Study on the Effects of Nonelectrolyte Additives on ... - ACS Publications

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Langmuir 2003, 19, 6668-6677

Study on the Effects of Nonelectrolyte Additives on the Phase, Thermodynamics, and Structural Changes in Micelles of Silicone Surfactants in Aqueous Solutions from Surface Activity, Small Angle Neutron Scattering, and Viscosity Measurements Saurabh S. Soni,† Nandhibatla V. Sastry,*,† Jayant V. Joshi,‡ Ekta Seth,§ and Prem S. Goyal‡ Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388 120, Gujarat, India, Inter University Consortium (IUC) for DAE Facilities, Mumbai Center, BARC, Trombay, Mumbai 400 085, India, and SSPD, Bhabha Atomic Research Center (BARC), Trombay, Mumbai 400 085, India Received March 5, 2003. In Final Form: May 22, 2003 Association properties of two silicone surfactants based on poly(dimethylsiloxane)-graft-polyethers in aqueous solutions of 2-butoxyethanol, poly(ethylene glycol), and glucose were determined using various techniques such as surface tension, small-angle neutron scattering, and viscosity. Dilute solution phase diagrams were also constructed, and cloud points were measured for different concentrations of both the surfactants in additive aqueous solutions. The thermodynamic parameters for the micellization were obtained from the temperature-dependent data on critical micelle concentration. The influence of given additive on the micellization of silicone surfactants was monitored from the changes in the free energy of micellization values for the surfactant solutions in water and water + additives. The cloud points and the critical micelle concentration values for the surfactant solutions were found to be decreased in the presence of the selected three additives. The analysis of the changes in the relative permittivities and partial molar volumes for the surfactant solutions in the presence of water and water + additives suggests that the solvent environment around the surfactant solute molecules is different in mixed solvent systems vis-a-vis water and the micellization and surface activity of silicone surfactants were dictated predominantly by the preferential hydration of cosolute additives. The analysis of small angle neutron scattering curves for the surfactant aqueous solutions in the presence of additives showed that the micelles formed have oblate ellipsoidal shape at 30 °C with, however, increased characteristic axial ratios than those in pure water. The increase in the concentration of additives has also increased the size of the micelles. The increase in temperatures corresponding to the values close to the turbid boundaries in the phase diagrams caused a transition from the oblate to the disklike micellar shape. The changes in the hydration values of the micellar associates were monitored from the dilute solution viscosity measurements. The dehydration of the micelles in the presence of additives at 30 °C and also at elevated temperatures occur from the interior to the fringe of the core-outer shell parts and to the outer shell successively.

1. Introduction Silicone surfactants (SS) are of novel and specialty agents and consist poly(dimethylsiloxane)s as the hydrophobic part alongside a hydrophilic moiety, and the latter can be nonionic, ionic, and zwitterionic in nature.1-4 The nonionic hydrophilic groups are made of oxyethylene or grids of oxyethylene and oxypropylene units. SS are available in different molecular architectures such as graft (rake or comb) like, trisiloxane, linear, or branched. SS with nonionic hydrophilic moieties not only share many common features with conventional low molar mass nonionic surfactants but also possess the following properties unique only to them.4 SS are (i) equally surface †

Sardar Patel University. ‡ Inter University Consortium. § Bhabha Atomic Research Center. (1) Kollmeier, H. J.; Langenhagen, R. D. Organo Polysiloxane Copolymers; Goldschmidt Informatiert; Special Publication No. 63; Th. Goldschmidt AG: Essen, Germany, Vol. 4184. (2) TegoprenesSilicone Surfactants. Products, Data and Information, Technical Brochure; Th. Goldschmidt AG: Essen, Germany, 1990. (3) Hill, R. M. Siloxane Surfactants; Hill, R. M., Ed.; Surfactant Science Series 86; Marcel Dekker: New York, 1999; Chapter 1, pp 1-47. (4) Hill, R. M. Siloxane Surfactants. In Encyclopedia of Physical Sciences and Technology; Meyers, R. A., Ed.; Academic Press: London, 2002; Vol. 14, pp 783-804,

active in water as well as in nonaqueous solvents such as mineral oils and polyols etc., (ii) lower surface tension of water to as low as 20 mN m-1, and (iii) remain as liquids even with very high molecular weights. SS have been widely used as foam stabilizers for polyurethanes, foam controlling agents for diesel fuel, deaerates and better wetting agents in ink, paint and coating formulations, and adjuants for effective spreading and penetration of herbicides on plant leafs.3 Despite their extensive use, only a few investigations are available in the literature on the surface active, phase, and association behavior of these interesting amphiphilic copolymeric surfactants.5-12 (5) Snow, S. A.; Fenton, W. N.; Owen, M. J. Langmuir 1990, 6, 385; 1991, 7, 868. (6) Jarvis, N. L. J. Colloid Interface Sci. 1969, 29, 647. (7) Gradzielski, M.; Hoffmann, H.; Robisch, P.; Ulbrich, W.; Grunning, B. Tenside, Surfactants, Deterg. 1990, 27, 366. (8) Gentle, T. E.; Snow, S. A. Langmuir 1995, 11, 2905. (9) Hoffmann, H.; Ulbricht, W. Surface Activity and Aggregation Behaviour of Siloxane Surfactants. In Silicone Surfactants; Hill, R. M., Ed.; Surfactant Science Series 86; Marcel Dekker: New York, 1999; p 97. (10) Yang, L.; Alexandridis, P. J. Phys. Chem. B 2002, 106, 10845. (11) Soni, S. S.; Sastry, N. V.; Aswal, V. K.; Goyal, P. S. J. Phys. Chem. B 2002, 106, 2606. (12) Soni, S. S.; Sastry, N. V.; John, G.; Bohidar, H. B. J. Phys. Chem. B 2003, 107, 5382-5390.

