US 2019/0271885 Al  3  Sep.5,2019  [0065]  For example, in a case where at least one of a  specific acid or a salt of the specific acid is included in a  retardation layer, decomposition of the ester group is sup› pressed.  [0074]  Furthermore, in a case where at least one of the  specific acid or a salt of the specific acid is included in the  retardation layer, the total content of the specific acid and a  salt thereof in the retardation layer is not particularly limited,  but from the viewpoint that the wet heat resistance of the  optical film is more excellent, the total content is preferably  0.10% to 5.00% by mole, and more preferably 0.20% to  2.50% by mole, with respect to the liquid crystal compound  represented by Formula  (I)  used for the formation of the  retardation layer. In addition, the specific amount of the total  content of the specific acid and a salt thereof in the retar› dation layer is preferably 0.25 to 12.3 nmol/cm 2 .  [0066]  In addition, components included in each of the  layers easily migrate in a wet heat environment. As a result,  usually, even in a case where at least one of a specific acid  or a salt of the specific acid is included in an alignment layer  disposed adjacent to the retardation layer, at least one of a  specific acid or a salt of the specific acid included in the  alignment layer easily migrates into the retardation layer in  the wet heat environment, and thus, the retardation layer is  in an acidic environment, whereby decomposition of an  ester group derived from the liquid crystal compound  included in the retardation layer is suppressed.  [0075]  Moreover, for example, in a case where only the  specific acid is included and a salt of the specific acid is not  included in the alignment layer, the total content is calcu› lated, considering the content of a salt of the specific acid in  the alignment layer as 0.  [0076]  The specific acid is an acid having the pKa of -10.0  or less.  [0067]  Hereinafter, the optical film of the embodiment of  the present invention will be described with reference to  drawings. FIG.  1  shows a cross-sectional view showing a  first embodiment of the optical film. Further, the drawings in  the present invention are schematic views, and the thickness  relationship, the positional relationship, and the like of each  of the layers do not necessarily match actual ones, which is  also applied to the following drawing.  [0077]  The pKa of the specific acid is any value of -10.0  or less, and from the viewpoint that the wet heat resistance  of the optical film is more excellent, the pKa is preferably  -11.0 or less, and more preferably -12.0 or less. The lower  limit is not particularly limited, but from the viewpoint that  the wet heat resistance of the optical film is more excellent,  the lower limit is preferably -20.0 or more, and more  preferably -18.0 or more.  [0078]  A salt of the specific acid is a compound in which  one or more hydrogen ions included in the specific acid are  substituted with cations such as metal ions and ammonium  ions. The salt may be either a so-called inorganic or an  organic salt.  [0079]  The type of the metal ion is not particularly limited,  and examples of the metal ion include an ion of a metal  selected from the group consisting of an alkali metal and an  alkaline earth metal.  [0068]  FIG.  1  is a schematic cross-sectional view showing  the first embodiment of the optical film of the present  invention. In FIG.  1,  an optical film  lOA  includes an  alignment layer  12  and a retardation layer  14  disposed  adjacent to the alignment layer  12.  [0069]  Hereinafter,  each of members and materials  included in the optical film will be described in detail.  [0070]  First, the specific acid and a salt thereof included in  the optical film will be described in detail.  [0071]  At least one of the specific acid or a salt of the  specific acid is included in at least one of the alignment layer  or the retardation layer. Specifically, at least one of a specific  acid or a salt of the specific acid may be included in only one  of the alignment layer and the retardation layer, or at least  one of the specific acid or a salt of the specific acid may be  included in both of the alignment layer and the retardation  layer. One or both of the specific acid and a salt thereof may  be included in one of the respective layers or may be  included in each of the layers.  [0080]  Examples of the ammonium ion include NH$_{4 }$ +  and  N(R)$_{4 }$ +  (R represents a hydrocarbon group).  [0081]  The pKa is an acid dissociation constant, and a  smaller value thereof indicates a higher acid strength.  [0082]  In the present specification, the pKa is calculated  based on the following procedures (i) to (iv). That is, in a  case where the pKa of the specific acid can be calculated in  (i), the pKa calculated with (i) is taken as the pKa of the  specific acid. In a case where the pKa cannot be calculated  with (i), a trial of calculation of the pKa is made with (ii),  and in a case where the pKa can be calculated with (ii), the  value is taken as the pKa of the specific acid. In addition, in  a case where the pKa cannot be calculated with (ii), a trial  of calculation of the pKa is made with (iii), and in a case  where the pKa can be calculated with (iii), the value is taken  as the pKa of the specific acid. Further, in a case where the  pKa cannot be calculated with (iii), a trial of calculation of  the pKa is made with (iv), and in a case where the pKa can  be calculated with (iv), the value is taken as the pKa of the  specific acid.  [0083]  (i) The pKa value based on a Hammett substituent  constant and the database of values in a well-known docu› ment is determined by computation using the following  software package 1.  [0084]  (Software Package 1)  [0085]  Advanced Chemistry Development (ACD/Labs)  Software VS.14 for Solaris (1994-2007 ACD/Labs).  [0086]  As the pKa of a superacid which is computable  with the software package 1, a value obtained by rounding  [0072]  Moreover, a presence or content of the specific acid  and a salt thereof included in the alignment layer and the  retardation layer can be measured by using time-of-flight  secondary ion mass spectrometry (TOP-SIMS). Further, as  described in detail in later paragraphs, the amounts of the  specific acid and a salt thereof included in the alignment  layer and the retardation layer can also be calculated from  the amounts of the specific acid or a salt thereof, and an acid  generator that generates the specific acid, which are used.  [0073]  In a case where at least one of the specific acid or  a salt of the specific acid is included in the alignment layer,  the total content of the specific acid and a salt thereof in the  alignment layer is not particularly limited, but from the  viewpoint that the wet heat resistance of the optical film is  more excellent, the total content is preferably 0.10% to  5.00% by mole, and more preferably 0.20% to 2.50% by  mole, with respect to the liquid crystal compound repre› sented by Formula  (I)  used for the formation of the retar› dation layer. Further, a specific amount of the total content  of the specific acid and a salt thereof is preferably 0.25 to  12.3 nmol/cm 2  in the alignment layer. 