Effect of Organized Assemblies, Part VII ... - ACS Publications

Feb 13, 2009 - Centre of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur UniVersity, Jyoti. Vihar-768 019, Orissa, India...
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Ind. Eng. Chem. Res. 2009, 48, 3403–3409

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Effect of Organized Assemblies, Part VII: Adsorption Behavior of Polyoxyethylated Nonyl Phenol at Silica-Cyclohexane Interface and Its Efficiency in Stabilizing the Silica-Cyclohexane Dispersion Pramila K. Misra,*,† Uma Dash,† and Ponisseril Somasundaran‡ Centre of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur UniVersity, Jyoti Vihar-768 019, Orissa, India, and Henry Krumb School of Mines, Columbia UniVersity, New York 10027

The adsorption behavior of a series of nonionic polyoxyethylated nonyl phenol surfactants having same hydrocarbon chain length but varying oxyethylene chain has been studied spectrophotometrically at the silica-cyclohexane interface. The adsorption density is found to be independent of the number of oxyethylene units throughout the adsorption isotherms, except at higher surface coverage, where the adsorption density decreases with the increase in the number of oxyethylene units. The settling rate of the silica particles in cyclohexane decreases with the increase in adsorption density as well as oxyethylene chain length. On the other hand, the sediment volume decreases with the increase in the adsorption density, with a minimum for the surfactant having the least number of oxyethylene units. Surfactants with a longer oxyethylene chain partitions preferentially to the bulk solvent at higher concentrations, as a result of the change in the polarity of the medium. On the basis of adsorption density, settling rate, and sediment volume data, a model for the mechanism of stabilization of silica-cyclohexane dispersion has been proposed. The aromatic ring of the surfactant is assumed to be anchored on the silica surface through specific interaction with silanol hydrogen, while its nonyl and oxyethylene chains are suspended in the bulk cyclohexane solution. The protruding oxyethylene chain offers a strong steric resistance for particle-particle association leading to the stabilization of the silica-water dispersion. 1. Introduction Surfactant organizes in solutions and at solid-liquid interfaces to give a variety of surfactant assemblies. The structural hierarchy of these surfactant assemblies helms several applications in industries and engineering, pharmaceuticals, agriculture and food technology, chemical processes, and so forth. In particular, the formation and stability of colloidal dispersions in polar and nonpolar media are largely affected by the organization, orientation, and packing pattern at the interfaces which in turn determines the efficiencies of the products like paint, pharmaceuticals, personal care, pesticides, and industrial fuels.1-7 Besides, the stability of the dispersions also depends on the interfacial properties of the particle as well as on the nature of the surrounding medium.8,9 In most of the cases, selective dispersion of the particles is difficult due to their strong interaction among themselves rather than with the surrounding medium leading to the flocculation and settling of the particles. Several interfacial phenomena such as dispersion and flotation have been tuned5-10 successfully by modifying the interfacial characteristics of the minerals through adsorption of surfactants and some polymers. In this regard, along with the extent of adsorption, orientation and the molecular packing of the adsorbed surfactant at the interface are also important in determining the efficiency of surface modification.9,11-13 In the present work we have studied the adsorption behavior of a series of polyoxyethylated nonyl phenols at the silicacyclohexane interface and investigated the stability of the silicacyclohexane dispersion in cyclohexane on subsequent adsorption. Polyoxyethylated nonyl phenols, NP-X, were chosen because of their strong tendency to adsorb on silica surface * Corresponding author. E-mail: [email protected]. † Sambalpur University. ‡ Columbia University.

