Collapse of preformed cobalt stearate film on water surface

Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

Contents lists available at ScienceDirect

Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Collapse of preformed cobalt stearate film on water surface Sarathi Kundu ∗ Department of Materials Science, S.N. Bose National Centre for Basic Sciences, JD Block, Sector III, Salt Lake City, Kolkata 700 098, India

a r t i c l e

i n f o

Article history: Received 23 April 2009 Received in revised form 8 July 2009 Accepted 13 July 2009 Available online 21 July 2009 Keywords: Cobalt stearate Langmuir monolayer Monolayer collapse Horizontal deposition X-ray reflectivity AFM

a b s t r a c t Preformed cobalt stearate (CoSt) molecules form a film on the water surface, which with barrier compression shows multilayers of different heights that are evidenced from the structures of the films deposited on hydrophilic silicon (0 0 1) substrates by using a horizontal deposition technique at different positions of the surface pressure ()–specific molecular area (A) isotherm. In-plane morphology and out-of-plane structures are obtained from the atomic force microscopy (AFM) and X-ray reflectivity studies. Electron density profiles (EDPs), extracted from the reflectivity data, show that the monolayer coverage is maximum when  is far before the collapse point (c ) but with barrier compression domains of multilayers start to form even before c . After c , two different bilayer repeat distances have been observed from the two different series of the Bragg peaks implying the formation of domains by both the tilted and untilted CoSt molecules. Far after c , reflectivity decreases rapidly and morphology of the deposited films changes totally. Structures before and after c of the CoSt film have also been obtained by changing the pH of the subphase water. From all the structural information it is clear that the preformed CoSt film collapses in a different way in comparison with the collapse of the standard cobalt stearate monolayer where cobalt stearate molecules were formed at the air–water interface. Reasons for obtaining different structures on the water surface with barrier compression have been proposed. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Stearic acid molecules form the Langmuir monolayer of cobalt stearate at the air–water interface when the acid molecules were spread on the water surface containing the cobalt ions in the subphase water. This is the standard process of forming Langmuir monolayers of fatty acid salts [1–3]. Langmuir–Blodgett (LB) films have been deposited from this cobalt stearate monolayer for studying the magnetic properties in low dimension [4] and for preparing sulfide layers within the organic media [5]. Cobalt stearate monolayer has also been used as a model system to study the collapse behavior of a 2D system [6]. Structures and the related collapse mechanisms for the different Langmuir monolayers have already been proposed [7–11]. Molecular mechanisms of monolayer collapses have also been investigated by molecular dynamics simulation [12,13]. For the cobalt stearate monolayer, mainly monolayer to bimolecular layer transformation takes place after the collapse point (c ) that has been obtained from the X-ray reflectivity studies [6,14] of the deposited films on Si (0 0 1) substrates [6,14]. With more and more barrier compression after c , surface pressure increases slowly and nearly parallel ridge like morphology forms on the water surface that has been observed from the AFM studies.

∗ Fax: +91 33 2335 3477. E-mail addresses: [email protected], [email protected]. 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.07.024

Actually, at the air–water interface, monolayer-forming molecules remain in the ‘asymmetric’ configuration where the heads containing the divalent ions touch the water surface and the tails stand out in air (shown by a cartoon in Fig. 1a). Except such stable asymmetric monolayer (AML), there is another stable configuration of layers on the water surface that has been observed when the bimolecular layer is formed. In the bimolecular layer, lower layer is AML but in the upper molecular layer, the heads are in the middle and the two tails are on the two sides. Thus with these ‘symmetric’ molecules the symmetric molecular layer (SML) is formed on the AML. For standard Langmuir monolayer, conversion from AML to AML + SML takes place for ‘constant pressure’ collapse whereas from AML to multilayer of SMLs on AML takes place for ‘constant area’ collapse which are shown by cartoons in Fig. 1b and c respectively. Actually, there are two main collapse behaviors as observed from surface pressure ()–specific molecular area (A) isotherms: either  falls suddenly after c or it remains nearly constant after c . The former is called the ‘constant area’ collapse while the latter is called ‘constant pressure’ collapse [14]. Coexistence of ‘monolayer’, ‘bilayer’ and ‘trilayer’ structures after the collapse of the fatty acid salt monolayer have also been observed on water surface using X-ray and neutron reflectivity techniques [15,16]. Coexistence of these monolayer, bilayer or multilayers after monolayer collapse is not the same as the coexistence of liquid expanded (LE) and liquid condensed (LC) or liquid condensed (LC) and solid (S) domains as usually observed in the standard Langmuir monolayer. Because in

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

197

isotherms, X-ray reflectivity and AFM studies help to obtain structural evolution with barrier compression. Reasons for obtaining different structures on the water surface with barrier compression have been proposed. 2. Materials and methods 2.1. Materials

Fig. 1. Cartoons depicting the (a) asymmetric molecular layer (AML) (b) bimolecular layer, i.e., symmetric molecular layer (SML) on top of the asymmetric molecular layer (AML) and (c) multilayer, i.e., SMLs on top of the AML. Area per molecule is same for both the bimolecular layer and multilayer configurations.