10.1021/la034389q CCC: $25.00 © 2003 American Chemical Society Published on Web 07/12/2003

Association Properties of Two Silicone Surfactants

The small amount of information on the thermodynamic and association behavior suggests that silicone surfactants form micelles in water with a nonspherical (ellipsoidal) shape. Our interest in silicone surfactant micelles has been to characterize their structural features and understand the correlation between the changes in the micellar characteristics and clouding behavior using various methods such as small-angle neutron scattering (SANS),11 dynamic light scattering (DLS),12 surface tension, and viscosity.11,12 It was found that the micelles in water are oblate ellipsoidal in shape with a core containing hydrophobic parts and an outer shell of hydrophilic units. The micelles were found to grow along the semimajor axis and the association number (SANS) as well as mass average association number (DLS) increased with the increase in temperature. The micellar growth has been thought to be mainly facilitated by a preferential dehydration of the micellar associates at elevated temperatures. Besides temperature rise, similar effect of dehydration of micellar associates can be achieved by adding cosolutes such as simple electrolytes13-21 as well as nonelectrolytes13,19,21,22 which can compete for water. A perusal of literature showed that few studies have been done to consider the effects of simple uni-univalent electrolytes based on common cation and different halide anions and nonelectrolytes such as urea, ethanol, formamide ,and glycerol on the micellization behavior of amphiphilic triblock copolymers based on oxyethylene (E)-oxypropylene (P)-oxyethylene (E). Following general observations were reported in these studies. The critical micellization temperatures (CMTs) and cloud points (CP) of EPE copolymers showed either a decrease13-16,19,21 or an increase13,14,19,21,22 depending upon the hydration ability of the added cosolute and its solvent quality with reference to the hydrophilic E parts. With regard to the changes in various micellar properties, it has been mostly reported that both the size of the micelles and the association number decrease when CP values show an increase in the presence of cosolute additive.21-23 Bahadur et al.15 have reported that the addition of electrolyte cosolute, i.e., potassium fluoride (in 1.0 M), did not produce any change in the size of E78P30E78 copolymer micelles, but the association number for E13P30E13 micelles showed enormous rise of about 42.5 times in the presence of 1.0 M KF.14 These studies indicate that the proportion of hydrophobic/hydrophilic block lengths is also one of the key factors for the induced changes in the micellar properties of copolymers. More studies in this direction are needed to come up with a generalization. The experimental data on the micellar properties of EPE copolymers in the presence of 1.0 M KF and at elevated temperatures (maximum up to close to CP) have also been (13) Pandya, K.; Lad, K.; Bahadur, P. J. Macromol. Sci.-Pure Appl. Chem. 1993, A30, 1. (14) Bahadur, P.; Pandya, K.; Almgren, M.; Li, P.; Stilbs, P. Colloid Polym. Sci. 1993, 271, 657. (15) Bahadur, P.; Li, P.; Almgren, M.; Brown, W. Langmuir 1992, 8, 1903. (16) Attwood, D.; Collett, J. H.; Tait, C. J. Int. J. Pharm. 1985, 26, 25. (17) Penders, M. H. G. M.; Nilsson, S.; Picuiell, L.; Lindman, B. J. Phys. Chem. 1994, 98, 5508. (18) Alexandridis, P.; Athanassiou, V.; Hatton, T. A. Langmuir 1995, 11, 2442. (19) Alexandridis, P.; Holzwarth, J. F. Langmuir 1997, 13, 6074. (20) Jorgensen, E. B.; Hvidt, S.; Brown, W.; Schillen, K. Macromolecules 1997, 30, 2355. (21) Desai, P. R.; Jain, N. J.; Sharma, R. K.; Bahadur, P. Colloids Surf., A 2000, 178, 57. (22) Alexandridis, P.; Yang, L. Macromolecules 2000, 33, 5574. (23) Lin, Y.; Alexandridis, P. Langmuir 2002, 18, 4220.

Langmuir, Vol. 19, No. 17, 2003 6669 Chart 1. General Structure of Silicone Surfactants

found to be very scarce and few studies14,15,20 have shown that the increase in temperature resulted in higher hydrodynamic radius,14,15 association number,15 and even induced sphere-to-rod transition in the micellar shape.20 We could find only one recent report on monitoring the effect of cosolutes (ethanol, formamide, and glycerol) on the micellar properties of amphiphilic siloxane-polyether-graft copolymer in aqueous solutions.23 The DLS data showed that addition of ethanol reduced the micellar size significantly, while addition of glycerol or formamide did not show such a strong effect. More detailed investigations are needed to understand the complex effects due to added cosolutes on the colloidal behavior of amphiphilic copolymers in general and silicone surfactants in particular. In this study, we report the effect of three nonelectrolyte additives, namely, 2-butoxyethanol (box), poly(ethylene glycol) with a molecular weight of 6000 g‚mol-1 (PEG 6000), and glucose as cosolutes, on the phase, surface active, thermodynamic, and association behavior of two poly(dimethylsiolxane)-graft-polyethers in water. We first describe the changes in the features of dilute solution phase diagrams and also report the critical micelle concentration (cmc) and other surface active properties. Then we analyze the SANS curves for eliciting the information on the changes in the size and shape of the micelles due to the added cosolutes as well as increased temperature. Finally an attempt will be made to monitor the hydration and dehydration effects using viscosity measurements. The effects due to the change in the additive concentrations and also temperature will be discussed in terms of surfactant solute-water and cosolutes-water interactions. 2. Experimental Section 2.1. Materials. The silicone surfactants were obtained as gift samples from Th. Goldschmidt AG, Germany. The general molecular structure of silicone surfactants is shown in Chart 1. Values of numbers m and n represent the polyether modified hydrophilic methylsiloxane and hydrophobic poly(dimethylsiloxane) units and the values of x and y denote the number of oxyethylene and oxypropylene units in the polyether grid. The values of m, n, x, and y are 5, 13, 12, and 0 and 5, 20, 10, and and 4 for SS-1 and SS-2 surfactants, respectively.2 These products are of commercial origin with m, n, x, and y being average numbers and the branch group A is statistically distributed over the silicone chain. The surfactants were used as received without any further purification. 2-Butoxyethanol, glucose, and poly(ethylene glycol) were of Analytical Reagent grade chemicals and used as received without any further purification. Water, freshly distilled from an all Pyrex glass still was used in preparing stock solutions (of 10% (w/v)) in stoppered glass vials. The dilutions were made as required from the fresh stock solutions. 2.2. Methods. The dilute solution phase diagrams have been constructed by reproducible visible observations in the surfactant solutions taken in a long glass tube following the warming and cooling cycles. The surface tensions of the surfactant solutions were measured by the drop weight method using a modified stalagmometer.24 The surface tensions calculated by this method (24) Jain, D. V. S.; Singh, S. Indian J. Chem. 1972, 10, 629.

