Effect of t-octylphenoxylpolyethoxyethanol (TX-100) on the dilute aqueous solution phase diagrams, surface activity and micellization behavior of non-ionic silicone surfactants (SS) in aqueous media

Journal of Molecular Liquids 177 (2013) 215–224

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Effect of t-octylphenoxylpolyethoxyethanol (TX-100) on the dilute aqueous solution phase diagrams, surface activity and micellization behavior of non-ionic silicone surfactants (SS) in aqueous media N.V. Sastry ⁎, S.H. Punjabi, I.R. Ravalji Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India

a r t i c l e

i n f o

Article history: Received 4 July 2012 Received in revised form 8 September 2012 Accepted 19 September 2012 Available online 8 October 2012 Keywords: Silicone surfactants TX-100 Surface activity Dissociation effect Mixed micelles

a b s t r a c t Dilute aqueous solution phase diagrams, surface tension, dynamic light scattering (DLS) and dilute solution viscosity measurements have been made in aqueous solutions of nonionic silicone surfactants (SS) in presence of t-octylphenoxylpolyethoxyethanol (TX-100). Critical micelle concentration, thermodynamic parameters of association, interfacial characteristics such as pC20, surface excess, Γmax , area per adsorbed molecule, a1s along with the size and shape and intrinsic viscosities of the associates have been determined and discussed to ascertain the effect of TX-100 as an additive in SS aqueous solutions. The importance of hydrophobic/hydrophilic (hp/hl) ratio of the SS rather than their molecular architecture and molar mass on their overall interaction with TX-100 micelles has been reflected in the above parameters for the mixture solutions. It was observed that the mixed species in SS aqueous solutions in TX-100 are more hydrophilic in nature and the overall association process in the mixture solutions is entropy driven. TX-100 addition to SS micellar solutions induces the dissociation of SS micelles. As compared to the room temperature, the mixed micelles at elevated temperatures were large in size suggesting that the ellipsoidal micelles in mixed state grow along their major axis that is typically characteristic of the micelles of the individual nonionic SS or TX-100 surfactants at temperatures close to the turbid point. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Silicone surfactants (SS) are prepared by modifying the silicone compounds. Even though the silicone compounds in general are self-emulsifying and provide softening, conditioning and treatment effects when mixed with other compounds in complex product formulation, one of the major disadvantage of the silicone compounds however is that by their chemical nature, they are just water insoluble oily materials. Therefore their application in water based formulations severely gets limited. In order to achieve the deliverability of silicone compounds in aqueous media, attempts have been made to modify the silicone compounds by incorporating hydrophilic functional groups. In some instances, silicone is incorporated into a surface-active agent with a polyoxyalkylene portion of the molecule and/or a hydrocarbon portion of the molecule. These water soluble modified silicone compounds are called as silicone surfactants and have emerged as specialty surfactants [1,2]. Silicone surfactants have excellent chemical stability and low skin irritation. These special features of SS have logically attracted the practicing surfactant chemists. In fact SS have been widely used as wetting and spreading agents, lubricants, as water— in oil emulsifiers, better wetting agents in inks, ⁎ Corresponding author. Tel.: +91 2692 226856x224; fax: +91 2692 236475. E-mail address: [email protected] (N.V. Sastry). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.09.014

paints and coatings, agricultural adjutants in effective spreading and penetration of herbicides onto plant leafs, compatibilizers between water and hydrocarbon oils in shampoo and showering gels, wetting, dispersing and foam control agents in diesel fuel defoaming and emulsifiers, foam and parting agents in the production of elastomeric polyurethane fibers, surface modifying additives for polymers, textile auxillaries, etc. [1,2]. The traditional oil soluble part of the conventional surfactant molecules is a higher alkyl or fatty chain and therefore the silicone surfactants substitute (or add) the hydrocarbon based hydrophobicity with silicone based hydrophobicity. The molecular architecture of silicone surfactants in general may consist of permethylated siloxane group coupled to one or more polar groups. Several polar groups such as nonionic (polyoxyethylene or polyoxyethylene/poloxypropylene or carbohydrates), anionic sulfates, cationic quaternary ammonium salts, zwitterionic betaines, etc. have been successfully incorporated. Besides these architectural variations, silicone surfactants are also designed to be polymeric as well as non-polymeric. Graft-type or rake type or ABA or BAB copolymers are the examples of polymeric architecture, while linear, or branched or cyclosiloxanes represent the nonpolymeric architecture [3,4]. The silicone backbone part in the silicone surfactants and the presence of methyl groups on silicone atoms enable them to possess very unusual and unique properties i.e. high surface activity both in aqueous and non-aqueous media (such as toluene, methanol, 2-butoxyethanol, etc.), ability to lower the surface tension of water up

216

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

to 20 mNm −1 and exist in pure liquid state even to high molar mass range, etc. [4]. In the applications cited above, the general physical properties such as adsorption at surfaces/interfaces, phase and aggregation behavior in a given solvent (or water) media are highly relevant [3,4]. The aggregation properties of silicone surfactants in aqueous media are yet to be fully understood. Few available literature studies indicate that non-ionic type silicone surfactants in water form bilayer phases and vesicles [1,5–10] rather than usual well known self-associated micellar structures, liquid crystalline and gel phases formed by oxyethylene– oxypropylene-oxyethylene (or EO/PO/EO) amphiphilic copolymers or hydrocarbon surfactants. Our own research group has reported the surface adsorptive and aggregation behavior of graft-like non-ionic polyether modified polydimethylsiloxanes in water as well as in presence of hydrophilic additives through surface tension, small angle neutron scattering, dynamic light scattering and dilute solution viscosity measurements [11–13]. Our results indicate that the polyoxyalkylene group in SS is the main reason for the inverse temperature solubility or clouding behavior, exhibited by SS in aqueous solutions. The micellar associates of SS in aqueous media were of poly-disperse in nature with an oblate ellipsoidal shape at room temperature. The micellar size and association number (i.e. number of SS molecules in a micelle) systematically increased resulting into large structures of higher geometry namely discor rod-like either with the increase in temperature or by the addition of hydrophilic non-electrolytes. The micellar growth and the appearance of clouding in aqueous solutions are found to be closely related with each other [13,14]. Silicone surfactants are seldom used in formulation systems as the sole material and often the mixed systems of silicone surfactants and fatty alkyl chain or hydrocarbon chain based conventional surfactants are invariably employed together. Since the surface active properties of silicone and hydrocarbon surfactants differ widely, it would be very interesting to investigate the colloidal behavior in mixture solutions of these two types of surfactants. Hill [15] had determined the critical micelle concentration (CMC) and surface active parameters for several mixtures of non-ionic silicone surfactants (based on either trisiloxanes or comb-like polyether modified poly(dimethylsiloxanes) and simple classical hydrocarbon surfactants such as sodium dodecyl sulphate (SDS), dodecyltrimethylammonium bromide (DTAB) and a non-ionic oxyethylene dodecyl ether, C12H25(EO)17. The CMCs of the mixtures were analyzed in terms of regular solution theory and the nature of interaction between the two components was reported to be dependent highly on the ionic character of the hydrocarbon surfactants. Silicone surfactant + cationic surfactant mixtures were found to behave ideally while the SS+ anionic surfactant or+ nonionic surfactant mixtures exhibited either negative or positive deviations from ideal mixing respectively. The large deviations in the later mixtures were attributed to the probable unfavorable interactions between the two components. Sastry and Hoffmann [16] had also reported detailed static light scattering measurements on series of mixtures of ionic (anionic, cationic) and non-ionic silicone surfactants with anionic SDS, cationic tetradecyl- trimethylammonium bromide (TTAB) and zwitterionic tetradecyltrimethylaminoxide (C14DMAO). The scattered intensities of light in forward and backward mode were recorded and the total micellar weight was determined. It was suggested that (i) the mixture solutions in general consisted associates of anisotropic and large structures, (ii) SDS interacts strongly with the anionic and zwitterionic silicone surfactants leading to mixed micelle formation, (iii) the cationic TTAB interaction with cationic silicone surfactant is weak and their mixed micelles resembled more of silicone surfactant type and (iv) cationic TTAB addition to the zwitterionic silicone surfactant led to the formation of micelles with reduced size. The zwitterionic C14DMAO addition in excess amount however had a destabilizing effect on the silicone surfactants micelles. These results indicate that the solution behavior in mixed systems of silicone and classical hydrocarbon surfactants is complex in nature and highly depends both on the molecular

