Alkylbenzenes in Diiodomethane. A Novel, “Primitive” Micelle-Forming

One of the few examples of a “structurally primitive” low molar mass system showing properties similar to those of conventional surfactants in wat...
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Alkylbenzenes in Diiodomethane. A Novel, “Primitive” Micelle-Forming Surfactant System P. D. I. Fletcher* and R. J. Nicholls Surfactant Science Group, Department of Chemistry, University of Hull, Hull HU6 7RX, U.K. Received July 23, 1999. In Final Form: October 4, 1999 We have investigated the extent to which alkylbenzenes exhibit surfactant properties in diiodomethane (DIM) as solvent. As a result of its dipole moment and high refractive index, DIM has a high cohesive energy density arising from dipole-dipole and dispersion forces. DIM has a high affinity for the benzene group of an alkylbenzene solute but low affinity for the alkyl chain. These different affinities endow alkylbenzenes with amphiphilic properties in DIM solution, causing them to adsorb at the DIM-air surface and to aggregate. Surface tensiometry measurements show that the adsorption of alkylbenzenes at the DIM-air surface increases with the alkylbenzene chain length, consistent with the notion that the antipathy between the DIM solvent and the alkyl chain of the alkylbenzene provides the main driving force for adsorption. Ellipsometry measurements confirm the adsorption of octyl- and hexylbenzenes at the DIMair surface and also indicate multilayer adsorption of octylbenzene at high concentrations. Using dynamic light scattering, it is shown that octylbenzene forms aggregates in DIM at concentrations higher than 5 mol %. The apparent aggregate hydrodynamic radius increases from 0.6 nm at low concentrations to 1.5 nm at high concentrations.

Introduction The adsorption and aggregation properties of surfactants arise from their amphiphilic, diblock molecular structure in which a solvophobic group (the “tailgroup”) is coupled to a solvophilic group (the “headgroup”). For a conventional surfactant in water as solvent, the solvophobic tailgroup is commonly an alkyl chain and the solvophilic headgroup is generally polar or ionic or both. In this case, the solvophobicity derives from the hydrophobic effect,1 which drives adsorption of the surfactant at the air-water surface and aggregation into micelles and a rich variety of different microstructures. The solvophilic attraction between the water solvent and the polar/ionic headgroup plays an important role in limiting the aggregation to give micelles as opposed to complete phase separation, which occurs when the solvophilicity is weak.2 Clearly, the adsorption and aggregation properties of surfactants are intimately related to the nature and strength of the intermolecular interactions between the solvent and each of the two “blocks” within the surfactant molecule. Much research effort has been aimed at relating these interactions and surfactant properties with a view to obtaining a detailed and predictive theoretical understanding of surfactant behavior.3-7 For conventional surfactants in water, the complexity of intermolecular interactions to be considered (including hydrophobic, hydrogen bonding, ion-ion, ion-dipole, dipole-dipole, and * Author for chem.hull.ac.uk.

correspondence.

E-mail:

P.D.Fletcher@

(1) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; John Wiley & Sons: New York, 1980. (2) See, for example, Evans, D. F.; Wennerstrom, H. The Colloidal Domain Where Physics, Chemistry and Biology Meet, 2nd ed.; WileyVCH: New York, 1999. (3) Smit, B.; Hilbers, P. A. J.; Esselink, K.; Rupert, L. A. M.; van Os, N. M.; Schlijper, A. G. Nature 1990, 348, 624. (4) Smit, B.; Hilbers, P. A. J.; Esselink, K. In Structure and Dynamics of Strongly Interacting Colloids and Supramolecular Aggregates in Solution; Chen, S.-H. et al., Eds.; Kluwer: Amsterdam, 1992; p 519. (5) Karaborni, S.; van Os, N. M.; Esselink, K.; Hilbers, P. A. J. Langmuir 1993, 9, 1175. (6) Hariharan, A.; Harris, J. G. J. Chem. Phys. 1994, 101, 4156. (7) Klopfer, K. J.; Vanderlick, T. K. Colloids Surf., A 1995, 96, 171.