selectively13-17 and mildness.18 These surfactants have the same hydrocarbon chain length (nonylphenyl unit) but different oxyethylene units (X ) 5, 7.5, 10). The hydrophobicity of these polyoxyethylated alkyl phenol surfactants with the same hydrocarbon chain length depends essentially on the length of oxyethylene chains, the surfactants with longer oxyethylene chain being relatively less hydrophobic19-23 than the short chain surfactant. 2. Experimental Section Synthetic silica (Johnson Matthey company) used in the study has a BET surface area of 5.8 m2/g and an average particle size of 2.5 µm. Polyoxyethylated p-nonyl phenols obtained from Nikko Chemicals, Japan, were used as received. The absence of a minimum in the surface tension-concentration curve suggests these surfactants to be pure enough for the present purpose.19-21 2.1. Adsorption Studies. A 15 mL graduated test tube containing 1 g of silica sample was heated at 200 °C and was kept in a desiccator for 5-6 h. This water free silica was stirred with surfactant solution of various concentrations in cyclohexane for 12-15 h at 23 ( 0.2 °C followed by centrifuging the sample for 20 min at a speed of 300 c/s (30 000g). The supernatant was analyzed by UV analysis at 223 nm (λmax NP series) using Shzimadzu-1201A UV-visible spectrophotometer. The adsorption density was calculated by the depletion technique as described elsewhere.13 2.2. Settling Rate. Settling rate measurements were done by monitoring the rate of descent of the upper interface of silica-cyclohexane suspension in the presence and absence of various concentrations of the surfactants.7 The suspensions were shaken several times in the graduated test tube, and then the height of the uppermost solid-liquid interface was recorded

10.1021/ie801283f CCC: $40.75  2009 American Chemical Society Published on Web 02/13/2009

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Figure 1. Plot of adsorption density versus residual concentration of NP-X at silica-cyclohexane interface [S/L ) 0.067, temp ) 23 ( 0.2 °C].

Figure 2. Schematic diagram of the orientation of NP-10 at the silica-water interface.

until the same interface was fully settled. The rate was determined from the slope of interface height versus time at 23 ( 0.2 °C. The data were the average of at least five set of measurements. 2.3. Sediment Volume. The suspensions were kept undisturbed overnight at 23 ( 0.2 °C, and the sediment volumes were measured. 2.4. Area/Molecule. The area/molecule for NP-5 and NP7.5 was determined from the π-A isotherm using a LangmuirBlodgett film supplied by APEX, Kolkata. A volume of 250 µL of the 0.01 mM surfactant in chloroform was spilled on the trough for this purpose, and the pressure was applied to get the isotherm. 3. Results and Discussion 3.1. Adsorption Behavior of Surfactants. Figure 1 represents the adsorption isotherms of the nonionic surfactants, NPX, on silica from cyclohexane solutions. The adsorption isotherms are Langmuir type, and all of them nearly overlap up to 0.6 mM of residual concentration. The adsorption density is low at the beginning for each of the surfactants and sharply rises to attain a plateau at around 1.98 × 10-10 mol cm-2 for NP-5 and NP-7.5. The adsorption density of NP-10, however, decreases at higher concentrations after attaining a maximum adsorption density of 1.84 × 10-10 mol cm-2 at 0.6 mM residual concentration. Sharp increase in adsorption at very low concentrations, that is, at the foot of the adsorption isotherm, observed in all cases indicates that there is high affinity of the surfactants for the silica surface. The surfactant usually adsorbs through electrostatic interaction, chemical bonding, and H-bonding interactions13,24,25 on a polar surface like silica. In our earlier work13,19 we have noted that these polyoxyethylated alkyl phenols adsorb on silica from aqueous solutions through two factors: (i) H-bonding between the hydrogen of the silanol group and the oxygen of the oxyethylene chains (Figure 2) and (ii) the lateral association of the hydrophobic tails leading to the formation of twodimensional aggregates, hemimicelles (Figure 3). The surfactant with the least number of oxyethylene units has therefore minimum adsorption density at low surface coverage due to a smaller number of oxyethylene units and maximum at plateau region, that is,at higher surface coverage due to less strain in packing at the adsorbed monolayer because of its small size.

Figure 3. Schematic representation of NP-10 hemimicelle at the silica-water interface.

Absence of this trend in the present case suggests that neither the hydrogen bonding nor lateral interaction is the driving force for adsorption of these surfactants on silica from cyclohexane solutions. In organic solvents, electrostatic forces are also not considered to play the pronounced role in determining adsorption.26,27 The approximate flat value of adsorption density at the plateau for all of the surfactants under investigation, however, indicates that irrespective of their differences in polyoxyethylene units, the surfactants anchor on the silica surface through a common moiety or in similar pattern which would overcome the differences of the oxyethylene chain length. From our earlier work28 we have seen that such a type of surfactant aggregates form micelles in aqueous solution in which each of the surfactant monomers orients itself in such a way that the oxyethylene chain folds to give the appearance of a match-stick model with the polyoxyethylene chain as the head of the match stick (Figure 4). Such conformation of the oxyethylene group at the silica-cyclohexane interface may result in same adsorption density for these surfactants if the adsorption occurs through the oxyethylene chain. In our earlier studies we have seen that at the air-water interface these categories of surfactants bind to the water molecule through H-bonding with its oxyethylene units to form a monolayer20 (Figure 5), and therefore, a linear correlation is obtained between the area per molecule with the oxyethylene number. The area/

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Figure 4. Schematic representations of the match stick arrangement surfactant monomer in a nonionic polyoxyethylated alkyl phenol micelle.