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agreed within (0.2% of the values for pure organic liquids reported in the literature.25 The SANS experiments were carried out on the micellar solutions of silicone surfactants, prepared by dissolving known amounts of surfactant and additive in D2O and using an indigenously built SANS spectrometer at the DHRUVA Reactor, (Trombay, India).26 The D2O (with at least 99.5 atom % purity) was obtained from the heavy water division of BARC, Mumbai, India. The use of D2O instead of H2O for preparing the solutions provides a very good contrast between the associates of solute and the solvent in the SANS experiment. The solutions were held in 0.5 cm path length UV-grade quartz sample holders with tight-fitting Teflon stoppers, sealed with Parafilm. The sample to detector distance was 1.8 m for all the runs. The spectrometer makes use of a BeO filtered beam and has a resolution (∆Q/Q) of about 30% at Q ) 0.05 Å-1. The angular distribution of the scattered neutron is recorded using a one-dimensional positionsensitive detector. The accessible wave transfer Q ()4π sin 0.5θ/ λ, where λ is the wavelength of the incident neutrons and θ is the scattering angle) range of this instrument is between 0.02 and 0.3 Å-1. The mean neutron wavelength was λ ) 5.2 Å. The measured scattered intensities of neutrons were corrected for the background, empty cell scattering, and sample transmission. The intensities then were normalized to absolute cross-section units.26 Thus, plots of dΣ/dΩ vs Q were obtained. The uncertainty in the measured scattering intensities is less than 10%. The experimental points are fitted using a nonlinear least-squares method as discussed in the next section. The flow times of surfactant solutions and water + additives were obtained by using Ubbelohde suspended level viscometers. Two viscometers were used to record flow times in the range of 130-360 s, thus avoiding any kinetic corrections. Shear corrections were not taken into consideration because obtained intrinsic viscosities were always less than 3 dL g-1. The flow volume was greater than 5 mL, making drainage corrections unimportant. Viscometers were suspended in thermostatic water baths (ISREF, India) maintained at constant temperature accurate to (0.01 °C. The densities of aqueous surfactant solutions were measured by using a high precision Anton Paar density meter DMA 5000. The measured densities have uncertainty of (0.0005%.

3. Results and Discussion 3.1. Dilute Solution Phase Diagrams. The dilute solution phase diagrams for SS-1 and SS-2 surfactant solutions (in concentration range of 0.1-10.0% (w/v)) have been constructed in the presence of different amounts of three nonelectrolyte additives, namely, box, PEG 6000 and glucose as cosolutes. These additives were chosen so that cloud points of respective surfactant solutions always showed a decrease. The phase diagrams of SS-1 and SS-2 surfactant solutions in aqueous solutions11 as well as in the presence of the additives have been characterized by the following common features. A given clear solution at room temperature when warmed turned turbid, becomes cloudy, and upon further heating dense clouds started appearing across the entire solution volume. The representative phase diagrams for both the surfactants in 6% (w/v) box and 10% (w/v) PEG 6000 and glucose aqueous solutions are depicted in parts a-f of Figure 1. Despite sharing several common features, scrutiny of the phase diagrams showed that the addition of 6% (w/v) box and 10% (w/v) PEG 6000 and glucose as cosolutes decreased the cloud points to a maximum of 25-26 °C and systematically lowered the boundary lines between fully dense clouds and cloudy, cloudy and turbid, and turbid and clear regions. The additives, box and glucose in respective amounts of 6 and 10% (w/v) even induced a macroscopic phase separation at elevated temperatures. The effect of (25) Riddick, J. A.; Bunger, W. B.; Sakano, T. K. Organic Solvents, 4th ed.; Wiley-Interscience: New York, 1986; Vol. II. (26) Aswal, V. K.; Goyal, P. S. Curr. Sci. 2000, 79, 947.

Soni et al.

Figure 1. Dilute solution phase diagrams of SS-1 and SS-2 surfactant solutions in water + additives: (a, b) 6% (w/v) box; (c, d) 10% (w/v) PEG 6000; (e, f) 10% (w/v) glucose; (b) final cloud points.

increasing concentration of additives on the phase changes was also monitored for SS-2 surfactant solutions. The proportions of hydrophobic to hydrophilic segment (hp/hl) in individual SS-1 and SS-2 surfactants are calculated to be 0.3 and 1.0, respectively. Thus SS-2 molecules are more hydrophobic in nature than molecules of SS-1. The increased amounts of the three additives further decreased the cloud points and shifted the boundary lines to lower temperatures. The striking feature with the PEG 6000 was that, its addition in 20% (w/v) amount caused phase separation at higher temperatures in contrast to the absence of such separation in 10% (w/v) PEG 6000 aqueous solutions. The decreasing tendencies observed for the three additives need to be accounted and explained. The characteristic features and changes noted in the phase diagrams are expected to be implicitly related to the interactions between the surfactant molecules and water as well as cosolute molecules and water and the changes in the micellar associates. We have also measured the changes in the relative permittivities, r, of surfactant solutions in pure water and water + additive systems. The r values for the mixed solvent system of water + box at both the temperatures of 30 and 35 °C are found to be lower than that of pure water, while water + PEG 6000 and + glucose mixed solvent systems showed increased r values. This shows that the water + box system provides a less polar solvent medium while the other two mixed systems offer a high polar solvent environment. We have also calculated the change in the relative permittivities of surfactant solutions, δr (δr ) r(w+a) - r(w)) (where the


Langmuir, Vol. 19, No. 17, 2003 6671 Table 1. Partial Molar Volumes of Surfactant Solute at Infinite Dilution, VO∝, and Volumes of Transfer from Water to Additives, VO∝(wfa), for SS-1 and SS-2 Surfactant Solutions at 30 °C SS-1

SS-2

solvent system

Vφ∝

Vφ∝(wfa)

Vφ∝

Vφ∝(wfa)

water 6% (w/v) box 10% (w/v) PEG 6000 10% (w/v) glucose

3370.17 4243.44 3458.66 3662.81

873.27 88.49 292.64

4867.52 5803.24 4962.08 5192.96

935.48 94.72 325.44

F and F0 are the densities of surfactant solution and solvent system, respectively. Vφ values at 30 °C were fitted to a linear equation of following type to calculate the partial molar volumes at infinite dilution, Vφ∝.

