Adsorption Isotherms and Structure of Cationic Surfactants Adsorbed

Nov 13, 2013 - The adsorption isotherms and aggregate structures of adsorbed surfactants on smooth thin-film surfaces of mineral oxides have been stud...
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Adsorption Isotherms and Structure of Cationic Surfactants Adsorbed on Mineral Oxide Surfaces Prepared by Atomic Layer Deposition Thipvaree Wangchareansak,*,† Vincent S. J. Craig,† and Shannon M. Notley‡ †

Department of Applied Mathematics, Research School of Physics and Engineering, Australian National University, Canberra, 0200 ACT, Australia ‡ Faculty of Life and Social Sciences, Swinburne University of Technology, Hawthorn, 3122 VIC, Australia S Supporting Information *

ABSTRACT: The adsorption isotherms and aggregate structures of adsorbed surfactants on smooth thin-film surfaces of mineral oxides have been studied by optical reflectometry and atomic force microscopy (AFM). Films of the mineral oxides of titania, alumina, hafnia, and zirconia were produced by atomic layer deposition (ALD) with low roughness. We find that the surface strongly influences the admicelle organization on the surface. At high concentrations (2 × cmc) of cetyltrimethylammonium bromide (CTAB), the surfactant aggregates on a titania surface exhibit a flattened admicelle structure with an average repeat distance of 8.0 ± 1.0 nm whereas aggregates on alumina substrates exhibit a larger admicelle with an average separation distance of 10.5 ± 1.0 nm. A wormlike admicelle structure with an average separation distance of 7.0 ± 1.0 nm can be observed on zirconia substrates whereas a bilayered aggregate structure on hafnia substrates was observed. The change in the surface aggregate structure can be related to an increase in the critical packing parameter through a reduction in the effective headgroup area of the surfactant. The templating strength of the surfaces are found to be hafnia > alumina > zirconia > titania. Weakly templating surfaces are expected to have superior biocompatibility.

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INTRODUCTION Surfactants are used for the facile modification of surfaces. These amphiphilic molecules adsorb to interfaces, lowering the surface energy and producing changes in properties such as wettability, adhesion, friction, and lubrication. Hence, surfactants are of great importance in areas such as detergency, minerals processing (which includes flotation), and emulsion stabilization. Bulk solution properties of surfactants have been extensively studied.1 Entropic effects drive the self-assembly of surfactants at concentrations beyond the critical micelle concentration (cmc). The shape and size of the aggregated structure largely depend on the physical dimensions of the individual molecules as well as the forces between the molecules.2 The shape represented by these features is compared to a cylinder with the same basal area in order to arrive at the critical packing parameter, which can be used to infer the form of the aggregated structure including micellar, lamellar, and bicontinuous phases. The structure of surfactant aggregates adsorbed at the solid− liquid interface is somewhat related to the structure in solution.3 Clearly, however, the nature of the interface, whether hydrophilic or hydrophobic, positively or negatively charged or indeed uncharged, will influence the structure of the adsorbed surfactant layer. The chemistry of the surfactant will also play a © 2013 American Chemical Society

role in both ionic and nonionic amphiphiles undergoing different interactions with the solid−liquid interface, which leads to less-predictable aggregate structure formation than in bulk solution. For ionic surfactants, at low concentrations, the driving force for adsorption to the solid−liquid interface is predominantly of electrostatic origin.4,5 However, at concentrations typically 10 to 100 times below the cmc, the surface charge is neutralized, and adsorption at higher concentrations proceeds because of favorable hydrophobic interactions between surfactant molecules, hence the formation of admicelles6,7 leading to cooperative adsorption. This concentration is defined as the critical surface aggregation concentration, csac. Because of the favorable interactions between a surface and surfactant molecules, the maximal surface excess is reached below the critical micelle concentration (cmc). We suggest that the term maximal coverage concentration (mcc) be used to define the concentration at which the maximal surface excess is reached. The cmc is a solution property, and in practice, the surface has little effect on the mcc when it is above the csac because under Received: February 21, 2013 Revised: November 13, 2013 Published: November 13, 2013 14748