architecture of silicone surfactant and the ionic character of mixture components. In terms of morphological features, the micelles of silicone surfactants are similar to the micelles of simple ionic or nonionic surfactants [17]. Therefore it is believed that the mixed systems of SS of different architecture + ionic or non-ionic hydrocarbon surfactants are well suited as model systems for monitoring the interactions and morphological changes in the micelles of mixed systems. Moreover the performance properties of micelles in the mixed systems can also be fine tuned or modulated by just changing the composition of mixtures. In fact the mixed micelles have been employed in various biological, technological and industrial fields of applications such as drug loading and delivery systems, paint formulations, cosmetics, detergents, wetting agents, etc. Most pertinent points that need to be generally addressed (as far as silicone and hydrocarbon surfactant mixtures are concerned) are: how and why the micelles of silicone surfactant are dissolved or destabilized in presence of conventional hydrocarbon surfactants; which morphological form the mixed micelles are made of. How the architecture of silicone surfactants affect the interaction between the two components, etc. The present study attempts to address few of these issues. We have selected two non-ionic silicone surfactants with either a graft-like or trisiloxane architecture and explored the effect of a non-ionic hydrocarbon surfactant, t-octylphenoxylpolyethoxyethanol (TX-100) on solution phase characteristics, surface active and association features of SS in water through dilute aqueous solution phase diagrams, surface tension, dynamic light scattering (DLS) and dilute solution viscosity measurements in aqueous media. Triton X-100 is specially selected because it is a polyoxyethlyne based surfactant and hence is highly water soluble. It is a viscous liquid with high boiling point (270 °C). It has been used in many already reported applications covering a wide range of different disciplines. Its utility as a wetting agent in the microscopy and histology laboratory and in certain staining protocols [18] and also in the cleaning of diamond knives [19] is well reported. Its ability to wet silicone wafers to enhance and speed up certain surface related procedures and operations is also documented [20]. It is also often used in the lowest possible concentration as an aid for dissolution of protease in water [21]. Other established uses of TX-100 are in solubilizing membrane proteins in their native state in conjunction with zwitterionic detergents [22], in extraction of DNA along with lysis buffer [23], in dispersion of carbon materials for preparing soft composite materials [24] and in formulations for emulsion polymerization, etc. [25]. In all these applications, the adsorption at a given surface and association in the bulk media play a major role. There is a need to find solution conditions in which the surfactant performance of TX-100 can be enhanced. Therefore the understanding of colloidal behavior in SS-TX-100 mixed systems would be very promising and of great help in establishing the optimum conditions for inventing such novel and high performing mixture formulations of technical and application value. 2. Experimental 2.1. Materials SS (with a common structure shown in Chart 1) were obtained as gift samples from Th. Goldschmidt AG, Germany. m and n represent the polyether modified methylsiloxane and hydrophobic poly (dimethylsiloxane) units and x and y denote the number of (oxyethylene) and (oxypropylene) units in the polyether grid. The values of m, n, x and y are summarized in Table 1. It can be seen that SS are based on polyether modified polydimethylsiloxanes. That hydrophilicity of the silicone surfactants is due to x units of oxyethylene (EO) in a given sample. These products are of industrial origin with an average formula and therefore the values of m, n, x and y are average numbers, and the branch groups are statistically

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

Chart 1. Structure of silicone surfactants.

distributed over the silicone chain. The silicone surfactants were used as received without further purification. TX-100 was an Aldrich product with a stated purity of 98% (on mass basis) and was used as received.

3. Methods The stock solutions of silicone surfactant in desired concentrations were first prepared by dissolving their known amount in triple distilled water under swirling on a magnetic stirrer to ensure homogenous and thorough mixing. A stirring period of 60–90 minutes was found to be sufficient. After allowing the stock solution to stand for a day, dilutions were made to get the other concentrations. The solutions were always stored in stoppered glass vials. We did not find any hydrolysis in the silicone surfactant solutions at least for 4 weeks. The stock solution of TX-100 was prepared by just simple dissolution of its required amounts with a care to minimize the foam formation. The solution was allowed to stand for a few hours till the over head foam (if any) is fully settled and the stock solutions were further diluted to the desired concentration. The effect of TX-100 as an additive on surfactant properties of SS in water was monitored either by preparing the individual SS in TX-100 aqueous solutions or by mixing the solutions of individual SS or TX-100 in different volume ratios. The dilute aqueous solution phase diagrams were constructed by plotting the temperature corresponding to the change of appearance of solution, as a function of silicone surfactant concentrations. The solutions were taken in to a corning glass long tube placed in a water bath. The temperature of the bath was raised with a heating rate of 2 °C per minute. The boundary temperatures corresponding to the appearance of turbidity, cloudiness, full dense clouds, etc. in the solutions were noted by visible observation. The surface tension of solutions was determined at three different temperatures by drop weight method using a modified stalagmometer [26]. The stalagmometer assembly consists of pyrex glass bulb of spherical shape with a capillary tube attached at filling and dropping ends. The capillary tube at the dropping end is blown into a two fold U shape and the tip of the end is grounded in the form a fine cone. By this way, the drops formed were broken under their own weight and uniform drop shape and size is also ensured. A weighing bottle (thoroughly stoppered) is attached to the dropping end through a rubber septum. The same is attached to a tube containing a capillary through which the drops are formed. The stalagmometer assembly along with the pre-dried and -weighed bottle were lowered in to a thermostatic water bath maintained at the desired temperature controlled

Table 1 Molecular characteristics of silicone surfactants (SS) and TX-100.

SS-1 SS-3 TX-100

Type

m

n

x

y

% EO

Molar mass (g/mol)

hp/hl* ratio

Comb Trisiloxane Linear

5 1

13 0

12 12

0 3

100 80

4360 980 646

0.3 0.5 0.5

⁎ hp hydrophobic,

hl

hydrophilic.