Vφ ) Vφ∝ + Sv m

Figure 2. Deviations in relative permittivities, δr, as a function of concentrations of (a) SS-1 and (b and c) SS-2 surfactant solutions in water + additives: (a and b) (b) 6% (w/v) box, (+) 10% (w/v) PEG 6000, (9) 10% (w/v) glucose at 30 °C, (2) 6% (w/v) box (35 °C), (1) 10% (w/v) PEG 6000, (×) 10% (w/v) glucose at 45 °C; (c) (b) 3% (w/v) box, (+) 20% (w/v) PEG 6000, (9) 20% (w/v) glucose at 30 °C, (2) 3% (w/v) box (35 °C), (1) 20% (w/v) PEG 6000, (×) 20% (w/v) glucose at 45 °C.

subscripts w and w+a indicate water and water + additive). The variation of δr as a function of surfactant concentrations is depicted in parts a-c of Figure 2. The δr values were found to be large and negative when the water + box (3 and 6% (w/v)) system was used as solvent. Contrary to this, positive δr values were noted when the surfactant solutions were dissolved in water + PEG 6000 and + glucose (in 10 and 20% (w/v) amounts). Both the negative as well as positive magnitude of δr systematically showed further decrease with the increase in the surfactant concentration from 0.5 to 10.0% (w/v). To gain further information on the nature of solvent environment around surfactant molecules, apparent molar volumes of solute, Vφ, were calculated using the relation

Vφ )

() (

)

1000(F - F0) M F mFF0

(1)

where M ) molar mass of solute, m ) molality of solution,

(2)

where Vφ∝ is the partial molar volume at infinite dilution and Sv is the experimental slope. The utility of Vφ∝ values for the surfactant solutions in water and mixed solvent systems is that one can define the standard function of transfer (for partial molar volume) to get valuable information on the difference in the interaction of surfactant molecules with water when pure water or water + additives are used as solvents. The standard function of transfer, Vφ∝(wfa) ) Vφ∝(w+a) - Vφ∝(w). The values of Vφ∝ and Vφ∝(wfa) are summarized in Table 1. A perusal of the data on transfer functions reveals that its value is larger and positive for the water + box system. The positive magnitude gets further reduced by about three times in water + glucose systems. The water + PEG 6000 system however has the least positive values for the standard transfer function. The positive value for the transfer functions clearly indicates that the interactions between surfactant molecules and water are less when box, glucose, and PEG 6000 are used as cosolutes in the mixed solvent systems. On the basis of the relative magnitude of the transfer functions, the weakening of the surfactant solute and water interactions followed the order box > glucose > PEG 6000. This is consistent with the fact that the δr for box + water solvent system as well as surfactant solutions in box + water systems were negative, and hence the overall solvent media has to be less polar resulting into a less favorable environment for the interaction between hydrophilic part of surfactant molecules and water. It has also been reported that the water molecules strongly interact with 2-butoxyethanol,27-32 -OH, and the oxyethylene groups of PEG33,34 and also with the hydroxyl groups of glucose. This competitive interaction between the surfactant solute and the additive cosolute for the water around them would also induce the breaking of hydrogen bonds between the oxyethylene parts of surfactant and water. These effects combined are perhaps responsible for the observed lowering in the cloud points and also the boundary lines in the phase diagrams of surfactant solutions. (27) Pal, A.; Singh, Y. P. J. Chem. Eng. Data 1996, 41, 425. (28) Page, M.; Huot, J.-Y.; Jolicoeur, C. J. Chem. Thermodyn. 1993, 25, 139. (29) Fioretto, D.; Marini, A.; Onori, G.; Palmeeri, L.; Santucci, A.; Socino, G.; Veridini, L. Conference Procedings, Vol. 43, WaterBiomolecule Interactions, SIF, Bolgna, Italy, 1993. (30) Koga, Y. J. Phys. Chem. 1992, 96, 10465. (31) Kaatze, U.; Pottel, R.; Schumacher, A. J. Phys. Chem. 1992, 96, 6017. (32) Siu, W.; Koga, Y. Can. J. Chem. 1989, 67, 671. (33) Begum, R.; Matsuura, H. J. Chem. Soc., Faraday Trans. 1997, 93, 3839. (34) Schott, H. J. Chem. Eng. Data 1966, 11, 417.

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Table 2. Values of Critical Micelle Concentrations, cmc’s, cmc/C20, Surface Excess Concentration, Γm, Area per Surfactant Molecules, a1s, and Surface Pressure, πcmc, at Different Temperatures for SS-1 and SS-2 Surfactant Solutions in Water + Different Additives temp (°C)

cmc (g dL-1)

Γm × 106 (mol m-2)

cmc/C20

a1s (Å2)

πcmc (mN m-1)

