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Do clays absorb preservatives, rendering the preservatives unavailable in a cosmetic product.
Hello everyone. I recently had a customer tell me that she was taught in a formulating class, that clays will absorb preservatives in a skin care product … being taught that this makes the preservative unavailable to the rest of the formula.
In over 20 years, I’ve never heard of this. Considering the millions of products on the market that contain many types of clays, I am very sceptical. Am I missing something?
I would really appreciate any input on this issue. Thanks in advance.Sandra
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12 Replies
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I don’t know about literal absorption, but some preservatives are ‘used up’ by dealing with inherent burden of the microbes commonly found in something like clay
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Benzoic and maybe other organic acids reportedly absorbed by clays and may have issues with EDTA as preservative adjunct.
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It depends on the type of clay. Several 2:1 clay minerals of the phyllosilicate group like montmorillonite and vermiculite have a high CEC and hence a high affinity for cationic ions, they act like cation exchange resins. Highly porous silicates (e.g. diatomaceous earth and zeolites) tend to adsorb (not absorb, that would be a misnomer) everything like molecular sponges and may, depending on many factors (like presence of cationic polymers), even have preferences for anionic species such as organic preservatives available in salt form (e.g. benzoate, sorbate, salicylate, anisate, levulinate…). Only a few minerals like kaolinite and iron-rich minerals have a noteworthy affinity for anions. Obviously, CEC and AEC are also pH dependent.Apart from that, the main issue with clay minerals and preservation is not or at least not solely due to loss of free preservative due to adsorption but the often high content of trace elements within these minerals which serve as bug food and/or overcharge the added chelates. -
This has been a strong topic over the years and thanks to both @Pharma and the always informative @PhilGeis for the intel here. I had learned years ago that kaolin adsorbed (thanks again Pharma) preservatives too, so that it is advisable to always add all preservatives as early as possible in the process stream if you are adding clays of any kind. I’ve always wondered whether that was true or not. Can either of you weigh in? Should preservatives be added before or after clay addition?
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As long as there is diffusion, order of addition won’t matter. Also, ‘monoprotic ions’ aren’t bound very strongly to clay minerals (which is generally a good thing here).Pre-wetting/saturating clays (or any other porous or ion exchanging material) with as many ingredients not serving as preservatives increases the chance that ‘unimportant’ molecules will bind first and hopefully stay bound = occupy the spaces where preservatives might potentially compete for. But then again, a liquid system is always in equilibrium. Given the long period of time a cosmetic product may lay around before being used up, equilibrium will be achieved and a preservative having higher affinity than whatever was originally bound will occupy its place.A workaround may be to disperse such materials in the oil phase to render them hydrophobic and hence hampering electrostatic surface interactions. Alas, this approach is not always feasible.Another strategy is to add ions with high affinity for clay minerals. Drawback here is that commonly used salts are quite efficient at killing emulsion stability, saturating chelates, or are unwanted for other reasons (such as aluminium, copper, iron, phosphate, or nitrate).Affinity series for cations: Al
3+> H+> Ca2+> Mg2+> K+= NH4+> Na+Affinity series for anions: PO43-> SO42-> Cl-> NO3-Sorry for the poor formatting, I don’t know how to format sub-/superscript.
A problem with this approach is that the ion affinity series neglect organic molecules and there is no straightforward formula to predict their interactions with clay minerals (unlike inorganic ions, they tend to bind without hydration shells which also makes them bind tighter). -
Hey Matt - not aware of data but prob less effective exposure if addition is after clay is in formulation. But that also leaves previous formulation protected only by GMP’s/time/temp etc. and demands late addition doesn’t run afoul other formula interactions.
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I have found that it makes a huge difference if the clay is irradiated or not. Clay that is simply dried as opposed to dried + irradiated is much more prone to microbial contamination.
It would make sense to add some preservative prior to the addition of the clay and another aliquot after addition of the clay to ensure that you get some preservative pentration the interstitial spaces of the clay and some preservative in “free” solution.
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I’ve a clay product that failed lab test due to bacteria count. Would baking clay at 180 degrees Celsius for 1 or 2 hours help to kill those nasties? What about using UV sterilizer? What is the best treatment needed to EFFECTIVELY STERILIZE the clay before incorporating into another products?
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Gamma irradiation is the best method for clay. Dry heat 180C for 2-3 hours would theoretically do it - if you get penetration and the material is dry.
Please don’t use “nasties” - no need for the mommy blog BS.
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The alternative word to “nasties” is “microbes”
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Thanks Perry and PhilGeis!
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Thanks Perry and PhilGeis!
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