the Langmuir monolayer, molecules are always in the AML configuration but their packing, tilt angles, and tilt azimuths are different [17]. Collapse processes are generally irreversible, but certain mixed monolayers collapse reversibly, such as those of lung surfactant. Corresponding surface tension regulation is fundamental for lung operation [18–20]. Collapse processes are also found in biological cells, for instance in the response of membranes to stress, during membrane fusion and fission [21,22], which are sometimes triggered by divalent ions [23]. Langmuir film of cobalt stearate molecules on water surface can also be prepared in some other way, like the formation of ferric stearate film on the water surface where ferric stearate molecules were prepared through stepwise chemical reaction in bulk and then were spread on the water surface from the chloroform solution. However, very few works have been done on such preformed amphiphilic molecules [24,25]. Bimolecular layer (AML + SML) film has been observed by these preformed ferric stearate molecules on water surface form zero surface pressure. This bimolecular layer has some special properties. The film was found to reduce the surface tension of water by almost two orders of magnitude [26]. The upper molecular layer (SML) was found to grow in density with barrier compression following the –A isotherm curve [27] and in Langmuir–Blodgett (LB) method only monolayer was deposited in each up and down stroke of the substrate from this bimolecular layer [28]. On the other hand, preformed two-tailed cobalt stearate molecules form a film on the water surface from which multilayer deposition was not possible by using LB method [25]. In this article, a film of preformed cobalt stearate (CoSt) molecules has formed on the water surface, and lateral compression induced structural and morphological changes have been obtained after depositing the film on hydrophilic Si (0 0 1) substrates. Depositions were done in a modified version of the inverted Langmuir–Schaefer (MILS) technique [6,14,26] from pure water (pH ∼ 5.5) and from the water subphase at pH ∼ 6.8. Electron density profiles (EDPs), extracted from the reflectivity data, of these deposited films give the out-of-plane structures. AFM images give the changes in the in-plane morphology and heights of the deposited films. Combined information extracted from the –A

Cobalt stearate was prepared through stepwise reactions like ferric stearate [24]. At first sodium stearate was prepared by adding sodium hydroxide (Merck, 99%) in hot Milli-Q water (resistivity 18.2 M cm) containing stearic acid (Sigma, 99.9%) in appropriate amounts. Sodium hydroxide was added until the medium was slightly alkaline (pH ∼ 7.0–7.5). Sodium stearate was completely soluble in hot water. Measured amount of cobalt chloride (Merck, 99%) solution was then added in the freshly prepared sodium stearate solution in hot condition so that CoSt is formed and collected after filtration. As CoSt is insoluble in water at all temperatures, it is then washed repeatedly with hot Milli-Q water to remove unreacted sodium stearate and other water-soluble impurities. It is then washed with benzene (SLR, 99.8%) to remove unreacted stearic acid and other organic impurities. Fourier transform infrared (FTIR) spectra of the purified CoSt sample were collected in the 670–4000 cm−1 range with a Spectrum GX (PerkinElmer) spectrometer in the attenuated total reflection (ATR) mode at a resolution of 4 cm−1 . The presence of strong bands corresponding to carboxylate asymmetric and symmetric stretch modes indicate a large conversion of the fatty acid to the metal-bearing salt. Presence of very weak bands corresponding to COOH deformation and stretch and to hydroxyl stretch indicates, respectively, negligible amounts of free acid and hydroxyl group in the sample. 2.2. Deposition of the film To measure the –A isotherm of the CoSt monolayer and to transfer the film on to hydrophilic silicon (0 0 1) substrates, CoSt molecules were spread from a 0.6 mg/mL chloroform solution in a Langmuir trough (KSV 5000) on Milli-Q water at room temperature (∼21 ◦ C). The pH of the pure subphase water was ∼5.5. A platinum Wilhelmy plate was used to measure the surface pressure of the CoSt film on water surface. CoSt films were compressed with a speed of ∼0.58 Å2 /molecule/min. A modified version of the inverse Langmuir–Schaefer method of horizontal deposition was employed to transfer the films [6,14,26]. Hydrophilic silicon substrate was kept horizontally in a homemade L-shaped Teflon substrate holder, which was attached to the clip of the trough dipper and immersed into the water. CoSt molecules were then spread on the water surface from the solution (0.6 mg/mL) with the same amount as was spread at the time of all isotherm measurements. Depositions were done at 3 mN/m, 18 mN/m, 21 mN/m, 25 mN/m, and 32 mN/m surface pressures (indicated by arrows in Fig. 2) at room temperature with the substrate starting from below the films. Depositions were also done at 40 mN/m and 64 mN/m surface pressures (indicated by arrows in Fig. 2) at room temperature when the pH of the subphase water was ∼6.8, nearly at the same pH where stearic acid molecules were spread on water subphase containing the cobalt ions in the subphase water. Upward speed of the substrate holder was 0.5 mm/min for all transfers. 2.3. AFM and X-ray reflectivity measurement Surface topography of the CoSt films was studied through an AFM (Auto probe CP, Park Scientific) in contact mode using a silicon nitride cantilever (with spring constant 0.05 N/m) and pyramidal