Figure 5. Schematic representation of polyoxyethylated nonyl phenol monomer at the air-water interface.

molecule of both NP-5 and NP-7.5 determined from their π-A isotherms is also found to fit to the same plot of the NP series (Figure 6) which clearly envisages that these series of surfactant molecules anchor onto the water surface with alkyl chain suspended vertically at the air-water interface, adsorption being dependent on the oxyethylene chain length only. During the adsorption studies of the polyoxyethylated alkyl phenol at the silica-water interface13 we have also found the molecular parking areas to be dependent on the length of the oxyethylene unit (Figure 7). However, the intercept in the X-axis (Figure 7) at around seven oxyethylene units suggests that the lower members of NP series have negligible adsorption on the silica surface from aqueous solution as long as the adsorption through H-bonding with the oxyethylene group is concerned. The absence of the postulated H-bonding may possibly be due to the unavailability of a sufficient number of silanol groups at a suitable distance for the formation of hydrogen bonds with short oxyethylene chains. The common moieties available in the surfactants are (i) nonyl chain and (ii) phenyl nucleus. The adsorption through interaction of the nonyl chain with the silica surface, however, can be excluded since the surface of silica is polar. In the nonpolar medium, acid-base interactions between the solute and the

Figure 6. Plot of area/molecule as a function of oxyethylene unit for polyoxyethyleted nonyl phenol surfactant at the air-water interface.

Figure 7. Plot of area/molecule as a function of oxyethylene unit for polyoxyethyleted nonyl phenol surfactant at the silica-water interface.

Scheme 1

substrate have been proposed to be the dominant adsorption mechanism. These mechanisms of adsorption are comprised essentially of dipolar interactions involving an electron or proton transfer between the solute and the substrate molecules resulting in the formation of an acid-base adduct at the interface,26,27 or there may be interaction of the electron clouds of π-bonds with hydrogen.29,30 The plane of electron clouds forming the π-bonds in the phenyl nucleus is perpendicular to the plane of the flat ring of the carbon atoms, and hence these groups upon adsorption on the silica surface may arrange themselves in a plane. This arrangement may promote the specific interactions between the surface hydroxyl groups of silica and the π electrons of the phenyl ring in close proximity, the interaction being to a certain extent similar to a hydrogen bond30 (Scheme 1). Such interaction through the common moiety would, therefore, yield the constant value of adsorption density at the plateau for surfactants under investigation. Formation of the bilayer at high

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Figure 8. ESR spectra of 5-doxyl stearic acid in the presence of water (A) and cyclohexane (B) and silica-cyclohexane interface in the presence (C) and absence (D) of NP-5 [S/L ) 0.067, temp ) 23 ( 0.2 °C].