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Table 1. ALD Parameters Used for the Deposition of Mineral Oxides onto Silica Substrates for the Surfaces Used in the Imaging Study and the Sensitivity Parameter As Employed in OR Measurements material

precursor

growth temperature/°C

number of cycles

thickness/nm

roughness/nm

sensitivity parameter As

Al2O3 TiO2 HfO2 ZrO2

(Al(CH3))2 Ti(OCH(CH3)2)4 Hf [(CH3)2N]4 Zr[(C2H5)(CH3)N]4

200 250 100 175

952 280 121 141

108 5 12 2

0.22 0.24 0.53 0.79

−0.027 −0.024 −0.035 −0.026

purification. Milli-Q water was used to prepare surfactant solutions with the pH adjusted to 6 with an appropriate amount of NaOH or HCl. Atomic layer deposition (ALD) was used to prepare all of the mineral oxide surfaces on silicon wafers (100) supplied by MEMC, USA with a native oxide layer thickness of ∼2 nm. Surface Preparation. ALD is a self-limiting gas-phase deposition technique that can be used to produce a conformal film of controlled thickness.28,29 The process is detailed in recent reviews, so only brief details are given here.30−32 Substrates with reactive sites such as hydroxyl groups are first exposed to water vapor, which primes the surface, followed by purging of the chamber with nitrogen. The mineral oxide precursor is then injected in the gaseous form, which results in the formation of a monolayer through a self-limiting reaction with the hydroxyl groups of the surface. Excess precursor is purged from the system before water vapor is reintroduced to complete the formation of metal oxide bonds and produce additional reactive sites (hydroxyl groups). In this way, the oxide can be constructed layer-bylayer with precise control of the thickness of the deposited film. Furthermore, the crystallinity and through this the surface roughness can be controlled via the deposition temperature. In this study, deposition conditions were chosen that gave rise to amorphous films because these are smoother.33 Table 1 gives the parameters used to prepare the mineral oxide surfaces deposited using ALD. We have previously reported the characterization of these films.27 X-ray Photoelectron Spectroscopy (XPS) Characterization of Thin Films. The films prepared using ALD were studied using XPS to determine the surface chemistry and relative abundance of the atomic species. Furthermore, the peak positions in the spectra can indicate the nature of the chemical bonding to the metal ion. A Kratos Axis Nova with a monochromated Al Kα X-ray source was used to determine the spectra for the hafnia, alumina, and zirconia films. Previously, XPS was performed on the titania surfaces used in this study.34 Charge neutralization was used in all measurements. Quantification of the atomic ratios was determined from survey spectra. The peaks were charge referenced to the C/CH binding energy at 280.5 eV. Prior to measurement, the surfaces were cleaned through sequential sonication in dichloromethane, isopropanol, ethanol, and finally deionized water. AFM Imaging. Adsorbed surfactant layers were imaged in soft contact mode20−22,24 using a Multimode III (Bruker, USA) Atomic force microscope. Standard contact Si3N4 cantilevers (Veeco, USA) (200 μm long, triangularly shaped with a spring constant of 0.07 N/m) were used. The spring constant was measured by the thermal noise method.35 The scan rate was typically between 5 and 10 Hz, and gains of between 1 and 2 were used. The concentration of CTAB was 1.8 mM, which is 2 times the critical micelle concentration (cmc for CTAB in water is 0.9 mM) and more than twice the mcc. This was chosen to ensure that an equilibrium surfactant structure is achieved quickly and to provide aggregates that are sufficiently robust to be imaged. Optical Reflectometry. The surface excess of CTAB adsorbed on ALD surfaces of titania, hafnia, zirconia, and alumina was measured using optical reflectometry with a purpose-built instrument.9,15 The technique, which is related to ellipsometry, measures the polarization of reflected light from the surface to determine the adsorbed mass per unit area. The baseline signal is obtained when the surface is bathed in solvent. Introduction of the surfactant leads to adsorption, which results in a change in the polarisition of the reflected beam. This is converted to surface excess in the usual manner11 using eq 1