198

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

Fig. 2. Surface pressure ()–specific molecular area (A) isotherms at room temperature (21 ◦ C): (i) stearic acid monolayer in presence of cobalt ions in the subphase water at pH ∼ 6.8 (blue line) (ii) preformed cobalt stearate (CoSt) film on pure water subphase, i.e., at pH ∼ 5.5 (black line) (iii) preformed cobalt stearate (CoSt) film on water subphase at pH ∼ 6.8 (red line). Compression speed is 0.58 Å2 /molecule/min for each isotherm. Arrows indicate the points at which films are deposition by MILS method. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

tip [29]. Scans were performed in constant force mode over several portions of the film for different scan areas from 5 ␮m × 5 ␮m to 30 ␮m × 30 ␮m. To minimize the damage to the organic films a low constant force (∼0.8 nN) was used. Reflectivity studies of the CoSt films were carried out using an X-ray diffractometer (D8 Discover, Bruker AXS) with Cu source (sealed tube) followed by a Göbel mirror to select and enhance the CuK˛ radiation (0 = 1.54 Å). The diffractometer has a 2-circle goniometer [(ω) − 2] with 1/4-circle Eulerian cradle as sample stage. The cradle has two rotational ( and ) and three translational (X, Y and Z) motions. The scattered beam was detected using NaI scintillation (point) detector. Measurements were done for  = 0◦ ,  = 0◦ and varying  and 2 in steps of milli-degrees. Instrumental resolution in the out-of-plane direction was 0.0014 Å−1 . The scattering plane is perpendicular to the sample face. Data were taken in the specular condition, i.e., incident angle is equal to the exit angle and both were in the scattering plane. Under specular condition the momentum transfer vector q = kf − ki (ki(f) = incident (scattered) wave vector) has only one non-vanishing component qz normal to the surface given by qz = (4/) sin , where  is the angle the incident X-ray beam makes with the surface [30]. 3. Results and discussion 3.1. Isotherm studies In Fig. 2, –A isotherms are shown for three different conditions. Fig. 2(i) is for the –A isotherm of stearic acid monolayer in presence of cobalt ions in the subphase water at pH ∼ 6.8. In this condition, cobalt stearate is forming at the air–water interface. At c ,  and A values are 55 mN/m and 20 Å2 respectively. Surface pressure after c slowly increases with decreasing A. –A isotherms obtained from the films formed by the preformed CoSt molecules on pure water (pH ∼ 5.5) and on water subphase at pH ∼ 6.8 are shown in Fig. 2(ii) and (iii) respectively. On pure water, CoSt film collapses at 26 mN/m and after forming a shallow peak surrounding that collapse point, isotherm shows nearly a plateau region up to a certain area per molecule and then the pressure again increases after further barrier compression. For this compression speed (0.58 Å2 /molecule/min), monolayer pressure starts

to rise from the zero value when A becomes ≈15 Å2 and c occurs at A ≈ 12 Å2 (for 0.22 Å2 /molecule/min compression speed corresponding A values are ≈15.7 Å2 and ≈13 Å2 respectively). For the same barrier compression speed (0.58 Å2 /molecule/min) and subphase pH, at which cobalt stearate was formed at the air–water interface, preformed CoSt film on water surface shows intermediate behavior. At this condition c occurs at higher surface pressure, i.e., at 60 mN/m like the –A isotherm of the cobalt stearate monolayer formed at the air–water interface [6,14]. But the difference is that the pressure starts to rise at A ≈8.5 Å2 and collapses at A ≈ 6.5 Å2 , i.e., the curve has shifted to the lower A value and after c surface pressure slightly decreases although the plateau exists within a very narrow region. From the values of the area per molecule at zero surface pressure it is clear that preformed CoSt molecules are not in the asymmetric molecular configuration like AML (shown in Fig. 1a) because area per molecule for this two-tailed amphiphilic molecule will be ∼40 Å2 as the single hydrocarbon tail area is ∼20 Å2 [31]. Molecules are also not in the SML configuration because a hydrophobic tail cannot stay on hydrophilic water surface spontaneously and if it happens the area per molecule will be ∼20 Å2 for the initial rising of the surface pressure. It thus implies that the bimolecular layer or multilayer structure (shown in Fig. 1b and c) has formed from very low surface pressure. Bimolecular layer and multilayer structures have been observed after the ‘constant area’ and ‘constant pressure’ collapse of the fatty acid salt Langmuir monolayers [14]. Preformed three-tailed fatty acid salt molecules [27] also form bimolecular layer structure from very low surface pressure. An interesting point is that on pure water preformed CoSt film collapses at a value of A where bimolecular layer can form because then three molecules will be in the two tail area, i.e., area per molecule will be ∼13.4 Å2 . On the other hand, when subphase pH is ∼6.8, preformed CoSt film collapses at a value of A where trimolecular layer can form because in that case five molecules will be in two tail area, i.e., area per molecule will be ∼8 Å2 . From the steep rise of the surface pressure before c it is clear that monolayer to bimolecular/trimolecular layer transformation is not taking place during this period. However, it can happen that the pressure change due to monolayer to multilayer transition in the pre-collapse region of the isotherm is so weak that it could not be detected by the Wilhelmy balance. Thus, from the –A isotherm studies, molecular configuration on water surface can be speculated in two different ways. One possibility is that the CoSt molecules are forming a nearly compact bimolecular layer, i.e., AML + SML (as shown in Fig. 1b) on pure water surface and a nearly compact trimolecular layer, i.e., AML + 2SML on water surface at pH ∼ 6.8. Another possibility is that for the same value of A, molecules may form AML + SMLs structures (as shown in Fig. 1c) where SMLs are multilayers on top of the monolayer (AML). However, there is a possibility of the formation of 3D aggregates by CoSt molecules. To obtain the actual configuration of the molecules further out-of-plane structural studies are necessary. For obtaining the morphological and out-of-plane structural information, CoSt films were deposited on hydrophilic silicon substrates by a modified version of the inverse Langmuir–Schaefer (MILS) method of horizontal deposition at different positions in the –A isotherms indicated by the arrows in Fig. 2. 3.2. AFM studies AFM images depicting the surface topography of all the preformed cobalt stearate (CoSt) films deposited from pure water subphase are shown in Fig. 3 (a–e) with the corresponding line profiles in the lower inset. Mainly the height information and the morphological changes have been shown for all the deposited films. Fig. 3a is for the film deposited at 3 mN/m where smooth film is obtained. Line profile gives the maximum height of ∼1 nm. Fig. 3b

