried out. The Y4,uvalues, calculated from these experimental data, are also reported in Table IX and shown in Figures 4 and 5 . As can be seen, no deviations from linearity have been found. Discussion Urea io Micellu Solutha The dependence of the cmc on the additive concentration (mu)has been correlated to the distribution De Liii, R.; Milioto, S.J. Solution Chem. 1987, 16, 767. (43)De Lisi, R.; T u m Lived, V.;Castagnolo, M.; Inglese, A. J. Solutlon Chem. 1986, 23, IS. (42)
5670 The Journal of Physical Chemistry, Vol. 95, No.14, 1991
0.8
1
A
Causi et al. 44.9 44.7 L
s
2
44.5
0-
>' 44.3
44.1
0.2
0.1
1
2
3
4
5
6
7
8
9
mu/ mol kg-'
Figure 6. Dependence of the critical micelle concentration on the urea concentration.
0.3 0.4 mS 1 mol kg-1
0.5
Figure 7. Dependence of the standard partial molar volume ( Vuo)and heat capacity (Ch0) of urea in dodecyltrimethylammonium bromide micellar solutions on the surfactant molality.
constant of the additive between the aqueous and the micellar phases ( K ) and to the Setchenov constant (Ks), which reflects the additive-unmicellized surfactant interactions in the aqueous by the equation
In
cmc,
= cmc,,,
2.3Ks
+ (1 + p ) K (1 +v)
mu
(13)
where c m c and cmc,,, represent the critical micelle concentrations in water and wateradditive mixture, respectively; /3 and Y are the degrees of ionization of the micellized and the unmicellized surfactant, respectively. Equation 13 has been applied to several water-surfactantadditive ternary systems for sufficiently low additive concentrations. The cmc increases or decreases depending on the nature of the additive. For example, the cmc values of different surfactants decrease on addition of medium chain while they increase if urea,11,46dimeth lurea,'*l' dimethylformamide," acetamide,'JI sucrose,' acetone,' *I1 and dioxane7are used. These opposite behaviors can be explained by examining the two contributions which appear in eq 13. While K is always positive, Ks can be positive or negative. As a consequence, an increase of the cmc with the additive concentration implies that the contribution related to the negative Setchenov constant predominates over that of distribution while a decrease of the cmc can be due to both contributions when Ks is positive or to the predominance of the distribution term on that of the interaction when Ks is negative. As is shown in Figure 6, a linear relation between the term at the left-hand side of eq 13 and the urea molality is satisfied up to 3 mol kg-l urea. On the other hand, as mentioned above, eq 13 is valid only for low additive concentration and this is not the case here. By applying eq 13 to the experimental data up to 3 mol kg-I urea and by using the literature value of -0.18 for Ks" and the averaged value of 0.28 for 8, we obtain K = 0.12. This value, which agrees with that of 0.19 calculated from data reported by Treiner," seems to exclude the solubilization of the urea in the micellar phase as suggested by Desnoyers et ala* for 0.2 mol kg-I urea in octylammonium bromide. The trends of the standard thermodynamic properties of urea ( YOy)in DTAB micellar solutions as functions of ms (Figure 7 ) lead to the same conclusion. In fact, the curve of the standard partial molar property of the additive in the micellar solutions as a function of mS can be simulated" through the distribution constant and the corre-
l
(44)Treiner, C. J . Colloid Inrerface Sci. 1983, 93, 33. (45) Shirahama, K.; Kashiwabara, T. J . Collold Interface Sei. 1971,36, 65. (46) knoyers, J. E.;HCtu, D.; Perron, 0. J . Solurion Chem. 1983, 12, 427. (47) De Lisi, R.; Milioto, S.Colloid Surf 1989, 35, 309.
0.1
0.2
0.3
0.4
0.5
0.6
mS /mol kg.1
Figure 8. Apparent molar volume of dodecyltrimethylammonium bromide in some water-urea mixtures as a function of the surfactant molality. 4 mu = O.?
+mu=7
."+e:. 0 0.1
0.2
. 0
0.3 0.4 0.5 mS / mol kg-1
4
0.6
Figure 9. Apparent molar heat capacity of dodecyltrimethylammonium bromide in water-urea mixtures as a function of the surfactant molality.
sponding properties of the additive in the aqueous and in the micellar phases. The lack of a curvature for Yo, vs ms indicates that urea is not distributed between the aqueous and the micellar phases. The fact that this linearity is observed at such high urea concentrations seems to suggest that urea is dissolved in the aqueous phase even at high mu.
