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Agilent Probing 3D Structures of Surfactants

1. Conclusion The three dimensional topography structure of organized DTAB sur factants on mica in aqueous solu tion above the cmc has been stud ied using AFM under different operation conditions In contact mode using pre contact electro static interaction the observed height of the DTAB aggregates was 1 8 0 2 A when a force near the middle of the electrostatic repul sive range was used the observed height was 5 6 0 5 A when the force was reduced to the mini mum In MAC Mode more than 2 nm high DTAB aggregates were clearly observed which is about half of the estimated height of the proposed DTAB aggregate cylin ders on mica surface This highly sensitive three dimensional topog raphy imaging capability will undoubtedly help to further resolve and understand adsorption mechanisms of a variety of self assemblies at the solid liquid interface References 1 S Manne J P Cleveland H E Gaub G D Stucky and P K Hansma Langmuir 10 1994 4409 2 T J Senden C J Drummond and P K kicheff Langmuir 10 1994 358 3 Wenhai Han S M Lindsay Tianwei Jing Appl Phys Lett 69 1996 4111 4 S S Berr J Phys Chem 91 1987 4760 5 H N Patrick G G Warr S Manne and I A Aksay Langmuir 15 1999 1685 6 R M Pashley and J N Israelachvili Colloids Surf 2 1981 169 www agilent com AFM Instrumentation from Agilent Technologies Agilent Technologies offers high pre cision modul
2. PO VUVVVUVVUVVVVYV 10 20 30 40 50 10 20 30 40 50 Position nm Position nm Caption Figure 1 a Typical force versus distance curve of a silicon nitride cantilever on mica in 40 mM DTAB aqueous solution During approach the tip experience double layer electrostatic repulsive force before it jumped onto the surface b Typical topography image of DTAB surfactants adsorbed on mica obtained in contact mode when the force was set near the middle of the double layer electrostatic repulsive range c Cross section along the line in b showing corrugation around 1 8 A d Image of DTAB surfactants on mica when the force was decreased to the mini mum right before the tip broke off from the surface e Cross section along the line in d showing corrugation around 5 6 A height above 8 A on the DTAB aggregate cylinders has been observed using any of the contact mode operations Figure 2 shows a couple of topog raphy images of organized DTAB aggregate cylinders on mica obtained in MAC Mode The period was measured to be consistent with that obtained in contact mode A cross section along the line in each image was plotted to display the heights of the cylin ders While each image showed a slightly different height and even the same image had different cor rugations at different locations the overall height of the DTAB cylinders was significantly higher than that obtained in contact mode as shown in Figure 1 Some of the
3. Wenhai Han Agilent Technologies Inc Probing the Three Dimensional Structure of Soft Organized Surfactants at the Solid Liquid Interface via Atomic Force Microscopy Application Note Introduction Atomic force microscopy AFM provides a unique direct visualiza tion tool to study the three dimen sional structure of adsorbed sur factants on solid surfaces In the limited body of reported topogra phy data however the heights of observed surfactant aggregates were significantly lower than expected values The reduced corrugation on an AFM topography image is usually due to the fact that the surface adsorbates are soft compared to the spring constant of the can tilever and the applied force is too high The technique commonly used to image weakly adsorbed surfactant aggregates in contact mode AFM is to maintain a force within the pre contact double layer electrostatic interaction range 1 2 Generally the force is on the order of 10 to 10 pN The difficulty encountered in obtaining uncompressed topography images on adsorbed surfactant aggregates seemed to suggest that such small forces were still too high for the soft surfactants Alternatively MAC Mode from Agilent Technologies enables precision control of the oscillation of a magnetically coated AFM can tilever and makes it possible to image with an average force well below 1 pN This lower force mini mizes the distortion of soft mole cules In our w
4. ar AFM solutions for research industry and education Exceptional worldwide support is provided by experienced application scientists and technical service personnel Agilent s leading edge R amp D laboratories ensure the continued timely introduction and optimization of innovative easy to use AFM technologies www agilent com find afm For more information on Agilent Technologies products applications or services please contact your local Agilent office The complete list is available at www agilent com find contactus Phone or Fax United States tel 800 829 4444 fax 800 829 4433 Canada tel 877 894 4414 fax 800 746 4866 China tel 800 810 0189 fax 800 820 2816 Europe tel 31 20 547 2111 Japan tel 81 426 56 7832 fax 81 426 56 7840 Korea tel 080 769 0800 fax 080 769 0900 Latin America tel 305 269 7500 Taiwan tel 0800 047 86 fax 0800 286 331 Other Asia Pacific Countries tel 65 6375 8100 fax 65 6755 0042 Email tm_ap agilent com Revised 09 14 06 Product specifications and descriptions in this document subject to change without notice Agilent Technologies Inc 2007 Printed in USA September 7 2007 5989 7395EN Our measure is your success rae Agilent Technologies