these circumstances the cmm is determined by surfactant− surfactant interactions. At solution concentrations below the cmc, the rate of adsorption is slow because monomers are required to pack into surface aggregates.6,8,9 This can result in adsorption proceeding at a pace orders of magnitude slower than at concentrations above the cmc, thus the presence of surface aggregates can dramatically influence the rate of surface equilibration in addition to the amount of adsorption. There have been many methods described for studying surfactant adsorption at the solid−liquid interface.10 Adsorption isotherms provide indirect evidence for surfactant aggregation and the structure is therefore interpreted with regard to the measured surface excess. Techniques such as ellipsometry and optical reflectometry11 also provide kinetic information in regard to the adsorption process. Both have been used extensively to study the adsorption of surfactants at solid− liquid interfaces.6,7,9,12−15 Spatial information on surfactant structure can also be determined from neutron reflection techniques16−18 as well as direct imaging using soft contact mode atomic force microscopy.13,14,19−25 Soft contact imaging has, however, generally been successful only at concentrations above the mcc as a result of the compliance of surfactant layers at sparsely covered surfaces and the resulting small repulsive forces. Significantly, the dynamics of adsorbed aggregates must be very much slower than those in the bulk as evidenced first by the ability to image aggregate structures with AFM and second by the reproducibility of the imaged structures. Clearly, adsorbed aggregates have lifetimes many orders of magnitude greater than the lifetime of a micelle in bulk solution.15 This can be attributed to the templating influence of the substrate, which acts to chelate groups of surfactant monomers together, promoting stable structures. The structure of surfactant aggregates on inorganic oxide surfaces is of particular importance in mineral-processing applications.26 Surfactants are typically added in froth flotation in order to change the wettability of the particles and hence modify the attachment to bubbles. Recent advances in the production of smooth films of mineral oxide surfaces have led to improved measures of the fundamental structure of adsorbed layers while minimizing the complicating factor of surface roughness. Atomic layer deposition (ALD) is one such technique that can produce films with low roughness suitable for soft contact imaging of surfactant aggregates.27 This study uses surfaces produced by ALD to determine the structure of a quaternary ammonium surfactant at the mineral oxide−water interface at concentrations above the mcc and the critical micelle concentration (cmc) on titania, hafnia, zirconia, and alumina surfaces. Additionally, the adsorption isotherms for CTAB on the same surfaces are obtained by optical reflectometry (OR).

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EXPERIMENTAL SECTION

Materials. Hexadecyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich Australia and used without further 14749

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ΔS S0A s

75.0 eV, suggests that no precursor remains after deposition The binding energies and the relative proportion of the metal ion to oxygen ratio suggest that mineral oxides are formed under these low-temperature deposition conditions. Furthermore, it is unlikely that any unreacted precursor is present at the interface because of the thorough cleaning protocol employed prior to the surfactant adsorption measurements. Previous studies have shown that the metal ion−oxygen peak is located at 75.0 eV (Al 2p), 17 and 19 eV (Hf 4f5/2 and 4f7/2), and 182 and 182.7 eV (Zr 3d5/2 and 3d3/2) for Al2O3, HfO2, and ZrO2, respectively, which is in agreement with the data presented here.38−40 The spectrum for the ZrO2 film also shows a prominent peak associated with HfO2 that was expected as a result of the very thin (∼2 nm) layer of deposited ZrO2 and the relative penetration depth of the XPS measurement. The optical reflectometry experiments are summarized in Figure 1, which depicts the measured adsorption isotherms for

(1)

where Γ is the adsorbed surface excess, S0 is the baseline ratio of the reflectivities of the p and s components (i.e., S0 = Rp/Rs (usually ∼1), ΔS is the change in polarization, and As is the sensitivity parameter obtained from the optical model. The optical model uses the Fresnel equations where each layer is characterized by a thickness and complex refractive index. A silicon wafer with an oxide layer thickness of 320 nm was employed as the substrate in these experiments, with the beam from a 632.8 nm He−Ne laser impinging on the surface at an angle of incidence of 71°. The dn/dC value of 0.149 cm3 g−1 was used for CTAB.6,36 The calculated sensitivity parameters employed are shown in Table 1. A sensitivity that is high in magnitude is preferred, but the optical configuration is also chosen such that the sensitivity parameter is insensitive to the surface excess and errors in the angle of incidence in order to minimize errors in the surface excess.