DTAB in Water-Urea Mixtures
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5671
TABLE I X Apparent Mdrr Volulacs rad H u t Capacities of Urea in Aqueous Dodccyltrimetbyhmmonium Bromide Solutium at 298 K O
0.21226 0.2953 0.50516 0.9844 1.0498 2.9532
m, = 0.05141 dn = 0.991854, Cpn= 4.15001 44.32 96 3.2w 44.36 99 4.9220 44.43 92 5.2213b 44.41 91 6.8901 44.46 96 1.2884b 44.58 100
44.69 44.10 44.85 44.15 44.98
100 103 103 106 101
d
dn = 0.998231, Cpo = 4.13458 44.36 93 4.8196 44.71 44.42 94 6.8315 44.15 44.59 99
0.2893 0.9643 2.8930
m, = 0.12 do = 0.998154, Cpn = 4.10819 44.35 81 4.8216 44.12 44.42 94 6.1503 44.15 44.60 99
-k E
.
800
cr
103 106
700 600
f
4
4
I I l ' r ' l ' l . 1 ' 1 ' 1 '
5
10
15 20
103 106
25
30
35
mslcmc
Figure 10. Apparent molar heat capacity of dodecyltrimethylammonium bromide in water-urea mixtures as a function of the reduced concen-
m, = 0.1996
m,
900
&
L?
0.2928 0.9159 2.9218
dn = 0.999185, Cpn= 4.05241 44.32 81 2.9505b 44.41 95 4.1102 44.42 94 5.2134b 44.49 99 6.5942 44.44 95 6.6 184b 101 44.64
+ mu=7
1mt
m, = 0.08
0.2826 0.291 I b 0.5 1396 0.9420 1.014ob 2.8260
1%
0 mu = 0.3
tration. 44.10 44.15 44.89 44.11 44.99
101 104 105 101 101
297
295
= 0.25
0.2185 0.9284 2.1853
dn = 1.000384, Cpo = 4.02683 44.42 I1 4.6422 44.15 44.48 93 6.4991 44.18 44.65 99
0.2108 0.9026 2.1018
m, = 0.35 do = 1.001594, Cpo= 3.96424 44.58 69 4.5130 44.19 44.55 91 6.3182 44.81 44.69 98
0.2635 0.8182 2.6345
m, = 0.45 dn = 1.002141, Cpn= 3.90398 44.61 65 4.3908 44.81 44.61 98 6.1412 44.83 44.12 100
0.2599 0.8664 2.5993
m, = 0.50 dn = 1.003293, Cpo = 3.81662 44.11 59 4.3322 44.84 44.64 98 6.0651 44.84 44.82 100
0.2565 0.8550 2.5651
m, = 0.55 dn = 1.003831, Cpo= 3.84981 44.15 IO 4.2151 44.84 44.61 91 5.9851 44.85 44.15 101
103 106
293 29 1
104 101
289 1
105
101
105 106
104
101
#For units see Table 111. bExperimental points.
Vdunns and Heat Capacities. Figures 8 and 9 show examples of the plot of V, and C, of DTAB in water-urea mixtures as functions of the surfactant concentration. In the premicellar region V, decreases with increasing mg, the slopes being, within the uncertainties (see Table X),independent of mu. In this region, C, values increase with mg at low mu and decrease at high mu. As expected, beyond the cmc V, increases and C, decreases tending to level off at high concentrations. The plots of C, vs ms show anomalies around 0.30.35 mol kg-I DTAB whose amplitude is always about 30 J K-'mol-' such as observed in pure water.42 Only for mu = 3 the peculiarity is found at 0.21 m. According to the literature," these peculiarities can be attributed to a micellar structural transition. On increasing the urea concentration, the V, vs ms curves are shifted toward larger values while C, vs ms curves are shifted toward lower values in the
2
3 4 5 mu/ mol kg-1
6
1
Figure 11. Partial molar volume of dodecyltrimethylammonium bromide in the micellar phase (Vs),in the aqueous phase (Vm),and in the standard state (V,") vs urea concentration.