coverage31 may not occur in the present case since the lateral hydrophobic interactions may be weakened by the interactions of the hydrophobic groups with the surrounding cyclohexane. In nonaqueous solvents, the surfactant molecules are assumed to adsorb as isolated monomer32 with the adsorption proceeding only up to a monolayer. 3.1.1. Evidence in Favor of Adsorption through Phenyl Nucleus. The following two experimental observations support adsorption through phenyl nucleus. (i) The polarity of the adsorbed layer was determined from electron spin resonance experiments using 5-doxyl stearic acid as the probe. A constant amount of probe (2× 10-5 M) was added to a mixture of 1 g of silica and 10-2 M of surfactant solution (equivalent to plateau concentration), and adsorption was carried out. The ESR spectra of both the supernatant and the slurry were recorded. Absence of ESR signal in the supernatant indicated complete transfer of the probe to the interface. The resultant slurry showed a spectrum, which was substantially anisotropic when compared with that of pure cyclohexane (Figure 8). The nature of the anisotropy is indicative of the probe residing in a rigid nonaqueous environment with restricted mobility28 and hence the adsorbed layer may be of nonpolar nature. (ii) Calculations based on apparent monolayer coverage at plateau yields a parking area of 85.1 Å2/molecule for NP-5 and NP-7.5. Using the Csp2-Csp2 and Csp2-H for benzene, 1.4 Å and 1.1 Å,33 respectively, the area of benzene is estimated to be 16.24 Å2. Considering the adsorption of the NP-10 through 10 oxyethylene units in a flat posture13,34 we have estimated the molecular parking area to be 66 Å2 for NP-10 on the silica surface. If the adsorption occurs through the oxyethylene chain interaction in a similar fashion the parking areas per molecule should be smaller than 66 Å2 for both NP-5 and NP-7.5 and should be in the order NP-7.5 > NP-5. But the estimated parking area per molecule not only is same for both the surfactants but also corresponds to five times of the single benzene area (16.24 Å2). The surface of silica has three types of surface silanols, isolated silanols, vicinal silanol, and geminal silanol.35 The geminal silanols and vicinal silanol are close enough to form H-bond with neighboring hydrogen atoms, whereas isolated silanols are free. The presence of two types of silanol at the silica surface with pKa values of 4.9 and 8.5 and surface population of 19 and 81%, respectively, have been shown.36 The silanols with the higher pKa value (8.5) are believed to be

Figure 9. Plot of sediment volume and settling rate versus adsorption density of the NP-5 silica-cyclohexane interface [S/L ) 0.067, temp ) 23 ( 0.2 °C].

those groups which are connected with each other through intramolecular H-bonding. The silanol groups with the lower pKa value (4.9) are believed to be isolated silanol groups with no H-bond with the neighbors and are, therefore, accessible for interaction with adsorbing groups. Consequently, only ∼20% is available for H-bonding with such groups. Hence, the parking area of five times that of the phenyl ring is therefore justifiable. The evolution of the adsorbed layer of the nonionic surfactants at silica-cyclohexane interface in the present case is, therefore, proposed to be due to interactions of the phenyl nucleus and silanol -OH group.29 At higher concentration the adsorption of NP-10 becomes negative, suggesting greater interaction of NP-10 molecules with one another in the bulk solution than with silica surface. It has been reported that with increase in the solvent polarity surfactant becomes more compatible with the solvent and hence preferably partitions into the solvent.37 The possibility of an inverted micelle in bulk solution cannot be ignored at this stage since the solubility of NP-10 is relatively less when compared to those of the other two surfactants. NP-5 and NP-7.5 do not exhibit this type of decrease in adsorption because they are more soluble in cyclohexane due to their short oxyethylene chain. The change from a positive to negative adsorption for adsorption of n-hexane on graphatized black with an increase in the benzene content in the bulk solution has been reported.38 3.2. Settling Rate. Usually the necessary criterion for the dispersion process is that a solid particle to be dispersed should be wetted sufficiently by the surrounding medium. Since the surface of silica is hydrophilic due to the presence of large number of -OH groups on its surface it readily settles down in cyclohexane. For stable silica dispersion, the surface of silica needs to be modified to enhance silica-cyclohexane interactions. A suitable dispersing agent has to be adsorbed onto the solid particle to produce sufficient energy barriers so that the particle will be dispersed in liquid medium.39 In order to determine the ability of the polyoxyethylated nonyl phenols as dispersing agents, the silica particles with adsorbed surfactants were again redispersed in the corresponding surfactant solution in cyclohexane and were allowed to settle. The settling rate was plotted as a function of adsorption density (Figures 9-11). The settling rate was found to decrease with increase in the adsorption density as well as increase in the oxyethylene number. In nonaqueous media steric barriers are generally prominently required to disperse solid particles since electrical barriers are less significant.39 The steric barriers may arise either due to the

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Figure 12. SEM picture of silica before adsorption. Figure 10. Plot of sediment volume and settling rate versus adsorption density of the NP-7.5 silica-cyclohexane interface [S/L ) 0.067, temp ) 23 ( 0.2 °C ].

Figure 13. SEM picture of silica after adsorption.