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

199

Fig. 3. AFM images of the preformed CoSt films of scan area 10 ␮m × 10 ␮m deposited at (a) 3 mN/m (b) 18 mN/m (c) 21 mN/m (d) 25 mN/m and (e) 32 mN/m surface pressure from pure water subphase. Insets show typical line profiles. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

is for the film deposited at 18 mN/m, i.e., before the collapse point (c ). Mainly scattered patches of the thicker heights are observed over the relatively flat surface. Maximum height obtained from the line profile is of ∼27 nm. Films deposited at 21 mN/m and 25 mN/m, i.e., after c are shown in Fig. 3c and d respectively. From images, it is clear that nearly the same morphology, i.e., large thicker patches over the flat surface exists. Line profiles show that the film heights are of ∼32 nm and ∼37 nm respectively. The film deposited at 32 mN/m is shown in Fig. 3e, where a major morphological change occurs. A granular morphology exists over the whole film area and the film has maximum height of ∼65 nm. 3.3. X-ray reflectivity studies All X-ray reflectivity data obtained from the films deposited from pure water (pH ∼ 5.5) and from water subphase at pH ∼ 6.8 are shown in Figs. 4a, 5a and 6a respectively. Bragg peaks indicated by

arrows imply the formation of periodic molecular structures in the out-of-plane direction. Tilted and untilted molecular packing are indicated by the blue and red arrows respectively. All reflectivity profiles are analyzed using the Parratt formalism [32] introducing finite interfacial width [33,34]. For satisfactory fitting of the reflectivity profiles coexistence of monolayer, bimolecular layer and multilayer domains is considered where monolayer domains are made by AML, bimolecular layer domains are made by AML + SML and multilayer domains are made by AML + SMLs. SMLs of two different thicknesses were introduced for the fitting of all Bragg peaks. Less thick SMLs are due to the tilted packing of the CoSt molecules in the out-of-plane direction and thus form tilted multilayer domains. Comparatively thicker SMLs are due to the untilted packing of the CoSt molecules in the out-of-plane direction and thus forming untilted multilayer domains. The total reflectivity from such an inhomogeneous film is assumed to consist of incoherent scattering from the different domains [15,16] and can be

200

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

Fig. 4. (a) Observed (open circles) and calculated (lines) X-ray reflectivity profiles of the four preformed CoSt films deposited at 3 mN/m, 18 mN/m, 21 mN/m and 25 mN/m surface pressures from pure water subphase. Reflectivity profiles and corresponding fits have been shifted vertically for clarity. Arrows indicate the Bragg peaks (blue for tilted molecules and red for untilted molecules). (b) Electron density profiles are extracted from reflectivity data using the model as described in the text: (i) AML (˛1 = 0.918), AML + SML (˛2 = 0.08) and AML + SMLs (˛3 = 0.002, tilted) (ii) AML (˛1 = 0.89), AML + SML (˛2 = 0.1), and AML + SMLs (˛3 = 0.01, tilted) (iii) AML (˛1 = 0.87), AML + SML (˛2 = 0.1) and AML + SMLs (˛3 = 0.03, tilted) and (iv) AML (˛1 = 0.78), AML + SMLs (˛2 = 0.19, tilted) and AML + SMLs (˛3 = 0.03, untilted). Tilted SMLs are shown by the blue lines while untilted SMLs are shown by the red line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

written as R(qz ) =

 i

˛i Ri (qz )

(1)

where ˙ i ˛i = 1 and the weight factor ˛i is for the i-th domain. Reflectivity profiles shown in Fig. 4a are nicely fitted by using Eq. (1) and the calculated reflectivity curves for the best fits are presented by the solid lines. Corresponding electron density profiles (EDPs) are shown in Fig. 4b. Reflectivity profile obtained from the film deposited at 3 mN/m (Fig. 4a-(i)) is fitted by using the coexistence of monolayer (AML), bimolecular layer (AML + SML) and multilayer (AML + 5SMLs) domains where the ˛1 , ˛2 and ˛2 values

are 0.918, 0.08 and 0.002 respectively with the error of 3–5% for each ˛ value. Three EDPs for the three coexisting domains are shown in Fig. 4b-(i). Thus, weight factor of the monolayer domain is very large in comparison with the bimolecular and multilayer domains. The SML layer density in the bimolecular layer is not compact. The addition of multilayer domain of very low coverage helps to fit the very weak peak around ∼0.45 Å−1 . This week peak becomes stronger in the subsequent reflectivity profiles obtained from the other films deposited at higher . The next film deposited at 18 mN/m (before c ) gives nearly the same reflectivity profile as the previous one but shows first, third and fifth order Bragg peaks at ∼0.15, ∼0.47 and ∼0.79 Å−1 respectively. This strongly implies the coexistence of the