217

to ± 0.05 °C. A 30-minute time of equilibration was always allowed. Then known number of drops (>20) of given solution and reference triple distilled water were allowed to fall into the weighing bottle in separate runs. The weight of solutions as well as triple distilled water (drawn from separate runs) was instantly recorded on a single pan analytical balance (with a weight accuracy of ±0.00001 g). The surface tension of the individual solution was then calculated from known values of surface tension of water, densities and weight of solution and water respectively The stalagmometer was tested for pure and reference organic liquids (carbon tetrachloride, n-hexane, etc.) and also sodium dodecyl sulfate aqueous solutions. Our determined surface tensions are found to agree with the literature reported values by ± 0.2% [27]. The DLS apparatus consisted of a laboratory goniometer with two arms. One of these arms housed the excitation source, which was a solid state frequency doubled Nd:YAG laser radiating at a wavelength of 532 nm. The other arm of the goniometer has the photo multiplier tube mounted on to it enabling angle dependent detection of scattered light. Experiments were carried out at a fixed scattering angle of 90°. The solutions were ultra centrifuged at ~ 10,000 rpm for about half an hour, then directly loaded into optical quality 5 cm 3 borosilicate cell and sealed. Then cell was held inside a homemade temperature controller in which the temperatures within a range of 15–75 °C could be maintained to ± 0.1 °C [28]. Scattered light from sample solutions was detected by the photo multiplier tube, and the photocurrent was suitably amplified and digitized before it was fed to a 1024 channel digital correlator (Brookhaven Instruments Inc., USA, model BI-9000AT). In all experiments, the difference between the measured and calculated baseline was not allowed to go beyond ±0.1%. The data that showed excessive baseline difference were rejected. All the data exhibited a single narrow distribution. The uni-modal particle sizes were determined through the CONTIN software provided by the Brookhaven instrument company. The flow times for the individual SS solutions in water or in TX-100 aqueous solutions were measured using Ubbelohde suspended level viscometers. Two viscometers were used to cover the flow time range of 130–360 s. Three consecutive flow times agreeing within ± 0.02 s were taken and the mean flow time was considered. Since the flow times recorded were always greater than 130 s, no kinetic corrections were invoked. 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 cm 3 making drainage corrections unimportant. Viscometers were suspended in thermostatic water baths maintained at a constant temperature maintained to ± 0.05 °C. 4. Results and discussion 4.1. Dilute aqueous solution phase diagrams for SS in water or in TX-100 aqueous solutions The dilute aqueous solution phase diagrams for SS-1 or SS-3 aqueous solution in presence of TX-100 were constructed. The solutions were prepared in two different ways. In the first set of solutions, a series of silicone surfactant (s) aqueous micellar solutions containing a fixed amount of TX-100 (i.e. 0.2 or 1.2 mM) were prepared and their solution phase characteristics were established. Two concentrations of TX-100 were selected such that one is close to its CMC (0.19 mM) and another one is far higher than its CMC. By this way, the effect of TX-100 as an additive on the solution phase characteristics of micellar solutions SS was assessed. In the second set of solutions, priorily prepared solutions of individual components i.e. SS or TX-100 of a given concentration were mixed in different volume ratios to obtain binary mixtures with varying mole fractions for each of the components. The representative dilute aqueous solution

218

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

100

a Phase seperation

90 Fullydense clouds

80 Cloudy Turbid

70

Clear

60

0

5

T/( C)

50

10

15

20

2

o

[SS-1] /(mM) 100

b 90 80 Phase seperation

70 Fullydense clouds

60 50 40

Cloudy Turbid

Clear

0

25

50

75

100

125

[SS-3] /(mM) Fig. 1. Dilute aqueous solution phase diagrams for silicone surfactants in 0.2 mM of TX-100 aqueous solutions: (●) final cloud points: (a) SS-1 and (b) SS-3.

phase diagrams for SS-1 or SS-3 solutions in presence of 0.2 mM of TX-100 are shown in parts a and b of Fig. 1 (Similar diagram for SS-1 or SS-3 in presence of 1.2 mM of TX-100 are shown in Fig. S1 of supplementary information). A perusal of Fig. 1 shows that turbidity sets in the solutions with the increase in the temperature and upon further warming the solutions become cloudy before fully dense clouds are formed throughout the solution at a temperature marked as cloud point (Cp). Solutions with high SS concentrations at elevated temperatures exhibited a clear phase separation. The profiles corresponding toss-1 or SS-4 in TX-100 aqueous solutions are distinctly different as far as the temperatures at which phase separation occurred and also the concentration range width corresponding to the phase separation. Not only the individual concentrations but

also its range (corresponding to phase separation) for SS-1 solutions were higher and broad as compared to the same for SS-3 solutions. Our previous studies showed that SS-1 solutions alone in water (1–10%) did not phase separate even at temperatures close to 95 °C [11]. SS-3 solutions in water (0.5–7.5%) however get phase separated (at temperatures of 90 °C to 72 °C) [12] and similarly TX-100 aqueous solutions (>3%) get phase separated at high temperatures [17]. SS-3 and TX-100 have a linear molecular structure and while SS-1 has a comb-like structure. SS-3 and TX-100 have similar hydrophobic to hydrophilic (hp/hl) ratios close to ≈ 0.5 and while the SS-1 sample has a hp/hl value of 0.3. Otherwise, the three of them have different molar mass. The data on Tp and Cp for the SS-1 or SS-3 solutions in water alone and in presence of TX-100 (as collected from Fig. 1 and Fig. S1) are listed in Table 2. A perusal of the data from the first half of the Table reveals that the Tp and Cp values in general increased slightly (except at very high SS concentrations). In fact, the solutions with very high SS-1 or SS-3 concentrations were characterized by less Cp values. The results clearly indicate that that TX-100 interacts strongly with SS-3 and the mixed systems as a whole have more hydrophilic character except at high SS-3 concentrations in which the SS-3 micelles seem to play a predominant role leading to a phase separation exhibited typically by SS-3 micellar solutions alone in water. The interesting observation is that SS-1 solutions in presence of TX-100 get phase separated and while SS-3 solutions under the same conditions remain clear in appearance. This indicates that TX-100 plays a dominant role in the solution phase characteristics in mixed state. In order to gain more insight into the relative contribution of each of the components to the phase characteristics of solutions in mixed state, the solution profiles of mixtures prepared across the composition (second set of mixtures solutions) were constructed and the same are shown in parts a and b of Fig. 2 and the data on Tp and Cp are listed in the second half of Table 3. It can be seen that both the transition temperatures displayed a maxima (in the lower mole fraction ratios of xTX/xSS-1) followed by a smooth decrease and while the same increased smoothly (in low mole fraction ratios) before plateauing in SS- 3–TX-100 mixtures. Otherwise, the individual transition temperatures at any given composition are higher for SS-1–TX-100 mixture solutions as compared to SS-3–TX-100 binary systems. It can also further be observed from part a of Fig. 2 that few initial mixtures of SS-1–TX-100 exhibited phase separation and while no such phase separation was noticed at all in SS-3–TX-100 mixtures. From these straight and simple observations, a comment can be made on the relative strength of interactions between SS and TX-100 components. Trisiloxane based SS-3 is relatively more hydrophobic (hp/hl = 0.5) than comb-like SS-1 silicone surfactant and accordingly the mixture solutions containing SS-1 have higher Tp and Cp than the mixtures containing SS-3, even though SS-1 has an higher molar mass (4.4 times) than SS-3. Therefore it can be inferred that the relative hp / hl value of a given silicone surfactant seems to be more important than their molar mass as far as the phase behavior in mixture solutions is concerned.