Figure 11. Plot of sediment volume and settling rate versus adsorption density of the NP-10 silica-cyclohexane interface [S/L ) 0.067, temp ) 23 ( 0.2 °C].

energy required to desolvate the portion of the adsorbed surfactant molecules extending into the dispersing medium as the particles approach each other or from the decrease in entropy of the system as the portion of these adsorbed surfactant molecules are restricted in their movement. In our proposed adsorption model, the surfactant with longer polyoxyethylene unit would, therefore, render higher steric hindrance. The settling rates for NP-5 and NP-7.5 are in line with their oxyethylene unit. For NP-10, at high concentrations the settling rate decreases, despite the fact that there is a substantial decrease in adsorption density in that range. The presence of water does affect suspension stability by changing the characteristics of the layer at the solid-liquid interface.40 Since the samples as a whole are free of water the decrease in settling rate could not account for the presence of water. At higher concentrations, the results can be accounted for by considering the influence of the surrounding bulk solution on the adsorbed surfactant.9,41 The polarity of the bulk solution increases due to the increase in NP-10 surfactant concentration in the bulk. This enhances

Figure 14. SEM picture of silica mentioned in Figure 13 after 400× magnification.

the interaction of the adsorbed layer of particles with the surrounding solvent and hence decreases settling of silica particles. 3.3. Sediment Volume. Usually sediment volume should increase with the increase in adsorption since the suspended oxyethylene unit would hinder the close approach of the particles. Nevertheless, the sediment volumes in the present case are found to decrease with increase in adsorption (Figures 9-11). As per the SEM of the original silica particles, shown in Figure 12, it has an average particle size around 2.5 µm, but on treating with surfactants, at the residual concentration corresponding to the plateau region, it is less than 0.1 µm (Figures 13 and 14). This is attributed to the breaking of the flocculated silica particles into small particles as the surfactant adsorbs. The packing of particles becomes more as the size decreases, leading to reduction of sediment volume. The minimum sediment volume value of 1.1 cm3 is obtained in the case of NP-5 due to the exposure of the short oxyethylene chain,

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Figure15.Orientationofployoxyethylated(5)nonylphenolatsilica-cyclohexane interface.

whereas in cases of NP-7.5 and NP-10, the values are approximately 1.5 cm3 and 1.8 cm3, respectively. 4. Conclusion The following conclusions are drawn on the stability of silica-cyclohexane dispersion: (i) Even though the NP-series adsorbs on the silica surface from the cyclohexane as well as in the aqueous solution, the mechanisms of adsorption in the two solvents differ markedly. In cyclohexane the adsorption is driven initially by the difference in polarity between the aromatic ring and that of the silanol -OH group. As a result of the presence of a large number of -CH2- groups, the polyoxyethylene chain can be pushed up from the silica surface toward the bulk cyclohexane solution. The formation of surface aggregates is not possible since the surfactant chain interacts well with the solvent. The proposed orientation of the surfactant species at the silica-cyclohexane interface would therefore be represented as shown in Figure 15. (ii) At higher concentrations the settling rate decreases since the approach of the particles is hindered due to the exposed oxyethylene chains. This hypothesis is further supported by relative decrease in the settling rate in the case of NP-10 since it can provide more steric hindrance due to its relatively longer oxyethylene chains. At even higher concentrations, the decrease in adsorption density does not affect the settling rate because the particle interacts strongly with the solvent. (iii) The reduction of sediment volume with increase in adsorption density is attributed to the breaking of flocculated silica particles as the surfactant adsorbs. Acknowledgment The authors thank the Department of Science and Technology, Govt. of India, for providing the BOYSCAST Fellowship to P.K.M. and JRF to U.D. (Project No. SR/S1/PC-39/2004 dated 14/03/2006). The financial support of the National Science Foundation, U.S.A., and the Department of Energy is also gratefully acknowledged. Support of the UGC(DRS) and DST(FIST) are also gratefully acknowledged. Literature Cited (1) Novotny, V. Applications of nonaqueous colloids. Colloids Surf. 1987, 24, 361–375. (2) Blier, A. In Stability of Ceramic Suspensions in Ultrastructure Processing Ceramics, Glasses and Composite; Hench, L. L., Ulrich, D. R., Eds.; J. Wiley and Sons: New York, 1984; p 391. (3) Mckay, R. B. Pigment Dispersions in Apolar Media. In Interfacial phenomena in Apolar Media; Eicke, H. F., Parfitt, G. D., Eds.; CRC Press: New York, 1987; p 361.

