1. Separation and Concentration of Ingredients:
* Temperature Fluctuations: Exposure to extreme temperatures (very hot or very cold) can destabilize the shampoo's emulsion, causing ingredients like thickeners, oils, and surfactants to separate. When this happens, some ingredients can concentrate, leading to a gluey or clumpy texture.
* Evaporation: If the shampoo bottle is left open or the cap isn't sealed tightly, water can evaporate. This increases the concentration of the remaining ingredients, especially thickeners, resulting in a viscous, glue-like consistency.
* Ingredient Interactions: Certain ingredients, when exposed to air or changes in temperature, can react with each other. This can cause polymerization (linking of molecules) or cross-linking, leading to a gel-like or gluey substance. This is more common in shampoos with complex formulations.
2. Microbial Growth:
* Contamination: Although shampoos contain preservatives, they aren't always foolproof. If bacteria, mold, or other microbes enter the shampoo (e.g., from contaminated water or fingers), they can grow and alter the shampoo's composition. This can produce byproducts that cause thickening or clumping. In these cases there is usually a smell, but not always.
3. Chemical Reactions and Degradation:
* Oxidation: Some ingredients, especially oils and fragrances, can oxidize when exposed to air and light. This can lead to changes in texture and potentially the formation of sticky or resinous substances.
* Expired Product: Over time, even well-formulated shampoos can degrade. The preservatives might become less effective, and the ingredients can break down, leading to changes in texture and performance.
* Incorrect Formulation: In rare cases, the manufacturer may have used an unstable formula or an incompatable ingredient.
4. Hard Water:
* Mineral Buildup: If you use very hard water (water with high mineral content), minerals can react with certain shampoo ingredients, especially sulfates or soaps. This can create a residue that builds up over time and can cause the shampoo to thicken and become less effective. This is not typically "gluey," but can cause issues.
What to do if your shampoo becomes gluey:
* Check the Expiration Date: If it's expired, it's best to discard it.
* Inspect for Contamination: Look for any signs of mold, discoloration, or an unusual odor. If present, discard the shampoo.
* Try to Re-Emulsify (If Minor): If the shampoo appears separated but otherwise seems okay, try shaking the bottle vigorously to see if you can remix the ingredients. You might add a *very* small amount of distilled water if it's extremely thick, but be cautious as this can dilute the preservatives too much.
* Consider New Shampoo: If unsure or the changes are significant, it's safest to replace the shampoo. The ingredients may have changed enough to make it no longer safe or effective.
* Store Properly: Keep shampoo bottles tightly sealed, away from extreme temperatures, and out of direct sunlight.
In summary: Thickening or glueing can happen from a combination of factors. The best way to prevent it is to properly store your shampoo, use it before the expiration date, and avoid contamination.