Why is sodium chloride not a humectant? ...and other questions

MaydayMayday Member
edited January 27 in Science
I've been thinking about and looking up these topics for a while, but haven't been sure of an answer. I found a number of related concepts, but can't seem to make sense of them together.
Related questions:
  • What makes a humectant a humectant? How is humectancy related to hygroscopy (attracting water), "water holding" capacity, and the vapor pressure of an aqueous solution (escaping molecules)?
  • What part of a molecule can we use to clue in on its ability to bind water? Is it the physical structure that determines the ability to bind water rather than solely the presence of polar groups on the outside? (Like EDTA with divalent metal ions; Hydroxypropyl Cyclodextrins with VOCs—the structure determines the function)
  • Could humectants more effective than PCA be found? What makes PCA more effective than glycerol on a mass basis? Is there a difference in how PCA vs. glycerol vs. PEG attract water molecules?
⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️

Comments

  • @Mayday Since sodium chloride is an organic salt, when in contact with water, its ionic structure is lost and you'll vahe only ions (solvated) wandering aroung the solution...it's like when you make an instant noodle soup: you first have the tightly bound chunk of noodles, but when you add them to water, they disperse.

    Humectants work in a different way: they have a consistent structure (made of covalent ions, which keep atoms together in solution), and they bind water through specific groups (in the case of polyols like glucose or glycerin, though hydroxyl groups, and in the case of PCA, through its carboxylic groups)...they are like a mop, sucking water from a wet surface, and loosing it when in contact with a dry surface. 

    Now, just a word about gycerol: it's more than what it is (or what it seems to be, hehe). 
  • Bill_TogeBill_Toge Member, Professional Chemist
    the other point to bear in mind is that sodium and chlorine ions carry an electrical charge, even when solvated in water, and will interact with the skin in a different manner to an uncharged molecule such as glycerine
    UK based formulation chemist. Strongest subjects: hair styling, hair bleaches, hair dyes (oxidative and non-oxidative) I know some stuff about: EU regulations, emulsions (O/W and W/O), toothpaste, mouthwash, shampoos, other toiletries
  • I had completely forgotten about ionic disassociation in this context.
    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
  • PharmaPharma Member, Pharmacist
    edited January 29
    Humectancy is independent of any charge or dissociation upon solubilisation.
    In case of PCA: PCA is NOT a humectant but PCA salts are.
    Is there a structural feature to predict humectancy? No, there is not.
    Is there another way to still get some approximation? Yes. A good humectant is usually (but not necessarily) a hygroscopic substance. A very good humectant usually shows deliquescenct, too. Note, being a hygroscopic substance does not automatically mean that it's also a good humectant! Nobody would consider concentrated sulphuric acid a humectant ;) .
    A copy-paste (Valery V Gorbatchuk on ResearchGate) because he put's it to the point:
    ...Hydroscopicity has a simple thermodynamic explanation but encompass too broad range of specific cases. It corresponds to a process of water transfer from its pure liquid through vapor phase to
    (1) surface of hydrophilic adsorbent (eg. celulose), or the crystalline phase of hydrate (eg. that of cyclodextrin or inorganic salts), or the product of chemical hydration, like with CaO.
    (2) liquid solution formed as a result of dissolution of a target solid substance in water vapor (eg. CaCl2, sugar, etc., )
    (3) solution in liquid substance (eg. TEG).
    Respectively, in the case of hydroscopicity, Gibbs energy of this transfer should be large negative. For this,
    (1) Adsorbent should have metal cations, or hydroxylic, or peptide groups on its surface, or any combination of these. In other words, there should be both proton-acceptor and proton-donor centers in its pores. A crystalline receptor should be able to include or coordinate water;
    (2) Solubility of a solid in water should be high; the data can be found in any handbook with properties of organic and inorganic substances;
    (3) The liquid should have a negative or small positive parameter of hydrophobicity LogP, where P is a 1-octanol/water partition coefficient, which can be found in various databases or calculated by software available online.

    This means:
    A) On can reason why a substance is hygroscopic but most people couldn't tell whether a substance shows hygroscopicity or not.
    B) Humectancy is a mixed physiological, physical, and chemical alteration of a complex organ (i.e. skin) induced by a substance applied to it by different means in different carriers under different climatic conditions and so on... All you could guess is that physiologically tolerated substances such as cosmetic ingredients which are hygroscopic (especially if deliquescent as well), very well water soluble or water miscible, and have a low equilibrium relative humidity are likely behaving as humectants.

    Sodium chloride is only slightly hygroscopic, not super well water soluble, and it has a high equilibrium relative humidity of ~75% (it's only abstracting water from air above a rH of >75%).
  • @Pharma Thanks so much for the explanation!!! I really appreciate it. I think I understand well enough, though this is the first I've heard of the octanol/water partition coefficient.
    My chemistry schooling is just high school and one college chemistry class a while back, and I don't think we covered these topics. I'm trying to fill in the gaps in both directions: bottom up (fundamentals) and top down (questions like these, relevant to functional cosmetic chemicals).
    ⚠️ I have a lot of ideas, but not much experience! Please keep this in mind when reading my suggestions. ⚠️
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