84 ( 4 80 ( 2 76 ( 3

47.8 ( 0.3 43.3 ( 0.3 36.6 ( 0.2

15 30 45

0.032 ( 0.003 0.016 ( 0.002 0.009 ( 0.003

1345 107 28

SS-1 6% (w/v) box 1.9 ( 0.1 2.1 ( 0.1 2.2 ( 0.1

15 30 45

0.048 ( 0.003 0.029 ( 0.001 0.020 ( 0.001

2098 291 52

10% (w/v) PEG 6000 1.5 ( 0.1 1.6 ( 0.1 1.6 ( 0.1

110 ( 6 107 ( 3 106 ( 4

46.4 ( 0.2 40.7 ( 0.4 35.6 ( 0.4

15 30 45

0.038 ( 0.002 0.020 ( 0.002 0.016 ( 0.001

1955 185 48

10% (w/v) glucose 1.8 ( 0.1 1.9 ( 0.1 1.9 ( 0.1

95 ( 3 90 ( 2 87 ( 2

46.9 ( 0.2 41.2 ( 0.2 32.0 ( 0.3

95 ( 3 89 ( 2 80 ( 3

48.6 ( 0.4 42.0 ( 0.3 35.6 ( 0.1

15 30 35

0.025 ( 0.002 0.017 ( 0.001 0.009 ( 0.003

292 96 15

SS-2 6% (w/v) box 1.8 ( 0.1 1.9 ( 0.1 2.1 ( 0.1

15 30 45

0.040 ( 0.003 0.026 ( 0.002 0.012 ( 0.003

498 68 17

10% (w/v) PEG 6000 1.6 ( 0.1 1.7 ( 0.1 1.8 ( 0.1

102 ( 3 97 ( 3 93 ( 2

47.0 ( 0.2 40.2 ( 0.2 32.8 ( 0.3

15 30 45

0.037 ( 0.001 0.020 ( 0.002 0.011 ( 0.003

448 54 15

10% (w/v) glucose 1.7 ( 0.1 1.8 ( 0.2 2.0 ( 0.2

97 ( 3 91 ( 4 84 ( 2

48.0 ( 0.4 40.4 ( 0.3 32.8 ( 0.3

3.2. Critical Micelle Concentrations (cmc) and Surface Active Properties. The cmc’s were determined from the surface tension, γ, vs logarithmic concentration profiles. The profiles were found to be “L” shaped with an initial steep fall in the surface tension values up to a characteristic surfactant concentration followed by near constant values. The cmc values, as extracted from the brake points in γ vs log C plots for the two surfactants in solutions of water + various additives and at 15, 30, and 45 °C along with cmc/C20, where C20 is the surfactant concentration at which γ value for water is reduced by 20 mN m-1, are listed in columns 2 and 3 of Table 2. The addition of any of the three additives resulted into lowering of cmc at the three temperatures. The maximum decrease in cmc values at 30 °C for SS-1 surfactant has been found to be 2.3, 1.2, and 1.8 times for 6% (w/v) box, 10% (w/v) PEG 6000 and glucose, and 2.0, 1.3, and 1.7 times for SS-2 surfactant under the same conditions. Similarly, the characteristic values of cmc/C20 were also lowered drastically for the surfactant solutions in the presence of additives. The decrease in cmc values clearly indicated that the water + additive mixed solvent facilitates the micelle formation at lower concentrations. The surface active parameters such as surface excess concentration, Γm, of copolymer molecules in the surface layer compared to the bulk, the surface area per molecule, a1s, in the surface monolayer were also calculated by a standard procedure.35 The Γm and a1s values for the aqueous solutions of both the surfactants in the presence of various additives and at different temperatures are given in columns 4 and 5 of Table 2. Column 6 of the same table shows the πcmc, which is called maximum surface pressure and is taken as the difference between the surface tension value of solvent system and the limiting value of surface tension beyond the cmc. A perusal of the data on various surface active parameters showed that the surface (35) Rosen. M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley and Sons: New York, 1989, Chapters 2, 3, and 5.

areas for surfactant molecules decreased considerably in water + additive mixed solvents (over pure water) hinting that the surfactant molecules are pushed to the surface due to the possible unfavorable environment for the strong interaction between the hydrophilic part of the surfactant and water, even though the mixed solvent systems of water + PEG 6000 and + glucose are more polar than pure water. The smaller a1s values for both the surfactants in the mixed solvent systems can be attributed to the predominance of preferential hydration by these cosolutes. As expected, the surface pressure, πcmc, values showed a marginal increase especially at 15 and 25 °C because of excess surfactant molecular concentration at the surface. 3.3. Thermodynamic Parameters of Micellization. The thermodynamic parameters, namely, free energy, ∆G°mic, enthalpy, ∆H°mic, and entropy, ∆S°mic, of micelle formation in additive aqueous solutions of both the surfactants, were calculated by standard relations.35 The effect of cosolutes on the thermodynamics of the micellar association process of the surfactant was calculated from the following relation

∆G°M ) ∆G°mic(w+a) - ∆G°mic(w)

(3)

The thermodynamic parameters of adsorption were obtained using the equation

∆G°ads ) ∆G°mic - πcmc/Γm

(4)

The standard state in the surface phase is always assumed to be the surface covered with a monolayer of surface active agent at a surface pressure equal to zero. A perusal of the sign and magnitudes of the various thermodynamic parameters showed that the observed negative ∆G°mic values have sole contribution from the positive ∆S°mic since the ∆H°mic are always found to be positive. The addition of any of the three cosolutes invariably decreased the negative ∆G°mic values. Thus, it can be concluded that the presence of three additives selected for our study in water


Langmuir, Vol. 19, No. 17, 2003 6673

renders the overall solvent quality selectively poorer toward the hydrophilic part of the silicone surfactants. The ∆G°M values were found to be small and negative in the three cases for the both the surfactant solutions. Interestingly, the ∆G°M values became 1.5-3-fold more negative with the increasing additive concentrations from 3 to 6% (w/v) for box and 10 to 20% (w/v) for PEG 6000 and glucose systems. The negative value of ∆G°M can be rationalized in terms of enhancement of interactions among surfactant molecules because of the preferential hydration of additives. The ∆G°ads values are more negative than the corresponding ∆G°mic values. This indicates that the surfactant molecules prefer to reside at the surface and compensating work has to be done for transferring the surfactant molecules from the surface to the micelles. A slight decrease in the negative magnitude of ∆G°ads has been found with the rise in the temperature for water as well as mixed solvent systems. 3.4. Small Angle Neutron Scattering (SANS). 3.4.1. SANS Data Analysis. The SANS experiment measures the coherent differential scattering cross section dΣ/dΩ for the sample. The expression for dΣ/dΩ for a unit volume of solution of monodisperse micelles is given by36,37

dΣ ) nmV2m(Fm - Fs)2{〈F2(Q)〉 + dΩ 〈F(Q)〉2[S(Q) - 1]} + B (5) where nm denotes the number density of the micelles and Fm and Fs are the scattering length densities of the micelles and the solvent, respectively. F(Q) is the single particle (intraparticle) form factor, S(Q) is the interparticle structure factor, and B is a constant term that represents the incoherent scattering background, which is mainly due to the hydrogen in the sample. The particle form factor F(Q) depends on the shape and size of the micelles. Expressions for F(Q) corresponding to different geometrical shapes are known38,39 and the same were previously applied by us for the ellipsoidal micelles of SS-1 and SS-2 surfactants in water.11 For the cylinder of radius R and length L ) 2l

F(Q) )

∫0π/2

Figure 3. SANS intensity profiles for 1% (w/v) SS-1 surfactant solution in D2O and D2O + additives at 30 °C.

the S(Q) usually shifts to higher Q values with an increase in surfactant solute concentration. It may be mentioned that the spatial arrangement of micelles and the S(Q) depend on the intermicellar interactions. In principle, interaction potential, V(r), between micelles could change with temperature of the solution. That is, a change in temperature could also result in changes in S(Q). The SANS distribution plots of dΣ/dΩ vs Q for SS-1 and SS-2 surfactant aqueous solutions at different additive concentrations at 30 °C and for 1% (w/v) SS-2 solutions at different temperatures are shown in Figures 3 and 4. The SANS intensities decreased monotonically with Q and there is no indication of any correlation peak. This suggests that for the surfactant concentrations reported in this work, intermicellar interference effects are negligible. Thus, we have assumed S(Q) ) 1 in further analysis. Then eq 5 will be reduced to

dΣ ) nmV2m(Fm - Fs)2〈F2(Q)〉 + B dΩ

(7)