premicellar region and toward higher values in the micellar region. So, it seems that for this latter property urea acts in opposite
directions depending on the surfactant region analyzed. Since this behavior could be a consequence of the dependence of the cmc on the urea concentration, in order to visualize more correctly the urea effect, Y, was plotted against the reduced concentration, Le., the ratio between the surfactant concentration and the cmc (ms/cmc). So, with ms/cmc as abscissa scale, the V, curves are again shifted toward higher values (not shown) while the C, curves are always shifted toward lower values (Figure 10). It is to be noted that for both properties urea strongly affects the premicellar region while hardly does the micellar one. As it is shown in Figures 11 and 12, the standard partial molar volumes ( P2)of DTAB in water-urea increase linearly with mu while the heat capacities (Cho) decrease with a small curvature. The dependence of Yo2 on mu reflects only the solutesolvent interactions since the standard partial molar property is given by the intrinsic and the solutesolvent interaction contributions. According to the literat~re,'~J~*'* these trends prove the role of urea in reducing the amount of ordered water around the hydrophobic groups of the surfactant. According to the Friedman-Krishnan the standard thermodynamic functions of transfer of DTAB from water (w)
d
(48) Benjamin, L. J. Colloid Inter ace Sei. 1966, 22, 386. (49) Friedman, H.L.; Kriahnan, V. J . Solution Chem. 1973, 2, 119.
5672 The Journal of Physical Chemistry, Vol. 95, No. 14, 1991
288.4b 294.9b 6Sb 2Y
-312
104Y 181oc
llloc 560" -550
Causi et al.
288.2 f 0.1 -8 f 12 288.3 295.30 7.0
288.9 f 0.4 -9 f 30 288.7 295.60 6.9
291.1 f 0.3 -45 f 17 289.4 296.35 6.9
293.0 f 0.2 -50 11 290.7 296.55 5.9
*
294.5 f 0.1 -39 f 6 292.4 296.55 4.2
29 1052 f 3 200 f 231 1064 563 -501
36 1008 f 7 912 457 1046 574 -472
33 962 f 4 -402 f 227 95 1 580 -371
30 908 f 5 -186 f 281 905 585 -322
27 892 f 6 -245 & 50 889 600 -289
'From ref 41. bFrom ref 43. CFromref 42. dCalculatcd from data in ref 42.
1
2
3
4
5
6
7
mu /mol kg-1 Figure 12. Partial molar heat capacity of dodecyltrimethylammonium bromide in the micellar phase (C,), aqueous phase (CPJ and in the standard state (Cn0)vs urea concentration.
to urea-water (w + u) mixtures can be expressed as functions ~ triplet YDTABY-,, of urea molality through the pair Y D T Aand interaction parameters
-
AYDDTA*(W w
+ u) = ~ Y ~ A B - u M+u3YDTAh-m: + (14)
On the basis of the results shown in Figures 11 and 12, the triplet DTAB-urea-urea interaction parameter is negligible for volumes while not for heat capacities. The pair DTAB-urea interaction rameter for volume has been evaluated to be 0.44 an3kg mol- This value is close to those we have evaluated from literature data for different electrolytes: 0.50 and 0.47 cm3 kg mol-' for nonyltrimethylammonium bromide (NTAB)" and tetrabutylammonium bromide (Bu4NB),%respectively. For heat capacities the values of -21 J K-'kg mol-' and 0.90 J K-' kg2 mor3 have kcn calculated for YmA, and YT-, respectively. ~ is close to that of -25 J K- kg mol-2 obtained The Y m Avalue for Bu4NB and twice that derived for NTAB (-13 J K-'kg mol-2). So, it seems that the nature of the electrolyte plays a more important role on the contribution to heat capacity due to the unlike solute molecules interaction than it does on the contribution to volume. The partial molar volume (V,) and heat capacity (Cp ) of the unmicellized surfactant at the cmc as functions of mu (bigures 11 and 12) reflect essentially the behaviors of the corresponding standard property. In particular, C is practically equal to Ch0 since Ac and Bc (see eq 12) are ne3igible at low mu and com-
pa.