dispersion interactions) is such that a complete and rigorous theoretical description of surfactants remains elusive. For this reason, there is interest in the extent to which surfactant properties are manifested in systems of simpler molecular structure exhibiting a less complex range of intermolecular interactions. One of the few examples of a “structurally primitive” low molar mass system showing properties similar to those of conventional surfactants in water is that of the semifluorinated alkanes (SFAs) in either alkane or perfluoroalkane solvents. SFAs, with a diblock architecture comprising a fluoro chain linked to a hydrocarbon chain, adsorb at the alkane-air surface and aggregate in alkane and perfluoroalkane solvents as a result of the antipathy between the hydrocarbon and fluorocarbon chains of the SFA and the solvent.8-23 Because the demixing tendency of hydrocarbon and fluorocarbon chains is considerably weaker than that for alkanes and water (the hydrophobic effect), SFAs in alkanes show only weak (8) Twieg, R. J.; Russell, T. P.; Siemens, R.; Rabolt, J. F. Macromolecules 1985, 18, 1361. (9) Rabolt, J. F.; Russell, T. P.; Siemens, R.; Twieg, R. J.; Farmer, B. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1986, 27, 223. (10) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1988, 110, 7797. (11) Hopken, J.; Pugh, C.; Richtering, W.; Moller, M. Makromol. Chem. 1988, 189, 911. (12) Gaines, G. L. Jr. Langmuir 1991, 7, 3054. (13) Hopken, J. Ph.D. Thesis, University of Twente, 1991. (14) Napoli, M.; Fraccaro, C.; Scipioni, A.; Alessi, P. J. Fluorine Chem. 1991, 51, 103. (15) Hopken, J.; Moller, M. Macromolecules 1992, 25, 2482. (16) Lo Nostro, P.; Chen, S.-H. J. Phys. Chem. 1993, 97, 6535. (17) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. Langmuir 1995, 11, 977. (18) Fulton, J. L.; Pfund, D. M.; McClain, J. B.; Romack, T. J.; Maury, E. E.; Combes, J. R.; Samulski, E. T.; DeSimone, J. M.; Capel, M. Langmuir 1995, 11, 4241. (19) Lo Nostro, P.; Ku, C. Y.; Chen, S.-H.; Lin, J.-S. J. Phys. Chem. 1995, 99, 10858. (20) Napoli, M. J. Fluorine Chem. 1996, 79, 59. (21) Binks, B. P.; Fletcher, P. D. I.; Thompson, R. L. Ber. BunsenGes. Phys. Chem. 1996, 100, 232. (22) Binks, B. P.; Fletcher, P. D. I.; Sager, W. F. C.; Thompson, R. L. J. Mol. Liq. 1997, 72, 177. (23) Binks, B. P.; Fletcher, P. D. I.; Kotsev, S. N.; Thompson, R. L. Langmuir 1997, 13, 6669.

10.1021/la990987m CCC: $19.00 © 2000 American Chemical Society Published on Web 11/24/1999

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Table 1. Physical Properties (20 °C) Used in Data Analysis property

DIM

BN

surface tension (mN m-1) 51.4 44.7 density (g cm-3) 3.321 1.4826 viscosity (Pa s) 2.80 × 10-3 refractive index (632.8 nm), 1.734 1.652 real part dipole moment (D) 1.22 1.55

C8B

C6B

0.858 1.484

0.861 1.486

-

-

a

The values were either measured as part of this work or taken from refs 25-29.