5. cope with a 10mm multipurpose scanner In contact mode stan dard silicon nitride cantilevers with a nominal spring constant of 0 03 nN nm were used In MAC Mode cantilevers of the same type coated with a magnetic film were used All images presented are topography images acquired at scan rates from 1 5 to 4 lines sec The scanner s z dimension was carefully calibrated using both a 100nm height calibration standard and the height of atomic steps on Au 111 surface Before imaging about 400 ml surfactant solution was applied to a large AFM liquid cell and equilibrated for 20 min utes The large amount of solution allowed stable scanning for many hours without evaporation related problems Results and discussion Figure 1 summarizes the results of DTAB surfactant aggregates adsorbed on mica in contact mode using the double layer electrostatic interaction Figure 1 a is a typical force versus distance curve Both approach red and withdrawal blue have been plotted During approach the tip experienced a double layer electrostatic interaction force before jumped into contact The interaction range was around 0 3 nN Organized DTAB aggregate images were obtained when the force was set inside the electrostatic range before the tip jumped into contact Figure 1 b shows a typical exam ple The force was set near the middle of the range about 200 pN from Figure 1 a The measured period of the parallel DTAB aggre gate
6. cylinders was 4 6 0 2 nm consistent with previous observa tions 4 The height of the cylin ders measured from a number of images acquired under similar conditions was only 1 8 0 2 A as shown in the cross section plot Figure 1 c along the line in 0 6 a 0 4 0 2 8 0 amp 0 2 0 4 0 6 0 10 20 30 40 50 Z position nm Approach Withdrawal Height nm oa L a a Figure 1 b Based on different measurements and calculations 3 4 5 the height of the pro posed DTAB aggregate cylinders on mica was estimated in the range of 4 5 nm A higher height of the organized DTAB aggregate cylinders was observed when the imaging force was decreased to the minimum right before the tip broke off from the surface Figure 1 d was such an example The organized DTAB aggregates at different locations even on a single scan line appeared to have different heights The cross section along a row of high DTAB aggregates as shown in Figure 1 e clearly demonstrated a noticeable height increase com pared to Figure 1 c The average height of those highly corrugated surfactant aggregates measured from a number of similar images was 5 6 0 5 A It is still signifi cantly lower than the estimated height of the DTAB aggregate Imaging with such a low force or even negative attractive force required a more stabilized condi tion than scanning with a normal double layer repulsive force No
7. cylinders were measured clearly above 2 nm as shown in Figure 2 d which is about half of the estimated height of the proposed DTAB aggregate cylin ders on mica surface Figure 2 c was captured 1 hour 43 minutes after Figure 2 a on the same Height nm sample The similar topography feature in both images indicated a stable condition for scanning as well as the structure The full height in the range of 4 5 nm of the adsorbed DTAB aggregate cylinders on mica was not observed One possible reason was that the operation condition used in MAC Mode still caused a certain amount of compression on the soft surfactants scanned Another possibility was that the tip might not have been able to reach the lower half of the DTAB cylinders because the soft flexible cylinders were closely packed In that case the observed three dimensional images of DTAB aggregates would be the actual adsorbed surfactant topography that could be detected by AFM Unlike deflection undisturbed three dimensional surface topogra phy can be used as direct evidence of the adsorption mechanism of the surfactant aggregates at the solid liquid interface Position in Caption Figure 2 Topography images a and c of DTAB surfactants adsorbed on mica in 40 mM aqueous solution obtained in MAC Mode Graphs b and d present cross section plots along the lines in a and c respectively showing corrugation of DTAB aggregates around 2 nm
8. ork topography structures of cationic surfactant aggregates dodecyltrimethylam monium bromide DTAB adsorbed on a negatively charged hydrophilic mica surface above the critical micelle concentration cmc have been studied under different imaging conditions In contact mode using double layer electrostatic interaction the measured heights of the organized aggregate images varied depending on the applied force They were nevertheless all significantly lower than the estimated length of the DTAB molecule In MAC Mode however corrugations near half of the proposed DTAB aggregate cylinders on mica have been obtained This result to the best of our knowledge represents the most realistic three dimensional topography structure directly observed on surfactant aggregates at the solid liquid interface 7 Agilent Technologies Experiment Water was pre filtered with a reverse osmosis system and then passed through a Milli Q system resulting in 18 Mg cm 1 final con ductivity DTAB was purchased from Sigma Aldrich 99 pure and then diluted to 40 mM solution about three times the cme 3 in Milli Q water before use All con tainers and sample contacting AFM parts including the liquid cell and cantilever holder were sonicated in detergent and then sonicated and rinsed with Milli Q water Mica was cleaved immediately before the solution was applied All data were acquired using an Agilent 5500 atomic force micro s

Agilent Probing 3D Structures of Surfactants

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