Table 2 Turbidity (Tp) and cloud points (Cp) for various concentrations of silicone surfactants in TX-100 aqueous solutions (concentration in mM). Tp/(°C) [SS-1]

[TX-100]

2.3 5.7 11.5 17.2 22.9

0 67 50 52 56

Cp/(°C)

0.2 72.0 70.0 67.0 65.5 65.0

1.2 75.0 71.5 68.0 66.0 64.0

0 79 76 79 86

Tp/(°C)

0.2 84.0 82.0 79.5 77.0 75.0

1.2 86.0 84.0 82.5 80.0 79.0

[SS-3]

[TX-100]

10.2 25.5 51.0 76.5 102

0 55.0 53.0 52.5 51.0

Cp/(°C)

0.2 56.0 54.0 52.0 50.0 48.0

1.2 57.0 55.0 53.0 51.0 49.0

0 58.5 59.0 60.0 62.0

0.2 61.0 59.0 57.5 56.0 55.0

1.2 63.0 61.0 59.0 57.0 55.0

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

100

219

60

a

a

Phase seperation

50

90 Fullydense clouds Cloudy

40

80 Turbid

30

70

0

5

10

15

20

25

30

T/( C)

60

Clear

20 1E-4

1E-3

0.01

0.1

4

1

o

60

b 70

b

50

Fullydense clouds

65

40 Cloudy

60 30

Turbid

55

Clear

20 1E-4

1E-3

0.01

0.1

1

4

60 50 0.0

0.2

0.4

0.6

0.8

c

1.0 50

Fig. 2. Dilute aqueous solution phase diagrams for silicone surfactants + TX-100 mixtures: (●) final cloud points; (a) SS-1 and (b) SS-3.

40 4.1.1. Surface tension isotherms and critical micelle concentration (CMC) of SS in TX-100 solutions In order to study the effect of TX-100 on the CMC, surface active and thermodynamic parameters of association for SS solutions in water, surface tension isotherms were constructed at three different temperatures for the SS aqueous solutions containing TX-100 as an additive. The two concentrations of TX-100 were kept far below its CMC in order to get a spread of surface tension measurements so that well defined isotherms can be obtained. The surface tension isotherms for SS-1 in TX-100 aqueous solutions at 15, 30 and 45 °C are shown in Fig. 3 (similar isotherms for SS-3 in TX-100 aqueous solutions at the three different temperatures are depicted in Fig. S2 of supplementary information). The surface tension profiles for the TX-100 and silicone surfactant aqueous solutions alone are also included in the figures for a comparison purpose. The surface tension measurements in very dilute solutions (of SS-1 or SS-3) could not be made due to the limitation in the drop weight method. Otherwise the profiles in general are of typical L shape with a well defined break point assigned to the CMC value for respective solution. A close scrutiny of the isotherms given in Fig. 3 and Fig. S2 reveals that the addition of TX-100 to SS-1 aqueous solutions increased the surface tension values and in fact at high TX-100 concentrations (selected in the present study), the same are almost close to that of TX-100 aqueous solutions at 15 °C. As compared to hydrocarbon based TX-100, the silicone surfactants effectively adsorb at the air/water

30

20 1E-4

1E-3

0.01

0.1

1

4

log CSS-1/(mM) Fig. 3. Surface tension versus log concentrations of SS-1 in TX-100 aqueous solutions at different temperatures (a) 15 °C (b) 30 °C and (c) 45 °C: TX-100 concentrations in mM: (□) 0, (●) 0.004, (▲) 0.039, (○) TX-100 alone in water.

interface and hence decrease the surface tension of water because of their inherent high hydrophobicity associated with the permethylated groups on silicone back bone [1,2]. Perusal of the surface tension isotherms for the same solutions at 30 °C reveals that the difference in surface tension values between the two individual surfactants gets narrowed down indicating that the TX-100 molecules dominate the surface active behavior in solutions at this temperature. Interestingly at 45 °C, the SS aqueous solutions containing high TX-100 concentration are characterized by values higher than that of TX-100 solutions alone in water indicating that the increase in temperature not only enhances the interaction between the two

220

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

surfactant components but also increases the role of TX-100 molecules. The surface tension values for SS-3 aqueous solutions containing TX-100 at (15 and 30) °C were close to that of SS-3 solution in water and while at 45 °C the same are close to TX-100 in water. This indicates that of SS-3 molecules dominate the adsorption at the interface at 15 and 30 °C. 4.1.2. CMC and surface active parameters The CMC and surface active parameters namely pC20, CMC/C20, surface excess concentration, Γmax and surface area occupied per molecule at air / water interface, a1s were calculated using the relations: pC20 =−log C20, where C20 is the concentration needed to reduce the surface tension of water Γ max ¼ −1=RT ½∂γ=∂lnCT:P

ð1Þ

s

a1 ¼ 1=NA Γ max

ð2Þ

where R is the gas constant, NA is the Avogadro's number, γ is the surface tension and C is the concentration of the silicone surfactant in solution. The data of CMC along with the various surface active parameters at three different temperatures for the individual SS or TX-100 surfactants and SS in aqueous TX-100 solutions are summarized in Table 4. A perusal of the data reveals that the CMC values of SS-1 in TX-100 aqueous solutions are slightly more than that of SS-1 in water and while CMC values of SS-3 in TX-100 aqueous solutions are close to that of SS-3 in water. This indicates that TX-100 molecules preferentially dominate the micellization process of SS-1. SS-3 molecules as such are more hydrophobic in nature and hence have weak affinity with water. Therefore SS-3 molecules have more interfacial free energy. In order to decrease the interfacial free energy, the hydrophobic molecules would coil themselves and minimize their exposure to water. Such coiling would however does not favor the micelle formation and therefore SS-3 has higher CMC values than SS-1 in water. Since the TX-100 addition to SS-3 did not result in drastic change of its CMC values, it can fairly be assumed that the hydrophilic oxyethylene units of SS-3 are mainly responsible for its interaction with TX-100. The CMC values of the individual SS or TX-100 in water and also SS solutions in TX-100 decreased with the increase in the temperature and such dependence of CMC on temperature is typical of non-ionic surfactants in water [29,30]. Therefore it is emphasized that the hydrophilic nature of SS molecules is enhanced in presence of TX-100 and has a major role in the overall micellization process. pC20 measures the efficiency of adsorption of a surfactant at the air / water interface. In general larger pC20 values indicate better adsorption at the interface i.e. higher efficiency to reduce the surface tension and also increased tendency for the micellization. Since the pC20 values for SS-3 aqueous solutions are smaller than that of SS-1, it would be stated that the propensity of SS-3 molecules to adsorb at the air/water interface is high and therefore its adsorption layers are better compact. TX-100 solutions in water are characterized by Table 3 Turbidity (Tp) and cloud points (Cp) data for silicone surfactants–TX-100 mixture solutions.

X

XSS-1

XTX-100

Tp (°C)

Cp (°C)

XSS-3

XTX-100

Tp (°C)

Cp (°C)

1 0.7274 0.5425 0.4088 0.3078 0.2216 0.1650 0.1131 0.0694 0.0311 0

0 0.2726 0.4575 0.5912 0.6922 0.7714 0.8350 0.8869 0.9306 0.9689 1

67 67 72 75 78 80 80 78 76 73 61

79 80 83 85 87 90 88 86 84 80 70

1 0.9223 0.8410 0.7536 0.6634 0.5686 0.4677 0.3618 0.2491 0.1280 0

0 0.0777 0.1590 0.2464 0.3364 0.4314 0.5323 0.6382 0.7509 0.8720 1

58 53 54 55 56 57 57 56 55 56 61

61 59 61 62 63 63 64 63 62 63 70

is the mole fraction.

