20 TESTING
0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 -0.10
1.00 100.00 10.000.00 1.000.000.00
n Sample 1 n Sample 2 n Sample 3 n Sample 4 n Sample 5 T (µs)
Figure 2: Correlation Function Overlay at 173° (0.5 µs first delay).
the correlation functions have at least one smooth decay and flatten out. There is evidence of multiple decays for Samples 1-4. To then interpret the size populations
present, the non-negatively constrained least squares (NNLS) algorithm within Particle Solutions was applied. As seen in Figure 3, on an intensity- weighted basis, there are particles in the 10 nm range for all samples and most also have populations exceeding 1 micron. These data suggest the presence of small micelles as well as aggregates or other species. As seen in Figure 4, on a number- weighted basis, every formulation has a dominating 10 nm population. There are also fewer populations in the micron range as compared to the intensity interpretation. This suggests that these formulations have a high percentage of particles in the 10 nm range as compared to those in the micron
Table 1: Zeta potential results. Sample 1 2 3 4 5
120.00 100.00 80.00 60.00 40.00 20.00 0.00
1.00 10.00 100.00 Diameter (nm) n Sample 1 n Sample 2 n Sample 3 n Sample 4 n Sample 5 Figure 4: NNLS by number overlay. PERSONAL CARE EUROPE 1.000.00 10.000.00
Zeta Potential (mV) -14.12 ± 0.53 -8.18 ± 0.90 -0.49 ± 0.36 -0.09 ± 0.15 -0.25 ± 0.38
120.00 100.00 80.00 60.00 40.00 20.00 0.00
1.00 10.00 100.00 Diameter (nm) n Sample 1 n Sample 2 n Sample 3 n Sample 4 n Sample 5 Figure 3: NNLS by Intensity Overlay.
range. Sample 3 only shows evidence of particles near 10 nm. These data suggest the ability of dynamic light scattering to quantify size differences among micellar water formulations. This insight may be useful in the formulation process.
PALS
By applying a voltage to a suspension of charged particles, the mobility of the particles can be measured. This mobility value is then used to calculate zeta potential. Zeta potential is determined by the charge near the surface of the micelle. Table 1 summarises the zeta potential, mobility, and conductance results. Samples 1 & 2 are negatively-charged. Zeta potential magnitude can be used to characterise suspension stability. The larger the magnitude of the zeta potential, the more stable the suspension due to the
Mobility [(µ/s) (V/cm)] -1.10 ± 0.04 -0.64 ± 0.07 -0.04 ± 0.03 -0.01 ± 0.01 -0.02 ± 0.03
Conductance (µS) 6095 6888 5851 1981 194
electrostatic repulsion between particles. The sign of the charge can also describe the behaviour of the suspension and the particles. Dirt is typically positively charged; applying negatively charged micelles may improve the efficacy of the formula. Samples 3-5 are all neutrally charged. Since Sample 5 was the control that did not contain any stabilisers or additives, this suggests these additives would be useful in modifying the charge of the micelles. Either Samples 3 & 4 may not contain additives that affect the charge of the micelles, or the formulators are intentionally creating a neutrally charged suspension. In general, neutrally charged suspensions are prone to agglomeration of particles which can cause these larger particles to settle out of solution and no longer yield effective formulas. The conductance value is directly
related to the ionic strength of a solution. Samples 1-3 contain much more salt than Sample 4. Sample 5 was the control and did not have any salt added and that is reflected in the measured conductance value. Samples 1 & 2 had the highest ionic strength and the largest charge magnitude. These observations are consistent with the fact that salt concentrations affect both size and charge, as manifested by zeta potential, of micelles.
These data suggest the ability of zeta potential measurements to quantify differences across formulations that may be able to characterise stability and efficacy. µRhe Microrheology is used to determine the viscoelastic properties of fluids. Microrheology allows the measurement of these properties at much higher frequencies than a traditional rheometer. This experiment is simple as it uses the same principles of DLS. A probe particle of a known size is chosen and added to the solution of interest so that it dominates the scattering signal. The motion of this probe throughout the solution is tracked and used to calculate rheological properties.
June 2018 1.000.00 10.000.00
Number
C (T)
Intensity
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