pensate at large 4. On adding 1 mol of urea, PPz and V, increase by about 0.9 and 0.6 cm3mol-', respectively; these values are about twice those that we have evaluated'8 from the data of NTAB in water and in 3.4 m water-urea mixture. Upon the addition of 1 mol of urea, the CH2group contribution to V, and v02 increase by about 0.15 and 0.1 cm3mol-', respectively. Other comparisons cannot be made because of the lack of literature data. As far as heat capacity is concerned, Singh et al.I9 studied sodium dodecyl sulfate (NaDS) in a 3.4 m urea solution at 303 K,and hexadecylpyridinium bromide (CPB) and hexadecylpyridinium chloride (CPC) in a 2.2 m urea solution at 313 and 303 K,respectively, while Desnoyers et a1.I8 have investigated NTAB in urea solution 3.4 m as a function of temperature. From these data, it can be derived that, on adding 1 mol of urea, C decreases by 14, 30 f 1 1,85 f 11, and 120 f 40 J K-'mol-' f o k A B , NaDS, CPB, and CPC, respectively. To a first approximation, despite the different nature of the polar head and counterion of the surfactant, and different temperature and urea concentration, the above data seem to indicate that the urea effect on Cpmdepends essentially on the surfactant alkyl chain length. To confirm this hypothesis, the C values of DTAB at 2.2muand 3 . 4 4 were calculated from the faowing polynomial function of C, vs mu Cpm= 1098 - 62mu + 4.6m:
(15)
From the C values at 0, 2.2mu, and 3.4mu, the addition of 1 mol of urea k d s a Cpmchange of -51 and -46 J K-' mol-' at 2.2mu and 3.4mu, respectively. It is to be noted that, from the derivative of eq 15, the urea effect on Cpmis different. Within the uncertainties, the value of -46 J K-'mol-' can be considered to agree with that of -30 J K-'mol-' for NaDS. From the values for NTAB and DTAB, the urea effect on the CHI contribution is -1 1 J K-'mol-' from which we can calculate the value of -95 J K-'mol-' for hexadecyltrimethylammonium bromide at 2.2mu. Since this value agrees with those for CPB and CPC, the above hypothesis, following which the urea effect is essentially sensitive to the apolar moiety of the surfactant, is supported. As shown in Figures 11 and 12, the partial molar volume of DTAB in the micellar phase at the cmc ( Vs)depends on the urea concentration up to 1 m and tends to a constant value at larger concentrations. In the first region, the addition of 1 mol of urea leads to an increase of V ,of 0.7 cm3 mol-' which is very close to that found for V,. As mentioned above, volumetric literature18 data concem only NTAB in urea solution 3.4 m. From these we have evaluated the increase of V,of 0.08 cm3 mol-' per mole of added urea which results in a value smaller than that of 0.44 cm3 mol-' for DTAB that we have derived from the value in water and that in mu = 3.4. The heat capacity of the micellid surfactant at the cmc (C,) increases in a linear manner upon the addition of urea with a slope of 5.1 f 0.6 J K-'mol-'. Within the uncertainties, the value of 5.1 J K-'mol-' for DTAB and NTAB18 can be considered equal to those of 14 i 8, 12 f 45, and 19 f 29 J K-' mol-' for NaDS,I9 CPB,I9 and CPC,I9 respectively. Therefore, it seems that the urea effect on C is not only independent on the nature of the polar moiety of B e sur-
5673
J. Phys. Chem. 1991,95, 5673-5676
7.5
-
The trend of AV, as a function of urea concentration is shown in Figure 13. If the value in pure water is excluded, the AV, does not depend on the urea concentration up to 3 mol kg'.The AV, in water reported in the literature (6.5 an3m01-l)'~ has been evaluated by taking for V, the value at infinite dilution instead of that at the cmc; from the plot of V, vs mu the extrapolated V, value in pure water is 288 cm3 mol-' and, hence, AV, is 6.9 an3mol-' in agreement with those up to 3 mol kg-I urea. Beyond this urea concentration, AV, strongly decreases. The weak effect of urea on AV, up to 3 mol kg-' is due to the parallel increase of V, and Vs with the urea concentration while the decrease reflects the fact that Vs tends to a constant value at high h.As far as the heat capacities of micellization (Figure 13) are concemed, they change with a curvature by increasing mu reflecting the behavior shown by C, since Cp,change linearly with 4. These results show that the interpretation of the properties of micellization can be ambiguous without knowledge of the prop erties of the surfactant in the aqueous and the micellar phases. In other words, this means that the interpretation of the thermodynamic properties of micellization obtained from the dependence of the cmc on temperature and/or pressure is not as effective as that based on direct determinations. Acknowledgment. We are grateful to the National Research Council of Italy (CNR, Progetto Finalizzato Chimica Fine 11) and to the Ministry of University and of Scientific and Technological Research (MURST) for financial support. Registry No. DTAB, 1119-94-4; urea, 57-13-6.
