Alterations promoted by acid straightening and/or bleaching in hair microstructures

C R R C Lima; R J S Lima; A C C Bandeira; R A A Couto; M V R Velasco; H N Bordallo; C L P Oliveira

doi:10.1107/S1600576723005599

Alterations promoted by acid straightening and/or bleaching in hair microstructures

J Appl Crystallogr. 2023 Aug 1;56(Pt 4):1002-1014. doi: 10.1107/S1600576723005599.

Authors

C R R C Lima¹, R J S Lima^{2

3}, A C C Bandeira¹, R A A Couto⁴, M V R Velasco⁵, H N Bordallo^{3

6}, C L P Oliveira¹

Affiliations

¹ Institute of Physics, University of São Paulo, São Paulo 05508-090, SP, Brazil.
² Academic Unit of Physics, Federal University of Campina Grande, Campina Grande 58429-900, PB, Brazil.
³ Niels Bohr Institute, University of Copenhagen, 2300 Copenhagen, Denmark.
⁴ Institute of Chemistry, University of São Paulo, São Paulo, Brazil.
⁵ Faculty of Pharmaceutical Sciences, University of São Paulo, São Paulo 05508-000, SP, Brazil.
⁶ European Spallation Source ESS ERIC, PO Box 176, SE-221 00 Lund, Sweden.

Abstract

Human hair is a biopolymer constituted mainly of keratin intermediate filaments, lipids, pigments and water. Cosmetic treatments usually interact with the hair at the molecular level, inducing changes in its components and modifying the physicochemical and mechanical properties of the fibers. Here, the effect of acid straightening on the morphology and ultrastructure of Caucasian hair was investigated by a group of complementary experimental methods: wide-, small- and ultra-small-angle X-ray scattering; high-resolution 3D X-ray microscopy; quasi-elastic neutron scattering and inelastic neutron scattering; thermogravimetry-mass spectrometry; and differential scanning calorimetry (DSC). X-ray diffraction patterns showed that acid straightening associated with a flat iron (∼180°C) changed the cortex of the fiber, shown by denaturation of the intermediate filaments (measured by DSC). The increase in the spacing of the lipid layers and the observation of the dehydration behavior of the fiber provided indications that water may be confined between these layers, while neutron spectroscopy showed alterations in the vibration mode of the CH₂ groups of the lipids and an increase of the proton (H⁺) mobility in the hair structure. The latter may be associated with the extremely low pH of the formulation (pH ≃ 1). Additionally, this investigation showed that bleached hair (one-time bleached) is more damaged by the action of acid straightening than virgin hair, which was shown by a threefold increase in the percentage of total porosity of the tresses. The obtained results demonstrate that the investigation approach proposed here can provide very important thermodynamic and structural information on induced changes of hair structure, and certainly can be applied for the evaluation of the action mode and efficiency of cosmetic treatments.

Keywords: X-ray scattering; acid straightening; biopolymers; bleaching; cosmetic treatments; damage; hair; microstructure; neutron spectroscopy.

Grants and funding

CRRCL and CLPO acknowledge financial support of this work by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Instituto Nacional de Ciência e Tecnologia dos Fluidos Complexos (INCT-FCx), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo, grant/award No. 2016/24531-3. RAAC acknowledges financial support by Fundação de Amparo à Pesquisa do Estado de São Paulo, grant/award No. 2011/50318-1. RJSL acknowledges financial support from the Federal University of Campina Grande (UFCG) for a research visit at the Niels Bohr Institute (NBI). RJSL and HNB also acknowledge the support for neutron and X-ray research given by DanScatt, the Danish Agency for Science, Technology and Innovation.