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RESULTS Our interest here is to characterize the surfactant layer in order to determine the influence of the substrate material on the morphology of the surfactant adsorption. We have employed flat surfaces with the lowest degree of roughness attainable and note that the roughness of the surface is another parameter that can strongly influence the adsorption and structure of adsorbed surfactant aggregates.37 Typically, the surfactant layer is soft and less than 3 nm thick; therefore, for all but the smoothest of substrate materials, the features associated with the surface roughness of the substrate will be convolved with the image of the surfactant layer. In Figure 2, we show AFM tapping mode images, conducted in air, of the four substrates used in this investigation. Although the surfaces are very smooth, with the roughness being comparable to that of the silicon wafer substrate, it is evident that the surfaces have different textures. In each case, the surfaces have previously been investigated by XRD27 and have been found to be amorphous; therefore, the texturing is not associated with the crystallinity of the substrate. All of the surfaces when clean are hydrophilic (water contact angle HfO2 > ZrO2 > Al2O3. This is consistent with the surface excess being governed by the magnitude of the negative charge on the surface, considering that when the materials are arranged in order of increasing isoelectric point the same sequence is achieved. What is significant is that the cationic surfactant adsorbs to the alumina surface at significant levels despite the alumina surface carrying a considerable positive charge. This is attributed to the presence of some negatively charged adsorption sites on the surface (presumably ionizable hydroxyl groups41) despite the overall positive charge and the influence of the attractive van der Waals interaction between the surfactant and the surface. This indicates that charge alone cannot be employed as an effective means to prevent surface fouling on oxide surfaces. Note that in all cases the latter stages of adsorption are driven by hydrophobic interactions against a repulsive electrostatic force as the overall surface charge becomes positive. Despite differences in the surface excess and structure of adsorbed surfactant, in each case the mcc is very similar (∼1 mM), indicating that it is primarily determined by the surfactant and the solution properties rather than the substrate. This is because the final stages of adsorption are determined primarily by hydrophobic interactions driving the formation of

Table 2. Metal, Oxygen, and Carbon Ratios Determined from XPS for ALD Surfaces Used in This Studya material

metal%

Al2O3

30.8 (Al 2p)

HfO2

23.9 (Hf 4d)

ZrO2

15.7 (Zr 3p) and 10.1 (Hf 4d)

O% 51.1 (O 1s) 55.3 (O 1s) 52.0 (O 1s)

C% 17.6 (C 1s) 16.0 (C 1s) 22.1 (C 1s)

N% negligible 4.6 (N 1s) negligible

a

Peak assignments for the calculation of the atomic percentage are in parentheses. 14750

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aggregate structures, which are opposed to some extent by repulsion between neighboring headgroups and are therefore largely independent of the nature of the interface. Contact-mode AFM images of ALD surfaces in equilibrium with aqueous CTAB at a concentration of twice the cmc (i.e., these images are captured in solution) are shown in Figure 3. Note that this concentration corresponds to the highest concentrations shown in the isotherm in Figure 1 and therefore the surface excess in each case varies from a low of 0.4 mg m−2 for alumina to a high of 1.95 mg m−2 for titania. At this surfactant concentration, which is far above the mcc, the surfactant adsorbs to the surface, resulting in maximal coverage for each surface. Comparing the images obtained to those in Figure 2 for the bare substrates, it is evident that the texture of

Figure 3. AFM representative deflection images of CTAB adsorbed layers on four different ALD-produced mineral oxide surfaces. The scan size is the same for each image at 300 nm × 300 nm.

Figure 2. Representative AFM tapping mode (deflection) images of ALD-deposited films in air for four different ALD-produced mineral oxide surfaces. The scan size is the same for each image at 300 nm × 300 nm. The rms roughnesses of the surfaces over this scan size were found from height images to be 0.22 nm (Al2O3), 0.24 nm (TiO2), 0.53 nm (HfO2), and 0.79 nm (ZrO2).

the surface has changed. This is attributed to the structure of the adsorbed aggregates of CTAB. It is also apparent that the aggregate structures differ considerably from one substrate to the next, showing the influence of the substrate on the structure of the adsorbed surfactant film. In Figure 4, the 2D Fourier transform of each total image is presented, which aids in the evaluation of the aggregate structure. We recognize that within an image the structures and their orientation vary; however, Fourier transforms of subregions were not useful in characterizing these subregions because of the level of noise. On a mica substrate, much clearer Fourier transforms are obtained than is possible for our amorphous surfaces because of the perfect atomic flatness of the substrate and the strong templating property of the crystalline mica.24 Alumina. On the alumina substrate, the surface appears to consist of aggregates that are locally aligned with neighboring aggregates, but there is little long-range order. This structure is superimposed on the roughness of the underlying substrate. The Fourier transform consists of two spherical rings. The

Figure 4. Two-dimensional Fourier transforms obtained from the images shown in Figure 3.