2

sin2(Ql cos β) 4J1 (QR sin β) Q2l2 cos2 β

Q2R2 sin2 β

sin β dβ (6)

where β is the angle between the axis of the cylinder and bisectrix. J1 is the Bessel function of order unity. The radius R and l are considered as fitting parameters in the calculations, the disk being a special case for a cylinder when L , R. It can be shown that for a cylindrical particle, F(Q) varies as 1/Q in the Q range of 1/l < Q < 1/R and as 1/Q2 for disklike particle in the Q range of 1/R < Q < 1/l. We would like to mention that while F(Q) at small Q is sensitive to the dimension of the major axis, its behavior at large Q is decided by the dimension of the minor axis of the micelles. S(Q), the interparticle structure factor, is decided by the spatial arrangement of micelles in solution. Usually S(Q) shows a peak at Qm ) 2π/D, where D is the average distance between the micelles. Thus the peak in (36) (a) Chen, S. H. Annu. Rev. Phys. Chem. 1986, 37, 351. (b) Chen, S. H.; Lin, T. L. Methods of Experimental Physics; Academic Press: New York, 1987; Vol. 23B, p 489. (37) (a) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (b) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (c) Hansen, J. P.; Hayter, J. B. Mol. Phys. 1982, 46, 109. (d) Hayter, J. B.; Penfold, J. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1851. (38) Goyal, P. S.; Rao, K. S.; Dasannacharya, B. A.; Kelkar, V. K. Physica B 1991, 174, 192. (39) Goyal, P. S. Phase Transitions 1994, 50, 143.

The micelles formed by the silicone surfactants consist of a core made of hydrophobic poly(dimethylsiloxane) (and poly(oxypropylene)) with an outer shell made up of hydrophilic poly(oxyethylene). The scattering length densities of hydrophobic parts of SS-1 and SS-2 surfactants, Fm, are calculated to be 0.154 × 1010 and 0.146 × 1010 cm-2, respectively. The scattering length density of D2O, Fs, has a value of 6.38 × 1010 cm-2. Thus, there is very good contrast between the hydrophobic core parts of the micelles and D2O. Since, the outer shell of the micellar associates is expected to be swollen extensively with water (D2O), the scattering contrast between the outer shell of the associates and solvent could be very poor. In view of this and as considered by others,40,41 we also assume that the F(Q) value depends mainly on the micellar core dimensions. 3.4.2. Dependence of SANS Distributions on Additive Concentrations. The measured SANS distribution for 1% (w/v) SS-1 surfactant solution at 30 °C is shown in Figure 3. The effect of the three additives, namely box, glucose, and PEG 6000, on the above curve is also shown in Figure 3. Results of SANS experiments on 1% (w/v) SS-2 surfactant solutions are shown in Figure 4. The effect (40) Mortensen, K.; Talmon, Y. Macromolecules 1995, 28, 8829. (41) Jain, N. J.; Aswal, V. K.; Goyal, P. S.; Bahadur, P. J. Phys. Chem. B 1998, 102, 8452.

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Soni et al. Table 3. Values of Semimajor Axis, b, Semiminor Axis, a, and Axial Ratio, b/a for the Oblate Ellipsoidal Micelles in 1% (w/v) SS-1 and SS-2 Solutions in D2O and D2O + Additives at 30 °C concn of additives, % (w/v)

a (Å)

b/a

SS-1 51 58 68 86

23 24 24 24

2.2 2.4 2.8 3.6

pure D2O 10% (w/v) 20% (w/v)

SS-2 PEG 6000 49 56 78

25 25 25

2.0 2.2 3.1

10% (w/v) 20% (w/v)

Glucose 71 97

25 25

2.8 3.9

Box 63 82

25 25

2.5 3.3

pure D2O 10% (w/v) PEG 6000 10% (w/v) glucose 6% (w/v) box

3% (w/v) 6% (w/v)

Figure 4. SANS intensity profiles for 1% (w/v) SS-2 surfactant solution in D2O and D2O + additives: (a), (c), and (e) at 30 °C and (b), (d), and (f) at different temperatures.

of increase in additive concentration on SANS distributions for SS-2 solutions corresponding to PEG 6000, glucose, and box at T ) 30 °C is shown in parts a, c, and e of Figure 4, respectively. The effect of temperature on SANS curves from the above solutions is shown in parts b, d, and f of Figure 4. In all the measured distributions, the dΣ/dΩ or intensity showed relatively strong Q dependence. The intensities at large Q values have been found to be almost independent of additive concentrations or type of additive. However, in the low Q range, the intensity values for 1% (w/v) SS-1 solutions increased with the addition of PEG 6000, glucose, or box (Figure 3). Similarly, the increase in the concentrations of individual additives produced considerable increase in the intensity values for 1% (w/v) SS-2 solutions (Figure 4a,c,e). The intensities, for example, became about 1.2-1.4 times more in magnitude with doubling of the concentration of the respective additive. In an earlier work,11 we have presented the detailed analysis of the SANS data for the above two surfactant aqueous solutions without additives. It was found that the SANS curves for surfactant solutions in D2O were best reproduced when the micellar associates were assumed to be oblate ellipsoidal in shape. On the basis of our previous results, we attempted to reproduce the experimental dΣ/dΩ values of Figures 3 and 4 using eq 7 with F(Q) calculations assuming oblate ellipsoidal, cylindrical, and disk shapes for the micelles. The experimental SANS distributions were found to be still best reproducible with the oblate shape for the micelles in 1% (w/v) SS-1 and SS-2 solutions in the presence of 3 and 6%

b (Å)