lowest of pC20 values. The pC20 values gradually decreased in presence of TX-100 at the three temperatures indicating that the adsorption characteristics of the mixed species would be similar to that of TX-100 in water. Similarly, the area adsorbed per molecule among the three individual surfactants followed the order: SS-1 > SS-3 > TX-100. The a1s values decreased in the presence of TX-100 indicating that TX-100 molecules also play a predominant role in determining the orientation and packing of the molecules at the air /water interface. CMC/C20 ratio is another important factor that indicates whether the molecular species have preference for adsorption at interface or for association in the bulk [31]. As compared to the TX-100 in water, the solutions of both of the silicone surfactants in water are characterized by large CMC/C20 values and this is not unexpected keeping in mind the inherent hydrophobic property of permethylated siloxane back bone moiety that push the SS molecules to adsorb preferentially at the air/water interface and result in effective decrease of the surface tension of water. The solutions of SS in TX-100 are marked by very much decreased CMC/C20 values and this supports our earlier conclusion that TX-100 molecules dominate the adsorption and micellization process in mixture solutions. The observed drastic decrease of CMC/C20 values with the increase in the temperature can be a direct consequence of large decrease in CMC values with the increase in the temperature. 4.1.3. Thermodynamic parameters of micellization in mixture solutions The standard free energy, enthalpy and entropy of micellization were calculated from the CMC data at different temperatures using the following relations: o

ΔG

Mic

  CMC ¼ R T ln ω

ð3Þ

where ω is the molarity of water, R is gas constant and T is the temperature, ΔH

o Mic

¼ −R T

2

  ∂ lnCMC ∂T

ð4Þ

and ΔS

o Mic

¼

ΔHo Mic −ΔGo Mic T

ð5Þ

The summary of the thermodynamic parameters for the individual surfactants as well as SS in TX-100 aqueous solutions is given in Table 5. It can be seen that ΔG omic is negative for the individual surfactants and also for the SS solutions in TX-100 indicating that the micelle formation is spontaneous. As compared to TX-100, the micellization of the silicone surfactants is characterized by more negative ΔG omic values. The TX-100 addition decreased the negative magnitude of ΔG omic. Hence it can be stated that the TX-100 decreases the micellization tendency of SS in water and the same is probably attributed to the enhanced hydrophilicity of the mixed species in solutions. The enthalpy of micellization, ΔH omic for the individual silicone surfactants is large and positive as compared to the TX-100 in water and the same can be attributed to presence of un-favorable repulsions among the hydrophobic permethylated silicone back bone segments. The presence of TX-100 even though decreased the positive magnitude of ΔH omic values and the SS solutions in TX-100 aqueous solutions are still characterized by ΔH omic values larger than that of for TX-100 in water. The increase in temperature in general decreased ΔG omic and ΔH omic values indicating that the micelle formation becomes more spontaneous and silicone segment– silicone segment repulsions are equally compensated by structure disruptions in water molecules at high temperatures. The TΔSomic term however was always found to be positive for the individual as well as for SS in TX-100 aqueous solutions and in fact the increase in the

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

221

Table 4 CMC, pC20, surface excess, Γm and area adsorbed per molecule, a1s for silicone surfactants in water and in TX-100 aqueous solutions at different temperatures. T/(°C) 15 30 45 15 30 45 15 30 45 15 30 45

CMC (mM)

pC20

CMC / C20

SS-1 in water⁎ 0.12 ± 0.02 7.3 2279 0.09 ± 0.02 6.8 625 0.06 ± 0.02 6.0 59 TX-100 in water 4.95 ± 0.02 4.9 4.02 ± 0.02 4.8 3.25 ± 0.02 4.8 SS-1 in 0.004 mM TX-100 0.37 ± 0.02 6.6 624 0.25 ± 0.02 6.2 192 0.16 ± 0.02 5.7 37.8 SS-1 in 0.039 mM TX-100 0.46 ± 0.02 6.1 219 0.34 ± 0.02 5.9 130 0.25 ± 0.02 5.5 34

Γm × 1010 (mol cm−2)

a1s (Ǻ2)

T/(K)

1.3 ± 0.1 1.3 ± 0.1 1.2 ± 0.1

129 ± 5 130 ± 5 133 ± 5

288.15 303.15 318.15

1.9 ± 0.2 2.0 ± 0.2 2.1 ± 0.2

87 ± 4 87 ± 3 87 ± 4

288.15 303.15 318.15

1.4 ± 0.1 1.4 ± 0.1 1.5 ± 0.1

119 ± 4 119 ± 4 110 ± 4

288.15 303.15 318.15

1.5 ± 0.1 1.5 ± 0.1 1.6 ± 0.1

110 ± 4 110 ± 4 104 ± 3

CMC (mM)

pC20

CMC/C20

SS-3 in water⁎ 3.37 ± 0.06 7.0 3438 2.14 ± 0.04 6.3 438 1.02 ± 0.02 5.4 25 SS-3 in 0.004 mM TX-100 3.47 ± 0.02 6.6 1467.8 1.94 ± 0.04 5.8 145 1.12 ± 0.03 5.3 22.2 SS-3 in 0.039 mM TX-100 3.57 ± 0.02 5.7 193 2.24 ± 0.04 5.7 115 1.22 ± 0.03 5.7 16.3

Γm × 1010 (mol cm−2)

a1s (Ǻ2)

1.9 ± 0.1 1.8 ± 0.1 1.8 ± 0.1

89 ± 2 92 ± 2 93 ± 2

1.9 ± 0.1 1.9 ± 0.1 1.9 ± 0.1

87 ± 4 87 ± 4 87 ± 4

1.9 ± 0.1 1.9 ± 0.1 1.9 ± 0.1

87 ± 5 87 ± 4 87 ± 2

⁎The CMC and surface active parameters for SS-1 and SS-3 in water at T = (15 and 45) °C are taken from our earlier work [11,12].

temperature made this term to be large and positive. The water around the oxyethylene groups is more structured as compared to the same in the bulk state. As micellization sets in, the individual surfactant molecules get organized within a micelle and hence displace the water molecules leading to the increased over all randomness or entropy values. Therefore it can be concluded that the addition of TX-100 to silicone surfactant solutions decrease the hydrophobic segmental repulsions without affecting the entropy contribution and therefore the association process for SS in TX-100 aqueous solutions is still remains entropy driven. 4.1.4. DLS measurements and morphological features of micelles of individual surfactants and their mixtures in aqueous solution DLS measurements were made to study the effect of TX-100 on the size and shape of the micelles of silicone surfactants in water. The chosen concentrations of TX-100 as an additive were same as in dilute aqueous solution phase diagram experiments. The mean translational diffusion coefficient, D̄ values for a series of either SS-1 or SS-3 aqueous solutions in presence of TX-100 were determined and the same found to increase linearly with the concentration (please see Fig. 4). The data could be fitted to an equation of type,  ð1 þ k C Þ D¼D o D

ð6Þ

 is the z-average translational diffusion coefficient at infinite where D o dilution, C is the concentration of the silicone surfactant and kD is the

 values were used to calcudiffusion second virial coefficient. The D o late the translational friction coefficient ft. using the relation,  ¼ kB T D o ft

ð7Þ

where kB is Boltzmann constant and T is absolute temperature. Once the ft values are known, the micellar dimensions can be estimated by giving allowance to a particular shape to the micelles. For example, assuming a spherical shape to the micelles, their hydrodynamic radius, Rh can be estimated using the relation, ft = 6πηRh as per the well known Stokes law. Similarly Tanford's formulae [32] as listed below can also be applied by assuming the micelles as prolate or oblate ellipsoids of revolution with semi axes b and a respectively,   2 2 1=2 6 π η b 1−a =b   ft ¼ 1=2 1þð1−a2 =b2 Þ ln a=b

ðfor prolate ellipsoidÞ

ð8Þ

ðfor oblate ellipsoidÞ

ð9Þ

and

ft ¼

6π η a tan−1



1=2 2 2 b =a −1
1=2 b2 =a2 −1

By giving trial values to the semi axes in an iterative manner, we had attempted to reproduce the ft values (as calculated from mean

