Of course I had to answer this question pronto! Thanks for the very insightful question Courtney!
Q1: How do scientists know that hair does not react in the pH range 4-9?The experiment (J Soc Cosmet Chem, pp 393-405, 1981) performed was to place hair into solutions of different pH (skip to the diagram if you wish - click it to enlarge). The scientists then made sure that the solution stayed at the fixed pH. If hair started reacting with acid, the solution would become less acidic meaning the scientists have to pump more acid into the solution (and the same for high pH i.e base/alkaline). If there is no reaction then little to no extra solution is required.
Conclusion: Hair when placed into a solution in the pH range of 4-9 does not change
Suppose hair is at a pH of 5 and you then use an alkaline soap at a pH of 8 to wash it. The hair does NOT get to the pH of 8. In actual fact it may change by about 0.4 to 0.5 units (see previous post). It is not cycling from 5 to 8 but rather 5 to 5.5.
Q2: Each pH unit is 100 times more powerful than the previous one.I am not sure that powerful is the right word but in essence, yes this is somewhat true (not exactly true but close enough). I could go into the ins and out of it but I am not sure how interesting that is. Yes it is true that the difference between two numbers on the pH scale is quite large but this does not mean that a pH of 5 is more powerful than a pH of 6. For hair, science tells us that a pH of 5 is no different from a pH of 6. However just to give you a different point of view, for blood the aim is a very narrow pH range somewhere between 7.3 and 7.5. In short pH is a scale that makes scientific sense and is relative to what you are using it for. (For geeks - pH is on a logarithmic scale).
Q3: How can hair resist change over such a large range?This is indeed fascinating. The truth is I have not seen a good explanation of why, there is only clear evidence is that it does. Theories as to why include the role of the oil layer of hair, the shielding effect from the cuticle, the presence of certain amino acids in the hair............the list goes on. I hate to disappoint but science is like that sometimes.......more questions than answers.
I can say that hair is really not that unique in this respect. There are many substances in the natural world which are designed to resist chemical attack. An example is your stomach which produces acids at a pH of 1-2 and manages to maintain its structure.
Side note:To be accurate, the pH range where hair is not seen to react is 4-10. The pH 9 cut off comes because in addition to the reaction tests, the scientists performed microscopic examination of hair. Over a pH of 9, the cuticle of the hair is seen to lift. Therefore the cut off point is moved to 9.
Second note, this study is for natural hair, if hair is bleached or relaxed, the pH range is narrowed by 2-3 units on the base side going to the range of pH 4-7/8
General info - The pH solutions were made from sodium hydroxide (base) and hydrochloric acid (acid).