- -2s
1
2
3
4
5
6
7
qlm0llrg.l
Figure 13. Volume ),'IA( and heat capacity (AC,) of miccllization of dodecyltrimethylammonium bromide as functions of the urea concen-
tration. factant, as observed for Cpm,but also on the alkyl chain length. From Vs and V, values at the cmc, the properties of micelli=tion Aym can be On the basis Of the peudophase transition they are given by
AY,
Ys - Y,
(16)
(52)
(51) De Lk,R.; Pmon,G.; Dcanoym, J. E. Can. J. Chem. 1980,58,959.
433.
Dcsnoyers, J. E.; De Lisi, R.; Perron, G. Pun Appl. Chem. Iw#), 52,
Detrimental Effect of Excess Lecithin on the Stability of Fluorocarbon/Leclthln Emulsions Marie-Pierre Krafft, Jean-Paul Rolland, and Jean C.Mess* Laboratoire de Chimie MolPculaire, Unit8 AssociPe au CNRS, UniversitP de Nice Sophia-Antipolis, Parc Valrose, 06034 Nice, Cedex, France (Received: April 4, 1990)
The formation and stability of concentrated lecithin-stabilized emulsions of perfluordecalin (1005%and 70% w/v, Le. 5 1.5% and 36.1% v/v) have been studied for lecithin/fluorocarbon ratios ranging from 2 to 11. The stability was found to 80 through a maximum for a lecithin/fluorocarbon ratio of 4 4 % . Emulsions prepared with lecithin/fluorocarbon ratios higher than 6% lead, after 5 months storage at 4, 25, and 50 O C , to increasingly larger particle sizes and wider size distributions. This detrimental effect of an exccss of surfactant on the emulsions' stability is shown to be related to the presence of increasing amounts of fluorocarbon-empty lecithin-based aggregates. These aggregates were identified by ESR as consisting of free lecithin vesicles.
Introduction
recently been achieved with the development of significantly more efficient, concentrated sterilizable emulsions of perfluorooctyl bromide with egg yolk phospholipids as the surfactant. These emulsions are destined to be used as contrast agents and blood substitutes and for other biomedical applications.)-' The present paper is concerned with the influence of the lecithin/fluorocarbon ratio on the stability of highly concentrated (70% and 100% w/v, i.e. 51.5% and 36.1% v/v) fluorocarbon
The stability of the first, recently approved, injectable fluorocarbon emulsion, Fluos01,~is low: it requires the product to be stored and shipped in the frozen state. This is inconvenient, limits its u~cd,and carried some risk of mishandling during the thawing of the emulsion and admixing of the annex solutions used to obtain the final, injectable preparation. The development of oxygen-carrying injectable fluorocarbon emulsions having a shelf life compatible with routine use in hospitals, and the understanding of the factors that contribute to stabilizing such emulsions, are therefore among the primary objectives of research in this field.- Considerable advances have
(3) R i a , J. G. Proc. 2nd World Surfactant Congr. (Parls, May 1988)
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(4) Rims, J. 0.;Arlen, C.;Greiner, J.; LeBlanc, M.; Manfredi, A.; Pace, S.;V a m n , C.;Zarif, L. In BIoodSubstIrutes; Chang,T. M.S.,Geyer, R.
P., Eda.;,Dekker: New York, 1989; p 421. (5) RICSS,J. 0. Artv Or ns, in press. (6) Long, D. C.;Long, M.;R i a , J. 0.; Follana, R., Burgan, A,; Mattrey, R. F. Blomater. Art* Cells, Artlf. Organs 1988, 16, 441. (7) Mattrey, R. F. AJR, Am. J . Rocntpnol. 1989, 152, 247.
Naito, R.; Yokoyama, K. Technical I@ormatIon SerIes No. 5 and 7 , Perfluomchetnlcal BloodSubstItute&The Green Cmr Crop.:Osaka, Japan, 1978, 1981; Package Incert of Fluorol, 1990. (1)
h
(2) R i a , J. 0. Curr. Surg. 1988, I S , 5.
0022-3654/91/2095-5673$02.50/0
(a
1991 American Chemical Society