(w/v) box and 10 and 20% (w/v) of PEG 6000 and glucose additives, at 30 °C. In our analysis the terms semiminor axis, a, and semimajor axis, b, were considered as adjustable parameters. Taking the a values of 23 and 25 Å for SS-1 and SS-2 micellar associates in pure D2O, b values were varied until the acceptable χ2 values (3-3.5) for the correlation of fitted and experimental intensities were obtained. The final values for the semimajor, b, semiminor, a, and characteristic axial ratios, b/a, as extracted from the above procedure are listed in Table 3. A perusal of data given in the table shows that the characteristic value of the semimajor axis, b, for SS-1 and SS-2 surfactant micelles successively become higher in the presence of 10% (w/v) PEG 6000, 10% (w/v) glucose, and 6% (w/v) box as cosolutes. The increase in the values of the semimajor axis, b, accordingly raised the axial ratios. The increased axial ratios of micellar associates directly indicate that the micelles grow along the semimajor axis, b. It was earlier mentioned that the additive molecules preferentially get hydrated and thus induce dehydration of the micelles. This dehydrated environment within the micellar associates facilitates the expansion of the hydrophobic core part. 3.4.3. Temperature Dependence. SANS measurements have also been made on 1% (w/v) SS-2 solutions in the presence of 6% (w/v) box and 10% (w/v) glucose, and PEG 6000 at 35, 45, and 50 °C, respectively. These temperatures are chosen such that they are very close to the phase boundary between clear and turbid regions of the phase diagrams. The SANS distribution curves are depicted in parts b, d, and f of Figure 4. It can be seen that the nature of the profiles at elevated temperatures, even though similar with those at lower temperature, show an increase in the intensity at low Q values. Also, the calculated SANS intensities based on the oblate ellipsoidal model were found to deviate largely from the measured values. The SANS intensity profiles could best be reproduced with a disklike shape to the micelles. The SANS intensities vs Q values are also plotted on a log-log scale in the Q range 0.019-0.07 Å-1 in Figure 5. The solid lines shown in the figure are the fitted straight lines. The slopes of these lines have been found to be -1.96 (for 1% (w/v) SS-2 aqueous solutions in 10% (w/v) PEG 6000 at 50 °C), -2.08 (for 1% (w/v) SS-2 aqueous solutions in 10% (w/v) glucose at 45 °C), and -2.03 (for 1% (w/v) SS-2 aqueous solutions in 6% (w/v) box at 35 °C). The slopes of ≈-2.0 in general are obtained when the micellar associates have


Langmuir, Vol. 19, No. 17, 2003 6675

Figure 5. A log-log plot of SANS intensities for 1% (w/v) SS-2 surfactant solution in D2O + additives in the Q range of 0.019 < Q < 0.07 Å-1: (9) 10% (w/v) PEG 6000 at 50 °C; (2) 10% (w/v) glucose of 45 °C; (b) 6% (w/v) box at 35 °C. Table 4. Values of Semimajor Axis, b, Semiminoar Axis, a, Axial Ratio, b/a, Radius, R, and Length, L, for Micelles of 1% (w/v) SS-2 Surfactant Solutions in D2O + Additives at Different Temperatures oblate ellipsoidal model temp (°C)

b (Å)

30

56

30 30

a (Å)

disk model b/a

temp (°C)

R (Å)

L (Å)

10% (w/v) PEG 6000 25 2.2 50

197

25

71

10% (w/v) glucose 25 2.8 45

208

30

82

6% (w/v) box 25 3.3 35

223

30

disklike shape.42 The micellar dimensions such as radius of the disk, R, and the thickness of the disklike micelles, L, are listed in Table 4. 3.5. Viscosity Measurements. Dilute solution viscosity measurements are quite useful in estimating the intrinsic viscosities, [η]. [η] measures the hydrodynamic volumes of the micellar associates. The intrinsic viscosities are also very handy for calculating the hydration, W, i.e., a gram of water bound to a gram of surfactant. The following equation has been widely used for calculating the hydration values,43,44

W ) υ˜ Fo{(100[η]/2.5υ˜ ) - 1}

(8)

where υ˜ , Fo, and [η] are the partial specific volume of surfactant, density of water, and intrinsic viscosity, respectively. The partial specific volume of silicone surfactants in solutions was calculated from the slopes of the linear plots obtained by the equation, F ) Fo + (1 - υ˜ Fo)C, where F is the density of the surfactant solution at a given concentration, C. The concentration dependence of reduced viscosities, ηsp/C (in very dilute concentration range), for SS-1 and SS-2 aqueous solutions in the presence of various amounts of additives and at different temperatures has also been monitored. The profiles of ηsp/C vs C are in general expected to be linear as per the Huggins relation; ηsp/C ) [η] + kH[η]2C. The same is observed for some cases, while in others the dependence showed rectilinear curvatures with initial shoot up in the ηsp/C values. The presence of the additives as cosolutes and the temperature rise in the (42) Aswal, V. K.; De, S.; Goyal, P. S.; Bhattacharya, S.; Heenan, R. K. J. Chem. Soc., Faraday Trans. 1998, 94, 2965. (43) Tokiwa, F.; Ohki, K. J. Phys. Chem. 1967, 41, 137. (44) Oncley, J. L. Ann. N. Y. Acad. Sci. 1949, 41, 121.

Figure 6. Concentration dependence of reduced viscosity for very dilute (a) SS-1 and (b) SS-2 surfactant solutions in water + additives (6% (w/v) box) at different temperatures. Points are experimental values, and the dashed curves are calculated using eq 9.

surfactant solutions are thought to induce the dehydration of the micelles, the surfactant molecules with depleted water around them could adsorb on the inner wall of the glass capillary of the viscometer. These adsorption effects would increase the flow time and hence the measured ηsp/C values at low concentrations are high and apparent. Ohrn45 considered this effect to relate the true and apparent values of ηsp/C by

(ηsp/C)* ) ηsp/C + ∆

(9)

where

∆ ≈ (ηr/C) (4alay/r) and where the asterisk indicates apparent value. The terms alay and r in the ∆ relation are the thickness of adsorbed layer and radius of the capillary of the viscometer used (0.300 mm in our case). Since one can equate ηr close to unity at low concentrations, true ηsp/C values can be estimated by inserting the chosen values for adsorbed layer thickness. The alay values in the range 100-600 nm were found to reproduce the trend in the reduced viscosities in the very dilute solution range of concentration. These adsorption effects, as explained above, can be seen and be appreciated in very low surfactant concentrations and at elevated temperatures. Two representative ηsp/C vs C profiles for SS-1 and SS-2 surfactant aqueous solutions in the presence of 6% (w/v) box are depicted in Figure 6. The observed sharp shoot up of ηsp/C in very dilute solutions is thus the direct consequence of the adsorption effects. The reduced viscosity vs concentration plots (corresponding to a higher concentration range) for SS-1 and (45) Ohrn, O. E. J. Polym. Sci. 1955, 17, 137.

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Soni et al.

