HAIR CARE 0.2 0.2 0.0 0.15 -0.2 -0.4 -0.6 Peak temperature: 233.09°C -0.8 0 Exo Up Figure 1: Typical DSC curve of virgin human hair
the cuticle’s smooth and closed structure, promoting healthier hair. Hair colouring is a popular cosmetic
procedure that alters the natural pigment of the hair shaft. However, maintaining the vibrancy and longevity of dyed hair can be a challenge. Hair colouring involves altering the hair’s internal structure to introduce synthetic pigments. The longevity and appearance of hair dye are influenced by various external factors. It is hypothesised that certain carboxylic
acids can penetrate the hair fibre, regulate water uptake, and bind with proteins. Evidence for the interesting effect of these acids comes from differential scanning calorimetry (DSC) experiments and water uptake (DVS) experiments.2 This article explores the chemical,
structural, and physical advantages of using low pH shampoos by looking at techniques in the literature using Dynamic Vapor Sorption (DVS) and Differential Scanning Calorimetry (DSC) to investigate the benefits of low pH formulations and the interaction between hair fibres and shampoo formulations.
25 Virgin 20 15 10 5 0 Figure 3: DVS adsorption isotherm for treated and untreated human hair6
www.personalcaremagazine.com 5% Citric acid
Scientific literature and measurements to support low pH claims Thermal properties of hair (DSC)-investigation of the denaturation properties of the ∝-helix Hair switches soaked in such acidic solutions showed a marked increase in protein denaturation temperature—in some instances by up to 10°C, suggesting a damage repair benefit for these carboxylic acids. The denaturation temperature is a measure of the thermal stability of the hair protein as a result of the biochemical environment, especially pH. Analysis of a DSC thermogram to determine
the transition peak temperature peak (Tp) (sometimes-termed maximum temperature (Tmax), or denaturation temperature (Td) – i.e. denaturing of the matrix proteins – showed the transition of protein from a native to a denatured conformation is accompanied by the rupture of inter- and intra- molecular bonds, and the process needs to occur in a cooperative manner to be discerned by DSC.3 Therefore, to some, the ability for an
ingredient or product to raise this value (no matter how slight) is an indication of ‘repair’ of the hair fibre structure.
Figure 2: Typical human hair DVS isotherm- adsorption and desorption This technique gives an indication of any
interaction between actives and hair proteins particularly the loss of structural integrity or the stabilisation of protein components. DSC analysis showed that hair treated with low pH shampoo exhibited higher thermal stability, as indicated by a higher denaturation temperature (Tg) of the ∝-helix, in contrast, hair treated with the high pH shampoo displayed a lower Tg and a broader thermal transition range, suggesting that the high pH had compromised the keratin structure, making the hair more prone to heat-induced damage. It is hypothesised that certain small molecules such as citric acid have the ability to modify the intrinsic properties of hair by penetration the hair fibre and possibly stabilising proteins by binding on to negative charged site. The improved thermal properties of low pH-
treated hair imply that it retains better structural integrity under both normal and elevated temperature conditions. These findings contribute to a growing body of evidence supporting the use of low pH shampoos as a gentler, more effective option for promoting long-term hair health.
DVS - moisture content. Do acids act to regulate water content in the hair fibre? Water is a fundamental component of hair, and it affects the physical and mechanical properties of hair. The formation of a tighter structure after cross-linking restricts water molecules, leading to a decrease in hysteresis.6
Soaking human hair
switches in a 5% citric acid solution decreased hair’s water uptake and furthermore slowed the release of the water during the drying process.
DVS desorption isotherm9 The results from DVS analysis show that hair treated with low pH shampoo demonstrated significantly higher moisture retention compared to hair treated with high pH shampoo. At both 30% and 70% relative humidity, the low pH- treated hair absorbed less water, suggesting that the low pH shampoo helped to close the hair cuticle more effectively, reducing moisture loss.10 Various hypotheses for this effect include:
Organic acids such as citric, tartaric, lactic have an affinity for hair, and a suggestion
April 2025 PERSONAL CARE 50
Temperature T (°C) 150
100 200 250 -0.05 Sample PP |(%) 0 0 10 20 30 40 50 60 70 80 90 100
Endothermic Peak
Enthalpy (normailzed): 6.3785 J/g Onset x 228.71°C
0.1 0.05 ■ Cycle 1 Scop ■ Cycle 1 Desorp ■ Cycle 2 Scop ■ Cycle 2 Desorp
89
Amount adsorbed (%0
Heat Flow (Normalized) Q (W/g)
Change in Mass (%) - Dry
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