I do not have the chemistry background to answer your question...but I will toss this out there.Typically (not always) BG will be less sticky. So some formulators will split the formula, as they feel things get too tacky with just glycerin. However...as many of you know...I'll buy the same ingredient from multiple suppliers .... just to compare. I did this with BG (when I was using it....no long am).... and when one of the repackers offered a non-petrol based version...I jumped all over it....and you guessed it....more gross than glycerin!I use glycerin at low levels...but love to support it with propanediol, betaine, and my all time fav....pentylene glycol. (I call that one...my secret weapon.)
It's better to do a combination since they have been shown to work better combined. Here's a screenshot from one of the test for a comparison done to compare individually. Link from my dropbox (since i can't find the PDF online strangely, please save if require in case i move it) -> https://www.dropbox.com/s/4r009ugoq0k077a/Moisturizing Effects of Diglycerol combined with glycerol.pdf?dl=0
@Abdullah it depends on multiple factors really. If you don't mind the natural/synthetic status, then the normal combination of butylene glycol + glycerin is fine enough in most cases, unless you want to use something more fanciful for specific purposes such as "natural derived" (eg, with propanediol or pentylene glycol), co-emulsification (eg, with pentylene glycol). From what i've seen, butylene glycol normally boosts the humectancy profile for the first 1 to 2 hours, which is similar to the Figure above. Good thing about the glycols, they would help boost preservation to a certain extent. Though just to mention, there's also 1,2-Hexanediol.
If you have to stick to one, then glycerin would be fine. Depending on the what you are trying to achieve and the suitability of the formulation, the choice can really be different. For example, butylene glycol can make sense if you are intending to give a short term / up front impression that your product is very hydrating but mainly the long term hydrating function is sustained by glycerin. This would deal more with the customer's perception.Alternatively, other humectants like sodium lactate would likely be a good choice but if your formulation is not electrolyte sensitive. Assuming that you are cost saving, then it may not work out since you likely will use carbomer as part of the formulation for stability purposes. The main issue with just using glycerin would be that it can be quite tacky depending on the percentage used and the humidity your intended users are at. In a high humidity climate, you don't need that much glycerin, but at lower humidity, you can consider adding more. So a mixture of humectants can be used to create different skin feels as required.
Performance wise Glycerin is the gold standard, Diglycerin is stickier and more expensive so you might not need it.If your product has a huge end price margin sure you can use any humectant combo you like, but if not I would say glycerin alone would be enough.
I am a poor resource for you...because I do not factor cost when I formulate... I only chase performance and texture.First.... I am crazy for fast breaking, wet feeling products that dry quickly with a cooling sensation, and feel as close to weightless as possible. Creams with a gel undertone. I expect them to feel like products one might have tried that cost $200+. So pentylene glycol brings an incredible 'wet' feel when added to a formula (more so than other glycols I have tried). So automatically a 'yes' for me. Compound this with other well known fast breaking components...and you can begin to imagine. However, I also use Pentylene G as part of a larger preservative program, as jemolian mentioned. I also use the afore mentioned 1,2-Hexanediol plus caprylyl glycol in everything I make. I think pharma has an affinity for the glycols...and has really pushed me down that path (so I use use several in each project)...hehehe.I think the point Jemolian makes about how humectants perform at different humidities.... is often overlooked. I produce products in a rain forest.... So I need them to be comfortable here...as well as in a dry climate. Based on feedback from personal and testers from around the globe, I have for the most part, hit the target I sought. I don't think that could have been achieved with a single humectant. That being said....if you are selling into a single climate....than one could hypothetically test the humectants against the climate, and make a match.I am not a huge fan/follower/believer in HA...but I use it in absolutely everything. Partly for claim...and partly...when I take it out....I notice it. I use a magical blend of wet and dry silicones....and I am suspecting a synergy in this area with HA. The moisturizing aspects of some of my projects....are greater than the sum of the parts. I do not use BG or Propylene glycol...probably more for consumer perception than any other reason. When I was evaluating PG against Propanediol... in my climate Propanediol clearly out performed it. This again....results will vary with climate and other factors. When I tested both BG and PG....they never really stood out in a formula....like Pentylene glycol.....that one will make you sit up and pay attention. Sidebar.... lots of glycols...can cause issues in emulsions....so better be using something pretty stout with them. I am typically starting with a core of 165, and flavoring it with a Montanov + GSC or a cationic (w/o GSC).I guess my final point on humectants would be.... who cares...if you don't support them with stellar barrier function. They're all short lived...if you don't lock them in and down.I break every project into the functional aspects....and build the supporting cast for each aspect, otherwise all is for naught.
Yes, so normally for a higher short term humectancy, the combination can be used, but if your humidity is average of 52%, then butylene glycol won't really make sense. You can take a look at the reference below, which is the normal table when you search for "humectant humidity" on google.
In terms of the 50+% humidity range, you can consider about 3% to 5% of glycerin. In my test, in an air conditioned room, at about 50% humidity, 2% glycerin USP isn't that tacky.
Regarding the pH, it depends on your overall formulation, but there's no issues with pH being at 4.5 - 5.5 to be honest. My preference is pH 5 - 5.5.
In this paper, The role of epidermal lipids in cutaneous permeability
Though, ultimately it's up to you and your formulations requirements really.
Butylene glycol for short term hydration is also less than glycerin. Isn't it? Yes, so normally for a higher short term humectancy, the combination can be used, but if your humidity is average of 52%, then butylene glycol won't really make sense. You can take a look at the reference below, which is the normal table when you search for "humectant humidity" on google. In terms of the 50+% humidity range, you can consider about 3% to 5% of glycerin. In my test, in an air conditioned room, at about 50% humidity, 2% glycerin USP isn't that tacky. Regarding the pH, it depends on your overall formulation, but there's no issues with pH being at 4.5 - 5.5 to be honest. My preference is pH 5 - 5.5. Natural skin surface pH is on average below 5, which is beneficial for its resident floraIn this paper, The role of epidermal lipids in cutaneous permeability
barrier homeostasis:For example, the activities of both b-glucocerebrosidase and acidic sphingomyelinase are optimal at or below pH 5.5, which is very similar to the pH of the stratum corneum. Conversely, many of the proteases in the stratum corneum have a pH optimum of 7 or higher;
therefore, their activities are decreased at the usual stratum
corneum pH of 5.5. If the pH of the stratum corneum is
increased, the activities of b-glucocerebrosidase and acidic
sphingomyelinase are reduced and the extracellular processing of glucosylceramides and sphingomyelins to ceramides
is impaired, leading to abnormalities in the structure of the
extracellular lipid membranes and decreased permeability
barrier function (4, 41–43). Furthermore, increases in stratum corneum pH stimulate protease activity, resulting in increased corneocyte desquamation (4, 41, 42).Though, ultimately it's up to you and your formulations requirements really.
Butylene Glycol is more versatile because it can be used as a solvent and less tacky than glycerin. It also a preservative booster. Sodium Lactate has a downside of being a strong electrolyte, so acrylic acid based polymeric thickeners and emulsifiers aren’t an option.
@Abdullah the table is from the book called "Handbook of cosmetic science and technology". You can find the PDF online or if you only want to see the chapter, it's on google books here -> https://books.google.com.sg/books?id=RIvOBQAAQBAJ&newbks=1&newbks_redir=0&lpg=PP1&pg=PA93#v=onepage&q&f=false
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