surfactant properties when compared with conventional surfactants of comparable chain length in water.23 Relative to low molar mass materials, strong surfactant properties are seen for block copolymers where, because of the long chain lengths, even small differences in solvent affinity for the individual monomer units of the different blocks can produce strong adsorption and aggregation.24 In this study we have investigated the surfactant properties of alkylbenzenes in (mainly) diiodomethane (DIM) as solvent. DIM has an unusually high refractive index (approximately 1.7) and a dipole moment of 1.2 D (Table 1). Because of these properties, DIM has a high cohesive energy density resulting from dispersion and dipole-dipole attractive forces. Alkylbenzenes consist of a linear alkyl chain of low refractive index (approximately 1.4) and low polarizability linked to a benzene ring of high refractive index (approximately 1.5) and high polarizability. Qualitatively, the benzene ring is anticipated to show a relatively strong affinity for DIM through dispersion and dipole-induced dipole interactions. The alkyl chain is expected to show negligible dipole-induced dipole interaction (because of its low polarizability) and relatively weak dispersion interactions with DIM (because of its low refractive index). Thus, the benzene group is expected to act as a surfactant headgroup with high affinity for DIM and the alkyl chain is expected to act as a surfactant tailgroup with low affinity for the DIM solvent. In this work we set out to test the expectation that the alkyl-aromatic diblock structure of alkylbenzenes may produce surfactant-like behavior in DIM. Experimental Section DIM was obtained from Lancaster (99%) and was columned twice over alumina to remove polar impurities. DIM was found to decompose over time with liberation of iodine, and samples varied in color from straw yellow when fresh to red when aged. The liquid-air surface tensions of fresh and aged samples were found to be identical (51.4 ( 0.1 mN m-1 at 20 °C), indicating that impurities resulting from the decomposition had negligible surface activity. All samples were measured promptly after preparation to minimize impurity effects. Hexylbenzene (C6B, Lancaster, 98%), octylbenzene (C8B, Aldrich, 99%), decylbenzene (C10B, Aldrich, 98%), dodecylbenzene (C12B, Aldrich, 97%), and hexylcylcohexane (C6cyc, Lancaster, 97%) were used as received. 1-Bromonaphthalene (BN, Lancaster, 98%) was columned twice over alumina before use to remove polar impurities. Solution-air surface tensions were measured with a Kruss K10 instrument using a Wilhelmy plate of roughened platinum. Ellipsometry measurements were made using a Plasmos SD2300 ellipsometer equipped with a HeNe laser of wavelength 632.8 nm. Samples for ellipsometry measurements were held in an open glass circular dish of 5 cm diameter sufficient to avoid distortion of the liquid surface due to capillary effects at the dish wall, mounted within a thermostated dish. Before each measurement, the DIM-air surface was sucked off using a Pasteur (24) See, for example, Jonsson, B.; Lindman, B.; Holmberg, K.; Fronberg, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, 1998.

pipette connected to a vacuum pump to remove surface contaminants. The sample table, isolated from the ellipsometer instrument body, was held on a Sandercock JRS active antivibration table. This careful vibration isolation was found to be necessary to produce data from liquid surfaces, which showed comparable precision with that obtained for solid surfaces. All ellipsometric results were the mean of at least 20 measurements. Dynamic light scattering (DLS) measurements were made using a Malvern 4700 instrument equipped with a Uniphase air-cooled argon ion laser of 488 nm wavelength (model 2013, operating at 70 mW power) and a Malvern 7032 correlator. Samples for DLS measurements were held within tightly stoppered 1-cm path length quartz fluorescence cuvettes (Hellma) mounted within a thermostated bath. A scattering angle of 90° was used for all DLS measurements. DLS measurements were collected under software control in 1- or 2-s bursts. The autocorrelation function for each burst was analyzed and either rejected or accepted depending on whether it contained artifact intensity contributions arising from “dust” contaminant particles within the scattering volume. Signal averaging with “dust” rejection was typically continued over 24 h to obtain autocorrelation functions of sufficient signal-to-noise ratio. All experiments were done at 20 °C.