Table 5. Partial Specific Volume, υ˜ , and Hydration, W, for SS-1 and SS-2 Surfactant Solutions in Water + Additives at Different Temperatures SS-1

SS-2

temp (°C)

υ˜ (mL g-1)

W (g of H2O/ g of copolymer)

υ˜ (mL g-1)

W (g of H2O/ g of copolymer)

30 35 40 45 50

1.0041 1.0060 1.0081 1.0103 1.0128

1.08 1.14 1.18 1.31 1.50

20 25 30 35

1.0183 1.0246 1.0330 1.0425

0.98 1.10 1.42 1.70

30 35 40 45 50 55

0.9438 0.9486 0.9525 0.9603 0.9668 0.9708

0.50 0.54 0.60 0.66 0.72 0.82

10% (w/v) PEG 6000 20 25 30 35 40 45

0.9250 0.9315 0.9389 0.9483 0.9535 0.9616

0.97 1.01 1.08 1.15 1.41 1.60

30 35 40 45 50

0.9682 0.9699 0.9719 0.9740 0.9762

0.70 0.75 0.79 0.85 1.09

0.9330 0.9410 0.9506 0.9688 0.9804

0.70 0.75 0.79 0.85 1.09

temp (°C)

6% (w/v) box

10% (w/v) glucose 20 25 30 40 45

Figure 8. Variation of intrinsic viscosity, [η], and Huggins constant, kH, with temperature for SS-1 surfactant solutions in water + additives (6% (w/v) box).

Figure 7. Concentration (dilute to moderate) dependence of reduced viscosity for (a) SS-1 and (b) SS-2 surfactant solutions in water + additives (6% (w/v) box) at different temperatures.

SS-2 aqueous solutions in the presence of additives and at different temperatures were also constructed. The plots in all the cases were in general found to be linear and adhered to Huggins relation. Two representative profiles of such plots are shown in Figure 7. Using the extrapolation procedure through leas-squares linear fits, two important quantities, namely, intrinsic viscosities, [η], and Huggins constants, kH, were calculated from all the plots. The indirect information on the micellar growth, dehydration, and micellar interactions can be gained from these two quantities. The representative variations of [η] and kH for SS-1 micelles in the presence of 6% (w/v) box are shown in Figure 8. [η] values of SS-1 and SS-2 surfactant micelles in different solution conditions in general were found to

increase with the rise in temperature for a fixed additive concentration and also with increase in additive concentration at a fixed temperature. The trend in kH values has been found to be reverse, i.e., they decrease under the same conditions. The increased [η] values indicate the micellar growth facilitated by micellar dehydration and worsening solvent quality of the mixed water + additive system. The fact that kH values become smaller or even less than unity in some cases at elevated temperatures indicates that the dehydration effects weaken the attractive interactions between the hydrophilic part of the molecules and solvent media to a considerable extent. The data of partial specific volumes, υ˜ , and hydration, W, for the surfactant micelles at different temperatures are presented in Table 5. A perusal of the columns 3 and 6 of the table reveals that the hydration of the micelles increases systematically with the temperature in the presence of the three additives. This is rather unexpected and needs to be accounted for properly. We propose the following explanation. It is possible that the core part of the micelles may not be totally dry and contain water at the inner side of the fringe between the core and the outer shell. The dehydration of the micelles may follow a preferential pattern in which water may be lost successively from the inner core to the fringe and to the outer shell parts. It is worth mentioning that at the maximum


temperatures at which the above measurements were made, the solutions are still clear and hold the solute molecules in solution. Thus, it is postulated that the dehydration under these conditions expels more water from within the micelles to the outer corona. 4. Conclusions The effect of three nonelectrolyte additives, namely, 2-butoxyethanol (box), poly(ethylene glycol) (PEG 6000), and glucose as cosolutes on the phase, thermodynamic, and association properties of two silicone surfactants based on poly(dimethylsiloxane)-graft-polyethers in aqueous solutions has been monitored from the construction of dilute solution phase diagrams and surface tension, small angle neutron scattering, and viscosity measurements. The characteristic changes that take place in these surfactant solutions revealed that cloud points and various boundary lines that separate clear and turbid, turbid and cloudy, and cloudy and dense clouds were decreased and shifted to the lower temperature in the presence of the additives. The addition of box, glucose, and 20% (w/v) PEG 6000 to the surfactant aqueous solutions resulted into macrophase separation at elevated temperatures. The transfer functions for the partial molar volumes of surfactant solutions in water and water + additives indicated that the additives preferentially hydrate themselves and thus weaken the interactions between oxyethylene parts of the surfactant molecules and water. The solvent environment for the surfactant molecules in the presence of additives has been found to be less favorable as more and more surfactant molecules were found to be pushed to the surface and the area per surfactant molecule became less. The worsening of solvent quality of water in the presence of additives is perhaps responsible for the observed decrease in the critical micelle concentrations. The thermodynamics of micellization of silicone surfactants in mainly governed by entropy of micellization. The influence of cosolute additives on the micellization was monitored from the calculation of free energy changes for the process in mixed solvent systems and water. The values of free energy of adsorption were also calculated. The analysis of these functions revealed that the additives

Langmuir, Vol. 19, No. 17, 2003 6677

facilitate the micelle formation at lower concentrations and at the same time-compensating work has to be done for the transfer of surfactant molecules from the surface to micellar associates. The characteristic changes in the structure of micellar associates were monitored from the SANS measurements. The analysis of SANS distributions at 30 °C showed that with the introduction of additives as cosolutes in to surfactant aqueous solutions, the oblate ellipsoidal micelles grow along the semimajor axis. The enlargement in the size can be attributed to the dehydration effects on the micelles by the additives due to their preferential hydration tendencies. Interestingly, the rise in the temperatures close to region of turbidity produced drastic changes in micellar parameters. The shape of the micelles has shown a oblate ellipsoidal-to-disklike transition at elevated temperatures. The radius and thickness of the disk become considerably larger as more and more surfactant molecules have been added to the micellar associates. The calculated changes in the hydration of micelles, W, revealed that W values increased when the additives are introduced in to the solution and the temperature is raised. This rather unexpected trend was explained by considering preferential or successive dehydration patterns for various parts of the micellar associates. Acknowledgment. The authors thank Dr. B. A. Dasannacharya, Director, IUCsIndore, for his useful suggestions and interest in this work. One of the authors (S.S.) thanks IUC-DAEF, Mumbai, for granting a research assistantship under a collaborative research scheme (Grant No. IUC/CRS-M-73/362-66). Supporting Information Available: Tables listing the data on relative permittivities, r, free energy of micellization, ∆G°mic, enthalpy of micellization, ∆H°mic, entropy of micellization, ∆S°mic, ∆G°M, and ∆G°ads for SS-1 and SS-2 surfactant solutions in water and water + additives at different temperatures and figures showing the variation of cloud points and surface tension with the surfactant concentrations for SS-1 and SS-2 surfactants in water and water + additive solutions. This material is available free of charge via the Internet at http://pubs.acs.org. LA034389Q

Study on the Effects of Nonelectrolyte Additives on ... - ACS Publications

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