(4) Fowkes, F. M.; Pugh, R. J. Steric and Electrostaticcontributions to the Colloidal Properties of Nonaqueous Dispersions. Polymer Adsorption and Dispersion Stability; ACS Symposium Series 240; American Chemical Society, Washington, DC, 1984; p 331. (5) Somasundaran, P. Interfacial Chemistry of Particulate flotation. In AdVances in interfacial phenomena of particulate/solution/gas systems; Grieves, R. B., Ed.; AIChE Symposium Series; AIChE: New York, 1975; Vol. 71, pp 1-15. (6) Somasundaran, P.; Healy, T. W.; Fuersteanau, D. W. Surfactant Adsorption at the Solid-Liquid Interface - Dependence of Mechanism on Chain Length. J. Phys. Chem. 1964, 68, 3652–3656. (7) Cambell, A.; Somasundaran, P. Use of pyrene spectroscopic probes in the study of colloidal systems. J. Colloid Interface Sci. 2000, 229, 257– 260. (8) Hanna, S.; Somasundaran, P. Flotation of salt type Minerals in Flotation. In A.M. Gaudin Memorial Volume; Fuersteanau, M. C., Ed.; Americam Institute of Mining, Metallurgical Petroleum Engineers Inc.: New York, 1976; Vol. 1, pp 197-272. (9) Misra, P. K.; Panigrahi, S.; Somasundaran, P. Organization of amphiphiles, Part VIII: Role of polyoxyethylated alkylphenols in optimizing the beneficiation of hydrophilic mineral. Int. J. Miner. Process. 2006, 80, 229–237. (10) Somasundaran, P.; Ananthapadmanavan, K. P. Physico-Chemical Aspects of Flotation. Trans. Indian Inst. Met. 1979, 32, 177–194. (11) Somasundaran, P.; Lee, L. T. Polymer-Surfactant Interactions in Flotation of Quartz. Sep. Sci. Technol. 1981, 16, 1475–1490. (12) Moudgil, B. M.; Soto, H.; Somasundaran, P. Adsorption of surfactants on minerals. In Reagents in Mineral Technology; Marcel Dekker: New York, 1987; p 79. (13) Misra, P. K.; Mishra, B. K.; Somasundaran, P. Organization of amphiphiles V. In situ fluorescence probing of the adsorbed layers of polyoxyethylated alkyl phenol at silica-water interfaces. J. Colloid Interface Sci. 2003, 265, 1–8. (14) Fu, E. Adsorption of anionic-nonionic surfactant mixtures on oxide minerals. Ph.D. thesis, Columbia University, New York, 1987. (15) Patryka, S.; Zains, S.; Lindheimer, M.; Brun, B. The adsorption of non-ionic surfactants on silica gel. Colloids Surf. 1984, 12, 255–270. (16) Lawrence, S. A.; Pilc, J. A.; Readman, J. R.; Sermon, P. A. Unexpectedly low extent of adsorption of non-ionic ethoxylated surfactants on alumina. J. Chem. Soc., Chem. Commun. 1987, 1035–1036. (17) Boomgaard, Th. V. D.; Tadros, Th. F.; Lyklema, J. Adsoption of non-ionic surfactants on lattices and silica in combination with stability study. J. Colloid Interface Sci. 1987, 116, 8–16. (18) Cox, M. F. Surfactants in Detergents and Cleaners. In Handbook for Formulators; Lange, K. R., Ed.; Hanser/Gardner Publications, Inc.: Cincinnati, 1994; p 43. (19) Misra, P. K.; Mishra, B. K.; Somasundaran, P. Organization of amphiphiles Part IV. Characterization of the microstructure of the adsorbed layer of decylethoxylene nonyl phenol. Colloids Surf. 2005, 252, 169–174. (20) Sahoo, L.; Misra, P. K.; Somasundaran, P. Organization of amphiphiles, Part-II-Surface activity of polyoxyethylated alkyl phenols at air-water interface. Indian J. Chem. 2002, 41A, 1402–1405. (21) Okano, T.; Tamura, T.; Abe, Y.; Tasuchida, T.; Lee, S.; Sugihara, G. Micellisation and adsorbed film formation of a binary mixed system of anionic/nonionic surfactants. Langmuir 2000, 16, 1508–1514. (22) Liu, J.-F.; Min, G.; Ducker, W. A. AFM study of adsorption of cationic surfactants and cationic polyelectrolytes at the silica water interface. Langmuir 2001, 17, 4895–4903. (23) Sahoo, L.; Sarangi, J.; Misra, P. K. Organization of Amphiphiles, Part1: Evidence in favour of premicellar aggregates through fluorescence spectroscopy. Bull. Chem. Soc. Jpn. 2002, 75, 859–865. (24) Huang, L.; Somasundaran, P. The change in structure of surfactant aggregates during adsorption/desorption processes and its effect on the stability of alumina suspension. Colloids Surf., A 1996, 117, 235–244. (25) Chander, P.; Somasundaran, P.; Turro, N. J. Fluorescence probe studies on the structure of adsorbed layer of dodecyl sulphate at the aluminawater interface. J. Colloids Interface Sci. 1987, 117, 31–46. (26) Fowkes F. M. Chemistry and Physics at Interfaces; American Chemical Society: Washington, DC, 1965; p 1. (27) Pugh, R. J. In Ceramic Powder Science III; Meessing, G. L., Hirano, S. I., Hausner, H., Eds.; American Ceramic Society: Westerville, OH, 1990; p 375. (28) Sahoo, L.; Misra, P. K. Characterization of the structure of polyoxyethylated alkyl phenol micelles at 300K. Sambalpur UniV. J. Sci. Technol. 2001, 13B, 18–22. (29) Synder, L. R. Interactions responsible for the selective adsorption of non-ionic organic compounds on alumina-comparisions on silica. J. Phys. Chem. 1968, 72, 489–495.