inner ring corresponds to long-range roughness and is attributed to the substrate. The outer ring corresponds to a distance of 10.5 ± 1.0 nm, which gives the average repeat distance between admicelles on the surface. We note here that the errors we report in the repeat distance are an estimate obtained from the error associated with the uncertainty in picking the maximum of the peak in the Fourier transform. A more rigorous treatment would involve the determination of the peak width at half height or a similar measure, but the noise in the data precludes such analysis here. This indicates that the admicelles are flattened and extended in shape. (The height of 14751

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Surface force measurements can be used to reveal the thickness of the adsorbed aggregates. With reference to Figure 5, it can be seen that the substrate influences the nature of the

the admicelle can be determined from the jump-in distance in a force vs separation measurement.) For alumina, the thickness was determined to be ∼1.0 nm from the analysis of the force data (see below). This is thinner than the layer thickness of the same surfactant on the other substrates. There is a very slight modulation as a function of angle in the brightness of the outer ring, indicating that there is some slight preference in the alignment of adjacent micelles for particular orientations. This is supported by the alignment seen between admicelles over small regions evident in the deflection image. See the Supporting Information for section profiles of CTAB on alumina. Titania. The titania substrate is extremely smooth, allowing the aggregate structures to be very clearly seen. The Fourier transform consists of a single ring that corresponds to an average repeat distance of 8.0 ± 1.0 nm. Thus, the admicelles are flattened, though smaller than those on the alumina surface. In the deflection image, it can be seen that in some regions there is preferential alignment between neighboring admicelles and lines of admicelles are formed. This is also evidenced in the Fourier transform as brighter spots in the ring at angles perpendicular to the alignment direction. The thickness of the aggregates on the titania surface determined from force data is 2.7 ± 0.3 nm. This value is consistent with bilayered aggregates as is the surface excess. See the Supporting Information for section profiles of CTAB on titania Hafnia. Although the hafnia substrate is quite smooth and has a regular texture, no clear images of surfactant aggregates could be obtained on this substrate. Accordingly, the Fourier transform shows no significant features. It is apparent that a bilayered aggregate is present on the surface, which is supported by the measured surface excess. The inability to obtain a clear image can be due to either an aggregate structure that is insufficiently rigid to be imaged or a lack of features that can be discerned by AFM. We surmise that the films are featureless bilayers because force measurements indicate that the film is robust. Indeed, the thickness of the aggregates on the hafnia surface could not be determined from force measurements directly because the films were too robust to be penetrated. From comparison with the iostherms of hafnia and zirconia, we estimate that the thickness of the surface bilayer is around 2.7 nm. This is also consistent with the molecular dimensions of the CTAB molecule and is supported by previous surface forces studies.25,42 Zirconia. Aggregates are considerably elongated on the zirconia substrate, forming structures that are shortened 2D analogues of wormlike micelles. Some of these aggregates can be seen to persist from nearly one corner of the image to the other. An inspection of the Fourier transform of the image reveals two bright spots (top left, bottom right) that correspond to the separation between adjacent wormlike admicelles. The separation is found to be 7.0 ± 1.0 nm whereas the admicelles are in some cases more than 100 nm long. There are also regions where the wormlike admicelles cannot be seen. In these regions, the wormlike admicelles may not have been sufficiently rigid to be imaged and the texture of the substrate was seen. Alternatively, these regions may have been occupied by aggregates of another form. The thickness of the aggregates on the zirconia surface determined from force data is 2.7 ± 0.3 nm, which is consistent with bilayered aggregates. See the Supporting Information for section profiles of CTAB on hafnia.

Figure 5. Force vs separation measured between the tip of the AFM cantilever and ALD substrates of titania, hafnia, zirconia, and alumina. The force and depth required to penetrate the surfactant layers are used to interpret the stability and thickness of the surface aggregates. The data for hafnia has been offset by 5.4 nm to reflect the presence of a bilayer on both the substrate and the AFM tip, which are not displaced during the measurement.

interaction forces, in particular, the nature of the force required to displace the surfactant layer. On approach, electrostatic repulsion is evident from ∼30 nm separation before an additional repulsion is measured. For titania and zirconia surfaces, the tip penetrates a layer of ∼5.4 nm thickness before a hard wall repulsion is measured. This is attributed to the removal of a bilayered aggregate from both surfaces, indicating that the aggregate thickness is ∼2.7 nm. No such push through was observed on hafnia surfaces, although the adsorption data reveals the presence of a bilayer. This is attributed to a more robust film that could not be penetrated. Consequently, we have translated the data by 5.4 nm such that the force curves have separation scales that are all relative to the substrate− substrate contact. On the alumina surface, a small attraction is seen at a separation of ∼8 nm before a push through of ∼3.7 nm is measured. If 2.7 nm is attributed to the bilayered aggregate on the tip as seen in other measurements, then a surface aggregate thickness of ∼1.0 nm is determined. This indicates that the aggregates on the alumina substrate are thinner, which is consistent with the lower surface excess found with optical reflectometry.