Table 5 Free energy, ΔGomic , enthalpy, ΔHomic and entropy, TΔSomic of micellization for silicone surfactants in water and in TX-100 aqueous solutions at different temperatures. T/(K) 288.15 303.15 318.15 288.15 303.15 318.15 288.15 303.15 318.15 288.15 303.15 318.15

ΔGomic (kJ mol−1)

ΔHomic (kJ mol−1)

SS-1 in water⁎ −31.1 ± 1.0 34.9 ± 0.5 −31.5 ± 1.0 39.3 ± 0.5 −36.4 ± 1.0 42.6 ± 0.5 TX-100 in water −22.3 ± 1.0 19.3 ± 0.5 −24.0 ± 1.0 21.4 ± 0.5 −25.8 ± 1.0 23.6 ± 0.5 SS-1 in 0.004 mM TX-100 −28.6 ± 0.3 37.8 ± 0.3 −31.0 ± 0.4 41.9 ± 0.3 −33.7 ± 0.5 46.1 ± 0.5 SS-1 in 0.039 mM TX-100 −28.0 ± 0.3 27.6 ± 0.3 −30.2 ± 0.3 30.6 ± 0.4 −32.5 ± 0.5 33.7 ± 0.3

TΔSomic (J mol−1)

T/(K)

57.6 70.8 95.4

288.15 303.15 318.15

41.7 45.4 49.3

288.15 303.15 318.15

66.4 72.9 79.8

288.15 303.15 318.15

ΔGomic (kJ mol−1)

ΔHomic (kJ mol−1)

SS-3 in water⁎ −28.8 ± 0.5 26.2 ± 0.2 −30.6 ± 0.5 28.4 ± 0.2 −34.9 ± 0.5 31.9 ± 0.2 SS-3 in 0.004 mM TX-100 −23.2 ± 0.3 51.9 ± 0.6 −25.9 ± 0.4 57.5 ± 0.5 −28.6 ± 0.5 63.3 ± 0.4 SS-3 in 0.039 mM TX-100 −23.1 ± 0.3 49.3 ± 0.6 −25.5 ± 0.4 54.6 ± 0.6 −28.4 ± 0.3 60.1 ± 0.6

55.6 60.8 66.2

⁎The CMC and surface active parameters for SS-1 and SS-3 in water at T = (288.15 and 318.15) K are taken from our earlier work [11,12].

TΔSomic (J mol−1) 54.7 60.4 66.8 75.1 83.3 91.9 72.4 80.0 88.4

222

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

6.0

5.5

5.0

4.5

4.0

3.5

3.0

0

20

40

60

80

100

[SS] /(mM) Fig. 4. Average translational diffusion coefficients versus concentration plots for silicone surfactants in TX-100 aqueous solutions at 25 °C: SS-1 in (■) 0.2 mM TX-100, in (●) 1.2 mM TX-100; SS-3 in (□) 0.2 mM TX-100 and in (○) 1.2 mM TX-100.

translational diffusion coefficients at infinite dilution). By this way it was finally noted that the micelles of individual SS in water or SS in TX-100 aqueous solutions are of oblate ellipsoidal shape with a definite set of b and a values. The summary of various micellar parameters is given in Table 6. It may be mentioned here that our previous DLS measurements [12] on SS-1 and SS-3 solutions in water also indi o , b and a cated that their micelles are of elliposoidal shape. The D values for SS-1 or SS- 3 micelles in water are also listed in Table 6 so that a clear comparison between the morphological characteristics of SS micelles in water as well as in TX-100 solutions can be made. 4.1.4.1. Micelles of individual surfactants in water. Let us examine the micellar features of the individual surfactants in water first. It can be seen from the data of Table 6 that the SS-3 micelles in water are characterized by large b/a ratio and high association number as compared to the SS-1 micelles in water. Therefore it can be stated that higher the hp/hl ratio, more asymmetric would be the micellar geometry and higher would be the association number because of close or parallel alignment of hydrophobic parts in the core part of the micelles. A further scrutiny of the data from Table 6 revealed that the increase of the temperature from 30 °C to 60 °C resulted in the elongation of SS ellipsoidal micelles in water. As far as the geometrical features of TX-100 micelles in water are concerned, there exist only few literature reports. Miya et al. [33], Ruiz et al. [34] and Phillies et al. [34,35] had interpreted DLS data by assuming a spherical shape to

the TX-100 micelles with hydrodynamic radii, Rh values of 48.4, 47.0, 45 and 43 Å respectively at 25 °C. Phillies et al. [35] and Li et al. [36] had also concluded that both the association number and micellar radii of TX-100 micelles increased with the increase in temperature (5 to 50 °C). Therefore it can be concluded that the increase in temperature induces dehydration of SS or TX-100 micelles in water. However our detailed small angle neutron scattering (SANS) measurements on TX-100 solutions in D2O and their analysis revealed that TX-100 forms at best micelles of ellipsoidal shape with an average semi major and minor axes values of 39.6 and 11.3 Å and a number averaged association number of 190 (at 30 °C) [17]. In our data analysis the micellar core was considered to be dry i.e. devoid of any water. The treatment of SANS data therefore ignores the hydration effects in water swollen micelles and while DLS data analysis gives allowance to water of hydration in micellar structures. The difference in the analytical treatment of DLS or SANS data can be responsible for the obtained differences in the size parameters for the same micellar system using the two independent methods. Not only this, the association number of 190 for TX-100 micelles (SANS) is also higher than the values of 100 [34,35] and 105 [37] and 120 [38] (all from DLS, last two at 25 °C). SANS intensities are number averaged and while light scattering data are weight averaged. 4.1.4.2. Effect of TX-100 on SS micelles in water. A perusal of the DLS results from Table 6 further reveals that TX-100 addition did not result any appreciable changes in the dimensions of SS-1 micelles at 30 °C. However at elevated temperatures close to the Tp values, semi major axis of ellipsoidal mixed micelles increased indicating that the micelles grow along this axis. Our previous analysis of SANS measurements for SS-1 solutions (2% (w/v)) in aqueous solutions containing 14 to 93 mM of TX-100 revealed that TX-100 presence not only systematically decrease the semi major axis of the ellipsoidal micelles but also induce the demicellization in such way that the mixed micelles predominantly consist of TX-100 molecules [17]. As mentioned earlier, even though SS-3 and TX-100 belong to a different class, their hp/hl ratio are close to 0.5. Therefore it can be stated that the interaction between two different surfactants in aqueous media with similar hp/hl ratio would be strong. Accordingly, the semi axes as well as the axial ratio for the mixed micelles of SS-3 and TX-100 are less and their mixed micelles have a predominant contribution from TX-100 molecules / micelles. Therefore it can be concluded that the concentration of TX-100 as well as hp/hl ratio of interacting surfactants together contribute to the strength of interaction between silicone and hydrocarbon based non-ionic surfactants. The DLS measurements on mixtures of SS and TX-100 systems prepared to cover the full range of mole fractions of each of the components are expected to yield more information about the effect of one of the components on the micelles of another. We estimated

Table 6 Diffusion coefficient, D, dry core radius, Ro, semi major axis, b, semi minor axis, a, axial ratio, b/a, for silicone surfactant solutions in water and TX-100 aqueous solutions at different temperatures. T/(°C) 30a 60a 50a a

30 63b 48b 30a 65b 50b

 o × 107 (cm2 s−1) D

Ro (Å)

