SILICONES 33
A/ethanol/PEG-12 Dimethicone) have been sufficiently checked to convince that the whole phase diagram is completely isotropic. In the lower gel level an obvious phase boundary is not encountered. The pseudo-tertiary diagram of (gel
B/ethanol/PEG-12 Dimethicone) is more complex due to 10 times more gelling content. The system points with ethanol < 60% are so viscous and cause mixing difficulty. For all practical purpose with WHO’s regulation of ethanol must be >60%, so ethanol levels between 60% to 80% are being focused. Two binary dilutions confirmed practical
formulation space in the diagram as in Figure 2. Although in theory one can go as high as 40% in PEG-12 Dimethicone, a 2- 10% is considered to be a reasonable range to show cost-sensitive benefits. The green box in Figure 2, therefore, is an effective formulation space but not exclusive where clear gels could be obtained. Exploration outside the box definitely has academic merit.
PEG/PPG-18/18 Dimethicone in hand sanitiser gel In comparison to a rather easy case in PEG- 12 Dimethicone work, PPG moiety in the molecule causes the HLB to drop to 8. It is expected to have somewhat narrower clear phase region which turned out to be true. In Figure 3, one can add as high as 82%
ethanol (point #1) to gel B without triggering phase separation. To incorporate PEG/PPG-18/18 Dimethicone, which is more hydrophilic than ethanol, one needs to reduce ethanol and increase water content. It is seen that the clear gel phase boundary moves along the curve from point #1 to point #2 where the composition has (60% ethanol, 12% PEG/PPG-18/18 Dimethicone, and 28% water). All the system points on the left side of the curve are clear gels and the ones on the right likely have multiple phases may experience sudden drop in viscosity with white precipitations (Fig 4). A pseudo-tertiary phase diagram has
two degrees of freedom which means a system point can be converted into (X, Y) coordinates for plotting. Anyone can use the following techniques in an Excel sheet to plot a good phase diagram without need to purchase special tools or spend time hand plotting using following equations (1) to (3):
(1) (2)
X=30.5 Y= 30.5
Y
+ right component% *top component%
In Figure 3, the right component is PEG/PPG-18/18 Dimethicone. The top
November 2020
Table 1: Two dimensional coordinates and composition of system points in the pseudo-tertiary phase diagram of (Gel B/ethanol/PEG/PPG-18/18 Dimethicone).
Phase Point Ethanol X Y
Gel B (10%) Carbomer /TIPA)
10% Carbo /TIPA in Water 0.0000
BRB 526 Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel Clear Gel
1.0000 0 0
Carbomer 940
TIPA Water Ethanol
PEG/PPG 18/18
Dimethicone 0.5000 0.8660254 0.00% 0.00% 0.00% 0.00% 100.00% 0.00% 100.00% 5.63% 4.37% 90.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 100.00%
0.4100 0.7101408 18.00% 1.01% 0.79% 16.20% 82.00% 0.00% 0.4150 0.7014806 18.00% 1.01% 0.79% 16.20% 81.00% 1.00% 0.4200 0.6928203 18.00% 1.01% 0.79% 16.20% 80.00% 2.00% 0.4250 0.6841601 18.00% 1.01% 0.79% 16.20% 79.00% 3.00% 0.4300 0.6754998 18.00% 1.01% 0.79% 16.20% 78.00% 4.00% 0.4340 0.6668396 18.00% 1.02% 0.79% 16.20% 77.00% 4.90% 0.4400 0.6581793 18.10% 1.02% 0.79% 16.09% 76.00% 6.00% 0.4450 0.6495191 18.00% 1.01% 0.79% 16.20% 74.00% 7.00% 0.4490 0.6408588 18.10% 1.02% 0.79% 16.29% 73.00% 7.90% 0.4500 0.6321985 18.50% 1.04% 0.81% 16.65% 72.00% 8.50% 0.4500 0.6235383 19.00% 1.07% 0.83% 17.10% 71.00% 9.00% 0.4500 0.614878 19.50% 1.10% 0.85% 17.55% 70.00% 9.50% 0.4500 0.6062178 20.00% 1.13% 0.87% 18.00% 69.00% 10.00% 0.4470 0.5975575 20.80% 1.17% 0.91% 18.72% 68.00% 10.20% 0.4440 0.5888973 21.60% 1.22% 0.94% 19.44% 67.00% 10.40% 0.4410 0.580237 22.40% 1.26% 0.98% 20.16% 66.00% 10.60% 0.4380 0.5715768 23.20% 1.31% 1.01% 20.88% 65.00% 10.80% 0.4350 0.5629165 24.00% 1.35% 1.05% 21.60% 64.00% 11.00% 0.4320 0.5542563 24.80% 1.40% 1.08% 22.32% 63.00% 11.20% 0.4290 0.545596 25.60% 1.44% 1.12% 23.04% 62.00% 11.40% 0.4260 0.5369358 26.40% 1.49% 1.15% 23.76% 61.00% 11.60% 0.4230 0.5282755 27.20% 1.53% 1.19% 24.48% 60.00% 11.80% 0.4200 0.5196152 28.00% 1.58% 1.22% 25.20% 80.00% 12.00%
Translucent Gel 0.4500 0.6928203 15.00% 0.85% 0.65% 13.50% 60.00% 5.00%
Gel with “Galaxy lump” 0.5000 0.5196152 20.00% 1.13% 0.87% 18.00% 80.00% 20.00%
Predpitation/ Liquid
0.5000 0.6928203 100.00% 0.56% 0.44% 9.00% 80.00% 10.00%
component is ethanol. The left component is not independent due to mass balance equation:
(3)
Right component% +
Left component% +
Top component% = 100%
The coordinates and %composition and
breakdown are summarised in Table 1. Prior to gel B work, system points are
checked for the gel A diagram (gel A/ethanol/PEG/PPG-18/18 Dimethicone). They are thin clear solutions due to low gelling agent load. With this knowledge on gel A and B on PEG/PPG- 18/18 Dimethicone, it is rather easy to freely
adjust the composition to formulate the right ethanol base hand sanitiser gel. The gelling agent levels in Table 1 are in
excess. One can easily reduce them by dilution with 60-80% ethanol solutions without imparting phase transition. Due to the limit of this paper, this content has been omitted. Please also note that phase boundary of a thick gel is hard to study due to insufficient mixing in a small vial. Sometimes it is found that a translucent gel turns clear after several days or the ‘galaxy’ type of flocculation disappeared. For high quality work, larger scale work and a stability study may be needed to confirm. The phase boundary on Figure 3 may move slightly to the right depending on observation length. PEG-10 Dimethicone also has its own characteristic phase diagram in hand
PERSONAL CARE EUROPE
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