Results and Discussion Relevant physical properties of DIM and other materials used in this study are collected together in Table 1. As a result of the dipole moment and unusually high refractive index, dipole-dipole and dispersion attractive forces between DIM molecules are high. The relatively high cohesive energy density of DIM is reflected in the value of the surface tension γ of DIM (51.4 mN m-1), which is considerably higher than for liquid alkanes (16 to 30 mN m-1, depending on chain length30). Adsorption of alkylbenzenes at the DIM-air surface, in which the solvated benzene group is retained in the DIM and the poorly solvated alkyl group protrudes into the air, involves replacement of the high surface energy DIM-air surface with an alkyl chain-air surface of lower surface energy. Thus, alkybenzene adsorption is predicted to be energetically favorable. Surface Tensions of Alkylbenzene Solutions. Figure 1 shows plots of γ versus ln(concentration) for C6B, C8B, C10B, and C12B solutions in DIM. Adsorption, and concomitant tension reduction, occurs at lower concentrations with increasing alkylbenzene chain length. The curves for C6B and C8B show break points indicative of aggregate formation, but no macroscopic phase separation is observed up to the highest concentrations tested. For C10B and C12B, increasing concentration gives macroscopic phase separation. Using the Gibbs equation, the surface excess concentration of alkylbenzene Γ can be obtained from the slopes of the plots according to

Γ)-

1 dγ kT d ln a

(1)

where k is the Boltzmann constant, T is the absolute temperature, and a is the activity of the adsorbing solute. (25) Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds; Elsevier: Amsterdam, 1950; Vol. 1. (26) Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds; Elsevier: Amsterdam, 1965; Vol. 2. (27) The Merck Index, 12th ed.; Budavari, S., Ed.; Merck & Co.: Whitehouse Station, NJ, 1996. (28) McClellan, A. L. Tables of Experimental Dipole Moments; W. H. Freeman: San Francisco, 1963. (29) Handbook of Chemistry & Physics, 62nd ed.; CRC Press: Boca Raton, FL, 1981. (30) Selected Values of Properties of Hydrocarbons and Related Compounds; A. P. I. Project 44, Chemical Thermodynamics Properties Centre: Texas, 1966, Vol. 2.

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Figure 1. Variation of surface tension with ln[alkylbenzene] in DIM at 20 °C.

Figure 3. Variation of ∆µ0ads with alkylbenzene chain length for adsorption at the DIM solution-air surface.

is estimated (using CPK molecular models) to be approximately 0.7 nm2, whereas a vertical orientation gives an area of approximately 0.3 nm2. Comparison with the observed value of A0 (0.37 nm2) suggests that, at high surface pressures, the molecular orientation in the surface is close to vertical. We note here that this analysis of the isotherms uses the assumption of ideal behavior in the bulk solutions made to calculate the surface excess concentrations. The ellipsometry results discussed later suggest that this assumption is not entirely valid in the higher concentration ranges. As discussed previously, the driving force for adsorption arises from the antipathy between the alkyl chains of the CnB and the DIM solvent and is expected to increase with chain length. To quantify the effect, we consider the standard free energies of adsorption ∆µoads as a function of chain length. Values of ∆µoads accompanying the adsorption of the alkylbenzenes from solution in DIM to the DIM-air surface were computed using the expression

∆µ0ads ) -RT ln(Π/X)0 Figure 2. Surface pressure versus area per molecule for octylbenzene adsorbed at the DIM-air surface. The solid line corresponds to the Volmer surface equation of state with Ao ) 0.37 nm2.

For concentrations below the concentrations corresponding to either aggregate formation or phase separation, we assume that the solutions behave approximately ideally, that is, that activity and concentration can be equated. The curves of Figure 1 were fitted to polynomial functions that were then differentiated to obtain Γ and the area A per adsorbed molecule (A ) 1/Γ) as a function of concentration. The variation of surface pressure Π (equal to γ0 - γ, where γ0 is the tension of the pure solvent) with A is shown for C8B films in Figure 2. As shown by the solid line of Figure 2, the Π-A isotherm for C8B shows good agreement with the Volmer surface equation of state:

Π(A - A0) ) kT

(2)

where the excluded area A0 has the value of 0.37 nm2. Thus, the adsorbed film behaves as a nonideal twodimensional “gas”. Using molecular models, the area occupied by a single C8B molecule lying flat on the surface