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

201

Fig. 5. (a) Observed (open circles) and calculated (lines) X-ray reflectivity profiles of the preformed CoSt film deposited at 32 mN/m surface pressure from pure water subphase. Arrows indicate the Bragg peaks (blue for tilted molecules and red for untilted molecules). (b) Electron density profiles extracted from reflectivity data using the model as described in the text: AML + SMLs (˛1 = 0.6, tilted) and AML + SMLs (˛2 = 0.4, untilted). Tilted SMLs are shown by the blue line and untilted SMLs are shown by the red line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

monolayer, bimolecular layer and multilayer domains and that the SML layer thickness is ∼41.0 Å. The three EDPs for the three coexisting domains are shown in Fig. 4b-(ii) where the ˛1 , ˛2 and ˛3 values are 0.89, 0.10 and 0.01 respectively. Maximum error for each ˛ is of 5–7% and nearly the same error in ˛ has been obtained from the fitting of all the reflectivity profiles. Maximum height obtained from the EDP is AML + 5SMLs which is little less than the maximum height obtained from the AFM line profile. Film deposited after c (at 21 mN/m) shows Bragg peaks at ∼0.15, ∼0.47 and ∼0.79 Å−1 respectively, which indicates the same SML thickness, i.e., ∼41.0 Å. Reflectivity profile is fitted also by the three coexisting domains and the corresponding EDPs are shown in Fig. 4b-(iii) where the ˛1 , ˛2 and ˛3 values are 0.87, 0.10 and 0.03 respectively. Higher value of ˛3 indicates the more transformation from monolayer to multilayer domains. One new but very weak peak has observed at ∼0.408 Å−1 (thickness ∼15.4 Å) that corresponds to the third order Bragg peak of another SML thickness of ∼46.2 Å which is little larger than the previous SML thickness (∼41.0 Å) and indicates the starting of the untilted multilayer domain formation. Here addition of more SMLs is not required but only the higher value of the weight factor (˛3 ) is sufficient for the fitting. Film deposited at 25 mN/m shows peaks at ∼0.153 Å−1 , ∼0.313 Å−1 , ∼0.472 Å−1 , ∼0.632 Å−1 , ∼0.79 Å−1 , ∼0.948 Å−1 and ∼1.1 Å−1 that correspond to the first, second, third, fourth, fifth, sixth and seventh order Bragg peaks of the SML layer thickness of ∼41.0 Å. Peaks observed at ∼0.408 Å−1 and

Fig. 6. (a) Observed (open circles) and calculated (lines) X-ray reflectivity profiles of the preformed CoSt film deposited at 40 and 64 mN/m surface pressures from water subphase at pH ∼ 6.8. Blue arrows indicate the Bragg peaks of tilted molecules. (b) Electron density profiles extracted from reflectivity data using the model as described in the text: (i) AML (˛1 = 0.907), AML + SML (˛2 = 0.09) and AML + SMLs (˛3 = 0.003, tilted) (ii) AML + SML (˛2 = 0.99) and AML + SMLs (˛3 = 0.01, tilted). Tilted SMLs are shown by the blue lines and untilted SML is shown by the red line. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

∼0.681 Å−1 correspond to the third and fifth order Bragg peaks of another SML thickness of 46.2 Å. Reflectivity profile is fitted considering the monolayer and the tilted and untilted multilayer domains. The three EDPs for the three above-mentioned coexisting domains are shown in Fig. 4b-(iv) where the ˛1 , ˛2 and ˛3 values are 0.78, 0.19 and 0.03 respectively. EDPs give eight tilted and five untilted SMLs on AML.

202

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

Reflectivity profile obtained from the film deposited at 32 mN/m is shown in Fig. 5a. The profile is not fitted using Eq. (1) as reflectivity has decreased rapidly. This may be due to the loss of repeat distances and due to the enhancement of interfacial roughness. At this stage, the coverages of the tilted and untilted domains become comparable and as a result, different head–head distances are possible in the out-of-plane direction that reduces the reflectivity drastically. Four weak peaks at ∼0.135 Å−1 , ∼0.155 Å−1 , ∼0.408 Å−1 and ∼0.472 Å−1 have obtained which correspond to the first and third order Bragg peaks of the two different SML layer thicknesses of ∼41 Å and ∼46.5 Å respectively. Reflectivity ratio for the two first order Bragg peaks are nearly the same and for this reason using the close values of ˛1 (tilted domains) and ˛2 (untilted domains) reflectivity profile has been generated. The generated curve, which is not following the experimental curve, is shown by the solid line in Fig. 5a and the corresponding EDPs are shown in Fig. 5b where the values of ˛1 and ˛2 are 0.6 and 0.4 respectively. Actually, the calculated profile is very far from the experimental reflectivity profile for all combinations of ˛1 and ˛2 values. Reflectivity profiles shown in Fig. 6a are for the films deposited from water subphase at pH ∼ 6.8. Reflectivity profiles are nicely fitted using Eq. (1) and the calculated reflectivity curves for the best fits are presented by the solid lines. EDPs are shown in Fig. 6b. Here a peak is observed at ∼0.47 Å−1 which is very weak but becomes relatively stronger after c . This peak corresponds to the third order Bragg peak of the tilted SML of thickness ∼41.0 Å. Reflectivity profile for the film deposited at 40 mN/m, i.e., before collapse point is fitted by the three coexisting domains of monolayer (AML), bimolecular layer (AML + SML) and tilted multilayer (AML + SMLs). EDPs are shown in Fig. 6b-(i) where the ˛1 , ˛2 and ˛3 values are 0.907, 0.09 and 0.003 respectively. The film deposited at 64 mN/m, i.e., after collapse is fitted by using bimolecular (AML + SML) and tilted multilayer (AML + SMLs) domains. A small peak at ∼0.79 Å−1 has also been observed which corresponds to the fifth order Bragg peak of tilted SML thickness. EDPs of the two coexisting domains are shown in Fig. 6b-(ii) where the ˛1 and ˛2 values are 0.99 and 0.01 respectively. After collapse, the density of the SML layer becomes totally compact in the bimolecular layer. This compact SML layer has formed by the untilted CoSt molecules.