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DISCUSSION As seen before with HOPG,43 mica,21 and silica25 surfaces, the aggregate structures that are formed44 on different substrates vary considerably. The role of the substrate can be related to the strength of the substrate templating action, whereby the templating action of the substrate is strong when the structures observed on the surface differ considerably from those seen in the bulk solution at the same concentration. Strong templating indicates strong interactions between the surface and the surfactant. For hydrophobic substrates such as HOPG, the templating is extremely strong and dominates the system to the extent that analogues of the structures observed on the surface are not found in bulk solution. In the case of HOPG, singlelayer crystalline aggregates are formed by the very strong 14752

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adsorption is complex and dependent on electrostatic, nonelectrostatic, and solvent interactions. The electrostatic component of the interaction will be dominated by interaction between the counterion and the surfactant headgroup and therefore will be unchanged for a given surfactant. The nonelectrostatic interactions are complex and dependent upon dispersion interactions between the ion and the surface and the interactions of both the ion and surface with water molecules. Akin to Hofmeister interactions, the strength of the interaction may be expected to depend on the law of matching water affinities proposed by Collins.47 This approach separates ions (or surface groups) into chaotropes or kosmotropes in reference to their interaction with water and their influence on the water structure. It is found that ions pair (or become closely associated) if both ions are from the same grouping (i.e., chaotrope−chaotrope or kosmotrope−kosmotrope pairs) whereas if one ion is a kosmotrope and the other is a chaotrope they will be repelled.48 The bromide ion is classified as a chaotropic ion and as such will strongly interact with a chaotropic surface. Silica surfaces are kosmotropic, and alumina surfaces are chaotropic,49 suggesting that bromide ions will strongly adsorb to alumina surfaces, which is consistent with alumina templating more strongly than silica, as is observed. At this stage, the chaotropic or kosmotropic nature of the hafnia and zirconia surfaces has not been established. Untreated titania surfaces are also chaotropic,50 but there is currently no measure of the relative degree of chaotropic tendency for different surfaces. Thus, the templating strength of a system involving ionic surfactants may be strongly dependent upon the nature of the counterion in addition to the surface and the monomer. If this is the case, then the nature of the ions in the buffer (in addition to the pH) may be important in controlling the strength of templating in biological systems. Recent simulations of SDS adsorbing to silica surfaces found that the sodium counterion strongly influences the adsorption.51 It should also be apparent that for the adsorption of surfactant molecules to occur, water molecules at the interface must be displaced. Interestingly, the enthalpy of desorption becomes less positive in the order found for surface templating. It is possible, then, that interfacial water molecules actively participate in the surfactant adsorption process with the energy gain associated with the immobilization of CTAB molecules at the surface also dependent on the interaction of water molecules with the surface. The strongly templating nature of graphite supports this idea. Wormlike admicelles formed on mica surfaces are seen to form domains that are oriented at set angles as a result of the underlying crystallinity of the mica surface. Therefore, the spacing of surface molecules or surface groups can also contribute to a reduction in the critical packing parameter in a manner similar to epitaxy. Because the surfaces in this study are amorphous, it is difficult to evaluate the role that the molecular ordering at the surface may be playing in the templating strength of the substrates because such order is only short-ranged. Although ALD may produce surfaces that are extremely smooth, it is possible that the surface roughness is still exerting a significant effect on the structure of the adsorbed aggregates. We note that the templating seen here is not of the same order as the surface roughness, though even roughness on a minor scale may influence the detailed structure of adsorbed material.