(3)

where (Π/X)0 is the slope of a plot of Π versus mole fraction X in the linear, low Π region. The standard states are, for the surface Π ) 1 mN m-1, and for the bulk a hypothetical state in which the product of the mole fraction and activity coefficient is unity. The variation of ∆µoads with alkylbenzene chain length is shown in Figure 3. The plot is linear, indicating that the driving force for adsorption increases (i.e., ∆µoads becomes more negative) by 0.4 kJ mol-1 for each additional methylene group of the alkyl chain. This value of the increment in ∆µoads corresponds to adsorption into a dilute monolayer where the alkyl chain is not entirely removed from contact with the DIM solvent. It is therefore expected to be somewhat lower than the increment in free energy of transfer of an alkyl chain from DIM solvent to an alkane solvent. For adsorption of linear alcohols at the water-air surface, the increment in standard free energy of adsorption (calculated using the same standard states) per alcohol methylene group is -2.7 kJ mol-1.31 For SFAs in alkane solvents, the standard free energy of transfer per -CF2- from alkane to a perfluoroalkane is approximately -1.1 kJ mol-1.23 Thus, (31) Clint, J. H.; Corkill, J. M.; Goodman, J. F.; Tate, J. R. J. Colloid Interface Sci. 1968, 28, 522.

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tive index and a lower polarizability. Thus, both the dipole-induced dipole and dispersion attractions with DIM should be reduced. To test this hypothesis, the behavior of hexylcyclohexane (C6cyc) was compared with that of C6B in DIM solvent. C6cyc was found to show macroscopic phase separation at concentrations above 1.3 mol %, whereas C6B showed weak aggregate formation (but remains single phase) at concentrations above 5 mol %. This observation is consistent with the cyclohexyl group having a lower affinity for DIM than the benzene group. Ellipsometry. The ellipsometric parameters Ψ and ∆ were determined for octylbenzene solutions in DIM at 20 °C. The experimental values were transformed into values of the apparent thickness of the adsorbed film of octylbenzene using standard ellipsometry theory.32 For the calculations the surface region was assumed to consist of a semiinfinite layer of DIM solution, a film of octylbenzene of variable thickness, and a semiinfinite layer of air. The refractive index for the CnB solutions in DIM were assumed to scale with the volume fraction of CnB (φCnB) in the solutions according to Figure 4. Variation of surface tension with ln[C12B] in BN (open circles) and DIM (filled circles) at 20 °C. The horizontal lines show the tensions of pure BN (dash-dotted) and DIM (dotted).

alkyl chains in DIM show weaker solvent antipathy than either alkyl chains in water or fluorocarbon chains in alkanes. BN has a higher dipole moment than DIM (1.55 D as compared with 1.22 D) but a lower refractive index (1.65 as compared with 1.73). The surface tension of BN is lower than that of DIM, suggesting the overall solvent-solvent cohesion is weaker for BN than for DIM. The lower refractive index of BN (affecting dispersion interactions) is expected to result in a weaker antipathy between the alkyl chains of alkybenzenes with BN than that seen for DIM. This expectation was tested by comparing the adsorption of C12B at the BN-air and DIM-air surfaces, as shown in Figure 4. Adsorption at the BN-air surface occurs at higher concentrations than for the DIM-air surface. Additionally, the slopes of the tension plots at the highest concentrations tested indicate a much lower maximum surface excess concentration for BN as compared with DIM. The weaker adsorption from BN solution is consistent with a reduced solvent-solvent cohesion and solvent-alkyl chain antipathy. For conventional surfactants in water, aggregation is promoted by the antipathy between the tail group and the solvent and opposed by the affinity of the headgroup for the solvent. A suitable balance of these opposing forces leads to the formation of small micellar aggregates. When the solvent-headgroup affinity is too small, as for example in the case of hexanol in water, the aggregation is unlimited and results in macroscopic phase separation rather than micelle formation. For C8B and C6B in DIM the tension plots (Figure 1) indicate that aggregation commences at concentrations above approximately 5 mol %. The limitation to the aggregation is thought to be a consequence of the affinity of the DIM for the benzene headgroup resulting from its high polarizability (strong dipole-induced dipole attraction with DIM) and high refractive index (high dispersion attractions with DIM). In addition, the molecular shapes of C8B and C6B are likely to favor the formation of small aggregates with the benzene groups on the exterior surface. Solvent-headgroup affinity is expected to be reduced if a cyclohexyl group is substituted for the benzene group. Compared with benzene, cyclohexane has a lower refrac-

