62nd Annual Report on Research 2017

Introduction

Mechanical stabilization of viscous or rigid surfactant films on oil droplets may contribute to kinetically-limited coalescence, thus slowing the separation of oil from water.  Fundamental understanding and characterization of this mechanical stabilization with respect to the presence of salts (coions and counterions) is incomplete.  With this ACS PRF grant funding, we have focused upon characterizing the rigidifying effect of coions and counterions on films of anionic sodium dodecyl sulfate (SDS) and cationic dodecyl amine hydrochloride (DAH) by measuring forces using atomic force spectroscopy and by micropipette aspiration of oil droplets.  We have found that these films behave similarly to their thicker lipid bilayer cousins, and that AFM is a useful tool for evaluating more film properties than strictly morphology of surfactant films.  We have published one research article (Langmuir, 2017) on this work and the second research article is in revision. 

Development of Protocols for Atomic Force Spectroscopy

The films were formed by simply preparing a mixture of the desired concentration of surfactant (5-15 mM for SDS or DAH) and allowing it to equilibrate with a freshly cleaved graphite surface for at least 30 minutes. Imaging was done with a Bruker Dimension 3100 using MSCT silicon nitride cantilevers of spring constant 0.05 N/m. The same tip was used throughout the run. In order to meet this condition, we changed the concentrations of surfactant and added ions while the tip was still in solution (but away from the surface). The tip remained over the same patch of surface throughout the experiment, as surface conditions can cause widespread variations in breakthrough properties even in the absence of added counterions. Figure 1 shows an example of a force curve from this work and how each quantity was taken from a curve.

Figure 1. Shown is a diagram displaying how each quantity measured was taken from a force curve. This curve is from the 10 mM SDS dataset in the absence of added salts.

Development of Micropipette Aspiration to Measure Mechanical Properties of Oil Droplets

We are currently using micropipette aspiration to measure the length deformation under pressure and surface tension of octane and dodecane droplets in the presence of added surfactant. Early results have revealed the presence of a second layer around droplets at high surfactant concentrations, whose mechanical properties are similar to a biological membrane (see Figure 2).

Figure 2.  Micropipette aspiration of hexane droplet with 20 mM of added SDS.

Results

The effect of adding various ions at mM quantities on the breakthrough forces for each SDS or DAH surfactant film was clearly discernable by comparing the breakthrough force distributions.  By analytical analysis of measured critical micelle concentration (CMC) values, we show that the cause of this strengthening was the change in free energy of formation of these micellar structures. As an example, addition of NaCl to DAH caused a distinct shift of the breakthrough force distribution to higher values (Figure 3A) representing a monotonic increase in film strength measured at the mean of each distribution (Figure 3B).  The divalent MgCl₂ first decreased film strength before causing a significant increase at higher concentrations. In cases where strengthening occurs, the strength of the film approached an upper limit roughly 60-70% greater than that of the initial 10 mM film.  We also performed similar measurements by adding H₃O⁺ and OH^- counterions to solution and by using mixtures of SDS and DAH.  It is important to note that the concentration of these ions corresponds to the bulk value and that concentrations are higher near the film due to the presence of a diffuse double layer.  

Figure 3.  (A) Breakthrough event distributions for 10 mM cationic DAH films with added NaCl (Cl- is the counterion), (B) Normalized breakthrough forces for 10 mM DAH and added NaCl. Error bars shown represent the width of each sample standard deviation.

We also examined how the force on the AFM tip changes as it approaches the SDS and DAH hemi-micellar films adsorbed to graphite, and analyze what the cause of longer range forces may be.    Our work demonstrates that long range forces on the AFM tip (as seen in Figure 4) are the result of some previously unreported surface structures, and that these structures can show steric forces similar to charged or neutral polymer brushes depending on the electrolyte environment.  In addition, we used previous theory developed for AFM to measure the Young’s modulus of these films and how it changes with added salt concentrations.  We used an estimate to show that the film has a Young’s modulus similar to those reported for lipid bilayers (~100 MPa).

Figure 4. Shown are representative force vs separation curves taken for 10 mM SDS. To make the differences more clear between graphs, each increase in concentration multiplies the true force by a factor of 5. The red (λ₁ ) and green (λ₂) lines represent the regions over which each decay length is dominant.

Impact

Our work with DAH and SDS surfactant films demonstrate how even small amounts of ions can have dramatic effects upon the mechanical properties and interfacial forces of interfacial surfactant films which may contribute to kinetically-limited separation of oil from water.

Educational Impact - Mentoring and Career Development

Training for both graduate and undergraduate students was provided.  Students were trained with state-of-the-art atomic force microscopy and micropipette aspiration equipment.  Three engineering graduate students (one URM)  and one female undergraduate student were supported financially on this grant while they performed this work.   A PhD student presented this work at the American Chemical Society Annual Spring Meetings of 2016 and 2017.  He is first author on one research article (Langmuir, 2017) on this work and the second research article is in revision.   In addition, four undergraduate students benefitted from supplies purchased with this grant used for their contributions to this project.  Funds from this grant have been crucial for developing a new line of atomic force microscopy investigation for the PI’s laboratory and in restoring the capacity to perform micropipette aspiration measurements.