3.4. Coexisting domains Out-of-plane structural and in-plane morphological evolution of the preformed CoSt film with barrier compression are obtained from the –A isotherms, AFM topography and X-ray reflectivity profiles. Weight factors ˛i give the coverages of the coexisting monolayer, bimolecular layer and tilted and untilted multilayer domains of all the deposited films. On pure water, domains of monolayer (AML), bimolecular layer (AML + SML) and multilayer (AML + SMLs) form by the tilted CoSt molecules when  is far before c . Monolayer coverage is very large (91.8%) in comparison with the bimolecular layer and multilayer coverages (8% and 0.2% respectively). Before c , the same monolayer, bimolecular layer and tilted multilayer domains are observed but corresponding coverages are 89%, 10% and 1% respectively. This indicates that the coverages of the bimolecular layer and multilayer domains have increased. After c , the monolayer, bimolecular layer and tilted multilayer domains get coverages of 87%, 10% and 3% respectively. The tilted SML thickness is ∼41 Å. Film deposited after plateau at 25 mN/m has three types of domains with 78% (monolayer domain), 19% (tilted multilayered domain) and 3% (untilted multilayer domains) coverages respectively. The untilted SML layer thickness is ∼46 Å. From the two SML thicknesses, one can calculate the tilt angle of the molecules in the tilted domain, which is ∼27◦ . Film deposited at 32 mN/m, i.e., far after plateau, shows only very weak Bragg peaks in the Xray reflectivity profile due to the presence of tilted and untilted SMLs. It implies that not only due to their comparable coverages but possibly also due to the formation of other types of tilted and untilted domains the film as a whole contains different head–head or tail–tail separations that introduce a huge interfacial roughness and the reflectivity reduces drastically. Growth of the domains with barrier compression is shown schematically in Fig. 7a. At very low surface pressure, monolayer (AML) coverage is maximum and it coexists with the less covered bimolecular layer (AML + SML) and tilted multilayer (AML + SMLs) domains as shown in Fig. 7a-(i). With film compression the tilted domain coverage increases and after c , in addition with the tilted domain, untilted domain also starts to grow but tilted domain coverage is more than the untilted domain coverage which is shown in Fig. 7a-(ii). Near after the plateau region, both the tilted and untilted domains coexist but still the

Fig. 7. Schematic representation of the layered structures (not in proper ratio) formed by the preformed CoSt film (a) on pure water: (i) before collapse, (ii) after collapse, (iii) far after collapse and (b) on water at subphase pH ∼ 6.8: (i) before collapse and (ii) after collapse.

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

203

tilted domain has more coverage. Far after the plateau region, both domains get comparable coverage as shown in Fig. 7a-(iii) with the possibility of other tilted and untilted domain formation (not shown) that losses the periodicity in the out-of-plane direction and reduces the reflectivity. Preformed CoSt film deposited from the water subphase at pH ∼ 6.8 also contains monolayer (AML), bimolecular layer (AML + SML) and tilted multilayer (AML + SMLs) domains before c . Corresponding domain coverages are 90.7%, 9.0% and 0.3% respectively. After collapse, only bimolecular layer (AML + SML) and tilted multilayer (AML + SMLs) domains coexist and their coverages are 99% and 1% respectively. The greater reduction in A in the –A isotherm at this subphase pH (∼6.8) is probably due to the formation of more multilayer domains. The growth of the domains, at this subphase pH, with barrier compression has been shown schematically in Fig. 7b. Before collapse, monolayer of AML coexists with the bimolecular layer (AML + SML) and tilted multilayer (AML + SMLs) domains, which is shown in Fig. 7b-(i). After collapse, monolayer to compact bimolecular layer (AML + SML) structure forms together with the tilted domains of AML + SMLs as shown in Fig. 7b-(ii). EDPs show that the preformed CoSt molecules are not sequentially transforming from a monolayer (AML) to a bimolecular layer (AML + SML) and then to a trimolecular layer (AML + 2SML) on pure water surface with barrier compression. Molecules are also not going into the water subphase with more and more barrier compression. Molecules prefer to form multilayered (AML + SMLs) structure of different heights. However, on water surface at pH ∼ 6.8, with barrier compression, molecules prefer to go from monolayer (AML) to a multilayer (AML + SMLs) structure forming a compact bimolecular layer (AML + SML). Structures of the films before and after collapse thus strongly depend on the subphase pH. All these structures are shown schematically in Fig. 7, not considering the exact coverages of the individual domains. 3.5. Proposed reasons It is clear from the structural analysis that the structure of the film formed by the preformed CoSt molecules on water surface is not the same as the structure of the cobalt stearate monolayer formed at the air–water interface. Presence of different cobalt–headgroup interactions inside the cobalt stearate molecules forming in different ways affects the structure formation. For preformed CoSt molecules, bonding between the Co2+ ions and the carboxylate (COO− ) headgroups become fixed before their spreading on the water surface as the reaction has already taken place in the bulk. These Co-bearing headgroups behave like dipoles and these dipoles most likely interact with the dipoles of the water molecules (shown by a cartoon in Fig. 8a) and with the dipoles of the other CoSt molecules (not shown) after their spreading on the water surface and the film forms. On the other hand, when reaction occurs at the air–water interface, carboxylate (COO− ) headgroups first take a preferred configuration after the spreading of the stearic acid molecules on the water surface and then Co2+ ions attach with the negatively charged carboxylate ions (COO− ) as the headgroups dissociate at pH ∼ 6.8 of the subphase water. In this case, the interaction between the negatively charged carboxylate (COO− ) ions with the positively charged Co2+ ions is most likely electrostatic (shown by a cartoon in Fig. 8b). As the electrostatic interaction is relatively stronger than the dipole–dipole interaction [35], cobalt stearate molecules formed at the air–water interface prefer to stay on the water surface and move slowly on the top of the compact monolayer (AML) with the barrier compression. As a result, mainly a bimolecular layer (AML + SML) forms after c . As dipole–dipole interaction is relatively weak, preformed CoSt molecules are not so stable on the pure water surface and with barrier compression move easily on top of the CoSt monolayer (AML) forming both the bimolecular layer

