Lateral diffusion of single polymer molecules at interfaces between water and oil

Lateral diffusion of polymer molecules at the interfaces between immiscible oil and water is investigated at the single molecular level. The interfaces between water and alkanes are chosen as the model systems and polyethylene oxide (PEO) is the model polymer. Fluorescence correlation spectroscopy is used to measure the interfacial diffusion of fluorescence-labeled PEO with its molecular weight ranging over more than an order of magnitude. It is discovered that the interfacial diffusion coefficient scales with the molecular weight by the exponent of −0.5. Detailed analysis shows that the PEO chain takes an ideal two-dimensional random coil conformation at these fluidic interfaces and the bigger contribution from water's hydrodynamic friction is discovered.


The effect of purification of the alkanes to the stability of the interfaces
All n-alkane samples used in the experiments (n-octane, n-dodecane, nhexadecane) with purity of ≥99% were purchased from Sigma-Aldrich. Deionized water with a resistivity of 18.2 MΩ·cm was obtained from a Milli-Q water purification system. Prior to use, alkanes were purified by column chromatography using a basic alumina stationary phase for several times to remove amphiphilic impurities. Purification of alkanes has been shown essential for the construction of the stable interface designed, as tracer amount of impurities can adsorb to the interface and consequently affects interfacial properties.
The interfacial tension of the two phases was measured using Wihelmy plate method (Tensiometer K100, Kruss, Germany). The platinum plate was cleaned with a flame before every measurements. The measurements of the interfacial tension for each alkane-water system was performed for more than 20 minutes. Figure S1 shows the value of interfacial tension of three alkane/water system as * Corresponding author: yangjf@iccas.ac.cn and jzhao@iccas.ac.cn.

Figure S1
The values of interfacial tension as a function of time for three alkane/water systems, before and after purification.
Electronic Supplementary Material (ESI) for RSC Advances. This journal is © The Royal Society of Chemistry 2020 a function of time. It is immediately noticed that after purification, the values of interfacial tension of n-octane and n-hexadecane become considerably higher and the values of three alkanes exhibit a right order. Also, the noticeable decay of interfacial tension with time before purification is largely suppressed by purification. Figure S2 shows the schematic of the FCS setup together with the sample cell for the measurements of interfacial diffusion at liquid-liquid interface.

The auto-correlation function of PEO diffusion at alkane/water interfaces
The typical normalized auto-correlation functions of diffusion of fluorescencelabeled PEO at alkane/water interfaces are provided in Figure S3, in which it is observed that the diffusion of PEO with larger molecular weight diffuses slower than the sample of smaller molecular weight. As an example, the PEO sample with the molecular weight of 5.0 kg mol 1 at interfaces between water and three alkanes are shown, telling that the interfacial diffusion is slower at interfaces formed between water and alkane with bigger carbon number.

Figure S2
The schematic diagram of fluorescence correlation spectroscopy setup for the measurements of lateral diffusion of PEO molecules at the alkane-water interfaces.

Evidence to prove that the adsorption at the interface alkane/water interface is by PEO molecules
Control experiments were conducted to check that the adsorption of PEO chain at alkane/water interface is by PEO molecules to exclude the possibility of adsorption induced by the fluorescent molecule attached to the chain end. The fluorescence intensities at the dodecane/water interface was measured by using  S4 two samples separately: PEO labeled with Alexa 488 fluorophore and the free Alexa 488 molecules along. Alexa-488 was chosen because of its hydrophilicity, which prevent its adsorption to the alkane/water interface. In these experiments, the concentration of free Alexa 488 was twice of that of labeled PEO. The data of the fluorescence intensities at the interface are shown in Figure S4. The data show that the intensity from the labeled PEO sample is ~50 times higher than that of free fluorophores, indicating that the adsorption is by PEO molecule itself. Figure S5a shows the diffusion coefficient ( s D ) of PEO at the interface between water and alkanes plotted as a function of the viscosity of the alkane phase ( oil  ).

Proof of Brownian motion of the PEO at water/alkane interfaces
The data of mean square displacement,  