If your product contains water then you will normally need a preservative to prevent the growth of harmful bacteria and microbes. In the UK and the EU before a product containing water can be sold or given away the formula must undergo preservative efficacy testing to ensure that the preservative is effective under various conditions.
Products containing a small percentage of water and high percentage of glycerin or sugar may be able to avoid the need for a preservative. I use a preservative in all of my leave in products that contain water. However, if I'm making a small batch of conditioner or shampoo that will be rinsed out there isn't any need for a preservative. Anhydrous products such as oils, butters and pomades do not require a preservative but you may need an antioxidant such as Vitamin E to prevent the oils from going rancid. You will also need an antioxidant if you use essential oils in your formula.
A friend of mine recently told me that she had been making her own products using recipes from YouTube. She told me she was using aloe vera mixed with water and a few essential oils. I questioned her on the use of a preservative and she said that she keeps the product in the fridge. I explained that refrigeration only protects the product while it's in the fridge but if you apply the product to your hair and leave it on for more than 24 hours the bacteria would have adequate opportunity to multiply on your head. She asked about adding water or aloe vera to store bought products that already contain preservatives. I explained that preservatives are normally used at approximately 1% of the formula, if you add other ingredients to the formula you are reducing the percentage of preservative and it may no longer be as effective.
The type of product you make will determine the type of preservative that you choose. Some preservatives work better in an emulsion so they would be unsuitable for water based lotions.
An emulsion is a product that contains both lipids (fats, waxes, butters and oils) and water. Lipids and water do not naturally mix with each other so if you want to create a cream, lotion or conditioner containing lipids and water you will need an emulsifier along with a preservative (a preservative is not needed if you are planning to rinse the product off of your hair quite quickly). Lecithin is a natural 'water in oil' emulsifier which means that it works well when a small amount of water is combined with a large amount lipids. Beeswax and Lanolin also have some 'water in oil' emulsification properties as they are able to hold on to small amounts of water helping to prevent our hair from drying out. Xanthan Gum and Tragacanth Gum are natural water thickeners which allow droplets of oil to be dispersed and suspended in the formula so they can function as natural 'oil in water' emulsifiers. However, these ingredients can only hold onto small amounts of oil before the formula starts to separate. Many formulations combine 'water in oil' emulsifiers with 'oil in water' emulsifiers to produce a stable emulsion.
The majority of emulsifiers found in store bought products are synthetic as these emulsifiers tend to produce more stable emulsions. Some synthetic emulsifiers have been found to have negative side effects so please do your research when experimenting with these ingredients. I have been making products with Behentrimonium Methosulfate (BTMS) since 2009 and I have had great results with this ingredient. BTMS is an excellent detangler so it's often used in hair conditioners. It's normally combined with Cetearyl Alcohol for it's emollient and thickening properties. In the wrong leave in formula BTMS can be a bit drying so it might take some experimenting until you find a formula that works well for your hair.
The bottom line is, if you make your own products containing water, rinse them off within a few hours or use a preservative.