Fig. 8. Cartoons show the different interactions at the air–water surface. (a) dipole–dipole interaction between the preformed CoSt headgroups and water molecules. (b) Electrostatic interaction between the carboxylate (COO− ) headgroups of stearic acid molecules and Co2+ ions dissolved in the water subphase. (c) Dipole–dipole and dipole–charge interactions between the preformed CoSt headgroups and water molecules and preformed CoSt headgroups and OH− ions at pH ∼ 6.8 of the subphase water.

(AML + SML) and multilayer (AML + SMLs) before and after c . But at higher subphase pH (∼6.8), due to the presence of OH− ions in the subphase water, there probably exist both dipole-dipole and dipolecharge interactions (shown by a cartoon in Fig. 8c). As the strength of the dipole-charge interaction is intermediate between the electrostatic and the dipole–dipole interaction [35], the film formed by

204

S. Kundu / Colloids and Surfaces A: Physicochem. Eng. Aspects 348 (2009) 196–204

the preformed CoSt molecules shows intermediate behavior at this higher subphase pH. Through dipole-dipole interaction, bimolecular layer (AML + SML) and multilayer (AML + SMLs) structures form with barrier compression. However, possibly due to the presence of the dipole-charge interaction, others CoSt molecules prefer to stay on the water surface and move slowly on the top of the monolayer (AML) with compression, and form a compact bimolecular layer (AML + SML) after c . As a result, compact bimolecular layer and relatively less compact multilayer domains form. Why at this high subphase pH (∼6.8), relatively large number of CoSt molecules form bimolecular layer (AML + SML) and multilayer (AML + SMLs) structures at very low surface pressure is still not clear. Probably the subphase pH changes the collapse nature of the film, as in the Langmuir monolayers of the fatty acid salts it has been observed that the nature of the monolayer collapse depends upon the subphase pH variation [36]. In situ study at the air–water interface is necessary for getting the information about the in-plane packing of these preformed CoSt molecules before and after the collapse of the film and also for getting the information about the configurations of the Co-bearing headgroups. 4. Conclusions Structures of the collapsed film formed by the cobalt stearate molecules strongly depend up on the way of the fatty acid salt formation. When the reaction occurs at the air–water interface, cobalt stearate monolayer collapses mainly from monolayer to bimolecular layer with the formation of parallel ridge like morphology. On the other hand, when the fatty acid salt formation takes place through the stepwise chemical reactions in bulk, the film of the preformed cobalt stearate (CoSt) molecules on the water surface collapses from monolayer to multilayer structures of different heights and with barrier compression the scattered multilayer domains morphology changes to the granular morphology. Possibly, the presence of electrostatic interaction between the Co2+ ions and the carboxylate (COO− ) headgroups of the strearic acid molecules at the air–water interface and dipole–dipole interaction between the Cobearing headgroups of the preformed CoSt molecules and the water molecules on the water surface are responsible for the two different types of collapse. X-ray and optical scattering and absorption studies of these films on the water surface are essentially the further studies needed to obtain a microscopic understanding of the interfacial and bulk chemistry and the supra molecular structures. Acknowledgements The author would like to acknowledge Prof. A. Datta and Prof. S. Hazra of surface physics division, Saha Institute of Nuclear Physics, Kolkata for their academic and experimental support. References [1] G.L. Gaines, Insoluble Monolayers at Liquid-Gas Interfaces, Interscience, New York, 1966. [2] M.C. Petty, Langmuir–Blodgett Films: An Introduction, Cambridge University Press, Cambridge, 1996. [3] M.K. Sanyal, M.K. Mukhopadhyay, M. Mukherjee, A. Datta, J.K. Basu, J. Penfold, Role of molecular self-assembling in Langmuir-Blodgett film growth, Phys. Rev. B 65 (2002) (033409-1-033409-4).