Ind. Eng. Chem. Res., Vol. 48, No. 7, 2009 3409 (30) Gerasimov, Y.; Dreving, V.; Eremin, E.; Kiselev, A.; Lebedev, V.; Panchenkov, G.; Shlygin, A. In Physical Chemistry; Gerasimov, Ya., Eds.; Mir Publishers: Moscow, 1974; Vol. 1; p 476. (31) Somasundaran, P.; Fuerstenau, D. W. Mechanism of Alkyl Sulfonate Adsorption at the Alumina-Water Interface. J. Phys. Chem. 1966, 70, 90– 96. (32) Krishnakumar, S.; Somasundaran, P. Adsorption of Aerosol-OT on Graphite from Aqueous and Non-Aqueous Media P. Colloids Surf. 1996, 117, 227–233. (33) March, J. AdVanced Organic Chemistry, Reactions, Mechanism and Structure, 2nd ed.; McGraw Hill: London, 1977; p 24. (34) Misra, P. K.; Somasundaran, P. Organization of amphiphiles VI. A comparative study of the orientation of polyozxyethylated alkyl phenols at the air-water and the silica-water interface. J. Surfactants Deterg. 2004, 7, 373–378. (35) Jal, P. K.; Patel, S.; Mishra, B. K. Chemical modification of silica surface by immobilization of functional groups of extractive concentration of metal ions. Talanta 2004, 62, 1005–1028. (36) Ong, S. W.; Zhao, X. L.; Eisenthal, K. B. Polarisation of water molecules at a charged interface: second harmonic studies of silica water interface. Chem. Phys. Lett. 1992, 191, 327–335.

(37) Krishnakumar, S.; Somasundaran, P. Aggregation Behavior of Aerosol OT in Nonaqueous Solvents and Its Desorption - An ESR Study. J. Colloid Interface Sci. 1994, 162, 425–430. (38) Gerasimov, Y.; Dreving, V.; Eremin, E.; Kiselev, A.; Lebedev, V.; Panchenkov, G. Shlygin, A. In Physical Chemistry; Gerasimov, Ya., Eds.; Mir Publishers: Moscow, 1974; Vol. 1, p 511. (39) Rosen, M. J. Surfactants and Interfacial Phenomenon; John Wiley and Sons: New York, 1978; p 270. (40) Malbrel, C. A.; Somasundaran, P. Effect of Water on the Dispersion of Colloidal Alumina in Cyclohexane Solutions of Aerosol OT. Langmuir 1992, 8, 1285–1290. (41) Reichardt, C. Solvatochromic dye as solvent polarity indicators. Chem. ReV. 1994, 94, 2319–2358.

ReceiVed for reView August 28, 2008 ReVised manuscript receiVed January 12, 2009 Accepted January 13, 2009 IE801283F