The interaction of polycaprolactone (PCL) with droplets of a microemulsion is studied with quasielastic light scattering and small angle X-ray scattering At constant droplet size we vary the PCL concentration and there is clear evidence for an increasing attractive interaction of the droplets from structural investigations with small-angle X-ray scattering (SAXS). The collective diffusion coefficient (
Quasielastic light scattering is a very useful method to characterize the structure of microemulsion, polymers, and nanoparticles in solution [
If the scattering particles of solutions are moving, fluctuations in the scattered intensity with time are directly reflecting the so-called Brownian particle motion of the scattering particles (caused by thermal density fluctuations of the solvent). This is the case because of a change in the interference pattern with changing interparticle position, and correspondingly a change in the detected scattered intensity were measured at a given scattering angle [
The microemulsions are a mixture of water oil and surfactant that they can form in different shapes as water droplets to oil solutions or oil droplets to the water solutions phase. The microemulsions are optically transparent and they are good for optical experiments.
The C12E5 microemulsion consists of nanometer-size oil (decane) nanodroplets in water matrix stabilized by the pentaethylene glycol monododecyl ether (C12E5). The structure and phase behavior of such microemulsions are well investigated. The mixture of microemulsion with polymer is an interesting topic on soft matter physics [
The PCL is a biopolymer that it is water-soluble and the solubility of PCL in water depends on the molecular weight and concentration of PCL [
In this work, we study the effect of concentration and length scale of polycaprolactone (PCL) on droplets of C12E5/water/decane by QELS and small angel X-ray scattering (SAXS).
The pentaethylene glycol monododecyl ether (C12E5) and n-decane were obtained from Sigma-Aldrich. Polycaprolactone (PCL) with molecular weight (M.W. = 5000 and 10000) was optioned from Polysciences company, Eppelheim, Germany. The microemulsion was prepared by weight, in terms of surfactant-oil mass ratio of 1.08 and the mass fraction of droplets (
Small angle X-ray scattering (SAXS) measurements were performed using the pinhole SAXS instrument at Nanolab company (KNL, Iran). A X’Pert Pro MPD small angle X-ray from PANalytical was employed to obtain SAXS patterns. The experiments were done at a fixed wavelength of
QELS measurements were performed using an ALV single-detector version compact goniometer system, from ALV-GmbH, Langen, Germany. The light source is a He-Ne laser, operating at a wavelength of 632 nm, with vertically polarized light. It has been seen that particles in dispersion are in a constant, random Brownian motion and that this causes the intensity of scattered light to fluctuate as a function of time. The correlator used in a QELS instrument will construct the correlation function
Microemulsions were prepared by mixing C12E5 with n-decane and water at a constant droplet mass fraction (0.2) and
First-order field correlation function versus time for a mixture of C12E5 microemulsion with PCL at droplet mass fraction (0.2) and
The collective diffusion of a mixture of C12E5 microemulsion with PCL at constant droplet mass fraction (0.2) and
From the inverse decay times, which vary linearly with the square of the momentum transfer, the collective diffusion coefficient (
The negative slope of collective diffusion coefficient as a function of droplet mass fraction of pure C12E5 microemulsion and polymer microemulsion shows attractive interaction between nanodroplets at microemulsion; see Figure
In Figure
A study with QELS shows that the mixture of C12–polyethylene oxide- (PEO-) C12 with C12E5 microemulsion induces a network between nanodroplets at C12E5 microemulsion that it is found that the increase of C12–PEO–C12 can produce fast and slow motions (collective diffusion coefficient) in the microemulsion [
The depletion effect was observed in the mixture of polyethylene glycol (PEG) with C12E5 microemulsion [
In order to characterize the structure of the pure as well as the PCL containing C12E5 microemulsions we applied small angle X-ray scattering. The scattered intensity as a function of
The SAXS experiment of a mixture of C12E5/water/n-decane PCL at a constant droplet mass fraction (0.2) and
The scattered intensity is recorded with a 2-dimensional detector at a distance of 150 cm from the sample leading to an accessible range of scattering vectors
In order to extract the structural information from the X-ray data, the following model will be employed. For analyzing the SAXS data’s a hard sphere model with an attractive interaction is used. The results of analyzing are presented in Table
The results of analyzing the SAXS experiments with hard sphere depletion model that is explained in the text; PDI is polydispersity of PCL/C12E5 microemulsion.
M.W. |
|
|
Size ratio | Core | Core + shell | PDI |
---|---|---|---|---|---|---|
5000 | 0.2 | 0.008 | 0.43 | 72 | 83 | 0.2 |
0.2 | 0.012 | 0.38 | 72 | 83 | 0.2 | |
0.2 | 0.025 | 0.31 | 72 | 83 | 0.2 | |
0.2 | 0.05 | 0.27 | 72 | 83 | 0.2 | |
0.2 | 0.08 | 0.24 | 72 | 83 | 0.2 | |
|
||||||
10000 | 0.2 | 0.007 | 0.51 | 72 | 83 | 0.2 |
0.2 | 0.01 | 0.48 | 72 | 83 | 0.2 | |
0.2 | 0.02 | 0.46 | 72 | 83 | 0.2 | |
0.2 | 0.051 | 0.44 | 72 | 83 | 0.2 | |
0.2 | 0.08 | 0.41 | 72 | 83 | 0.2 |
The results of analyzing show the droplets size of microemulsion is constant at
The size ratio (size ratio of nanodroplet over polymer) is changing from 0.43 to 0.24 with increase of polymer concentration with low molecular weight (M.W. = 5000) and it changes from 0.51 to 0.41 for high molecular weight of polymer (M.W. = 10000). For both cases with the increase of concentration the length scale is decreasing.
One possibility to obtain information about internal structure of the particles is the indirect Fourier transformation (IFT) technique, resulting in the pair density distribution function, PDDF. This calculation is performed completely model-free and requires only a rough estimate of the maximum dimension of the particle. This method is useful for dilute systems.
The influence of the interaction effects can be computed by simultaneous calculation of the form factor and the structure factor, leading to a PDDF limited to intraparticle contributions. This can be achieved by the generalized indirect Fourier transformation (GIFT) technique [
The GIFT with a model with a depletion potential is used to extract the structure factor (Figure
The structure factor of a mixture of C12E5/water/n-decane PCL at a constant droplet mass fraction (0.2) and
The structure factor of a mixture as a function of polymer mass fraction of C12E5/water/n-decane at
This behavior is similar to the mixture of PEG with C12E5 microemulsion [
SAXS and QELS have been used to study the mixture of different weight lengths and concentrations of PCL mixed with C12E5/water/n-decane microemulsion. The results show a depletion interaction due to the fact that the increase of amount of PCL can decrease the motion of nanodroplets inside microemulsion. The different molecular weights of PCL (5000 and 10000) have similar effect on attractive interaction. The SAXS data’s could describe the depletion model quantitatively. The result of SAXS with data’s shows the size of nanodroplets is constant but the size ratio is decreasing with increase of PCL in the microemulsion that it is due to aggregation of PCL in the solutions. The depletion effect can describe well the behavior of collective diffusion coefficient with increase of PCL concentrations.
The author declares that there is no conflict of interests regarding the publication of this paper.