[4] E. Hatta, T. Maekawa, K. Mukasa, Y. Shimoyama, Spin-glass behavior in CoSt2 Langmuir-Blodgett multilayer films, Phys. Rev. B 60 (1999) 14561–14564. [5] X. Luo, Z. Zhang, Y. Liang, Structure of cobalt stearate and cobalt sulfide-stearic acid Langmuir-Blodgett films, Langmuir 10 (1994) 3213–3216. [6] S. Kundu, A. Datta, S. Hazra, Growth of a collapsing Langmuir monolayer, Phys. Rev. E 73 (2006) (51608-1-051608-7). [7] H.E. Ries Jr., Stable ridges in a collapsing monolayer, Nature 281 (1979) 287–289. [8] E. Hatta, J. Nagao, Topological manifestations of surface-roughening collapse in Langmuir monolayers, Phys. Rev. E 67 (2003), 041604-1-041604-5. [9] E. Hatta, Th.M. Fischer, Modulation crack growth and crack coalescence upon Langmuir monolayer collapse, J. Phys. Chem. B 106 (2002) 589–592. [10] C. Gourier, C.M. Knobler, J. Daillant, D. Chatenay, Collapse of monolayers of 10,12-pentacosadiyonic acid: kinetics and structure, Langmuir 18 (2002) 9434–9440. [11] A. Angelova, D. Vollhardt, R. Ionov, 2D–3D transformations of amphiphilic monolayers influenced by intermolecular interactions: a Brewster angle microscopy study, J. Phys. Chem. 100 (1996) 10710–10720. [12] C.D. Lorentz, A. Travesset, Atomistic simulations of Langmuir monolayer collapse, Langmuir 22 (2006) 10016–10024. [13] S. Baoukina, L. Monticelli, H.J. Risselada, S.J. Marrink, D.P. Tieleman, The molecular mechanism of lipid monolayer collapse, Proc. Natl. Acad. Sci. U.S.A. 105 (2008) 10803–10808. [14] S. Kundu, A. Datta, S. Hazra, Effect of metal ions on monolayer collapses, Langmuir 21 (2005) 5894–5900. [15] D. Vaknin, W. Bu, S.K. Satija, A. Travesset, Ordering by collapse: formation of bilayer and trilayer crystals by folding Langmuir monolayers, Langmuir 23 (2007) 1888–1897. [16] W. Bu, D. Vaknin, Bilayer and trilayer crystalline formation by collapsing behenic acid monolayers at gas/aqueous interfaces, Langmuir 24 (2008) 441–447. [17] V.M. Kaganer, H. Möhwald, P. Dutta, Structure and phase transitions in Langmuir monolayers, Rev. Mod. Phys. 71 (1999) 779–819. [18] Z.D. Wang, S.B. Hall, R.H. Notter, Dynamic surface activity of films of lung surfactant phospholipids, hydrophobic proteins, and neutral lipids, J. Lipid Res. 36 (1995) 1283–1293. [19] B. Robertson, H.L. Halliday, Principles of surfactant replacement, Biochim. Biophys. Acta 1408 (1998) 346–361. [20] M.M. Lipp, K.Y.C. Lee, D.Y. Takamoto, J.A. Zasadzinski, A.J. Waring, Coexistence of buckled and flat monolayers, Phys. Rev. Lett. 81 (1998) 1650–1653. [21] R.M. Epand, H.J. Vogel, Diversity of antimicrobial peptides and their mechanisms of action, Biochim. Biophys. Acta 1462 (1999) 11–28. [22] L.V. Chernomordik, M.M. Kozlov, Protein-lipid interplay in fusion and fission of biological membranes, Annu. Rev. Biochem. 72 (2003) 175–207. [23] S.S. Vogel, J. Zimmerberg, Proteins on exocytic vesicles mediate calciumtriggered fusion, Proc. Natl. Acad. Sci. U.S.A. 89 (1992) 4749–4753. [24] A. Datta, M.K. Sanyal, A. Dhanabalan, S.S. Major, Formation of highly condensed ferric stearate monolayers at the air–water interface, J. Phys. Chem. B 101 (1997) 9280–9286. [25] S. Mukherjee, A. Datta, A. Giglia, N. Mahne, S. Nannarone, Chemistry at air/water interface versus reaction in a flask: tuning molecular conformation in thin films, Langmuir 25 (2009) 3519–3528. [26] A. Datta, S. Kundu, M.K. Sanyal, J. Daillant, D. Luzet, C. Blot, B. Struth, Dramatic enhancement of capillary wave fluctuations of a decorated water surface, Phys. Rev. E 71 (2005), 041604-1-041604-7. [27] S. Kundu, A. Datta, M.K. Sanyal, J. Daillant, D. Luzet, C. Blot, B. Struth, Growth of bimolecular films of three-tailed amphiphiles, Phys. Rev. E 73 (2006), 0616021-061602-6. [28] S. Kundu, Langmuir–Blodgett film from a bi-molecular layer at air–water interface, Colloids Surf. A 317 (2008) 618–624. [29] J.K. Basu, S. Hazra, M.K. Sanyal, Growth mechanism of Langmuir-Blodgett films, Phys. Rev. Lett. 82 (1999) 4675–4678. [30] J.K. Basu, M.K. Sanyal, Ordering and growth of Langmuir–Blodgett films: X-ray scattering studies, Phys. Rep. 363 (2002) 1–84. [31] D.K. Schwartz, Langmuir-Blodgett film structure, Surf. Sci. Rep. 27 (1997) 245–334. [32] L.G. Parratt, Surface studies of solids by total reflection of X-Rays, Phys. Rev. 95 (1954) 359–369. [33] J. Daillant, A. Gibaud, X-Ray and neutron reflectivity: principles and applications, Springer, Berlin, 1999. [34] M. Tolan, X-Ray Scattering from Soft Matter Thin Films, Springer, Berlin, 1999. [35] J. Israelachvili, Intermolecular & Surface Forces, Academic Press, New York, 1985. [36] S. Kundu, D. Langevin, Fatty acid monolayer dissociation and collapse: Effect of pH and cations, Colloids Surf. A 325 (2008) 81–85.