SS-1 in water⁎ 4.41 ± 0.15 3.94 ± 0.18 SS-1 in 0.19 mM TX-100 3.50 ± 1 3.90 ± 1 SS-1 in 1.2 mM TX-100 3.79 ± 1 4.21 ± 1

b (Å) 49 ± 3 70 ± 4

38 ± 1 67 ± 1

36 ± 1 63 ± 1

49 ± 1 114 ± 1

45 ± 1 105 ± 1

a (Å) 25 ± 1 25 ± 1

23 ± 1 23 ± 1

23 ± 1 23 ± 1

b/a 2.0 2.8

2.1 4.9

1.9 4.5

 o × 107 (cm2 s−1) D

Ro (Å)

b (Å)

A (Å)

b/a

72 ± 4

20 ± 1

3.6

160 ± 8

20 ± 1

8.0

39 ± 3

53 ± 1

22 ± 1

2.4

3.59 ± 1 SS-3 in 1.2 mM TX-100 3.53 ± 1

57 ± 5

93 ± 1

22 ± 1

4.2

37 ± 3

49 ± 1

22 ± 1

2.3

3.88 ± 1

55 ± 1

87 ± 1

22 ± 1

3.9

SS-3 in water⁎ 3.12 ± 0.06 2.26 ± 0.05 SS-3 in 0.19 mM TX-100 3.34 ± 1

⁎ The data on micellar properties are reproduced from our earlier work [12], a The diffusion coefficients extrapolated to zero Concentration of SS, b Diffusion coefficients at a single concentration of SS.

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

1.18

Table 7 Diffusion coefficient D, hydrodynamic radius Rh, of silicone surfactants–TX-100 mixture solutions at different temperatures. SS-1–TX-100

223

1.16

SS-3–TX-100

1.14

T, 30 °C XTX-100

 × 107 D (cm2 s−1)

Rh (Ǻ)

XSS-3

XTX-100

 × 107 D (cm2 s−1)

Rh (Ǻ)

1 0.7274 0.5425 0.4088 0.3078 0.2286 0.1650 0.1131 0.0694 0.0311 0 0.2286

0 0.2726 0.4575 0.5912 0.6922 0.7714 0.8350 0.8869 0.9306 0.9689 1 0.7714

3.89 4.20 4.47 4.78 5.15 5.69 6.68 6.41 6.56 6.88 7.11 5.40a 5.19b

64 58 55 52 47 43 40 38 37 36 34 80 129

1 0.9223 0.8410 0.7536 0.6634 0.5686 0.4677 0.3618 0.2491 0.1280 0 0.5686

0 0.0777 0.1590 0.2464 0.3364 0.4314 0.5323 0.6382 0.7509 0.8720 1 0.4314

3.44 3.64 3.88 4.20 4.45 4.77 5.10 5.72 6.11 6.47 7.11 4.49c 4.21d

71 67 63 58 55 51 48 43 40 38 34 78 113

a

50 °C, b75 °C, c 40 °C and d55 °C

the average translational diffusion coefficients for each of such mixtures at a given mole fraction value at 30 °C as well as at elevated temperatures (close to the Tp value of the corresponding mixture). Our attempt to obtain the size parameters from single point DLS data by assuming an ellipsoidal shape to the mixed micelles was not successful and at present we are unable to provide a satisfactory explanation for this (usually the D values extrapolated to zero concentration are much smaller). Therefore we have calculated the hydrodynamic radius Rh of the micelles in mixed systems using equation, Rh ¼

kB T 6Πη D

1.12

rel

XSS-1

ð10Þ

where kB =Boltzmann constant, T = absolute temperature, η =dynamic viscosity and D= diffusion coefficient. It is emphasized that the above equation is valid not only for spherical shaped micelles and also takes into account of the hydration shell. Since it was emphasized in the previous section that the micelles of individual SS or TX-100 or SS in TX-100 aqueous solutions in water are of ellipsoidal shape, Rh values obtained by Eq. (10) need to be taken only as an approximation. The trends in the variation of Rh as a function of mole fraction ratios of XTX-100/XSS-1

1.10 1.08 1.06 1.04 1.02

0

5

10

15

20

25

30

35

40

XTX / XSS Fig. 6. Variation of relative viscosity, ηrel as a function of mole function ratios for SS–TX-100 mixtures: (■) 25 °C, (□) 50 °C for SS-1 mixtures; (●) 25 °C and (○) 40 °C for SS-3 mixtures.

or XTX-100/XSS-3 are still highly useful and help understand the relative effects of each of the components on the overall geometrical features of mixed micelles to a fair degree. The DLS data derived micellar parameters of mixture systems at 30 °C as well as at elevated temperatures are summarized in Table 7. The variation of Rh vs. mole fraction ratio for mixture solutions is depicted in Fig. 5. It can be seen that the hydrodynamic radius of mixed micelles decreased sharply with the increase in the TX-100 proportion before it is almost plateaued at high mole fraction ratios. This indicates clearly that TX-100 addition systematically induces the dissociation of silicone surfactant micelles and the mixed micelles formed have smaller dimension. By comparing the inflection points obtained through the intersections as drawn through the experimental data points of Fig. 5, one can adjudge the extent of the demicellization effect. It is interesting to note that as compared to the SS-1 micelles, SS-3 micelles get dissociated at low TX-100 concentrations. Based on this as well as earlier observations, it can be concluded that the strength of interaction between SS and TX-100 surfactants depends highly on the hp/hl ratios. More hydrophobic the silicone surfactant, more would be its interaction with TX-100. An increase in the temperature resulted into an increase of the Rh values and this effect

2.4 70 2.2 2.0

sp /C ,

R h, Å

dl g

-1

60

50

1.8 1.6 1.4 1.2

40

1.0 30 0

0

20

40

60

80

100

120

[SS] /(mM) 5

10

15

X

TX

20

25

30

35

/X

SS

Fig. 5. Variation of hydrodynamic radii (Rh) as a function of mole fraction ratios for SS– TX-100 mixtures at 25 °C: (■) SS-1 and (□) SS-3.

Fig. 7. Variation of reduced viscosity, ηsp/C vs concentrations of silicone surfactants in TX-100 aqueous solutions at different temperatures: TX-100 concentration in mM: SS-1 in (□) 0.2 mM and (Δ) 1.2 mM of TX-100 at 25 0C; in (■) 0.2 mM and (▲) 1.2 mM of TX-100 at 60 °C: SS-3 in (○) 0.2 mM and (◊) 1.2 mM of TX-100 at 25 0C; in (●) 0.2 mM and + 1(♦) 1.2 mM of TX-100 at 45 °C.

224

N.V. Sastry et al. / Journal of Molecular Liquids 177 (2013) 215–224

Table 8 Effect of TX-100 on intrinsic viscosity, [η] and Huggins constant, kH on silicone surfactant aqueous micellar solutions at (25 and 60) °C. [η] (dl g−1)

[TX-100] (mM) T/(°C) SS-1 0 0.19 1.20 SS-3 0 0.19 1.20

kH

25

60

25

60

0.027 0.031 0.039

0.041 0.043 0.053

2.16 1.98 1.68

0.92 0.82 0.78

0.028 0.054 0.063

0.121 0.076 0.087

1.53 1.18 0.66

1.08 0.73 0.36

of SS–TX-100 mixed micelles with morphological features dominated by TX-100 molecular or micellar species. The interaction between the SS and TX-100 gets facilitated more when both the interacting components have same or close hp/hl values. Acknowledgements Authors are thankful to Board of Research in Nuclear Science (BRNS), Department of Atomic Energy, Mumbai for providing financial support under grant no. 2006/37/4/BRNS/893 and Prof. H. B. Bohidar, School of Physical Sciences, JNU, New Delhi for the DLS facility. Appendix A. Supplementary data

can be attributed to the loss of hydration of water associated with the oxyethylene parts of the surfactants. 4.1.5. Dilute solution viscosity measurements in the mixture solutions The relative viscosity, ηrel values for the SS–TX-100 mixture solutions are plotted as a function of the mole fraction ratio at 25, 40 and 50 °C respectively and the same are shown in Fig. 6. It can be seen that the ηrel values decreased sharply with the increase in the TX-100 proportion at different temperatures. The values plateaued beyond a mole fraction ratio value of ~ 0.5. The dimethylsiloxane moieties are more hydrophobic than the simple hydrocarbon chain of TX-100 molecules. Therefore the drastic decrease in the ηrel indicate that the mixed micelles have predominant contribution from TX-100 micelles. The reduced viscosity, ηsp/C vs. silicone surfactant concentration plots for SS in TX-100 aqueous solutions at 25 °C as well as at elevated temperatures (please see Fig. 7) are found to be linear and could be fitted to Huggin's equation: 2