hydrophobic attraction between the HOPG and the surfactant chains and the consequent strong lateral hydrophobic interactions between the chains. The templating action of mica is stronger than that of silica as evidenced by the wormlike admicelles and complete bilayer structures21 seen on mica at surfactant concentrations that produce only admicelles on silica and spherical micelles in bulk solution. What is not clear is the particular features of the surface that control templating. The surface charge and van der Waals interactions should contribute to interactions between the surfactant and the surface, but the atomic arrangements of the surface may also play a role through influencing the strength of surfactant chain−chain interactions. The templating strength of a surface will be important in determining the biocompatibility of a surface. A strongly templating surface is more likely to alter the primary structure of adsorbed proteins and result in a loss of protein function, whereas proteins adsorbing to a weakly templating surface are more likely to retain their tertiary structure and thereby retain functionality. From the information presented above, we can determine the relative strength of the templating action of the surfaces studied as alumina > zirconia > titania. This is based on how strongly the structures of the adsorbed aggregates deviate from the aggregate structure present in the bulk, which for CTAB is micelles. The admicelles of zirconia are wormlike, and for alumina, the coverage is low, resulting in admicelles that are sparse and interdigitated whereas the admicelles of titania that are elongated are comparatively perturbed less by the surface. It is not clear what surfactant structures are present on the hafnia surface from the image alone. The lack of any structure in the image and a surface excess and push through consistent with a bilayered structure indicate stronger templating action than that of zirconia; therefore, the trend is hafnia > alumina > zirconia > titania. This trend cannot be explained by the surface charge alone because the reported isoelectric point of alumina (∼8) is higher than those of hafnia (7−7.6), zirconia (6−7), and titania (5−6).45 However, the van der Waals forces may be important because the refractive index of the materials follows the order titania > zirconia > alumina > hafnia.46 The correlation is unlikely to be simple, though, because the stronger van der Waals attraction is associated with the weaker templating action. It is not explained by the surface excess that follows the order titania > hafnia > zirconia > alumina. Because a strongly templating surface leads to a change in the structure of surface aggregates, we can also evaluate templating in terms of the critical packing parameter2 of the adsorbed aggregates in comparison to that of the micelles present in bulk solution. At the concentration employed (2 × cmc), CTAB forms spherical micelles in solution corresponding to a critical packing parameter of ∼1/3. The critical packing parameter must increase in order to form flattened admicelles (cpp ≈ 1/3−1/2), wormlike admicelles (1/2−2/3), or a complete adsorbed bilayer (cpp ≈ 1). Because the volume of the surfactant chain is unchanged during adsorption, an increase in the critical packing parameter can be achieved only by a reduction in the effective headgroup area of the surfactant. We note here that the headgroup area is not determined by the physical size of the headgroup but by the repulsive interactions between headgroups that determine their proximity. A reduction in the headgroup area could result from an effective decrease in the headgroup charge, such as an increase in the number of counterions adsorbing to the surface or an increase in the density of surface sites of counter charge. Counterion 14753

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CONCLUSIONS The structure of adsorbed aggregates of CTAB has been observed by atomic force microscopy on alumina, titania, hafnia, and zirconia surfaces produced using atomic layer deposition. The surfaces strongly influence the admicelle organization at the surface. This interaction can be characterized by the strength of the templating influence of the surface in that a weakly templating surface will produce surface aggregates that are similar to those present in bulk solution, whereas a strongly templating surface will tend toward surface aggregates that are 2D analogues of structures that occur in the bulk at much higher concentrations. Alternatively the change in surface aggregate structure can be related to an increase in the critical packing parameter through a reduction in the effective headgroup area. The templating strength is found to be hafnia > alumina > zirconia > titania. The relatively weak templating action of titania may be significant in imparting the biocompatibility of titania surfaces because proteins adsorbing to weakly templating surfaces can be expected to maintain their tertiary structure and therefore function.

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ASSOCIATED CONTENT

* Supporting Information S

XPS survey scans of Al2O3 thin films and HfO2 thin films deposited on oxidized silicon wafers and ZrO2 deposition on top of a HfO2 layer on an oxidized silicon wafer. Highresolution scans of the Al 2p, Hf 4d, and Zr 3p peaks. Section profiles of CTAB on ZrO2, TiO2, and Al2O3 substrates. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61-26125-1585. Author Contributions

This manuscript was written through equal contributions of all of the authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Rick Walsh for preparing the ALD surfaces and Thomas Ameringer for performing XPS measurements. Discussions with Peter Kingshott are gratefully acknowledged. V.S.J.C. and S.M.N. gratefully acknowledge the support of the Australia Research Council through future fellowships. T.W. acknowledges the support of the Australian government through an Endeavor award. V.S.J.C. gratefully acknowledges the International Fine Particle Research Institute (IFPRI) for financial support.

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