n ) φCnBnCnB + (1 - φCnB)nDIM + xφCnB(1 - φCnB) (4) where nCnB and nDIM are the real components of the refractive index for pure CnB and DIM, respectively. The parameter x allows for a slight nonlinearity in the variation of n with φCnB and was determined from independent refractive index measurements using an Abbe refractometer. In principle, the colored impurity in the DIM solvent suggests that the imaginary component k (related to the absorption coefficient) of the DIM refractive index is nonzero. However, the magnitude of the absorption coefficient of DIM is low enough that k for the solvent could be equated to zero with negligible error in the analysis of the ellipsometry results. The octylbenzene film layer was assumed to have a refractive index equal to that of bulk liquid octylbenzene. All the layers were assumed to be isotropic. With these assumptions, the only adjustable parameter was the adsorbed film thickness, which was adjusted until agreement was obtained between measured and calculated values of Ψ and ∆. Because of the assumption concerning the refractive index of the adsorbed film, thickness values derived in this way must be regarded as effective values but are expected to be close to the absolute values. Ellipsometry measurements were made at an incidence angle of 57° with respect to the surface normal, approximately 3° lower than the Brewster angle for the pure solvent. Under these conditions, the accuracy of the film thickness determination is primarily determined by the precision of the measured ∆ values. The measured standard deviation of 100 measurements of ∆ for a single sample was found to correspond to an uncertainty in film thickness of about (0.02 nm. The measured reproducibility of different samples of the same composition was found to be slightly higher at approximately (0.05 nm, and this uncertainty is quoted in the results presented here. The variation of film thickness with octylbenzene concentration is shown in Figure 5 (upper plot) and is compared with thicknesses derived from the surface tension data. Values of Γ from surface tension were converted to film thicknesses by assuming that the octylbenzene-adsorbed film has a density equal to that of bulk liquid octylbenzene, that is, film thickness is the (32) McCracken F. L.; Passaglia, E.; Stromberg, R. R.; Steinberg, H. L. J. Res. Natl. Bur. Stand. (U.S.) 1963, 67A, 363.

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Figure 5. Variation of film thickness with octylbenzene (upper plot) and hexylbenzene (lower plot) concentration in DIM as determined using tensiometry and ellipsometry.

product of Γ and the molecular volume of C8B. Because the effective thickness values derived from tension and ellipsometry are both based on the assumption that the adsorbed film has properties (refractive index for ellipsometry and density for tension) equal to those of bulk alkylbenzene, comparison of the two data sets is expected to be reasonably valid. For C8B concentrations less than 1 mol %, the ellipsometric and tensiometric thicknesses show good agreement. For C8B concentrations higher than 1 mol % and approaching the onset of aggregation (approximately 5 mol %), the ellipsometric thicknesses are significantly higher than those from tension. This discrepancy may be a result of neglecting the possible nonideal behavior of the C8B solutions in applying the Gibbs adsorption equation to obtain Γ. For higher concentrations (where tension data is unobtainable) ellipsometry indicates a sharp increase in film thickness to a high plateau value of greater than 2 nm. Because the vertical, fully extended length of C8B is approximately 1.6 nm, the results suggest multilayer formation in this region. For many micelle-forming conventional surfactants in water, neutron reflection