ηsp =C ¼ ½η þ kH ½η C

ð11Þ

where kH is Huggin's constant and [η] is intrinsic viscosity. Fitting the experimental data to Eq. (11), the intrinsic viscosities and the Huggin's constant, kH were evaluated and the summary of the data is given in Table 8. A close examination of the data from the columns 2 and 3 shows that the intrinsic viscosity values increase slightly with the addition of TX-100 at both the temperatures. Similarly, the Huggins constant values (whose magnitude is related to the interaction of a solute with the solvent and are also correlated with the second virial coefficients) are less for the SS in TX-100 aqueous solutions as compared to the silicone surfactant aqueous solutions alone. Intrinsic viscosity gives an indirect measure of hydrodynamic volume of the solute and hence its value is highly dependent on the relative movements and hydration of the solute species. The larger the intrinsic viscosities, the higher would be the hydrophilic character and increase would be the mobility of solute species in aqueous media. At the same time, small kH values for the mixture solutions indicate enhanced interactions between the two components in the mixed state. 5. Conclusion The non-ionic silicone surfactants (SS) aqueous solutions in presence of TX-100, a hydrocarbon based non-ionic surfactant contain species of more hydrophilic character and hence are characterized by high cloud points. The surfactant properties of SS in water can be modulated by the addition of TX-100. The adsorption layers at air/ water interface in SS solutions in presence of TX-100 consist predominantly TX-100 molecules. The micelle formation in the mixture solutions (in the temperature range of 15–45 °C) is entropy driven. The individual SS or TX-100 in water form micelles of ellipsoidal revolution. The addition of TX-100 to SS aqueous solutions first result in the systematic dissociation of SS micelles followed by the formation

Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.molliq.2012.09.014. References [1] R.M. Hill, in: Meyers (Ed.), Third edition, Encyclopedia of Polymer Science and Technology, 14, 2002, pp. 793–804. [2] R.M. Hill, Silicone surfactants - New developments, Curr. Opin. Colloid Interface Sci. 7 (2002) 255–261. [3] R.M. Hill, Silicone Surfactants, All chapters, Marcel Dekker, New York, 1999. [4] I. Schlachter, G. Faldmann– Krane, in: K. Holmberg (Ed.), Novel Surfactants, 74, 1998, pp. 201–224. [5] H. Kuneida, M. Uddin, M. Horii, H. Furukawa, A. Harashima, The Journal of Physical Chemistry. B 105 (2001) 5419–5426. [6] R. Wagner, R. Strey, Langmuir 15 (1999) 902–905. [7] A. Strumer, C. Thunig, H. Hoffmann, Tenside Surfactants Detergents 31 (1994) 90–98. [8] H. Kunieida, H. Taoka, T. Iwanger, A. Harashima, Langmuir 14 (1998) 5113–5118. [9] S. Ahn, P. Alexandridis, Polymer Preprints 42 (2001) 169–172. [10] M. Gradzielski, H. Hoffmann, P. Robsich, W. Ulbricht, Tenside Surfactants Detergents 27 (1999) 366–379. [11] S.S. Soni, N.V. Sastry, V.K. Aswal, P.S. Goyal, The Journal of Physical Chemistry. B 106 (2002) 2606–2617. [12] S.S. Soni, N.V. Sastry, J. George, H.B. Bohidar, The Journal of Physical Chemistry. B 107 (2003) 5382–5390. [13] S.S. Soni, N.V. Sastry, J.V. Joshi, E. Seth, P.S. Goyal, Langmuir 19 (2003) 6668–6677. [14] S.S. Soni, S.H. Punjabi, N.V. Sastry, Colloids and Surfaces A: Physicochemical and Engineering Aspects 377 (2011) 205–211. [15] R.M. Hill, ACS Symposium Series 501 (1992) 278–286. [16] N.V. Sastry, H. Hoffmann, Colloids and Surfaces A: Physicochemical and Engineering Aspects 250 (2004) 247–261. [17] N.V. Sastry, S.H. Punjabi, V.K. Aswal, P.S. Goyal, Journal of Dispersion Science and Technology 33 (2012) 245–253. [18] D. Koley, A.J. Bard, Proceedings of the National Academy of Sciences of the United States of America 39 (2010) 16783–16787. [19] M.A. Hayat, Y. Zhang, Micron 29 (1998) 411–414. [20] J.M. De Simone, Science 297 (2002) 799–803. [21] M. Kielian, A. Helenius, The Journal of Cell Biology 101 (1985) 2284–2291. [22] N. Zhang, L. Li, Rapid Communications in Mass Spectrometry 18 (2004) 889–896. [23] M. Li, C. Yang, S. Tong, A. Weidmann, R.W. Compaus, Journal of Virology 76 (2002) 11845–11852. [24] J.L. Luo, Z.D. Duan, Advances in Materials Research 60–61 (2009) 475–479. [25] I. Capek, Advances in Colloid and Interface Science 110 (2004) 49–74. [26] D.V.S. Jain, S. Singh, Ind. J. Chem. 10 (1972) 629–631. [27] J.A. Riddick, W.B. Bunger, T.K. Sakano, in: 4th ed., Organic Solvents, Vol. II, Wiley-Inter-science, New York, 1986. [28] H.B. Bohidar, T. Berland, T. Jassang, T. Fedex, Journal of Review Scientific Instruments 58 (1987) 1422–1426. [29] Li-Jen Chen, Shi-Yow Lin, Chiuang-Chan Huang, En-Ming Chen, Colloids and Surfaces A: Physicochemical and Engineering Aspects 135 (1998) 175–181. [30] Kyang-Hee Lim Homg-UnKim, Bulletin of the Korean Chemical Society 24 (2003) 1449–1454. [31] P. Alexandridis, V. Athanassiou, S. Fukuda, T.A. Hatton, Langmuir 10 (1994) 2604–2612. [32] C. Tanford, in: Physical Chemistry of Macromolecules, Wiley, New York, 1961, p. 327. [33] K.Y. Miya, A.M. Jamieson, A. Sirivat, Polymer 40 (1999) 5741–5749. [34] C.C. Ruiz, Photochemical & Photobiological Sciences (2012), http://dx.doi.org/10: 1039/C2PP25049G. [35] G.D.J. Phillies, J. Stott, S.Z. Ren, The Journal of Physical Chemistry 97 (1993) 11563–11569. [36] K. Streletzky, G.D.J. Phillies, Langmuir 11 (1995) 42–47. [37] W. Brown, R. Rymden, J. van Stam, M. Almagren, G. Svensk, The Journal of Physical Chemistry 93 (1989) 2512–2519. [38] M. Li, Y. Rharbi, X. Huang, M.A. Winnik, Journal of Colloid and Interface Science 230 (2000) 135–139.