This is a question asked in a previous thread and I have collected a lot of info about it so I thought I'd write a discussion.
I'll start with Conditioning Agents for Hair and Skin, a book edited by Perry Romanowski (/u/thejoggler44) and Randy Schueller in 1999. AFAIK it's the only book specifically on the subject.
Helpfully in chapter 1 they provide a definition:
A product can be said to have conditioning properties if it improves the surface to which it is applied particularly if this improvement involves the correction or prevention of some aspects of surface damage
In plain English I believe this could be boiled down to
- Makes the surface look and feel better, like soft, shiny or smooth
- "Fixes" existing damage like smoothing down rough areas
- Protects against future damage, acting like a shield to prevent further damage like UV, heat, friction
In the context of hair, the chapter goes on to list some components of conditioning
- Structural factors: Mainly the damage to the cuticle, when the scales are chipped, raised, or broken by actions such as combing. The damage leaves the hair vulnerable to more damage, so to help with this conditioners help maintain a smoothing coating on the outside
- Compositional elements: Natural lipids can get stripped from hair, which can make it more brittle and prone to damage. Water, which usually is determined by humidity (explained in our myths guide) but the book says when hair is dried by heating it can impair this (histeresis), which can make hair less soft and more prone to breakage. Conditioner can help add these things (lipids and water) back to hair.
- Appearance: consumers tend to associate shine and color with health, and conditioner can add both to hair
- Tactile Perception: Conditioning can make the hair feel softer by smoothing the cuticle and assuring the proper level of lipids/moisture, also lubrication which makes the hair easier to groom and helps prevent grooming friction damage
- Physical properties: Like improving tensile strength, reducing static charges that cause flyaways, and frictional characteristics
Now let's go to the other main hair book Chemical and Physical Behavior of Human Hair by the late Clarence Robbins, last published in 2012
For hair conditioning products the principle function involves combability. Ease of combing depends primarily on lubrication of the fiber surface. This action is accomplished by the sorption or binding of lubricating or conditioning ingredients onto the hair surface.
This is a simpler definition that may be too simple in some ways, but gets to something important which is that hair conditioners deposit stuff on the hair surface. In modern conditioners this is often "cationic surfactant in combination with long-chain fatty alcohol or other lipid components", these are unique to modern chemistry.
Before the mid 20th century most conditioning was just applying oils and grease to dry hair. In the mid-twentieth century the advent of chemical treatments and shampoos meant they needed a product that could deposit on wet hair and not just be completely rinsed down the drain when rinsed.
Another issue with oil/grease is that damaged hair becomes hydrophilic (water loving) and that means oil is less likely to stick on it. Cationic surfactants get around this by being positively (cationically) charged, so they are attracted to the negative charges on the damage surface. This is also useful because they are going to be MORE attracted to those damaged areas than undamaged areas.
Finally my third book, Hair Care an Illustrated Handbook (2005) by Dr. Zoe Diana Draelos, which is unlike the previous sources targeted at dermatologists (MDs) not chemist formulators.
Its most simple definition is:
- Conditioners: Leave the hair soft and smooth after sebum removal
and later on expounds on it in a dedicated section
Conditioners are liquids, creams, pastes, or gels that mimic sebum in making the hair manageable, glossy, and soft. The role of conditioners goes beyond maintaining the appearance of healthy hair. Conditioners also attempt to recondition hair that has been damaged by chemical or mechanical trauma. Common sources of trauma include excessive brushing, hot blow-drying, permanent hair waves, hair straightening, hair bleaching, etc. Damage to the hair shaft can also occur through environmental factors such as exposure to sunlight, air pollution, wind, seawater, and chlorinated swimming pool water.26 This type of hair damage is technically known as ‘weathering.’ Obviously, since hair is nonliving tissue,
Healthy, undamaged hair is soft, resilient, and easy to disentangle. Unfortunately, the trauma caused by shampooing, drying, combing, brushing, styling, dyeing, and permanent waving damages the hair, making it harsh, brittle, and difficult to disentangle. Hair conditioners are designed to reverse this hair damage by decreasing static electricity, improving manageability, increasing hair shine, decreasing split ends, and improving hair flexibility.
As an aside a common online discussion goes along the lines of "if shampoo removes sebum, and conditioner adds it back, shampoo must be bad and if we stop using it we won't need conditioner" which understates the benefits of shampoo and also ignores the fact that conditioner corrects for a lot of non-shampoo related damage. It's talked about on our myths page.
Anyway, maybe that was helpful, keep in mind I am not a chemist or a formulator, nor do I work in the industry, just an enthusiast trained to read scientific research as part of my unrelated sciences degree and I also did some professional science writing in the past.
I would love any industry folks or scientists to weigh in, and feel free to ask followup questions