Fletcher and Nicholls

results indicate that Γ increases only slightly above the critical micelle concentration (cmc).33 One exception is seen with sodium bis(2-ethylhexyl)sulfosuccinate (AOT) solutions under conditions in which the bulk solutions are close to the micellar phase-lamellar liquid crystal phase transition. In this case studies have suggested the formation of multilayers.34 For the C8B/DIM system, it is relevant to note that the system is close to a phase separation boundary because phase separation (rather than micelle formation) is observed with C10B. We speculate here that the observed multilayer formation may represent a limited degree of surface-induced phase separation in the C8B/DIM system, which is a precursor to bulk phase transition. If this idea is correct, one would expect C6B solutions to show a lower extent of film thickening, as this system should be further from bulk phase separation. The lower plot of Figure 5 shows that the increase in C6B film thickness at high concentrations is indeed much smaller than for C8B. This observation lends support to the notion that the multilayer film formation is associated with proximity to bulk phase separation. Because the ellipsometric analysis does not, in principle, allow a unique solution for the surface structure to be found, several other models for the surface film were tested in addition to that described here. Calculations assuming a four-layer model consisting of DIM solution, a layer of benzene groups, a layer of octyl chains, plus air produced a qualitatively similar picture of the adsorption as seen in Figure 5. For concentrations higher than 5 mol %, including a surface DIM layer depleted of aggregates in the calculations was found not to agree with the experimental data. The model encompassed in the film thickness data of Figure 5 represents the simplest possible interpretation of the ellipsometric data that is consistent with the experimental data. Dynamic Light Scattering. In the DLS experiments, the intensity autocorrelation function G(t) as a function of correlator delay time t was measured for C8B/DIM solutions with concentrations above and below 5 mol %, where tension measurements indicate the onset of aggregate formation. For the DIM solutions the static scattering intensity was found to be irreproducible, owing to the variable color of the solvent resulting from decomposition. However, the intensity autocorrelation functions for fresh and aged solutions with visibly different colors were found to be identical, indicating that complications arising from the impurities can be safely neglected in the analysis of DLS. For noninteracting, spherical particles, G(t) decays exponentially according to

G(t) ) 〈I2〉 exp(-2DQ2t) + 〈I〉2

(5)

where I is the scattering intensity, D is the particle diffusion coefficient, and Q is the magnitude of the scattering vector given by

Q)

4πn sin(θ/2) λ

(6)

n is the solution refractive index, θ is the scattering angle, and λ is the wavelength of the incident light. The particle (33) Simister, E. A.; Thomas, R. K.; Penfold, J.; Aveyard, R.; Binks, B. P.; Cooper, P.; Fletcher, P. D. I.; Lu, J. R.; Sokolowski, A. J. Phys. Chem. 1992, 96, 1383. (34) Li, Z. X.; Lu, J. R.; Thomas, R. K.; Penfold, J. Faraday Discuss. 1996, 104, 127.

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Figure 6. ln[G(t)] versus delay time for 11.99 mol % octylbenzene in DIM at 20 °C. The solid line shows the best fit to a second-order polynomial.

Figure 7. Variation of the apparent z-average hydrodynamic radius with octylbenzene concentration in DIM at 20 °C.

hydrodynamic radius rH is obtained from D using the Stokes-Einstein equation.

Table 2. Summary of Octylbenzene Aggregate Parameters Derived from Dynamic Light-Scattering Measurements

kT rH ) 6πηD

(7)

where η is the solvent viscosity. Polydisperse particles with a monomodal distribution produce a nonexponential decay that may be analyzed using a cumulants expansion as described by Koppel.35

(

ln G(t) ) ln〈I2〉 - 2DmQ2t +

µ 2 t + ... + ln〈I〉2 (8) 2!

)

Dm is the mean diffusion coefficient and µ/(2DmQ2)2 is the normalized variance of the particle size distribution. Higher-order terms in the expansion yield additional moments of the particle size distribution but cannot generally be determined with precision and are neglected here. Conversion of Dm into the equivalent hydrodynamic radius yields the z-average radius. DIM solutions of C8B below 5 mol % showed autocorrelation functions with a fast principal decay component with a half-life of less than 1 µs. This component, probably arising from molecular motions of the DIM solvent, was removed from the autocorrelation functions of higher concentration samples by commencing the fit to eq 8 at delay times higher than 1 µs. A typical fit to eq 8 is shown for 11.99 mol % C8B in DIM in Figure 6 where it can be seen that the signal-to-noise ratio is sufficiently good that both Dm and µ can be obtained with reasonable precision. In principle, both the aggregate size and the interaggregate interactions change with octylbenzene concentration. To proceed further we assume that interactions can be neglected and that the particles are spherical. Because of these assumptions, the rH values obtained by the analysis described above must be regarded as apparent or effective values. Figure 7 shows the variation of the z-average rH with octylbenzene concentration. The aggregates grow progressively with increasing C8B concentration to reach a plateau radius value of approximately 1.5 nm. This type of aggregate growth is characteristic of weak aggregation and relatively small aggregation num(35) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814.

[C8B] z-average normalized Nagg Nagg (mole%) (rH/nm) variance (no solvation) (1 DIM per C8B) 6.08 8.08 10.01 11.99 14.97 18.99 22.00 25.09

0.68 1.01 0.91 1.21 1.37 1.51 1.42 1.28

0.187 0.220 0.120 0.119 0.042 0.034 0.041 0.005

4 12 9 20 29 39 33 24

3 9 6 15 22 28 24 17

bers, as also suggested by the fall in surface tension with concentration above the onset of aggregation at 5 mol % (Figure 1). The limiting apparent radius at high C8B concentrations (1.5 nm) appears reasonable when compared with the fully extended length of the C8B molecule of 1.6 nm. The small decrease in apparent radius at the highest concentrations tested may be due to weak repulsive interactions between the aggregates. The rH values were used to estimate aggregation numbers by dividing the micellar volume by the C8B molecular volume. Because the hydrodynamic radii include an unknown contribution from entrapped DIM solvent, two different calculations were made. First, it was assumed that no DIM solvent was present in the aggregates to give an upper limit to the aggregation number. Second, it was assumed that the aggregates contained 1 DIM solvent molecule per C8B molecule. It is likely that the true aggregation numbers lie between these two limits. Apparent z-average rH, normalized variances, and aggregation numbers for the different C8B concentrations are summarized in Table 2. Fully formed micelles of C8B (at concentrations >3 cmc) have an aggregation number of around 30 and show low polydispersity. It is noteworthy that C8B in DIM shows an increased tendency to form micelles of low polydispersity and relatively high aggregation number in comparison with SFAs in alkane or perfluoroalkane solvents23 even though the driving force for aggregation is larger in the latter systems. This observation is consistent with the intuitively reasonable notion that the molecular shape of C8B, with a large benzene headgroup and small octyl tailgroup, favors the formation of small micelles.

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Conclusions Alkylbenzenes in DIM form a novel, structurally primitive class of surfactants that exhibit adsorption and aggregation as a result of their alkyl/aromatic diblock structure. The system described here represents only the third class of low molar mass surfactant systems, with the first two being conventional surfactants in water (nonpolar/polar diblock architecture) and SFAs in either alkane or perfluoroalkane solvents (hydrocarbon chain/ fluorocarbon chain diblock architecture). Relative to the first two classes, alkylbenzenes in DIM exhibit only weak surfactant properties. Effects of changing the alkylbenzene

Fletcher and Nicholls

chain length, solvent variation, and substitution of the benzene headgroup are all rationalized in terms of considerations of the intermolecular forces. Acknowledgment. We are grateful to Dr. W. D. Cooper and Mr. P. Stevenson (Shell Research Ltd., Thornton, UK) and Prof. R. Aveyard (University of Hull) for helpful discussions. We thank Shell Research and the EPSRC (UK) for financial support. We also thank Dr. A. Stoyanov and Mr. A. McLaughlin of the University of Hull for making some of the light scattering and tension measurements. LA990987M